Negative Uniaxial Crystal Research Articles

High-energy single-longitudinal-mode Q-switched double-cladding fiber laser based on the injection seeding technique.

This paper demonstrates a high-energy, single-longitudinal-mode (SLM), actively Q-switched fiber laser based on the injection seeding technique. The large-mode-area double-cladding fiber is used as the gain medium to improve energy storage. Simultaneously, by using the linear electro-optic effect of the negative uniaxial crystal (β-B a B 2 O 4, BBO), a matching frequency-shift-free Q-switch with high damage threshold and high extinction-ratio is designed. Before reaching the stimulated Brillouin scattering threshold, the SLM Q-switched pulses can be achieved with energy higher than 15µJ over a wide range of repetition rates from 10 to 80kHz, and the maximum output power reaches 1.2W at the repetition rate of 80kHz, which may be the highest output pulse energy for such a SLM Q-switched fiber laser so far, to our best knowledge.

Separating radial and azimuthal polarizations of circular Airy vortex beam via uniaxial crystal.

Since Ciattoni A. et al. found that a particular circularly polarized beam propagating along the optical axis in a uniaxial crystal can generate a vortex with a reversed circular polarization, numerous studies of spin-orbit coupling in this polarization conversion process have been carried out. In this paper, from another perspective rather than the circular polarization conversion, for the first time we find that radial- and azimuthal-polarization components will be separated and finally focus on two separated focus points when circular Airy vortex beams propagate in a uniaxial crystal. Both the separation of the radial- and azimuthal-polarization components in positive and negative uniaxial crystals are investigated, and the physical mechanism of this phenomenon is explained in details. Moreover, the influences of the crystal length and birefringence on the separation of the radial- and azimuthal-polarization components are also discussed. Our results could offer deeper understanding of the propagation of light beam in uniaxial crystal and facilitate the flexible applications of circular Airy vortex beams.

Density Functional Study of the Electronic, Elastic and Optical Properties of Bi2O2Te

Abstract Layered crystal Bi2O2Te has recently been found to have high electron mobility and excellent thermoelectric properties for technical applications; however, its other properties are not well studied yet. In this work, the electronic, elastic and optical properties of Bi2O2Te are systematically studied using the density functional method. The results indicate that Bi2O2Te is a narrow band gap semiconductor. The gap is formed by the Te 5p orbital at the top of the valence band and the Bi 6p orbital at the bottom of the conduction band. There are both ionic and covalent interactions within the Bi–O layers, and these layers are linked by the ionic Bi–Te bonds forming the crystal. Bi2O2Te is mechanically stable but anisotropic. It is easy to fracture along the c axis under shear stress. Its shear modulus is far smaller than its bulk modulus, so shape deformation is easier to occur than pure volume change. Its melting point is predicted to be 1284.0 K based on an empirical formula. The calculated refractive index at zero frequency reveals that Bi2O2Te is a negative uniaxial crystal with a birefringence of 0.51, making it a potential tuning material for optical application. The characteristics and origins of other optical properties are also discussed.

Determination of the Subpicosecond Laser Pulse Chirp in the Middle IR Range Based on the Fourth Harmonic Noncollinear Generation

A new method is proposed to determine the subpicosecond laser pulse chirp in the middle IR range at the central wavelength of 10 μm, based on the generation of the second-harmonic pulses both by the bandwidth-limited and frequency-modulated subpicosecond pulses and the subsequent noncollinear generation of the fourth-harmonic radiation by the corresponding second-harmonic pulses. The time dependences are given of the instantaneous frequency of the frequency-modulated second-harmonic pulse at the central wavelength of 5 μm, generated in the field of the frequency-modulated subpicosecond IR pulse, propagating in the negative uniaxial AgGaS2 crystal along the direction of 61°36′ relative to the optical axis. These results can be used in designing a nonlinear optical phase correlator to determine the phase and time profile of the subpicosecond laser pulse in the middle IR range.

