Reflection Mode Research Articles

Circular polarizer based on twisted double [formula omitted] thin film

We unveil a highly efficient circular polarizer based on twisted double Td − WTe2 thin film in the transmissive and reflective modes. This polarizer can transform a linearly polarized incident wave into a circularly polarized one with extremely high extinction ratios by simply adjusting the orientation of the incident polarization plane. The proposed structure provides switching between the right-handed and left-handed circular polarizations (RCP and LCP) in the output wave by tuning the frequency and, in particular, the polarization direction of the incident wave. Furthermore, the twist angle of the thin films can control the handedness of the converted wave to be either LCP or RCP. Owing to the high efficiency of the polarization conversion and accessibility of the realization of the proposed structure, it may have potential applications as a polarization manipulator in the optical sensing, communication, and defense industry.

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.

Stretchable structural colors with polarization dependence using lithium niobate metasurfaces.

Independently tunable biaxial color pixels, composed of isolated nanosquare dimers, are demonstrated in this study. These pixels are capable of displaying a full range of colors under a linear-polarization dependent reflection mode. The metasurface is constructed by arranging LiNbO3 nanodimers on a PDMS substrate. By exciting a strong magnetic dipole (MD) resonance and effectively suppressing other multipolar resonances using surface lattice resonances, the researchers achieved a single reflection peak with a bandwidth of less than 9 nm and a reflective efficiency of up to 99%. Additionally, the stretchability of the PDMS substrate allows for active and continuous tuning of the metasurface by up to 40% strain, covering almost 150 nm of the visible light spectrum and enabling changes in reflection color. This metasurface holds potential applications in various fields, such as color displays, data storage, and anti-counterfeiting technologies.

Multipath-controlled bidirectional metasurface for multitasking polarization regulation and absorption.

In the design of metasurfaces, integrating multiple tasks into a single small unit cell and achieving regulation through various paths pose a serious challenge. In this paper, a multipath-controlled bidirectional metasurface (MCBM) is designed to achieve polarization regulation, perfect absorption and total reflection as multitasking functions. The findings demonstrate that under different excitation conditions, when co-planar polarized terahertz (THz) waves are incident normally on the metasurface, the MCBM can convert co-planar polarization to cross-polarization, co-planar polarization to circular polarization wave in reflection mode, and co-planar polarization to cross-polarization in transmission, respectively. When co-planar polarized THz waves are incident from the back side of the metasurface, the tasks of MCBM change to broadband perfect absorption, total reflection, and transmission co-planar polarization to cross-polarization conversion. Remarkably, all operating frequency bands of these tasks are very approximate. Additionally, the multitasking functions can be switched by altering the excitation conditions, and their performance can be regulated through multipath controls, such as the temperature, voltage, and polarization status. Our design provides an effective strategy for multipath-controlled multitasking integrated devices in the THz band.

Two-dimensional transmissive structural colors for high-security information encryption.

Structural colors produced from nanostructures have attracted much attention due to their promising advantages of long-term stability and high resolution. Many nanostructures like metasurfaces have been demonstrated to generate color information in the transmission or reflection mode. Here, a strategy of combining polarization-insensitive and polarization-sensitive transmissive structural color is proposed to realize convenient and diverse encrypted pattern designs. A two-dimensional metasurface, whose polarization characteristics are determined by the size of a nanobrick unit, is embedded inside an optical cavity to produce transmissive structural color. The polarization-insensitive transmissive structural color exhibits a wide color gamut and high excitation purity in all polarization states, while the polarization-sensitive transmissive structural color maintains the similar color appearance in x-direction polarization but appears nearly black in y-direction polarization. Combining these two transmissive structural colors can achieve diverse images designed at different polarizations instead of simply hiding the image in a specific polarization state. An image of "flower and flowerpot" using the generated colors is visually illustrated, which shows that the proposed transmissive structural colors would have great potential in the areas of security information encryption.

