INSTITUTE OF PHYSICS PUBLISHING JOURNAL OF PHYSICS D: APPLIED PHYSICS

J. Phys. D: Appl. Phys. 34 (2001) R1–R14 PII: S0022-3727(01)26660-1

TOPICAL REVIEW

Terahertz wave parametric sourceKodo Kawase1,4, Jun-ichi Shikata2 and Hiromasa Ito2,3

1 Kawase Initiative Research Unit, RIKEN, 2-1 Hirosawa, Wako 3510198, Japan2 Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira,Sendai 9808577, Japan3 Photo Dynamics Research Center, RIKEN, 519-1399 Aramaki-Aoba,Sendai 9800845, Japan

E-mail: kodo@riken.go.jp

Received 5 December 2001PublishedOnline at stacks.iop.org/JPhysD/34

AbstractWidely tunable terahertz (THz) wave generation by optical parametricprocesses based on laser light scattering from the polariton mode ofnonlinear crystals is reviewed. Using parametric oscillation of LiNbO3 orMgO-doped LiNbO3 crystal pumped by a nanosecond Q-switched Nd : YAGlaser, we have realized widely tunable coherent THz-wave sources in therange between 0.7 and 3 THz, with a simple configuration. For the efficientcoupling of the THz wave, a monolithic grating coupler or an Si-prism arraycoupler was used. We report the detailed characteristics of the oscillationand the radiation, including tunability, spatial and temporal coherency,uni-directivity, and efficiency. Further, Fourier transform limited THz-wavespectrum narrowing was achieved by introducing the injection seedingmethod. A linewidth of about 100 MHz (0.003 cm−1) was assured by theabsorption spectrum measurement of low-pressure water vapour. At thesame time, the THz-wave output was increased hundreds of times higherthan that of a conventional generator which has no injection seeder. Inaddition, a wider tunability was observed using a tunable diode laser as theinjection seeder. This room-temperature-operated, tabletop system promisesto be a new widely tunable THz-wave source that is suited to a variety ofapplications.

1. Introduction

In recent years, the generation of terahertz (THz) radiationby optical rectification or photoconductive switching has beenextensively studied using femtosecond laser pulses [1, 2].Applied research, such as time domain spectroscopy, makesuse of the high time resolution of THz waves and ultra broadbandwidth up to the THz region. In contrast, our researchfocuses on the development of tunable THz-wave sourceswith high temporal and spatial coherence using the resonantfrequency of ferroelectric crystal lattices. Specifically, widelytunable coherent sources have a wide range of applications,such as in material science, solid state physics, molecularanalysis, atmospheric research, bioscience, chemistry, gas

4 Author to whom correspondence should be addressed.

tracing, material testing, food inspection, differential imaging,etc. Novel tunable sources already exist in the sub-THz (several hundred GHz) frequency region, such as thebackward-wave oscillator (BWO). However, a widely tunableTHz-wave source has long been desired in the frequency regionabove 1 THz, where the tuning capability of a BWO rapidlydecreases.

Widely tunable sources covering the region between 1 and3 THz (with wavelengths from 100 to 300µm) are limitedto free electron lasers or photomixers. The University ofCalifornia at Santa Barbara (UCSB) plays a central rolein exploring the THz region with free electron lasers [3].However, the number of researchers who can access such anapparatus is limited, due to its large scale. In photomixers,two optical lasers are mixed to generate tunable THz waves

0022-3727/01/010001+14$30.00 © 2001 IOP Publishing Ltd Printed in the UK R1

Topical review

[4, 5]. This should become a reliable source if the limitationon the output due to the nondestructive limit of a photomixeris overcome. A p-type germanium (p-Ge) laser is anotherwidely tunable source in this region, but it is primarily for usein pure science since liquid He is required [6, 7]. Althoughthe aforementioned widely tunable sources can be used atthe laboratory level, they do not satisfy all of the needs ofresearchers interested in practical applications. Therefore,there is significant potential for an explosion of appliedresearch on the THz band if simple, convenient tunable sourcesare made available.

Many research efforts have been carried out a couple ofdecades ago concerning the generation of tunable coherent far-infrared radiation based on optical technology [8, 9]. Amongthem, THz oscillation and amplification were expected byNishizawa of our institute to be made available by utilizingthe resonant frequency of the crystal lattice or of the moleculeitself in 1963 [10, 11]. The efficient and widely tunableTHz generation had been reported in the pioneering worksof Pantell, Puthoff, and others in the late 1960s to theearly 1970s [12–14]. This is based upon tunable lightscattering from the long-wavelength side of the A1-symmetrysoftest mode in LiNbO3. The input (pump) photon at nearinfrared stimulates a near-infrared Stokes photon (idler) atthe difference frequency between the pump photon and thevibrational mode. At the same time, the THz wave (signal)is generated by the parametric process due to the nonlinearityarising from both electronic and vibrational contributions ofthe material. The tuning is accomplished by controlling thepropagation direction. Although the interaction is highlyefficient, it should be noted that most of the generated THzwaves are absorbed or totally reflected inside the crystal dueto a large absorption coefficient, as well as a large refractiveindex in the THz range. To allow the THz radiation, a cut exitwas made in the corner of the crystal.

It is to our surprise that no research has been reportedon this novel method since 1976 due to the alternative andsuccessful use of submillimetre molecular gas lasers. Inthese six years, we have developed an efficient and widelytunable source of coherent THz waves based on the principleof previous works, and far better characteristics by introducingthe monolithic grating coupler, the arrayed Si-prism coupler,doped crystals, and injection seeding.

2. Principles of operation

2.1. THz-wave parametric generation using a polariton

The generation of coherent tunable THz waves resultsfrom the efficient parametric scattering of laser light via apolariton (stimulated polariton scattering). A polariton isa quantum of the coupled phonon–photon transverse wavefield, and stimulated polariton scattering occurs when thepump excitation is sufficiently strong in polar crystals, such asLiNbO3, LiTaO3, and GaP, which are both infrared-active andRaman-active. The scattering process involves both second-and third-order nonlinear processes. Thus, strong interactionoccurs among the pump, the idler, and the polariton (THz)waves. LiNbO3 is one of the most suitable materials forgenerating THz waves efficiently because of its large nonlinear

kp

ki

θ kT

ω

ωTO

kT

ωp

ωi

ωT

ω = (c/n) k

k

θ

Photon-like(Parametric)

Figure 1. Dispersion relation of the polariton, an elementaryexcitation generated by the combination of a photon and a transverseoptical phonon (ωTO). The polariton in the low-energy regionbehaves like a photon at THz frequency. Due to the phase-matchingcondition as well as the energy conservation law which hold in thestimulated parametric process, the tunable THz wave is obtained bythe control of the wave vector kT. The inset shows the noncollinearphase-matching condition.

coefficient (d33 = 25.2 pm V−1, λ = 1.064µm [15]) andits transparency over a wide wavelength range (0.4–5.5µm).LiNbO3 has four infrared- and Raman-active transverse optical(TO) phonon modes, called A1-symmetry modes, and thelowest mode (ω0 ∼ 250 cm−1) is useful for efficient tunablefar-infrared generation because it has the largest parametricgain as well as the smallest absorption coefficient [16].

