Polarization-dependent Intensity Research Articles

Polarization-dependent intensity ratios in double resonance spectroscopy.

Double Resonance is a powerful spectroscopic method that unambiguously assigns the rigorous quantum numbers of one state of a transition. However, there is often ambiguity as to the branch (ΔJ) of that transition. Spectroscopists have resolved this ambiguity by using the dependence of the double resonance intensity on the relative polarization directions of pump and probe radiation. However, published theoretical predictions for this ratio are based upon a weak (i.e., non-saturating) field approximation. This paper presents theoretical predictions for these intensity ratios for cases where the pump field is strongly saturating in the two limits of transitions dominated by homogeneous or of inhomogeneous broadening. Saturation reduces but does not eliminate the magnitude of the polarization effect (driving the intensity ratio closer to unity) even with strong pump saturation. For the case of an inhomogeneously broadened line, such as when Doppler broadened linewidth dominates over the power-broadened homogeneous line width, a large fraction of the low pump power polarization anisotropy remains. This paper reports predicted polarization ratios for both linear and circular pump and probe field polarizations. The present predictions are compared with experimental measurements on CH4 ground state → ν3 → 3ν3 transitions recently reported by de Oliveira et al.63 and these are in better agreement than with the weak field predictions.

Strong, anisotropic, layer-independent second harmonic generation in multilayer SnS film.

Materials based on group IV chalcogenides exhibit extensive technologically important properties. Its unusual chemical bonding and off-centering of in-layer sublattices could cause chemical polarity and weakly broken symmetry, making optical field controlling feasible. Here, we fabricated large-area SnS multilayer films and observed unexpected strong SHG response at 1030 nm. The appreciable SHG intensities were obtained with an independence on layer, which is opposite to the generation principle of overall nonzero dipole moment only in odd-layer material. Taking GaAs for reference, the second-order susceptibility was estimated to be 7.25 pm/V enhanced by mixed-chemical bonding polarity. Further polarization-dependent SHG intensity confirmed the crystalline orientation of SnS films. The results imply surface inversion symmetry broken and nonzero polarization field modified by metavalent bonding should be the origin of SHG responses. Our observations establish multilayer SnS as a promising nonlinear material, and will guide in design of IV chalcogenides with improved optics and photonics properties for the potential applications.

Antiferromagnetic Spin Orientation and Magnetic Domain Structure in Epitaxially Grown MnN Studied Using Optical Second-Harmonic Generation

$\mathrm{Mn}\mathrm{N}$ is a centrosymmetric collinear antiferromagnet belonging to the transition-metal-nitride family with a high N\'eel temperature ($660\phantom{\rule{0.2em}{0ex}}\mathrm{K}$), a low anisotropy field, and a large magnetic moment ($3.3{\ensuremath{\mu}}_{B}$ per $\mathrm{Mn}$ atom). Despite several recent experimental and theoretical studies, the spin symmetry (magnetic point group) and magnetic domain structure of the material remain unknown. In this work, we use optical second-harmonic generation (SHG) to study the magnetic structure of thin epitaxially grown single-crystal (001) $\mathrm{Mn}\mathrm{N}$ films. Our work shows that spin moments in $\mathrm{Mn}\mathrm{N}$ are tilted away from the [001] direction and the components of the spin moments in the (001) plane are aligned along one of the two possible in-plane symmetry axes ([100] or [110]), resulting in a magnetic-point-group symmetry of 2/m1'. Our work rules out magnetic-point-group symmetries 4/mmm1' and mmm1' that have been previously discussed in the literature. Four different spin domains consistent with the 2/m1' magnetic-point-group symmetry are possible in $\mathrm{Mn}\mathrm{N}$. A statistical model based on the observed variations in the polarization-dependent intensity of the second-harmonic signal collected over large sample areas puts an upper bound of $0.65\phantom{\rule{0.2em}{0ex}}\ensuremath{\mu}\mathrm{m}$ on the mean domain size. Our results show that SHG can be used to probe the magnetic order in metallic antiferromagnets. This work is expected to contribute to the recent efforts in using antiferromagnets for spintronic applications.

Manipulating the polarization dynamics in a >10-GHz Er3+/Yb3+ fiber Fabry-Pérot laser.

