Highly localized continuous wave optical vortex with controllable orbital angular momentum orientation and topological charge.

We report an approach to simultaneously control orbital angular momentum (OAM) orientation and topological charge in highly localized optical vortices by employing a 4π focusing system. The required continuous wave illumination field in the pupil planes is derived by superimposing the radiation pattern of only one dipole placed at the focal point of the high numerical aperture lens and the corresponding tailored spiral phase factor. The topological charge and OAM orientation of the obtained focused fields are quantitatively evaluated based on the focal field distributions calculated by the Richards-Wolf vector diffraction integration theory. Results show that the OAM of the generated optical vortices can be tailored by changing the oscillation orientation of the mimic dipole and the topological charge of the superimposed spiral phase term. The presented method may find potential applications in optical trapping, optical tweezers, light-matter interaction, etc.

Dynamically changeable terahertz metamaterial absorbers with intelligent switch and high sensitivity and wide and narrow band perfect absorption.

In this work, we theoretically designed a dynamically changeable terahertz metamaterial absorber with intelligent switch and high sensitivity, wide band and narrow band perfect absorption based on the combination of Dirac semimetal (BDS) and vanadium dioxide (VO2). It features two methods for absorption adjustment: altering the Fermi energy level of BDS to modify the resonant frequency of the absorption peaks and utilizing the phase change of VO2 to regulate the absorption rate of the peaks. In addition, its rotational symmetric design ensures strong polarization-insensitivity. The simulation results demonstrate the presence of two narrowband absorption peaks and one mini-broadband absorption peak within the frequency range of 6.0-9.5 THz, all with absorption rates exceeding 90%. We provide an explanation of the absorption mechanism of the device, employing the relative impedance theory and localized surface plasmon resonance to analyze its electric field distribution. We also defined the refractive index sensitivity (S), which is SI = 378 GHz per RIU and SIII = 204 GHz per RIU. Our device possesses high sensitivity and two methods of adjusting absorption modes, which endow it with advantages in the fields of metamaterial absorbers, intelligent switch, and optical sensors.

Tunable broadband absorber based on a layered resonant structure with a Dirac semimetal.

With the development of science and technology, intermediate infrared technology has gained more and more attention in recent years. In the research described in this paper, a tunable broadband absorber based on a Dirac semimetal with a layered resonant structure was designed, which could achieve high absorption (more than 0.9) of about 8.7 THz in the frequency range of 18-28 THz. It was confirmed that the high absorption of the absorber comes from the strong resonance absorption between the layers, and the resonance of the localised surface plasmon. The absorber has a gold substrate, which is composed of three layers of Dirac semimetal and three layers of optical crystal plates. In addition, the resonance frequency of the absorber can be changed by adjusting the Fermi energy of the Dirac semimetal. The absorber also shows excellent characteristics such as tunability, absorption stability at different polarisation waves and incident angles, and has a high application value for use in radar countermeasures, biotechnology and other fields.

Design of Surface Plasmon Resonance-Based D-Type Double Open-Loop Channels PCF for Temperature Sensing.

Here, we document a D-type double open-loop channel floor plasmon resonance (SPR) photonic crystal fiber (PCF) for temperature sensing. The grooves are designed on the polished surfaces of the pinnacle and backside of the PCF and covered with a gold (Au) film, and stomata are distributed around the PCF core in a progressive, periodic arrangement. Two air holes between the Au membrane and the PCF core are designed to shape a leakage window, which no longer solely averts the outward diffusion of Y-polarized (Y-POL) core mode energy, but also sets off its coupling with the Au movie from the leakage window. This SPR-PCF sensor uses the temperature-sensitive property of Polydimethylsiloxane (PDMS) to reap the motive of temperature sensing. Our lookup effects point out that these SPR-PCF sensors have a temperature sensitivity of up to 3757 pm/°C when the temperature varies from 5 °C to 45 °C. In addition, the maximum refractive index sensitivity (RIS) of the SPR-PCF sensor is as excessive as 4847 nm/RIU. These proposed SPR-PCF temperature sensors have an easy nanostructure and proper sensing performance, which now not solely improve the overall sensing performance of small-diameter fiber optic temperature sensors, but also have vast application prospects in geo-logical exploration, biological monitoring, and meteorological prediction due to their remarkable RIS and exclusive nanostructure.

Highly sensitive sensing of a magnetic field and temperature based on two open ring channels SPR-PCF.

