Terahertz Spectral Range

Transcription

1 Inhalt 1. Einleitung 2. Wechselwirkung Licht-Materie 3. Bilanzgleichungen 4. Kontinuierlicher Betrieb 5. Relaxationsoszillationen 6. Güteschaltung 7. Modenkopplung 8. Laserresonatoren 9. Eigenschaften von Laserlicht 9.1 Kohärenz 9.2 Photonenstatistik 10.Lasertypen 10.1 Festkörperlaser 10.2 Flüssikgkeitslaser 10.3 Gaslaser 11. Anwendungen 11.1 Erweiterung des Frequenzbereiches Terahertz-Strahlung Nichtlineare optische Frequenzkonversion

2 Terahertz Spectral Range LaserphysikWS09/10 2

3 Absorption of the Atmosphere Atmospheric absorption spectrum Long distance atmospheric transmission is very challenging (but possible for ~100 m or so). LaserphysikWS09/10 3

4 Material Properties at THz Frequencies THz radiation is non ionizing no modification of chemical properties of organic compounds important for living cells charactereristic absorptions of almost all polar molecules specific detection of substances also for complex molecules detection of changes medicals, drugs, explosives Absorptionskoeffizient [b. E.] 1,0 0,8 0,6 0,4 0,2 0,0 H 2 O 0,5 1,0 1,5 2,0 2,5 Frequenz [THz] Absorption z/2[a. u.] CO + NO CO NO 0,6 0,8 1,0 1,2 Frequency [THz] LaserphysikWS09/10 4

5 Material Properties at THz Frequencies strong absorption of polar liquids (like water) small penetration depth for water containing substances quantitative determination of water content metals are not transparent for THz dielectrics (paper, plastics, textiles, etc.) are transparent detection of substances inside objects (through packages or cloths) LaserphysikWS09/10 5

6 Material Properties at THz Frequencies Higher spatial resolution in comparison with microwave imaging THz Dave Zimdars, Picometrix, Inc. High potential for applications fundamental research, nondestructive testing, security technology,. LaserphysikWS09/10 6

7 Material properties at THz frequencies Advantages: THz radiation is non ionizing dielectrics are transparent location of substances in packaging characteristic absorption of polar molecules higher spatial resolution compared to microwaves 1 THz 330 µm 33 cm -1 1 ps 6.4 mev Limitations: metals reflect THz strong absorption of polar liquids (water) LaserphysikWS09/10 7

8 Terahertz Technology for Sensing Applications THz Sources and Detectors LaserphysikWS09/10 8

9 Blackbody Radiation Natural sources of THz are very weak. Emissivity (J/m 2 ) K 300K T=30K Frequency (THz) LaserphysikWS09/10 9

10 Typical THz Sources short pulses (< 1 ps) broad band PCS, surface emitter, opt. rectification, pulsed long pulses (>10 ps) narrow band opt. rectification in PPLN, GaAs, OPOs, DFG, photomixing, continuous wave direct lasers: gas, Ge, QCL,. backward oszillators Electric field [a. u.] Spectral amplitude [b. E.] 1,0 0,8 0,6 0,4 0,2 0,0-0,2-0, Delay [ps] 0,5 1,0 1,5 2,0 2,5 3,0 Frequency [THz] Field amplitude [a. u.] Delay [ps] LaserphysikWS09/10 10

11 THz and Optical Parametric Generators THz picks up where OPG s leave off. Gavin D. Reid, University of Leeds, and Klaas Wynne, University of Strathclyde LaserphysikWS09/10 11

12 Mixing of Optical Sources Photomixer Electrodes LTG-GaAs Epitaxial Layer on One-Inch Semi-Insulating GaAs Substrate Silicon Hyper- Hemispherical Lens Pump Beams Spiral Antenna 10 µm Fiber Photomixer Terahertz Signal S. Duffy & K. McIntosh, MIT Lincoln Labs Diode Lasers LaserphysikWS09/10 12

