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Papers by Salla Gangi Reddy

Research paper thumbnail of Pancharatnam phase in non-separable states of light

Journal of the Optical Society of America B, 2016

We generate the non-separable state of polarization and orbital angular momentum (OAM) using a la... more We generate the non-separable state of polarization and orbital angular momentum (OAM) using a laser beam. The generated state undergoes a cyclic polarization evolution which introduces a Pancharatnam geometric phase to the polarization state and in turn a relative phase in the non-separable state. We experimentally study the violation of Bell-CHSH inequality for different Pancharatnam phases introduced by various cyclic polarization evolutions with linear and circular states as measurement bases. While measuring in linear bases, the Bell-CHSH parameter oscillates with Pancharatnam phase. One can overcome this dependence by introducing a relative phase in one of the projecting state. However for measurement in circular bases, the Pancharatnam phase does not affect the Bell-CHSH violation.

Research paper thumbnail of Selecting the pre-detection characteristics for fiber coupling of parametric down-converted biphoton modes

Optics Communications, 2017

Research paper thumbnail of Generating arbitrary cebits on the orbital angular momentum poincaré sphere

International Journal of Quantum Information, 2016

Research paper thumbnail of Non-diffracting speckles of a perfect vortex beam

Research paper thumbnail of Non-coaxial superposition of vector vortex beams

Applied Optics, 2016

Vector vortex beams are classified into four types depending upon spatial variation in their pola... more Vector vortex beams are classified into four types depending upon spatial variation in their polarization vector. We have generated all four of these types of vector vortex beams by using a modified polarization Sagnac interferometer with a vortex lens. Further, we have studied the non-coaxial superposition of two vector vortex beams. It is observed that the superposition of two vector vortex beams with same polarization singularity leads to a beam with another kind of polarization singularity in their interaction region. The results may be of importance in ultrahigh security of the polarization-encrypted data that utilizes vector vortex beams and multiple optical trapping with non-coaxial superposition of vector vortex beams. We verified our experimental results with theory.

Research paper thumbnail of Polarization state transformation using two quarter wave plates: application to Mueller polarimetry

Applied Optics, 2016

We show that there are a number of ways to transform an arbitrary polarization state into another... more We show that there are a number of ways to transform an arbitrary polarization state into another with just two quarter wave plates (QWPs). We have verified this geometrically using the trajectories of the initial and final polarization states corresponding to all the fast axis orientations of a QWP on the Poincaré sphere. The exact analytical expression for the locus of polarization states has also been given, and describes the trajectory. An analytical treatment of the equations obtained through matrix operations corresponding to the transformation supports the geometrical representation. This knowledge can be used to obtain the Mueller matrix by just using QWPs, which has been shown experimentally by exploiting projections of the output states on the input states.

Research paper thumbnail of Violation of Bell's inequality for phase-singular beams

Physical Review A, 2015

We have experimentally verified the violation of Bell's inequality for continuous variables of po... more We have experimentally verified the violation of Bell's inequality for continuous variables of position and momentum for beams with phase singularities. Given that Wigner distribution function (WDF) provides information about position and momentum simultaneously, it has been used to derive Bell's inequality for phase singular beams [Phys. Rev. A 88, 013830 (2013)]. We have measured the WDF taking recourse to Fourier transform of the two point correlation function for vortex beams. PACS numbers: 42.50.Tx, 42.25.Kb, 03.65.Ud Optical vortices, phase singularities of the field, are observed as dark spots in bright background. Being topological structures, they are robust and find applications in free space [1] and fiber communication [2]. For the vortex of topological charge n, the azimuthal phase variation is 2πn in a full rotation around the dark spot . The sense of rotation of phase provides the sign of its charge. The topological charge n can be considered as an important parameter for such beams. One of the main characteristics of these beams is that they carry an orbital angular momentum (OAM) of n per photon . The OAM carrying property has raised extensive interest in scientific community due to its unique applications in the fields of particle manipulation [5] and quantum information [6-8]. Vortex beams have been experimentally realized both in lasers [9] as well as electron beams . These beams form an infinite dimensional basis for applications such as quantum computation and cryptography . Moreover, increase in information entropy with the order of vortices can be utilised to encode more amount of information in these structures .

