Overview onTerahertz Generation and

Detection using Photoconductive Aperture antennaby

Tanumoy Saha

What is Terahertz Radiation?

Terahertz radiation, also called sub millimeter radiation, terahertz waves, terahertz light, T-rays, T-waves, T-light, T-lux, or THz, consists of electromagnetic waves at frequencies from 40GHz to 4THz

Generation of Terahertz Radiation

Photoconductive technique

Non-linear optical technique

Photoconductive technique•High energy photons creates charge carriers

•Biased electric field applied accelerates charge carriers

•Accelerating charge carriers emit radiation

V+

V-

Inte

nsity

of

Exci

tatio

n la

ser p

ulse

Curr

ent

dens

ityE TH

z

Time

Current density Change due to formation of Charge Carries for a short instant of time

Due to change in current density ETHz α dJ/dt

Requirements for Generating Broadband Terahertz radiation•A photoconductive Antenna

•Ultrashort Laser pulse source

•DC source for Bias Voltage

Setup for Generating Terahertz Radiation

Photoconductive Antenna•For our application we use Semiconductors (GaAs)

• Impurities are doped epitaxy is done for decreasing the life

time of the carriers

•Structural design and material properties of the Antenna

dictates efficiency of the THz radiaton that we will discuss in

the subsequent slides.

Types of Photoconductive Antennas on the Basis of

their Design

•Aperture antennas(Small and large compared to

wavelength)

•Spiral Antennas

•Bowtie Antennas

•Dipole Antennas

Photoconductive Aperture Antenna

Metal Contacts

Epitaxial layer(carriers in this layer has low life time then substrate layer)

substrate layer

LT-GaAs

SI-GaAs

l

Photoconductive Aperture Antenna

LT-GaAs

SI-GaAs

l

Where τr,epi= trapping time of the carriers in the epitaxial layer

R = intensity reflectivity of the surface

x = distance from surface of semiconductor to the observation point

n(x,t) = carrier density

V+ V-

Small Aperture Antenna(A<<λTHz)

LT-GaAs

SI-GaAs

l

V+ V-

Photoconductive Aperture Antenna

Where nepi = carrier conc in the epitaxial layer

Therefore we have

LT-GaAs

SI-GaAs

l

V+ V-

Photoconductive Aperture Antenna

Similarly for substrate layer we have

Photoconductive Aperture Antenna

LT-GaAs

SI-GaAs

l

V+ V-

In presence of biased field the time evolution of the velocity of carriers is given by

Where τrel = momentum relaxation time E = local electric field

LT-GaAsl

V+ V-

Photoconductive Aperture Antenna

Time evolution of Polarization is given by

Where τrec = recombination time of the carriers

J(t) = surface current density

SI-GaAs

Now by the use of Maxwells equation electric far field(i.e r>>λTHz) is given by

Where A = area of illumination of the excitation pulser = distance from the center of the antenna to observation pointJs(t) = surface induced current density = σ(t)Eeff(t)

Photoconductive Aperture Antenna

V+

LT-GaAsl

V-

SI-GaAs

EDC

Photoconductive Aperture Antenna

Large Aperture antenna (A >> λTHz)

Then using the above approximation we have

Where σs(t) = surface conductivity σd = threshold conductivity(conductivity at which substance transfers from dielectric to metallic)

EDC

Photoconductive Aperture Antenna

Surface conductivity is given by

Where I(t) is the instantaneous amplitude of the excitation pulseAnd v is the frequency of the excitation pulseAnd τ is the carrier life time

Photoconductive Aperture Antenna

Where I(t) is given by

Factors Effecting the efficiency of aperture THz-PCAs•Trapping time of Carriers: Trapping time governs the FWHM of the carrier density thereby that of current density J(t). Trapping time of the order of ps generate THz spectrum

•Effect of Laser pulse and Duration: High frequency and low duration pulse(order of femto-seconds) generate wideband terahertz radiation

•Effect of Electric field and Dipole apperture antenna: smaller aperture perfect dipole

Detection of Terahertz Radiation

Photoconductive technique

Non-linear optical technique

Exci

tatio

n la

ser p

ulse

Carr

ier

Den

sity

Tera

hert

z pu

lse

Curr

ent

dete

cted

time

Detection of Terahertz RadiationDynamics of the Carriers is same as discussed earlier, The only difference is that instead of bias field we have the time varying ETHz and we measure the time varying current which gives information of the frequency and amplitude of the THz radiation

Detection of Terahertz Radiation

FFT

Curr

ent

dete

cted

time

Ampl

itude

Frequency(THz scale)Frequency(THz scale)

Factors Effecting the efficiency of detector•Trapping time of Carriers: Trapping time governs the FWHM of the carrier density i.e the effective region of detection

So for better detection τtrap<1/wTHz

•Effect of Laser pulse and Duration: Amplitude dictates the rate of formation of effective charge carriers and so its density thereby increasing the resolution of detection

•Dipole apperture antenna: small aperture more effective detection as it acts like perfect dipole

Conclusion

So in making terahertz antennas we focus on factors effecting1. Life time of the carriers2. Mobility of the Carriers3. Density of carriers

Reference

1. Broadband THz Generation from Photoconductive Antenna by Qing Chang1, Dongxiao Yang1,2, and Liang Wang12. Terahertz Photoconductive Antennas: Principles and Applications byDaryoosh Saeedkia 3. COMPARISON OF TERAHERTZ ANTENNAS by Di LI , and Yi HUNAG4. Terahertz Spectroscopy Principles and Applications by Brian J. Thompson5. Wikipedia6. Electricity and Magnetism by DJ Griffiths7. Solid State physics by Charles Kittel