Terahertz sensing and imaging based on carbon nanotubes: - - PowerPoint PPT Presentation

Terahertz sensing and imaging based on carbon nanotubes:

Frequency-selective detection and near-field imaging

Yukio Kawano RIKEN, JST PRESTO

ykawano@riken.jp http://www.riken.jp/lab-www/adv_device/kawano/index.html

１． １．THz detector THz detector： ：

Frequency-tunable THz detector using a carbon nanotube

２． ２．Near Near-

• field THz

field THz imaing imaing： ：

On-chip near-field THz probe integrated with a detector

３． ３．THz imaging application to semiconductor research THz imaging application to semiconductor research

Simultaneous imaging of THz radiation and voltage

４． ４．Summary Summary

Outline

ＳｉO2 film

B

Si-lens 2DEG (Sample) 2DEG (Electrometer) 2DEG (THz detector)

Voltage THz radiation

THz absorber

Deleted image

What is terahertz (THz) wave？

Detector, Source, Imaging, Spectroscopy.... All basic components remain undeveloped

Wave (Electronics) Light (Optics) THz (1012Hz) undeveloped

Radio astronomy Biochemical spectroscopy Medicine Solid-state physics

Related fields:

Phonon Energy gap of superconductors Impurity level of semiconductors Energy spacing due to quantum confinement Landau level

EC +∆E

G S D Tunnel barrier

Quantum dot

Single electron charging energy 10~50meV (=THz)

Carbon Nanotube Quantum Dot Feature 1・・・Single electron transistor Feature 2・・・Photon-assisted tunneling

N N+1 N N+1 N N+1 N+1

L λ

calculation

strong

• ff

hf

strong

• ff

strong

• ff

hf hf

New current signals via photon detection

Why can a carbon nanotube be used as a THz detector?

Photon-assisted tunneling: Tien-Gordon model

Photon sidebands via combination with AC electric field

N N+1 N N+1 N N+1 N+1

L λ

calculation

strong

• ff

hf

strong

• ff

strong

• ff

hf hf

Quantum dot (QD)： Generation of new satellite currents

New energy levels are formed at intervals of nhf The current follows the Bessel function of the illuminated power

Semiconductor QD: Microwave (GHz) Microwave (GHz) region In our work: Carbon nanotube QD THz THz region

102-103 higher

THz gas laser THz gas laser Cryostat with Cryostat with an optical window an optical window

Experimental setup

1.5K

Continuous oscillation Frequency tunable

Carbon Nanotube Quantum Dot

N++-Si (back gate) Source Drain SiO2 Tunnel barrier

CNT

CNT

Quantum dot

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

THz irradiation effect: THz frequency dependence

5.0 4.5 4.0 3.5 3.0 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) Current (pA)

VSD=1mV T=1.5K

Satellite currents by THz irradiation Linear dependence

• n THz-photon energy

Evidence for: THz photon-assisted Tunneling (Frequency-tunable THz detection)

4 8 12 16 20 4 8 12 16 20

Photon energy (meV) κ∆VG (meV)

slope 1

• Y. Kawano et al.,
• J. Appl. Phys.,

103, 034307 (2008)

THz irradiation effect: THz power dependence

VSD=0.5mV T =2.5K f =2.5THz

14 12 10 8 6 4 2 Current ( pA )

• 520
• 510
• 500
• 490
• 480
• 470

Vlotage ( mV )

TH

(ar

Gate voltage (V) Current (pA)

10 8 6 4 2 Current (pA) 0.8 0.4 0.0 THz power (arb. units)

eak eak

Main peak

Main Satellite

Current vs THz power

Satellite peak

Theoretically, the current follows the Bessel function

• f the illuminated power

Main Satellite

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 3.5 3.0 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) Current (pA) 量子ドット GaAs/AlGaAs Source Drain SiO2 Tunnel barrier ナノチューブ

THz

CNT QD

THz

VSD=0.5mV T =2.5K f =2.5THz

14 12 10 8 6 4 2 Current ( pA )

• 520
• 510
• 500
• 490
• 480
• 470

Vlotage ( mV )

THz p

(arb. u

Gate voltage (V) Current (pA)

VSD=0.5mV T =2.5K f =2.5THz

14 12 10 8 6 4 2 Current ( pA )

• 520
• 510
• 500
• 490
• 480
• 470

Vlotage ( mV )

THz p

(arb. u

Gate voltage (V) Current (pA)

14 12 10 8 6 4 2 Current ( pA )

• 520
• 510
• 500
• 490
• 480
• 470

Vlotage ( mV )

THz p

(arb. u

Gate voltage (V) Current (pA)

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 tunability tunability (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 dot

Carbon nanotube Quantum point contact

１． １．THz detector THz detector： ：

Frequency-tunable THz detector using a carbon nanotube

２． ２．Near Near-

• field THz

field THz imaing imaing： ：

On-chip near-field THz probe integrated with a detector

３． ３． THz imaging application to semiconductor research THz imaging application to semiconductor research： ：

Simultaneous imaging of THz radiation and voltage

４． ４．Summary Summary

Outline

ＳｉO2 film

B

Si-lens 2DEG (Sample) 2DEG (Electrometer) 2DEG (THz detector)

Voltage THz radiation

THz absorber

Deleted image

THz imaging applications

• Nondestructive Inspection
• Materials Science
• Astronomy
• Medicine

Semiconductor Superconductor Organic conductor Carbon nanotube etc.

