Dark-field spin Hall effect of light.

While an optical system's symmetry ensures that the spin Hall effect of light (SHEL) vanishes at normal incidence, the question of how close to the normal incidence can one reliably measure the SHEL remains open. Here we report simulation and experimental results on the measurement of SHEL at $\sim 0.12^\circ$ away from normal incidence in the Fourier plane of a weakly focused beam of light, reflected at an air-glass interface. Measurement of transverse spin-shift due to $< 0.05^\circ$ polarization variation in the beam cross section along the X- and Y-directions is achieved in the dark-field region of the reflected beam. Our ability to measure the SHEL at near-normal incidence with no moving optomechanical parts and significantly improved sensitivity to phase-polarization variations is expected to enable several applications in the retro-reflection geometry including material characterization with significant advantages.

Spin-orbit coupling mediated transverse spin mode rotation in a uniaxial crystal.

We demonstrate topological features in a spin-orbit coupled inhomogeneously polarized beam of light due to propagation of a linearly polarized focused Gaussian beam through a tilted-rotated (θ-ϕ) quartz crystal plate. The crystal plate is kept in a polarization interferometer, and transverse and longitudinal phase difference is introduced between the o- and e-wave-beams via (θ-ϕ) variation. The curvature in the phase difference, originating at a phase saddle, at the stem of an intensity forklet pattern, enables continuous rotation of the output two-lobe intensity pattern as a function of (θ-ϕ). The transverse spin-shift of the rotating output beam shows variation in both magnitude and slope. Such a study of exploring topological features arising due to spin-orbit coupling in simple optical systems is of fundamental interest and is expected to open up potential applications in the investigation of material anisotropy and polarization-sensitive sensing.

Wave dislocation line threaded polarization interferometer.

Constructing a closed-circuit polarization interferometer, wherein a wave dislocation line can be visualized to thread the parameter space, is a topic of fundamental and applied research interest. Proposed by Berry [Proc. R. Soc. A463, 1697 (2007)10.1098/rspa.2007.1842] in the scalar wave domain, this universal phenomenon is simulated and experimentally demonstrated in the vector domain using a rotated-tilted quartz crystal plate in a polarization interferometer. The phase difference between overlapping ordinary and extraordinary paraxial ray beams passing through the crystal plate is varied continuously. The appearance of ±1 dislocation number spiral- and saddle-type topological structures in the complex Stokes phase is a result of satisfying ± π/2 phase difference between the ray beams and around the zero-crossings of the Stokes parameters.

Generalized matrix transformation formalism for reflection and transmission of complex optical waves at a plane dielectric interface.

We describe a generalized formalism, addressing the fundamental problem of reflection and transmission of complex optical waves at a plane dielectric interface. Our formalism involves the application of generalized operator matrices to the incident constituent plane-wave fields to obtain the reflected and transmitted fields. This formalism, though physically equivalent to Fresnel formalism, has greater mathematical elegance and computational efficiency as compared to the latter. We utilize exact 3D electric-field expressions, which enable us to seamlessly analyze waves of miscellaneous wavefront shapes and properties using the single formalism, along with appropriately retaining the geometric phase and wavefront curvature information. We demonstrate our formalism by obtaining and analyzing the reflected and transmitted fields in a simulated Gaussian beam model.

Amplified measurement of weak optical activity using a spin-phase-gradient beam.

Development of alternate techniques to polarimetry for the measurement of weak optical rotation, with improved sensitivity, is becoming increasingly important as one understands the role of chirality in drug design and synthesis and the fundamentals of chiral light-matter interaction. We demonstrate here an optical amplification scheme using a spin-phase-gradient beam to measure ultra-small optical rotation angle (4 mdeg), with a sensitivity of 220 µdeg/µm, due to dilute (mg/mL) dextro-rotatory sugar solution. A Soleil-Babinet compensator is used to generate a tunable spin-phase-gradient beam which enables us to achieve high measurement sensitivities. Theoretical formalism of the technique leads us to the possibility to realize much higher measurement sensitivity of up to 10 µdeg/µm by tuning-in the experimental parameters.

Chiral dynamics of exceptional points in weakly absorbing biaxial crystal.

In this Letter, we report the chiral dynamics of optical exceptional points (EPs), the singular axes in weakly absorbing biaxial crystal, arising due to a fine balance between the birefringence and dichroism. Taking advantage of the coincidence of the singular axes and C-point singularity, the EPs are characterized using conoscopic Stokes polarimetry. We observe that the two optic axes of the biaxial crystal split into two pairs of singular axes upon introducing weak dichroism which, upon application of a transverse electric field, follows a helical trajectory to coalesce and disappear. Our results show a fine control on the chiral behavior of EPs due to an electrogyration effect, which controls the retardance gradient and the fast-axis orientation in the crystal.

