Towards high-performance polarimeters with large-area uniform chiral shells: a comparative study on the polarization detection precision enabled by the Mueller matrix and deep learning algorithm.

Polarization detection and imaging technologies have attracted significant attention for their extensive applications in remote sensing, biological diagnosis, and beyond. However, previously reported polarimeters heavily relied on polarization-sensitive materials and pre- established mapping relationships between the Stokes parameters and detected light intensities. This dependence, along with fabrication and detection errors, severely constrain the working waveband and detection precision. In this work, we demonstrated a highly precise, stable, and broadband full-Stokes polarimeter based on large-area uniform chiral shells and a post-established mapping relationship. By precisely controlling the geometry through the deposition of Ag on a large-area microsphere monolayer with a uniform lattice, the optical chirality and anisotropy of chiral shells can reach about 0.15 (circular dichroism, CD) and 1.7, respectively. The post-established mapping relationship between the Stokes parameters and detected light intensities is established through training a deep learning algorithm (DLA) or fitting the derived mapping-relationship formula based on the Mueller matrix theory with a large dataset collected from our home-built polarization system. For the detection precision with DLA, the mean squared errors (MSEs) at 710 nm can reach 0.10% (S1), 0.41% (S2), and 0.24% (S3), while for the Mueller matrix theory, the corresponding values are 0.14% (S1), 0.46% (S2), and 0.48% (S3). The in-depth comparative studies indicate that the DLA outperforms the Mueller matrix theory in terms of detection precision and robustness, especially for weak illumination, small optical anisotropy and chirality. The averaged MSEs over a broad waveband ranging from 500 nm to 750 nm are 0.16% (S1), 0.46% (S2), and 0.61% (S3), which are significantly smaller than those derived from the Mueller matrix theory (0.45% (S1), 1% (S2), and 39.8% (S3)). The optical properties of chiral shells, the theory and DLA enabled mapping-relationships, the combination modes of chiral shells, and the MSE spectra have been systematically investigated.

9-bit spin- and wavelength-encoded hologram using a hybrid metasurface.

Metasurface holographic encryption, with high resolution and strong concealment, is promising for information security. However, traditional metasurface holographic encryption is limited by spin-polarization channels and monochromaticity, restricting the level of security and information capacity. In this Letter, we propose a 9-bit spin- and wavelength-encoded hologram achieved by encoding phase and amplitude information into full spin polarization and primary color channels using a hybrid metasurface. Our hybrid metasurface structure can break the limitations of traditional metasurfaces, significantly increasing the number of encoded channels by an order of magnitude. We believe that this strategy can provide a new, to the best of our knowledge, approach to high-capacity information encoding and encryption.

Multiplicative-noise-multiplexing holography with ultrahigh capacity and low cross talk.

Optical multiplexing technologies, by utilizing various dimensions of light, can effectively expand the information capacity and density for holography but may also lead to multiplexing cross talk. Here, we propose and demonstrate a novel, to our knowledge, multiplicative-noise-multiplexing holography by utilizing the orthogonality between multiplicative noises as a multiplexing dimension. The results prove that this holography can provide a new multiplexing dimension, significantly enhancing information capacity and effectively lowering cross talk. This promising scheme for ultrahigh-capacity holography has the potential to address the limitations of traditional holographic multiplexing technologies.

Polarization-encoded optical secret sharing based on a dielectric metasurface incorporating near-field nanoprinting and far-field holography.

Metasurface encryption with high concealment and resolution is promising for information security. To improve the encryption security, a polarization-encoded secret sharing scheme based on dielectric metasurface by combining the secret sharing method with nanoprinting and holography is proposed. In this encryption scheme, the secret image is split into camouflaged holograms of different polarization channels and shares a total of 24-1 encryption channels. Benefiting from the secret sharing mechanism, the secret image cannot be obtained by decoding the hologram with a single shared key. Specifically, the secret hologram of a specific channel in the far field can be obtained by specifying the optical key, acquiring the near-field nanoprinting image to determine the combination order for the shared key, and decoding using multiple shared keys. The secret sharing encryption scheme can not only enhance the security level of metasurface encryption, but also increase the number of information channels by predefining camouflage information. We believe that it has important potential applications in large-capacity optical encryption and information storage.

