Terahertz sensing and imaging based on carbon …...1. THz detector : Frequency-tunable THz...
Transcript of Terahertz sensing and imaging based on carbon …...1. THz detector : Frequency-tunable THz...
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Terahertz sensing and imaging based on carbon nanotubes:Frequency-selective detection
and near-field imaging
Yukio Kawano
RIKEN, JST PRESTO
[email protected]://www.riken.jp/lab-www/adv_device/kawano/index.html
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1.1.THz detectorTHz detector::Frequency-tunable THz detector using a carbon nanotube
2.2.NearNear--field THz field THz imaingimaing::
On-chip near-field THz probe integrated with a detector
3.3.THz imaging application to semiconductor researchTHz imaging application to semiconductor researchSimultaneous imaging of THz radiation and voltage
4.4.SummarySummary
Outline
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiationTHz absorberDeleted image
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What is terahertz (THz) wave?
Detector, Source, Imaging, Spectroscopy....All basic components remain undeveloped
Wave (Electronics)
Light (Optics)
THz (1012Hz)undeveloped
Radio astronomyBiochemical spectroscopy Medicine Solid-state physics
Related fields: PhononEnergy gap of superconductorsImpurity level of semiconductorsEnergy spacing due to quantum confinementLandau level
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EC+∆E
G
S D
Tunnel barrier
Quantum dot
Single electron charging energy
10~50meV (=THz)
Carbon NanotubeQuantum Dot
Feature 1・・・Single electron transistor
Feature 2・・・Photon-assisted tunneling
N
N+1
N
N+1NN N+1N+1
LLλλ
calculation
strong
off
hf
strong
off
strong
off
hfhf
New current signalsvia photon detection
Why can a carbon nanotube be used as a THz detector?
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Photon-assisted tunneling: Tien-Gordon modelPhoton sidebands
via combination with AC electric field
N
N+1
N
N+1NN N+1N+1
LLλλ
calculation
strong
off
hf
strong
off
strong
off
hfhf
Quantum dot (QD):Generation of new satellite currents
New energy levels are formed at intervals of nhfThe current follows the Bessel function of the illuminated power
Semiconductor QD: Microwave (GHz)Microwave (GHz) region
In our work: Carbon nanotube QD THzTHz region
102-103
higher
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THz gas laserTHz gas laser
Cryostat with Cryostat with an optical windowan optical window
Experimental setup
1.5K
Continuous oscillationFrequency tunable
Carbon NanotubeQuantum Dot
N++-Si(back gate)
Source Drain
SiO2
Tunnel barrier
CNT
CNT
Quantum dot
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Transport properties (without THz irradiation)
・ thermal enregy @ 1.5 K: kBT~ 0.15 meV
・ Charging energy: EC = 9.1 meV
・ 0-D level spacing: ∆E = 2.1 meV
・ tunnel rate: Γ = 10 MHz(for 1.6 pA)
・ tunnel barrier height: φB ~ 5 meV
・ photon energy: hf = 10.3 meV(for f = 2.5 THz)
THz
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THz irradiation effect: THz frequency dependence
5.0
4.5
4.0
3.5
3.0
2.5
2.0
1.5
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0.5
0.0
x10
-12
-0.80 -0.75 -0.70 -0.65
THz off
1.4THz
1.6THz
4.2THz
2.5THz
Gate voltage (V)
Cur
rent
(pA
)VSD=1mVT=1.5K
Satellite currents by THz irradiation
Linear dependence on THz-photon energy
Evidence for:THz photon-assisted
Tunneling(Frequency-tunable
THz detection)
0 4 8 12 16 200
4
8
12
16
20
Photon energy (meV)
κ∆V
G(m
eV)
slope 1Y. Kawano et al., J. Appl. Phys., 103, 034307 (2008)
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THz irradiation effect: THz power dependence
VSD=0.5mVT =2.5Kf =2.5THz
14
12
10
8
6
4
2
0
Curr
ent
( pA
)
-520 -510 -500 -490 -480 -470Vlotage ( mV )
TH(ar
Gate voltage (V)
Cur
rent
(pA
) 10
8
6
4
2
0
Curr
ent
(pA
)
0.80.40.0THz power (arb. units)eak
eak
Main peak
MainSatellite
Current vs THz power
Satellite peak
Theoretically, the current follows the Bessel function
of the illuminated powerMain
Satellite
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Performance as a THz detector
(2) Sensitivity:100-1000 times larger than a conventional Si bolometer
(3) Operation temperature:Carbon nanotube quantum dot: ~4K (in principle, ~20K)Earlier highly sensitive detector: < 0.3K
(1) Frequency bandwith:Frequency tunable in 1.4-4.2THz
