Natural Birefringence Research Articles

Ultrasensitive optical electric field sensor with temperature compensation for point-wise and quasi-distributed sensing.

A point-wise and quasi-distributed optical sensing technique with the Vernier effect is proposed and achieved by multiplexing Fabry-Perot interferometers (FPIs). The FPIs are fabricated by LiNbO3 (LN) crystals of varying lengths to enable simultaneous measurement of electric field (E-field) and temperature. Compared to the traditional bulk-type optical E-field sensors, this innovative sensor enables E-field measurement without being limited by half-wave voltage and effectively avoids the influence of natural birefringence. By an algorithm based on the Fourier transform, sub-FPIs are addressable, and their interference spectra can be demodulated independently, where any two FPIs are paired to generate the fundamental Vernier effect (FVE) or harmonic Vernier effect (HVE). Thus, the measurement sensitivities of FPIs can be significantly improved by monitoring the spectral shift of the envelope in the superimposed spectra. The quasi-distributed sensing experiment is conducted using three cascaded FPIs, yielding E-field sensitivities of 2.84 nm/E (FVE) and 3.37 nm/E (HVE), with a standard deviation (STD) of spectrum variation after temperature compensation below 3.19 × 10-3. The proposed multiplexing method is significant for practical applications of the E-field quasi-distributed measurement.

Self-injection-locked second-harmonic generation at 532 nm in high-Q Fabry-Perot micro-cavities

Based on the combination of DFB laser self-injection-locking (SIL) and external-cavity frequency-doubling, we have successfully demonstrated, for the first time to our knowledge, the self-injection-locked second-harmonic generation (SHG) at 532 nm by adopting the high-Q Fabry-Perot (FP) micro-cavities. With the DFB incident pump power of 79 mW at 1064 nm, the maximum output SHG power of 20.7 mW at 532 nm was achieved from the 11 mm long plane-concave FP external-cavity of 0.12 mL volume with the 10 mm long PPLN crystal. The corresponding output SHG efficiency of 26.2 % and the Q-factor of 56.5 M both exceeded all known SIL-SHG results reported, and the SIL SHG output power was one order of magnitude higher than the previous maximum of over 2 mW. The reliable SIL has been realized by utilizing the natural birefringence of the intracavity nonlinear crystal to provide laser feedback, and the intrinsic linewidth of the SIL pump laser at 1064 nm was about 30 Hz. Moreover, by replacing the PPLN crystal with the LN and KTP crystals, the output SHG power of 6.1 mW and 2.7 mW at 532 nm were respectively obtained with the Q-factors of 29.3 M and 27.6 M. Our new SIL-SHG design with the high-Q FP micro-cavity paves the way to a new type of the field-deployable ultra-narrow-linewidth visible and near-visible lasers of low size, weight, and power (SWaP).

Optical manipulation of ratio-designable Janus microspheres

Angular optical trapping based on Janus microspheres has been proven to be a novel method to achieve controllable rotation. In contrast to natural birefringent crystals, Janus microspheres are chemically synthesized of two compositions with different refractive indices. Thus, their structures can be artificially regulated, which brings excellent potential for fine and multi-degree-of-freedom manipulation in the optical field. However, it is a considerable challenge to model the interaction of heterogeneous particles with the optical field, and there has also been no experimental study on the optical manipulation of microspheres with such designable refractive index distributions. How the specific structure affects the kinematic properties of Janus microspheres remains unknown. Here, we report systematic research on the optical trapping and rotating of various ratio-designable Janus microspheres. We employ an efficient T-matrix method to rapidly calculate the optical force and torque on Janus microspheres to obtain their trapped postures and rotational characteristics in the optical field. We have developed a robust microfluidic-based scheme to prepare Janus microspheres. Our experimental results demonstrate that within a specific ratio range, the rotation radii of microspheres vary linearly and the orientations of microsphere are always aligned with the light polarization direction. This is of great importance in guiding the design of Janus microspheres. And their orientations flip at a particular ratio, all consistent with the simulations. Our work provides a reliable theoretical analysis and experimental strategy for studying the interaction of heterogeneous particles with the optical field and further expands the diverse manipulation capabilities of optical tweezers.