The principle of tunable THz-wave generation is asfollows. Polaritons exhibit phonon-like behaviour in theresonant frequency region (near the TO-phonon frequencyωTO). However, they behave like photons in the nonresonantlow-frequency region (figure 1), where a signal photon atTHz frequency (ωT) and a near-infrared idler photon (ωi)are created parametrically from a near-infrared pump photon(ωp), according to the energy conservation law ωp = ωT + ωi

(p: pump; T: THz; i: idler). In the stimulated scatteringprocess, the momentum conservation law kp = ki + kT

(noncollinear phase-matching condition; see the insets offigure 1 also holds. This leads to the angle-dispersivecharacteristics of the idler and THz waves. Thus, a coherentTHz wave is generated efficiently by using an optical resonatorfor the idler wave, and continuous and wide tunability isaccomplished simply by changing the angle between theincident pump beam and the resonator axis.

2.2. Theory of THz-wave parametric gain

In stimulated polariton scattering, four fields mutuallyinteract: the pump Ep, idler Ei, THz wave ET, and ionicvibration Q0 (lowest A1 mode). The parametric gaincoefficients for the idler and THz waves are obtained bysolving the classical coupled-wave equations that describe thisphenomenon. Assuming a steady state and no pump depletion,the coupled-wave equations are written as [16–18](

∇2 +ω2

c2εT

)ET = −ω2

c2χPEpE

∗i

[∇2 +

ω2i

c2

(εi + χR|Ep|2

)]Ei = −ω2

i

c2χPEpE

∗T

(1)

where ωi (=ωp − ω) and ω denote the frequencies of the idlerand THz wave, respectively, εβ (β = T, i) is the permittivity inthe material (LiNbO3), and c is the velocity of light in vacuum.

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The nonlinear susceptibilitiesχP andχR denote parametric andRaman processes, respectively, and are expressed as

χP = dE +S0ω

20

ω20 − ω2

· dQ (2)

χR = S0ω20

ω20 − ω2 − iω�0

· d2Q (3)

where ω0, S0, and �0 are the eigenfrequency, oscillatorstrength, and damping coefficient (or linewidth) of the lowestA1-symmetry phonon mode, respectively. The coefficientsdE (=16πd33) and dQ denote the second- and third-ordernonlinear processes, which originate from electronic and ionicpolarization, respectively. According to the rate equationanalysis, the expression of dQ in cgs units is given by

dQ =[

8πc4np(S33/L��)0

S0h̄ω0ω4i ni(n̄0 + 1)

]1/2

(4)

where nβ (β = p, i) is the refractive index and n̄0 =(exp[h̄ω0/kT ] − 1)−1 (h̄: Planck constant; k: Boltzmannconstant; T : temperature) is the Bose distribution function.The quantity (S33/L��)0 denotes the spontaneous Raman(Stokes) scattering efficiency of the lowest A1-symmetryphonon mode, whereS33 is the fraction of incident power that isscattered into a solid angle�� near a normal to the optical pathlength L, and is proportional to the scattering cross section.

The coupled-wave equations (1) can be solved usingthe plane wave approach, and analytical expressions of theexponential gain for the THz wave and idler are

gT = gi cosφ = αT

2

[

1 + 16 cosφ

(g0

αT

)2]1/2

− 1

(5)

where φ is the phase-matching angle between the pump andthe THz wave, g0 is the low-loss limit, and αT is an absorptioncoefficient in the THz region. In cgs units, they are written as

g0 =(

πωωiIp

2c3nTninp

)1/2

χP (6)

αT = 2|Im kT| = 2ω

cIm

(ε∞ +

S0ω20

ω20 − ω2 − iω�0

)1/2

(7)

The low-loss parametric gain g0 has the same form as theparametric gain in the optical region [19], but the nonlinearsusceptibility χP, which involves both second- and third-orderprocesses (equation (2)), is almost entirely determined by thethird-order (ionic) dQ term (more than 80% contribution).

The physical meaning of the susceptibility χP is explainedas follows. According to the simple classical picture, a polarcrystal, such as LiNbO3, is considered to be an ensemble ofindividual molecular systems, where each molecule consists ofnuclei bonded together and surrounded by an electron cloud.When the crystal is irradiated by the pump laser polarized alongthe z-axis of the crystal, the relatively light electron cloudabsorbs the pump energy and follows the incident field, andthe electronic dipole moment appears due to the displacementof the electron charge cloud with respect to the nuclei. Thisis the origin of the second-order nonlinearity expressed by thedE (or d33) coefficient. Next, some of the energy absorbed by

0 1 2 3Frequency [THz]

500 300 200 150 100

TH

z-w

ave

Gai

n [c

m–1

]

T= 77K150MW/cm2

T= 300K

300MW/cm2

300MW/cm2

150MW/cm2

Wavelength [µm]

1000

0

5

10

Figure 2. Calculated gain coefficient for the parametric THz-wavegeneration using LiNbO3 crystal pumped at 1.064µm. The gain isenhanced by cooling the crystal due to the reduced absorption loss atTHz frequency.

the electrons is transferred to the nuclei, and two nuclei beginto vibrate (i.e. phonons are created) because the A1-symmetrymodes are infrared-active. This phenomenon leads to the ionicdipole moment or the third-order nonlinearity expressed by theRaman susceptibility χR (Stokes process). Finally, the ionicvibration modulates the electronic vibration, and an electron–ion interaction occurs because the vibrations are along thez-axis. Thus, the three-photon parametric scattering processinvolving the pump, Stokes (idler), and THz (polariton) wavesinvolves both second- and third-order nonlinearities.

Figure 2 shows the calculated parametric gain gT forLiNbO3 at typical pump intensities. A gain in the order ofseveral cm−1 is feasible in the frequency domain up to 3 THzat room temperature, and the gain is enhanced by cooling thecrystal [20]. The decrease in the linewidth �0 of the lowestA1-symmetry phonon mode [21] makes the major contributionto the enhancement at low temperature, because αT is nearlyin proportion to �0 (equation (7)). The reduced linewidthreduces the absorption coefficient αT at THz frequencies,enhancing the parametric gain gT, which is a monotonicallydecreasing function of the absorption coefficient (equation (5)).Physically, when the polariton damping caused by randomthermal activation is reduced and the polariton has a longerlifetime, an efficient parametric interaction occurs. It is alsopossible to increase the parametric gain, either by increasingthe pump intensity or by using a shorter-wavelength pumpsource, because the gain is a monotonically increasing functionof g0, which is proportional to [(ωp − ωT) · Ip]1/2.

3. THz-wave parametric oscillator (TPO) withmonolithic grating coupler

Widely tunable THz-wave generation was successfullydemonstrated utilizing a grating coupler fabricated on thesurface of an LiNbO3 crystal which was pumped by aQ-switched Nd : YAG laser. In this section, we describe thedetailed characteristics of the oscillation and the radiationincluding tunability, spatial and temporal coherency, anddirectivity.