In this work, we report on the vector and scalar soliton dynamics that result from inevitable fiber birefringence in an 8-mm Er3+/Yb3+ fiber based Fabry-Férot (FP) laser that has a free spectral range of up to 12.5 GHz. The generation of polarization-evolving vector solitons can largely degrade the performance of application systems, and the underlying mechanisms and manipulation technologies are yet to be explored. To realize the transition from vector to scalar (linearly polarized) state, we here incorporate the polarization selection effect (PSE) in the simulation model and the numerical results verify that only a small amount of PSE is sufficient for manipulating the soliton dynamics. It also reveals that, prominent polarization-dependent intensity discrimination can be acquired via geometry-induced oblique incidence to the Bragg mirror of the semiconductor saturable absorber mirror (SESAM), and we obtain switchable operating states by tilting the SESAM in the experiments. These efforts create a feasible method to manipulate high-repetition-rate pulse and may shed light on understanding the dissipative soliton dynamics in ultrafast fiber FP lasers.

Spatial mapping of a low-frequency combination Raman mode in twisted bilayer graphene

The exotic properties of twisted bilayer graphene (tBLG) are profoundly affected by interlayer interactions, which can be sensitively probed by the low-frequency Raman modes. However, conventional Raman spectroscopy lacks spatial resolution and, hence, can hardly disclose spatial-dependent inhomogeneous properties of interest. Herein, we demonstrate spatial mapping of low-frequency Raman modes in tBLG via micro-Raman spectroscopy (mRS) imaging. In our constructed tBLG, the shear (C) and out-of-plane breathing (ZO′) modes are directly mapped out, and, more importantly, a regional-dependent, highly localized awakening of the combination C+ZO′ mode is clearly visualized. Through a set of mRS-based analyses, including correlation of Raman G- and 2D-mode frequencies, G-band splitting, and polarization-dependent Raman intensity distributions, we reveal that its turning on/off correlates with the mechanical strain distributions in a sensitive fashion. The spatial mapping of this unique combination mode offers an indicator for scrutinizing the elusive strain in the tBLG system or the like.

Characterizing Raman modes and gas sensing features of functionalized tetragonal graphyne quantum dots: A first principles study

This theoretical work has availed the first-principles method to critically explore the characterizing Raman spectra of tetragonal graphyne quantum dots. The Raman spectra of these T-graphyne systems possess the fingerprint of both sp and sp2 bonded atoms. We have explored that sp atoms predominate in determining intense Raman peaks over 2000 cm−1. Group theoretical analysis also provides a different insight into the occurrence of Raman active modes in these systems. Furthermore, the energy preference guides us to substitutionally dope boron (B) and nitrogen (N) atoms into the most stable α-T graphyne structure. The doping process enhances the system's stability and reduces the energy gap to a value of 1.29 eV. The concentration and position of the BN pair in T graphynes can be identified from the intense BN stretching mode at ∼ 1890 cm−1. Moreover, angle-dependent polarized Raman analysis depicts the effect of parallel and perpendicular polarization on the vibrational modes of T-graphyne for different incident laser wavelengths of 532 nm, 633 nm, and 785 nm. The anisotropy in the period of polarization-dependent intensities can be used to identify the crystal orientation direction. Firm absorption peaks in the UV–Vis spectra are also discussed from the allowed electronic transitions. Furthermore, the BN doped α-T graphyne structure possesses a large dipole moment of 5.2 Debye and can adsorb carbon-monoxide (CO) molecules with fascinating recovery time. The I–V response of the adsorbed T-graphyne system exhibit reasonably good sensing behaviour within a bias range of 0.28–0.29 V. Our results on vibrational signatures, and gas sensing applications will serve as an essential benchmark for graphyne-related research.

Enhancement of terahertz radiation from a filament by using circularly polarized two-color laser fields

We investigate the terahertz radiation from a filament in two-color laser fields with various polarizations. It has been experimentally demonstrated that the efficiency of terahertz emission in co-rotating two-color circularly polarized laser fields is eight times as strong as that in linearly polarized two-color laser fields with parallel polarization. This enhancement is explained based on the polarization-dependent clamping intensity inside a filament and the dynamics of electrons ionizing from a single atom or molecule. Our finding provides a simple method to improve the THz intensity from an air filament in two-color fields.