Deeply subwavelength mid-infrared phase retardation with α-MoO3 flakes

Phase retardation is a cornerstone of modern optics, yet, at mid-infrared (mid-IR) frequencies, it remains a major challenge due to the scarcity of simultaneously transparent and birefringent crystals. Most materials resonantly absorb due to lattice vibrations occurring at mid-IR frequencies, and natural birefringence is weak, calling for hundreds of microns to millimeters-thick phase retarders for sufficient polarization rotation. Here, we demonstrate mid-IR phase retardation with flakes of α-MoO3 that are more than ten times thinner than the operational wavelength, achieving 90 degrees polarization rotation within one micrometer of material. We report conversion ratios above 50% in reflection or transmission mode, and wavelength tunability by several micrometers. Our results showcase that exfoliated flakes of low-dimensional crystals can serve as a platform for mid-IR miniaturized integrated low-loss polarization control.

Second-order topological magneto-optical effects in noncoplanar antiferromagnets

The second-order magneto-optical effects, represented by Voigt and Sch\"{a}fer-Hubert effects, are effective methods to detect the spin textures in antiferromagnets, whereas the previous studies are usually limited to collinear antiferromagnets. In noncollinear antiferromagnets, the spin textures characterized by spin chirality have been revealed to play a critical role in many exciting physics. In particular, the first-order topological magneto-optical effects originated from scalar spin chirality have been discovered recently. In this work, using the first-principles calculations and group theory analysis, we generalize the first-order topological magneto-optical effects to the second-order cases, that is, topological Voigt and Sch\"{a}fer-Hubert effects, by taking the noncoplanar 3Q spin state of $\gamma$-Fe$_{x}$Mn$_{1-x}$ alloy as an example. The conventional Voigt and Sch\"{a}fer-Hubert effects are comparatively studied in the collinear 1Q and 2Q spin states of $\gamma$-Fe$_{x}$Mn$_{1-x}$ alloy. In addition, the natural linear birefringence due to crystal anisotropy is discussed in the strained 1Q, 2Q, and 3Q states, and a unique fingerprint for experimentally distinguishing the second-order topological magneto-optical effects and natural linear birefringence is identified. Our work brings a topological insight into the second-order magneto-optical effects in noncoplanar antiferromagnets and also provides $\gamma$-Fe$_{x}$Mn$_{1-x}$ as an attractive material platform for future experimental exploration.

Optimization of the electrospun-aligned microfiber composite with liquid crystal for terahertz wave variable phase shifters

AbstrcatIn this study, electrospun microfibers (MFs) were produced and used as key materials to optimize the composite with liquid crystal (LC) for terahertz (THz) wave phase shifters. These MFs, with an average diameter of 1.4 μm, were produced using the polymer solution concentration of 14 wt% and spinning voltage of 13 kV. When MFs were combined with LC, the measured results of the electrical characteristics showed that 14 wt% MFs provided an outstanding solution to the problem of a significant reduction in the natural birefringence of pure LC. The birefringence in the THz frequency range of the composite using 14 wt% MFs approached 90% compared with that using pure LC. Additionally, the decay time drastically shortened from hundreds of seconds for pure LC to hundreds of milliseconds for MF/LC composite and was independent of device thickness.

A simple and comprehensive electromagnetic theory uncovering complete picture of light transport in birefringent crystals

Birefringence production of light by natural birefringent crystal has long been studied and well understood. Here, we develop a simple and comprehensive rigorous electromagnetic theory that allows one to build up the complete picture about the optics of crystals in a friendly way. This theory not only yields the well-known refraction angle and index of ellipse for birefringence crystal, but also discloses many relevant physical and optical quantities that are rarely studied and less understood. We obtain the reflection and transmission coefficient for amplitude and intensity of light at the crystal surface under a given incident angle and show the electromagnetic field distribution within the crystal. We derive the wavefront and energy flux refraction angle of light and the corresponding phase and ray refractive index. We find big difference between them, where the phase refractive index satisfies the classical index of ellipse and Snell’s law, while the ray refractive index does not. Moreover, we disclose the explicit expressions for the zero-reflection Brewster angle and the critical angle for total internal reflection. For better concept demonstration, we take a weak birefringent crystal of lithium niobate and a strong birefringent crystal tellurium as examples and perform simple theoretical calculations. In addition, we perform experimental measurement upon z-cut lithium niobate plate and find excellent agreement between theory and experiment in regard to the Brewster angle. Our theoretical and experimental results can help to construct a clear and complete picture about light transport characteristics in birefringent crystals, and may greatly facilitate people to find rigorous solution to many light–matter interaction processes happening within birefringent crystals, e.g., nonlinear optical interactions, with electromagnetic theory.

