How fast can light and matter be made to interact? 'Almost instantaneously' is the answer provided in the latest study of semiconductor structures embedded in an optical microcavity.
In recent decades, few things have changed as rapidly as the way in which we communicate with one another over distance. Starting from basic telephony, we have moved on to the Internet and web-based communication: e-mail, instant messaging, blogging, Facebook and Twitter. Rarely do we realize, however, that we owe the speed and pervasiveness of web-based communication to our ability to switch light on and off swiftly and conveniently in semiconductor devices. Fast-forward into the future, when transferring increasing amounts of data will require manipulation of light at even greater speeds, and at least one crucial question will need to be addressed: how fast can light–matter interactions, the building blocks of switches and light modulators, be turned on and off? On page 178 of this issue, Günter et al.1 show that ultrastrong coupling of light and matter can be turned on practically instantaneously, significantly faster than a single oscillation cycle of the light involved in the process.
In their experiment, Günter and colleagues studied a semiconductor structure comprising 50 quantum wells — thin layers of gallium arsenide (GaAs) — separated by aluminium gallium arsenide (AlGaAs) barriers. This stack of quantum wells was sandwiched between two wider layers of bulk semiconductors (see Fig. 1 of the paper1 on page 179). Each quantum well confines electrons to two energy sub-bands within the quantum well's conduction band (the uppermost energy band that is separated from the valence band by a forbidden band gap). This two-state system forms the matter part of the light–matter system; the wider cladding layers form an optical microcavity that confines the system's light component, the cavity mode.
An electron in the lower-energy sub-band will be excited into the upper sub-band by absorbing light at the appropriate frequency, whereas an electron in the upper sub-band can revert to the lower sub-band by spontaneously emitting light. The energy difference between the two sub-bands is determined by the thickness of the quantum wells, and was chosen to be 113 millielectronvolts (meV); the period of the corresponding light (the cavity mode) necessary to trigger the intersub-band transition is 37 femtoseconds (1 femtosecond is 10−15 seconds). In semiconductors, intersub-band transitions are distinguished from other optical transitions by their strong coupling to light and their narrow spectral width (in this case, 5 meV). This makes them fairly straightforward to observe as narrow and pronounced dips or peaks in light spectra.
The natural state of the semiconductor structure is an undoped state, that is, no electron occupies either the lower or upper sub-bands, and even ambient temperature is not enough to provide a noticeable population of electrons in either sub-band. So to couple light and matter, electrons need to be injected into the conduction sub-bands. The higher the electron density, the stronger the coupling (the coupling strength follows a square-root dependence on the density). In earlier experiments2,3,4, electrons were provided through extrinsic doping or an applied voltage.
Observation of the magnitude of the light–matter coupling is done by measuring the reflection of broadband light from the microcavity5,6,7. The reflected light spectrum depends on the strength of the coupling. A weakly coupled system exhibits only one reflection resonance — a pronounced dip in the spectrum (at 113 meV in Günter and colleagues' system1). Strong and ultrastrong coupling, meanwhile, result in two reflection resonances that are clearly separated from each other by up to 20% of the original resonance's energy, and that are located on either side of this resonance (see Fig. 3 of the paper1 on page 180).
But let's restate the question: how fast can strong light–matter coupling be turned on or off? To address this question, the authors1 used a high-energy light pulse of 12-femtosecond duration that could excite electrons from the valence band into the lowest-energy sub-band within that timescale. They found that the strongly coupled light–matter system switches on almost instantaneously, not at its own intrinsic speed (the oscillation of its light field) but at the speed of the light pulse used to pump the electrons (12 femtoseconds compared with 37 femtoseconds): as soon as the electrons reach the conduction sub-bands, the light–matter coupling is established. What's more, the switch is done non-adiabatically rather than adiabatically — that is, the system's coupled quantum state does not emerge gradually but rather switches on abruptly.
Günter and colleagues' work is excellent news in many ways. First, the observation that light–matter coupling can be tuned in less than one cycle of light means that a number of non-adiabatic, non-thermal-equilibrium light–matter phenomena, which belonged solely to the niche of theory, can now be tested in experiments. For example, the observation of the emission of virtual photons lodged in every optical cavity8 should now be possible. Second, the fact that the experiments are conducted at ambient temperature and pressure makes them accessible to a wider group of experimentalists. Finally, the authors' demonstration of ultrafast switching of ultrastrong light–matter coupling in semiconductor-based structures may, in the long run, prove very useful for communication applications.
Günter, G. et al. Nature 458, 178–181 (2009).
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Semiclassical study of intersubband cavity polaritons: Role of plasmonic and radiative coupling effects
Physical Review B (2013)
Physical Review B (2009)