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Topical review

3.1. TPO

Tunable THz-wave generation, using nonlinear opticalmethods, has been widely reported [8, 9] where difference-frequency mixing between two laser sources was utilized,though the conversion efficiency observed was poor. Incontrast, higher conversion efficiency was obtained bysimultaneous Raman and parametric oscillation, utilizing thepolariton mode scattering of LiNbO3 based on the 248 cm−1

A1-symmetry soft mode [12–14]. This method used a singlefixed optical source, and was performed at room temperature.The idler (Stokes) and signal (THz) waves were generated fromthe pump (near-infrared) wave in the direction consistent withthe noncollinear phase-matching condition inside the LiNbO3

crystal. The idler wavelength was longer than the pumpwavelength by a few nanometres. A widely tunable coherentTHz wave could easily be generated by slightly changingthe incident angle of the pump beam. The nonlinearityarises from both the electronic and vibrational contributionsof the material. Although the interaction between waves wasgenerated by stimulated oscillation, most of the generated THzwave was absorbed or totally reflected inside the crystal due tothe material’s large absorption coefficient and large refractiveindices (5.2 at the THz range [22]). Therefore, it was ratherdifficult to couple out the THz radiation efficiently to the freespace. In previously reported work [14], a specially preparedcrystal was used, one corner of which was cut and polishedat the proper angle to allow the THz radiation to emergeapproximately normal to the exit surface, so that the problem oftotal internal reflection was avoided. In the work reported here,a grating structure on the surface of LiNbO3 was employed tocouple out the THz wave directly to the free air space withalmost thousand times higher efficiency [23, 24].

3.2. Experimental methods

Our experimental setup is shown in figure 3. A 3.5 mmthick LiNbO3 z-plate was cut to a dimension of 50 (x) ×10 (y) × 3.5 (z)mm3. Two end surfaces in the x-planewere cut parallel, polished, and antireflection (AR)-coated foroperation at 1.07µm. The grating coupler was fabricatedon the y-surface by the precise machining using a DISCOcutter (DAC-2SP/86) as shown in figure 4, and the gratingpitch and length were 125µm and 10 mm, respectively. Fourdifferent depths of grating, 20, 40, 60, and 100µm, wereformed on the sample to investigate the coupling efficiency.The crystal was placed inside the 15 cm long cavity as shown

kp

ki

kTφ

δ

LiNbO3

idler pump

M1

M2grating coupler

HR

HR

z x

y

THz-waveET

Ei

Ep

Figure 3. Experimental cavity arrangement for the TOP using amonolithic grating coupler on the y-surface of the LiNbO3 crystal.

in figure 3, which was resonated by the idler wave using twohigh-reflection mirrors, M1 (f = ∞) and M2 (f = 10 m).Both mirrors were half-area-coated, so that only the idlerwave could resonate and the pump beam propagate through theuncoated area without scattering. The mirrors and crystal weremounted on a rotating stage, and tunability was obtained byrotating the stage slightly to vary the angle of the resonator withrespect to the pump wave. The pump source was a Q-switchedNd : YAG laser (SOLAR LF113, 1.064µm) whose electricfield was along the z-axis of the crystal. The pump power,pulsewidth, and repetition rate were 30 mJ/pulse, 25 ns, and16.7 Hz, respectively. The pump beam entered the x-surfaceof the crystal and passed through the LiNbO3 crystal close tothe surface of the grating coupler to minimize the absorptionloss of the THz radiation (α > 10 cm−1). A near-infraredidler oscillation around 1.07µm was clearly recognized byits oscillating spot above a threshold pump power density ofabout 130 MW cm−2. The THz-wave radiation was monitoredwith a 4 K Si bolometer (Infrared Laboratories). In order tosuppress its response at infrared regions, a Yoshinaga filter wasinstalled inside. A white polyethylene lens (f = 60 mm) wasset in front of the bolometer to focus the THz-wave radiation.A Schottky barrier diode [25] was also utilized to detect the

d = 20 µm

d = 40 µm

d = 60 µm

d =100 µm

Grating Pitch = 125 µmNumber of Lines = 80, Grating Length = 10 mm

Figure 4. Magnified view of the grating coupler formed on they-surface of the LiNbO3 crystal.

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Topical review

temporal characteristic of THz waves whose time response wasfaster than 10 GHz. At the same time, the intensity of the idlerwas measured by a power meter and recorded together withthe bolometer output in a computer via a digital oscilloscope.

As shown in the inset of figure 3, the THz wave wasgenerated in the direction satisfying the noncollinear phase-matching condition. Here, kj is the wave vector with j = p, i,and T, indicating the pump, idler, and THz waves, respectively.As the relationship kp > ki � kT holds, the angle φ betweenthe pump and idler waves is small (φ ≈ 1◦), while the angle δbetween the idler and THz waves is large (δ ≈ 65◦).

3.3. Oscillation characteristics

By varying the incident angle of the pump beam from 2.1◦ to0.9◦, the angle φ between the pump and idler inside the crystalwas changed from approximately 1◦ to 0.5◦. As the phase-matching angle changed, the idler and the THz wavelengthswere tuned from 1.072 to 1.068µm and 140 to 310µm,respectively, as depicted in figure 5. The angle δ betweenthe THz and idler inside the crystal changed from 66◦ to 65◦.We also found that LiTaO3 was capable of oscillating in thesame cavity. In the case of LiTaO3, we needed to make theincident angle of the pump larger than that of LiNbO3.

For the 25 ns pulsewidth of the pump wave, thepulsewidths of the idler and THz waves were about 10 ns.Hence, the THz wave consists of several tens of thousandsof cycles during the oscillation time, so that the coherencywas sufficient. The THz wavelength and its linewidth weremeasured by a scanning Fabry–Perot etalon consisting of twoNi metal mesh plates with 65µm grid. Figure 6 shows anexample of the measurement. The displacement of one ofthe metal mesh plates corresponds directly to half of thewavelength. The free spectral range (FSR) of the etalon wasabout 100 GHz, and the linewidth was measured to be almost20 GHz. The polarization of the THz wave was measured to beparallel to the z-axis of the crystal using a wire grid polarizer,as shown in figure 7, coinciding with those of the pump and

TH

z w

avel

engt

h [µ

m]

incident angle of pump [deg.]

0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2100

150

200

250

300

350

Figure 5. Tuning characteristic between the incident angle of thepump to the x-surface of the crystal normal, generated THzwavelength. The solid curve indicates the calculated tuning curve.

idler waves. The THz-wave output from a 10 mm long gratingcoupler was measured to be 3 mW at peak with a pump powerof 34.5 mJ/pulse. Accordingly, we could obtain almost 15 mWof output from the grating coupler on the whole y-surface ofthe crystal (50 mm length). We also made a cut exit at the endcorner of the crystal; then we only got 5µW at best, underthe same conditions. In comparison to the cut exit, the gratingcoupler had an efficiency of better than ×1000. Further, almosta hundred times higher THz-wave output can be obtained bycooling the crystal to liquid nitrogen temperature [20].

3.4. Radiation characteristics of the grating coupler

Figure 8 shows the variation of the diffraction angle θ (of theTHz wave to the grating plane) with wavelength. Therelation between the grating period & and the N th-orderradiation angle θN to the grating surface is given by θN =cos−1(nT cos −δNλT/&), where δ is the incident angle tothe grating and nT is the refractive index of LiNbO3 at THz.The generated wavelengths ranged between 180 and 270µm,while the radiated angles varied from 45◦ to 80◦. This was

d = 85µm(= λTHz / 2)

20 GHz

1750 1800 18501700

tran

smitt

ed T

Hz-

wav

e [a

.u.]

etalon length [µm]

Figure 6. Example of the wavelength and linewidth measurementusing the scanning Fabry–Perot etalon consisting of metal meshplates.

