arXiv subjects

Hema Ramachandran

Publications and source records attributed to Hema Ramachandran.

At least 19 recordsLinked to original sources

arXiv2021

Imaging through fog using quadrature lock-in discrimination

We report experiments conducted in the field in the presence of fog, that were aimed at imaging under poor visibility. By means of intensity modulation at the source and two-dimensional quadrature lock-in detection by software at the receiver, a significant enhancement of the contrast-to-noise ratio was achieved in the imaging of beacons over hectometric distances. Further by illuminating the field of view with a modulated source, the technique helped reveal objects that were earlier obscured due to multiple scattering of light. This method, thus, holds promise of aiding in various forms of navigation under poor visibility due to fog.

eess.IV↗

arXiv2019

An all-optical technique enables instantaneous single-shot demodulation of images at high frequency

High-frequency demodulation of wide area optical signals in a snapshot manner is a technological challenge that, if solved, could open tremendous perspectives in 3D imaging, free-space communications, or even ballistic photon imaging in highly scattering media. We present here a novel snapshot quadrature demodulation imaging technique, capable of estimating the amplitude and phase of light modulated from a single frame acquisition, without synchronization of emitter and receiver, and with the added capability of continuous frequency tuning. This all-optical setup relies on an electro-optic crystal that acts as a fast sinusoidal optical transmission gate and which, when inserted in an optimized optical architecture, allows for four quadrature image channels to be recorded simultaneously with any conventional camera. We report the design, experimental validation and examples of potential applications of such a wide-field quadrature demodulating system that allowed snapshot demodulation of images with good spatial resolution and continuous frequency selectivity, at modulation frequencies up to 500 kHz; no fundamental impediment in modulating/demodulating in the range 100-1000 MHz range is foreseen.

physics.ins-det↗

arXiv2018

Measurements of spin properties of atomic systems in and out of equilibrium via noise spectroscopy

We explore the applications of spin noise spectroscopy (SNS) for detection of the spin properties of atomic ensembles in and out of equilibrium. In SNS, a linearly polarized far-detuned probe beam on passing through an ensemble of atomic spins acquires the information of the spin correlations of the system which is extracted using its time-resolved Faraday-rotation noise. We measure various atomic, magnetic and sub-atomic properties as well as perform precision magnetometry using SNS in rubidium atomic vapor in thermal equilibrium. Thereafter, we manipulate the relative spin populations between different ground state hyperfine levels of rubidium by controlled optical pumping which drives the system out of equilibrium. We then apply SNS to probe such spin imbalance nonperturbatively. We further use this driven atomic vapor to demonstrate that SNS can have better resolution than typical absorption spectroscopy in detecting spectral lines in the presence of various spectral broadening mechanisms.

physics.atom-ph↗

arXiv2016

Theoretical optimal modulation frequencies for scattering parameter estimation and ballistic photon filtering in diffusive media

The efficiency of using intensity modulated light for estimation of scattering properties of a turbid medium and for ballistic photon discrimination is theoretically quantified in this article. Using the diffusion model for modulated photon transport and considering a noisy quadrature demodulation scheme, the minimum-variance bounds on estimation of parameters of interest are analytically derived and analyzed. The existence of a variance-minimizing optimal modulation frequency is shown and its evolution with the properties of the intervening medium is derived and studied. Furthermore, a metric is defined to quantify the efficiency of ballistic photon filtering which may be sought when imaging through turbid media. The analytical derivation of this metric shows that the minimum modulation frequency required to attain significant ballistic discrimination depends only on the reduced scattering coefficient of the medium in a linear fashion for a highly scattering medium.

physics.optics↗

arXiv2015

Real-time quadrature lock-in discrimination imaging through scattering media

Numerous everyday situations like navigation, medical imaging and rescue operations require viewing through optically inhomogeneous media. This is a challenging task as photons, instead of traversing ballistically, propagate predominantly diffusively due to random multiple scattering off the inhomogenieties. Imaging {\it in real-time} with ballistic light under continuous-wave illumination is even more challenging due to the extremely weak signal, necessitating voluminous data-processing. In this paper, we report imaging through strongly scattering media in real-time and at rates several times the critical flicker frequency of the eye, so that motion is perceived as continuous. Two factors contributed to the speedup of {\it more than three orders of magnitude} over conventional techniques - the use of a simplified algorithm enabling processing of data on the fly, and the utilisation of task and data parallelization capabilities of typical desktop computers. The extreme simplicity and low cost promises great utility of this technique. As an example, navigation under poor visibility is examined.

physics.optics↗

arXiv2015

Time-delayed intensity-interferometry of the emission from ultracold atoms in a steady-state magneto-optical trap