Electronic, optical, infrared, and elastic properties of KCdCO3F from first principles

KCdCO3F is a newly synthesized promising ultraviolet nonlinear optical crystal, but its structure is disputed and its fundamental properties have not been well studied. Here our first-principles study indicates that the structure with the space group P6¯c2 is energetically more stable than the P6¯m2 phase. We systematically investigated its electronic, optical, vibrational, infrared, and elastic properties. The results reveal that KCdCO3F is a direct-band-gap insulator with rather flat bands below the Fermi level. Analyses of its partial density of states revealed that the top (bottom) of its valence (conduction) band is formed by the O 2p (Cd 5s) orbital. It is a negative uniaxial crystal with ionic-covalent nature. Both infrared-active and Raman-active modes exist at its Brillouin zone center, and ions contribute more to its static dielectric constants. Its optical spectra in the visual and infrared ranges were studied, and their origins were revealed. Calculations indicate that KCdCO3F is mechanically stable but anisotropic since it is more vulnerable to shear stress and is easy to cleave along the c axis.

Crystal Growth and Optical Properties of Beryllium-Free Nonlinear Optical Crystal K3Ba3Li2Al4B6O20F

A beryllium-free nonlinear optical (NLO) crystal K3Ba3Li2Al4B6O20F (KBLABF) with sizes up to 18 × 13 × 8 mm3 was successfully grown by the top-seeded solution growth method using LiOH-H3BO3–BaF2 flux. The key limiting factors on the crystal morphologies and sizes were carefully analyzed. Meanwhile, by using the minimum deviation technique, the refractive index dispersion was measured and fitted well by the Sellmeier equations. The results suggest that KBLABF is a negative uniaxial crystal with moderate birefringence of 0.06050–0.06834 at a wavelength range from 363 to 852.1 nm. Furthermore, the type-I phase matching range for fundamental (second harmonic) light is down to 486 (243 nm) in theory, which indicates that KBLABF crystal could achieve 532 and 266 nm light generation by direct second-harmonic generation. As compared with the famous NLO crystals β-BaB2O4 and CsLiB6O10, KBLABF crystal exhibits a smaller walk-off angle and larger acceptance angle at the wavelength of 532 nm while having a comparable...

Spatial asymmetry of optical parametric fluorescence with a divergent pump beam and potential applications.

The spatial distribution of the optical parametric fluorescence generated in a negative uniaxial nonlinear crystal is asymmetric with respect to the pump when the pump beam has a slight divergence angle. The formation mechanism of this phenomenon and the influence of parameters were analyzed and discussed from a theoretical standpoint. Moreover, two potential applications of this phenomenon were experimentally demonstrated, showing the temporal contrast improvement of ultra-intense lasers and the intensity enhancement of special light sources induced by the optical parametric generation.

Second harmonic generation of off axial vortex beam in the case of walk-off effect

Process of off axial vortex beam propagating in negative uniaxial crystal is investigated in this work. Firstly, we get the formulae of the normalized electric field and calculate the location of vortices for second harmonic beam in two type of phase matching. Then, numerical analysis verifies that the intensity distribution and location of vortices of the first order original vortex beam depend on the walk-off angle and off axial magnitude. It is shown that, in type I phase matching, the distribution of vortices is symmetrical about the horizontal axis, the separation distance increases as the off axial magnitude increases or the off axial magnitude deceases. However, in type II phase matching, the vortices are symmetrical along with some vertical axis, and increase of the walk-off angle or off axial magnitude leads to larger separation distance. Finally, the case of high order original off axial vortex beam is also investigated.

A novel numerical technique for simultaneous calculations of type II phase matching angle and temperature for second harmonic generation in negative uniaxial crystals

In the present work, the phase matching type II conditions using birefringence for second harmonic generation (SHG) in negative uniaxial crystals were studied. A novel numerical method allowing the simultaneous and reliable calculations of the type II phase matching angle and temperature versus fundamentals wavelengths for SHG in Lithium Niobate (LiNbO3) was proposed. The angles and temperatures were determined for values less or equal to 5×10−6 of uncertainty. Indeed, SHG efficiency curves at fundamental frequency f=1.6 THz obtained for various error values highlighted the validity and accuracy of the calculations. The proposed method was also applied in the case of the type I phase matching SHG using both the same crystal and precision. A good agreement between the analytical and numerical results was obtained.

