Kalaichelvi Saravanamuttu

Diffraction rings due to spatial self-phase modulation in a photopolymerizable medium

Diffraction rings due to spatial self-phase modulation of a continuous wave, visible laser beam were observed in a photopolymerizable organosiloxane medium. Self-phase modulation originates from laser initiated free-radical polymerization and corresponding changes in the refractive index of the medium. This study focuses on the differences in the photoresponse of the organosiloxane relative to other nonlinear optical materials and the opportunities that they provide to probe previously inaccessible properties of the self-induced diffraction rings. Specifically, the noninstantaneous response of the organosiloxane enabled diffraction rings to propagate through long distances ( length) in the medium without disruption from optical self-focusing. It was moreover possible to monitor the temporal evolution of the rings, and thereby gain direct insight into the dynamics of self-phase modulation. Furthermore, changes in refractive index due to polymerization were permanent and provided a direct view of the conical trajectory of the diffraction rings through the medium.

An experimental study of the interactions of self-trapped white light beams in a photopolymer

Interactions of two parallel-propagating and mutually incoherent white light beams were examined in a photopolymerizable organosiloxane. The beams fused when separated by a distance corresponding to the width of each beam but at separation distances⪢beam width, formed two self-trapped beams that repelled each other. At separation distances<beam width, they suffered filamentation but ultimately fused into a single self-trapped beam.

Spontaneous and sequential transitions of a Gaussian beam into diffraction rings, single ring and circular array of filaments in a photopolymer

A Gaussian beam propagating in a photopolymer undergoes self-phase modulation to form diffraction rings and then transforms into a single ring, which in turn ruptures into a necklace of stable self-trapped multimode filaments. The transitions of the beam between the three distinct nonlinear forms only occur at intensities where the beam-induced refractive index profile in the medium slowly evolves from a Gaussian to a flattened Gaussian.

Spontaneous formation of 3-D optical and structural lattices from two orthogonal and mutually incoherent beams of white light propagating in a photopolymerisable material

The spontaneous generation of 3-D lattices by an orthogonal pair of white light beams propagating in a polymerisable organosiloxane medium is described. Lattice formation occurs through polymerisation-induced modulation instability of the beams, each of which spontaneously divides into multiple self-trapped filaments. Dynamic interactions between the beams lead to a 3-D lattice with near-cubic symmetry, where each lattice point corresponds to the intersection of an orthogonal pair of filaments. Three different lattice types were found. Each leaves an indelible imprint in the photopolymer, which transforms from an isotropic and homogeneous medium into a 3-D microperiodic lattice constructed entirely of intersecting cylindrical waveguides.

Optical self-trapping in a photopolymer doped with Ag nanoparticles: a single-step route to metallodielectric cylindrical waveguides

A cw, visible laser beam self-traps by initiating free-radical polymerization in an organosiloxane photopolymer doped with a well-characterized distribution of Ag nanoparticles. The self-trapped beam propagates over long distances (≫Rayleigh range) without diverging and permanently inscribes a cylindrical metallodielectric waveguide containing a dispersion of Ag nanoparticles. The self-trapped beam evolves from single-mode to multimode guidance over time; the effects of nanoparticle concentration on multimode dynamics were investigated. These findings open room temperature, soft polymer-based pathways where self-action effects including self-trapping and modulation instability can be exploited to spontaneously generate three-dimensional metallodielectric single or multiple cylindrical waveguides.

An optochemically organized nonlinear waveguide lattice with primitive cubic symmetry

We describe the first example of a primitive cubic lattice assembled spontaneously from three mutually orthogonal and intersecting arrays of cylindrical, multimode waveguides. The lattice is generated in a single, room-temperature step with separate (mutually incoherent) incandescent light bulbs. To demonstrate its potential as a nonlinear photonic lattice, we generated a self-trapped lattice beam of incoherent white light. These two findings open entirely new experimental opportunities to study the behavior of spatially and temporally incoherent, polychromatic lattice solitons in 3-D Bravais lattices.