Research paper thumbnail of Divergence of optical vortex beams

Applied Optics, 2015

We show, both theoretically and experimentally, that the propagation of optical vortices in free ... more We show, both theoretically and experimentally, that the propagation of optical vortices in free space can be analyzed by using the width (w(z)) of the host Gaussian beam and the inner and outer radii of the vortex beam at the source plane (z = 0) as defined in Optics Letters 39, 4364-4367 (2014). We also studied the divergence of vortex beams, considered as the rate of change of inner or outer radius with the propagation distance, and found that it varies with the order in the same way as that of the inner and outer radii at zero propagation distance. These results may be useful in designing optical fibers for orbital angular momentum modes that play a crucial role in quantum communication.

Research paper thumbnail of Generalized orbital angular momentum Poincaré sphere

International Conference on Optics and Photonics 2015, 2015

Research paper thumbnail of Propagation of an arbitrary vortex pair through an astigmatic optical system and determination of its topological charge

Journal of the Optical Society of America A, 2014

We embed a pair of vortices with different topological charges in a Gaussian beam and study its e... more We embed a pair of vortices with different topological charges in a Gaussian beam and study its evolution through an astigmatic optical system, a tilted lens. The propagation dynamics is explained by a closed-form analytical expression. Furthermore, we show that a careful examination of the intensity distribution at a predicted position past the lens can determine the charge present in the beam. To the best of our knowledge, our method is the first non-interferometric technique to measure the charge of an arbitrary vortex pair. Our theoretical results are well supported by experimental observations.

Research paper thumbnail of Spatial intensity distribution of optical vortex beams

Workshop on Recent Advances in Photonics (WRAP), 2013

Research paper thumbnail of Higher order optical vortices and formation of speckles

Optics Letters, 2014

We have experimentally generated higher order optical vortices and scattered them through a groun... more We have experimentally generated higher order optical vortices and scattered them through a ground glass plate that results in speckle formation. Intensity autocorrelation measurements of speckles show that their size decreases with an increase in the order of the vortex. It implies an increase in the angular diameter of the vortices with their order. The characterization of vortices in terms of their annular bright ring also helps us to understand these observations. The results may find applications in stellar intensity interferometry and thermal ghost imaging.

Research paper thumbnail of Experimental generation of ring-shaped beams with random sources

Optics Letters, 2013

We have experimentally reproduced ring shaped beams from the scattered Laguerre-Gaussian and Bess... more We have experimentally reproduced ring shaped beams from the scattered Laguerre-Gaussian and Bessel-Gaussian beams. A rotating ground glass plate is used as a scattering medium and a plano convex lens collects the scattered light to generate ring shaped beams at the Fourier plane. The obtained experimental results are supported with the numerical results and are in good agreement with the theoretical results proposed by Wang, Cai and Korotkova (Opt. Exp. 17, 22366 (2009)).

Research paper thumbnail of Spatial distribution of spontaneous parametric down-converted photons for higher order optical vortices

Optics Communications, 2014

We make a source of entangled photons (SEP) using spontaneous parametric down-conversion (SPDC) i... more We make a source of entangled photons (SEP) using spontaneous parametric down-conversion (SPDC) in a non-linear crystal and study the spatial distribution of photon pairs obtained through down-conversion of different modes of light including higher order vortices. We have observed that for the Gaussian pump, the thickness of the SPDC ring varies linearly with the radius of pump beam. While for the optical vortex beams, two concentric SPDC rings are formed, however, it happens above a critical radius of the vortex beam. The FWHM of SPDC rings increases with increase in the order of optical vortex beams. The presence of a critical beam width for the optical vortices as well as the observed full width at half maximum of the SPDC rings are supported by our numerical results also.

Research paper thumbnail of Measuring the Mueller matrix of an arbitrary optical element with a universal SU(2) polarization gadget

Journal of the Optical Society of America A, 2014

We propose a new method for determining the Mueller matrix of an arbitrary optical element and ve... more We propose a new method for determining the Mueller matrix of an arbitrary optical element and verify it with three known optical elements. This method makes use of two universal SU(2) polarization gadgets to obtain the projection matrix directly from the experiment. It allows us to determine the Mueller matrix without precalibration of the setup, since the generated polarization states are fully determined by the azimuths of the wave plates. We calculate errors in determining the Mueller matrix and compare with other techniques.