Spatial and spectral properties of entangled photons from spontaneous parametric down-conversion with a focused pump

The spatial and spectral properties of entangled photons from spontaneous parametric down-conversion (SPDC) with a focused pump are investigated with theoretical analysis and numerical simulation. Here, the spatial and spectral properties of down-converted photons are fully calculated without using the transverse momentum conservation assumption. We have obtained the spatial and spectral properties of SPDC photons under focused pumping in various SPDC configurations, including type-I, type-II for both positive and negative uniaxial crystals. Our result not only helps to understand the effect of pump focusing in SPDC better but also can find use in application of SPDC in realizing quantum information protocols.

Generalized Jones matrix method for homogeneous biaxial samples.

The generalized Jones matrix (GJM) is a recently introduced tool to describe linear transformations of three-dimensional light fields. Based on this framework, a specific method for obtaining the GJM of uniaxial anisotropic media was recently presented. However, the GJM of biaxial media had not been tackled so far, as the previous method made use of a simplified rotation matrix that lacks a degree of freedom in the three-dimensional rotation, thus being not suitable for calculating the GJM of biaxial media. In this work we propose a general method to derive the GJM of arbitrarily-oriented homogeneous biaxial media. It is based on the differential generalized Jones matrix (dGJM), which is the three-dimensional counterpart of the conventional differential Jones matrix. We show that the dGJM provides a simple and elegant way to describe uniaxial and biaxial media, with the capacity to model multiple simultaneous optical effects. The practical usefulness of this method is illustrated by the GJM modeling of the polarimetric properties of a negative uniaxial KDP crystal and a biaxial KTP crystal for any three-dimensional sample orientation. The results show that this method constitutes an advantageous and straightforward way to model biaxial media, which show a growing relevance for many interesting applications.

Frequency doubling effect of off-axial vortex beam in the case of weak walk-off

By using the truncated equations of the coupled wave, the expressions of the normalized electric field and the normalized intensity of the frequency doubling beam are derived in the cases with and without walk-off effect, caused by the off-axial vortex beam propagating through the negative uniaxial crystal. The influences of the off-axial magnitude, the weak walk-off angle and the crystal length on the output frequency doubling beam are mainly investigated. It is shown that while the walk-off angle is neglected, the two dark cores of the frequency doubling beam coincide with the point (0, 0); but while the weak walk-off angle is taken into account, the dark cores move along the direction where the walk-off effect is generated, and the two coincident dark cores separate in the direction perpendicular to the direction where the walk-off effect is generated on the cross-section. Especially, the distance the dark cores move is related to the off-axial magnitude, the weak walk-off angle and the crystal length. But the separation distance of the two dark cores is only related to the weak walk-off angle and the crystal length. The exact details show that when the off-axial magnitude increases, the distance the dark cores move along the direction where the off-axial magnitude is generated increases at the same time, but the separation distance has no connection with he off-axial magnitude. When the weak walk-off angle and the crystal length increase, the distance the dark cores move and the separation distance of the two dark cores increase. In addition, it can be found by comparison that when the crystal length reduces, the distance the dark cores move will decrease. And also the separation distance of the two dark cores will reduce, which is induced by the walk-off effect and the off-axial magnitude. Therefore, by reducing the crystal length, the output frequency doubling beam can be modified to a certain degree.

Optical anisotropy and blue-shift phenomenon in tetragonal BiFeO3

We study the uniaxial optical anisotropy of tetragonal BiFeO3 using first-principles density-functional theory and the Heyd–Scuseria–Ernzerhof hybrid functional in the near infrared to near ultraviolet range, and compare the results with related experiment and theory. It is found that the dielectric function is overall blue shifted compared with that of rhombohedral BiFeO3, and the charge transfer excitations are ∼0.3 eV higher than those of the rhombohedral counterpart, which is caused by the symmetry breaking of FeO6 octahedral. Furthermore, it is a negative uniaxial crystal and shows significant birefringence.