A surface plasmon resonance (SPR) sensor comprising photonic crystal fiber (PCF) is designed for magnetic field and temperature dual-parameter sensing. In order to make the SPR detection of magnetic field and temperature effectively, the two open ring channels of the proposed sensor are coated with gold and silver layers and filled with magnetic fluid (MF) and Polydimethylsiloxane (PDMS), respectively. The sensor is analyzed by the finite element method and its mode characteristics, structure parameters and sensing performance are investigated. The analysis reveals when the magnetic field is a range of 40-310 Oe and the temperature is a range of 0-60 °C, the maximum magnetic field sensitivity is 308.3 pm/Oe and temperature sensitivity is 6520 pm/°C. Furthermore, temperature and magnetic field do not crosstalk with each other's SPR peak. Its refractive index sensing performance is also investigated, the maximum sensitivity and FOM of the left channel sensing are 16820 nm/RIU and 1605 RIU-1, that of the right channel sensing are 13320 nm/RIU and 2277 RIU-1. Because of its high sensitivity and special sensing performance, the proposed sensor will have potential application in solving the problems of cross-sensitivity and demodulation due to nonlinear changes in sensitivity of dual-parameter sensing.

Design of two-dimensional sampled Bragg grating for a curved waveguide.

Due to the ability of changing light propagation path direction, curved waveguide Bragg grating (CWG) plays an important role in photonic integrated circuits. In this paper, we proposed a cascaded sampled Bragg grating on tilted waveguide (CSBG-TW) structure to equivalently realize CWG. As an example, by designing two-dimensional (2D) sampled gratings, the direction of +1st sub-grating vector in CSBG-TW can be changed. Then if a curved waveguide is divided into several sections of tilted waveguide, we can keep the grating direction being always parallel to the longitudinal direction of each section of tilted waveguide, while the basic grating is uniform. Hence, the required CWG can be equivalently realized, and the light responses such as reflection Bragg wavelength shift and backward mode convert caused by the tilted grating in curved waveguide can be compensated for. The results show that the sampling structures of CSBG-TW is micro-scale and the difference between reflection intensity between the CSBG-TW with four section tilted waveguide and CWG as design target is less than 0.1 dB. Compared with CWG, the CSBG-TW allows convenient holographic exposure and the wavelength can be accurately controlled. Therefore, the CSBG-TW can be used in various photonic integrated devices that require changing propagation paths.

Realization of 18.97% theoretical efficiency of 0.9 µm thick c-Si/ZnO heterojunction ultrathin-film solar cells via surface plasmon resonance enhancement.

In this work, we demonstrate that the performance of c-Si/ZnO heterojunction ultrathin-film solar cells (SCs) is enhanced by an integrated structure of c-Si trapezoidal pyramids on the top of a c-Si active layer and Al pyramids in the active layer on the Al back electrode. The top c-Si trapezoidal pyramid (TTP) increases the absorption of short wavelengths by lengthening the propagation distance of incident light and coupling the incident light into photonic modes in the active layer. The bottom Al pyramid (BP) improves the overall optical absorption performance especially for the long wavelength band by forming the surface plasmon resonance (SPR) mode in the active layer. As a result, the average absorption in the entire wavelength range (300-1400 nm) reaches 93.16%. The optimized short-circuit current density (Jsc) and photoelectric conversion efficiency (PCE) of ultra-thin film c-Si/ZnO SCs are 41.94 mA cm-2 and 18.97%, respectively. Moreover, the effect of different illumination angles on the optical absorption of the SCs was explored. The SCs have good absorption when the incident angles are in the range from 0 degrees to 60 degrees. Furthermore, the underlying mechanism for the enhancement of photon absorption in the SCs was discussed through careful analysis of the electric field intensity profile at different wavelengths. It was found that the electric field tends to concentrate around the bottom pyramids and top trapezoidal pyramids even for the long-wave band, which results in an excellent light-trapping performance.

A switchable terahertz device combining ultra-wideband absorption and ultra-wideband complete reflection.

