XIIth Semiconducting and Insulating Materials Conference, June 30 - July 5, 2002, Smolenice Castle, Slovakia

Few-Cycle THz Pulse Generation And Spectroscopy Of Semiconductor Nanostructures

K. Unterrainer, R. Bratschitsch, T. Müller, R. Kersting*, J.N. Heyman+, G. Strasser

Institute for Solid State Electronics, Technical University Vienna, A-1040 Vienna, Austria

*)Rensselaer Polytechnic Institute, Troy, NY12180

+)Macalester College, St. Paul, MN 55105

During the past years several groups showed that ultrafast photoexcitation of semiconductors generates coherent THz radiation. We present a new emission process which overcomes the limitations of the conventional generation schemes (intensity, frequency, tuning). We use n-doped GaAs layers where extrinsic electrons are confined between the substrate and the surface depletion field. In addition, modulation doped parabolic quantum wells are used where the THz emission frequency can be designed by the well dimensions. The THz radiation is generated by excitation of the GaAs structures with femtosecond laser pulses. The emitted THz pulses are detected by a bolometer detector using an auto correlation technique or through electro-optic detection.

From the n-doped layers we observe intense, coherent THz radiation. The THz emission is generated by coherent plasma oscillations of the extrinsic electrons inside the layer. Femtosecond laser excitation of the GaAs leads to an ultrafast screening of the surface depletion field.Thus, the extrinsic electrons respond to the fast field change by starting coherent plasma oscillations. The frequency of the THz emission increases with higher doping - according to the square root dependence of the plasma frequency on the carrier concentration /1/. Since the emission spectrum shows no dependence on excitation density we conclude that the emission results exclusively from the coherent oscillation of the extrinsic electrons. In all experiments we observe temporally and spatially coherent THz radiation with intensities of up to 100 nW.

The dephasing time of the plasma oscillation is limited to <1ps due to the mobility of the electrons in the layer. A longer dephasing time can be achieved by modulation doping. We used modulation doped, parabolic GaAs/AlGaAs quantum wells with widths in the range of 1200 - 2000 Å and carrier sheet densities of 1.7x1011 - 5x1011 cm-2. The spectrum of the emitted THz radiation consists of two components, a broad one around 0.8 THz and a narrow one (FWHM: 0.3 THz) with a center frequency of 2.54 THz /2/. The narrowband emission results from the intersubband plasmon oscillation of the carriers inside the PQW. The observed frequency corresponds very well to the designed frequency given by , where  is the depth, and L is the width of the well. The dephasing times are increased by about one order of magnitude compared to the "bulk" GaAs layers.

Using these new THz generators we perform THz time domain spectroscopy to investigate transitions between quantized states in semiconductor nanostructures /3/. In addition, this new THz emission technique is applied to study the quantized energy levels in lower dimensional structures.

[1]R. Kersting, K. Unterrainer, G. Strasser, H.F. Kauffmann, E. Gornik, Phys. Rev. Lett., 79, 3038 (1997).

[2]R. Bratschitsch, T. Müller, R. Kersting, G. Strasser, and K. Unterrainer, Appl. Phys. Lett., 76, 3501 (2000).

[3]R. Kersting, R. Bratschitsch, G. Strasser, J. N. Heyman, and K. Unterrainer, Opt. Lett., 25, 272 (2000).