Naturally occurring van der Waals heterostructure lengenbachite with strong in-plane structural and optical anisotropy

Lengenbachite is a naturally occurring layered mineral formed with alternating stacks of two constituent PbS-like and M2S3-like two-dimensional (2D) material layers due to the phase segregation process during the formation. Here, we demonstrate to achieve van der Waals (vdW) heterostructures of lengenbachite down to a few layer-pair thickness by mechanical exfoliation of bulk lengenbachite mineral. The incommensurability between the constituent isotropic 2D material layers makes the formed vdW heterostructure exhibit strong in-plane structural anisotropy, which leads to highly anisotropic optical responses in lengenbachite thin flakes, including anisotropic Raman scattering, linear dichroism, and anisotropic third-harmonic generation. Moreover, we exploit the nonlinear optical anisotropy for polarization-dependent intensity modulation of the converted third-harmonic optical vortices. Our study establishes lengenbachite as a new natural vdW heterostructure-based 2D material with unique optical properties for realizing anisotropic optical devices for photonic integrated circuits and optical information processing.

Effects of the Linear Polarization of Polariton Condensates in Their Propagation in Codirectional Couplers

We report on the linear polarization of polariton condensates in a codirectional coupler that allows evanescent coupling between adjacent waveguides. During the condensate’s formation, polaritons usually acquire a randomly oriented polarization, however, our results reveal a preferential orientation of the linear polarization along the waveguide propagation path. Furthermore, we observe polarization-dependent intensity oscillations in the output terminal of the coupler, and we identify the mode beating between the linear-polarized eigenmodes as the origin of these oscillations. Our findings provide an insight into the control of the polarization of polariton condensates, paving the way for the development of spin-based polaritonic architectures where condensates propagate over macroscopic distances.

Effect of thickness and stacking order on Raman spectrum of layered CrCl3

Layer- and stacking-dependent physical properties discovered in van der Waals materials have attracted a lot of attention recently. In this work, we have investigated the Raman spectrum of layered magnetic CrCl3 with different atomic thickness and stacking order from first principles. Our study indicates Raman shift of R and C2/m phase is near degenerate except that the first and fourth Raman resonant modes exhibit a marked peak difference about 3 cm−1, which can be understood by stacking-dependent vibrational eigenvectors. Moreover, we also find that both frequency and intensity of Raman modes of few-layer CrCl3 increase with the number of layers simultaneously. Furthermore, the Raman tensor and polarization-dependent Raman intensity are also evaluated in details, which are found to be in good agreement with available experiments. Our results proposed several distinctive signals to identify layer number and stacking order of layered CrCl3, which will provide an important reference to further experimental study.

Single-shot measurement of polarization state at low light field using Mueller-mapping star test polarimetry

Polarization measurement is often completed in the scenes with low signal-to-noise ratio (SNR) in some fields, such as biomedical sensing and remote sensing, resulting in the requirement for the polarimeter to conduct the real-time measurement of the polarization state at low light field. However, accurate measurement of the polarization state at low light field is challenging due to background-noise interference. Herein, we develop a Mueller-mapping star test polarimetry (MMSTP) method by quantitatively linking the Mueller matrix of a space-variant birefringence device and the polarization-dependent intensity distribution. Combined with a normalized least-squares method that accounts for the spatial distribution characteristics of photons at low light field, in addition to a low-pass fast-Fourier-transform filter, the proposed method can calculate polarization states from a single irradiance image at low light field with higher accuracy. Experimental results show that the proposed method can significantly improve the measurement accuracy of polarization states compared with the classical division-of-time Stokes polarimeters at low light field.

A new imaging concept in spin polarimetry based on the spin-filter effect.