0.0

0.2

0.4

0.6

0.8

1.0

polarizer: wire grid

ETHz // Epump // Eidler // z-axis

tran

smitt

ed T

Hz-

wav

e [a

.u.]

angle between z-axis of crystal and polarizer [deg.]

–150 –120 –90 –60 –30 0 30 60 90 120 150

Figure 7. Measured polarization of the generated THz wave using awire grid polarizer.

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Topical review

150 200 250 300

diff

ract

ion

an

gle

θ [

de

g.]

wavelength [µm]

0

50

100

Figure 8. Diffracted angle θ of THz wave to the grating plane. Thesolid line shows a calculated result.

0 10 20 30 40 50

zero

zero

FWHM =12 mm

FWHM = 8 mmd =150 mm

d = 300 mm

TH

z in

tens

ity [r

el.]

Shift of bolometer [mm]

Figure 9. Intensity cross section for the horizontal direction at300 mm (upper curve) and 150 mm (lower curve) from the gratingcoupler.

in good agreement with our prior calculation. The intensitydistribution of the generated beam was Gaussian-like bothalong and perpendicular to the grating, as can be seen infigure 9. FWHM spot sizes of 8 and 12 mm were observedalong the grating direction (x) at the measured points of 150and 300 mm from the grating, which corresponds to a beamdivergence of 0.8◦. Along the z-axis, the divergence wasdetermined by the pump spot size, and it was 1.5◦. Theseresults are in good agreement with diffraction theory.

The THz-wave absorption coefficient of LiNbO3 at certainwavelengths was also measured. Keeping the angle conditionbetween the pump and the idler constant, the THz-waveintensity was measured as a function of crystal displacement dalong the y-axis. Figure 10 indicates the relation between thedistance d and the intensity of the THz wave radiated fromthe grating coupler. Then the absorption coefficient was givenby the slope to be αe = 21.2 cm−1 for λTHz = 213µm, andαe = 51.9 cm−1 for λTHz = 184µm in another measurement;both were in good agreement with the previously reportedvalues [22].

1

10

αe= 21.2 cm–1

λTHz= 213 µm

inte

nsity

of

TH

z-w

ave

[a.u

.]

d (mm)

0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

Figure 10. Plot of the output power from the grating coupler versusthe distance d travelled by the THz wave in LiNbO3 before itreaches the grating plane.

4. Arrayed Si-prism coupler for a TPO

In generating a THz wave by parametric oscillation in anLiNbO3 crystal, some way of extracting the THz wave from thecrystal is necessary, since the refractive index of the LiNbO3

crystal for the THz wave is large enough to cause total internalreflection. We introduced an Si-prism coupler (n ≈ 3.4) toextract the THz wave generated inside a nonlinear crystal,thereby substantially improving the exit characteristics [26].This section describes the characteristics of the oscillation,and a novel coupling method for THz waves using an arrayedSi prism [27]. Using the arrayed-prism coupler, there is a six-fold increase in coupling efficiency and a 40% decrease in thefar-field beam diameter, compared with using a single-prismcoupler. We also discuss the negative effect of the free carriersat the Si-prism surface excited by the scattered pump beam,and the positive effect of cavity rotation on the uni-directivityof THz-wave radiation from the Si prism.

4.1. Experimental methods

The basic configuration of the source consisted of a Q-switchedNd : YAG laser (LOTIS LS-2136, 1.064µm) and a parametricoscillator, as shown in figure 11. The LiNbO3 crystal usedin the experiment was cut from a wafer 5 (thick) × 65 (x) ×6 (y)mm3. The x-surfaces at both ends were mirror-polishedand antireflection-coated. The y-surface was also mirror-polished, in order to minimize the coupling gap between theprism base and the crystal surface, and to prevent scatteringof the pump beam, which excites a free carrier at the Si-prismbase. The pump wave passed through the crystal close to they-surface, to minimize the travel distance of the THz waveinside the crystal. The idler wave was amplified in an oscillatorconsisting of flat mirrors with a half-area HR coating. In theexperiment, the beam diameter, pulsewidth, and repetition ofthe pump wave were 1.5 mm, 25 ns, and 50 Hz, respectively,and the typical excitation intensity was 30 mJ/pulse. Themirrors and crystal were installed on a precise, computer-controlled rotating stage (Nanoradian Stage, Harmonic DriveSystems Inc.) for the precise tuning.

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Topical review

rotating stage

pump LiNbO3

idler

THz-waveSi-prism array

HR

φ

kp

ki

kTδ

HR

HR

Q-swNd: YAG TEM001064nm35mJ/pulse 50pps

Figure 11. TPO with an Si-prism array. The Si-prism array was introduced to increase the output, and to reduce the diffraction angle of theTHz wave by increasing the coupling area. An array of seven right-angle Si-prism couplers was placed on the y-surface of LiNbO3. Thetotal base length was 8 mm × 7 = 56 mm.

A Si-prism array was introduced to obtain higherTHz-wave output by increasing the coupling area. An arrayof seven Si-prism couplers was placed on the y-surface ofLiNbO3, as shown in figure 11. The right-angle prismswere fabricated from high-resistivity Si (ρ > 1 k� cm, α ∼=0.6 cm−1); each was cut from a bulk Si crystal, usinga precise diamond cutter, to dimensions of 8.0 (base) ×6.1 (face) × 5.1 (side) × 5.0 (thickness)mm, and the angleswere 50–40–90◦. The total base length was 8 mm × 7 =56 mm. A prism opening angle of ξ = 40◦ was chosen sothat the THz wave would emerge almost normal to the prismface (6.1×5.0 mm2). The base of the prisms was pressed by aspecially designed holder against the y-surface of the LiNbO3

crystal to maximize coupling efficiency.

4.2. Experimental results

Typical input–output characteristics of a TPO with an Si-prismarray are shown in figure 12, in which the oscillation thresholdwas 18 mJ/pulse. With a pump power of 34 mJ/pulse, theTHz-wave output from the prism array was 192 pJ/pulse(∼=19.2 mW at the peak), calibrated using the sensitivity of thebolometer. Since the Si-bolometer output becomes saturated atapproximately 5 pJ/pulse, we used several sheets of thick paperas an attenuator after calibration. The minimum sensitivity ofthe Si bolometer is approximately 1 fJ/pulse; therefore, thedynamic range of measurement using the TPO as a source is192 pJ/1 fJ, which exceeds 50 dB. In the case of single-prismcoupling, the typical output was about 30 pJ/pulse (3 mW atthe peak) at best under similar conditions: THz wavelength180µm and idler output 1 mJ/pulse. In comparison, the prismarray was capable of emitting more than six times as muchTHz-wave energy as the single-prism coupling due to seventimes wider base area.