Upconversion of Broadband Infrared Radiation into Visible Light for Different Pumping Parameters

The influence of pump radiation parameters such as the polarization and the spectral width of infrared radiation on the conversion of broadband radiation in lithium niobate crystals was investigated. The spectra of converted radiation were calculated for two types of phase matching in the negative uniaxial crystal by taking into account the convergence of the light beam in the crystal. Experimental spectra were obtained and compared with the calculated spectra.

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Phase matching of four-wave interactions of SRS components in birefringent SRS-active crystals

A new method has been proposed for achieving wave vector matching in four-wave interactions of frequency components upon SRS in birefringent SRS-active crystals. The method ensures anti-Stokes wave generation and enables a substantial reduction in higher order Stokes SRS generation thresholds. Phase matching directions in BaWO4 SRS-active negative uniaxial crystals and SrWO4 SRS-active positive uniaxial crystals have been found in the wavelength range 0.4 — 0.7 μm.

An imaging technique using rotational polarization microscopy for the microstructure analysis of carbon/carbon composites

A novel image analysis technique was proposed for microstructure investigation of carbon/carbon (C/C) composites. The rotational polarization microscopy was developed to meet the special imaging requirements. The samples of C/C composites were observed in reflection polarized light microscope, where the analyzer was rotated instead of the stage, and the polarizer was taken out. The bireflectance of like-graphite negative uniaxial crystal was analyzed. It was the theoretic foundation of image collection and data processing. The analyzer was rotated through 36 × 10° intervals without any movement of the specimen. The polished cross-section of C/C composites took micrographs at each analyzer orientation. All image data collected from the same field of view were processed by image registration and image fusion. The synthesized images were obtained by calculating the maximum and minimum gray values and their differences at each point of the million pixels at 18 orientations of the analyzer. They are unique and quite reliable to be applied to analyze the microstructure of C/C composites. Subsequently, image segmentation was performed, and the feature parameters of each component were calculated. Good agreement was found between the results from image analysis and experimental data.

Design and analysis of a polarization interference imaging spectrometer with expanded field of view

To obtain straight interference fringes over larger field of view and thus acquire high-accuracy interference data, a wide field of view polarization interference imaging spectrometer based on a combined Savart plate is presented. The Savart plate is composed of positive and negative uniaxial crystals. The relationship between the optical path difference and incident angle is given. By numerical modeling and analysis examples with similar spectral resolution and total thickness, The simulation results show that the field of view to obtain straight interference fringes for the combined Savat plate is ten times as large as that for the conventional Savart plate.

Design and analysis of wide-field-of-view polarization imaging spectrometer

Several novel designs of wide-field-of-view polarization imaging spectrometers based on combined Savart polariscopes are presented. By numerical modeling and analysis, we show that the field of view can be extended when the polariscopes are made of the same uniaxial crystal or positive and negative uniaxial crystals are combined. The designs with increased fields of view enable the acquisition of undistorted interferogram and high étendue for the spectrometer systems.

FOCUSING OF ELECTROMAGNETIC WAVE FROM HYPERBOLIC LENS INTO A UNIAXIAL CRYSTAL WITH AN ARBITRARY ORIENTATION OF THE OPTICAL AXIS IN THE PRESENCE OF ABERRATIONS

We derive integral representations suitable for studying the focusing of electromagnetic waves through a symmetrically hyperbolic focusing lens into uniaxial crystal in the presence of cylindrical and coma aberrations using Maslov's method. The uniaxial crystal used is the negative crystal LiNbO3. Numerical computations are made to obtain the results for focused flelds inside negative uniaxial crystal with several difierent orientations of the optical axis in the plane of incidence. The efiects of aberrations inside uniaxial crystal and isotropic medium are also noted. The results are compared with those obtained by Kirchhofi-Huygens integral and Maslov's method which are in good agreement.