Ergodic rate analysis and power allocation schemes for a novel active simultaneous transmission and reflection reconfigurable intelligent surface‐aided wireless network

SummaryIn this work, we propose the ergodic rate analysis and power allocation schemes for a novel active simultaneous transmits and reflects reconfigurable intelligent surface (STAR‐RIS) aided downlink wireless network. Here, the system model comprises of a single antenna source node (S) that uses the ES protocol to broadcast information to two users (T user and R user) via active STAR‐RIS. Each active STAR‐RIS element can function simultaneously in transmission and reflection modes owing to this ES protocol. We first derive the closed‐form analytical expressions for the system's ergodic rate. An optimization problem is formulated with a total power constraint to enhance the ergodic rate of the proposed system. A computationally inexpensive particle swarm optimization‐based power allocation (PSO‐PA) scheme and the Lagrange multiplier method (LMM) are adopted to solve the described optimization problem and to find the optimized power allocation factors for the source node and the active STAR‐RIS that can enhance the ergodic rate. Additionally, a comparison of the suggested PSO‐PA scheme, the LMM approach, and the equal power allocation (EPA) scheme is shown in terms of ergodic rate. Our results unveil that the proposed active STAR‐RIS system outperforms the passive STAR‐RIS and conventional RIS (transmitting/reflecting) aided systems. Further, the ergodic rate can be significantly improved with the proposed PSO‐PA and LMM schemes over the EPA scheme.

Method—Setups and Protocols for Operando Batteries Experiments at Variable Temperatures Applied to CaxTiS2 and Na3−xV2(PO4)3

In this paper, we present two thermalized setups for in situ operando X-ray diffraction data collection in temperature ranging from 0° to 100 °C: a modified Leriche cell and a coin cell chamber. Both setups proved proper thermalization and representative electrochemistry. While the coin cell chamber may only be used in transmission mode, the modified Leriche cell can also be used in reflection mode. Two case studies are presented as example of operation: calcium insertion in TiS2 and sodium (de-)insertion in Na3−xV2(PO4)3. For the former, heating at 60 °C enables faster kinetics to form a solvent co-intercalated phase upon reduction, a process which is reversible upon reoxidation. For the later, Na3−xV2(PO4)3 is known to form a low temperature phase at ca 12 °C. Thanks to the thermalized setups, operando X-ray diffraction patterns were collected while cycling on the low temperature phase.

Closed-Form Global Optimization of Beyond Diagonal Reconfigurable Intelligent Surfaces

Reconfigurable intelligent surfaces (RISs) allow controlling the propagation environment in wireless networks by tuning multiple reflecting elements. RISs have been traditionally realized through single connected architectures, mathematically characterized by a diagonal scattering matrix. Recently, beyond diagonal RISs (BD-RISs) have been proposed as a novel branch of RISs whose scattering matrix is not limited to be diagonal, which creates new benefits and opportunities for RISs. Efficient BD-RIS architectures have been realized based on group and fully connected reconfigurable impedance networks. However, a closed-form solution for the global optimal scattering matrix of these architectures is not yet available. In this paper, we provide such a closed-form solution proving that the theoretical performance upper bounds can be exactly achieved for any channel realization. We first consider the received signal power maximization in single-user single-input single-output (SISO) systems aided by a BD-RIS working in reflective or transmissive mode. Then, we extend our solution to single-user multiple-input multiple-output (MIMO) and multi-user multiple-input single-output (MISO) systems. We show that our algorithm is less complex than the iterative optimization algorithms employed in the previous literature. The complexity of our algorithm grows linearly (resp. cubically) with the number of RIS elements in the case of group (resp. fully) connected architectures.

Reflective mode conversions between extensional and flexural waves by ultrathin oblique anisotropic tri-component resonators

In this letter, we report a theoretical study for the conversions between extensional and flexural modes for Lamb waves. The incident extensional waves can be completely converted into reflected flexural waves, or vice versa. The used structure is oblique anisotropic tri-component resonators with locally dipolar resonators. Without special optimization designs, the thickness of conversion device is much smaller than the working wavelengths (1/150 and 1/15 for the extensional and flexural waves, respectively). Our proposed design is more feasible for low-frequency application scenarios.