A small portion of the pump beam was reflected from theend mirror or scattered at the crystal edge and shone on theemitting face of the Si prism, generating a free carrier thatstrongly absorbed the THz wave. An HR mirror was thereforeinstalled in front of the last Si prism (rightmost in figure 11) tointercept the reflected pump beam, as illustrated in figure 11.The HR mirror needed to be close to the y-surface of theLiNbO3 crystal to intercept the pump beam, as only 10µJ cm−2

of pump is enough to generate the free carrier in Si [28].Without the HR mirror, the THz-wave output from the last Siprism decreased to 10−2–10−3 of the output with the HR mirror

TH

z-w

ave

ener

gy [

pJ/p

ulse

]

pump energy [mJ/pulse]

0

40

80

120

160

200

1050 15 20 25 30 35

Figure 12. Typical input–output characteristics of a TPO with anSi-prism array. By increasing the prism base area, the THz-waveoutput to more than six times that of a single-prism coupling wasobtained.

in place. Because of the difficulty in perfectly shielding thescattered pump, it was difficult to obtain maximum THz-waveoutput (∼3 mW) using a single-prism coupling, even with theHR mirror. On the other hand, it is much easier to extractmaximum THz-wave output (∼20 mW) from the arrayedprism, because the last prism acts as a perfect auxiliary shieldfor the scattered pump.

The spatial intensity distribution of the THz-waveradiation was measured by transversely shifting a Si bolometerwith a 1.4 mm wide incident slit. The beam pattern anddiffraction of the THz wave in the z-plane are shown infigures 13 and 14 for the single- and arrayed-prism couplers,respectively. In both figures, d indicates the distance betweenthe prism coupler and the slit. In both measurements, the THzwavelength was 170µm and the angle between the THz beamand the y-surface of the crystal was 50◦.

With single-prism coupling, both the near- and far-fieldpatterns were Gaussian-like, as shown in figure 13, and thediffraction angle in the far field was measured to be 1.4◦.With arrayed-prism coupling, the far-field pattern was almostGaussian-like, whereas the near-field pattern was asymmetric,as shown in figure 14. In the near field, higher output wasobserved from the prisms closest to the pump exit surface(right in figures 11 and 14), because as the distance between thepump and the y-surface of the crystal shortens, the absorptionloss of the THz wave decreases. The output of each prismwas distinguishable at d < 10 cm, whereas the beam pattern

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Topical review

d = 5 cm

d =10 cm

d = 20 cm

d = 40 cm

d = 60 cm

d =100 cm

d = 0.6 cm

TH

z-w

ave

inte

nsi

ty [

a.u.

]

Slit position [mm]

0

0

0

0

0

0

0

–80–60–40–20 0 20 40 06 80

Figure 13. Intensity cross section of the THz wave in the horizontaldirection at distance d from a single Si-prism coupler. Themeasured diffraction angle of the THz wave in the far field was 1.4◦.

0

0

0

0

0

0

0

0

d = 5cm

d =10 cm

d = 20 cm

d = 40 cm

d = 60 cm

d =100 cm

d =130 cm

d = 0.6 cm

TH

z-w

ave

inte

nsi

ty [a

.u.]

Slit position [mm]–80 –60 –40 –20 0 20 40 06 80

Figure 14. Intensity cross section of the THz wave in the horizontaldirection at distance d from the Si-prism array. Comparing theFWHM at d = 100 cm, the diameter of the THz wave was 40%smaller than that of a single-prism coupler.

became continuous at d > 20 cm. The diffraction angle of theTHz wave emitted from the prism array coupler was apparentlysmaller than that of single-prism coupling, comparing theFWHM of the beam pattern at distance d = 100 cm. Atd = 100 cm, the FWHM is 58 mm for the single-prismcoupling and 34 mm for the arrayed-prism coupling. Thefar-field diffraction angle is decided by the emitting aperturewidth and the wavelength. The smaller diffraction angle wasobtained by the prism array due to the seven times wideremitting aperture than the single prism.

In previous experiments [14], a cut exit was used to avoidtotal internal reflection, as illustrated in figure 15. A cut exit

100 150 200 250 300 350 400

(∆θp of prism coupler)-(cavity rotation)

∆θp of prism coupler

∆θc of cut exit

Cha

nge

of r

adia

tion

dire

ctio

n [d

eg.]

THz-wavelength [µm]

∆θpλ1

λ2

prism coupler

LiNbO3

cut exit

∆θ∆θc

λ1

λ2

LiNbO3

–4

–2

0

2

4

6

8

10

12

14

Figure 15. Calculated radiation angle changes for two differentTHz-coupling methods: cut exit and Si-prism coupler. The dottedand broken lines indicate the radiation angle changes for the cut exit,�θc, and Si-prism coupler, �θp, respectively. The solid line showsthe actual change in THz-beam direction when observed fromoutside the TPO. Since the TPO cavity can be angle-tuned byrotating the stage in the direction counter to �θp, the actual anglechange becomes much smaller than �θp. The changes in theradiation angle are set at zero for λTHz = 200µm, for comparison.

was made at the corner of the LiNbO3 crystal, so that theTHz wave emerged approximately normal to the exit surface.In this case, the refractive index dispersion of LiNbO3 andchange of the phase-matching angle, δ, directly influenced theTHz-wave direction change, �θc. On the other hand, when anSi-prism coupler is used, radiation is almost in one directionand variation in the phase-matching angle is substantiallyreduced. Using the phase-matching conditions, the outputangle of the THz wave θp yields [29]

sin θp = ncT sin

[arcsin

(ni

2ncT

+niλT

2ncT(λT − λp)

− n2Tλp

2ncTni(λT − λp)

+(n2

p − n2i )λ

2T

2ncTniλp(λT − λp)

)− ξ

]

where λp and λT are the pump and THz wavelength, np,ni, and nT are the refractive indices of the pump, idler, andTHz waves in LiNbO3, nc

T is the refractive index of the THzwave in silicon, and ξ denotes the opening angle of the Siprism. Figure 15 shows the calculated changes in radiationangle for these two methods of coupling. The changes inthe radiation angle are set at zero for λTHz = 200µm, forcomparison. The dotted and broken lines indicate changesin the radiation angle for the cut exit, �θc, and Si-prismcoupler, �θp, respectively. The solid line shows the changein THz-beam direction when observed from outside the TPO.It is important to note that since the TPO cavity can be angle-tuned by rotating the stage in the direction counter to �θp,the actual angle change becomes much less than �θp. For thetuning range of 100–420µm, �θc = 16.5◦, �θp = 4.0◦, andthe actual change, �θp − (cavity rotation),= 1.5◦.

An example of the tuning range using the Si prism is shownin figure 16 as the solid line. In the figure, stars indicate the

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Topical review

150 180 210 240 270 3000

1

2

3

4

5

cut exit

prism coupler

linewidth

TH

z-w

ave

Inte

nsity

[a.

u.]

wavelength [µm]

Figure 16. Measured THz-wave intensity dependence on thewavelength for the prism coupler (——) and the cut exit (–×–×–×). Inthe case of the prism coupler, the bolometer position was fixed;meanwhile, in the case of the cut exit, the bolometer position had tobe shifted point to point each time to measure, since the radiationangle varied as it tuned.

data obtained by the cut exit at the end corner of the LiNbO3

crystal. In the case of the cut exit, the bolometer position hadto be shifted point to point each time to measure, since theradiation angle varied as it tuned. By comparing the tuningranges of both methods, the radiation direction was assured tobe almost constant by use of the Si prism.

The THz-wave output can be increased by increasing thepump beam diameter in the z-axis direction, so that the THzwave is coupled out from a wider area of the y-surface of thecrystal. In this case, the diffraction angle of the THz wave in thevertical direction decreases, since it is decided by the diameterof the pump beam.

