A two-laser trick that renders opaque media transparent can be achieved in systems of tiny optical resonators — with potentially profound consequences for optical communication and information processing.
The discovery of electromagnetically induced transparency (EIT) — an unusual effect that occurs when two laser beams interact within an optical material — and the use of novel techniques to fabricate ever smaller structures to control light have been recent exciting developments in optical physics. Writing in Physical Review Letters, Xu et al.1 neatly combine the two, demonstrating an on-chip, all-optical analogue of EIT based on the response of coupled optical microresonators. The result may open up untrodden pathways in photonics, offering prospects of smaller, more efficient devices for the manipulation and transmission of light.
As originally implemented2, EIT involves the interaction of laser light with a collection of atoms. It relies on the fact that when an incident photon has a ‘resonant’ energy equal to the difference in the energies of two levels of an atom, the photon can be absorbed by that atom and its energy used to excite the atom into the higher energy state. When two separate laser fields drive two such atomic transitions that share the same upper level (Fig. 1a, overleaf), destructive interference between the pathways connecting the upper level with the lower levels allows the quantum-mechanical probability that the atom is in the upper level to vanish. As there are no atoms in the upper level, there has been no absorption of the applied fields. The atom is thereby rendered ‘transparent’ to the applied laser fields in an extremely narrow frequency range (Fig. 1b). This interference process has analogies in classical physics, where the coupling between two oscillators results in a reduction in the amplitude of their oscillation3.
The EIT phenomenon can be used to greatly strengthen nonlinear optical effects — such as the dependence of a material's refractive index on the intensity of the incident light — on which the operation of many photonic devices depend. In general, such nonlinear effects can be achieved by using as the source of the applied field a frequency-tunable laser tuned close to the resonance frequency of an atomic transition. Unfortunately, absorption of the laser light also increases at exactly these frequencies, and this effect undoes much of the benefit of working close to resonance. With two laser sources, however, quantum interference can be used to ensure that atomic absorption is eliminated, while retaining a large nonlinear response.
A downside of the ‘traditional’, atomic EIT effect — and more generally of nonlinear optics based on atomic resonance — is that it can be implemented only for light in a very small range of frequencies near fixed atomic transitions. An alternative way of achieving the transmission characteristics of EIT, as investigated by Xu and colleagues1, uses small devices called optical microresonators4 (Fig. 1c), whose resonance frequency depends on their physical size. In such devices, resonance frequencies occur whenever a whole number of wavelengths of the incident light fits into the resonator. Moreover, destructive interference occurs between light emerging from two coupled resonators.
Early microresonators were simply aerosols containing particles of varying sizes, but modern fabrication procedures produce individual resonators with highly controllable resonance frequencies4. These resonators come in various forms: ring shapes; disks or spheres, in which the light skims across the outer surface as sound does in a ‘whispering gallery’; or less obvious forms, such as defects in an otherwise perfect photonic crystal. Their small size makes them ideally suited to perform operations on light analogous to those performed by components of silicon chips on electrons, and numerous applications have been proposed for them5,6,7.
Several methods5,8,9 for combining EIT and optical microresonators to obtain EIT-like resonances in integrated optical systems were proposed in 2004, and a laboratory demonstration of such an effect was published last year10. In that experiment, EIT-like transmission spectra were observed in two interacting microresonators in the form of glass spheres approximately 400 micrometres in diameter that supported resonances of the whispering-gallery type.
Xu et al.1 take this advance a stage further. Using modern nanofabrication procedures, they manufactured a pair of micro-ring resonators coupled to parallel waveguides on a silicon-on-insulator substrate typical of an electronic chip. The refractive index of the silicon that made up the waveguides and the rings (3.45) was significantly greater than that of the silicon oxide that surrounded them on all sides (1.46). This meant that light could be confined in rings of a very small diameter through the process known as total internal reflection. EIT-like behaviour was demonstrated by measuring the transmission at different frequencies of a laser beam injected into the system. The authors constructed several such devices, where the distance between the micro-rings was varied from device to device, and thus the spectral location of the EIT-like transmission peak could be adjusted.
So what good is all this? As the name indicates, EIT is primarily a way of rendering transparent an otherwise highly absorbing material, and so lends itself to applications such as the construction of long-distance optical transmission lines. The dispersion characteristic that results from EIT has already allowed ‘slow light’ propagation to be achieved11,12,13 at speeds just a small fraction of the normal speed of light. Optical ‘buffers’ to temporarily store light are an obvious application of such slow-light technology.
Perhaps most importantly, EIT resonances can be much narrower than those of indi-vidual resonators, whether these are atoms or microresonators. As a resonance frequency can typically be measured only to an accuracy given by some fixed fraction of the resonance linewidth, narrowed linewidths could be useful for performing precise optical measurements of quantities such as magnetic field strength14. The ability to synthesize such components on an optical chip suggested by Xu and colleagues' innovation1 is a crucial step towards the development of such integrated optical devices.
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Chinese Science Bulletin (2013)