With a penchant for integrated photonics and miniaturization, the fabrication of micron sized optical elements using precision laser pulse management is drawing attention due to the possibility of minimizing tolerances for collateral material damage. The work presented here deals with the design, fabrication and characterization of a range of diffractive optics - gratings, grids and Fresnel zone plates – on transparent and metallic samples. Their low volume, light weight, transmission bandwidth, high damage threshold and flexible design make them suited for replacing conventional refractive optical elements. Our one-step, mask-less, 3-D laser direct writing process is a green fabrication technique which is in stark contrast to currently popular Photo-lithography based micro-structuring. Our method provides scope for modifications on the surface as well as within the bulk of the material. The mechanism involved in the fabrication of these optics on transparent and thin metallic substrates differ from each other. Our studies show that both amplitude and phase versions of micro-structures were achieved successfully with performances bearing ~98% accuracy vis-a-vis theoretical expectations.
Filamentation in gases due to high power femtosecond pulses results from the combined action of the optical Kerr effect (giving rise to self-focusing) and plasma formation (giving rise to defocusing) that confines optical energy in a small region over a distance longer than the Rayleigh range. Since the discovery of N<sub>2</sub> as a potential gain medium, which subsequently led to the formation of nitrogen lasers, it has held a keen interest due to its potential in achieving lasing by remote excitation. Recently, Yamanouchi and coworkers demonstrated lasing action in N<sub>2</sub> in the forward as well the backward directions along the femtosecond pulse propagation. In the present work, we have focused on excitation of N<sub>2</sub><sup> +</sup> (corresponding to the 391nm spectral feature) and have measured spectral narrowing. We have investigated the influence exerted by the incident pulse power and gas pressure for incident pulses of durations 40 fs and 10 fs in forward and backward detection modes. Spectral narrowing that occurs for N<sub>2</sub> gas at 391 nm shows a dependence on the incident pulse duration. Pressure threshold for different incident powers for lasing has been established. Increase in the signal intensity on varying the incident power is ascribed to amplified spontaneous emission (ASE). White-light-seeded lasing in N<sub>2</sub><sup> +</sup> is generated by a Ti:sapphire femtosecond laser for different focusing. The lasing lines peak over the trail of the incident broadband spectra.
Single-cell micro-Raman spectroscopy has been applied to explore cell differentiation in single, live, and malignant cells from two tumor cell lines. The spectra of differentiated cells exhibit substantial enhancement primarily in the intensities of protein peaks with concomitant decrease in intensities of O─P─O asymmetric stretching peaks in DNA/RNA. Principal component analyses show that the spectral score of differentiated cells tends to asymptotically approach that of spectra obtained from normal neural stem cells/progenitors. This lends credence to the notion that the observed spectral changes are specific to differentiation, since upon differentiation, malignant cells become less malignant and tend toward benignity.
The birefringence of a red blood cell (RBC) is quantitatively monitored as it becomes infected by a malarial parasite. Large changes occur in the cell’s refractive index at different stages of malarial infection. The observed rotation of an optically trapped, malaria-infected RBC is not a simple function of shape distortion: the malarial parasite is found to itself exercise a profound influence on the rotational dynamics by inducing stage-specific birefringence. Our measurements shed new light on the competition between shape- and form-birefringence in RBCs. We demonstrate the possibility of using birefringence to establish very early stages of infected parasites and of assessing various factors that contribute to birefringence in normal and infected cells. Our results have implications for the development and use of noninvasive techniques of quantifying changes in cell properties induced by malaria disease pathology.
We investigate the physics of an optically trapped red blood cell under physiological conditions. When a single, live red
blood cell, is placed in an optical trap, the normal biconcave disk shaped cell is observed to undergo a folding action
and thereby take up a rod like shape. If such an RBC has any shape anisotropies due to perturbation through malarial
infection or hyperosmotic stress, it is observed to rotate in the linearly polarised laser field. Finally when such an
optically trapped RBC is exposed to a shear flow, a tank treading like behaviour of the red blood cell membrane is
visualised (wherein the RBC membrane revolves around the central body of the cell). The tank treading motion of a red
blood cell held stationary in the optical trap allows for the dynamics to be viewed in a prolonged manner without the
usage of earlier constraints such fast imaging system.
A laser-based method has been developed for experimentally probing single red blood cell (RBC) buckling and determining RBC membrane rigidity. Our method combines a liquid flow cell, fluorescence microscopy, and an optical-trap to facilitate simple measurements of the shear modulus and buckling properties of single RBCs, under physiological conditions. The efficacy of the method is illustrated by studying buckling behavior of normal and Plasmodium-infected RBCs, and the effect of Plasmodium falciparum–conditioned medium on normal, uninfected cells. Our simple method, which quantifies single-RBC deformability, may ease detection of RBC hematological disorders.
The first studies of the propagation of ultrafast (<45 fs) pulses of intense infrared light through protein media reveal that supercontinuum (white light) generation is severely suppressed in the presence of the protein -amylase, a potential stress marker in human saliva. The continuum suppression capacity is attributed to the electron scavenging property of the protein.
Propagation of ultrashort pulses of intense, infrared light through transparent medium gives rise to a visually spectacular phenomenon known as supercontinuum (white light) generation wherein the spectrum of transmitted light is very considerably broader than that of the incident light. We have studied the propagation of ultrafast (<45 fs) pulses of intense infrared light through biological media (water, and water doped with salivary proteins) which reveal that white light generation is severely suppressed in the presence of a major salivary protein, &agr;-amylase.