Method for designing ultra-wideband absorbers based on water-filled Fabry-Perot cavity with continuously varying cavity length.

Broadband and efficient terahertz (THz) absorbers are crucial for various applications in sensing, imaging, detecting, and modulation. Although recent studies have reported a series of THz metamaterials for enhanced absorption, achieving high absorption across the entire ultrabroad terahertz band remains challenging. We propose a novel, to the best of our knowledge, method to design ultra-wideband terahertz absorbers using a water-filled Fabry-Perot cavity with continuously varying cavity length. Our design achieves over 90% absorption across an ultrabroad terahertz band ranging from 0.26 to 30 THz. Furthermore, the design method can be extended to the visible, infrared, and microwave regimes. We believe that our method will inspire further studies and applications of ultra-wideband absorbers.

Mitigation of the small-scale self-focusing effect by a rotating laser beam in a high-power laser.

Intense lasers tend to produce nonlinear effects during propagating through the nonlinear media, which greatly limits the output power and beam quality of lasers. The approach against small-scale self-focusing (SSSF) of high-power lasers (HPLs) is proposed by using rotating beams generated by the coherent superposition of two vortex beams with opposite topological charges and frequency shift. The propagation model of rotating beams in the nonlinear medium is established, and the SSSF effects of the non-rotating and rotating beams are numerically simulated and comparatively analyzed. The results show that, compared with the non-rotating beam, the rotating beam can contribute to the reduction of the breakup integral and mid-high frequency components of the HPLs.

Bifunctional metasurface for high-efficiency terahertz absorption and polarization conversion.

A reconfigurable metasurface with a switchable function, broad band, high efficiency, and ultra-compact size is crucial for the development of efficient and compact devices. We propose a bifunctional metasurface that utilizes vanadium dioxide (V O 2) and graphene to achieve high-efficiency absorption and polarization conversion (PC) in the terahertz (THz) range. In our design, an extra dielectric layer is added on the top of V O 2 and graphene. It is worth pointing out that the presence of the additional dielectric layer greatly enhances the coupling of the wave in the Fabry-Perot cavity, resulting in remarkable improvement in absorption and PC efficiency. Furthermore, by controlling the working state of V O 2 and graphene, the functionality of the metasurface can be flexibly switched among absorption, cross-polarized conversion, and linear-to-circular PC (LTC). Simulation results indicate that the metasurface works in the absorption mode when V O 2 is in a metal state, and it can efficiently absorb THz waves at 2.0-7.0 THz with a remarkable relative bandwidth of 111.1%. Furthermore, the absorption is over 98.4% under a normal incident case and still maintains over 90% with an incident angle of 50° at 2.8-7.0 THz. Importantly, by changing the conductivity of V O 2, the absorption can be flexibly adjusted, allowing for tuning the absorption between 10% and 98.4%. When V O 2 is in an insulator state, the function of the designed metasurface is altered to PC mode, and it can efficiently convert incident linearly polarized (LP) waves into cross-polarized waves with a PC ratio exceeding 95% at 1.8-3.4 THz when the Fermi level of graphene is 1 eV. When switched to the LTC mode, it can convert incident LP waves into right-circularly polarized waves with ellipticity less than -0.95 at 1.7-2.1 THz and into left-circularly polarized waves with ellipticity greater than 0.90 at 2.7-3.0 THz when the Fermi level of graphene is 0.55 eV.

Mitigation of atmospheric turbulence effect by light beams carrying self-rotating wavefront.

We propose an approach against the turbulence-induced degradation by using laser beam with self-rotating wavefront. Such laser beam, generated by the coherent combination of vortex beams with different helical charges and central angular frequencies, can introduce coupling of its wavefront in spatial and temporal domain, that is, periodic wavefront rotation. When the wavefront rotation is faster than the airflow, the laser beam can travel through the inhomogeneity and anisotropy of air in the azimuthal direction within the time interval of airflow. The wavefront distortion caused by the turbulent atmosphere is therefore rotated and gradually smoothed as the laser beam travels. After the laser propagating through the turbulent atmosphere, the total wavefront distortion becomes centrosymmetric with lower peak-to-valley (PV) value. Such smoothed wavefront distortion can dramatically eliminate the turbulence-induced degradation of laser beams, especially beam centroid drift. We believe that this approach can lead to new trend in remote sensing, free-space optical communication, lidar, etc.