Defect inspection of space shuttles Imaging of cancer cells

Far-infrared image of Magellanic clouds

Energy gap

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 Near Near-

• field probe

field probe

Wavelength Irradiation Sample

For obtaining optical images For obtaining optical images beyond the diffraction limit beyond the diffraction limit

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

Several pages have been deleted because they contain unpublished data.

１． １．THz detector THz detector： ：

Frequency-tunable THz detector using a carbon nanotube

２． ２．Near Near-

• field THz

field THz imaing imaing： ：

On-chip near-field THz probe integrated with a detector

３． ３． THz imaging application to semiconductor research THz imaging application to semiconductor research： ：

Simultaneous imaging of THz radiation and voltage

４． ４．Summary Summary

Outline

ＳｉO2 film

B

Si-lens 2DEG (Sample) 2DEG (Electrometer) 2DEG (THz detector)

Voltage THz radiation

THz absorber

Deleted image

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： Semiconductor Superconductor Organic conductor Carbon nanotube etc.

• S. Shikii et al.,

APL 74, 1317 (1999)

Photon energy corresponding to 1THz(wavelength: 300µm)： ～4meV

Physical properties： Phonon Energy gap of superconductor Impurity state of semiconductor Landau level Charge density wave etc.

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： Semiconductor Superconductor Organic conductor Carbon nanotube etc.

• S. Shikii et al.,

APL 74, 1317 (1999)

Photon energy corresponding to 1THz(wavelength: 300µm)： ～4meV

Physical properties： Phonon Energy gap of superconductor Impurity state of semiconductor Landau level Charge density wave etc.

Study of spatial properties of a two-dimensional electron system on a semiconductor Simultaneous imaging of THz radiation and voltage

In our work

Combined system of a THz microscope and an electrometer

ＳｉO2 film

B

Si-lens

2DEG (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

Motivation:

Electron density mapping for each Landau level

Density of state Energy

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

Combined system of a THz microscope and an electrometer

ＳｉO2 film

B

Si-lens

2DEG (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

B

SiO2 film

Equivalent circuit Setup

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

• f Applied Physics Letters

Capacitive coupling between two 2DEGs Large magnetoresistance oscillation → Highly sensitive detection Low impedance → High speed detection

Imaging of Imaging of voltage distributions voltage distributions

2D electron (Sensor) 2D electron (Sample)

Mapping of voltage & THz cyclotron emission

1 2 3 4 5 6 7 4 8 12 16 20 R2t (kΩ) B (T)

+

－

B

• 20μ

A 70μ A 140μ A

1mm 2.8mm

1 2 3 4 5 6 7 4 8 12 16 20 R2t (kΩ) B (T)

+

－

B

• B
• 20μ

A 70μ A 140μ A

1mm 2.8mm

• Y. Kawano et al., Phys. Rev. B 70, 081308(R) (2004).

Ground-state electrons Excited-state electrons

DOS E EF DOS E EF

－

+

－

Ionized impurity scattering Period: 0.05～0.2µm

• Ground

Ground-

• state electrons

state electrons

Acoustic phonon scattering

Drift 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

• Excited

Excited-

• state electrons

state electrons

Macroscopic size effect of THz emission images

• Y. Kawano et al., Phys. Rev. Lett. 95, 166801 (2005).

Width 20µm 300µm 1200µm （Length: 4mm） Size effect arising from a long equilibrium length

• f excited electrons

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 Graphen

Electron Hole

Direct probing of electron transport and energy dissipation

Summary

5.0 4.5 4.0 3.5 3.0 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) Current (pA)

(1) Carbon (1) Carbon nanotube nanotube THz detector THz detector (2) On (2) On-

• chip near

chip near-

• field THz probe

field THz probe Deleted image (3) Simultaneous imaging of THz radiation and voltage (3) Simultaneous imaging of THz radiation and voltage

ＳｉO2 film

B

Si-lens 2DEG (Sample) 2DEG (Electrometer) 2DEG (THz detector)

Voltage THz radiation

THz absorber

Ground-state electrons Excited-state electrons