5.0
4.5
4.0
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3.0
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2.0
1.5
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0.5
0.0x1
0-12
-0.80 -0.75 -0.70 -0.65
THz off
1.4THz
1.6THz
4.2THz
2.5THz
Gate voltage (V)
Cur
rent
(pA)
量子ドット
GaAs/AlGaAs
Source Drain
SiO2
Tunnel barrier
ナノチューブ
THz
CNT
QDTHz VSD=0.5mV
T =2.5Kf =2.5THz
14
12
10
8
6
4
2
0
Curr
ent
( pA
)
-520 -510 -500 -490 -480 -470Vlotage ( mV )
THz p(arb. u
Gate voltage (V)
Cur
rent
(pA
)
VSD=0.5mVT =2.5Kf =2.5THz
14
12
10
8
6
4
2
0
Curr
ent
( pA
)
-520 -510 -500 -490 -480 -470Vlotage ( mV )
THz p(arb. u
Gate voltage (V)
Cur
rent
(pA
)
14
12
10
8
6
4
2
0
Curr
ent
( pA
)
-520 -510 -500 -490 -480 -470Vlotage ( mV )
THz p(arb. u
Gate voltage (V)
Cur
rent
(pA
)
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量子ポイントコンタクト カーボンナノチューブ量子ポイントコンタクト カーボンナノチューブ量子ポイントコンタクト カーボンナノチューブ量子ポイントコンタクト カーボンナノチューブ量子ポイントコンタクト カーボンナノチューブ量子ポイントコンタクト カーボンナノチューブ
Future improvement(2) Frequency (2) Frequency tunabilitytunability
(3) THz camera(3) THz camera
N. R. Franklin et al., APL 81, 913 (2002)
Fabrication of a double quantum dot
(1) Sensitivity(1) Sensitivity
Readout of a single THz-excited electron by quantum point contact
Two-dimensional array of many carbon natnobues
VgL
VgR
hf
左右のゲートを独立に制御
source drain
source drain
hf1
hf2
Single dot Double dotCarbon nanotubeQuantum point
contact
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1.1.THz detectorTHz detector::Frequency-tunable THz detector using a carbon nanotube
2.2.NearNear--field THz field THz imaingimaing::
On-chip near-field THz probe integrated with a detector
3.3. THz imaging application to semiconductor researchTHz imaging application to semiconductor research::Simultaneous imaging of THz radiation and voltage
4.4.SummarySummary
Outline
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiationTHz absorberDeleted image
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THz imaging applications● Nondestructive Inspection
● Materials Science● Astronomy
● Medicine
SemiconductorSuperconductorOrganic conductorCarbon nanotubeetc.
Defect inspection of space shuttles Imaging of cancer cells
Far-infrared image of Magellanic clouds
Ene
rgy
gap
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Towards improvement in spatial resolution: Near-field technique
Localized electromagnetic field(Evanescent field)
1) Aperture type: Small aperture (tapered optical fiber or wave guide)2) Apertureless type: Small scatterer (STM/AFM probe)
Resolution:determined by the tip size
NearNear--field probefield probe
Wavelength
Irradiation
Sample
For obtaining optical imagesFor obtaining optical imagesbeyond the diffraction limitbeyond the diffraction limit
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Why is the development of near-field THz imaging difficult?
Microwave region
Waveguide, Coaxial cable
Visible and near-infrared regions
Resolution: 20µm(λ/200)
N. Klein et al., J. Appl. Phys. 98, 014910 (2005)
Sample
Scattered wave
Evanescent wave
Resolution: Several tens of nm(~λ/100)
Optical fiber
THz region:Lack of high transmission wave line Low sensitivity of commonly used detectors
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Several pages have been deleted because they contain unpublished data.
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1.1.THz detectorTHz detector::Frequency-tunable THz detector using a carbon nanotube
2.2.NearNear--field THz field THz imaingimaing::
On-chip near-field THz probe integrated with a detector
3.3. THz imaging application to semiconductor researchTHz imaging application to semiconductor research::Simultaneous imaging of THz radiation and voltage
4.4.SummarySummary
Outline
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiationTHz absorberDeleted image
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THz imaging application to materials science
For example;Supercurrent mapping
by THz irradiation
Direct probing of spatial properties of excited states in the meV spectrum
Materials:
SemiconductorSuperconductorOrganic conductorCarbon nanotubeetc.
S. Shikii et al., APL 74, 1317 (1999)
Photon energy corresponding to 1THz(wavelength: 300µm):
~4meV
Physical properties:
PhononEnergy gap of superconductorImpurity state of semiconductorLandau levelCharge density waveetc.