Photolytic formation of Ag nanoparticles in oligomeric organosiloxanes: new photolithographic routes to metallodielectric microperiodic structures

A single-step, room temperature photolithographic route to fabricate periodic metallodielectric microstructures is reported. Ag nanoparticles in organosiloxane sols were first generated through the well-established method of AgCl photoreduction. Evolution of the shape, size and distribution of Ag nanoparticles in the fluid sols upon irradiation with white light was characterised in detail through optical extinction spectroscopy and transmission electron microscopy. Ag nanoparticles were extremely stable in sols, which were subsequently processed into films, gels and monoliths. The application of these composites in the fabrication of optical gratings containing a uniform distribution of Ag nanoparticles was demonstrated through both photomask and interference lithographic techniques. This technique provides accessible routes to a range of metallodielectric microperiodic structures, which could enable experimental verification of theoretically predicted optical properties including enhanced photonic band gaps.

A black beam borne by an incandescent field self-traps in a photopolymerizing medium.

We report that a self-trapped black optical beam that is spatially and temporally incoherent forms spontaneously in a nascent photopolymerization system. The black beam inscribes a permanent cylindrical channel, which prevents the propagation of visible light even under passive conditions (in the absence of polymerization). The finding opens a powerful new mechanism to manipulate light signals from incoherent sources such as LEDs through selective suppression of light propagation. This contrasts with approaches employed by photonic crystals and optical waveguides, which concentrate and guide light intensity within spatially localized regions. The self-trapped black beam forms when a broad incandescent beam bearing a negligible depression was launched into a photopolymerizable medium. Because of refractive index changes caused by polymerization, the depression narrows, deepens, and continually rejects the visible spectrum of light until it stabilizes as a black beam that propagates over long distances (≫ effective Rayleigh range) without significant divergence. As refractive index changes due to polymerization are irreversible, the cylindrical region occupied by the self-trapped black beam is inscribed as a black channel waveguide in the medium.

Optochemical self-organisation of white light in a photopolymerisable gel: a single-step route to intersecting and interleaving 3-D optical and waveguide lattices

We report a minute-long, single step, room temperature and spontaneous route to complex 3-D optical lattices and their corresponding waveguide architectures. The method exploits the instability and spontaneous filamentation of broad light beams propagating through a photopolymerisable medium and in this way, combines the ease and elegance of self-organisation with the precision and directionality that characterise beam-directed lithography. We found that when launched into a photopolymer, an orthogonal pair of white incandescent beams spontaneously divided into light filaments that self-organised into a hexagonal lattice. When made to intersect, the orthogonal hexagonal lattices generated a 3-D optical lattice with BCC symmetry and when made to interleave, yielded a lattice with the woodpile arrangement. These optical lattices permanently inscribed microstructures composed of thousands of discrete cylindrical, multimode and polychromatic waveguides. The structural lattices spanned a volume of ≅1000 mm3, at least an order of magnitude greater than volumes that are typically constructed through lithography or assembled through self-organisation processes. The densely packed and multidirectional waveguide lattices could serve as fundamentally new and remarkably inexpensive devices with highly efficient, angle-independent light collection and guiding capability. As a result, they hold significant potential to improve the light-harnessing capabilities of photovoltaic devices and separately, to function as nonlinear photonic crystals.

Collective dynamics of populations of weakly correlated filaments of incoherent white light

We examined the dynamics of two populations of self-trapped filaments of spatially and temporally incoherent white light. The populations consisted of (i) independent filaments generated through self-trapping of incandescent speckles, and (ii) co-dependent filaments created through modulation instability of a broad incandescent beam. Both filament populations were positionally stable in conditions where individual pairs of self-trapped beams interact strongly. Both also acquired significantly broad intensity distributions, which were independent of their parent optical fields; a small but persistent number of high-intensity filaments was identified in both cases. These studies provide accessible routes to weakly correlated ensembles, insight into their collective behaviour such as self-stabilization and self-selected intensity distributions, and reveal intriguing similarities between the dynamics of two populations of different origins.