Terahertz broadband tunable multifunctional metasurface based on VO2

A switchable multi-function terahertz metasurface employing phase transition material vanadium dioxide (VO2) is presented and investigated. By varying the temperature, the hybrid structure can switch between transmission, absorption, and reflection modes in broadband ranges. When the temperature is below 340 K and VO2 is in the insulating state, perfect polarization conversion is demonstrated. Efficient asymmetric transmission (AT) exceeding 0.7 is simultaneously achieved with an ultra-wide bandwidth of 3.6THz. When VO2 is in the metal phase, it shows different modulation characteristics for x and y-polarized waves. The structure can absorb over 90% of y-polarized waves from 3.56 THz to 7.2 THz (bandwidth, 3.64 THz), while 85% of x-polarized waves are reflected from 1 THz to 9 THz (bandwidth, 8 THz). Compared with other related published works, the designed structure makes significant progress in integrated functionalities, operating bandwidth, and working efficiency. It shows great potential for use in terahertz dynamic control and multifunctional integrated systems.

Self-absorption and investigation of excited carrier dynamics in two-dimensional perovskite scintillator

The PEA2PbBr4 scintillator exhibits varied performance in photoluminescence (PL) and radioluminescence (RL) profiles. PL in the reflectivity mode shows emission peaks at 414 and 434 nm, whereas PL and RL in the transmission mode exhibit only one peak at 434 nm. Temperature-dependent PL spectra measurements confirm that self-absorption contributes to this difference. PEA2PbBr4 displays decay times of 1.3 ns (90%) and 4.4 ns (10%) for PL and 11.5 ns (89%) and 47 ns (11%) for RL. To explain the distinct decay times, we construct two models for simulating laser and radiation excitation and investigate carrier dynamics using non-adiabatic molecular dynamics. Our findings suggest that the relatively long radiative recombination process of carriers contributes to the slow RL decay time.

Ionic-resolution protoacoustic microscopy: A feasibility study.

Visualizing micro- and nano-scale biological entities requires high-resolution imaging and is conventionally achieved via optical microscopic techniques. Optical diffraction limits their resolution to ∼200 nm. This limit can be overcome by using ions with ∼1 MeV energy. Such ions penetrate through several micrometers in tissues, and their much shorter de Broglie wavelengths indicate that these ion beams can be focused to much shorter scales and hence can potentially facilitate higher resolution as compared to the optical techniques. Proton microscopy with ∼1 MeV protons has been shown to have reasonable inherent contrast between sub-cellular organelles. However, being a transmission-based modality, it is unsuitable for in vivo studies and cannot facilitate three-dimensional imaging from a single raster scan. Here, we propose proton-induced acoustic microscopy (PrAM), a technique based on pulsed proton irradiation and proton-induced acoustic signal collection. This technique is capable of label-free, super-resolution, 3D imaging with a single raster scan. Converting radiation energy into ultrasound enables PrAM with reflection mode detection, making it suitable for in vivo imaging and probing deeper than proton scanning transmission ion microscopy (STIM). Using a proton STIM image of HeLa cells, a coupled Monte Carlo+k-wave simulations-based feasibility study has been performed to demonstrate the capabilities of PrAM. We demonstrate that sub-50 nm lateral (depending upon the beam size and energy) and sub-micron axial resolution (based on acoustic detection bandwidth and proton beam pulse width) can be obtained using the proposed modality. By enabling visualization of biological phenomena at cellular and subcellular levels, this high-resolution microscopic technique enhances understanding of intricate cellular processes.

Quantitative determination of ademetionine in tablets utilizing ATR-FTIR and partial least squares methods approaches

In this study there were utilized a combination of Fourier transform infrared spectroscopy in attenuated total reflection mode (ATR-FTIR) and partial least squares (PLS) regression method to develop quantitative models for determining the concentration of ademetionine in commercial tablets. The established and validated models were specifically designed for a commercial product containing ademetionine 1,4-butandiesulfonate. The coefficient of determination for the developed model was 0.999. Relative standard deviation (RSD) does not exceed 1.6% for repeatability and intermediate accuracy, which meets the international ICH and AOAC requirements for the method performance. The validation results effectively confirmed that this method is suitable and meets the current requirements for analytical methods in drug quality control. Consequently, this approach can be used for routine ademetionine analysis in pharmaceutical products and has the potential to be applied to other active pharmaceutical ingredients (APIs) in drug quality control.

