arXiv subjects

A. Srinivasa Rao

Publications and source records attributed to A. Srinivasa Rao.

12 recordsLinked to original sources

arXiv2026

Origin of Experimental Realization of Vector Beams by Superposition Technique: Geometric Phase

Optical vector beam generation based on the superposition technique [Phys. Rev. Lett. 107(5), 053601 (2011)] has attracted significant interest in both fundamental and applied optics due to its simplicity and cost-effectiveness. In this approach, the superposed modes exist in orthogonal polarization states and therefore do not interfere with each other. When optical gadgets are employed for characterization, the corresponding polarization distribution can be observed. In this work, we report that the characteristic (non-uniform) polarization distribution of vector beams is not inherently present in their field distribution. Instead, the observed polarization features arise during characterization as a consequence of the geometric phase difference introduced between the superposed modes by the optical gadgets. This analysis opens a new direction in the understanding of various classes of vector beams and offers potential advances in the field of structured beam optics.

physics.optics↗

arXiv2026

Pancharatnam Berry Phase as the Origin of Vector Nature Observed in Hermite Gaussian Superposition States

Superposition of orthogonal Hermite Gaussian (HG) modes in orthogonal linear polarization states is one of the techniques used for the experimental realization of lower order optical vector beams and vector vortex lattices. To date, it has been widely believed that the vector nature arising from this technique originates from the spatial intensity distribution of the superposed HG modes. Here, we report that the vector characteristics observed during the characterization of vector modes generated via HG mode superposition arise from the azimuthally inhomogeneous intensity distribution and the polarization-dependent Pancharatnam Berry (PB) phase. Our analytical calculations confirm that the vector nature is not inherently present in the superposition state; rather, it becomes observable due to polarization-based optical elements used in the characterization process. This insight provides a fundamental clarification of the physical origin of the experimentally observed vector nature in HG-mode superposition.

physics.optics↗

arXiv2026

Evolution of Geometric Phase of light since 1956: A Catalog Review

The geometric phase of light is a fascinating phenomenon in optics and arises whenever there is a change in the polarization state of light. It is a fundamentally well-established concept and has recently found extensive applications, particularly in the development of geometric phase elements that enable efficient manipulation of light. In this tutorial review, we discuss the evolution of the geometric phase of polarization on the Poincaré sphere, from its inception by Shivaramakrishnan Pancharatnam in 1956 to its recent advances and applications. This review article aims to focus on core papers related to the geometric phase of polarization rather than providing an exhaustive literature survey. In this review, first, we introduced the basic parameters and corresponding parameter spheres involved in the geometric phase of light. Then, we provide an in-depth analysis of geometric phase in polarization modes, spatial modes, vector modes, and electromagnetic fields. A brief discussion of applications of the geometric phase is also provided. The intriguing explanation given in this review can awaken new ideas related to the geometric phase of light and can open new directions in fundamental and applied optics. Finally, the tutorial is structured as a comprehensive catalog of the geometric phase of light.

physics.optics↗

arXiv2024

Realization of Non-diffracting and Self-healing Optical Skyrmions

Optical skyrmions formed in terms of polarization are topological quasi-particles and have garnered much interest in the optical community owing to their unique inhomogeneous polarization structure and simplicity in their experimental realization. These structures belong to the Poincaré beams satisfying the stable topology. We theoretically investigated the non-diffracting and self-healing Poincaré beams based on the superposition of two orthogonal Bessel modes by mode matching technique. These Poincaré beams are topologically protected, and we suggest them as optical skyrmions. These optical skyrmions are quasi-skyrmions and their range of propagation depends on the range of superposed Bessel modes. The polarization structure of these optical skyrmions has no change upon the propagation. A necessary experimental configuration is suggested to realize variable order skyrmions in Bessel modes experimentally. This work can provide a new direction for the generation of skyrmions with completely new textures and features with respect to existing skyrmions originating from Laguerre-Gaussian modes.

physics.optics↗

arXiv2024

Origin, Experimental Realization, Illustrations, and Applications of Bessel beams: A Tutorial Review

