Terahertz sensing and imaging based on carbon nanotubes:

Transcription

1 Terahertz sensing and imaging based on carbon nanotubes: Frequency-selective detection and near-field imaging Yukio Kawano RIKEN, JST PRESTO

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

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

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

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

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

7 Transport properties (without THz irradiation) THz thermal 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)

8 THz irradiation effect: THz frequency dependence V SD =1mV T=1.5K Current (pa) x κ V G (mev) slope Photon energy (mev) 1.4THz THz off 1.6THz 4.2THz 2.5THz Gate voltage (V) Y. Kawano et al., J. Appl. Phys., 103, (2008) Satellite currents by THz irradiation Linear dependence on THz-photon energy Evidence for: THz photon-assisted Tunneling (Frequency-tunable THz detection)

9 THz irradiation effect: THz power dependence Current (pa) Current ( pa ) Satellite Main V SD =0.5mV T =2.5K f =2.5THz Vlotage ( mv ) Gate voltage (V) TH (ar eak eak Current vs THz power Current (pa) Main peak THz power (arb. units) Satellite peak Theoretically, the current follows the Bessel function of the illuminated power Main Satellite

10 Performance as a THz detector THz 量子ドット QD Source Drain SiO 2 ナノチューブ CNT Tunnel barrier GaAs/AlGaAs Current (pa) x THz 2.5THz 1.6THz 1.4THz THz off Gate voltage (V) Current ( (pa) ) 14 V SD =0.5mV T =2.5K 12 f =2.5THz Gate Vlotage voltage ( mv ) (V) THz (arb. u (1) Frequency bandwith: Frequency tunable in THz (2) Sensitivity: times larger than a conventional Si bolometer (3) Operation temperature: Carbon nanotube quantum dot: ~4K (in principle, ~20K) Earlier highly sensitive detector: < 0.3K

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

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

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

14 Towards improvement in spatial resolution: Near-field technique For obtaining optical images beyond the diffraction limit Wavelength Near-field probe Irradiation Resolution: determined by the tip size 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)

15 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, (2005) Evanescent wave Scattered wave Sample Resolution: Several tens of nm(~λ/100) Optical fiber Resolution: 20µm(λ/200) Waveguide, Coaxial cable THz region: Lack of high transmission wave line Low sensitivity of commonly used detectors

16 Several pages have been deleted because they contain unpublished data.

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

18 THz imaging application to materials science Photon energy corresponding to 1THz(wavelength: 300µm): ~4meV 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. Physical properties: Phonon Energy gap of superconductor Impurity state of semiconductor Landau level Charge density wave etc. S. Shikii et al., APL 74, 1317 (1999)

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

20 Combined system of a THz microscope and an electrometer Y. Kawano et al., Phys. Rev. B 70, (R) (2004). B SiO 2 film 2DEG (Electrometer) Voltage 2DEG (Sample) Si-lens THz absorber THz radiation 2DEG (THz detector) Electrometer, Sample, THz detector: fabricated from GaAs/AlGaAs heterostructure wafers

21 Motivation: Electron density mapping for each Landau level Energy Landau level 2) 1) Ground state (Intra-level scattering) 2) Excited state (Inter-level scattering) ~10meV (THz) How are the two states distributed? 1) Density of state No method for separate imaging Our technique: Combination between THz microscope and electrometer THz imaging --- Spectroscopic information Voltage imaging --- Transport information

22 Combined system of a THz microscope and an electrometer Y. Kawano et al., Phys. Rev. B 70, (R) (2004). B SiO 2 film 2DEG (Electrometer) Voltage 2DEG (Sample) Si-lens THz absorber THz radiation 2DEG (THz detector) Electrometer, Sample, THz detector: fabricated from GaAs/AlGaAs heterostructure wafers

23 Scanning electrometer Imaging of voltage distributions Y. Kawano et al., Appl. Phys. Lett. 84, 1111 (2004). Y. Kawano et al., Appl. Phys. Lett. 87, (2005). Setup Equivalent circuit B SiO 2 film 2D electron (Sensor) = ΔQ C I sen ΔR V(x,y) 2D electron (Sample) I sam Selected as a cover page of Applied Physics Letters CV(x,y)=ΔQ ΔR Capacitive coupling between two 2DEGs Large magnetoresistance oscillation Highly sensitive detection Low impedance High speed detection

24 Mapping of voltage & THz cyclotron emission Y. Kawano et al., Phys. Rev. B 70, (R) (2004) mm R 2t (kω) mm B 20μA 70μA 140μA B (T) Ground-state electrons Excited-state electrons

25 Separate distributions of ground-state and excited-state electrons E Ground-state electrons E Excited-state electrons µm - + E F 2800µm E F DOS Ionized impurity scattering Period: 0.05~0.2µm DOS Acoustic phonon scattering Drift velocitye/b Scattering timeτ = (m/s) 10~100 (ns) =30~300µm Local behavior Non-local behavior

26 Macroscopic size effect of THz emission images Y. Kawano et al., Phys. Rev. Lett. 95, (2005). Width 20µm 300µm 1200µm (Length: 4mm) Size effect arising from a long equilibrium length of excited electrons

27 Future perspective: Research on Graphen with Near-field THz Imaging 2D electron on Graphen Surface 2D electrons: compatible with near-field techniques Wide-band energy spectrum (several to several tens THz) Dirac particle Electron-hole symmetry Electron Hole Direct probing of electron transport and energy dissipation K. S. Novoselov et al., Nature 438, 7065 (2005)

28 Summary (1) Carbon nanotube THz detector (2) On-chip near-field THz probe Current (pa) x THz 2.5THz 1.6THz 1.4THz THz off Gate voltage (V) Deleted image (3) Simultaneous imaging of THz radiation and voltage B SiO 2 film 2DEG (Electrometer) Voltage Si-lens THz absorber 2DEG (Sample) THz radiation 2DEG (THz detector) Ground-state electrons Excited-state electrons