Terahertz functional devices have been instrumental in the development of terahertz technology. Moreover, the advent of metamaterials has greatly contributed to the advancement of terahertz devices. However, most of today's metamaterials in the terahertz band exhibit poor performance and are mono-functional. This greatly limits the scalability and application potential of the devices. To achieve diversification and tunability of device functionality, we propose a combination of metamaterial structures and vanadium dioxide film. A metamaterial absorber based on the thermotropic phase change material VO2 has been designed. Flexible switching of absorption performance (complete reflection and ultra-broadband perfect absorption) can be achieved through temperature adjustment. Moreover, the perfectly absorbed bandwidth is a staggering 3.3 THz. The thermal tuning of spectral absorbance has a maximal range of 0.01 to 0.999. The shift in absorption properties is explained by the phase change process of vanadium oxide (MIT). The electric field intensity on the absorber surface at different temperatures was monitored and analysed as a way to correlate the VO2 film phase transition process. The impedance matching theory is applied to explain the high level of absorption generated by the absorber. Finally, the effects of the structural parameters on the performance of the absorber are analysed. This work will have many applications in the terahertz field and offers a wide range of ideas for the design of terahertz-enabled devices.

Thermal tuning of terahertz metamaterial absorber properties based on VO2.

We present a novel, structurally simple, multifunctional broadband absorber. It consists of a patterned vanadium dioxide film and a metal plate spaced by a dielectric layer. Temperature control allows flexible adjustment of the absorption intensity from 0 to 0.999. The modulation mechanism of the absorber stems from the thermogenic phase change properties of the vanadium dioxide material. The absorber achieves total reflection properties in the terahertz band when the vanadium dioxide is in the insulated state. When the vanadium dioxide is in its metallic state, the absorber achieves near-perfect absorption in the ultra-broadband range of 3.7 THz-9.7 THz. Impedance matching theory and the analysis of electric field are also used to illustrate the mechanism of operation. Compared to previous reports, our structure utilizes just a single cell structure (3 layers only), and it is easy to process and manufacture. The absorption rate and operating bandwidth of the absorber are also optimised. In addition, the absorber is not only insensitive to polarization, but also very tolerant to the angle of incidence. Such a design would have great potential in wide-ranging applications, including photochemical energy harvesting, stealth devices, thermal emitters, etc.

Unemployment Insurance and Opioid Overdose Mortality in the United States.

Over the past two decades, opioid overdose deaths contributed to the dramatic rise in all-cause mortality among non-Hispanic Whites. To date, efforts among scholars to understand the role of local area labor market conditions on opioid overdose mortality have led to mixed results. We argue the reason for these disparate findings is scholars have not considered the moderating effects of income support policies such as unemployment insurance. The present study leverages two sources of variation-county mass layoffs and changes in the generosity of state unemployment insurance benefits-to investigate if unemployment benefits moderate the relationship between job loss and county opioid overdose death rates. Our difference-in-differences estimation strategy reveals that the harmful effects of job loss on opioid overdose mortality decline with increasing state unemployment insurance benefit levels. These findings suggest that social policy in the form of income transfers played a crucial role in disrupting the link between job loss and opioid overdose mortality.

Design of Ultra-Narrow Band Graphene Refractive Index Sensor.

The paper proposes an ultra-narrow band graphene refractive index sensor, consisting of a patterned graphene layer on the top, a dielectric layer of SiO2 in the middle, and a bottom Au layer. The absorption sensor achieves the absorption efficiency of 99.41% and 99.22% at 5.664 THz and 8.062 THz, with the absorption bandwidths 0.0171 THz and 0.0152 THz, respectively. Compared with noble metal absorbers, our graphene absorber can achieve tunability by adjusting the Fermi level and relaxation time of the graphene layer with the geometry of the absorber unchanged, which greatly saves the manufacturing cost. The results show that the sensor has the properties of polarization-independence and large-angle insensitivity due to the symmetric structure. In addition, the practical application of testing the content of hemoglobin biomolecules was conducted, the frequency of first resonance mode shows a shift of 0.017 THz, and the second resonance mode has a shift of 0.016 THz, demonstrating the good frequency sensitivity of our sensor. The S (sensitivities) of the sensor were calculated at 875 GHz/RIU and 775 GHz/RIU, and quality factors FOM (Figure of Merit) are 26.51 and 18.90, respectively; and the minimum limit of detection is 0.04. By comparing with previous similar sensors, our sensor has better sensing performance, which can be applied to photon detection in the terahertz band, biochemical sensing, and other fields.

Ultra-wideband and wide-angle perfect solar energy absorber based on Ti nanorings surface plasmon resonance.