Over the past 36 years much research has been carried out on Bessel beams (BBs) owing to their peculiar properties, viz non-diffraction behavior, self-healing nature, possession of well-defined orbital angular momentum with helical wave-front, and realization of smallest central lobe. Here, we provide a detailed tutorial review on BBs from their inception to recent developments. We outline the fundamental concepts involved in the origin of the BB. The theoretical foundation of these beams was described and then their experimental realization through different techniques was explored. We provide an elaborate discussion on the different kinds of structured modes produced by the BB. The advantages and challenges that come with the generation and applications of the BB are discussed with examples. This tutorial review provides reference material for readers who wish to work with non-diffracting modes and promotes the application of such modes in interdisciplinary research areas.

physics.optics↗

arXiv2019

Detection of excited state absorption cross-section of porphyrin through cw and femto-second laser pump-probe technique

We report on direct detection of excited states absorption cross-section using dual wavelength pump-probe technique. Also, we experimentally demonstrate using porphyrin composite molecules (porphyrin derivatives such as 5,10,15,20-meso-tetrakis phenyl porphyrin (H2TPP), 5,10,15,20 - meso-tetrakis(4-hydroxyphenyl) porphyrin (H2TPP(OH)4)). The cw laser at 761 nm wavelength is used as a pump to maintain excited state population. Changes in the population of excited states lead to the change in transmission are monitored using femto-second probe pulses of 130 fs width and repeated at a 1kHz rate with central wavelength around 800 nm. Transmittance changes due to excited state population are modeled using rate equation approach. The effect of the absorption on the transmitted pulse shape has been discussed as a function of fluence. Obtained excited state absorption cross-sections of H2TPP and H2TPP(OH)4 doped boric acid glass (BAG) films are 4.9X10-18 cm2 and 1.2X10-17 cm2 respectively.

physics.optics↗

arXiv2018

A Method to Determine the Ultrafast Laser Beams Spot Size

From the standard TEM00 Gaussian beam profile equations, we have derived the equations for beam waist at lens focus as a function of variable spot size. In this process, we obtained two equations for beam waist and the validity of these equations is studied with respect to Rayleigh length. The physical validity of the equations has been theoretically checked and also experimentally verified with He-Ne laser at 632 nm. The derived equations are useful for the estimation of the spot size of high peak power lasers (those spot sizes less than few mm). In complicated experimental setup by inserting a beam splitter, we can walk out the some of the beam and the spot size of this beam at the particular position can be measured by normal methods (pinhole and knife-edge techniques). By using our derived equations we can know the spot size of the beam at any position in the experimental setup form the measured spot size.

physics.gen-ph↗

arXiv2018

Third order nonlinear study of ZnO nano particles under femto-second laser illumination

ZnO nano particles are synthesized at different percentages of Ag dopant by facile precipitation method. Two photon absorption (2PA) cross-section and nonlinear refractive index of ZnO nano particles are studied with 130 femto-second laser pulses at a repletion rate of 1 kHz with central wavelength of 532 nm by z-scan technique. The 2PA cross-section of ZnO nano particles are obtained by theoretical calculations, using rate equation approach. Both absorption cross-section and nonlinear refractive index increased with increasing the Ag dopant percentage. We have observed an increase of order of 104 in the nonlinear refractive index in the presence of femto-second laser excitation with reference to nano-second laser excitation.

physics.optics↗

arXiv2018

Nonlinear Spectroscopic Study of Porphyrin Under cw and Femto-second Laser Pulse Excitation