5. Injection-seeded THz-waveparametric generator

Coherent tunable THz waves were successfully generatedusing an injection-seeded THz-wave parametric generator(IS-TPG) based on laser light scattering from the A1-symmetrypolariton mode of MgO : LiNbO3 crystals. A THz-wavespectrum narrowing to the Fourier transform limit wasachieved by injection seeding the idler wave (near-infraredStokes). This resulted in a THz-wave output approximately300 times higher than that of a conventional TPG, whichhas no injection seeder. In addition, a wide tunabilityfrom 0.7 to 2.4 THz was observed using a tunable diodelaser as an injection seeder. A resolution of less than100 MHz (0.003 cm−1) was assured by the absorptionspectrum measurement of low-pressure water vapour. Thiscompact system operates at room temperature and promises tobe a new, widely tunable THz-wave source.

5.1. IS-TPG

We have researched a TPO and a TPG using LiNbO3 orMgO : LiNbO3 crystals. The TPO has proved to be a usefulcoherent THz-wave source that operates at room temperature.It is continuously tunable in the 1–3 THz range in one operationand can emit peak powers of up to several tenths of a mW. The

kp

ki

kT

MgO:LiNbO3idlerseed for idler

THz-waveSiprism array

telescope

MgO:LiNbO3

pump

attenuator

TPX cylindrical lens

Q-swNd:YAGlaser

tunable LD or

Figure 17. Setup used for our experimental IS-TPG. The pump wasa single longitudinal mode Q-switched Nd : YAG laser (1.064µm),and the seed for the idler was a CW Yb-fibre laser (1.070µm) ortunable laser diode (1.066–1074µm).

difference between a TPO and a TPG is that the former has anidler cavity while the latter does not. The THz-wave linewidthof a conventional TPG exceeds 500 GHz and the THz-waveoutput is much smaller than that from a TPO. Therefore, wepreviously concentrated our efforts on the development of aTPO system, although its linewidth was tens of GHz.

This section is primarily concerned with new work onTPGs, which appear to perform better than TPOs. TheTPG spectrum was narrowed to the Fourier transform limitof the pulsewidth by introducing injection seeding to theidler [30, 31]. The purity of the THz-wave frequency wasdramatically improved to �ν/ν < 10−4. Simultaneously, theoutput obtained was several hundred times higher than thatof a conventional TPG. In addition, wide tunability and fineresolution were demonstrated using a tunable seeder. Evenin the optical region, injection seeding to a nanosecond (ns)optical parametric generator (OPG) has not been reported untilrecently [32], due to the limit of parametric gain.

5.2. Experimental setup

Figure 17 shows the setup of our experimental IS-TPG.Arrangements were tested using one, two, and three nonlinearcrystals, 65 mm in length. The maximum THz-wave outputwas obtained when two crystals (5 mol% MgO-doped LiNbO3)were used in series. The TPG efficiency of an MgO : LiNbO3

is several times higher [16] than that of a nondoped LiNbO3.Both crystals were cut into 65 × 6 × 5 mm (x × y ×z-axis) pieces. The x-surfaces were polished and antiref-lection-coated. An array of seven Si-prism couplers was placedon the y-surface of the secondary MgO : LiNbO3 crystal forefficient coupling of the THz wave as shown in figure 17. Thepump used was a single longitudinal mode (SLM) Q-switchedNd : YAG laser (Spectron SL404T, wavelength: 1.064µm;energy: <50 mJ/pulse; pulsewidth: 15 ns; beam profile:TEM00). The pump beam diameter was decreased to 0.8 mmusing a telescope in order to increase the power density. Thepump power density was<530 MW cm−2 at the crystal surfaceand could be varied with an attenuator. The pump beamwas almost normal to the crystal surfaces as it entered thecrystals and passed through the crystal close to the y-surface.A continuous-wave (CW) SLM Yb-fibre laser (wavelength:1.070µm fixed; power: <300 mW) or tunable diode laser(wavelength: 1.066–1.074µm; power: 50 mW) was used asan injection seeder for the idler. Observation of the intense

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Topical review

idler beam easily confirmed the injection-seeded THz-wavegeneration. The polarizations of the pump, seed, idler, andTHz waves were all parallel to the z-axis of the crystals. TheTHz-wave output and temporal waveform were measured witha 4 K Si bolometer and a Schottky barrier diode detector,respectively.

5.3. Power enhancement

Energy enhancement of the THz and idler waves by injectionseeding is shown in figures 18(a) and (b), respectively. TheTHz and idler outputs are roughly proportional to each other.Comparison of the output from 0 and 200 mW seeding enabledus to determine that the THz-wave and idler energy increasedby factors of nearly 300 and 500, respectively. The maximumconversion efficiency was achieved when the pump and seedbeams almost fully overlapped at the incident surface of thefirst MgO : LiNbO3 crystal, as shown in figure 17. This wasconfirmed by the fact that initial excitation is an essentialfeature of injection seeding. The maximum THz-wave outputof 900 pJ/pulse (peak > 100 mW) was obtained with apump of 45 mJ/pulse and a seed of 250 mW. In our previousstudies, the maximum THz-wave output from a conventionalTPG and a TPO was 3 and 190 pJ/pulse, respectively [27].The Si bolometer became saturated at about 5 pJ/pulse, sowe used several thick calibrated papers as an attenuator. Asthe minimum sensitivity of the Si bolometer was almost

1

10

100

1000

seed power

TH

z-w

ave

ene

rgy

[pJ/

puls

e]

pump energy [mJ/pulse]

idle

r e

ne

rgy

[mJ/

pu

lse

]

pump energy [mJ/pulse]

(a)

(b)

200 mW100 mW

30 mW0 mW

seed power 200 mW100 mW

30 mW0 mW

25 30 35 40 45 50

25 30 35 40 45 50

0.01

0.1

1

Figure 18. Input–output characteristics of an IS-TPG, showingenergy enhancement of the (a) THz wave (190µm) and (b) idler(1.070µm) by a factor of several hundreds with injection seeding.

1 fJ/pulse, the dynamic range of the injection-seeded TPGsystem was 900 pJ to 1 fJ ∼ 60 dB, which is sufficient formost applications. The dynamic range can be significantlyincreased using a lock-in amplifier.

Figure 19 shows the THz-wave (a) and idler (b) outputsas functions of the seed power. The outputs began to saturatewith a seed power of almost 100 mW. A relatively high seedpower was required in this experiment because the seed energydid not fully contribute to the idler generation. The seedand idler beams were spatially separated from each other asshown in figure 20. This is because the pump and seed beamswere spatially separated inside the secondary MgO : LiNbO3

crystal (see figure 17), and because most of the idler energywas generated inside the secondary MgO : LiNbO3 crystal.

0

1

2

3

idle

r en

ergy

[m

J/pu

lse]

0

200

400

600

800

41mJ 39mJ 36mJ

pump energy

41mJ 39mJ 36mJ

pump energy

TH

z-w

ave

ener

gy [p

J/pu

lse]

seed power [mW]

(a)

(b)

0 50 100 150 200 250 300

seed power [mW]0 50 100 150 200 250 300

Figure 19. THz wave (a) and idler (b) outputs as a function of seedpower.

idlerseed

3mm

Figure 20. Beam profiles of the seed and idler at a distance of160 cm from the crystal end. They are separated from each otherbecause most of the idler output is generated inside the secondaryMgO : LiNbO3 crystal (see figure 17).

