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Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antenna

Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antenna. Itoh lab Michitaka Bitoh H. Shimosato et al. Ultrafast Optics V , (Springer, Berlin, 2007) pp. 317-323 . Outline. Introduction THz region THz time-domain spectroscopy (THz-TDS)

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Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antenna

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  1. Ultrabroadband detection of THz radiation and the sensitivity estimation of photoconductive antenna Itoh lab Michitaka Bitoh H. Shimosato et al. Ultrafast Optics V, (Springer, Berlin, 2007) pp. 317-323

  2. Outline • Introduction THz region THz time-domain spectroscopy (THz-TDS) THz emission THz detection Motivation • Experimental results and discussions ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary

  3. THz region 0.1~100THz 33K 30K 27K 24K 17.5K 6K Terahertz region 10GHz 0.1THz 1THz 10THz 100THz In THz region light ・intermolecular vibration ・lattice oscillation ・superconducting gap etc electric wave 30mm 3mm 300mm 30mm 3mm 1THz = 1012Hz ⇔ 300mm ⇔ 4.1meV 30K<Tc<33K In THz region, a depletion of the real part of the conductivity which correspond to the opening of a superconducting gap can be observedlike this. Real part of conductivity σ1(ω) for the MgB2 film normalized to its normal state value σ1N(40K).

  4. Beamsplitter Probe pulse fs pulse laser Pump pulse Delay stage THz emitter THz detector Sample THz Time-domain Spectroscopy (THz-TDS) phase(位相) Complex refractive index Amplitude(振幅) From n and κ, many optical coefficient can be obtained. Fresnel coefficients tvs, tsv

  5. fs laser pulse Backward THz wave Forward THz wave transient current J(t) Decay time of J : ~100fs ETHz(ω):~10THz THz emission from PC antenna Bias (LT-GaAs substrate)

  6. THz emission via DFG DFG (Difference-frequency generation) ・・・差周波発生 Differential frequency generation fs laser pulse NL crystal THz wave I(t,ω,Δω) ETHz(t,ωTHz) ω3 =0　⇒ optical rectification Δω Integration of different frequency Intensity Intensity Δω ∬ 0 frequency 0 frequency Frequency spectrum of THz wave from NL crystal Frequency spectrum of fs pulse laser

  7. fs laser pulse THz wave transient current J(t) t = t2 > t1 t = t3 > t2 t = t1 THz detection using PC antenna A t : time delay

  8. THz region 0.1~100THz Motivation 10GHz 0.1THz 1THz 10THz 100THz electric wave light spectral range of THz-TDS YBCO:Δ=4.9THz MgB2:Δ=3.4THz superconducting gap lattice vibration Authors demonstrated 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 mid-infrared region.

  9. Outline • Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation • Experimental results and discussions Ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary

  10. A Setup LT-GaAs substrate 10fs 400um 5um emitter detector

  11. Water vapor absorption The reflection of THz radiation by phonon (between LT-GaAs and the air) The PC antenna emission and detection THz radiation from 0.1 to 25 THz was observed monotonously (except around 8 THz)

  12. Phonon absorption of GaSe crystal Absorption by CO2 DFG generation and the PC antenna detection THz radiation was observed up to 100THz (except for the frequency range from0.1 to 10 THz)

  13. Outline • Introduction THz region THz emission THz detection THz time-domain spectroscopy (THz-TDS) Motivation • Experimental results and discussions ultrabroadband detection from 0.1 to 100 THz Investigation of the sensitivity of the PC antenna • Summary

  14. 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.

  15. 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 (experimental date) Calibrated power spectrum of the THz radiation Extracted power spectrum from…

  16. 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 N(t): number of photocarriers e: the elementary electric charge μ: the electron mobility σ: optical conductivity According to the convolution theorem of the Fourier transformation: Correspond to the sensitivity The carrier number N(t) is described by the following equation: I(t): the shape of probe laser pulse R(t): time response of carriers This is transformed into the following equation:

  17. The PC antenna sensitivity (model calculation) Authors assumed the shape of probe pulse as the following equation 10fs τd=10(fs) τd : the pulse width R(t) is estimated from the transient reflection measurement. I(t)and R(t) are transformed into I(ω) and R(ω) respectively

  18. Experimental date and calculated one Good agreement between experimental data and calculated one with τd=10fs. By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna. Noise level This results indicates the feasibility of ultrabroadband detection over 130 THz using the PC antenna if broader and stronger THz radiation obtained.

  19. 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.

  20. My study Intensity (a.u.) Time (fs) By using 5fs pulse laser, the more sensitive and broadband detection is expected with the PC antenna.

  21. My study DAST (4-N ,N-dimethylamino-4’-N’- methyl stilbazolium tosylate)

  22. Absorption of DAST itself My study THz radiation from 100 to 170 THz was obtained

  23. My study There is good agreement between experimental data 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

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