Single Beam Transmittance (SBT) was used as nonlinear spectroscopic tool to investigate the absorption cross-sections and lifetimes of Tetra Phenyl porphyrin (H2TPP) and its OH- group derivative (H2TPP(OH)4) doped in boric acid glass (BAG). We have used 671 nm wavelength as exciting wavelength for both CW (incident intensity up to 1010 W/cm2) and femto-second laser pulse (up to fluence of 102 mJ/cm2). Under cw laser excitation, H2TPP doped BAG demonstrates Double Saturable Absorber (DSA) behavior whereas H2TPP(OH)4 doped BAG act as Revere Saturable Absorber (RSA). Rate equation model espouses to extract the spectroscopic parameters from the experimental data. Excited state life times and absorption cross-sections were obtained as parameters for theoretical fit on SBT data. Porphyrin molecules act as four level systems under cw laser excitation, whereas in the presence of femto-second laser excitation they act as a two level system. We have derived the equations for transmitted energy through the material in the presence of femto-second laser illumination. Both systems viz., H2TPP and H2TPP(OH)4 doped BAG behaves as saturable absorbers when excited by femto-second laser pulses.

physics.optics↗

arXiv2018

Generation of high peak power, segmented and Bessel beams of tunable range without on-axis intensity modulation

We propose and experimentally demonstrate a novel experimental scheme to generate high peak power, segmented, smooth, zero-order Bessel beams with tunable range. Illuminating the axicon with hollow Gaussian beams (HGBs) of different orders we have generated Bessel beams of varying range at different positions away from the axicon. The presence of dark core at the center of the HGBs removes the effect of imperfection in the axicon tip. As a result, the entire power of the input beam is transformed into zero-order Bessel beam without any on-axis intensity modulation. We observe the decrease in range and increase in peak power of the zero-order Bessel beam with the order of HGBs. Controlling the superposition of the HGBs of different orders to the axicon we have demonstrated the increase in the range of the Bessel beam. The current technique can also produce Bessel beams of different intensity distribution including single peak or multiple peak Bessel beams. Using single-pass second harmonic generation in nonlinear crystals of different lengths we have further verified the increase of peak power of the Bessel beam with the order of the HGBs and also the increase in the range of the Bessel beam due to the superposed HGBs. The generic experimental scheme can be used at different wavelengths and timescales (continuous-wave to ultrafast).

physics.optics↗

arXiv2018

Nonlinear absorption study in four and five energy level systems

Nonlinear absorption in four and five energy level systems have been studied with the aid of steady-state rate equation approach. We report on the tunability of saturable and reverse saturable absorption as a function of spectroscopic parameters (lifetimes and absorption cross-sections). Detailed information is given on the estimation of spectroscopic parameters in the nonlinear absorption spectroscopy. The exhaustive graphical analysis of this article can provide the brief idea about transmittance curves of the nonlinear absorption to the experimentalists. In four and five level cascade models, simultaneously saturable and reverse saturable absorption can be generated.

physics.optics↗

arXiv2018

Controlled generation, manipulation and frequency conversion of broadband orbital angular momentum spectrum of asymmetric optical vortex beams

We report on generation and control of tunable, broad orbital angular momentum (OAM) spectrum of a vortex beam. Using two spiral phase plates (SPPs), we have converted the Gaussian beam of Yb-fiber femtosecond laser at 1064 nm into optical vortices of orders, l=1-6, with conversion efficiency >95%. By adjusting the transverse shift of the SPPs with respect to the incident Gaussian beam axis, we have transformed the symmetric (intensity distribution) optical vortex of order, l, into an asymmetric vortex beam of broad OAM spectrum of orders, l, l-1, l-2,..,0. While the position of the SPPs determines the weightage of the OAM modes, however, the weightage of the higher OAM modes transfer to lower OAM modes with the shift of SPPs and finally resulting a Gaussian beam (l=0). Using single pass frequency doubling of asymmetric vortices in a 5-mm-long bismuth triborate crystal, we have transferred the pump OAM spectra, l, l-1, l-2, ..., 0, into the broad spectra of higher order OAM modes, 2l, 2l-1, 2l-2, ...,0 at green wavelength, owing to OAM conservation in nonlinear processes. We have also observed that the conversion efficiency of the frequency-doubled vortices of all orders increases with the increase in the asymmetry of the pump vortices. Using pump power of 4.6 W, we have generated symmetric vortices at green wavelength of output powers, 1.2, 0.76, 0.47, 0.4, 0.32, and 0.3 W and orders of ls=2, 4, 6, 8, 10, and 12, respectively.

physics.optics↗