テラヘルツ時間領域分光法と 基礎科学への応用

Transcription

1 2007/ 7/ 9 ERL 研究会 コンパクト ERL が拓く世界 KEK, Tsukuba テラヘルツ時間領域分光法と 基礎科学への応用 Department of Physics, Kyoto University Koichiro Tanaka SCOPE project ATT OC Advanced Terahertz Technologies adapted for Optical Communication JSPS project Exploitation of Organic Electronic Materials of Potential Dynamic Switches for Nonequilibrium Condensed Matter Sciences 1

2 講演内容 テラヘルツテクノロジーとは何か? 時間領域テラヘルツ分光法の基礎 時間領域テラヘルツ分光法による物性測定 有機材料, 強誘電体 半導体 超伝導体水 水溶液 非線型テラヘルツ分光の必要性と戦略 Kyoto University

3 Importance of terahertz frequency region Radio wave THz region Light wave Dielectric constants ε 1 Intermolecular vibration Large molecule vibration Molecular Vibration Rotation Relaxation mode Spin excitation Electronic Transition X-ray X diffraction 線回折 0 ε 2 1m 100mm 10mm 1mm 100μm 10μm 1μm 100nm 10nm 1nm 0.1nm Radio wave Wavelength Infrared Visible Kyoto University

4 Absorption of Liquid Water Absorption Coefficient (cm -1 ) Microwave cooker Frequency (THz) THz EW MW IR Vis. UV 10m 1m 100mm 10mm 1mm 100μm 10μm 1μm 100nm 10nm Wavelength Jackson, Classical Electrodynamics 4

5 THz-Imaging THz x y THz B. B. Hu and M. C. Nuss, Opt. Lett. 20, 1716 (1995) Kawase (Nagoya/Riken), Nikon 5

6 講演内容 テラヘルツテクノロジーとは何か? 時間領域テラヘルツ分光法の基礎 時間領域テラヘルツ分光法による物性測定 有機材料, 強誘電体 半導体 超伝導体水 水溶液 非線型テラヘルツ分光の必要性と戦略 Kyoto University

7 .. THz-TDS の進展 - フェムト秒レーザーの進歩 Photoconductor Si SI-GaAs a-si RD-SOS LT-GaAs 10 4 Mode-locked Nd:glass laser YAG laser Flashlamp dye laser PC antenna femtosecond laser with Battery (IMRA) 10 3 Pulse Width (fsec) E-O sampling CW mode-locked dye laser Self-mode locked Ti : Sapphire laser Free space E-O sampling Wavelength (μm) CPM ring dye laser 第 3 世代フェムト秒システム 10 Pulse Complession Year 1 Er ファイバーレーザー Yb glass laser Diode laser + 希土類 Kyoto University

8 THz-detection Generation Detection Photocurrent LT-GaAs Dipole antenna Bias Voltage LT-GaAs E(t) A Photo-conducting antenna χ (2) EO crystal χ (2) Optical rectification E(t) λ/4 P Electro Optic sampling Kyoto University

9 THz-wave generation with second-order non-linear optical process E(t) χ (2) Pulse width: Δτ Δτ Δω = 1 P (2) (2) ( t) = ε 0χ E1( t) E2 ( t) Sum frequency Differential frequency E1( t) = E2( t) = (2) 2 P ( t) = A ( χ (2) A( e iωt + e iωt (2ω, ω, ω) e ) i2ωt + χ (2) (0, ω, ω)) 1 I(t) Δτ Δω = 1 Ω < Δω P (2) ( t) SHG Optical rectification P (2) (2ω) P (2) 2 (2) = A χ ( Ω, ω + Ω, ω)) e (0) iωt E THz (t) 0.5 Δτ Kyoto University

10 Phase matching condition ZnS ZnSe ZnTe CdTe 1.56 μm GaP InP GaAs Phase match condition: k(ω+ω) - k(ω) - k(ω) = 0 Including dispersion: refractive index in THz region = V = V THz phase Coherent length: c l = c 2 f n Wavelength (μm) optical Index of group velocity THz visible group n g Coherent Length (mm) ZnTe ω=800 nm Frequency (THz) Nahata et al., APL 69, 2321 (1996). 10

11 THz-wave detection LT-GaAs Photocurrent A EO crystal χ (2) E(t) Photo-conducting antenna E(t) λ/4 P Electro Optic sampling 1 Δτ current Time (ps) THz J ( Δ τ ) = eμ dte ( t) N( t Δτ ) Kyoto University

