Birefringent Optical Elements Research Articles

Deep ultraviolet hydrogel based on 2D cobalt-doped titanate

Birefringent optical elements that work in deep ultraviolet (DUV) region become increasingly important these years. However, most of the DUV optical elements have fixed birefringence which is hard to be tuned. Here, we invent a birefringence-tunable optical hydrogel with mechano-birefringence effect in the DUV region, based on two-dimensional (2D) low-cobalt-doped titanate. This 2D oxide material has an optical anisotropy factor of 1.5 × 10–11 C2 J−1 m−1, larger than maximum value obtained previously, leading to an extremely large specific magneto-optical Cotton-Mouton coefficient of 3.9 × 106 T−2 m−1. The extremely large coefficient enables the fabrication of birefringent hydrogel in a small magnetic field with an ultra-low concentration of 2D oxide material. The hydrogel can stably and continuously modulate 303 nm DUV light with large phase tunability by varying the strain (compression or stretching) from 0 to 50%. Our work opens the door to design and fabricate new proof-of-concept DUV birefringence-tunable element, as demonstrated by optical hydrogels capable of DUV modulation by mechanical stimuli.

Lifetime prediction of nanogratings inscribed by a femtosecond laser in silica glass.

This paper is dedicated to the lifetime prediction of Type II modifications (i.e., nanogratings) written in silica glass using an infrared femtosecond laser. Herein we report accelerated aging experiments of such nanogratings through the monitoring of their characteristic linear birefringence signature. Based on the master curve formalism, we demonstrate that these laser-induced nanostructures can survive for 200 hours at 1100°C. Under the reported processing conditions and after a dedicated passivation treatment, the estimated lifetime of the birefringent optical elements is beyond 10 years at 800°C with a minor erasure of 7%.

Deep-learning-based colorimetric polarization-angle detection with metasurfaces

Polarization plays a key role in both optics and photonics. Generally, the polarization states of light are measured with birefringent or dichroic optical elements paired with a power meter. Here we propose a direct polarization detection method based on colorimetric asymmetrical all-dielectric metasurfaces to obtain the polarization angles of the incident light. The independently tunable periods and diameters along the x and y axes enables double-layer nanopillars to realize high-performance dual-color palettes with arbitrary combinations under orthogonal polarization states. The polarization detection network based on residual networks is used to deeply learn the regulations between color palette variations and incident polarization angles, which can accurately recognize extremely slight polarization variations in about 1 s with an accuracy of 81.4% within 0.7° error and 99.5% within 1.4° error. Our strategy significantly improves the compactness of polarization detection, and it can be readily expanded to polarization distribution measurement and colorimetric polarization imaging on an intelligent platform.

Modeling light interference figures and isogyres of uniaxial and biaxial birefringent optical elements

Using polarization fields, light interference figures and isogyres of some uniaxial and biaxial birefringent optical elements are modeled. The effects of diattenuation, and linear and circular birefringence are included, and the Airy spiral is displayed for various cases. The two branches of the Airy spiral are shown to separate and surround separately the melatopes in the case of biaxial elements with linear and circular birefringence. A connection and discussion with the aberration field of astigmatism, in particular binodal, is presented.

Poincaré sphere-based technique for tuning flat-top fiber comb filter

We propose a universal method for designing flat-top passband and wavelength tuning of transmission spectrum of a fiber comb filter based on a polarization-diversified loop structure using Poincaré sphere representation. The proposed method gives a clear visualization of the evolution of polarization state through birefringent optical elements comprising the filter, and the design results are experimentally verified. Our method can also suggest which optical components are necessary for a specifically desired transmission spectrum of the comb filter.

Shape control of femtosecond-laser-induced birefringent structures by controlling spherical aberration

Birefringent optical elements can be fabricated by inscribing structures of nanogratings inside a silica glass by focusing femtosecond (fs) laser pulses. When birefringent optical elements are used as phase retarders, the retardance of the birefringence and transmittance are important properties to determine the quality as a phase retarder. Our target of application of fs-laser-induced birefringent structures inside a silica glass is a polarization imaging filter, which can capture the polarization distribution of detected light in a real-time. Larger retardance and higher transmittance of a filter are required to acquire polarization with higher accuracy and sensitivity. Therefore, it is necessary to elucidate the relationship between laser processing conditions and the induced birefringent structures. In this study, the authors have investigated fs-laser induced birefringent structures under various processing conditions. The authors have confirmed that the retardance and transmittance of the induced structures depend on the focusing depth from the glass surface. The authors also demonstrate that spherical aberration due to a refractive mismatch between the air and glass can be changed by modulating the spatial phase distribution of fs laser pulses with a spatial light modulator (SLM) and birefringent structures with larger birefringence could be fabricated at a desired depth inside a silica glass by using an SLM.