An accurate measurement of the bunching of photons in the fluorescent emission from an ultracold ensemble of thermal 87Rb atoms in a steady-state magneto-optical trap is presented. Time-delayed-intensity-interferometry (TDII) performed with a 5-nanosecond time resolution yielded a second-order intensity correlation function that has the ideal value of 2 at zero delay, and that shows coherent Rabi oscillations of upto 5 full periods - much longer than the spontaneous emission lifetime of the excited state of Rb. The oscillations are damped out by ~150ns, and thereafter, as expected from a thermal source, an exponential decay is observed, enabling the determination of the temperature of the atomic ensemble. Values so obtained compare well with those determined by standard techniques. TDII thus enables a quantitative study of the coherent and incoherent dynamics, even of a large thermal ensemble of atomic emitters.

physics.atom-ph↗

arXiv2013

Classical Light Sources with Tunable Temporal Coherence and Tailored Photon Number Distributions

We demonstrate the generation of classical incoherent light with electronic control over its temporal characteristics and photon number distribution. The tunability of the temporal coherence is shown, under both classical and quantum detection, through second order correlation ($G^2(τ)$) measurements. The tailoring of desired classical photon number distributions is illustrated by creating two representative light sources - one thermal and the other a specific classical, non-Gaussian state. Such generation of classical light sources, quite different from existing natural light sources, is likely to be a useful resource in quantum information processing. As a particular application in this direction we outline how a non-Gaussian state generated in this manner may be mixed with an appropriate non-classical Gaussian state at a beamsplitter, to generate non-Gaussian entanglement.

quant-ph↗

arXiv2012

Optical phase noise engineering via acousto-optic interaction and its interferometric applications

We exercise rapid and fine control over the phase of light by transferring digitally gen- erated phase jumps from radio frequency (rf) electrical signals onto light by means of acousto-optic interaction. By tailoring the statistics of phase jumps in the electrical signal and thereby engineering the optical phase noise, we manipulate the visibil- ity of interference fringes in a Mach-Zehnder interferometer that incorporates two acousto-optic modulators. Such controlled dephasing finds applications in modern experiments involving the spread or diffusion of light in an optical network. Further, we analytically show how engineered partial phase noise can convert the dark port of a stabilised interferometer to a weak source of highly correlated photons.

physics.optics↗

arXiv2012

Classical light analogue of the nonlocal Aharonov-Bohm effect

We demonstrate the existence of a non-local geometric phase in the intensity-intensity correlations of classical incoherent light, that is not seen in the lower order correlations. This two-photon Pancharatnam phase was observed and modulated in a Mach-Zehnder interferometer. Using acousto-optic interaction, independent phase noise was introduced to light in the two arms of the interferometer to create two independent incoherent classical sources from laser light. The experiment is the classical optical analogue of the multi-particle Aharonov-Bohm effect. As the trajectory of light over the Poincare sphere introduces a phase shift observable only in the intensity-intensity correlation, it provides a means of deflecting the two-photon wavefront, while having no effect on single photons.

quant-ph↗

arXiv2011

An information theoretic study of number-phase complementarity in a four level atomic system

We study number-phase uncertainty in a laser-driven, effectively four-level atomic system under electromagnetically induced transparency (EIT) and coherent population trapping (CPT). Uncertainty is described using (entropic) knowledge of the two complementary variables, namely, number and phase, where knowledge is defined as the relative entropy with respect to a uniform distribution. In the regime where the coupling and probe lasers are approximately of equal strength, and the atom exists in a CPT state, there is coherence between the ground states, and correspondingly large phase knowledge and lower number knowledge. The situation is the opposite in the case where coupling laser is much stronger and the atom exists in an EIT state. We study these effects also in the presence of a higher-order nonlinear absorption, which is seen to produce a dephasing effect.

quant-ph↗

arXiv2011

Light scattering from a magnetically tunable dense random medium with weak dissipation : ferrofluid

We present a semi-phenomenological treatment of light transmission through and its reflection from a ferrofluid, which we regard as a magnetically tunable system of dense random dielectric scatterers with weak dissipation. Partial spatial ordering is introduced by the application of a transverse magnetic field that superimposes a periodic modulation on the dielectric randomess. This introduces Bragg scattering which effectively enhances the scattering due to disorder alone, and thus reduces the elastic mean free path towards Anderson localization. Our theoretical treatment, based on invariant imbedding, gives a simultaneous decrease of transmission and reflection without change of incident linear polarisation as the spatial order is tuned magnetically to the Bragg condition, namely the light wave vector being equal to half the Bragg vector (Q). Our experimental observations are in qualitative agreement with these results. We have also given expressions for the transit (sojourn) time of light and for the light energy stored in the random medium under steady illumination. The ferrofluid thus provides an interesting physical realization of effectively a "Lossy Anderson-Bragg" (LAB) cavity with which to study the effect of the interplay of spatial disorder, partial order and weak dissipation on light transport. Given the current interest in propagation, optical limiting and storage of light in ferrofluids, the present work seems topical.

physics.optics↗

arXiv2009

Localization of light by magnetically tuned correlated disorder: Trapping of light in ferrofluids