Research paper thumbnail of Pancharatnam phase in non-separable states of light

Journal of the Optical Society of America B, 2016

We generate the non-separable state of polarization and orbital angular momentum (OAM) using a la... more We generate the non-separable state of polarization and orbital angular momentum (OAM) using a laser beam. The generated state undergoes a cyclic polarization evolution which introduces a Pancharatnam geometric phase to the polarization state and in turn a relative phase in the non-separable state. We experimentally study the violation of Bell-CHSH inequality for different Pancharatnam phases introduced by various cyclic polarization evolutions with linear and circular states as measurement bases. While measuring in linear bases, the Bell-CHSH parameter oscillates with Pancharatnam phase. One can overcome this dependence by introducing a relative phase in one of the projecting state. However for measurement in circular bases, the Pancharatnam phase does not affect the Bell-CHSH violation.

Research paper thumbnail of Selecting the pre-detection characteristics for fiber coupling of parametric down-converted biphoton modes

Optics Communications, 2017

Research paper thumbnail of Generating arbitrary cebits on the orbital angular momentum poincaré sphere

International Journal of Quantum Information, 2016

Research paper thumbnail of Non-diffracting speckles of a perfect vortex beam

Research paper thumbnail of Non-coaxial superposition of vector vortex beams

Applied Optics, 2016

Vector vortex beams are classified into four types depending upon spatial variation in their pola... more Vector vortex beams are classified into four types depending upon spatial variation in their polarization vector. We have generated all four of these types of vector vortex beams by using a modified polarization Sagnac interferometer with a vortex lens. Further, we have studied the non-coaxial superposition of two vector vortex beams. It is observed that the superposition of two vector vortex beams with same polarization singularity leads to a beam with another kind of polarization singularity in their interaction region. The results may be of importance in ultrahigh security of the polarization-encrypted data that utilizes vector vortex beams and multiple optical trapping with non-coaxial superposition of vector vortex beams. We verified our experimental results with theory.

Research paper thumbnail of Polarization state transformation using two quarter wave plates: application to Mueller polarimetry

Applied Optics, 2016

We show that there are a number of ways to transform an arbitrary polarization state into another... more We show that there are a number of ways to transform an arbitrary polarization state into another with just two quarter wave plates (QWPs). We have verified this geometrically using the trajectories of the initial and final polarization states corresponding to all the fast axis orientations of a QWP on the Poincaré sphere. The exact analytical expression for the locus of polarization states has also been given, and describes the trajectory. An analytical treatment of the equations obtained through matrix operations corresponding to the transformation supports the geometrical representation. This knowledge can be used to obtain the Mueller matrix by just using QWPs, which has been shown experimentally by exploiting projections of the output states on the input states.

Research paper thumbnail of Violation of Bell's inequality for phase-singular beams

Physical Review A, 2015

We have experimentally verified the violation of Bell's inequality for continuous variables of po... more We have experimentally verified the violation of Bell's inequality for continuous variables of position and momentum for beams with phase singularities. Given that Wigner distribution function (WDF) provides information about position and momentum simultaneously, it has been used to derive Bell's inequality for phase singular beams [Phys. Rev. A 88, 013830 (2013)]. We have measured the WDF taking recourse to Fourier transform of the two point correlation function for vortex beams. PACS numbers: 42.50.Tx, 42.25.Kb, 03.65.Ud Optical vortices, phase singularities of the field, are observed as dark spots in bright background. Being topological structures, they are robust and find applications in free space [1] and fiber communication [2]. For the vortex of topological charge n, the azimuthal phase variation is 2πn in a full rotation around the dark spot . The sense of rotation of phase provides the sign of its charge. The topological charge n can be considered as an important parameter for such beams. One of the main characteristics of these beams is that they carry an orbital angular momentum (OAM) of n per photon . The OAM carrying property has raised extensive interest in scientific community due to its unique applications in the fields of particle manipulation [5] and quantum information [6-8]. Vortex beams have been experimentally realized both in lasers [9] as well as electron beams . These beams form an infinite dimensional basis for applications such as quantum computation and cryptography . Moreover, increase in information entropy with the order of vortices can be utilised to encode more amount of information in these structures .

