Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Generation and detection of ultrabroadband terahertz radiation
Document related concepts
Transcript
Generation and detection of ultrabroadband terahertz radiation Y. C. Shen, et al., Appl. Phys. Lett. 85, 164 (2004) Itoh laboratory Taisuke Katashima Contents ・Introduction of THz wave ・Applications of THz wave ・Generation and detection of THz wave ・Previous experiments ・Experiments ・Experimental results ・Summary What is a THz wave ? THz region:0.1~10THz Visible X-ray γ-ray Microwaves Frequency(Hz) 103 killo Example 106 mega Radio 109 giga 1012 tera Radar 1015 peta ・Wavenumber 1021 zetta Optical Medical Astrophysics ・Frequency: 1THz=1012Hz ・Wavelength: 1THz 1018 exa 300µm 33cm-1 Applications of THz ・THz spectroscopy ・THz Time-domain Spectroscopy (TDS) ・THz imazing ・3D-rendered imaging ・Imaging of teeth, cancerous tissue and flames ・・・ etc. ・Security screening THz -TDS splitter mirror We can get the amplitude and phase of electromagnetic wave at the same time!! The principle of THz-TDS THz wave At x=d n0d Ei ( ) E0 exp i c n~( )d Et ( ) E0 exp i c Et ( ) T ( ) Ei ( ) Refractive index: n( ) 2 Refractive index : n0 E t () ~n Sample 0 Ei () d x ( ) t ( ) i ( ) c c ( ) n0 : ( ) ln T ( ) Extinction cofficient d 2d Complex index of refraction : n~( ) n( ) i ( ) THz-imaging http://www.teraview.com/home_index.htm Imaging of teeth THz imaging is safer than X-ray imaging http://www.agri.tohoku.ac.jp/thz/jp/41_h15.htm Security screening Generation and detection of THz wave Two major elements to generate THz wave with using fs laser ・photoconductive antenna (PC antenna) ・nonlinear crystal(NL crystal) Generation: Photoconductive antenna fs laser pulse THz wave Photoconductive antenna i (t ) ETHz (t ) t fs laser pulse is focused on the gap Gap Generation of carriers Transient current Electrode Emission of THz wave Detection:Photoconductive antenna THz wave fs laser pulse J (t ) ETHz (t ) à preamplifier ①fs laser pulse is focused on the gap ②Generation of carriers Lock-in amp PC ③Carriers are accelerated by THz electric field Carriers movement Current signal of ETHz(t) Experiments: previous study Y. C. Shen, et al Appl., Phys. Lett. 83, (2003) Generation: LT-GaAs photoconductive antenna Detection: 20-µm-thick ZnTe crystal using electro-optic sampling (EO sampling) Frequency components over 30THz were observed!! Experimental results:previous study (1)Fourier-transform amplitude spectrum ・Over 30THz were observed!! Low frequency components were not observed!! The dips at 5.2THz and the peaks at 6.2THz are TO and LO phonon resonance in ZnTe, respectively. (TO =Transverse-optical, LO =Longitudinal-optical ) This spectral distribution up to 8THz is not ideal for spectroscopy. Purpose of this study ・Generate and detect ultrabroadband THz wave To reduce the phonon resonances of ZnTe crystal （This complicated measurement of THz signals in previous study） Using LT-GaAs PC emitter and receiver instead of PC emitter and ZnTe receiver ・Apply this system for spectroscopy Sample: Maltose pellet Measuring method: detecting transmitted light Experiments ・LT-GaAs PC antenna Gap: 400μm Gap 0.53-mm-thick GaAs substrate 1.0μm-thick LT-GaAs layer ・Samples Electrode ・Mixed maltose polycrystalline power with polyethylene powder pellet (in a mass ratio 1:10) ・Thickness 1.3mm Experimental set up Parabolic mirror PC antenna PC antenna sample Lock-in amp à PC Pump beam:400mW fs pulsed Ti-sapphire Probe beam:30mW Beam splitter delay stage ・15fs duration ・Repetition 76MHz ・Center wavelength 790nm : Vacuum-tight box purged with dry nitrogen gas Experimental results ・Low frequency components were observed. They can’t be observed in previous study. ・Distinct dips at 5.2THz and peaks at 6.2THz were reduced!! Distinct dips at 8.0THz and peaks at 8.7THz are TO and LO phonon resonance in GaAs. (a) Temporal THz wave form We can observe smooth spectral distribution up to 8THz!! Experimental results (spectroscopy) ・Conventional THz-TDS・・・frequency range 0.3~3.0THz ・This system・・・・・・・・・・・frequency range 0.3~8.0THz pure polyethylene 14 vibrational modes are observed at room temperature maltose/polyethylene Corresponding to interand intra-molecular interactions maltose Fourier-transform amplitude spectrum of pure polyethylene and maltose/polyethylene Summary ・PC-generation/PC-detection scheme leads to smooth spectral distribution up to 8THz. ・PC-detection provides about 8 times better signal-to-noise-ratio than EO-detection up to frequencies in excess of 8THz. PC-generation/PC-detection scheme is an ideal system for THz-TDS in the frequency range 0.3-8.0THz. Comparison with THz-TDS and other spectroscopy Fourier-transform infrared spectroscopy:5vibrational modes Raman spectroscopy:14vibrational modes Neutron inelastic scattering spectroscopy:all vibrational modes can be observed in principal THz-TDS: 19 vibrational modes of cytidine were observed in the frequency range 1-20THz mary Generation:Optical rectification of NL crystal Optical rectification fs laser pulse: 1 second-order non-linear optical effects 2 3 THz wave fs laser pulse: ①fs laser pulses falls on the NL-crystal Non-linear crystal ②Nonlinear polarization generates ( 2) pi (ω3) j .k x , y , z ijk (3 ; 1 2 ) j (1 )k (2 ) ③Electromagnetic wave ω3 is emitted ＊ω3= ω1-ω2 Detection: Optical rectification of EO crystal EO crystal λ/4 plate THz wave :preamplifier beam splitter photo diode :lock-in amp :delay stage :sampling pulse (fs-laser pulse) PC ・Electric field of THz induces anisotropy of refraction index in a crystal sampling pulse proves change in intensity (b) Abstract ・Recent years terahertz science and technology have developed ・THz technology Longitudinal-optical (LO) phonon resonance in ZnTe Experimental results:previous study (1)Temporal THz waveform Full width at half maxima is 40fs It’s the shortest THz pulse with using PC emitter (2)Fourier-transform amplitude spectrum Over 30THz are observed!! ・The dips at 5.2THz TO phonon resonance in ZnTe (TO =Transverse-optical) ・The peaks at 6.2THz LO phonon resonance in ZnTe (LO =Longitudinal-optical ) 3D-renderd image(two plastic cylinders) Optical image Refractive index of each cross sectional slice Surface-rendered image ・Carrier lifetime Emitter:0.1ps Reciever:10ps ・Electrodes Reciever:Ti/Pd/Au Emitter:NiCr/Au E t () d ~ exp in () n 0 E i () c
