Acousto-optic modulator-based improvement in imaging through scattering media.

Reduced visibility is a common problem when light traverses through a scattering medium, and it becomes difficult to identify an object in such scenarios. What we believe to be a novel proof-of-principle technique for improving image visibility based on the quadrature lock-in discrimination algorithm in which the demodulation is performed using an acousto-optic modulator is presented here. A significant improvement in image visibility is achieved using a series of frames. We have also performed systematic imaging by varying the camera parameters, such as exposure time, frame rate, and series length, to investigate their effect on enhancing image visibility.

Varying coordination modes of amide ligand in group 12 Hg(II) and Cd(II) complexes: synthesis, crystal structure and nonlinear optical properties.

Reactions of the amide ligand, H2L (H2L = N,N'-bis[2-(2-pyridyl)methyl]pyridine-2,6-dicarboxamide) with CdCl2 and Hg(CH3COO)2, in 1 : 1 ratio, at 298 K yield dimeric [Hg(L)]2 (1) and trimeric [Cd3(H2L)4Cl6] (2), respectively. In 1, the H2L is coordinated to Hg(II) via six N-atoms of central and terminal pyridines as well as of deprotonated amido groups, whereas the carbonyl groups remain free. However, in 2, the H2L is coordinated to Cd(II) through terminal pyridine N atoms and O atoms from carbonyl groups, whereas the nitrogen atoms of the central pyridine, two terminal pyridine and of all amido groups remain free. Molecular structures of 1 and 2 are confirmed by single crystal X-ray studies. The varying coordination modes of H2L give rise to different electrochemical behavior of 1 and 2, which has also been rationalized by theoretical calculations. Moreover, nonlinear optical (NLO) behavior of both complexes has been investigated using ultra-short femtosecond laser pulses, which ensures that the NLO response is exclusively from their electronic component.

A search for the sulphur hexafluoride cation with intense, few cycle laser pulses.

It is well established that upon ionization of sulphur hexafluoride, the SF6(+) ion is never observed in mass spectra. Recent work with ultrashort intense laser pulses has offered indications that when strong optical field are used, the resulting "bond hardening" can induce changes in the potential energy surfaces of molecular cations such that molecular ions that are normally unstable may, indeed, become metastable enough to enable their detection by mass spectrometry. Do intense, ultrashort laser pulses permit formation of SF6(+)? We have utilized intense pulses of 5 fs, 11 fs, and 22 fs to explore this possibility. Our results are negative: no evidence is discovered for SF6(+). However, multiply charged sulphur and fluorine ions from highly charged SF6(q+) ions are observed that enable us to resolve the controversy regarding the kinetic energy release accompanying formation of F(+) fragment ions. Quantum chemical computations of field-distorted potential energy curves of SF6 and its molecular ion enable us to rationalize our non-observation of SF6(+). Our findings have implications for high harmonic generation from SF6 in the few-cycle regime.

On the birefringence of healthy and malaria-infected red blood cells.

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.

Seventh-harmonic generation from tightly focused 2 µm ultrashort pulses in air.

We report generation of third, fifth and seventh harmonics from air by using tightly focused, ultrashort pulses of short-wave infrared (2 µm) radiation. We have measured the third- and fifth-harmonic efficiencies to be 5×10(-5) and ~1.4×10(-5), respectively, with the ratio of fifth-to-third-harmonic efficiency being close to 0.28. Our experimental results provide confirmation of expectations of the higher-order Kerr effect model.

Anomalies in the motion dynamics of long-flagella mutants of Chlamydomonas reinhardtii.

Chlamydomonas reinhardtii has long been used as a model organism in studies of cell motility and flagellar dynamics. The motility of the well-conserved '9+2' axoneme in its flagella remains a subject of immense curiosity. Using high-speed videography and morphological analyses, we have characterized long-flagella mutants (lf1, lf2-1, lf2-5, lf3-2, and lf4) of C. reinhardtii for biophysical parameters such as swimming velocities, waveforms, beat frequencies, and swimming trajectories. These mutants are aberrant in proteins involved in the regulation of flagellar length and bring about a phenotypic increase in this length. Our results reveal that the flagellar beat frequency and swimming velocity are negatively correlated with the length of the flagella. When compared to the wild-type, any increase in the flagellar length reduces both the swimming velocities (by 26-57%) and beat frequencies (by 8-16%). We demonstrate that with no apparent aberrations/ultrastructural deformities in the mutant axonemes, it is this increased length that has a critical role to play in the motion dynamics of C. reinhardtii cells, and, provided there are no significant changes in their flagellar proteome, any increase in this length compromises the swimming velocity either by reduction of the beat frequency or by an alteration in the waveform of the flagella.