13 Hyperhemispherical Lens Total internall reflection: = arcsin(n-1) ~ 17.1 Increasing the collection angle LaserphysikWS09/10 13

14 Photonic Terahertz Technologies THz spectral range: 100 GHz < < 20 THz 3 mm < < 15 µm Photonic THz Sources Photoconductive switches Surface emitter Nonlinear optics DFG, OPO, OPG Lasers QCL, Ge, Gas D. Auston D. Grischkowski X.-C. Zhang B y J. Faist, Q. Hu, F. Tredicucci, z E D LaserphysikWS09/10 14

15 Photonic THz Technologies Opto-electronics: Optical: photoconductive switches, surface emitter (magnetic field enhanced) optical rectification, optical rectification in PPLN, difference frequency generation, OPOs, quantum cascade lasers, THz lasers Detectors: photoconductive switches opto-electronic methods coherent detection (amplitude and phase) bolometer golay cell, etc. incoherent detection (intensity only) LaserphysikWS09/10 15

16 Terahertz Technology for Sensing Applications Photoconductive Switches LaserphysikWS09/10 16

17 Photoconductive Switch as Emitter 80 m 2 P E( t) 2 t P( t) N( t) e x( t) + - Semiconductor chip (GaAs) with applied voltage between electrodes - + Laser pulse generates free carriers Accelerated carriers emit electromagnetic wave (THz pulse) LaserphysikWS09/10 17

18 Photoconductive Switch as Emitter Generation Transient photoconductivity: 80 µm Ultrashort pulse generates free carriers in semiconductor Acceleration of free carriers by external electric field Transient photocurrent j(t) Radiation of a THz-pulse Laser Laserintensität intensity I(t) I(t) Stromdichte j(t) Current density j(t) V E( t) j j( t) t P t D. Grischkowsky et al. Time t Zeit LaserphysikWS09/10 18

19 Photoconductive Switch as Detector Coherent detection Femtosecond pulse Photoconductive antenna THz wave THz Pulse train A Ampere meter No current without voltage Current depends on field strength of THz pulse However: fs pulse is much shorter than THz pulse (100 x) Only the instantenous field is accelerating the charges fs Pulse train Temporal sampling of the THz pulse via averaging over many pulses LaserphysikWS09/10 19

20 THz Antenna Current I(t) Photoconductive antenna: Ultrashort pulse generates free carriers in semiconductor Acceleration of free carriers by electric field of synchronized THz pulse THz pulse leading nir pulse Transiente photocurrent j(t) proportional to THz field Current ampl. Lock-In Measurement of phase and amplitude RyMess Time t THz pulse trailing nir pulse LaserphysikWS09/10 20

21 THz Antenna Current I(t) Photoconductive antenna: Generation THz pulse leading nir pulse t j ( ) E( t) g( t ) dt finite length of NIR pulse carrier lifetime averaging over electric field dipole structure also determines frequency response Current ampl. Lock-In RyMess Time t THz pulse trailing nir pulse LaserphysikWS09/10 21

22 Photoconductive Switches as Detector fs-nir Generation ps-thz Repetition rate Sampling rate MHz, 1 10 khz LaserphysikWS09/10 22

23 Photoconductive Switches as Detector t = 0 fs-nir Generation Generation I t t ps-thz Repetition rate MHz, t = 0 Sampling rate 1 10 khz LaserphysikWS09/10 23

24 Photoconductive Switches as Detector t = 0 fs-nir Generation Generation t I t ps-thz Repetition rate MHz, t = 0 Sampling rate 1 10 khz LaserphysikWS09/10 24

25 Photoconductive Switches as Detector t = 0 fs-nir Generation Generation t I t ps-thz Repetition rate MHz, t = 0 Sampling rate 1 10 khz LaserphysikWS09/10 25

26 Photoconductive Switches as Detector t = 0 fs-nir Generation Generation t I t ps-thz Repetition rate MHz, t = 0 Sampling rate 1 10 khz LaserphysikWS09/10 26