A tunable single-longitudinal-mode wedge Nd:YVO4 laser with a YVO4 wave-plate

We report a tunable single-longitudinal-mode laser based on a wedged Nd:YVO4 gain medium and a YVO4 wave-plate. By taking advantage of the natural birefringence of YVO4 and adopting different cutting methods, a wedged Nd:YVO4 crystal acts as an ideal polarizer by the alignment sensitivity of the optical resonator, and a YVO4 crystal is used as a wave-plate by adjusting the temperature. A theoretical analysis of the single- longitudinal-mode operation of such a laser configuration is presented. The wavelength can be continuously tuned by the temperature at approximately 0.025 nm/°C, and the temperature tuning ranges of single-longitudinal-mode operation at pump powers of 1 W and 2 W are from 12 °C to 30 °C and from 11 °C to 26 °C, respectively. The maximum single-longitudinal-mode output power with linear polarization is 762 mW at a pump power of 2.2 W with a slope efficiency of 40%.

High Power SESAM Mode-Locked Laser Based on Yb<sup>3+</sup>:YAlO<sub>3</sub> Bulk Crystal

Yttrium aluminium perovskite YAlO3 (YAP) crystal, doped with rare-earth ions, has been extensively studied as a diode-pumped laser host material. The wide interest to rare-earth ions doped YAP crystals is explained by its good thermal and mechanical properties, high natural birefringence, widely used Czochralski growth method. The aim of this work was to study the Yb3+:YAlO3 crystal as an active medium for high power mode-locked laser.Yb3+-doped perovskite-like aluminate crystals have unique spectroscopic and thermooptical properties that allowed using these crystals as an active medium of high power continuous wave (CW) and modelocked (ML) bulk lasers with diode pumping.growth method. The aim of this work was to study the Yb3+:YAlO3 crystal as an active medium for high power mode-locked laser.In our work spectroscopic properties of Yb:YAP crystal and laser characteristics in CW and ML regimes are investigated. Maximum output power of 4 W with optical-to-optical efficiency of 16.3 % and 140 fs pulse duration have been obtained for Yb:YAP E //c-polarization with 10 % output coupler transmittance. Tunability range as wide as 67 nm confirms high promise of using Yb:YAP crystal for lasers working in wide spectral range.

Precise Method for Measuring the Quadratic Electro-Optic Effect in Noncentrosymmetric Crystals in the Presence of Natural Birefringence.

The application of the improved dynamic polarimetric method for the measurement of the quadratic electro-optic effect in NH4H2PO4 (ADP) crystal with the light beam propagating perpendicularly to its optical axis is presented. This technique can be applied in noncetrosymmetric crystals in the presence of natural birefringence even when the fast and slow rays diverge slightly, causing them to only partially interfere. The method allows for minor errors in cutting and orientation of the crystal samples, resulting in deviations from configurations in which the crystal symmetry vetoes the linear electro-optic effect. The occurring contribution of the linear effect, if it is not too large, not only does not exclude the measurement of the quadratic effect, but increases its accuracy. The method does not require any prior compensation for the natural birefringence. Its sensitivity allows for quadratic electro-optic effect measurements in ferroelectrics in temperatures significantly different from the phase transition temperature or in paraelectric crystals, for which this effect is relatively small.

Broadband terahertz transmissive quarter-wave metasurface

Polarization conversion devices are key components in spectroscopy and wireless communications systems. Conventional terahertz waveplates made of natural birefringent materials typically suffer from low efficiency, narrow bandwidth, and substantial thickness. To overcome the limitations associated with conventional waveplates, a terahertz quarter-wave metasurface with enhanced efficiency and wide bandwidth is proposed. The transmissive quarter-wave metasurface is rigorously designed based on an extended semi-analytical approach employing network analysis and genetic algorithm. Simulation results suggest that the design can achieve linear-to-circular polarization conversion with a 3-dB axial ratio relative bandwidth of 53.3%, spanning 205 GHz–354 GHz. The measurement results confirm that the proposed design enables a 3-dB axial ratio from 205 GHz to at least 340 GHz with a total efficiency beyond 70.2%, where the upper frequency bound is limited by the available experimental facility. This quarter-wave metasurface can cover an entire terahertz electronics band and can be scaled to cover other nearby bands under the same convention, which are technologically significant for future portable systems.