Field-controllable Spin-Hall Effect of Light in Optical Crystals: A Conoscopic Mueller Matrix Analysis.

Electric-field applied perpendicular to the direction of propagation of paraxial beam through an optical crystal dynamically modifies the spin-orbit interaction (SOI), leading to the demonstration of controllable spin-Hall effect of light (SHEL). The electro- and piezo-optic effects of the crystal modifies the radially symmetric spatial variation in the fast-axis orientation of the crystal, resulting in a complex pattern with different topologies due to the symmetry-breaking effect of the applied field. This introduces spatially-varying Pancharatnam-Berry type geometric phase on to the paraxial beam of light, leading to the observation of SHEL in addition to the spin-to-vortex conversion. A wave-vector resolved conoscopic Mueller matrix measurement and analysis provides a first glimpse of the SHEL in the biaxial crystal, identified via the appearance of weak circular birefringence. The emergence of field-controllable fast-axis orientation of the crystal and the resulting SHEL provides a new degree of freedom for affecting and controlling the spin and orbital angular momentum of photons to unravel the rich underlying physics of optical crystals and aid in the development of active photonic spin-Hall devices.

Ultrashort vortex from a Gaussian pulse - An achromatic-interferometric approach.

The more than a century old Sagnac interferometer is put to first of its kind use to generate an achromatic single-charge vortex equivalent to a Laguerre-Gaussian beam possessing orbital angular momentum (OAM). The interference of counter-propagating polychromatic Gaussian beams of beam waist ωλ with correlated linear phase (ϕ 0 ≥ 0.025 λ) and lateral shear (y 0 ≥ 0.05 ωλ) in orthogonal directions is shown to create a vortex phase distribution around the null interference. Using a wavelength-tunable continuous-wave laser the entire range of visible wavelengths is shown to satisfy the condition for vortex generation to achieve a highly stable white-light vortex with excellent propagation integrity. The application capablitiy of the proposed scheme is demonstrated by generating ultrashort optical vortex pulses, its nonlinear frequency conversion and transforming them to vector pulses. We believe that our scheme for generating robust achromatic vortex (implemented with only mirrors and a beam-splitter) pulses in the femtosecond regime, with no conceivable spectral-temporal range and peak-power limitations, can have significant advantages for a variety of applications.

Isogyres - Manifestation of Spin-orbit interaction in uniaxial crystal: A closed-fringe Fourier analysis of conoscopic interference.

Discovered in 1813, the conoscopic interference pattern observed due to light propagating through a crystal, kept between crossed polarizers, shows isochromates and isogyres, respectively containing information about the dynamic and geometric phase acquired by the beam. We propose and demonstrate a closed-fringe Fourier analysis method to disentangle the isogyres from the isochromates, leading us to the azimuthally varying geometric phase and its manifestation as isogyres. This azimuthally varying geometric phase is shown to be the underlying mechanism for the spin-to-orbital angular momentum conversion observed in a diverging optical field propagating through a z-cut uniaxial crystal. We extend the formalism to study the optical activity mediated uniaxial-to-biaxial transformation due to a weak transverse electric field applied across the crystal. Closely associated with the phase and polarization singularities of the optical field, the formalism enables us to understand crystal optics in a new way, paving the way to anticipate several emerging phenomena.

Direct patterning of vortex generators on a fiber tip using a focused ion beam.

The realization of spiral phase optical elements on the cleaved end of an optical fiber by focused ion beam milling is presented. A focused Ga+ ion beam with an acceleration voltage of 30 keV is used to etch continuous spiral phase plates and fork gratings directly on the tip of the fiber. The phase characteristics of the output beam generated by the fabricated structures measured via an interference experiment confirmed the presence of phase singularity in the output beam. The devices are expected to be promising candidates for all-fiber beam shaping and optical trapping applications.

Generation of optical vortex dipole from superposition of two transversely scaled Gaussian beams.

We propose a distinct concept on the generation of optical vortex through coupling between the amplitude and phase differences of the superposing beams. For the proof-of-concept demonstration, we propose a simple free-space optics recipe for the controlled synthesis of an optical beam with a vortex dipole by superposing two transversely scaled Gaussian beams. The experimental demonstration using a Sagnac interferometer introduces the desired amount of radial shear and linear phase difference between the two out-of-phase Gaussian beams to create a vortex pair of opposite topological charge in the superposed beam. Flexibility to tune their location and separation using the choice of direction of the linear phase difference and the amount of amplitude difference between the superposing beams has potential applications in optical tweezers and traps utilizing the local variation in angular momentum across the beam cross section.

Effect of residual phase gradients in optical null interference.

