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 Time resolved spectroscopists and condensed matter physicists have used THz as a probe of free electrons in time- resolved studies for some time 1 Fedrici et al, IEEE Spectrum, 44, 47

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

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

THz Generation Propagation Direction

InSb Experiment Effective mass: m e Band Gap: 0.17 eV (Si band gap = 1.12 eV) 1 THz photon: 4 meV<<170 meV Z X

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

Avalanche Multiplication E Field 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 Electron energy drops below ionization threshold after only a few collisions w/o field = conduction band e - = valence band e -

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

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

Why So Much Amplification? Measuring very small currents Considerable noise

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

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

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

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) Fit data to impact ionization model (or some other mechanism) Further develop ultrafast detector

Acknowledgements Lindenberg Group DOESLACSULI