Photoelastic Characterization of Residual Strain Distribution in Commercial Off-Axis SiC Substrates

Two-dimensional distribution of strain-induced birefringence has been selectively characterized in commercial 4° off-axis wafers of 4H-silicon carbide (SiC) crystal by using an originally-developed imaging polariscope. The natural birefringence, which is often observed in uniaxial crystals, was eliminated by optimizing the light incident angle so that the light propagates parallel to the c-axis inside the crystal. The eliminated natural birefringence was at least twice as large as that of the strain-induced ones, which demonstrates the validity of the elimination process. A typical retardation map under optimal incidence revealed various distributions of strain-induced birefringence, such as local concentrations due to the structural crystal defects and gradual increase or relief due to the thermal stress during the crystal growth and cooling processes. The maximum retardation was 0.28, which corresponds to 7.6 × 10−5 in strain terms, and 30 MPa in stress terms. Because of its sufficient sensitivity to the strain-induced birefringence and elimination capability in the natural birefringence, it is concluded that our technique is useful to characterize the residual strain distribution in commercial off-axis SiC substrates.

Engineering optical anisotropy in nonlinear crystals with ultrafast light

Photonic technology is widely based on anisotropic (and) nonlinear materials, which allow light modulation and parametric light conversion. Because the number of naturally occurring crystals is limited, there is a growing demand for artificial metamaterials with optical properties specifically tailored to a given application. Here, we utilize the top-down method to synthesize sub-wavelength periodic nanostructures inside a uniaxial optically nonlinear crystal (lithium niobate, LiNbO3) by irradiating it with multiple femtosecond laser pulses. By superimposing form-birefringence associated with the light-induced nanostructures onto natural birefringence of the host crystal we create macroscopic domains of a biaxial metamaterial embedded into otherwise uniaxial medium.

Active Terahertz Anisotropy and Dispersion Engineering Based on Dual-frequency Liquid Crystal and Dielectric Metasurface

Active broadband terahertz (THz) phase difference and its dispersion engineering based on the combination of dielectric metasurface and dual-frequency liquid crystal (DFLC) has been demonstrated, which can realize broadband achromatic wave plates and active manipulation by tuning the alternating ( AC ) frequency of electric field from 1 to 90 kHz. The artificial anisotropy of the dielectric metasurface is introduced to offset the tunable natural birefringence of the DFLC, so that the effective tuning range of the composite metadevice has been significantly improved, across from 0 to 1.0π, thus realizing the conversion from linear polarization (LP) light to arbitrarily polarized light. Moreover, a negative-dispersion phase difference of the dielectric metasurface can be achieved in the range of 0.69–1.4 THz by designing proper geometric parameters, which can be compensated with the positive-dispersion phase difference of DFLC to realize an achromatic device with broadband zero dispersion. The results show that the device can be tuned from OFF state without polarization conversion to a half-wave plate in 0.97–1.3 THz with the polarization conversion ratio (PCR) of over 97%, or to a quarter-wave plate in 0.67–1.3 THz with the PCR of over 96% by changing the AC frequency of the electric field. The active AC frequency tuning characteristics, anisotropic enhancement and broadband dispersion compensation show its great potential in THz application systems.

Measurement of quadratic electrogyration effect in KDP crystals for light propagating along the optical axis

In this paper, a method of measuring the β31 coefficient of the quadratic electrogyration effect in potassium dihydrogen phosphate (KDP)-type crystals is proposed. It is shown that this very weak effect can be decoupled from other stronger effects occurring simultaneously in the real measuring system, even when small inaccuracies in crystal cutting and alignment are taken into account. Theoretical and numerical analyses are illustrated by experimental data obtained for the KDP crystal. High sensitivity, which is required for successful measurements at room and higher temperatures, has been achieved by using the polarimetric method for a light beam propagating along the optical axis of the crystal in combination with the lock-in technique and a new measurement procedure that compensates for the contribution of other effects such as electro-optic effects, natural birefringence and dichroism. It was found that the β31 coefficient measured at a wavelength of 632.8 nm decreases linearly from 2.53 × 10−21 m2 V−2 at a temperature of 298 K to 2.08 × 10−21 m2 V−2 at 343 K.

Analysis of collagen fibers in keratocystic odontogenic tumor and ameloblastoma: A polarizing microscopic study.

