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

Outline １． THz detector ： １． THz detector ： Frequency-tunable THz detector using a carbon nanotube ２． Near ： ２． imaing ： Near- -field THz field THz imaing On-chip near-field THz probe integrated with a detector ３． THz imaging application to semiconductor research ３． THz imaging application to semiconductor research Simultaneous imaging of THz radiation and voltage ４． Summary ４． Summary 2DEG (Electrometer) Voltage B Ｓｉ O 2 film 2DEG (Sample) Si-lens Deleted image THz absorber THz radiation 2DEG (THz detector)

What is terahertz (THz) wave ？ THz (10 12 Hz) Light Wave undeveloped (Optics) (Electronics) Related fields: � Phonon � Radio astronomy � Energy gap of superconductors � Impurity level of semiconductors � Biochemical spectroscopy � Energy spacing due to � Medicine quantum confinement � Solid-state physics � Landau level Detector, Source, Imaging, Spectroscopy.... All basic components remain undeveloped

Why can a carbon nanotube be used as a THz detector? Feature 1 ・・・ Single electron transistor Carbon Nanotube Quantum Dot Feature 2 ・・・ Photon-assisted tunneling L λ Quantum dot S D N+1 N+1 N N+1 N+1 G N N Tunnel barrier calculation hf hf hf Single electron New current signals charging energy E C off off off via photon detection + ∆ E strong strong strong 10~50meV (=THz)

Photon-assisted tunneling: Tien-Gordon model Quantum dot (QD) ： Photon sidebands Generation of new satellite currents via combination with AC electric field L λ N+1 N+1 N N+1 N+1 N N calculation hf hf hf � New energy levels are formed at intervals of nhf off off off � The current follows the Bessel strong strong strong function of the illuminated power Semiconductor QD: Microwave (GHz) Microwave (GHz) region 10 2 -10 3 higher In our work: Carbon nanotube QD THz THz region

Experimental setup Carbon Nanotube Quantum Dot Cryostat with Cryostat with CNT an optical window an optical window Quantum dot CNT Source Drain SiO 2 N++-Si Tunnel barrier (back gate) 1.5K THz gas laser THz gas laser � Continuous oscillation � Frequency tunable

Transport properties (without THz irradiation) THz ･ thermal enregy @ 1.5 K : k B T~ 0.15 meV ･ Charging energy : E C = 9.1 meV ･ 0-D level spacing : ∆ E = 2.1 meV ･ tunnel rate : Γ = 10 MHz (for 1.6 pA) ･ tunnel barrier height : φ B ~ 5 meV ･ photon energy : hf = 10.3 meV (for f = 2.5 THz)

THz irradiation effect : THz frequency dependence 20 Y. Kawano et al., κ∆ V G (meV) V SD =1mV slope 1 16 5.0 J. Appl. Phys., 12 T =1.5K 103, 034307 (2008) 8 4.5 4 0 � Satellite currents 0 4 8 12 16 20 4.0 Photon energy (meV) by THz irradiation 3.5 4.2THz � Linear dependence Current (pA) 3.0 on THz-photon energy -12 2.5 2.5THz x10 2.0 Evidence for: 1.6THz THz photon-assisted 1.5 Tunneling 1.0 1.4THz (Frequency-tunable THz detection) 0.5 THz off 0.0 -0.80 -0.75 -0.70 -0.65 Gate voltage (V)

THz irradiation effect: THz power dependence Satellite 14 V SD =0.5mV Current vs THz power Main T =2.5K 12 f =2.5THz Main peak 10 Current (pA) Current ( pA ) 10 Current (pA) 8 8 6 4 6 TH 2 (ar Satellite peak 4 0 0.0 0.4 0.8 2 THz power (arb. units) eak eak 0 -520 -510 -500 -490 -480 -470 Vlotage ( mV ) Gate voltage (V) Theoretically, the current follows the Bessel function Main of the illuminated power Satellite

Performance as a THz detector 5.0 4.5 4.0 THz THz 14 14 14 V SD =0.5mV V SD =0.5mV 量子ドット QD 3.5 T =2.5K T =2.5K 12 12 12 4.2THz f =2.5THz f =2.5THz Current (pA) Source Drain 3.0 Current ( pA ) Current ( pA ) Current ( pA ) Current (pA) Current (pA) Current (pA) ナノチューブ 10 10 10 CNT -12 8 8 8 2.5 2.5THz SiO 2 x10 6 6 6 THz p THz p THz p 2.0 (arb. u (arb. u (arb. u Tunnel barrier GaAs/AlGaAs 4 4 4 1.6THz 1.5 2 2 2 1.0 1.4THz 0 0 0 -520 -520 -520 -510 -510 -510 -500 -500 -500 -490 -490 -490 -480 -480 -480 -470 -470 -470 Vlotage ( mV ) Vlotage ( mV ) Vlotage ( mV ) 0.5 Gate voltage (V) Gate voltage (V) Gate voltage (V) THz off 0.0 -0.80 -0.75 -0.70 -0.65 Gate voltage (V) （１） Frequency bandwith ： Frequency tunable in 1.4-4.2THz （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

