The advent of long-wavelength coherent sources, such as free-electron lasers (FELs), optical parametric amplifiers (OPAs), and Terahertz (THz) antennas, has created a new class of opportunities to study low-frequency phenomena in solids in the time domain and/or high-intensity regimes. In particular, far-infrared (FIR) pulses can directly excite low-energy dynamics in bulk and quantum-confined semiconductors, e.g., cyclotron resonance (CR), internal transitions of shallow donors and excitons, phonons, and intersubband transitions. In addition, small photon energies enhance the ponderomotive potential energy while minimizing interband absorption and sample damage, leading to extreme nonlinear optical behavior in semiconductors. Furthermore, intraband FIR/THz spectroscopy is independent of whether the states involved are interband-active or not, thus providing a rare opportunity to directly probe nonradiative (or "dark") states.

Free electron lasers are unique sources for tunable and coherent light pulses. Particularly in the FIR spectral range (20-100 µm), they provide intense, coherent, and ultrashort pulses, making novel low-energy spectroscopy experiments possible. Previously, we performed a series of time-resolved and nonliner optical experiments in the FIR at the Stanford Free-Electron Laser Center, where we developed and emplyed various two-color, time-resolved experimental setups utilizing a synchronized Ti:Sapphire-FEL system in conjunction with a 9 Tesla superconducting magnet. Currently, we are using the FEL at the Electron Linear accelerator with high Brilliance and Low Emittance (FELBE) located at the Forschungszentrum Dresden-Rossendorf, Germany, where we are collaborating with the group of Prof. Manfred Helm.

For our most recent work usnig the FELBE, see the following: W. D. Rice et al., "Time-Resolved Photoluminescence Quenching in Semiconductor Quantum Wells Using a Terahertz Free-Electron Laser," International Workshop on Optical Terahertz Science and Technology (OTST 2011), Santa Barbara, California, March 13-17, 2011.

Review Article:

J. Kono, "Ultrafast and Nonlinear Spectroscopy of Solids with Small Energy Photons,"Jpn. J. Appl. Phys. 41 (2002), Suppl. 41-1, pp. 76-87. (full text)

Regural Journal Articles:

G. A. Khodaparast et al., "Relaxation of Quasi-2D Electrons in a Quantizing Magnetic Field Probed by Time-Resolved Cyclotron Resonance," Phys. Rev. B 67, 035307 (2003). (abstract, full text)

G. A. Khodaparast et al., "Terahertz Dynamics of Photo-generated Carriers in Ferromagnetic InGaMnAs," J. Appl. Phys. 93, 8286 (2003). (abstract, full text)

M. A. Zudov et al., "Terahertz Magneto-spectroscopy of Transient Plasmas in Semiconductors," J. Appl. Phys. 94, 3271 (2003). (abstract, full text)

M. A. Zudov et al., "Time-Resolved, Nonperturbative, and Off-Resonance Generation of Optical Terahertz Sidebands from Bulk GaAs," Phys. Rev. B 64, 121204(R) (2001). (abstract, full text)

J. Kono et al., "Picosecond Time-Resolved Cyclotron Resonance in Semiconductors," Appl. Phys. Lett. 75, 1119 (1999). (abstract, full text)

K. B. Nordstrom et al., "Excitonic Dynamical Franz-Keldysh Effect," Phys. Rev. Lett. 81, 457 (1998). (abstract, full text)

S. K. Singh et al., "Saturation Spectroscopy and Electronic-State Lifetimes in a Magnetic Field in InAs/AlGaSb Single Quantum Wells," Phys. Rev. B 58, 7286 (1998). (abstract, full text)

T. Inoshita, J. Kono, and H. Sakaki, "Theory of terahertz/near-infrared optical mixing in quantum wells in strong magnetic fields," Phys. Rev. B 57, 4604 (1998). (abstract, full text)

J. Kono et al., "Resonant Terahertz Optical Sideband Generation from Confined Magnetoexcitons," Phys. Rev. Lett. 79, 1758 (1997). (abstract, full text)

J. Cerne et al., "Near-Infrared Sideband Generation Induced by Intense Far-Infrared Radiation in GaAs Quantum Wells," Appl. Phys. Lett. 70, 3543 (1997). (abstract, full text)

J. Cerne et al., "Terahertz Dynamics of Excitons in GaAs/AlGaAs Quantum Wells,” Phys. Rev. Lett. 77, 1131 (1996). (abstract, full text)