Collagen forms an integral part of connective tissue and maintains its structural integrity. It has natural birefringence which is attributed to the arrangement of its fibers and is enhanced by special stains such as picrosirius red through polarizing microscopy. The polarization colors differ according to the fiber thickness and pattern of arrangement which in turn related to aggressiveness. Hence, the present study was conducted to evaluate collagen fibers in keratocystic odontogenic tumor (KCOT) and ameloblastoma using polarizing microscopy. This study aims to compare and correlate different types and patterns of collagen fibers in KCOT and ameloblastoma using picrosirius red stain under polarizing microscopy to delineate their aggressiveness. The color, thickness, and orientation of collagen fibers in the KCOTs (n = 15) and ameloblastomas (n = 15) were studied histochemically by staining the sections with picrosirius red and examined under polarizing microscope using image analyzer software. When collagen fiber bundles in KCOT and ameloblastoma were compared, significant difference was noted between yellowish-orange collagen fiber bundles, but no significant difference was observed between greenish-yellow and orange-red collagen bundles. With respect to orientation and organization, the results are statistically significant (P < 0.05). The connective tissue stroma of KCOT could be regarded not just as a structural support but as a functional part of the lesion. In KCOT, the thin, parallel, and loosely arranged greenish-yellow collagen fibers may be attributed to its high recurrence rate and biological aggressiveness.

Influence of the Polarization Mode Dispersion on Propagation of Ultrashort Optical Pulses in the Spun Light Guides with Very Weak Linear Birefringence and Random Inhomogeneities

Although the spun light guides were developed in early 1980s, their quantity production has started only a few years ago. At present, the spun light guides with both strong and weak natural (unperturbed) linear birefringence are produced. The former are used for creating sensors of various physical parameters, mainly, current and magnetic field. The latter are very promising for use in fiber communication lines, since they have a considerable advantage over the conventional telecommunication light guides. In this work, we consider the influence of the polarization mode dispersion due to random inhomogeneities in the single-mode fiber light guides (SMFLGs) on propagation of ultrashort optical pulses in the fiber communication lines, which are based on the SMFLGs with very weak linear birefringence and strong regular twisting (such SMFLGs are called telecommunication spun light guides). The dependences of evolution of the envelope function of the ultrashort optical pulses and their spectra on the length of a spun light guide with very weak linear birefringence and random inhomogeneities are obtained by the numerical-simulation method. It is shown that an increase in the pulse width is proportional to the SMFLG length rather than occurs as per the diffuse law, as it is the case in the absence of torsion. It is established that the pulse widening to a certain (rather long) length with increasing length of the spun light guide occurs much slower than that in the untwisted light guides and faster for very long lengths. It is also shown that the relative intensity of radiation which is pumped from one orthogonal polarization mode to another over the depolarization length of nonmonochromatic radiation in the telecommunication spun light guides is small compared with unity, whereas an opposite situation is observed in the conventional telecommunication light guides.

Polarization switching realized in the continuous-wave and acousto-optic Q-switched pulse Er:Yb:LaMgB5O10 lasers at 1556 and 1568 nm.

By using the natural birefringence of crystalline quartz, the switching of a 1568 nm laser with polarization parallel to the Z optical indicatrix axis to a 1556 nm laser with polarization parallel to the Y optical indicatrix axis was observed in both continuous-wave and acousto-optic Q-switched pulse lasers based on an X-cut Er:Yb:LaMgB5O10 crystal, when the alignment of the output mirror in a 976 nm diode-end-pumped plano-concave resonator was precisely tilted. For the continuous-wave regime, a 1568 nm laser with a maximum output power of 500 mW and slope efficiency of 16%, as well as a 1556 nm laser with a maximum output power of 400 mW and slope efficiency of 13.5% were realized, respectively. For the Q-switched regime, a 1568 nm pulse laser with an energy of 144 μJ and width of 300 ns, as well as a 1556 nm pulse laser with an energy of 168 μJ and width of 270 ns, were obtained respectively by precisely tilting the alignment of the output mirror, when the absorbed pump power was 4.0 W and pulse repetition frequency was 0.5 kHz.

Evaluation of dermal collagen stained with picrosirius red and examined under polarized light microscopy

The special picrosirius red staining highlights the natural birefringence of collagen fibers when exposed to polarized light. The results from birefringence allow to evaluate the organization of the collagen fibers in the tissues. The authors intend to elucidate all steps to obtain and capture images of histological sections stained with picrosirius red and evaluated under polarized light microscopy, as well as possible artefacts that may occur.

Generation of Orthogonally Polarized Mode-Locked Lasers at Wavelength of 1342 nm

A mode-locked laser with orthogonal polarization at the wavelength of 1342 nm is experimentally achieved by using the natural birefringence of the Nd:YVO4 crystal. A total output power of 2.64 W could be obtained at an incident power of 14.4 W and the pulse durations are measured to be 15.1 and 16.9 ps with pulse repetition rates of 6.0 and 6.45 GHz for the laser output along the π - and σ -polarization, respectively. The mode-locked output is further found to exhibit a complex temporal trace with beat frequencies originated from the different central frequencies and pulse repetition rates.