1 / 16

Measurements of High-Field THz Induced Photocurrents in Semiconductors

Measurements of High-Field THz Induced Photocurrents in Semiconductors. Michael Wiczer University of Illinois – Urbana-Champaign Mentor: Prof. Aaron Lindenberg. THz Radiation. 1 THz ~ 33 cm -1 ~ .3 mm ~ 4 meV Few mainstream applications until recently Absorption measures free electrons

emily
Download Presentation

Measurements of High-Field THz Induced Photocurrents in Semiconductors

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Measurements of High-Field THz Induced Photocurrents in Semiconductors Michael Wiczer University of Illinois – Urbana-Champaign Mentor: Prof. Aaron Lindenberg

  2. THz Radiation • 1 THz ~ 33 cm-1 ~ .3 mm ~ 4 meV • Few mainstream applications until recently • Absorption measures free electrons • Time resolved spectroscopists and condensed matter physicists have used THz as a probe of free electrons in time-resolved studies for some time 1Fedrici et al, IEEE Spectrum, 44, 47

  3. THz Radiation • Half-cycle pulse • Watch a test particle in short times • Quasi-DC field 25 fs 300fs

  4. THz Generation Experiment Space Ti:Sapph Oscillator (800 nm) 25 fs pulses @ 10 nJ 100 MHz rep. rate Regen. Amplifier (800 nm) Chirped Pulse Amplification 50 fs pulses @ 1 mJ 1 KHz rep. rate Frequency Doubling BBO crystal Plasma in air (non-linear optical medium) THz propagation (collimated and focused)

  5. THz Generation Propagation Direction

  6. InSb Experiment • Effective mass: 0.014 me • Band Gap: 0.17 eV (Si band gap = 1.12 eV) • 1 THz photon: 4 meV<<170 meV X Z

  7. InSb Experiment • Measure transmission as beam focuses • Total incident intensity constant • Transmission drops!!! • Generate free carriers?

  8. Avalanche Multiplication • Lose one band-gap of kinetic energy each collision • Require field to be present for longer than collision timescale • Electron energy drops below ionization threshold after only a few collisions w/o field E Field = conduction band e- = valence band e-

  9. Photoconductive Antenna Silver paint electrodes on surface of InSb • Measure currents • Ultrafast detector! • If electron lifetimes are much longer than pulse width, current indicates free carriers generated during pulse duration. Gap ~500 μm

  10. Photoconductive Antenna Current Amplifier 1 pA/V ~4s output time const Lock-in amplifier Time const. 100s Optical Shutter Modulate beam at .2 Hz

  11. Why So Much Amplification? • Measuring very small currents • Considerable noise

  12. Current Measurement • Transverse scan looks like a beam • Suggest THz induced current X Z

  13. Current Measurement • Initial longitudinal scan much broader than transmission z-scan • Scattered IR light induced broadening? • Requires repetition X Z

  14. Conclusions • Non-linear effect has been observed and characterized with bolometer • Tentative measurements of photocurrents have been recorded • We believe to have a prototype ultrafast photodetector

  15. Further Investigation • Repeat current measurement with current induced by scattered IR light eliminated • Apply a voltage bias to isolate free-carrier measurement • Complete our physical picture of this process • Fit data to impact ionization model (or some other mechanism) • Further develop ultrafast detector

  16. Acknowledgements • Lindenberg Group • DOE • SLAC • SULI

More Related