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THz imaging application to materials science
For example;Supercurrent mapping
by THz irradiation
Direct probing of spatial properties of excited states in the meV spectrum
Materials:
SemiconductorSuperconductorOrganic conductorCarbon nanotubeetc.
S. Shikii et al., APL 74, 1317 (1999)
Photon energy corresponding to 1THz(wavelength: 300µm):
~4meV
Physical properties:
PhononEnergy gap of superconductorImpurity state of semiconductorLandau levelCharge density waveetc.
Study of spatial properties of a two-dimensional electron system on a semiconductor
Simultaneous imaging of THz radiation and voltage
In our work
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Combined system of a THz microscope and an electrometer
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiation
Electrometer, Sample, THz detector: fabricated from GaAs/AlGaAs heterostructure wafers
Y. Kawano et al., Phys. Rev. B 70, 081308(R) (2004).
THz absorber
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Motivation: Electron density mapping for each Landau level
Density of state
Ene
rgy
1)
2)
Landau level 1) Ground state (Intra-level scattering)
2) Excited state (Inter-level scattering)~10meV
(THz) How are the two states distributed ?
No method for separate imaging
Our technique:Combination between THz microscope and electrometer
THz imaging --- Spectroscopic information
Voltage imaging --- Transport information
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Combined system of a THz microscope and an electrometer
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiation
Electrometer, Sample, THz detector: fabricated from GaAs/AlGaAs heterostructure wafers
Y. Kawano et al., Phys. Rev. B 70, 081308(R) (2004).
THz absorber
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B
SiO2 film
Equivalent circuitSetup
CV(x,y)=ΔQ ΔR
=Isam
C
V(x,y)
Isen
ΔQ ΔR
Y. Kawano et al., Appl. Phys. Lett. 84, 1111 (2004). Y. Kawano et al., Appl. Phys. Lett. 87, 252108 (2005).
Scanning electrometer
Selected as a cover page of Applied Physics Letters
Capacitive coupling between two 2DEGsLarge magnetoresistance oscillation→ Highly sensitive detectionLow impedance → High speed detection
Imaging ofImaging ofvoltage distributionsvoltage distributions
2D electron (Sensor)
2D electron (Sample)
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Mapping of voltage & THz cyclotron emission
0 1 2 3 4 5 6 70
4
8
12
16
20
R2t
(kΩ
)
B (T)
+-
B●
20μA70μA
140μA
1mm
2.8mm
0 1 2 3 4 5 6 70
4
8
12
16
20
R2t
(kΩ
)
B (T)
+-
B● B●
20μA70μA
140μA
1mm
2.8mm
Y. Kawano et al., Phys. Rev. B 70, 081308(R) (2004).
Ground-state electrons
Excited-state electrons
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DOS
E
EF
DOS
E
EF
- +-
Ionized impurity scattering
Period: 0.05~0.2µm
● GroundGround--state electronsstate electrons
Acoustic phonon scatteringDrift velocityE/B ×Scattering timeτ
=3×103 (m/s)×10~100 (ns)=30~300µm
2800µm
1000µm+
Separate distributions of ground-state and excited-state electrons
Local behavior Non-local behavior
● ExcitedExcited--state electronsstate electrons
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Macroscopic size effect of THz emission imagesY. Kawano et al., Phys. Rev. Lett. 95, 166801 (2005).
Width
20µm
300µm
1200µm
(Length: 4mm)
Size effect arising from a long equilibrium length of excited electrons
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Future perspective:Research on Graphen with Near-field THz Imaging
K. S. Novoselov et al., Nature 438, 7065 (2005)
Surface 2D electrons:compatible with near-field techniques
Wide-band energy spectrum (several to several tens THz)
Dirac particle
Electron-hole symmetry
2D electron on GraphenElectron
Hole
Direct probing of electron transport and energy dissipation
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Summary
5.0
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2.5
2.0
1.5
1.0
0.5
0.0
x10
-12
-0.80 -0.75 -0.70 -0.65
THz off
1.4THz
1.6THz
4.2THz
2.5THz
Gate voltage (V)
Cur
rent
(pA)
(1) Carbon (1) Carbon nanotubenanotube THz detectorTHz detector (2) On(2) On--chip nearchip near--field THz probefield THz probe
Deleted image
(3) Simultaneous imaging of THz radiation and voltage(3) Simultaneous imaging of THz radiation and voltage
SiO2 filmB
Si-lens2DEG (Sample)
2DEG (Electrometer)
2DEG (THz detector)
Voltage
THz radiationTHz absorber
Ground-state electrons Excited-state electrons