The concept of an imaging-type 3D spin detector, based on the combination of spin-exchange interactions in the ferromagnetic (FM) film and spin selectivity of the electron-photon conversion effect in a semiconductor heterostructure, is proposed and demonstrated on a model system. This novel multichannel concept is based on the idea of direct transfer of a 2D spin-polarized electron distribution to image cathodoluminescence (CL). The detector is a hybrid structure consisting of a thin magnetic layer deposited on a semiconductor structure allowing measurement of the spatial and polarization-dependent CL intensity from injected spin-polarized free electrons. The idea is to use spin-dependent electron transmission through in-plane magnetized FM film for in-plane spin detection by measuring the CL intensity from recombined electrons transmitted in the semiconductor. For the incoming electrons with out-of-plane spin polarization, the intensity of circularly polarized CL light can be detected from recombined polarized electrons with holes in the semiconductor. In order to demonstrate the ability of the solid-state spin detector in the image-type mode operation, a spin detector prototype was developed, which consists of a compact proximity focused vacuum tube with a spin-polarized electron source [p-GaAs(Cs,O)], a negative electron affinity (NEA) photocathode and the target [semiconductor heterostructure with quantum wells also with NEA]. The injection of polarized low-energy electrons into the target by varying the kinetic energy in the range 0.5-3.0 eV and up to 1.3 keV was studied in image-type mode. The figure of merit as a function of electron kinetic energy and the target temperature is determined. The spin asymmetry of the CL intensity in a ferromagnetic/semiconductor (FM-SC) junction provides a compact optical method for measuring spin polarization of free-electron beams in image-type mode. The FM-SC detector has the potential for realizing multichannel 3D vectorial reconstruction of spin polarization in momentum microscope and angle-resolved photoelectron spectroscopy systems.

Polarization-dependent relative intensity noise of fiber supercontinuum sources

Detailed experimental polarization-dependent relative intensity noise (RIN) measurements of all-normal dispersion (ANDi) and conventional supercontinuum (SC) sources are presented. We show that the polarization-maintaining ANDi fiber suppresses polarization noise, is robust against pump polarization fluctua- tions, and allows ultra-low noise SC generation with RIN identi- cal to the pump laser (< 0.05%).

Broadened Bandwidth Amplified Spontaneous Emission from Blue GaN-Based Short-Cavity Superluminescent Light-Emitting Diodes

We report broad bandwidth blue superluminescent light-emitting diodes (SLEDs) based on a short-cavity active region. The dependencies of amplified spontaneous emission (ASE) output power and gain bandwidth on cavity length were investigated in devices whose gain medium consists of a ridge waveguide with embedded InGaN/GaN quantum wells sandwiched between one etched facet coated with a high reflectivity distributed Bragg mirror and one cleaved facet with an anti-reflection coating. 250 μm-long blue SLEDs exhibit a spectral bandwidth up to 7.5 nm at 1.72 mW output power at a wavelength of 427 nm. As cavity length decreases, the bandwidth gradually broadens up to 15 nm for the shortest, 40 μm-long, SLED devices. ASE is confirmed by current-dependent electroluminescence spectra and polarization-dependent emission intensity measurements. The optical features of those short-cavity devices could be helpful for designing broad bandwidth SLEDs aiming for various applications such as optical coherence tomography, next generation displays, on-chip biosensing and imaging.

Numerical and Experimental Characterization of Radiation Mode of 45° Tilted Fiber Grating

In this paper, we have numerically and experimentally investigated the radiation mode of 45° tilted fiber grating (45° TFG). By introducing the axial distributions into the volume current method, we have established a three-dimensional theoretical model to analyze spatial distributions of the radiation mode of 45° TFG, and measured the radiation mode spatial distributions in experiment. Both numerical and experimental results show that the radiation mode of 45° TFG exhibits an exponential reduction along the fiber axial direction, and a quasi-Gaussian profile along the radial direction of fiber. Additionally, we have also measured the detailed polarization-dependent azimuthal intensity distributions of radiation modes of both s- and p-polarized core modes. The degree of polarization of radiation mode is around 99.886%, which is independent of the length and the polarization extinction ratio of the grating. Moreover, for the first time, we have experimentally observed a very weak p-polarized component existing in the radiation mode, which has matched well with the simulation results.

Polarization-Insensitive Remote Electro-Optic Modulation for Distributed Antenna System Using a Novel Polarization Scrambling Method

This study investigates and experimentally demonstrates a novel polarization scrambling method to solve the polarization dependence of remote electro-optic modulation in the fiber-based distributed antenna system. Theoretically, if the polarization state of a monochromatic light is scrambled at a particular depth, the direct current component of the photocurrent would be independent of the polarization perturbation from the fiber link. A proof of concept experiment is performed in which a 500-MHz simulated polarization perturbation from the fiber is successfully suppressed by more than 40 dB. We then apply the proposed scrambling method to a 5-km remote electro-optic modulation fiber link and significantly reduce polarization-dependent intensity fluctuation from 42 to 0.6 dB. Furthermore, we evaluate the proposed scrambling method for remote photonic radio-frequency (RF) mixing, where the fluctuation of the converted signal is stabilized within 0.9 dB.