Nondestructive evaluation of mechanical heart valve disc thickness degradation due to accelerated durability test using THz-TDS measurements

ABSTRACT Quality assurance and control during the mechanical heart valve components’ production process is paramount to mitigate most of the valve-related complications arising from valve replacement. Nondestructive evaluation of heart valve implants with complex profiles during production is challenging due to the biocompatibility requirements and clean room assembly conditions. This study first tested the UHMWPE disc of TTK Chitra mechanical heart valve models TC1 and TC2 using Terahertz imaging in reflection mode. The UHMWPE discs were raster scanned in X-Y directions to obtain a Terahertz-C scan to locate the macro defects due to wear out, and to distinguish between the true positive and true negative samples. The results were verified with digital microscope studies. Second, this study investigates a unique relationship between Terahertz imaging in reflection mode and heart valve disc degradation due to accelerated durability study. Third, the variation of disc surface roughness due to accelerated durability test was studied by Gaussian curve fitting analysis of fast Fourier transform of the Terahertz reflections. Finally, a comparison between the TC1 and TC2 heart valve model’s wear-out foot print was carried out using Terahertz C-scan. Our results imply that Terahertz time-domain spectroscopy has promising applications in NDE measurements of biocompatible implants.

Regularized cost function in wavefront shaping for advancing the contrast of structured light.

The cost function in the iterative optimization algorithms is one of the sensitive optimization controllers that plays a crucial role in feedback based wavefront shaping for constructing well-resolved complex structured light through scattering media. There has been a trade-off between resolution and the contrast enhancement of the structured light in wavefront shaping. We have developed an ℓ 2-norm based quadratic cost function (L2QN) and proposed a regularized cost function (RCF) for advancing the contrast and maintaining the high resolution of structured light. Both the simulations and experiments have been performed, and it has been found that the proposed RCF significantly advances the contrast and structural uniformity for focusing light through scattering media as well as for diffused reflection mode. The potential applications of the method demonstrated in this study can be extended into holographic displays, structured light illumination microscopy, photo-lithography, photothermal treatments, dosimetry, laser materials processing, and energy control inside and outside an incubation system.

Transmission–reflection-integrated multifunctional metasurfaces for multiplexed acoustic wave manipulation

Considering the extraordinary wavefront modulation properties, acoustic metasurfaces have been extensively utilized to achieve powerful wave-manipulation functionalities. The next-generation acoustic metasurfaces are urgently required to encode more information capacity and process an increasing number of signal channels in a compact device, which needs more degrees of freedom for multifunctional wavefront modulation. In this study, subwavelength monolayer transmission–reflection-integrated metasurfaces (TRIMs) are systemically designed through a gradient-free topology optimization method to simultaneously realize diverse acoustic functions, such as beam steering, focusing, splitting, and diffusion, in reflection mode and transmission mode. Both numerical and experimental results demonstrate the desired wave-manipulation performance of the metasurfaces. In addition, a dual-frequency multiplexed TRIM is also numerically achieved for exploring the integration of multiple degrees of freedom and tunable function switching, which promise many unprecedented applications in integrated medical imaging, underwater wireless telecommunications, on-chip signal processing, etc.

Design, Optimization, and Experimental Evaluation of Slow Light Generated by π-Phase-Shifted Fiber Bragg Grating for Use in Sensing Applications.

This paper describes design, theoretical analysis, and experimental evaluation of a π-Phase-Shifted Fiber Bragg Grating (π-PSFBG) inscribed in the standard telecom fiber for slow light generation. At first, the grating was designed for its use in the reflection mode with a central wavelength of 1552 nm and a pass band width of less than 100 pm. The impact of fabrication imperfections was experimentally investigated and compared to model predictions. The optical spectra obtained experimentally show that the spectral region used for slow light generation is narrower (less than 10 pm), thus allowing for too-low levels of slow light optical-output power. In the next step, the optimization of the grating design was conducted to account for fabrication errors, to improve the grating's spectral behavior and its temporal performance, and to widen the spectral interval for slow light generation in the grating's transmission mode. The targeted central wavelength was 1553 nm. The π-PSFBG was then commercially fabricated, and the achieved parameters were experimentally investigated. For the region of (1551-1554) nm, a 15-fold increase in the grating's pass band width was achieved. We have shown that a pair of retarded optical pulses were generated. The measured group delay was found to be ~10.5 ps (compared to 19 ps predicted by the model). The π-PSFBG operating in its transmission mode has the potential to operate as tunable delay line for applications in RF photonics, ultra-fast signal processing, and optical communications, where tunable high precision delay lines are highly desirable. The π-PSFBG can be designed and used for the generation of variable group delays from tens to hundreds of ps, depending on application needs.

