Figure 1. Conceptual diagram showing difference between single- and two-photon interactions that connect the same valence and conduction bands.
Using light to control the behavior of electronics has many potential applications, but most efforts in this area have had the drawback of requiring the fabrication of complicated devices to turn incoming photons into a useful signal. At the Univ. of Toronto, however, researchers have demonstrated a way of creating electrical currents within a simple bulk semiconductor, using the light itself to specify the direction of the desired current. The validity of their approach was recently demonstrated when broadband terahertz radiation was detected coming from the circuit. This seems to be a telltale sign that the phenomenon they were investigating -- coherent control -- was, in fact, responsible for the currents they produced.
Coherent control (also sometimes known as quantum interference control) is a process that relies on the subtleties of quantum mechanics. The coherence involved is not so much that of the light beams used to do the control (though that is important) but the coherence between the electrons that are produced.
Figure 2. Two beams of light create currents in this simple circuit, fabricated with GaAs grown at low temperature. Researchers used 90-fs pulses at a 250-kHz repetition rate with average powers of 10 mW (1550 nm) and 400 µW (775nm). The result was 3 nW of THz power.
It is relatively easy to produce charge carriers in a semiconductor using light. If the photons have the correct wavelength (or are in the correct range) they can collide with an electron and excite it so that it jumps out of the valence band, across the band gap, and into the conduction band (leaving behind a hole). However, unless there is something else going on, these charge carriers don't really go anywhere; they simply diffuse in all directions until the electrons and holes are eventually reunited.
The Univ. of Toronto group found a way to use quantum interference to break this symmetry.1-3 The idea is based on the difference between a two-photon reaction and a single-photon reaction. In the former, two photons of a low frequency (ωω) collide with an electron at roughly the same time, thus allowing it to jump across a band gap that requires more energy than either photon has alone. In the latter, a single photon with double the frequency (2ωω) achieves the same end on its own.
Figure 3. The radiation produced is phase controllable and, here, is broadband around 4 THz. The frequency can be raised by shortening the length of the light pulses used in its production. Inset: the corresponding interferogram.
But not quite. It turns out that, in either case, the states the electrons jump to are slightly different (the wavefunctions have different parity; Figure 1). As a result, the effect of the two-photon and one-photon interactions undergoes interference, so that carrier creation in some areas is suppressed and in other areas is enhanced. This is the basis of coherent control. In choosing the phase difference between the two beams of light, the quantum interference parameters are also set -- in this case to travel preferentially in a particular direction. Thus, a flowing current is created (Figure 2). The terahertz radiation is a signature of this current production and is related to the phases of the two light beams used (Figure 3).4
According to researcher Daniel Côté, the radiation produced is doubly useful. The immediate advantage is that the characteristics of the emissions give the Toronto group a diagnostic tool that should allow them to better understand and optimize the process of coherent control. However, in the longer term, he says, the production of terahertz emissions could become an end in itself.
1. R. Atanasov, A. Haché, J.L.P. Hughes, H.M. van Driel, and J.E. Sipe, Coherent Control of Photocurrent Generation in Bulk Semiconductors, Phys. Rev. Lett. 76 (10), pp. 1703-1706, 4 March 1996.
2. A. Haché, Y. Kostoulas, R. Atanasov, J.L.P. Hughes, J.E. Sipe, and H.M. van Driel, Observation of Coherently Controlled Photocurrent in Unbiased, Bulk GaAs, Phys. Rev. Lett. 78 (2), pp. 306- 309, 13 January 1997.
3. A. Haché, J.E. Sipe, and H.M. van Driel, Quantum Interference Control of Electrical Currents in GaAs, IEEE J. Quantum Electronics 34 (7), pp. 1144-1154, July 1998.
4. D. Côté, J.M. Fraser, M. DeCamp, P.H. Bucksbaum, H.M. van Driel, THz emission from coherently controlled photocurrents in GaAs, App. Phys. Lett. 75 (25), 3959- 3961, 20 December 1999.
Sunny Bains is a scientist and writer based in London, UK.