Matter in the presence of a high-intensity AC electric field exhibits various intriguing, sometimes non-intuitive, phenomena. For example, strongly laser-driven atomic gases have shown such diverse phenomena as the AC Stark effect (or the Autler-Townes effect), electromagnetically-induced transparency, self-induced transparency, above-threshold ionization, high-order (>100) harmonic generation, etc. Condensed matter, semiconductors in particular, provides a viable alternative to gases for further exploring coherent non-perturbative phenomena. We can expect, e.g., 1) the effect of quantum confinement , 2) the explicit coupling of the spatially periodicpotential of the solid to the temporally periodic potential imposed by the AC field, and 3) various many-body effects – i.e., phenomena that cannot be observed in gases. Theoretical studies have predicted fascinating new phenomena such as band gap oscillations, the appearance of new gaps, field-induced electronic phase transitions, laser-assisted electron-electron attraction, etc. However, such effects have not been observed to date because of the unavoidable sample damage due to the very high intensity required using conventional near-infrared (NIR) or visible lasers.

Here, we are using intense mid-infrared (MIR) radiation generated by an optical parametric amplifier (OPA) to study non-perturbative phenomena in semiconductor systems. The use of long-wavelength light results in 1) an increased ponderomotive potential energy (which is proportional to the square of the wavelength), i.e., less intensity is required for observing the predicted effects and 2) lower probability for multi-photon interband absorption. Both of these help avoid the damage problem. Also, the low dispersion in between far-infrared (FIR) phonon absorption and NIR interband absorption in semiconductors allows excellent phase-matchingover long distances, leading to extreme nonlinear optical phenomena. Therefore, the MIR is an ideal wavelength range in which to study multi-photon, non-perturbative effects in semiconductors. The intense MIR fields directly , coherently and strongly drive charge carriers in semiconductors (sometimes to the regime where the effective mass approximation is no longer valid). The effects of this extreme driving will manifest themselves as dramatic modifications in interband optical properties.

In the dynamic Franz-Keldysh effect (DFKE), for example, ultrafast band structure changes are induced in a semiconductor in the presence of a strong laser electric field. These changes include absorption below the band edge and oscillatory behavior of the absorption above. We have observed these salient features of the DFKE unambiguously (see Figs. 1-3 below). The transmission below the bulk GaAs band edge, which was measured using white light as a probe, showed a dramatic decrease. The induced above-bandgap transparency, which exists only during the pulse width of the pump field, represents a new type of coherent phenomenon, different from electromagnetically-induced transparency or self-induced transparency. Similar measurements on other semiconductors, especially on quantum nanostructures including quantum wells and carbon nanotubes, will explore further exotic phenomena and the feasibility of ultrafast all-optical switching devices.

Fig. 1: Induced sub-bandgap absorption for GaAs at different pump wavelengths. The effect increases in intensity with increasing wavelength despite the fact that the laser intensity decreases with increasing wavelength , demonstrating the importance of the ponderomotive potential, which increases quadratically with increasing wavelength.

Fig. 2: Time-dependence of the below band edge absorption, demonstrating the ultrafast (i.e., coherent or virtual) nature of the DFKE.

Fig. 3: Above band edge oscillation and transparency observed in a GaAs film.

References:

A. Srivastava, R. Srivastava, J. Wang, and J. Kono, “Laser-Induced Above-Bandgap Transparency in GaAs,”Physical Review Letters 93, 157401 (2004). (abstract, full text)

A. Srivastava and J. Kono, “Ultrafast Photoinduced Above-Bandgap Transparency in GaAs due to the Dynamic Franz-Keldysh Effect,” to be published in: Quantum Electronics and Laser Science Conference, OSA Technical Digest (Optical Society of America, Washington DC, 2003). (full text)

J. Kono and A. H. Chin, “Extreme Midinfrared Nonlinear Optics in Semiconductors” (invited paper), in: Proceedings of the 26 th International Conference on Infrared and Millimeter Waves, edited by O. Portugall and J. Leotin (ISBN 2-87649-035-8, 2003), pp. 4-9 – 4-14. (full text)

A. H. Chin, J. Kono, and G. S. Solomon, “Absence of exciton quenching in the presence of strong fields at high frequencies,” Physical Review B 65, 121307(R) (2002). (abstract, full text)

O. G. Calderón, A. H. Chin, and J. Kono, “Multiphoton processes in the presence of self-phase modulation,”Physical Review A 63, 053807 (2001). (abstract, full text)

A. H. Chin, O. G. Calderón, and J. Kono, “Extreme midinfrared nonlinear optics in semiconductors,” Physical Review Letters 86, 3292 (2001). (abstract, full text)

A. H. Chin, J. M. Bakker, and J. Kono, “Ultrafast electro-absorption at the transition between classical and quantum response,” Physical Review Letters 85, 3293 (2000). (abstract, full text)

K. B. Nordstrom, K. Johnsen, S. J. Allen, Jr., A.-P. Jauho, B. Birnir, J. Kono, T. Noda, H. Akiyama, and H. Sakaki, “Excitonic dynamical Franz-Keldysh effect,” Physical Review Letters 81, 457 (1998). (abstract, full text)

K. B. Nordstrom, K. Johnsen, S. J. Allen, Jr., A.-P. Jauho, B. Birnir, J. Kono, T. Noda, H. Akiyama, and H. Sakaki, “Observation of the dynamical Franz-Keldysh effect,” physica status solidi (b) 204, 52 (1997). (full text)