Frequency-multiplexed multifunctional coding metasurface based on transmission and reflection modes

In this work, a frequency-multiplexed multifunctional coding metasurface is proposed, which can realize focusing lens at the lower frequency and multiple scattering beams at the higher frequency. The two functions can be manipulated independently. The proposed metasurface is composed of the top metal patch with a circular ring, two fan rings and two rectangle patches (CFRP), the bottom metal patch with C-shaped and rectangular metal slots (CRS) and a dielectric substrate. By changing the opening direction and size of CRS and the tilt direction and fan ring size of CFRP, the amplitude and phase of transmission and reflection can be modulated, respectively. The transmission mode works at 7 GHz, which can form focal points in different focal planes. The reflection mode works at 17.5 GHz, which can form multiple scattered beams in different directions in the far field. To further verify its practical feasibility, the proposed metasurface is fabricated and measured, the experimental results are in good agreement with the numerical simulation results. The proposed metasurface provides new possibilities to develop multifunctional devices.

Dual-band terahertz reflective-mode metasurface for the wavefront manipulation of independent linear and circular polarization waves

Metasurfaces (MSs) are being extensively researched owing to their ability to modulate the polarization and wavefront of electromagnetic (EM) waves in a flexible manner, which usually offer significant advantages including ultra-thinness, low losses, and easy fabrication. However, conventional MSs typically operate well only with a single polarization. Here, we propose a novel design strategy for a terahertz (THz) reflective-mode MS that relies on a single unit-cell arrangement combining propagation phase and geometric phase. Our designed MS can achieve multiple wavefront manipulations in reflection mode, not limited to circular polarization (CP) transformation, but also enabling linear polarization (LP) conversion. The MS we propose consists of a periodic array of bilayered metal patterned resonator structures sandwiched by a dielectric substrate. The metallic resonator is made of the outer single-split-ring (SSR) and C-shaped slot (CSS), inner double-split-ring (DSR), and its complementary structure. With this design, the MS is capable of converting a LP wave to its orthogonal counterpart at lower frequency (f1=0.7THz) after reflection. Additionally, at higher frequency (f2=1.4THz), the proposed MS can also convert the right-handed CP (RCP) to left-handed CP (LCP) upon reflection or vice versa. The 2π phase full coverage of the orthogonal LP and CP waves can be achieved independently and simultaneously by adjusting the opening and orientation angles of the SSR based on propagation phase, and orientation angle of the DSR based on geometric phase. We numerically demonstrate beam deflection, planar focusing, and the vortex beam for both reflected orthogonal LP and CP waves with three representative MSs to provide proof of concept. These findings reveal the great potential for multifunctional devices for dual-polarization in imaging and communication systems.

Multifunctional coding metasurface for focusing and beam deflection based on polarization selection

ABSTRACT In this paper, a full-space multifunctional coding metasurface (MS) with reflection and transmission integration is proposed to realize the independent manipulation of electromagnetic waves at two different frequencies. The unit consists of two layers of dielectric substrate and three layers of metal patch, the upper reflective layer is composed of a ring and a split ring (URSR), the middle layer is a split ring groove (MSR), and the lower transmission layer is a split ring (LSR). The transmission mode works at 13.4 GHz, incoming linear polarization (LP) waves to realize beam deflection with a deflection angle of 34°. While the reflection mode works at 17.9 GHz, beam focusing to achieved under the action of circular polarization (CP) waves. In order to verify the feasibility of the design, a circularly polarized antenna is placed at 80 mm in the metasurface -Z direction, and the gain increases from 7.5 dB to 22.5 dB. Meanwhile coding metasurface and circularly polarized microstrip antennas is fabricated and measured, and the results are consistent with the simulation results. The proposed method provides the possibility of high-rate transmission of information of multifunctional devices in communication systems.