Platinum-coated Core-Shell Gold Nanorods as Multifunctional Orientation Sensors in Differential Interference Contrast Microscopy.

We characterized the optical properties of single platinum-coated core-shell gold nanorods (Pt-AuNRs) under dark-field (DF) and differential interference contrast (DIC) microscopy. Furthermore, we examined their potential use as multifunctional orientation probes. Longitudinal surface plasmon resonance damping was observed for single Pt-AuNRs due to Pt metals coated on the AuNR surface under single-particle scattering spectroscopy. Despite the strong plasmon damping with a much-decreased scattering intensity, DIC microscopy allowed us to detect single Pt-AuNRs with much higher sensitivity. We found polarization-dependent DIC images and intensities of single Pt-AuNRs, which allowed us to determine their orientation angle under DIC microscopy. Therefore, we report that single Pt-AuNRs can be used to develop multifunctional orientation probes under DIC microscopy.

Three-dimensional polarization aberration functions in optical system based on three-dimensional polarization ray-tracing calculus

The polarization aberrations of a complex optical system with multi-element lens have been investigated using a 3D polarization aberration function. The 3D polarization ray-tracing matrix has been combined with the optical path difference to obtain a 3D polarization aberration function, which avoids the need for a complicated phase unwrapping process. The polarization aberrations of a microscope objective have been analyzed to include, the distributions of 3D polarization aberration functions, diattenuation aberration, retardance aberration, and polarization-dependent intensity on the exit pupil. Further, the aberrations created by the field of view and the coating on the distribution rules of 3D polarization aberration functions are discussed in detail. Finally a novel appropriate field of view and wavelength correction is proposed for a polarization aberration function which optimizes the image quality of a multi-element optical system.

Characterizing the optical properties of single palladium-coated core-shell gold nanorods as multifunctional orientation probes.

Bimetallic core-shell gold nanorods are promising multifunctional orientation probes due to the synergetic effect of two different metals, as examined under an optical microscope. Herein, we performed a single particle study to characterize the optical properties of palladium-coated core-shell gold nanorods (Pd-AuNRs) using scattering-based dark-field (DF) and differential interference contrast (DIC) microscopy, and to examine their use as multifunctional orientation probes in biological and physical studies. The longitudinal surface plasmon resonance (SPR) was strongly damped with the increased SPR linewidth for single Pd-AuNRs due to Pd metals coated on the AuNR surface under single particle scattering spectroscopy. Despite the strong plasmon damping with much reduced scattering intensity, DIC microscopy allowed us to detect single Pd-AuNRs with much higher sensitivity. We found polarization-dependent DIC images and intensities of single Pd-AuNRs, which enabled the determination of their orientation angle under DIC microscopy. Furthermore, we found that DIC polarization anisotropy can be obtained from the orthogonally polarized bright and dark intensities for more accurate and reliable angle measurements of the Pd-AuNR probes. The results support the idea that bimetallic Pd-AuNRs can be used to develop multifunctional orientation probes.

Strong Second-Harmonic Generation in Atomic Layered GaSe.

Nonlinear effects in two-dimensional (2D) atomic layered materials have recently attracted increasing interest. Phenomena such as nonlinear optical edge response, chiral electroluminescence, and valley and spin currents beyond linear orders have opened up a great opportunity to expand the functionalities and potential applications of 2D materials. Here we report the first observation of strong optical second-harmonic generation (SHG) in monolayer GaSe under nonresonant excitation and emission condition. Our experiments show that the nonresonant SHG intensity of GaSe is the strongest among all the 2D atomic crystals measured up to day. At the excitation wavelength of 1600 nm, the SHG signal from monolayer GaSe is around 1-2 orders of magnitude larger than that from monolayer MoS2 under the same excitation power. Such a strong nonlinear signal facilitates the use of polarization-dependent SHG intensity and SHG mapping to investigate the symmetry properties of this material: the monolayer GaSe shows 3-fold lattice symmetry with an intrinsic correspondence to its geometric triangular shape in our growth condition; whereas the bilayer GaSe exhibits two dominant stacking orders: AA and AB stacking. The correlation between the stacking orders and the interlayer twist angles in GaSe bilayer indicates that different triangular GaSe atomic layers have the same dominant edge configuration. Our results provide a route toward exploring the structural information and the possibility to observe other nonlinear effects in GaSe atomic layers.