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 ・Previous experiments ・Experiments ・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 300µm ・Wavenumber 33cm-1 Microwaves Visible X-ray γ-ray Frequency(Hz) 103 106 109 1012 1015 1018 1021 zetta killo mega giga tera peta exa Example Radio Radar Optical Medical Astrophysics ・Frequency: 1THz=1012Hz ・Wavelength: 1THz 300µm ・Wavenumber 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

We can get the amplitude and phase of electromagnetic wave 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 Refractive index Sample x d ・Φはx=0の時の位相である。 Refractive index: Extinction cofficient:

THz-imaging Imaging of teeth Security screening http://www.teraview.com/home_index.htm http://www.agri.tohoku.ac.jp/thz/jp/41_h15.htm Imaging of teeth Security screening THz imaging is safer than X-ray imaging

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 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 à preamplifier ①fs laser pulse is focused on the gap Lock-in amp ②Generation of carriers 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.

（This complicated measurement of THz signals in previous study） 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 ・Samples 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 à PC Lock-in amp sample delay stage Parabolic mirror Lock-in amp PC antenna PC antenna à sample PC Pump beam:400mW Probe beam:30mW Beam splitter delay stage fs pulsed Ti-sapphire ・15fs duration : Vacuum-tight box purged with dry nitrogen gas ・Repetition 76MHz ・Center wavelength 790nm

Experimental results ・Low frequency components were observed. (a) Temporal THz wave form 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. 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 inter- and 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 second-order non-linear optical effects fs laser pulse: THz wave fs laser pulse: Non-linear crystal ①fs laser pulses falls on the NL-crystal ②Nonlinear polarization generates ＊ω3= ω1-ω2 ③Electromagnetic wave ω3 is emitted

Detection: Optical rectification of EO crystal λ/4 plate EO crystal THz wave :preamplifier photo diode beam splitter :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

Abstract (b) ・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) Refractive index of each cross sectional slice Optical image Surface-rendered image

・Carrier lifetime Emitter:0.1ps Reciever:10ps ・Electrodes Emitter:NiCr/Au Reciever:Ti/Pd/Au