Samarendra K. Mohanty

Measurement of optical transport properties of normal and malignant human breast tissue

We report measurement of optical transport parameters of normal and malignant (ductal carcinoma) human breast tissue. A spatially resolved steady-state diffuse reflectance technique was used for measurement of the reduced scattering coefficient (mu(s)?) and the absorption coefficient (mu(a)) of the tissue. The anisotropy parameter of scattering (g) was estimated by goniophotometric measurements of the scattering phase function. The values of mu(s)? and mu(a) for malignant breast tissue were observed to be larger than those for normal breast tissue over the wavelength region investigated (450-650 nm). Further, by using both the diffuse reflectance and the goniophotometric measurements, we estimated the Mie equivalent average radius of tissue scatterers to be larger in malignant tissue than in normal tissue.

Comet Assay Measurements of DNA Damage in Cells by Laser Microbeams and Trapping Beams with Wavelengths Spanning a Range of 308 nm to 1064 nm

Abstract Mohanty, S. K., Rapp, A., Monajembashi, S., Gupta, P. K. and Greulich, K. O. Comet Assay Measurements of DNA Damage in Cells by Laser Microbeams and Trapping Beams with Wavelengths Spanning a Range of 308 nm to 1064 nm. Radiat. Res. 157, 378–385 (2002). DNA damage induced in NC37 lymphoblasts by optical tweezers with a continuous-wave Ti:sapphire laser and a continuous-wave Nd:YAG laser (60–240 mW; 10–50 TJ/m2; 30–120 s irradiation) was studied with the comet assay, a single-cell technique used to detect DNA fragmentation in genomes. Over the wavelength range of 750–1064 nm, the amount of damage in DNA peaks at around 760 nm, with the fraction of DNA damage within the range of 750–780 nm being a factor of two larger than the fraction of DNA damage within the range of 800–1064 nm. The variation in DNA damage was not significant over the range of 800–1064 nm. When the logarithm of damage thresholds measured in the present work, as well as values reported previously in the UV range, was plotted as a function of wavelength, a dramatic wavelength dependence became apparent. The damage threshold values can be fitted on two straight lines, one for continuous-wave sources and the other for pulsed sources, irrespective of the type of source used (e.g. classical lamp or laser). The damage threshold around 760 nm falls on the line extrapolated from values for UV-radiation-induced damage, while the data for 800–1064 nm fall on a line that has a different slope. The change in the slope between 320 and 340 nm observed earlier is consistent with a well-known change in DNA-damaging mechanisms. The change observed around 780 nm is therefore suggestive of a further change in the mechanism(s). The data from this work together with our previous measurements provide, to the best of our knowledge, the most comprehensive view available of the DNA damage produced by microfocused light.

Digital holographic microscopy for quantitative cell dynamic evaluation during laser microsurgery

Digital holographic microscopy allows determination of dynamic changes in the optical thickness profile of a transparent object with sub-wavelength accuracy. Here, we report a quantitative phase laser microsurgery system for evaluation of cellular/ sub-cellular dynamic changes during laser micro-dissection. The proposed method takes advantage of the precise optical manipulation by the laser microbeam and quantitative phase imaging by digital holographic microscopy with high spatial and temporal resolution. This system will permit quantitative evaluation of the damage and/or the repair of the cell or cell organelles in real time.

Dynamics of Interaction of RBC with optical tweezers.

It has recently been shown that a red blood cell (RBC) can be used as optically driven motor. The mechanism for rotation is however not fully understood. While the dependence on osmolarity of the buffer led us to conclude that the osmolarity dependent changes in shape of the cell are responsible for the observed rotation, role of ion gradients and folding of RBC to a rod shape has been invoked by Dharmadhikari et al to explain their observations. In this paper we report results of studies undertaken to understand the dynamics of a RBC when it is optically tweezed. The results obtained support our earlier conjecture that osmolarity dependent changes in shape of the cell are responsible for the observed rotation.

Self-rotation of red blood cells in optical tweezers: prospects for high throughput malaria diagnosis

A simple and sensitive approach for detection of malarial parasite in blood samples is demonstrated. The approach exploits our finding that, in hypertonic buffer, a normal red blood cell (RBC) rotates by itself when trapped by an optical tweezers. The rotational speed increases linearly at lower trap-beam powers and more rapidly at higher powers. In contrast, under the same experimental conditions, RBC having a malarial parasite does not rotate. The rotational speeds of other RBCs from malaria-infected sample are of an order of magnitude less than that for normal RBC and also increase much more slowly with an increase in trap beam power than that for normal RBC. The difference in rotational speeds could be exploited for the diagnosis of malaria.

Optical binding between dielectric particles

We report observation of optical binding between two dielectric particles with dimensions less than the wavelength of the interacting light. The observed dependence of the separation of optically bound Rayleigh particles on the polarization of the trapping beam is in agreement with earlier theoretical predictions.

In-Depth Activation of Channelrhodopsin 2-Sensitized Excitable Cells with High Spatial Resolution Using Two-Photon Excitation with a Near-Infrared Laser Microbeam

We used two-photon excitation with a near-infrared (NIR) laser microbeam to investigate activation of channelrhodopsin 2 (ChR2) in excitable cells for the first time to our knowledge. By measuring the fluorescence intensity of the calcium (Ca) indicator dye, Ca orange, at different wavelengths as a function of power of the two-photon excitation microbeam, we determined the activation potential of the NIR microbeam as a function of wavelength. The two-photon activation spectrum is found to match measurements carried out with single-photon activation. However, two-photon activation is found to increase in a nonlinear manner with the power density of the two-photon laser microbeam. This approach allowed us to activate different regions of ChR2-sensitized excitable cells with high spatial resolution. Further, in-depth activation of ChR2 in a spheroid cellular model as well as in mouse brain slices was demonstrated by the use of the two-photon NIR microbeam, which was not possible using single-photon activation. This all-optical method of identification, activation, and detection of ChR2-induced cellular activation in genetically targeted cells with high spatial and temporal resolution will provide a new method of performing minimally invasive in-depth activation of specific target areas of tissues or organisms that have been rendered photosensitive by genetic targeting of ChR2 or similar photo-excitable molecules.

Organization of microscale objects using a microfabricated optical fiber

We demonstrate the use of a single fiber-optic axicon device for organization of microscopic objects using longitudinal optical binding. Further, by manipulating the shape of the fiber tip, part of the emanating light was made to undergo total internal reflection in the conical tip region, enabling near-field trapping. Near-field trapping resulted in trapping and self-organization of long chains of particles along azimuthal directions (in contrast to the axial direction, observed in the case of large tip cone angle far-field trapping).

Controlled induction, enhancement, and guidance of neuronal growth cones by use of line optical tweezers

We report an optical tweezers based approach for efficient and controlled manipulation of neuronal growth cones. The approach exploits asymmetric transverse gradient force created in a line optical tweezers to transport actin monomers in the desired growth direction. With this approach induction of artificial growth cones from the neuronal cell body and enhancement of the growth rate of the natural growth cones have been achieved. The use of this approach to bring two growth cones into close proximity for establishing a neuronal connection is also discussed.

In depth fiber optic trapping of low-index microscopic objects

We demonstrate that a focused beam through a microaxicon built on the tip of a single mode optical fiber can trap low-index objects at much larger depths as compared to the vortex beam tweezers generated using high numerical aperture microscope objectives. The measured transverse trapping force for low-index objects in Mie regime was found to depend on particle size and on distance of trapped objects from the fiber tip. While axial movement of trapped low-index objects was achieved by variation in trap beam power, transportation in three dimensions was achieved by maneuvering the fiber.