12 Non-linear THz spectroscopy EO crystal State of the art in our laboratory (Kumiko YAMASHITA, 2005) : E(t) Estimation was made by EO sampling technique. ΔI I = Γ = l λ 3 2π n r41 E THz = 0.2 Optical pump: λ=800nm 485 mw (485 μj /pulse) ZnTe(l=1mm n=3.2 r 41 = m/v) P THz = 100 pj/pulse, 10 nj/cm 2, 10 kw/ cm 2, E THz ~1kV/cm=100kV/m χ (2) λ/4 P Electro Optic sampling Typically, for the non-linear spectroscopy in visible region, we need a MW/cm 2 class laser. 12

13 Electric FIeld(arb. units) Time-domain spectroscopy (TDS) is a powerful tool in THz region 0 5 Time (ps) 10 Power (log scale) Fourier 10 0 transformation 0.0 ~ iφ ( ω ) 1 E( ω ) = E( ω) e = E( t) e 2π Frequency(THz) iωt dt 2π 2 π Phase (rad) Pulse measurements High sensitivity. cf. FTIR Electric field measurements Complex dielectric constants. E(ω), φ(ω) 13 Kyoto University 2006

14 Terahertz Time-Domain Spectroscopy (THz-TDS) ~ ~ Esam( ω ) t ( ω) = ~ Eref ( ω ) = tvs tsv exp i = tvs tsv exp i tvs ( n ~ ( ω) 1) ωd) c d tsv ( n 1) ωd) κωd ) exp c c sample n ~ = n + iκ t vs t sv 2 = n~ ( ω) n ~ ( ω) = n~ ( ω) + 1 ~ E ref ~ E sam ( ω) ( ω) Complex refractive index t vs, t sv : Fresnel coefficients ( vacuum sample) ( sample vacuum) Kyoto University

15 Compact THz-ATR spectrometer Ellipsoid mirror detector ATR prism emitter Ellipsoid mirror emitter detector 30mm ATT OC Double-ellipsoid mirrors with a semispherical ATR prism. 15 Advanced Terahertz Technologies adapted for Optical Communication

16 講演内容 テラヘルツテクノロジーとは何か? 時間領域テラヘルツ分光法の基礎 時間領域テラヘルツ分光法による物性測定 有機材料, 強誘電体 半導体 超伝導体水 水溶液 非線型テラヘルツ分光の必要性と戦略 Kyoto University

17 Importance of terahertz frequency region Radio wave THz region Light wave Dielectric constants ε 1 Intermolecular vibration Large molecule vibration Molecular Vibration Rotation Relaxation mode Spin excitation Electronic Transition X-ray X diffraction 線回折 0 ε 2 1m 100mm 10mm 1mm 100μm 10μm 1μm 100nm 10nm 1nm 0.1nm Plasma oscillation Radio wave Wavelength Infrared Visible Kyoto University

18 Mode-softening in KTaO 3 crystal Ichikawa, Tanaka et al.: Physical Review B 71(2005) Reference (x1/10) Electric Field [arb.u.] KTaO3 300K 100K 49K 30K 12K (x10) ε Mode-softening Delay of the pulse(phase shift) Decrease of the intensity Increase of the period THz region E THz Band-limiting TO 1 mode ω Time [ps] ω Kyoto University

19 Dielectric function in KTaO 3 Ichikawa et al.: Physical Review B 71(2005) A single Lorentz Model ~ ε ( ω ) = ε + ( ε 0 ε ) Ω Ω ω iγ ω Fitting parameters Ω 0 γ 0 : Mode frequency : Damping ε 0 :Static dielectric constant Kyoto University

20 Static dielectric constant Ichikawa et al.: Physical Review B 71(2005) Kyoto University

21 アミノ酸結晶の吸収 30x Tyrosine 50K 300K Frequency [THz] 80x Arginine 50K 300K 谷他 アミノ酸分子結晶のテラヘルツ時間領域分光 より引用 Frequency [THz] Kyoto University

22 isolated bases DNA: first step cytosine (C) cm -1 C dc dcmp 2.0 deoxycytidine (dc) normalized α deoxycytidinemonophosphate (dcmp) frequency [THz] B. M. Fischer, M. Walther, and P. Uhd Jepsen, Phys. Med. Biol (2002) Courtesy of Peter Uhd Jepsen 22