27 Components of a Broadband THz System Laser Emitter Detector THz Optics fs Laser, diode laser pumped/diode laser/ir laser Photoconductive switch (surface emitter, PPLN, ) Photoconductive switches, EOS Depending on application Emitter Probe Probe Receiver optics Delay Fs/ps Laser Delay Beam splitter LaserphysikWS09/10 27

28 Empfängerchip Empfängerchip THz System receiver (SOS) transmitter (GaAs) absorption cell Empfängerchip Si - lens Empfängerchip receiver transmitter current ampl. Lock-In lens chopper DC-Bias (10 kv/cm) beam splitter RyMess delay fs laser: Puls length: Average power: Repetition rate: ~ fs ~ mw ~ MHz LaserphysikWS09/10 28

29 Terahertz Technology for Sensing Applications Time Domain Spectroscopy LaserphysikWS09/10 29

30 THz Time Domain Spectroscopy (TDS) Measuring principle Start End Emitter Receiver Time Start arrival Start LaserphysikWS09/10 30

31 THz Time Domain Spectroscopy (TDS) Measuring principle Start End Emitter Receiver Time Start arrival Start LaserphysikWS09/10 31

32 THz Time Domain Spectroscopy (TDS) Measuring principle Start End smaller amplitude delay absorption and/or thickness thickness and/or index of refraction Emitter Receiver Time Start Start arrival additional modulations spectroscopic information LaserphysikWS09/10 32

33 THz Time Domain Spectroscopy (TDS) Measuring the electric field in the time domain Coherent detection with high S/N ratio Informationen about: Amplitude/Intensity Time delay Spectral content electric field [arb. u.] amplitude delay time spectral features echos time [ps] reference with sample LaserphysikWS09/10 33

34 THz Time Domain Spectroscopy (TDS) Time domain direct measurement of electric field (amplitude and phase) Pulse duration < 1 ps Signal-to-Noise > 10 3 :1 (30 ms integration time) Fourier Transformation Frequency domain Spectral amplitude Phase information Usable bandwidth 100 GHz < < 4 THz LaserphysikWS09/10 34

35 THz Time Domain Spectroscopy (TDS) (a) (b) Intramolecular Electric Field [a.u.] (c) Time Delay Reference Sample FFT (d) Reference Sample Frequency [THz] E-3 1E Amplitude Spectrum [a.u.] THz-Messtechnik und Systeme Abs.coeff. [cm -1 ] Refractive Index Intermolecular Frequency [THz] Frequency [THz] 1.55 LaserphysikWS09/10 35

36 Electromagnetic Waves in Matter The interaction of electromagnetic radiation with matter: permittivity ~ ~ 2 r n n i 2 ε r : Relative permittivity n: refractive index : extinction coefficient ~ i kx t Plane wave in vacuum: (k = ω / c) dispersion absorption Plan wave in matter: TDS measures: E E 0 e ~ i n~ kx t in kx kx i t E E e 0 ~ E t ~ E E 0 i nkx t ~ sample E e i n~ 0 e i kx t ref E 0 e ~ e e 1 kx e LaserphysikWS09/10 36

37 Electromagnetic Waves in Matter TDS measures ~ E i nkx t ~ sample E e i n~ 0 t ~ e i kx t E ref E 0 e ~ 1 kx Conventional spectroscopy measures: T P P e i sample ref ~ sample ref sample ref * n 1 kx i n 1 kx 2 kx x e ~ E ~ E ~ ~ E ~ E e Absorption is seen, but phase information is lost: No information on real part of refractive index, n. * * e n(ω) can be derived from (ω) with the Kramers-Kronig Kronig relation in some cases and not completely. LaserphysikWS09/10 37