Research paper thumbnail of Divergence of optical vortex beams

Applied Optics, 2015

We show, both theoretically and experimentally, that the propagation of optical vortices in free ... more We show, both theoretically and experimentally, that the propagation of optical vortices in free space can be analyzed by using the width (w(z)) of the host Gaussian beam and the inner and outer radii of the vortex beam at the source plane (z = 0) as defined in Optics Letters 39, 4364-4367 (2014). We also studied the divergence of vortex beams, considered as the rate of change of inner or outer radius with the propagation distance, and found that it varies with the order in the same way as that of the inner and outer radii at zero propagation distance. These results may be useful in designing optical fibers for orbital angular momentum modes that play a crucial role in quantum communication.

Research paper thumbnail of Generalized orbital angular momentum Poincaré sphere

International Conference on Optics and Photonics 2015, 2015

Research paper thumbnail of Propagation of an arbitrary vortex pair through an astigmatic optical system and determination of its topological charge

Journal of the Optical Society of America A, 2014

We embed a pair of vortices with different topological charges in a Gaussian beam and study its e... more We embed a pair of vortices with different topological charges in a Gaussian beam and study its evolution through an astigmatic optical system, a tilted lens. The propagation dynamics is explained by a closed-form analytical expression. Furthermore, we show that a careful examination of the intensity distribution at a predicted position past the lens can determine the charge present in the beam. To the best of our knowledge, our method is the first non-interferometric technique to measure the charge of an arbitrary vortex pair. Our theoretical results are well supported by experimental observations.

Research paper thumbnail of Spatial intensity distribution of optical vortex beams

Workshop on Recent Advances in Photonics (WRAP), 2013

Research paper thumbnail of Higher order optical vortices and formation of speckles

Optics Letters, 2014

We have experimentally generated higher order optical vortices and scattered them through a groun... more We have experimentally generated higher order optical vortices and scattered them through a ground glass plate that results in speckle formation. Intensity autocorrelation measurements of speckles show that their size decreases with an increase in the order of the vortex. It implies an increase in the angular diameter of the vortices with their order. The characterization of vortices in terms of their annular bright ring also helps us to understand these observations. The results may find applications in stellar intensity interferometry and thermal ghost imaging.

Research paper thumbnail of Experimental generation of ring-shaped beams with random sources

Optics Letters, 2013

We have experimentally reproduced ring shaped beams from the scattered Laguerre-Gaussian and Bess... more We have experimentally reproduced ring shaped beams from the scattered Laguerre-Gaussian and Bessel-Gaussian beams. A rotating ground glass plate is used as a scattering medium and a plano convex lens collects the scattered light to generate ring shaped beams at the Fourier plane. The obtained experimental results are supported with the numerical results and are in good agreement with the theoretical results proposed by Wang, Cai and Korotkova (Opt. Exp. 17, 22366 (2009)).

Research paper thumbnail of Spatial distribution of spontaneous parametric down-converted photons for higher order optical vortices

Optics Communications, 2014

We make a source of entangled photons (SEP) using spontaneous parametric down-conversion (SPDC) i... more We make a source of entangled photons (SEP) using spontaneous parametric down-conversion (SPDC) in a non-linear crystal and study the spatial distribution of photon pairs obtained through down-conversion of different modes of light including higher order vortices. We have observed that for the Gaussian pump, the thickness of the SPDC ring varies linearly with the radius of pump beam. While for the optical vortex beams, two concentric SPDC rings are formed, however, it happens above a critical radius of the vortex beam. The FWHM of SPDC rings increases with increase in the order of optical vortex beams. The presence of a critical beam width for the optical vortices as well as the observed full width at half maximum of the SPDC rings are supported by our numerical results also.

Research paper thumbnail of Measuring the Mueller matrix of an arbitrary optical element with a universal SU(2) polarization gadget

Journal of the Optical Society of America A, 2014

We propose a new method for determining the Mueller matrix of an arbitrary optical element and ve... more We propose a new method for determining the Mueller matrix of an arbitrary optical element and verify it with three known optical elements. This method makes use of two universal SU(2) polarization gadgets to obtain the projection matrix directly from the experiment. It allows us to determine the Mueller matrix without precalibration of the setup, since the generated polarization states are fully determined by the azimuths of the wave plates. We calculate errors in determining the Mueller matrix and compare with other techniques.