Wavefront reconstruction method for aero-optical distortion based on compressed sensing.

The wavefront sensor plays an important role in the adaptive optics (AO) system for aero-optical distortion correction. However, the bandwidth of the current data interfaces of wavefront sensors, as one of the key factors, limits applications of the AO system in extremely high-frequency aero-optical distortion correction, leading to unsatisfactory performance. In this paper, a framework for wavefront data compression using compressed sensing is established to improve the correction ability of the AO system, and a disturbed Zernike gradient dictionary (DZGD) learning over the k-singular value decomposition algorithm is proposed for achieving good performance in the compression of aero-optical wavefront data. Based on the proposed DZGD, a method for aero-optical distortion data compression and wavefront reconstruction is developed that can efficiently reduce the amount of data in the information channel without degradation of the correction effect in aero-optical distortion correction. The compressibility of aero-optical distortions over the DZGD is analyzed in detail by numerical simulations. In addition, the selection criteria of the measurement matrix and the anti-noise characteristic of the method are also discussed. Data compression using our method is feasible and highly adaptable in the correction of aero-optical distortions, and exhibits stronger resistance against detector noise compared with using the conventional dictionary.

Effective optical smoothing scheme to suppress laser plasma instabilities by time-dependent polarization rotation via pulse chirping.

In this paper, we propose a novel effective optical smoothing scheme to suppress laser plasma instabilities (LPIs) by time-dependent polarization rotation (TPR) on a picosecond timescale. The polarization rotation with time-dependent frequency is generated by the superposition of chirped light pulses with dynamic frequency shift and counter-rotating circular polarization. Compared to light without polarization rotation or pulse chirping, such superposed light with TPR has a broader spectrum and lower temporal coherence. Using the one-dimensional fluid laser-plasma-instability code (FLAME) and PIC simulation, TPR is demonstrated working well in suppressing parametric backscattering, which provides an effective approach to suppress LPIs. In the meantime, a significant improvement of irradiation uniformity of the chirped pulses is achieved by the introduction of proper spatial phase modulation and grating dispersion.

Mitigation of thermal blooming by rotating laser beams in the atmosphere.

A novel method, to the best of our knowledge, for mitigating thermal blooming by using the rotating beam when propagating in the atmosphere is proposed. The rotating beam, generated by coherent superposition of two vortex beams with opposite topological charges and frequency shift, can directly modulate the heat source in time and then mitigate the thermal blooming in the atmosphere. The theoretical model of the rotating beam propagating in the atmosphere has been established, and the thermal blooming effects of the rotating beam and the conventional nonrotating beam through the atmosphere have been analyzed and compared. Results indicate that, compared to the nonrotating beam propagating in the atmosphere, the rotating beam is less affected by the thermal blooming and exhibits outstanding performance in mitigating the thermal blooming effect, resulting in smaller beam expansion, less shift of the beam centroid position, and better beam quality than those of the nonrotating beam.

Formation mechanism of the nanostructure in laser streaming phenomenon.

Laser streaming is a phenomenon in which liquid streaming is driven directly from the laser through an in situ fabricated nanostructure. In this study, liquid streaming of a gold nanoparticle suspension driven by a pulsed laser was studied using a high-speed camera. The laser streaming formation time, streaming velocity, and relative energy conversion efficiency of laser streaming was measured for different nanoparticle concentrations, focal lens position, laser powers, and laser repetition rates. In addition to the laser intensity, which played a significant role in the formation process of laser streaming, the optical gradient force was found to be an important approach involved in the transport and provision of nanoparticles during the formation of laser streaming. This finding facilitated a better understanding of the formation mechanism of laser streaming and demonstrated the possibilities of a new potential laser etching technique based on nanosecond lasers and nanoparticle suspensions. This result can also expand the application of laser streaming in microfluids and other fields that require lasers to move macroscopic objects at relatively high speeds.

Analysis of the smoothing performance of a rapid polarization rotation smoothing scheme.

