J. P. Kaminski

Far‐infrared cavity dump coupling of the UC Santa Barbara free‐electron laser

A Nd:YAG laser‐induced semiconductor switch for far‐infrared (FIR) radiation was mounted inside a free‐electron laser (FEL) wiggler cavity to act as an intracavity output coupler. By timing the FEL to the Q‐switched laser, the FIR switch could be turned on during the intracavity laser saturation period. In addition to demonstrating the generation of short, high peak power pulses, the cavity dump coupler (CDC) was used for the first time to image the intracavity mode structure of a FEL and to perform steady‐state saturation spectroscopy studies in the FIR.

Free‐electron laser study of the nonlinear magnetophotoconductivity in n‐GaAs

The University of California at Santa Barbara free‐electron laser was used to investigate the kinetics of electrons bound to shallow donors in n‐GaAs by saturation spectroscopy. The resonant photothermal conductivity arising from 1s–2p+ shallow donor excitations in a magnetic field was measured at intensities greatly exceeding that of earlier investigations and saturation of bound‐to‐free photoionization transitions was achieved. The impurity resonance photoconductive signal shows a distinct intensity dependence caused by competing bound‐to‐free transitions which saturate differently. This permits a more detailed evaluation of the electron recombination kinetics than was previously possible, yielding the ionization probability of the 2p+ state, the transition time of electrons from the 2p+ level to the gound state, and the recombination time of free carriers.

First lasing of the UCSB 30 μm free-electron laser

Abstract Third harmonic lasing has been achieved in a Free-Electron Laser specifically designed to operate in that mode. So far, 40 W at 41 μm wavelength and 5.16 MeV beam energy has been measured. One of the most difficult challenges has been suppression of lasing at the fundamental wavelength. This is currently achieved with an intracavity cesium iodide restrahlen filter, but eventually, the original grating design will be needed to provide full tunability. Operation of this FEL demands electron beam stability and control system accuracy at the limits of the present system. Further progress in the development of this FEL may require significant improvement in these areas.

Terahertz frequency response of an In0.53Ga0.47As/AlAs resonant‐tunneling diode

We have measured the room‐temperature, broad‐band, terahertz response of a high‐speed In0.53Ga0.47As/AlAs resonant‐tunneling diode from 120 GHz to 3.9 THz using the free‐electron lasers at UCSB. The ‘‘rectified’’ response is measured with a conventional probe station by using the tungsten probe tip as a whisker antenna. Normalizing the rectified response in the resonant‐tunneling regime with the off‐resonant response we remove the extrinsic frequency dependence controlled by the antenna and the RC time constant and measure an intrinsic relaxation time.

Generation of subnanosecond high power far infrared pulses using a FEL pumped passive resonator

Abstract By cavity-dumping a small passive, external resonator, subnanosecond far infrared pulses with enhanced peak powers were created. The resonator was pumped by radiation produced by the UC Santa Barbara free electron laser and subsequently cavity-dumped using a fast optical semiconductor switch.

Materials science in the far-IR with electrostatic based FELs

Abstract A technology gap exists between ∼ 100 GHz and ∼ 10 THz. Free-electron lasers (FELs), driven by high quality, relatively low energy electron beams from electrostatic accelerators, and capable of generating kilowatts of coherent, tunable radiation, are ideally suited to explore the enabling science for future technology in this spectral range. We describe two experiments that use terahertz “optical rectification” to measure i) the intensity and temperature dependent energy relaxation in quantum wells and ii) the intrinsic relaxation of resonant tunneling diodes. Both benefit from the power and tunablilty of the UCSB FELs.

Terahertz response of resonant tunneling diodes

Abstract We have measured the broad-band terahertz response of state of the art InGaAs/AlAs and InAs/AlSb resonant tunneling diodes from 180 GHz to 3.6 THz using the free-electron lasers at UCSB. A tungsten whisker antenna in a conventional probe station is used to couple the far-infrared radiation into the device. Normalizing the resonant tunneling response with the off-resonant response allows us to circumvent the much slower RC time constant of the device and consequently enables a measurement of the relaxation time due to the quantum inductance.

Current applications using the UCSB free electron laser

Abstract Using the far-infrared radiation produced by the UCSB FEL, various biological science and solid physics experiments have been performed. As a particular example, magneto-absorption measurements on the shallow donor system in n-GaS are presented.

Terahertz response of an InGaAs/AlAs resonant tunnelling diode

We have measured the broad-band terahertz response of a state-of-the-art InGaAs/AlAs resonant tunnelling diode from 120 GHz to 3.9 THz using the free-electron lasers at the University of California, Santa Barbara. A tungsten whisker antenna in a conventional probe station is used to couple the far-infrared radiation into the device. By normalizing the rectified response in the resonant tunnelling regime with the off-resonant response we are able to remove the antenna frequency effects and the frequency dependence controlled by the much slower RC time constant and measure the relaxation time due to the quantum inductance.

Far infrared response of thin film Bi2Sr2CaCu2O8 using a free electron laser

The far infrared response of granular thin-film Bi2Sr2CaCu2O8 superconductor has been investigated using long (≈5 μs) but sharply truncated free electron laser pulses in the frequency range between 50 cm−1 and 125 cm−1. Under constant current bias, a fast response and a slow bolometric signal component could be identified in this energy range, which is below the BCS energy gap (≈ 200 cm−1). Measurements of the power dependences of the signal voltages showed that both the fast and the thermal responses are consistent with the predictions of the resistively shunted Josephson junction model.