Ultrabroadband terahertz generation using DAST single crystal APPLIED PHYSICS LETTERS 97, 021105-1 I.Katayama, R.Akai, M.Bito, H.Shimosato, K.Miyamoto, H,Ito and M.Ashida Ashida lab. Takuya Iwasa Contents • • • • • • Introduction Motivation Experimental setup Results & discussion Summary My work THz Spectroscopy System Time Domain Spectroscopy（THz-TDS） • THz-TDS uses short pulses of broadband THz radiation. • The transmitted THz electric field is measured coherently, which provides both high sensitivity and time-resolved phase information. E THz wave t Probe laser t Time delay THz-TDS pump probe system Principle of THz-TDS Electromagnetic wave Wavelength 30mm Micro wave 10GHz Frequency 3mm 300μm Submilimeter wave 0.1THz 1THz THz region 30μm Far infrared 10THz 3μm Near infrared 100THz 300nm optical 1PHz broader • THz radiation is from 1011 to 1013Hz approximately. • If we generate broader radiation, we can measure from THz region to optical region in continuity. THz emitter Difference frequency generation Nonlinear crystal THz wave fs laser Intensity Intensity Fourier transform ω2 ω1 frequency 3 1 2 • A 400-μm-thick DAST crystal was used as nonlinear crystal. • A DAST crystal is an organic compound crystal and has large nonlinear optical coefficient.(非線形光学係数） ω３ frequency DAST crystal THz detector Photoconductive antenna Current meter A J (t ) e E (t ' ) N (t 't )dt' μ: mobility of electron fs laser N: the number of carrier THz wave Motivation • To generate the terahertz wave using a DAST crystal by considering the refractive index dispersion. • To detect the THz wave generated by a DAST crystal using monochromator and THz-TDS. Experimental setup (Spectral Phase Interferometry for Direct Electric-field Reconstruction) BaF2 (a positive dispersion medium) pulse image Positive chirping Short wavelength Long wavelength Positive dispersion medium Time lag Group velocity dispersion (郡速度分散) Results & discussion The data of laser phase spectra observed using SPIDER. BaF2 thickness ― 0mm ― 2mm ― 4mm 0mm-thick BaF2 Long wavelength 2mm-thick BaF2 0mm-thick BaF2 Short wavelength Negative chirping Positive chirping Results & discussion BaF2 thickness ― 0mm ― 2mm ― 4mm The best of these data is the one of negative chirping. Experimental setup Chirp mirror Ti:Sapphire laser BS Pump pulse Chopper BaF2 plate DAST 400mm Delay stage Band-pass filter PC antenna Probe pulse I/V amp lock-in amp PC Results & discussion In this graph, very high frequency part is present. Results & discussion BaF2 thickness ― 0mm ― 2mm ― 4mm The Fourier transform of the electric waveform. (dotted line) Results & discussion 0mm-thick BaF2 2mm-thick BaF2 DAST (Large positive dispersion) Short wavelength Long wavelength The region of generating High-frequency THz wave Short wavelength Long wavelength The energy is absorbed a lot. Summary • In the DAST crystal, the group velocity dispersion is important for the generation of the ultrahigh-frequency terahertz wave. • A waveform is detected by using photoconductive antenna. My work I try to measure a wave form by using this optical system when there is a sample in front of PC antenna. Monochromator Concave mirror Slit Diffraction grating HgCdTe and InAs Detector Slit THz pulse SPIDER法 (Spectral Phase Interferometry for Direct Electric-field Reconstruction) Sum frequency I (t) I () (t) Intense strongly-chirped pulse from Ti: sapphire amplifier Nonlinear Crystal Replica2 I (t) Interferogram Replica1 I () Pulse to be measured and its duplication SPIDER interferogram S () E() E( ) 2 E()E( ) cos () ( ) 2 2 2 21
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