Ultrabroadband Detection of THz radiation and the sensitivity estimation of Photoconductive antenna Itoh lab Michitaka Bitoh Outline • Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation • Experimental results and discussion ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary Terahertz region 10GHz 0.1THz 1THz 10THz 100THz THz region 0.1~100THz electric wave 30mm 3mm 300mm 30mm 3mm 1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV In THz region light ・intermolecular vibration ・lattice oscillation ・superconducting energy gap etc Transmitted waveform through MgB2 (b)6K(●)20K(○)27K(◆)30K(◇)33K(■) T=6K(solid line), T=40K(dashed line) (c)6K(●)20K(○)25K(◆)30K(◇)36K(■) THz emission from PC antenna fs laser pulse Backward THz wave Forward THz wave transient current J (t) Bias J (t ) ETHz (t ) t lifetime of carrier : ~1ps THz emission via DFG DFG (Difference-frequency generation) ・・・差周波発生 fs laser pulse NL crystal Differential frequency generation THz wave 1 10 fs ～ I(t,ω,Δω) 3 1 2 2 ETHz(t,ωTHz) ω3 =0 ⇒ optical rectification Integration of different frequency ∬ 0 frequency Intensity Intensity Δω ωTHz 0 Frequency spectrum of frequency Frequency spectrum of THz wave fs pulse laser from NL crystal THz detection using PC antenna fs laser pulse J t t t E t dt t : electrical conductivity THz wave E t : incident electric field A transient current J(t) t : time delay t = t1 t = t2 > t1 t = t3 > t2 THz Time-domain Spectroscopy (THz-TDS) Beamsplitter fs pulse laser Delay stage 1 ~ E ( ) 2 Probe pulse Pump pulse THz emitter E THz ~ ~ Eref Esam THz detector (t )e it dt E ( ) e 位相 振幅 ~ E ~ t ~sam E ref n~ 1d t vs t sv expi c i ( ) Fresnel coefficients tvs, tsv 2 t vs ~ (vacuum sam ple) n 1 2n~ t sv ~ ( sam ple vacuum) n 1 Sample Complex reflective index ~ n i n From n and κ, many optical coefficient can be obtained. Complexdielectricconstant: ~ 1 i 2 1 n 2 2 , 2 2n Absorptioncoefficient : 4 / Opticalconductivity : 2 / 4 Motivation Authors suggested the detection sensitivity beyond 100 THz from a model calculation by using 10 fs laser, and they also aimed to detect ultrabroad THz radiation with ultrashort pulse laser. They demonstrate ultrabroadband detection from 0.1 to 100 THz using the PC antenna detector, in two generation method. Investigation of the sensitivity of the PC antenna for midinfrared region. Outline • Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation • Experimental results and discussion Ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary Setup 10fs LT-GaAs substrate 400um emitter 5um detector A The PC antenna emission and detection Water vapor absorption The reflection of THz radiation by phonon (between LT-GaAs and the air) THz radiation from 0.1 to 25 THz was observed monotonously (except around 8 THz) DFG generation and the PC antenna detection Phonon absorption of GaSe crystal Absorption by CO2 THz radiation was observed up to 100THz (except for the frequency range from0.1 to 10 THz) Outline • Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation • Experimental results and discussion ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary The PC antenna sensitivity (experimental date) Response function of this system This system can detect the frequency up to 95 THz. The response around 70 THz is distorted by the absorption by CO2. The PC antenna sensitivity (experimental date) Calibrated power spectrum of the THz radiation Extracted power spectrum from… Because of the absorption of CO2, this region is unreliable. Calibrated power spectrum of the THz radiation was obtained. From (a) and (b), the sensitivity of PC antenna can be calculated. The PC antenna sensitivity (model calculation) The current by the incident THz radiation at delay time t is described as E(t): the incident electric field J t t t E t dt N(t): number of photocarriers e: the elementary electric charge e N t t E t dt μ: the electron mobility σ: electrical conductivity t eN t According to the convolution theorem of the Fourier transformation: J N E Correspond to the sensitivity The carrier number N(t) is described by the following equation: N t I t Rt t dt This is transformed into the following equation: N I R I(t) I(t): the shape of probe laser pulse R(t): time response of carriers Intencity(a.u.) -20 0.0 0.5 0 20 Time(fs) R(t) N(t) 1.0 1.5 Time(ps) 2.0 The PC antenna sensitivity (model calculation) 2 1.76t I t sec h d Intencity(a.u.) Authors assumed the shape of probe pulse1.0 as the following equation τd : the pulse width 0.8 0.6 10fs 0.4 τd=10(fs) 0.2 0.0 -30 -20 -10 0 10 20 30 R(t) is estimated from the transient reflection measurement. Time(fs) Rt A exp 5.8t B exp 2.4t A 0.2069, B 0.6589 I(t) and R(t) are transformed into I(ω) and R(ω) respectively Experimental date and calculated one Good agreement between experimental date and calculated one. Noise level To use 5fs pulse laser, the more sensitive detection is expected with the PC antenna. This results indicates the feasibility of ultrabroadband detection over 130 THz using the PC antenna if broader and stronger THz radiation obtained. Summary • By using the PC antenna for detection, spectrum response from 0.1 to 100 THz except around 8 THz was obtained using a combination of PC antenna emitter and a GaSe crystal. • The sensitivity of the PC antenna from a model calculation shows that the PC antenna is capable of detecting ultrabroadband THz radiation beyond 100 THz with an advanced setup. My study To use 5fs pulse laser, the more sensitive detection is expected with the PC antenna. My study My study Absorption of DAST itself THz radiation from 100 to 170 THz was obtained My study There is good agreement between experimental date and model calculation when the pulse duration is 5fs. Future plan is ・・・ ・To detect THz wave at broader frequency range with this system ・To measure something with this system THz Time-domain Spectroscopy (THz-TDS) fs pulse laser Delay stage Beamsplitter Probe pulse Pump pulse THz emitter • THz-TDS can be carried out at room temperature and ambient air. • Temporal waveform is detected. • The complex refractive index of the sample can be estimated without Kramers-Kronig analysis. THz detector Sample ~ n i n refractive index extinction coefficient complex dielectric constant absorption coefficient optical conductivity Various optical constants are obtained easily !
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