The addition of optical amplification to waves propagating in random media leads to a fascinating class of materials called ‘random lasers’. In a diffusive scattering medium, the effect manifests as narrowing in the emission spectra coupled with enhancement in intensity. In a localizing medium, sub-nanometre bandwidth radiation occurs as a result of amplification of Anderson localized modes. We have demonstrated a new class of random lasers that emit sub-nanometre bandwidth radiation, which originates from selective amplification of rare non-localized modes. This peculiar phenomenon has no counterpart in electronic transport, obviously because of the fact that electrons, unlike photons, cannot be amplified.
Near-field Scanning Optical Microscope (NSOM)
To capture variations in light intensity over distances smaller than the wavelength, one needs to employ a near-field imaging technique. This results from the fact that common, conventional far-field imaging techniques suffer from diffraction limitations, which forbid smaller-than-wavelength resolution. We shall use a technique called Near-field Scanning Optical Microscope (or NSOM in short) based on a vibrating tuning fork to achieve sub- wavelength imaging, aiming for a resolution of 100 nm or better. This method enables one to measure the topography of the nanostructured sample, and simultaneously the light distribution within, to yield an unambiguous correlation between the structure and light distribution. This is not measurable with any other imaging technique.
Mesoscopic phenomena in disordered media
In a scattering medium, some very exotic behaviour can be expected due to the interplay of scattering of waves and their interference. With light, we have an advantage of designing our media such that absorption length-scales become rather large, and, with the aid of sophisticated optical sources and detectors, such mesoscopic phenomena are measurable. As examples, coherent backscattering of light, or Anderson localization of light are direct consequences of interference of waves in disorder, and are measurable with optical techniques.
When dielectric materials are periodically structured at the lengthscales of optical wavelengths, they respond to light as would an atomic crystal to electrons. Photonic band gaps open up as a result of the periodicity in this so-called ‘photonic crystals’. This enables control on the flow of light within the material. Using specialised designs, one can engineer photonic devices like microresonators, splitters and so on. Using computational techniques, higher-level photonic devices can be designed and, thereafter, fabricated out of semiconductor materials. Some specialized designs of photonic crystals have also been observed to show negative refraction and superlensing effects.