The mean free path of light ($l^*$) calculated for elastic scattering on a system of nanoparticles with spatially correlated disorder is found to have a minimum when the correlation length is of the order of the wavelength of light. For a typical choice of parameters for the scattering system, this minimum mean free path ($l^*_{min}$) turns out to satisfy the Ioffe-Regel criterion for wave localization, $l^*_{min} \sim λ$, over a range of the correlation length, defining thus a stop-band for light transmission. It also provides a semi-phenomenological explanation for several interesting findings reported recently on the transmission/ reflection and the trapping/storage of light in a magnetically tunable ferrofluidic system. The subtle effect of structural anisotropy, induced by the external magnetic field on the scattering by the medium, is briefly discussed in physical terms of the anisotropic Anderson localization.

cond-mat.dis-nn↗

arXiv2007

Possible EIT-like effects in strong-field photodissociation of carbon disulphide

CS$_2$ molecules are spatially aligned upon irradiation by intense (1-100 TW cm$^{-2}$), 35 ps pulses of 512 nm or 355 nm light. When both colours are simultaneously present, spatial alignment disappears. We draw analogy with weak-field illumination of an atom by two colours wherein EIT (electromagnetically induced transparency) inhibits absorption by virtue of dipoles not being induced in two-colour fields. In the strong field scenario, molecular alignment is a consequence of a dipole being induced in the direction of the applied electric field. It follows, therefore, that when dipoles are not induced, no spatial alignment is to be expected.

physics.atom-ph↗

arXiv2005

Levy statistical fluctuations from a Random Amplifying Medium

We report the studies of emission from a novel random amplifying medium that we term a ``Levy Laser'' due to the non-Gaussian statistical nature of its emission over the ensemble of random realizations. It is observed that the amplification is dominated by certain improbable events that are ``larger than rare'', which give the intensity statistics a Levy like ``fat tail''. This, to the best of our knowledge, provides the first experimental realization of Levy flight in optics in a random amplifying medium.

physics.data-an↗

arXiv2004

Fluorescence from doubly driven four-level atoms - A density matrix approach

The unusually narrow features in the fluorescence from Rubidium-85 driven by cooling and repumper laser fields, reported in an earlier experiment [1] are explained on the basis of a four-level density matrix calculation. Quantum effects alter the efficiency of atom transfer by the probe (repumper) laser to the levels connected by the pump (cooling) laser. This combined with the double resonance condition [1], results in velocity selection from co-propagating and counter propagating probe and pump beams resulting in narrow fluorescence peaks from a thermal gas at room temperature.

cond-mat↗

arXiv2004

Observation of narrow fluorescence from doubly driven four-level atoms at room temperature

Unusually narrow fluorescence peaks are seen from Rubidium-85 atoms under the action of two driving laser fields that are in a three dimensional molasses configuration. One of the lasers is held at a fixed detuning from the "cooling" transition, while the other is scanned across the "repumping" transitions. The fluorescence peaks are split into symmetric pairs, with the seperation within a pair increasing with the detuning of the cooling laser. For large detunings additional small peaks are seen. A simple model is proposed to explain these experimental observations.

cond-mat↗

arXiv1999

Density-Matrix approach to a Strongly Coupled Two-Component Bose-Einstein Condensate

The time evolution equations for average values of population and relative phase of a strongly coupled two component BEC is derived analytically. The two components are two hyper-fine states coupled by an external laser that drives fast Rabi oscillations between these states. Specifically, this derivation incorporates the two-mode model proposed in [1] for the strongly coupled hyper-fine states of Rb. The fast Rabi cycle is averaged out and rate equations are derived that represents the slow dynamics of the system. These include the collapse and revival of Rabi oscillations and their subsequent dependence on detuning and trap displacement as reported in experiments of [1]. A proposal to create stable vortices is also given.

cond-mat↗

arXiv1999

The backscattering of polarised light from turbid media - An analysis of the azimuthal intensity variations and its implications for the position of the source of diffusing radiation

We study the azimuthal variation in the backscattered intensity that is seen when polarised light is scattered by a turbid medium. We present experimental observations of these intensity variations in colloidal suspensions over a wide range of optical densities. For a medium composed of spherical scatterers, we have developed using Mie scattering theory and Monte Carlo simulations of photon transport, a model which calculates the constant intensity contours of the backscattered intensity. Comparisons of calculated and experimentally obtained contours show very good agreement. To our knowledge, this is the first model to provide a quantitative comparison with experimental data. Close to the exact backscattering direction, where we have made our intensity measuements, we show that the patterns are formed by what we have called `reflected snake photons'. These are photons that have been backscattered once and have maintained their direction of propagation thereafter until they exit the medium. We also find that the reflected snake photons originate within a depth of about $4l^{*}$ from the point of entry of the incident beam, where $l^{*}$ is the photon transport mean free path. Further, in a novel approach, we have used these patterns as a probe of the assumptions underlying the diffusion approximation and present new results on the position of the apparent source of diffusing radiation within the medium. Possible applications are also discussed.

cond-mat.dis-nn↗