23 Evanescent wave for p-polarization θ in > θ c Evanescent wave in ATR Internal total reflection condition = Sin 1 n n H x 0 n ( ) 1 k sinθin H y = B exp βz expi x ωt n2 H 0 z TM mode Ex A E ( ) eva = n1 k sinθin Ey 0 exp βz expi x ωt n2 E C z 2 1 E in 1 β θ in E eva k // prism n 1 n 2 E out z Kyoto University 2006 x Comparable TM mode with with large the k // interaction length of water. Enable β 1 50μm to excite surface Best sensitivity modes. for water. exponentially decaying away from the interface waves 23

24 Surface plasmon in semiconductor with THz-ATR H. Hirori, M. Nagai, and K. Tanaka, Optics Express, 13, (26), (2005). Kyoto University

25 Dielectric constant in doped semiconductor InAs n-type n~10 17 Attenuated Total Reflection ω << γ ε ( ω) = 2 ε bω p i γ 1 = ω σ i ω H. Hirori, M. Nagai, and K. Tanaka, Optics Express, 13, (26), (2005). Kyoto University

26 Pump and probe spectroscopy How many-particle interactions develop after ultrafast excitation of electron-hole plasma R. Huber, F. Tauser, A. Brodschelm, M. Bichler, G. Abstrelter and A. Leitenstorfer Nature. Vol.414 (2001) 286 Kyoto University

27 Superconducting gap Far-infrared optical conductivity gap in superconducting MgB 2 films R. A. Kaindl, M. A. Carnahan, J. Orestein, D. S. Chemla, H. M. Christen, H. Y. Zhai, M. Paranthaman and D. H. Lowndes Phys. Rev. Lett. 88 (2002) nm MgB 2 薄膜における透過波形 6K( 実線 ) と 40K( 破線 ) Inset は 100nm MgB 2 薄膜 ( ) と 200nm MgB 2 薄膜 ( ) の抵抗の温度変化 40K の値で規格化した透過率 (b) 膜厚 100nm MgB 2 6K( ), 20K( ) 27K( ), 30K( ), 33K( ) (c) 膜厚 200nm MgB 2 (6K, 20K 25K, 30K, 36K) 6K( ), 20K( ) 25K( ), 30K( ), 36K( ) Kyoto University

28 TD-ATR spectroscopy in water 8 Water 6 Temperature-controlled Si ATR prism Re[ε] 4 2 Im[ε] K 293K 262K 265K (Ice) Frequency (THz) Nagai, et al., Int. J. IRMMW, 27, 505 (2006). Kyoto University

29 Dielectric Constants in Water Relative dielectric const ε 2 ε S ε1 1+ iωτ Water 20 C D ε 1 Kaatze Alison Hasted This work Hasted Orientational relaxation 8 ps,20 GHz Intermolecular Frequency [Hz] ε A + 2 ω ω + iωγ 2 0 stretching mode 200 fs, 5.4 THz 29

30 講演内容 テラヘルツテクノロジーとは何か? 時間領域テラヘルツ分光法の基礎 時間領域テラヘルツ分光法による物性測定 有機材料, 強誘電体 半導体 超伝導体水 水溶液 非線型テラヘルツ分光の必要性と戦略 Kyoto University

31 Non-linear THz spectroscopy State of the art of the THz pulse power using fs laser Pump power THz power Ti: Al 2 O 3 laser /pulse average /pulse Electric field (V/cm) Amplifier@1KHz 650 mw 650 μj 0.1μW 125 pj 1 KV/cm Oscillator with magnetic field (Sarukura 1500 mw 650 mw nj nj 650 μw 110 μw 8.125pJ 1.375pJ 0.25 KV/cm 0.1 KV/cm Cherenkov radiation with LiNbO mw 300μ J 0.1 mw 100 nj 30 KV/cm MIT group (2007): 1μJ (10 15 photons/pulse@1khz) 31

32 Non-linear THz spectroscopy Cherenkov radiation Cherenkov angle 32

33 Non-linear THz spectroscopy Basic idea of velocity matching by pulse front using Cherenkov effect with non-linear crystal A. G. Stepanov, J. Hebling and J. Kuhl et al., Appl. Phys. Lett. 83, 3000 (2003). 33

34 Non-linear THz spectroscopy High power single cycle THz generation using tilted femtosecond light sources J. Hebling and J. Kuhl et al., Appl. Phys. B 78, (2004). v n gr vis gr v >> << v n ph THz THz 34

35 Non-linear THz spectroscopy High power single cycle THz generation using tilted femtosecond light sources : Recent status A. G. Stepanov and J. Heebling et al., Optics Express (2005). Energy efficiency 5x10-4 Quantum Efficiency: 10% P THz = 100 nj/pulse, 10 nj/cm 2, 10 MW/ cm 2, E THz ~30kV/cm=3MV/m 35