38 Time Domain Spectroscopy of CO Electric field [a. u.] Absorptionskoeffizient /2 [cm -1 ] Delay [ps] 0,12 0,10 0,08 Absorption 0,06 0,04 0,02 0,00 0,5 1,0 1,5 2,0 2,5 FFT Spectral amplitude [a. u.] CO spectrum devided by reference spectrum: Frequenz [THz] Dispersion k [rad/cm -1 ] 0,06 0,5 1,0 1,5 2,0 2,5 Frequency [THz] Dispersion 0,04 0,02 0,00-0,02-0,04 0,5 1,0 1,5 2,0 2,5 Frequenz [THz] LaserphysikWS09/10 38

39 Time Domain Spectroscopy: FFT o Fast Fouriertransformation (FFT) N 1 n 0 n m i N k E (m ) E(n t) e N 2 k 0, 1, 2,... o Step width: o Spectral resolution - frequency domain - time domain t 1 N t o Zero filling E E k k 1 (2 1) t,..., E (2 ) t 0 k 1 k 2 (2 1) t,..., E (2 ) t 0 interpolation between neighboring data points LaserphysikWS09/10 39

40 Time Domain Spectroscopy: spectral resolution o Resolution depends on number of data points: 1 N t reduction of data points increase in line width 1,140 1,145 1,150 1,155 1,160 1,165 Absorption [(35 cm) -1 ] 2,0 1,5 1,0 0, ps 533,5 ps 267 ps 133,5 ps 67 ps 2,0 1,5 1,0 0,5 0,0 0,0 1,140 1,145 1,150 1,155 1,160 1,165 Frequency [THz] LaserphysikWS09/10 40

41 Time domain Spectroscopy: spectral resolution 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 Absorption [(35 cm) -1 ] 0,20 0,15 0,10 0, ps 0,20 0,15 0,10 0,05 Absorption [(35 cm) -1 ] 0,20 0,15 0,10 0, ps 0,20 0,15 0,10 0,05 0,00 0,00 0,00 0,00-0,05-0,05 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 Frequency [THz] -0,05-0,05 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 Frequency [THz] 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 Absorption [(35 cm) -1 ] 0,20 0,15 0,10 0, ps 0,20 0,15 0,10 0,05 Absorption [(35 cm) -1 ] 0,20 0,15 0,10 0,05 67 ps 0,20 0,15 0,10 0,05 0,00 0,00 0,00 0,00-0,05-0,05-0,05-0,05 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 0,75 0,80 0,85 0,90 0,95 1,00 1,05 1,10 Frequency [THz] Frequency [THz] LaserphysikWS09/10 41

42 THz Spectroscopy of CO and NO Feldstärke [b. E.] Feldstärke [b. E.] Messung Rechnung Verzögerung [ps] Messung Rechnung Absorption /2 [cm -1 ] Dispersion [rad/cm] Absorption /2 [cm -1 ] Dispersion [rad/cm] 500mbar NO 0,02 0,00 0,3 0,4 0,5 0,6 0,7 0,8 0,06 0,04 0,02 0,00 0,3 0,4 0,5 0,6 0,7 0,8 Frequenz [THz] 400mbar CO 0, ,3 0,4 0,5 0,6 0,7 0,8 0,9 LaserphysikWS09/10 Verzögerung [ps] Frequenz [THz] 42 0,06 0,04 0,04 0,02 0,00 0,3 0,4 0,5 0,6 0,7 0,8 0,9 0,04 0,02

43 THz Spectroscopy of CO and NO Electric Feldstärke field [a. [b. u.] E.] Delay [ps] 3 Delay [ps] Verzögerung [ps] Rechnung calc. Messung meas. FFT Absorption /2 [cm -1 ] Dispersion [rad/cm] 0,06 0,04 500mbar NO 0,02 0,00 0,3 0,4 0,5 0,6 0,7 0,8 0,06 0,04 0,02 0,00 0,3 0,4 0,5 0,6 0,7 0,8 Frequenz [THz] Frequency [THz] Absorption z/2 [a. u.] 2 CO NO 1 0 0,6 0,8 1,0 1,2 Frequency [THz] LaserphysikWS09/10 43