In order to improve the irradiation uniformity on the target plane and reduce the parametric backscattering in indirect-drive inertial confinement fusion facilities, a rapid polarization rotation smoothing scheme is proposed. In this scheme, there is a central wavelength shift between adjacent beams in a laser quad. Two of these beams are transformed into left-rotating circularly polarized beams by using polarization control plates, and the other two are transformed into right-rotating circularly polarized beams. Moreover, conjugate spiral phase plates are adopted to transform the beams into conjugate Laguerre-Gaussian beams. As a result of the frequency beat among the beams with counter-rotating polarization and conjugate spiral phases, both the intensity distribution and polarization of the focal spot would rotate rapidly on the picosecond timescale. The physical model of the rapid polarization rotation smoothing scheme is built up, and the smoothing performance affected by the parameters of spiral phase plates, polarization control errors, wavelength shift, and wavefront distortions is analyzed in detail. The results indicate that the rapid polarization rotation smoothing scheme has a relatively wide tolerance to the polarization errors and the spiral phase plates, and can improve the smoothing performance effectively with the combination of the smoothing by spectral dispersion scheme.

Broadband negative-refractive index terahertz metamaterial with optically tunable equivalent-energy level.

An optically tunable terahertz negative-refractive index metamaterial (NIM) is proposed. The NIMs are composed of two aluminum rings and two photosensitive ring-shaped silicon apertures coaxially coated on the both sides of Teflon substrate. The NIMS are also designed to realize wide incident angle, polarization insensitivity, and tunability. Similar to the real atom, the unit cell of NIMs is equivalent to the Teflon nucleus surrounded by top and bottom resonator electrons, which indicates that the equivalent-energy level of NIMs can be dynamically controlled by the resonator electrons, once the scale of substrate nucleus is fixed. Using the LC-circuit model, the dynamic control of the equivalent-energy level of NIMs is studied in detail. Simulation results indicate that the transmission of NIMs is tuned from lowpass to highpass when the conductivity of silicon is increased, and the corresponding phase at lower frequency can be continually tuned. Correspondingly, the negative refractive index of NIMs represents dynamically tunable property, and the tunable negative refraction is simulated by classical wedge prism model. Besides, the phase flow indicates that the direction of phase velocity of NIMs is negative for the single-negative index.

Ultrafast smoothing scheme for improving illumination uniformities of laser quads.

The beam smoothing technology, smoothing by spectral dispersion, plays an important role in improving the illumination uniformities of the lasers in inertial confinement fusion facilities. However, due to the limitations of the modulation frequency of the electro-optic modulator, the uniformity of the lasers approaches an asymptotic value after tens of picoseconds that are much longer than the response time of laser plasma instabilities. To the best of our knowledge, this is the first time that an ultrafast smoothing approach for improving the uniformity of a laser quad in both radial and azimuthal directions in the picosecond scale was proposed. Among the four individual beams in a quad, two of them were smoothed by independent ultrafast focal zooming, and the rest were transformed into Laguerre-Gaussian (LG) beams that carry same topological charges with opposite signs. The focal spots of these two LG beams were coherent superposed, and their intensity distributions rotated rapidly in a period of several picoseconds. As a result of the focal zooming and rotation, an ultrafast and significant improvement of the uniformity of a laser quad was achieved.

Fatigue characteristics of deformable mirrors caused by fatigue of lead zirconate titanate.

The correction ability of deformable mirrors (DMs) with different fatigued degrees has been studied by quantitatively analyzing the fitting wavefront, the times-diffraction-limit factor ß, and the wavefront power spectral density (PSD). The results indicate that the fatigue of DMs can be ignored when the DM continually works below 105 cycles, while the performance of the DM degrades gradually if the loading cycles of the DMs exceed 105. Consequently, the peak-valley (PV) value of the fitting wavefront of the DM decreases with the increase of the fatigue degree of the DM, resulting in the increase of the PV value of the corresponding residual wavefront. Meanwhile, the larger PV value of the distorted wavefront or the higher proportion of the high-spatial-frequency components gives rise to the larger residual wavefront after the existence of the fatigue of the PZT actuators, the worse corrected beam quality, and the lower correction ability of the DM. The correction ability of the DM composed of different piezoelectric ceramics has also been evaluated by the ß factor of the corrected laser beam, which indicates that for the given loading cycle, the fatigue degree and the correcting ability of the DM composed of different piezoelectric ceramics are different.

