Terahertz Charge Dynamics in Semiconductors James N. Heyman Macalester College St. Paul, MN

Selective Liberal Arts College in St. Paul, MN

What would a non-selective college be like?

Background -Terahertz radiation -Ultrafast THz pulses - Spectroscopy

Electromagnetic Spectrum radio  wave IR visible UV X-raygamma-ray far-IR mid-IRnear-IR eV0.04 eV0.4 eV1 eV frequency (Hz) energy (h ) rotations of molecules vibrations in solids vibrations in molecules super- conductors eletrons in heterostructures Free carriers in semiconductors

Generation of a THz Pulse 100 fs visible pulse THz pulse A V Photoconductor (SI-GaAs) Auston Switch

THz Spectroscopy: Molecular Spectroscopy of CO J I   I

Outline THz spectroscopy of semiconductors - Free carriers Conductivity Plasma Oscillations -Quantum Wells Intersubband charge oscillations

Semiconductors Si N Si-atoms 4N filled bonding states 4N empty anti- bonding states CB VB - Semiconductor + light Si

Motivation What can we learn from THz Spectroscopy of Semiconductors? Conductivity on picosecond timescales Fast electronic switching Fast opto-electronic elements

THz Generation: Ultrafast Current Probe J(t) J  E rad n t See also: Zhang, et. Al., APL 56, 1011, 1990

Experiment: THz Emission and Transmission Spectroscopy Laser System: : 800nm Pulse duration: 10 fs Repetition rate: 80 MHz Pulse energy: 10 nJ 10 fs laser THz emitter Delay stage Sample Gated Detector

Conduction on picosecond timescales from THz emission Ballistic Transport Drift Velocity Overshoot See also Leitenstorfer, et. Al., PRB 61, 2000

What Drives the Photocurrent? (a) Surface E - Field(b) Hot Carrier Diffusion (photo-Dember field) THz electromagnetic emission by coherent infrared-active phonons Dekorsy, et. al., PRB 53, (1996) Study of terahertz radiation from InAs and InSb Gu, et. al., JAP 91, (2002) Te InAs E C E v e h E C E v h e

drift diffusion Comparing Drift and Diffusion: A Simple Model f2f

THz emission from GaAs at f and 2f Schottky / i / n Schottky / i / p - Heyman, et. al., APL 83, 5476, 2003

THz Plasma Oscillations

k [001] B InAs n-GaAs THz Emission in a Magnetic Field Pump laser: = 0.8 W  = 800nm  = 140 fs Bolometer Sample 77K Black Body Chopper X X X X X X X Ti:S Laser Calibration

Plasma Oscillations (B = 0) k=0 plasma oscillation 3D SHO {x, y, z} {R, L, z}

Coupled cyclotron-plasma oscillation Uniform plasma of electrons in B-field: k=0 excitations B  B pp

Model B=5, ne=1.5, nlh = 0.5, nhh=1 initial conditions model I(  ) E radiation (t) THz NIR Calculate j(t) E

n-GaAs (n = 1.8·10 16 cm -3 ) THz power vs. B k [001] B - Heyman, et. al., PRB 64, , 2001

THz Source InAs Source B = 1.8T

THz spectroscopy of semiconductors -Quantum Wells Intersubband charge oscillations

Semiconductors GaAs 1.42 eV AlAs 2 eV InAs 0.3 eV Conduction Band Valence Band

2 1 material B material A CB VB

Intersubband Absorption: modulation-doped GaAs/AlGaAs QW - Heyman, et. al., APL 72, 644, ‘98

THz Emission from Parabolic Q.W. G334 (B=0) 800 nm 760 nm see also: Bratschitsch, et. al. APL 72, 644, 2000

QW’s in B-Field: Magneto-spectroscopy of 2DEG’s   Coupled oscillators: In plane - cyclotron Perpendicular - intersubband

THz emission vs. Magnetic Field to surface) [001] B Experiment Model see also: Some, Nurmikko, APL 65, 3377, 1994

Intersubband lifetime FELIX Free-Electron Laser 10 ps pulses 13.9 meV delay stage Ge:Ga FIR detector sample in optical cryostat APL 68, 20, 1996 See also Heyman, et.al., Phys. Rev. Lett. 74, 2682 (‘95).

FEL     HeNe Upconversion Spectrometer for UCSB Free Electron Laser

Collaborators U. M. Physics Paul Crowell Technical University of Vienna Thomas Mueller Karl Unterrainer Gottfried Strasser Macalester Pelagia Neocleous (‘01) Damon Hebert (‘02) Christine Lind (‘02) Hans Wrage (‘02) Nelson Coates (‘04) Alex Reinhardt (‘05) Funding National Science Foundation

n-GaAs (n = 2·10 16 cm -3 ) E(t) E(  )

Quantum Wells and Heterostructures material B material A conduction band valence band

THz probe 800nm pump Carrier Lifetime Measurement