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Topical review

–20 0 200.0

0.5

1.0

idler

depleted pump

depleted THz

pump

inte

nsity

[a.u

.]

time [nsec.]

Figure 21. Temporal waveforms of the pump (1.064µm), idler(1.07µm), and THz wave (190µm). The pump energy was 45 mJand the seed power was 250 mW. This is the largest pumpdepletion (28.4%) observed during our research on TPGsand TPOs.

On the other hand, when one crystal was used, 10 mW ofseed power was enough to obtain idler saturation because thepump and seed beams were not separated. Therefore, it isimportant to somehow confine the pump and seed beams in along interaction volume, in order to decrease the required seedpower and to increase the efficiency. In figure 20, the idlerbeam pattern was expanded in the z-axis direction probablydue to the photorefractive effect inside the crystal. The anglebetween the idler beam and the crystal x-surface normal wasalmost 1.5◦, proving that the cavity effect of the crystal surfaceshas no relation to this parametric generation.

Figure 21 shows examples of temporal waveforms of thepump, idler, and THz wave using a pump energy of 45 mJand a seed power of 250 mW. The pulsewidths of the pumpand idler are 15 and 4 ns, respectively. The observed pumpdepletion (28.4%) was the largest depletion encountered duringour TPG/TPO research. The THz waveform was also found tobe depleted, probably due to back conversion of the pump. Thesecond peak of the pump waveform in figure 21 is due to backconversion, and the product ofEp andEi resulted in the secondpeak of the THz waveform. Depletion of the THz waveformwas not observed with pump energies below 35 mJ/pulse. TheTHz waveform began to deplete as the pump energy increased,although the THz-wave energy continued to increase, as shownin figure 18, due to the pulsewidth expansion.

Figure 22 shows the THz-wave beam pattern in thehorizontal (upper) and vertical (lower) directions, respectively,at a distance of ∼40 cm from the Si-prism array. The beampattern was nearly Gaussian and had a diameter of 7 mm,which is suitable for many applications. The original verticaldivergence was about 6◦, as determined from the pump beamdiameter and the wavelength according to diffraction theory.A cylindrical lens (f = 30 mm) made of polymethylpentene(PMP or ‘TPX’) was used, as shown in figure 17, to collimatethe THz-wave divergence in the vertical (z-axis) direction. Asfor the horizontal direction, the beam diameter decreased as itpropagated, due to the phased array-like effect of the Si-prismarray [27]. Furthermore, using a short focus TPX or Si lens,the THz beam can be tightly focused into spot of diameter∼0.5 mm.

–30 –20 100–10 20 30

Vertical

FWHM = 7mm

Horizontal

FWHM = 7mm

0

0

TH

z-w

ave

inte

nsity

[a.u

.]

slit position [mm]

Figure 22. Beam pattern of the THz wave in the horizontal (upper)and vertical (lower) directions, at a distance of ∼40 cm from theSi-prism array.

5.4. Spectrum narrowing

Figure 23 shows the effect of injection seeding on idlerspectrum narrowing. The dotted line indicates the idlerspectrum of a conventional TPG without injection seeding, andthe solid line indicates the idler spectrum of an injection-seededTPG. The resolution limit of the spectrum analyser used was0.2 nm, so the real idler spectrum was much narrower than thatshown in this figure. Using a solid etalon, the idler spectrumwas assured to be less than 1 GHz.

The THz wavelength and linewidth were measured using ascanning Fabry–Perot etalon consisting of two Ni metal mesheswith a 65µm grid. Figure 24 shows the transmitted THz-wavepower as a function of etalon spacings of (a) ∼80 mm and(b) ∼210 mm. Figure 24(a) demonstrates the stability of thespectrum and output during the 20 min scan. The displacementbetween the two periods (190µm) directly corresponds to thewavelength. The merit of an injection-seeded TPG lies in itsoutput stability due to the mode-hop-free characteristic, sinceit has no cavity. On the other hand, as with an injection-seededTPO [33], the cavity length must be actively controlled tomatch the seed wavelength in order to stabilize the output.In figure 24(b), the FSR of the etalon is 750 MHz and theTHz-wave linewidth was measured to be less than 200 MHz(0.0067 cm−1), which is our measurement resolution limit.Since the etalon spacing was up to 210 mm, the THz-wavepulse (3.4 ns) made less than three round trips in the etaloncavity; thus, the resolution is inevitably limited.

The Fourier transform limit of the spectral width wascalculated from the pulse shape of the THz wave as measuredby SBD. The typical pulsewidth of the THz wave was3.4 ns, as shown in figure 25(a), and was almost identicalto that of the idler, which was measured with a high-speedphotodetector. Figure 25(b) shows the power spectrum of theTHz wave calculated from the upper graph, and indicates thatthe linewidth was 136 MHz. In this calculation, we ignoredany fluctuations in the background noise near the zero level infigure 25(a). Figures 24 and 25 confirmed that the linewidthof the THz wave was narrowed to near the Fourier transformlimit. The THz-wave linewidth was still more than 10 GHz,

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Topical review

0.0

0.5

1.0

spectrometerresolution

corresponding THz wavelength [µm]

not seeded

injection seeded

idle

r in

tens

ity [a

.u.]

idler wavelength [µm]

1.066 1.068 1.070 1.072 1.074

284 901 143 114567

Figure 23. Narrowing of the idler (1.07µm) spectrum by injectionseeding. The dotted and solid lines indicate the idler spectrum of aconventional TPG and an IS-TPG, respectively. The resolution limitof the spectrum analyser used was 0.2 nm, so the real idler linewidthwas much narrower than in this figure.

80.0 80.5 81.0 81.5 82.00.0

0.5

1.0λTHz=190µm

tran

smitt

ed T

Hz-

wa

ve p

ower

[a.

u.]

spacing of metal mesh etalon [mm]

210.6 210.7 210.8 210.90.0

0.5

1.0 ∆νTHz<200MHz FSR=750MHz

tra

nsm

itte

d T

Hz-

wa

ve p

ow

er

[a.u

.]

spacing of metal-mesh etalon [mm]

(a)

(b)

Figure 24. THz linewidth and wavelength measured with ascanning Fabry–Perot etalon consisting of two metal mesh plates.(a) The stability of the spectrum is demonstrated and thedisplacement between the two periods (190µm) correspondsdirectly to the wavelength. (b) The FSR of the etalon is 750 MHzand the linewidth of the THz wave is measured to be less than200 MHz (0.0067 cm−1), which is our measurement resolution limit.

even with an injection-seeded TPG that used a multi-frequencyNd : YAG laser as the pump. Thus, both the pump and seedmust be SLM lasers to obtain the transform-limited THz wavewith a TPG.

0

200

400

600

THz pulsewidth∆τ = 3.4 nsec.

SB

D o

utpu

t [m

V]

time [nsec.]

0.0

0.5

1.0 THz power spectrumFWHM = 136 MHz

frequency distribution [MHz]

inte

nsity

[a.u

.]