Solar energy absorption is a very important field in photonics. The successful development of an efficient, wide-band solar absorber is an extremely powerful driver in this field. We propose an ultra-wideband (UWB) solar energy absorber composed of a Ti ring and SiO2-Si3N4-Ti thin films. In the range of 300-4000 nm, the wide band has an absorption efficiency of more than 90% and can reach 3683 nm, and it has four absorption peaks with a high absorptivity. Moreover, the weighted average absorption efficiency of the solar absorber under AM 1.5 is maintained above 97.03%, which indicates it has great potential for use in the field of solar energy absorption. Moreover, we proved that the polarization is insensitive by analyzing the absorption characteristics at arbitrary polarization angles. For both the transverse electric (TE) and transverse magnetic (TM) modes, the UWB absorption is maintained at more than 90% in the wide incidence angle range of 60°. The UWB solar energy absorber has great potential for use in a variety of applications, such as converting solar light and heat into electricity for public use and reducing the side effects of coal-fired power generation. It can also be used in information detection and infrared thermal imaging owing to its UWB characteristics.

Three-band perfect absorber with high refractive index sensing based on an active tunable Dirac semimetal.

In this paper, we designed a three-band narrowband perfect absorber based on bulk Dirac semi-metallic (BDS) metamaterials. The absorber consists of a hollow Dirac semi-metallic layer above, a gold layer below and a photonic crystal slab (PCS) in the middle. The study found that the terahertz wave absorber achieved three perfect absorption rates of more than 95% in the range of 1 to 2.4 THz. The minimum bandwidth (FWHM) is 0.02 THz, and the maximum quality factor (Q) is 106. A reasonable explanation of high absorption can be obtained by impedance matching, electric dipole and other principles. The absorption spectra of the two polarizations show different responses at different incident angles. In addition, we also obtained the influence of the structural parameters of the upper layer of the metamaterial on the absorption performance. We defined the refractive index sensitivity (S) with a maximum sensitivity of 0.1525 THz RIU-1 and a highest quality factor (FOM) of 4.26 in the refractive index range of 1 to 1.8. The maximum adjustable range is 0.06 THz in the Fermi energy range of 60 to 140 meV. Because of its excellent characteristics, our absorber will have good development prospects in the fields of optical switching, biochemical imaging, and space detection.

A four-band and polarization-independent BDS-based tunable absorber with high refractive index sensitivity.

A four-band terahertz tunable narrow-band perfect absorber based on a bulk Dirac semi-metallic (BDS) metamaterial with a microstructure is designed. The three-layer structure of this absorber from top to bottom is the Dirac semi-metallic layer, the dielectric layer and the metal reflector layer. Based on the Finite Element Method (FEM), we use the simulation software CST STUDIO SUITE to simulate the absorption characteristics of the designed absorber. The simulation results show that the absorption rate of the absorber is over 93% at frequencies of 1.22, 1.822, 2.148 and 2.476 THz, and three of them have achieved a perfect absorption rate of more than 95%. We use the localized surface plasmon resonance (LSPR), impedance matching and other theories to analyze its physical mechanism in detail. The influence of the geometric structure parameters of the absorber and the incident angle of electromagnetic waves on the absorption performance has also been studied in detail. Due to the rotational symmetry of the structure, the designed absorber has excellent polarization insensitivity. In addition, the maximum adjustable range of absorption frequency is 0.051 THz, which can be achieved by changing the Fermi energy of BDS. We also define the refractive index sensitivity (S), which is 39.1, 75.4, 119.1 and 122.0 GHz RIU-1 for the four absorption modes when the refractive index varies in the range of 1 to 1.9. This high-performance absorber has a very good development prospect in the frontier fields of bio-chemical sensing and special environmental detection.

13 J direct-liquid-cooled solid-state disk laser with an unstable cavity.

A 13 J direct-liquid-cooled solid-state disk laser with an unstable cavity is developed and demonstrated in this paper. The output energy of the resonant cavity is first analyzed according to the gain level in a stable cavity. At the optimum gain level, a magnification of 1.3 in the unstable cavity is achieved. Experimentally, a concave-convex mirror is used as the cavity mirror. At a magnification of 1.3, a repetition frequency of 100 Hz, and a pulse width of 350 µs, a single-pulse energy output of 13.2 J is obtained, corresponding to an optical-optical efficiency of 22% and a slope efficiency of 27.6%. The x-axis beam quality factor ß is 4.7, and the y-axis beam quality factorß is 16.6.

On-chip optical narrowband reflector based on anti-symmetric Bragg grating.

We propose an on-chip optical narrowband reflector (NBR) based on two cascaded Bragg gratings (BGs). A π phase shifted anti-symmetric Bragg grating (π-PS-ASBG) and a rear uniform Bragg grating (UBG), are in-line connected. The π-PS-ASBG provides a hybrid mode resonance between the even- and odd TE (TE0 and TE1) modes, while the UBG is used as a rear reflector to reflect the TE0 mode that transmitted from the π-PS-ASBG. Different from traditional UBG, the reflection bandwidth decreases when the coupling coefficient increases. The calculated 3-dB bandwidth is 0.16 nm when the whole grating length is 400 µm. The proposed NBR can be applied in the cases requiring narrow reflection such as narrow linewidth semiconductor lasers.

