Venkatesh Gopal

Measuring cellular structure at submicrometer scale with light scattering spectroscopy

We present a novel instrument for imaging the angular distributions of light backscattered by biological cells and tissues. The intensities in different regions of the image are due to scatterers of different sizes. We exploit this to study scattering from particles smaller than the wavelength of light used, even when they are mixed with larger particles. We show that the scattering from subcellular structure in both normal and cancerous human cells is best fitted to inverse power-law distributions for the sizes of the scattering objects, and propose that the distribution of scattering objects may be different in normal versus cancerous cells.

The morphology of the rat vibrissal array: a model for quantifying spatiotemporal patterns of whisker-object contact.

In all sensory modalities, the data acquired by the nervous system is shaped by the biomechanics, material properties, and the morphology of the peripheral sensory organs. The rat vibrissal (whisker) system is one of the premier models in neuroscience to study the relationship between physical embodiment of the sensor array and the neural circuits underlying perception. To date, however, the three-dimensional morphology of the vibrissal array has not been characterized. Quantifying array morphology is important because it directly constrains the mechanosensory inputs that will be generated during behavior. These inputs in turn shape all subsequent neural processing in the vibrissal-trigeminal system, from the trigeminal ganglion to primary somatosensory (“barrel”) cortex. Here we develop a set of equations for the morphology of the vibrissal array that accurately describes the location of every point on every whisker to within ±5% of the whisker length. Given only a whiskers identity (row and column location within the array), the equations establish the whiskers two-dimensional (2D) shape as well as three-dimensional (3D) position and orientation. The equations were developed via parameterization of 2D and 3D scans of six rat vibrissal arrays, and the parameters were specifically chosen to be consistent with those commonly measured in behavioral studies. The final morphological model was used to simulate the contact patterns that would be generated as a rat uses its whiskers to tactually explore objects with varying curvatures. The simulations demonstrate that altering the morphology of the array changes the relationship between the sensory signals acquired and the curvature of the object. The morphology of the vibrissal array thus directly constrains the nature of the neural computations that can be associated with extraction of a particular object feature. These results illustrate the key role that the physical embodiment of the sensor array plays in the sensing process.

Precision Rotation Sensing and Interferometry Using Slow Lght

We show how a combination of pulsed and CW excitation enhance the sensitivity of a Mach-Zehnder interferometer and a Sagnac rotation sensor by the group index, which can be as high as ten million

Imaging and measurement of cell structure and organization with submicron accuracy using light scattering spectroscopy

Light scattering spectroscopy (LSS) is a promising optical technique developed for quantitative characterization of tissue morphology as well as in vivo detection and diagnosis of disease such as early cancer. LSS employs a wavelength dependent component of light scattered by epithelial cells and other tissues to obtain information about subcellular structure. We present two novel modalities of LSS, LSS imaging and scattering angle sensitive LSS (a/LSS). LSS imaging provides quantitative information about the epithelial cell nuclei, such as nuclear size, degree of pleomorphism, hyperchromasia, and amount of chromatin. It allows mapping these histological properties over wide areas of epithelial lining. We show that LSS imaging can be used to detect precancerous lesions in optically accessible organs. Using a/LSS, which enables characterization of tissue components with sizes smaller than the wavelength of light, we show that the number of subcellular components with the sizes between 30 nm and few microns scales with the size according to an inverse power-law. We show that the size distribution exponent is an important parameter characterizing tissue organization, for example the balance between stochasticity and order, and has a potential to be applicable for early cancer diagnosis and characterization.

Light-shift imbalance induced blockade of collective excitations beyond the lowest order

Current proposals focusing on neutral atoms for quantum computing are mostly based on using single atoms as quantum bits (qubits), while using cavity induced coupling or dipole–dipole interaction for two-qubit operations. An alternative approach is to use atomic ensembles as quantum bits. However, when an atomic ensemble is excited, by a laser beam matched to a two-level transition (or a Raman transition) for example, it leads to a cascade of many states as more and more photons are absorbed [R.H. Dicke, Phys. Rev. 93 (1954) 99]. In order to make use of an ensemble as a qubit, it is necessary to disrupt this cascade, and restrict the excitation to the absorption (and emission) of a single photon only. Here, we show how this can be achieved by using a new type of blockade mechanism, based on the light-shift imbalance (LSI) in a Raman transition. We describe first a simple example illustrating the concept of light-shift imbalance induced blockade (LSIIB) using a multi-level structure in a single atom, and show verifications of the analytic prediction using numerical simulations. We then extend this model to show how a blockade can be realized by using LSI in the excitation of an ensemble. Specifically, we show how the LSIIB process enables one to treat the ensemble as a two-level atom that undergoes fully deterministic Rabi oscillations between two collective quantum states, while suppressing excitations of higher order collective states.

Controllable anomalous dispersion and group index nulling via bi-frequency Raman gain for ultraprecision rotation sensing

Using a bi-frequency pumped Raman amplifier in a Rb vapor cell, we demonstrate experimentally the condition for achieving a null group index, necessary for fast-light enhanced rotation sensing in a passive cavity Sagnac gyroscope.

Visualizing the invisible: the construction of three low-cost schlieren imaging systems for the undergraduate laboratory

We describe the construction and operation of three low-cost schlieren imaging systems that can be fabricated using surplus optics and 80/20, an aluminium extrusion based construction system. Each system has a different optical configuration. The low cost and ease of construction makes these systems highly suitable for high-school and undergraduate laboratories. Undergraduate students responded enthusiastically to the experience of assembling and operating these systems. This experience also served as an introduction to issues in optical design, helping the students gain an intuition for geometrical optics.

Deterministic quantum storage, communication and computing with atomic ensembles using light-shift imbalance induced blockade of collective excitations

Collective excitation, the leading technique for quantum information processing, suffers from limitations due to higher order excitations. We describe a blockade mechanism, based on light-shift imbalance in a Raman transition that eliminates this shortcoming.