Analysis of illumination uniformity affected by small-scale self-focusing of a pump beam in the radial smoothing scheme.

In the radial smoothing (RS) scheme, small-scale self-focusing (SSSF) can degrade beam quality of a pump beam with a Gaussian pulse train, resulting in the degradation of the smoothing performance of the RS scheme. Considering SSSF of the pump beam in an optical Kerr medium (OKM), the propagation model of the laser beam in the RS scheme has been developed, and the effects of the characteristics of the pump beam and the thickness of the optical Kerr medium on the RS performance have further been numerically simulated and analyzed. The results show that SSSF decreases the illumination uniformity in the RS scheme by inducing distorted wavefront modulation of the pump beam. Consequently, the beam quality of the pump beam should be controlled to avoid the degradation of the RS performance affected by SSSF. In addition, the peak intensity of the pump beam and the thickness of the OKM should be reasonable to ensure RS performance while mitigating the impact of SSSF.

Effects of the DLA Light Source Quality on the Beam Quality of Combined Beam in Spectral Beam Combining Systems with Grating-External Cavity.

In Diode Laser Array (DLA) spectral beam combining (SBC) systems with grating-external cavity, the beam quality of the combined beam tends to degrade due to comprehensive function of factors involving divergence angle of light, positional deviation and direction angle deviation caused by "smile" effect of emitters, etc.. Based on the consideration of the effects of the divergence angle and "smile" effect of DLA emitters on the beam propagation characteristics, the light propagation model of DLA SBC systems with grating-external cavity has been built up, and the effects of divergence angle and the positional deviation and direction angle deviation caused by "smile" effect of DLA emitters on the beam quality of the combined beam in SBC systems have been analyzed quantitatively. The results indicate that, the beam quality of the combined beam would be significantly affected by the DLA light source quality, i.e., the bigger the divergence angle and the direction angle deviation caused by "smile" effect are, the worse the beam quality of the combined beam is. The positional deviation of emitters in beam-combination direction caused by "smile" effect has little impact on the beam quality of the combined beam, whereas the positional deviation in non beam-combination direction would significantly degrade the beam quality of the combined beam. In practical applications, measures should be taken to improve the DLA light source quality in order to eliminate the effect on the beam quality of the combined beam

Radial smoothing for improving laser-beam irradiance uniformity.

Laser-beam irradiation uniformity is a key issue in inertial confinement fusion research. We propose a radial smoothing (RS) approach in which the speckle in a focal plane is smoothed by the radial redistribution through fast focal zooming. This focal zooming is generated by introducing the periodical spherical wavefront modulation to the laser beam, based on an optical Kerr medium and its pump laser with the temporal profile of a Gaussian pulse train. The utilization of RS significantly improves the laser-beam uniformity without obvious impact on the performance of the high-power laser system.

A broadband tunable terahertz negative refractive index metamaterial.

A strategy to greatly broaden negative refractive index (NRI) band, reduce loss and ease bi-anisotropy of NRI metamaterials (MMs) has been proposed at terahertz frequencies. Due to the symmetric structure of the MM, the transmission and refractive index are independent to polarizations of incident radiations, and a broadband NRI is obtainable for the range of the incident angle from 0° to 26°. In addition, THz MMs' properties such as transmission, phase and negative refraction exhibit a real-time response by controlling the temperature. The results indicate that the maximum bands of the negative and double-negative refraction are 1.66 THz and 1.37 THz for the temperature of 40 °C and 63 °C, respectively. The figure of merit of the MMs exceeds 10 (that is, low loss) as the frequency increases from 2.44 THz to 2.56 THz in the working temperature range, and the maximum figure of merit is 83.77 at 2.01 THz where the refractive index is -2.81 for a given temperature of 40 °C. Furthermore, the negative refraction of the MMs at the low loss band is verified by the classical method of the wedge, and the symmetric slab waveguide based on the proposed MM has many unique properties.