Saravanamuttu Research Group

Research areas

Patterns of clouds in the sky, wind-sculpted striations in the desert sand, spots and stripes of animal skin are examples of patterns that emerge spontaneously in nature. The physical laws that govern spontaneous pattern formation in these dissimilar systems are strikingly similar. Any system that is unstable (prone to change) will spontaneously transform in an attempt to reach its most stable state (equilibrium). Patterns emerge if during its evolution towards equilibrium, the system is subjected to two opposing forces. Our research examines such nonlinear phenomena when a system simultaneously undergoes optical and chemical changes. Because the optical and chemical properties of such systems can be precisely controlled, they provide convenient pathways to examine complex nonlinear processes. This research, while consistent with studies of nonlinear light propagation established by optical physicists and engineers, contribute new chemical perspectives to this forty-year old field.

Nonlinear phenomena in photochemical systems

Consider the example of a narrow beam of light. Regardless of whether it originates from the sun or a laser, the beam will naturally broaden (diffract) while travelling through most transparent materials including air, glass and water. In certain types of materials however, the same beam of light can trigger a transformation - such as a chemical reaction – that causes the refractive index along the path of the beam to increase. A nonlinear situation arises in which the natural tendency of the beam to diffract is always opposed by refraction, which forces the beam to focus. Our research in the past four years has revealed that these conditions elicit a richly diverse suite of nonlinear phenomena including spontaneous pattern formation.

Our principal findings include self-trapped beams of light, which travel without broadening in certain photochemical systems over unexpectedly long distances. Self-trapping occurs when the beam becomes entrapped within the narrow channel of increased refractive index inscribed along its path of propagation. We have established a comprehensive understanding of the dynamics and properties of self-trapped beams and provided first experimental confirmation of several theoretical predictions of their behaviour. We have found self-trapped beams of both laser and separately, of white light and learnt that their properties are fundamentally different. Waves composing a laser beam possess the same wavelength and are strongly correlated to each other. Exactly the opposite is the case for white light, which is said to be incoherent because it is composed of an entire spectrum of wavelengths and has extremely poor correlation. Once self-trapped, a laser beam exhibits different and discrete optical modes – like the vibrational modes of a drum skin or the harmonics of a violin string. The optical modes of a self-trapped white light beam however fluctuate so rapidly that only a time-averaged or smoothed profile of the entire beam can be observed. (It is therefore particularly remarkable that such a rapidly fluctuating and incoherent wavepacket collectively self-traps and propagates as a single entity.) By applying our knowledge of the competition between diffraction and refraction in self-trapping behaviour, we have recently been able to demonstrate the spontaneous formation of patterns in large uniform beams of white light.

Interactions of self-trapped beams

Scientists have always sought to understand the interactions between particles – whether they be atoms or planets. Like these bodies of matter, self-trapped beams of light can also interact with each other. Under specific conditions, a pair of self-trapped beams can attract, repel and even appear to orbit about each other. Because we can now generate self-trapped beams in photochemical systems, we can also study their interactions. Our objectives are to establish the basic principles that govern self-trapped beam interactions through experiments that ask questions such as (a) is there any exchange of energy between interacting beams? (b) are there any changes to the fundamental properties of a self-trapped beam such as its spectrum, coherence and modes? (c) what type of structures are inscribed in the photochemical medium as a result of these interactions? (d) are self-trapped beams restricted to pairwise interactions or do interactions extend to multiple (> 2) self-trapped beams? (e) how do these interactions compare with interactions of particles in other systems?

Spontaneous pattern formation: is coherence important?

Our discovery that a uniform white light beam transforms spontaneously into an ordered pattern – although explicable through the mathematics of nonlinear light propagation – is counterintuitive. Periodic patterns of light when two or more laser beams, which are mutually coherent, interfere. Even in his seminal experiment, Young used a pair of slits to introduce mutual coherence into two beams of sunlight, which in turn produced an interference pattern and revealed the wave-nature of light. An understanding of optical pattern formation processes could bridge the two extremes of periodic interference patterns and periodic patterns that form spontaneously across a single broad beam of incoherent white light. Studies are planned to systematically tune the (spatial) coherence of a beam from excellent to weak and to monitor the resulting effects on properties of the pattern such as symmetry, periodicity and degree of order.

Theory development and modelling

Our experimental research, while confirming several predictions of light propagation models, also revealed new forms of nonlinear phenomena (higher order optical modes, white light phenomena, spontaneous pattern formation). We plan to develop theoretical models to simulate these phenomena; initial approaches will seek numerical solutions to problems cast through the nonlinear Schrödinger equation for nonlinear light propagation.

New materials for nonlinear light propagation and interactions

Experiments in photopolymers alone have revealed richly diverse self-trapping phenomena. An even greater variety of phenomena may be discovered by expanding the search to different classes of photochemical systems. To this end, we will study self-trapping phenomena in two different and well known classes of chemical systems (i) photosensitive liquid crystalline media and (ii) photoisomerizing azobenzene systems. These systems also exhibit photoinduced index changes and thus satisfy the basic prerequisite for self-trapping. Our focus will be on fundamental differences in their photoresponse relative to photopolymers that could lead to new forms of self-trapped species; the effects of wavelength of sensitivity, magnitude, anisotropy and reversibility of refractive index, timescale of response will be examined in detail. The application in optical switching, erasable and reconfigurable and birefringent self-induced waveguides will also be studied.

Spontaneous self-inscription of complex 3-D optical waveguides and microperiodic structures

Refractive index changes induced during self-trapping phenomena result in a variety of permanently self-inscribed structures including 3-D cylindrical waveguides (formed during self-trapping) and microperiodic structures (during pattern formation). The application of these structures as self-inscribed passive and active optical devices will be studied. Particular emphasis will be given to the development of a new technique “White-light based Spontaneous Lithography of 3-D Microperiodic Structures”. Here, pattern formation in multiple beams (maximum of 3) propagating at different orientations to each other and its potential in spontaneously forming 3-D ordered microperiodic structures will examined. Unlike holographic lithography, which relies on the interference of coherent laser beams, this technique relies on the spontaneous behaviour of the optical field itself and requires only incoherent (and cheap) white light from an incandescent source. Resulting structures can be further modified by introducing active optical components (metal nanoparticles, chromophores) for further functionality.