Binita Ghosh

Ion-Beam-Synthesized Colloidal Silver Nanoclusters in Crystalline Sapphire as Third-Order Optical Material

Silver ion implantation in single-crystalline sapphire has given rise to the formation of silver nanoparticle-sapphire composites, which have been imaged using transmission electron microscopy, and confirmed using linear optical absorption and Rutherford backscattering spectrometry. Nonlinear refractive index and two-photon absorption of these nanocomposites have been observed using Z-scan and Anti-resonant Interferometric Nonlinear Spectroscopy (ARINS) in the close proximity of surface plasmon resonance (SPR) wavelength of silver nanoclusters (~400 nm) and at ~807 nm, respectively. Both sign and value of the nonlinear parameters were determined, and the third-order optical susceptibility (χ(3)) of the composites has been found to be significant. Such metal nanocomposites in glasses and sapphires having appreciable χ(3) with temporal responses in picosecond to femtosecond time domain have great relevance to futuristic switching materials in nanophotonics.

Ion beam synthesis of metal-quantum dots for photonic applications

Although electronics technologies have made great advances in device speed, optical devices can function in the time domain inaccessible to electronics. In the time domain less than 1 ps, optical devices have no competition. Photonic or optical devices are designed to switch and process light signals without converting them to electronic form. The major advantages that these devices offer are speed and preservation of bandwidth. The switching is accomplished through changes in refractive index of the material that are proportional to the light intensity. The third-order optical susceptibility, χ(3), known as the optical Kerr susceptibility which is related to the non-linear part of the total refractive index, is the nonlinearity which provides this particular feature. Future opportunities in photonic switching and information processing will depend critically on the development of improved photonic materials with enhanced Kerr susceptibilities, as these materials are still in a relatively early stage of development. Different glass systems are now under investigation to increase their nonlinearity by introducing a variety of modifiers into the glass-network. Ion implantation is an attractive method for inducing colloid formation at a high local concentration unattainable by the melt-glass fabrication process and for confining the non-linearities to specific patterned regions in a variety of host matrices. Recent works on metal-ion implanted colloid generation in bulk fused silica glasses have shown that these nanocluster-glass composites under favourable circumstances have significant enhancement of χ(3) with picosecond to femtosecond temporal responses.

Optical Nonlinearities of Colloidal Metal Quantum Dot - Glass Composites for Nanophotonics

Since the inception of lasers, nonlinear optics has been a rapidly growing field of research in recent decades. It is based on the study of effects and phenomena related to the interaction of intense coherent light radiation with matter. In other words, nonlinear optics (NLO) is the branch of optics that describes the behaviour of light in nonlinear media, that is, the media in which the dielectric polarization P responds nonlinearly to the electric field E of the light. This nonlinearity is typically and only observed at very high light intensities (values of the electric field comparable to interatomic electric fields, typically 108 V/m) such as those provided by pulsed lasers. Such high powers of laser beams made it possible, for the first time, to observe that the effect of light on a medium can indeed change its optical properties, e.g. refractive index or absorption. When this happens, the light itself also gets affected by this change in a non-linear way; for example, the non-linear response of the material can convert the laser light into new colours, both harmonics of the optical frequency and sum and difference frequencies. Typically, only a laser light is sufficiently intense to modify the optical properties of a material system. In fact, the beginning of the field of nonlinear optics is taken to be the discovery of second-harmonic generation by Fraken et al. in 1961 [1], shortly after the demonstration of the first working laser by Maiman in 1960 [2]. The nonlinear effect comes essentially from ‘quantum confinement’ effect. In an unconfined (bulk) semiconductor, an electron-hole pair is typically bound within a characteristic length, called the ‘exciton Bohr radius’. This is estimated by replacing the positively charged atomic core with the hole in the Bohr formula. If the electron and hole are constrained further, then properties of the semiconductor change. Besides confinement in all three dimensions i.e. ‘Quantum Dot’ other quantum confined semiconductors include (1) quantum wires, which confine electrons or holes in two spatial dimensions and allow free propagation in the third (2) quantum wells which confine electrons or holes in one dimension and allow free propagation in two dimensions. Nonlinear optical effect in semiconductor quantum dots was observed in the minuscule crystals of semiconductor material composed of various compounds of chemicals such as cadmium, zinc, tellurium, selenium, sulphur, etc. of sizes less than 500 nanometers. These semiconductor nanoparticles or ‘quantum dots’ have been found to react to electricity or light by emitting their own light across the visible range of wavelengths from 470 to 730 nm.

Anti-Resonant Interferometric Nonlinear Spectroscopy (ARINS) study of metal nanocluster-glass composites

Linear and nonlinear optical properties of copper and gold nanoclusters in fused silica glasses synthesized by 200 KeV Cu+ and 1.5 MeV Au+ ion implantation at a dose of 3x1016 ions/cm2 have been studied. UV-Vis spectroscopy has revealed prominent linear absorption bands at characteristic surface plasmon resonance (SPR) frequency signifying appreciable formation of copper and gold colloids in glass matrices. Third-order optical properties of the nanocluster-glass composite materials have been studied by Z-Scan and Anti Resonant Interferometric Nonlinear Spectroscopy (ARINS) techniques. The sign of the nonlinear refraction is readily obtained from Z-scan signature. The ARINS technique utilizes the dressing of two unequal-intensity counter-propagating pulsed beams with differential nonlinear phases, which occur upon traversing the sample. This difference in phase manifests itself in the intensity-dependent transmission. The nonlinear refractive index, nonlinear absorption coefficient and the real (and imaginary) parts of the third-order optical susceptibility have been extracted from ARINS data. Results of the investigation of the nonlinear refraction using the above two techniques and the possible mechanisms responsible for the nonlinear optical responses are presented in the current paper.