Selected Topics in Photonics

This study describes the effect of temperature on binding parameters as well as mode of binding between coumarin 152 (C152) and human serum albumin (HSA). Site marker competitive experiment, Förster resonance energy transfer (FRET), and molecular docking study show that C152 binds to the digitoxin binding site in domain III of HSA. The binding constant calculated at the single molecular level experiment matches well with the ensemble average measurement, which indicate that at even low dye concentration the binding reaction proceeds with equal probability. Further, FRET and molecular dynamic simulation confirm that the binding location of C152 is independent on temperature (278 K to 323 K). It has been revealed that the binding affinity of C152 to HSA was almost unaffected until 298 K; afterward, it decreases continuously on increasing temperature forming two distinct regions. Thermodynamic parameters for association indicate that strong electrostatic and hydrophobic interactions are operational at lower temperature region, whereas hydrogen bonding predominates at higher temperate region.

A new method of utilizing photothermal effect at nano-volume dimensions to measure viscosity is presented here that can, in turn, provide the surrounding temperature. Our measurements use high repetition rate, low average power, femtosecond laser pulses that induce photothermal effect that is highly influence by the convective mode of heat transfer. This is especially important for absorbing liquids, which is unlike the typical photothermal effects that are due to such ultrashort pulses. Typical thermal processes involve only conductive mode of heat transfer and are phenomenological in nature. Inclusion of convective mode results in additional molecular characteristics of the thermal process. We measure traditional thermal lens with femtosecond pulse train through geometric beam divergence of a collimated laser beam co-propagating with the focused heating laser beam. The refractive index gradient in the sample arising from a focused heating laser creates a thermal lens, which is measured. On the other hand, the same heat gradient from the focusing heating laser beam generates a change in local viscosity in the medium, which changes the trapped stiffness of an optically trapped microsphere in its vicinity. We use co-propagating femtosecond train of laser pulses at 1560 and 780 nm wavelengths for these experiments. We also show from the bulk thermal studies that use of water as sample has the advantage of using conductive mode of heat transfer for femtosecond pulse train excitation.

The ability of fluorescence imaging to identify early physiological changes in tissue places it suitably for use in pre-cancer diagnosis. We have presented a review of the broad classification of tomographic schemes with respect to this problem. We have articulated some of the main motivations, issues, and potential solutions for the use of fluorescence optical tomographic systems for limited data in vivo imaging. We have also reported recent results obtained by us in our ongoing work of developing a frequency domain FOT system for early cervical cancer diagnosis.

A new fs-laser anneal technique of isolating InAs/GaSb T2SL p-i-n photodetector array is identified. The fs anneal isolation improvement in 200–400 μm pixels due to spatially selective Quantum Well intermixing is confirmed by FTIR measurements. A 128 × 128 array of 8 μm square pixels is fabricated with SU8 polymer isolation after reactive ion etching pixel delineation. Photo-response of 8 and 200 μm pixels under ns pulsed condition shows a peak responsivity of ~0.03A/W and 0.2 mA/W, respectively, at λ ~ 3.7 μm. Effort is underway to integrate this work with that of DRDO for larger and more efficient T2SL MWIR photodiode arrays.

Information transmission through fiber-optic channel is subjected to several impairments such as chromatic and polarization mode dispersion, nonlinear phase noise due to interaction of amplifier noise with fiber Kerr nonlinearity, and nonlinear effects such as self- and cross-phase modulation. In addition, laser phase noise and frequency offset between signal and local oscillators also degrade the received signal quality. Unless these impairments are mitigated, the performance of high data rate optical communication systems is degraded. We describe two approaches to mitigate fiber impairments in high data rate coherent optical communication systems. In the first approach, nonlinearity and dispersion in either fibers or semiconductors is used to undo the effects of transmission fiber on the optical carrier. We propose the use of mid-span spectral inversion, realized using counter-propagating dual pumped four-wave mixing in fibers, to mitigate dispersion and nonlinearity in 40 Gbps QPSK systems. We describe our work on realizing optical phase conjugation in semiconductor optical amplifiers. In the second approach, the optical signal is sampled after coherent reception and processed using digital signal processing algorithms to mitigate dispersion and nonlinearity. We describe Kalman filters to estimate and track phase noise in 100 Gbps QPSK systems. We also describe radial basis function neural network equalizer to mitigate nonlinearity in 80 Gbps 16 QAM CO-OFDM systems.

The motivation for research on photonic nanostructures is twofold. One is to attempt the miniaturization of standard optical devices and the other is to explore novel functionalities, otherwise unknown in bulk optical media. This article describes the work carried out using very inexpensive fabrication methods on the development of a few active and passive devices based on 1-D, 2-D, and 3-D photonic crystals, all with feature sizes below 300 nm.

We consider parametric downconversion (PDC)—a nonlinear optical process in which a pump photon gets annihilated to generate a pair of photons, termed as signal and idler, which are entangled in various degrees of freedom. In this paper, we consider the polarization degree of freedom and study how the polarization coherence properties of the pump photon get transferred to the signal-idler photons to manifest as two-qubit polarization entanglement. We show that for any generation process that is non-postselective and entropy-nondecreasing, the entanglement concurrence \( C(\rho ) \) of the generated two-qubit signal-idler state satisfies an intrinsic upper limit with \( C(\rho ) \le (1 + P)/2 \), where P is the degree of polarization of the pump photon. We further show that for the restricted class of two-qubit states having only two nonzero diagonal elements, the upper limit on the concurrence is the degree of polarization itself, that is, \( C(\rho ) \le P \). This study shows that the manifestation of the pump correlations as entanglement in the signal-idler state is dependent on the dimensionality of the computational subspace on which the state has support. This formalism could pave the way to inferring bounds on high-dimensional entanglement, for which no universally accepted measure exists.

Droplets are encountered in several natural systems, i.e. dew formation, cloud formation and practical applications such as inkjet printer, condenser, protein crystallization systems, digital microfluidic systems, disease diagnosis, droplet lenses, nano-patterning and droplet-based manufacturing systems. The internal hydrodynamics of droplets influence the behaviour and performance of these applications. Visualization of internal dynamics inside droplets is challenging due to the small-scale and curvature effect. The present study reports the ongoing work carried out at IITK on the interesting fluid flow dynamics inside droplets. Some of the case studies related to evaporating droplets, like a single evaporating droplet, two evaporating droplets, drying pattern and protein crystallization, have been reported. Marangoni stresses and buoyancy-driven Rayleigh convection are primarily responsible for motion inside droplets. The internal hydrodynamics inside a droplet shows several complexities irrespective of its simple symmetrical geometry.