(a)

(b)

–20 –10 0 10 20

–300 –200 –100 0 100 200 300

Figure 25. (a) Temporal THz-wave output measured by the SBD,and (b) calculated Fourier transform limit of the spectral width fromthe measured temporal THz waveform. The typical pulsewidth ofthe THz wave was 3.4 ns (a), and the calculated linewidth was136 MHz (b).

5.5. Wide tunability

It was possible to tune the THz wavelength using an externalcavity laser diode as a tunable seeder. A wide tunabilityfrom 125 to 430µm (frequency: 0.7–2.4 THz; wave number:23–80 cm−1) was observed as shown in figure 26 by changingboth the seed wavelength and the seed incident angle. Opensquares and closed circles indicate the tunability of the THz andidler waves, respectively. Both crystals were MgO : LiNbO3

in this experiment. The wavelength of 430µm (0.7 THz) wasthe longest ever observed during our study of TPGs and TPOs.In the longer-wavelength region, the angle between the pumpand idler becomes less than 1◦; thus it is difficult for the TPOto oscillate only the idler inside the cavity without scatteringthe pump. In the shorter-wavelength region, the THz output iscomparatively smaller than the idler output, due to the largerabsorption loss inside the crystal.

The absorption spectrum of low-pressure (<1 torr) watervapour was measured to demonstrate the continuous tunabilityand the high resolution of the IS-TPG. The absorption gascell used was an 87 cm long stainless light pipe with TPXwindows at both ends. Figure 27 shows an example ofmeasurements at around 1.92 THz, where two neighbouringlines exist. A resolution of less than 100 MHz (0.003 cm−1)was clearly shown. In fact, it is not easy for FTIR spectrometersin the THz-wave region to demonstrate a resolution better than0.003 cm−1 because of the instability of the scanning mirrorfor more than a meter. The system is capable of continuoustuning at high spectral resolution in 4 GHz segments anywhere

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Topical review

100 200 300 400 500

0.01

0.1

1

idler wavelength [nm]

THz wavelength [µm]

TH

z ou

tput

[nJ

/pul

se] THz-wave

0.01

0.1

1

10

idle

r ou

tput

[m

J/pu

lse]

idler wave

1075.4 1069.7 1067.8 1066.8 1066.3

Figure 26. Wide tunability of an IS-TPG. Open squares and closedcircles indicate the tunability of the THz and idler waves,respectively.

0.0029 cm–1

97MHz

water vapour, <1 torr

63.99379 cm–1

(523 432)64.02296 cm–1

frequency [THz]

tran

smis

sion

[a.

u.]

frequency [cm–1]

63.96 63.98 64.00 64.02 64.04 64.060.0

0.5

1.01.9188 1.9194 1.9200 1.9206 1.9212 1.9218

(523 432)

Figure 27. Example of the absorption spectrum measurementof low-pressure (<1 torr) water vapour at around 1.919 THz.A resolution of less than 100 MHz (0.003 cm−1) was clearly shown.

in the 0.7–2.4 THz region. The range of continuous tuning iscurrently restricted by the mode hop of the tunable laser diode.Since there is no cavity to be slaved, continuous tuning isextendible, in principle, to the full tunability of the IS-TPG byusing a mode-hop-free seeder, such as a Littman-type externalcavity diode laser.

Figure 28 shows the change in THz-wave output as afunction of the seed incident angle. In this experiment, the seedwavelength (1.07µm) and THz wavelength (190µm) werefixed, and the calculated noncollinear phase-matched anglewas 1.43◦. Here, it is important that injection seeding wasnot overly sensitive to the seed incident angle. In addition,the linewidth was assured to be less than 200 MHz at anydeviated incident angle. From this, we see that continuoustuning is possible, to some extent, by simply varying the seedwavelength without having to adjust the incident angle. Inpractice, the tuning in figure 27 was produced without changingthe seed incident angle. Tuning without mechanical movementwill lead to a stable and compact spectroscopic system. Evenwhen the incident angle must be varied for wide tuning suchas in figure 26, there is no requirement to precisely controlthe angle due to this tolerance. On the other hand, as withthe injection-seeded TPO [33], the incident angle must be

seed 1.07 µm (fix) seed 220 mW seed 150 mW seed 100 mW seed 50 mW seed 10 mW

TH

z-w

ave

inte

nsity

[m

V]

Angle between seed and pump [deg.]

0

200

400

600

1.1 1.2 1.3 1.4 1.5 1.6 1.7

Figure 28. Variation in THz-wave output as a function of seedincident angle. The seed wavelength (1.070µm) and generated THzwavelength (190µm) were confirmed to be constant. The angle ofincidence shows significant tolerance.

precisely controlled so that it is always perpendicular to thecavity mirror.

6. Conclusion

We reviewed the widely tunable THz-wave generation basedupon the optical parametric process. We have demonstrateda TPO using monolithic grating couplers in section 3.Measurements on tunability, coherency, power, polarization,radiation angle, and divergence have been accomplished,proving this method to be suitable for many applicationfields. These include spectroscopy, communication, medicaland biological applications, THz imaging, and so forth. Insection 4, an efficient TPO was demonstrated by introducingan arrayed Si-prism coupler. The output was more thansix times greater than with a single-prism coupling. Thediffraction angle observed using the prism array was smallerthan with single-prism coupling, because of the phased-array-like effect. Furthermore, we explained the uni-directionalTHz-wave radiation from the Si-prism coupler.

In section 5, we have demonstrated a high spectralresolution IS-TPG. We measured the power enhancement,spectrum narrowing, and tunability of this IS-TPG. Incomparison with a conventional TPG without injectionseeding, the output was increased from 3 to 900 pJ/pulse, andthe linewidth was decreased from >500 GHz to ≈100 MHz.Wide tunability up to 430µm (0.7 THz) was assured usinga tunable seeder, and fine tuning was demonstrated by THzspectroscopy of low-pressure water vapour.

Further improvement of our system is possible. AsOPGs and OPOs have improved tremendously in the lastdecade, the use of TPGs and TPOs shows great potentialto move towards a lower threshold, higher efficiency, andwider tunability. A lower threshold and a narrower linewidthcan be expected using a nonlinear optical waveguide and alonger pump pulsewidth, respectively. Operation in otherwavelength regions, through proper crystal selection, shouldalso be possible. Success in this will prove the practicality ofa new widely tunable THz-wave source, the IS-TPG, that willcompete with free-electron lasers and p-Ge lasers. For tunableTHz-wave applications, the simplicity of the wave source isan essential requirement since cumbersome systems do not

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Topical review

encourage new experimental thoughts and ideas. Comparedwith the available sources, the present parametric method hassignificant advantages in compactness, tunability, and ease ofhandling.

Acknowledgments

The authors would like to acknowledge the continuingguidance and encouragement of Prof. J Nishizawa. Theauthors thank Dr H Minamide and Dr K Imai of the PhotoDynamics Research Center, RIKEN, and Dr H Komine ofTRW Space & Technology Division for useful discussions;Prof. K Mizuno of the Research Institute of ElectricalCommunication, Tohoku University, for providing theSchottky barrier diodes; C Takyu for his excellent work coatingthe crystal surface, and T Shoji for polishing the crystalssuperbly. This work was supported in part by a Grant-in-Aidfor Developmental Scientific Research (No 12555105) fromthe Ministry of Education, Science, and Culture of Japan.

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