Optical torque reversal and spin-orbit rotational Doppler shift experiments.

We report on optical rotational Doppler frequency shift experiments in the context of a counter-intuitive optomechanical phenomenon that is the angular analog of so-called negative optical radiation forces, which involves spin-orbit scattering of light. In practice, spin-orbit opto-mechanical effects arising from the interaction between polarized light and azimuthally varying birefringent optical elements are retrieved from mechano-optical experiments that involve spatial of the medium. Two kinds of experiments (single-beam and two-beam geometries) are performed and both approaches are discussed in the framework of previous dynamical geometric phase and rotational Doppler shift experiments based on spin and/or orbital angular momentum of light.

Divided-pulse compression with gas-filled hollow-core fiber for generation of high-energy few-cycle pulses

We study theoretically a pulse compression method with gas-filled hollow-core fiber (HCF) based on pulse division. The input pulse is first divided temporally into a sequence of almost identical subpulses by birefringent optical elements that are designed to have nearly zero group delay dispersion. Then, these subpulses are coupled into gas-filled HCF for spectrum broadening independently. Last, the subpulses are recombined into one pulse by the birefringent elements and compressed temporally. This method is demonstrated to be suitable for compressing ultrafast pulses with energies far above the millijoule to few-cycle level. Several key issues on successfully implementing this method are analyzed quantitatively, and the limitations are also discussed.

Broadband anisotropy of femtosecond laser induced nanogratings in fused silica

We demonstrate operation of femtosecond laser imprinted birefringent optical elements from 200 to 2100 nm. The absorption losses in the UV can substantially reduced by the post-annealing of the structures. The birefringence is also accompanied with the weak dichroism which is related to the polarization dependent scattering. The transition from the isotropic modification to nanostructures is accompanied by occurrence of ODC(II) defect.

Femtosecond versus picosecond laser machining of nano-gratings and micro-channels in silica glass

The ability of 8 picosecond pulse lasers for three dimensional direct-writing in the bulk of transparent dielectrics is assessed through a comparative study with a femtosecond laser delivering 600 fs pulses. The comparison addresses two main applications: the fabrication of birefringent optical elements and two-step machining by laser exposure and post-processing by chemical etching. Formation of self-organized nano-gratings in glass by ps-pulses is demonstrated. Differential etching between ps-laser exposed regions and unexposed silica is observed. Despite attaining values of retardance (>100 nm) and etching rate (2 μm/min) similar to fs pulses, ps pulses are found unsuitable for bulk machining in silica glass primarily due to the build-up of a stress field causing scattering, cracks and non-homogeneous etching. Additionally, we show that the so-called "quill-effect", that is the dependence of the laser damage from the direction of writing, occurs also for ps-pulse laser machining. Finally, an opposite dependence of the retardance from the intra-pulse distance is observed for fs- and ps-laser direct writing.

High and stable photoinduced anisotropy in guest–host polymer mediated by chromophore aggregation

We present a novel materials concept for optical inscription of stable birefringent optical elements into guest-host type polymers by making use of chromophore aggregation. The method is based on photoalignment of azobenzene chromophores, the aggregation of which leads to significant enhancement and stabilization of the photoinduced birefringence. The obtained order parameter of the molecular alignment (0.3) in combination with the exceptional thermal stability of the anisotropy renders the material system unique among amorphous azobenzene-containing polymers and provides a route toward designing efficient photoresponsive optical elements through the guest-host type approach.

The use of symmetry to correct Larmor phase aberrations in spin echo scattering angle measurement

Spin echo scattering angle measurement (SESAME) is a sensitive interference technique for measuring neutron diffraction. The method uses waveplates or birefringent prisms to produce a phase separation (the Larmor phase) between the "up" and "down" spin components of a neutron wavefunction that is initially prepared in a state that is a linear combination of in-phase up and down components. For neutrons, uniformly birefringent optical elements can be constructed from closed solenoids with appropriately shaped cross sections. Such elements are inconvenient in practice, however, both because of the precision they demand in the control of magnetic fields outside the elements and because of the amount of material required in the neutron beam. In this paper, we explore a different option in which triangular-cross-section solenoids used to create magnetic fields for SESAME have gaps in one face, allowing the lines of magnetic flux to "leak out" of the solenoid. Although the resulting field inhomogeneity produces aberrations in the Larmor phase, the symmetry of the solenoid gaps causes the aberrations produced by neighboring pairs of triangular solenoids to cancel to a significant extent. The overall symmetry of the SESAME apparatus leads to further cancellations of aberrations, providing an architecture that is easy to construct and robust in performance.

