THz experiments at the UCSB FELs and the THz Science and Technology Network. Mark Sherwin UCSB Physics Department and Institute for Quantum and Complex Dynamics
UCSB Center for Terahertz Science and Technology UCSB Free- Electron Lasers UCSB Free- Electron Lasers Free-Electron Laser User s Lab Free-Electron Laser User s Labs
User s lab Free-Electron Lasers
User labs and output characteristics 6 Mev accelerator control room lab lasers transport system THz Intensity ~ 5 µs 5 kw 0.12-4.8 10 12 Hz (0.5-20 mev, 4-160 cm -1 ) Pulsed at ~ 1 Hz
Outline High-field THz experiments at UCSB Terahertz electro-optics in semiconductor quantum wells Rabi oscillations of electrons bound to Hydrogenic donors in GaAs QuickTime and a TIFF (Uncompressed) decompressor are needed to see this picture. Sam Carter Victoria Ciulin Chad Wang Larry Coldren Alex Maslov, MSS Terahertz Science and Technology Network
Motivation for THz electro-optics Spectroscopy of strongly-driven quantum system Voltage-controlled all-optical µe ~ hω THz wavelength conversion for wavelength-division multiplexing ω THz ω NIR
The Terahertz in fiber-optic communications Optical amplifier bands 25 THz 207 THz 201 196 191 186 182 THz S+ S C L L+ 1.45 µm 1.55 µm 1.65 µm 5 THz channel 0.1 THz
Sample and experimental geometry NIR sapphire E THz ~2 µm 50 GaAs/AlGaAs quantum wells, absorbing substrate removed sapphire NIR + sidebands (to 0.85 m monochromator) e2 e1 h1 h2
THz modulation of optical properties: sidebands Laser/100 n=+1 n=-2 (x100) n=-1 (x10) 1.6 THz +2(x10) +3 (x100)
Conclusions for THz electro-optics THz sidebands: easy to explore using high-power, accelerator-based FEL source With careful engineering, can envision chip-based wavelength conversion with QCL source.
Rabi oscillations of electrons bound to shallow donors Bryan Cole (now @ TeraVision) Jon Williams (now @ Caltech) Samples: Colin Stanley, U. of Glasgow Support: DARPA/QUIST Tom King (N. Zealand) Matt Doty Now@NRL
Terahertz quantum electrodynamics for quantum information processing in semiconductors ~100 µm THz cavity in 2-D photonic band gap structure Interband laser for controlling and reading out individual qubit Qubits (shallow donor impurities or doped quantum dots)
Two-state system in a resonant oscillating field 1> and 0> coupled with strength g 1> 0> Probability of 1> 1 0.8 0.6 0.4 0.2 Pulse duration (NMR language) 0 š 2š 3š 4š Resonant excitation 0>-i 1> i 1> -i 1> Detuned excitation 0 0> - 0> 0> -0.5 0 0.5 1 1.5 2 2.5 time (gt)
Hydrogenic donors Example: Si in GaAs Effective mass approx.: H atom w. m-> m*=0.067 Dielectric const. -> 13 Ry->Ry*~ (m*/m 0 ) (1/ε 2 )13.6 ev ~4 mev Bohr radius (m 0 /m*)εa 0 10 nm Si atom Samples & experiment - Epitaxial GaAs, N D -N A =10 14 cm -3 - From Prof. Colin Stanley, U. Glasgow photocurrent THz photoconductor
Photocurrent spectrum (B=0) Lines inhomogeneously-broadened 1s-2p Si S 30 1s-continuum 1s-continuum
B-dependence of Hydrogenic levels in GaAs 3 12 Frequency (10 12 Hz) 1.5 0 1> 2.5 THz 6 0 Energy (mev) Free-Electron Laser -1.5 0> -6 Data: Stillman and Wolfe, 1969 Calculations: B. Tom King (following Larsen, 1968)
THz Switches Silicon n = 3.42
THz Switches Silicon n = 3.42 e h plasma Regenerative Amplifier 150 fs >1mJ per switch THz Intensity Time
Short THz Pulses:Pulse Slicer Regenerative Amplifier NIR Delay Stage FEL Output ~ 4 µs 1 kw FIR Beam Stop FIR pulse shape Beam Stop
Sliced pulses detector signal (a.u.) 1 ns / div F. A. Hegmann, MSS et. al., Appl. Phys. Lett. 76, 262 (2000)
Sample geometry, RO in impurities Magnetic field THz field To pulse amplifier and 750 MHz digital oscilloscope Grid-type ohmics B. Cole, MSS et. al., Nature 410, 60 (2001)
Mechanism for resonant photoresponse Continuum 2 p + Resonant THz excitation (photon) Recombination (phonon) Ionization->photoconductivity (phonon) 1s
Single photocurrent pulse excitation + ionization recombination continuum population photocurrent B. Cole, MSS et. al., Nature 410, 60 (2001)
Rabi oscillation
Rabi oscillation Dependence on THz electric field
Rabi oscillation Dependence on detuning Resonant
Effect of continuum 3 Coupling 2p + to continuum Intensity-dependent dephasing? Optical Stark effect? 12 Frequency (10 12 Hz) 1.5 0 2.5 THz 6 0 Energy (mev) -1.5-6
Outline High-field THz experiments at UCSB Terahertz electro-optics in semiconductor quantum wells Rabi oscillations of electrons bound to Hydrogenic donors in GaAs Terahertz Science and Technology Network
Experiments under way 3 12 Frequency (10 12 Hz) 1.5 0 6 0 Energy (mev) 1 THz -1.5-6 2p - state reduced THz-induced decoherence ->many Rabi oscillations
THz Science and Technology Network Transparent organization. Lower barriers to entry into THz research. Connect researchers in academia, industry, government labs. Make appropriate technology available to researchers. Share and disseminate knowledge. Grow the field to realize its potential.
Broad-band, low power THz sources Narrow-linewidth, low power. Molecular gas laser, multiplied microwave source, backward wave oscillator, photomixer, quantum cascade laser, Broad-band, high power. Narrow-linewidth, high power. FEL
Network in operation Idea for THz experiment Oklahoma:Time-domain THz Spectroscopy Ohio-State: High-resolution Spectroscopy. UCSB: Narrow-linewidth high-power FEL radiation. Network meeting Jefferson Labs: Coherent Synchrotron Radiation.
Immediate issues for Network Set up organization (by-laws, web-site, elected officers,... ) Build up membership (esp. outside of physics) Web site, word of mouth, presentations at professional society meetings. Acquire funding network co-ordinator travel funds facility support funds Funds for exploratory collaborations using table-top apparatus Potential sources of funds Government Industry Membership dues (small)