Future improvement (2) Frequency tunability tunability (2) Frequency (1) Sensitivity (1) Sensitivity Double dot Quantum point Single dot Carbon nanotube 量子ポイントコンタクト 量子ポイントコンタクト 量子ポイントコンタクト 量子ポイントコンタクト 量子ポイントコンタクト 量子ポイントコンタクト カーボンナノチューブ カーボンナノチューブ カーボンナノチューブ カーボンナノチューブ カーボンナノチューブ カーボンナノチューブ hf contact hf 1 hf 2 source drain V gR drain source V gL Readout of a single THz-excited electron by 左右のゲートを独立に制御 quantum point contact Fabrication of a double quantum dot (3) THz camera (3) THz camera Two-dimensional array of many carbon natnobues N. R. Franklin et al ., APL 81 , 913 (2002)

Outline １． THz detector ： １． THz detector ： Frequency-tunable THz detector using a carbon nanotube ２． Near ： ２． imaing ： Near- -field THz field THz imaing On-chip near-field THz probe integrated with a detector ３． THz imaging application to semiconductor research ： ３． THz imaging application to semiconductor research ： Simultaneous imaging of THz radiation and voltage ４． Summary ４． Summary 2DEG (Electrometer) Voltage B Ｓｉ O 2 film 2DEG (Sample) Si-lens Deleted image THz absorber THz radiation 2DEG (THz detector)

THz imaging applications ● Medicine ● Nondestructive Inspection Imaging of cancer cells Defect inspection of space shuttles ● Astronomy ● Materials Science Far-infrared image of Magellanic clouds Semiconductor Superconductor Organic conductor Energy gap Carbon nanotube etc.

Towards improvement in spatial resolution: Near-field technique Near- Near -field probe field probe For obtaining optical images For obtaining optical images beyond the diffraction limit beyond the diffraction limit Wavelength Resolution: determined by the tip size Irradiation Sample Localized electromagnetic field (Evanescent field) 1) Aperture type: Small aperture (tapered optical fiber or wave guide) 2) Apertureless type ： Small scatterer （ STM/AFM probe ）

Why is the development of near-field THz imaging difficult? Visible and near-infrared regions Microwave region N. Klein et al., J. Appl. Phys. 98 , 014910 (2005) Evanescent Scattered wave wave Sample Resolution: Several tens of nm （～λ /100 ） Resolution: 20 µ m （λ /200 ） Optical fiber Waveguide, Coaxial cable THz region: � Lack of high transmission wave line � Low sensitivity of commonly used detectors

Several pages have been deleted because they contain unpublished data.

Outline １． THz detector ： １． THz detector ： Frequency-tunable THz detector using a carbon nanotube ２． Near ： ２． imaing ： Near- -field THz field THz imaing On-chip near-field THz probe integrated with a detector ３． THz imaging application to semiconductor research ： ３． THz imaging application to semiconductor research ： Simultaneous imaging of THz radiation and voltage ４． Summary ４． Summary 2DEG (Electrometer) Voltage B Ｓｉ O 2 film 2DEG (Sample) Si-lens Deleted image THz absorber THz radiation 2DEG (THz detector)

THz imaging application to materials science For example; Photon energy corresponding Supercurrent mapping to 1THz(wavelength: 300 µ m) ： by THz irradiation ～ 4meV Direct probing of spatial properties of excited states in the meV spectrum Physical properties ： Materials ： � Phonon � Semiconductor � Energy gap of superconductor S. Shikii et al., � Superconductor APL 74 , 1317 (1999) � Impurity state of semiconductor � Organic conductor � Landau level � Carbon nanotube � Charge density wave etc. etc.

THz imaging application to materials science For example; Photon energy corresponding Supercurrent mapping to 1THz(wavelength: 300 µ m) ： by THz irradiation ～ 4meV In our work Direct probing of spatial properties of Simultaneous imaging of THz radiation and voltage excited states in the meV spectrum Study of spatial properties of a two-dimensional Physical properties ： Materials ： electron system on a semiconductor � Phonon � Semiconductor � Energy gap of superconductor S. Shikii et al., � Superconductor APL 74 , 1317 (1999) � Impurity state of semiconductor � Organic conductor � Landau level � Carbon nanotube � Charge density wave etc. etc.