Polarization-splitting common-path interferometer based on a zero-twist liquid crystal display

We present a compact optical polarization-splitting common-path interferometer based on a zero-twist liquid crystal display (LCD). The LCD is encoded with a diffraction grating pattern and illuminated with a polarization state with both horizontal and vertical components. The polarization component perpendicular to the director axis of the liquid crystal molecules is not affected by the LCD and forms the reference beam. However, the polarization component parallel to the director axis is diffracted at an angle determined by the period of the grating. By imposing an analyzer polarizer, these two beams create an interferogram that can either display retardance patterns encoded onto the LCD or analyze external birefringent optical elements. The programmability of the system allows new ways of increasing the utility of the interferograms. Experimental results are provided, including the visualization of optical vortices with different and opposite topological charges.

Elementary Matrix-Based Vector Optical Network Analysis

An alternative formulation of the elementary matrices, which was originally defined by Jones, is proposed. The matrix formulation is based on frequency derivatives instead of the originally considered spatial derivatives. It is demonstrated that the modified elementary matrices are well suited for the analysis of optical systems with multiple optical waves created by modulation. We illustrate this new approach by applying it to a novel interferometric measurement system capable of complete characterization of optical devices. Exemplary measurements of optical power, polarization state, and both chromatic and polarization mode dispersions are given for an optical device composed of a molecular gas absorption cell and birefringent optical elements

Polarization Ray Tracing in Twisted Liquid Crystal Systems

In this paper it will be presented different ray trace models that are able to simulate uniaxial birefringent optical elements with twisted structure. There exist already methods to calculate the optical behavior of birefringent elements (including angular dependence) such as the Jones matrix method [2]. But those methods are not based on ray tracing and cannot be linked together with other optical components such as lenses for example. The aim of this work is to enable ray trace simulations of optical systems containing complex birefringent elements and extended sources. The developed models are applied on problems that are related to twist liquid crystal Wollaston prism.

Polarization Ray Tracing in Twisted Liquid Crystal Systems

In this paper it will be presented different ray trace models that are able to simulate uniaxial birefringent optical elements with twisted structure. There exist already methods to calculate the optical behavior of birefringent elements (including angular dependence) such as the Jones matrix method [2]. But those methods are not based on ray tracing and cannot be linked together with other optical components such as lenses for example. The aim of this work is to enable ray trace simulations of optical systems containing complex birefringent elements and extended sources. The developed models are applied on problems that are related to twist liquid crystal Wollaston prism.

Conoscopic Alignment Methods for Birefringent Optical Elements in Fusion Lasers

The alignment of birefringent crystals for use as wave plates and nonlinear frequency converters in high-power, low-repetition-rate laser systems, such as those used for laser fusion, is greatly facilitated through the use of convergent light (conoscopy). In such laser systems, a long shot cycle is necessary to allow laser amplifiers time to thermally equilibrate in order to produce beams of adequate wavefront quality. It is therefore essential that crystal parameters such as phase-matching angle and location of principle axes be determined off-line, and a method developed to easily transfer the alignment to the fusion laser system. Conoscopic methods1–3 have been used for the off-line alignment of two such devices in the 60-beam OMEGA4 laser system because of the simplicity and accuracy of such techniques. Both devices, of which 60 each are required, currently incorporate 30-cm-diam × 1-cm-thick KDP plates with the crystalline optic axis at 59° to the surface normal.

Investigation of rough surfaces and transparent birefringent samples with Mueller-matrix scatterometry

Measurements made with an automated angle-resolved Mueller-matrix scatterometer are described. The instrument uses incident-polarization electro-optical modulation, division-of-amplitude photopolarimetry, and software-implemented Fourier-transform analysis of the detected signals to determine the scattered Mueller matrix of the sample. The measurement time is approximately 1 s per scattering angle. Applications to the control of surface roughness and structure on rough steel sheets (galvanized and uncoated) and of the properties of transparent birefringent optical elements (liquid-crystal devices) are discussed.

Two-photon-induced birefringence in polysilanes

Chain scission induced by two-photon exposure produces a permanent birefringence in polysilane polymer films, which can be used for the formation of birefringent optical elements. Spectroscopy of the effect for poly(di- n-hexylsilane) reveals an unexpected sharp resonance at 580 nm. A comparison of several polysilanes indicates the effect only occurs in polymers with symmetric or near symmetric sidegroups. Models to explain these observations are introduced and discussed.

Two-Photon Processes in Substituted Polysilanes

Abstract Chain scission induced by two-photon exposure produces a permanent birefringence in polysilane polymer films, which can be used for the formation of birefringent optical elements. Spectroscopy of the effect for poly(di-n-hexylsilane) reveals an unexpected sharp resonance at 579 nm. Models to explain these observations are introduced and discussed