Condition for invariant spectrum of an electromagnetic wave scattered from an anisotropic random media.

Within the accuracy of the first-order Born approximation, sufficient conditions are derived for the invariance of spectrum of an electromagnetic wave, which is generated by the scattering of an electromagnetic plane wave from an anisotropic random media. We show that the following restrictions on properties of incident fields and the anisotropic media must be simultaneously satisfied: 1) the elements of the dielectric susceptibility matrix of the media must obey the scaling law; 2) the spectral components of the incident field are proportional to each other; 3) the second moments of the elements of the dielectric susceptibility matrix of the media are inversely proportional to the frequency.

Compact high power mid-infrared optical parametric oscillator pumped by a gain-switched fiber laser with "figure-of-h" pulse shape.

We demonstrate a compact high power mid-infrared (MIR) optical parametric oscillator (OPO) pumped by a gain-switched linearly polarized, pulsed fiber laser. The gain-switched fiber laser was constructed with a piece of Yb doped polarization maintaining (PM) fiber, a pair of fiber Bragg gratings written into the matched passive PM fiber and 6 pigtailed pump laser diodes working at 915 nm with 30 W output peak power each. By modulating the pulse width of the pump laser diode, simple pedestal-free pulse shape or pedestal-free trailing pulse shape ("figure-of-h" as we call it) could be achieved from the gain-switched fiber laser. The laser was employed as the pump of a two-channel, periodically poled magnesium oxide lithium niobate-based OPO system. High power MIR emission was generated with average output power of 5.15 W at 3.8 µm channel and 8.54 W at 3.3 µm channel under the highest pump power of 45 W. The corresponding pump-to-idler conversion efficiency was computed to be 11.7% and 19.1%, respectively. Experimental results verify a significant improvement to signal-to-idler conversion efficiency by using "figure-of-h" pulses over simple pedestal-free pulses. Compared to the master oscillator power amplifier (MOPA) fiber laser counterpart, the presented gain switched fiber laser is more attractive in OPO pumping due to its compactness and simplicity which are beneficial to construction of OPO systems for practical MIR applications.

Spectral shifts of evanescent waves generated by the scattering of polychromatic light in the near field.

Evanescent waves generated by the scattering of polychromatic light using a spatially deterministic medium and their near-zone spectral properties, to the best of our knowledge, have not been addressed in the literature. On the basis of the first-order Born approximation, we formulate expressions for the near-zone scattered wave by assuming that the scattering potential of the medium is of the Gaussian profile. The dependence of the spectral shift on effective sizes of the scattering potential (ESSP) is shown using numerical simulations. It is indicated that the increase of either ESSP or near-field diffraction length can induce spectral shifts in scattered evanescent waves.

Actively Tunable "Single Peak/Broadband" Absorbent, Highly Sensitive Terahertz Smart Device Based on VO2.

In recent years, the development of terahertz (THz) technology has attracted significant attention. Various tunable devices for THz waves (0.1 THz-10 THz) have been proposed, including devices that modulate the amplitude, polarization, phase, and absorption. Traditional metal materials are often faced with the problem of non-adjustment, so the designed terahertz devices play a single role and do not have multiple uses, which greatly limits their development. As an excellent phase change material, VO2's properties can be transformed by external temperature stimulation, which provides new inspiration for the development of terahertz devices. To address these issues, this study innovatively combines metamaterials with phase change materials, leveraging their design flexibility and temperature-induced phase transition characteristics. We have designed a THz intelligent absorber that not only enables flexible switching between multiple functionalities but also achieves precise performance tuning through temperature stimulation. Furthermore, we have taken into consideration factors such as the polarization mode, environmental temperature, structural parameters, and incident angle, ensuring the device's process tolerance and environmental adaptability. Additionally, by exploiting the principle of localized surface plasmon resonance (LSPR) accompanied by local field enhancement, we have monitored and analyzed the resonant process through electric field characterization. In summary, the innovative approach and superior performance of this structure provide broader insights and methods for THz device design, contributing to its theoretical research value. Moreover, the proposed absorber holds potential for practical applications in electromagnetic invisibility, shielding, modulation, and detection scenarios.