A scheme to study the effect of residual phase gradients in an optical interference between two out-of-phase Gaussian beams is proposed. In a Sagnac interferometer configured to provide a null output, a variable linear phase swept across the null point unfolds an optical field rotation due to an apparently negligible residual phase gradient present orthogonal to the linear phase sweep. As the optical beam that rotates around its propagation axis carries orbital angular momentum, the experimental results presented in this Letter could provide an insight into the momentum change associated with the energy redistribution in the fundamental phenomenon of optical interference.

Polarimetric measurement method to calculate optical beam shifts.

Stokes polarimetry measurements are carried out to calculate the spatial and angular Goos-Hänchen and Imbert-Fedorov shifts of a Gaussian beam reflected at glass-air interface, by measuring the phase difference between the TE and TM components and the amplitude of reflection. Variation of the beam shifts as a function of input beam polarization is also measured. The results obtained here are in good agreement with the theoretical predictions and the results obtained using a position sensitive detector. The polarimetric measurement method is accurate, independent of the intensity distribution of the beam, and opens up a new method to study the beam shift problem.

Topological structures in the Poynting vector field: an experimental realization.

Experimental measurements of the nonplanar phase of scalar optical beams is achieved using polarization singularities and Stokes parameters. The product of beam intensity and its phase gradient is calculated to get the Poynting vector distribution in the beam cross section. Using these we propose a scheme and experimentally obtain generic fundamental spiral, node, and saddle topological structures in the Poynting vector field.

Plasmon-mediated vectorial topological dipole: formation and annihilation.

We report the formation and annihilation of vectorial topological dipole mediated by plasmons in structure-free metal surfaces using a polarization singular optical beam. The measured far-field behavior is due to the strong angular and polarization dependence of the reflection coefficient and the phase shift experienced by the spatially inhomogeneous polarized beam around the plasmon resonance. The threshold polarization ellipticity (σth) anticipated for the dipole formation is experimentally verified.

Manifestation of the Gouy phase in vector-vortex beams.

Experimental measurements of the twirl and changes in the anisotropy of the constant intensity ellipse, and the rotation of the polarization singular lemon pattern a generalized vector-vortex beam experiences around the two foci due to the converging and diverging conical waves and in between, are presented and interpreted as being due to the universal form of the Gouy phase, φ(G)=mπ/2.

All-optical thermo-plasmonic device.

We demonstrate an all-optical thermo-plasmonic effect to switch/modulate the surface plasmon resonance signal intensity excited at the metal-air interface. This optically addressed thermo-plasmonic measurement scheme is suitable to amplify very small changes in the complex dielectric constant (ε(m)(T)) of thin gold (Au) film, induced by the Ar(+) laser. The predominant contributions due to small but highly repeatable transient photo-thermal effects in the complex metal dielectric constant is confirmed to be the reason behind the highly reproducible all-optical thermo-plasmonic device performance presented here.

Generation of spirally polarized propagation-invariant beam using fiber microaxicon.

We present here a fiber microaxicon (MA)based method to generate spirally polarized propagation-invariant optical beam. MA chemically etched in the tip of a two-mode fiber efficiently converts the generic cylindrically polarized vortex fiber mode into a spirally polarized propagation-invariant (Bessel-type) beam via radial dependence of polarization rotation angle. The combined roles of helico-conical phase and nonparaxial propagation in the generation and characteristics of the output beam from the fiber MA are discussed.

Experimental investigation of link between growth and decay of fiber Bragg gratings.

We report here an experimental investigation for establishing and quantifying a link between the growth and decay characteristics of fiber Bragg gratings. One of the key aspects of our work is the determination of the defect energy distribution from the grating characteristics measured during their fabrication. We observe a strong correlation between the growth-based defect energy distribution and that obtained through accelerated aging experiments, paving the way for predicting the decay characteristics of fiber Bragg gratings from their growth data. Such a prediction is significant in simplifying the postfabrication steps required to enhance the thermal stability of fiber Bragg gratings.

Rotational Doppler-effect due to selective excitation of vector-vortex field in optical fiber.

Experimental demonstration of rotational Doppler-effect due to direct and simultaneous excitation of orthogonal elliptically-polarized fundamental and vortex modes in a two-mode optical fiber is presented here. The rotation frequency and the trajectory of the zero-intensity point in the two-mode fiber output beam measured as a function of analyzer rotation matches with the S-contour of polarization singularity in the beam, identified via Stokes parameter measurement. The characteristics of the S-contour around the C-point in the output beam is also measured as a function of rotating Dove prism and half-wave plate - Dove prism combination to highlight the role of polarization modifying components on the observed rotational Doppler-effect of vector-vortex beams.