Enhancing the strength of an optical trap by truncation.

Optical traps (tweezers) are beginning to be used with increasing efficacy in diverse studies in the biological and biomedical sciences. We report here results of a systematic study aimed at enhancing the efficiency with which dielectric (transparent) materials can be optically trapped. Specifically, we investigate how truncation of the incident laser beam affects the strength of an optical trap in the presence of a circular aperture. Apertures of various sizes have been used by us to alter the beam radius, thereby changing the effective numerical aperture and intensity profile. We observe significant enhancement of the radial and axial trap stiffness when an aperture is used to truncate the beam compared to when no aperture was used, keeping incident laser power constant. Enhancement in trap stiffness persists even when the beam intensity profile is modulated. The possibility of applying truncation to multiple traps is explored; to this end a wire mesh is utilized to produce multiple trapping that also alters the effective numerical aperture. The use of a mesh leads to reduction in trap stiffness compared to the case when no wire mesh is used. Our findings lead to a simple-to-implement and inexpensive method of significantly enhancing optical trapping efficiency under a wide range of circumstances.

Axicon-based writing of waveguides in BK7 glass.

We report on the direct writing of waveguide structures using an axicon lens to focus a 40 fs laser pulse within BK7 glass. The written structures are characterized for waveguiding action: waveguiding action for 635 and 1550 nm light and propagation loss as low as 0.19 dB/cm is measured. Loss values decrease with laser exposure time for incident energy of 300 µJ, indicating enhancement of index contrast. At higher energies, a reverse trend is obtained: higher loss values are obtained as index contrast degrades with an increase in exposure time.

Tank treading of optically trapped red blood cells in shear flow.

Tank-treading (TT) motion is established in optically trapped, live red blood cells (RBCs) held in shear flow and is systematically investigated under varying shear rates, temperature (affecting membrane viscosity), osmolarity (resulting in changes in RBC shape and cytoplasmic viscosity), and viscosity of the suspending medium. TT frequency is measured as a function of membrane and cytoplasmic viscosity, the former being four times more effective in altering TT frequency. TT frequency increases as membrane viscosity decreases, by as much as 10% over temperature changes of only 4°C at a shear rate of ∼43 s(-1). A threshold shear rate (1.5 ± 0.3 s(-1)) is observed below which the TT frequency drops to zero. TT motion is also observed in shape-engineered (spherical) RBCs and those with cholesterol-depleted membranes. The TT threshold is less evident in both cases but the TT frequency increases in the latter cells. Our findings indicate that TT motion is pervasive even in folded and deformed erythrocytes, conditions that occur when such erythrocytes flow through narrow capillaries.

Shape anisotropy induces rotations in optically trapped red blood cells.

A combined experimental and theoretical study is carried out to probe the rotational behavior of red blood cells (RBCs) in a single beam optical trap. We induce shape changes in RBCs by altering the properties of the suspension medium in which live cells float. We find that certain shape anisotropies result in the rotation of optically trapped cells. Indeed, even normal (healthy) RBCs can be made to rotate using linearly polarized trapping light by altering the osmotic stress the cells are subjected to. Hyperosmotic stress is found to induce shape anisotropies. We also probe the effect of the medium's viscosity on cell rotation. The observed rotations are modeled using a Langevin-type equation of motion that takes into account frictional forces that are generated as RBCs rotate in the medium. We observe good correlation between our measured data and calculated results.

Flagella-generated forces reveal gear-type motor in single cells of the green alga, Chlamydomonas reinhardtii.

Optically trapped single cells of the biflagellated, green alga, Chlamydomonas reinhardtii, rotate. The rotational dynamics of trapped wild-type and mutant cells show that functional flagella play a decisive role: the entire flagellar apparatus (central microtubules, radial spokes, and dynein arms) is involved. Any aberration in this apparatus leads to non-functionality, indicating a gear-type mechanism. The translational and rotational motions of the wild-type and mutant cells do not differ significantly. Optical forces alone do not play a vital role in the rotational dynamics of this cellular motor, making them useful as probes of the internal dynamics without external influence.

Measuring erythrocyte deformability with fluorescence, fluid forces, and optical trapping.

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.

Suppression of ultrafast supercontinuum generation in a salivary protein.

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 alpha-amylase, a potential stress marker in human saliva. The continuum suppression capacity is attributed to the electron scavenging property of the protein.