Absorber In Cavity Research Articles

Mode-locked soliton pulse generation using copper phthalocyanine absorber in long erbium-doped fibre laser cavity

We present the development of a mode-locked erbium-doped fibre laser (EDFL) utilizing Copper Phthalocyanine (CuPc) as a saturable absorber (SA). CuPc exhibits unique optical properties, including its high nonlinear optical response and rapid recovery time. To seamlessly integrate CuPc into the laser cavity, we employed a simple embedding technique within a polymer film. The SA demonstrates a modulation depth of 3.1% and a saturation intensity of 17 MW/cm2. Operating within a pump power range of 90.6 to 122.9 mW, the laser achieves mode-locked operation, producing a soliton pulse train with a fundamental frequency of 1.82 MHz and a pulse width of 6.4 ps at a wavelength of 1561.5 nm. It attains a maximum output power of 13.97 mW and a pulse energy of 7.67 nJ. These results underscore CuPc's potential as a promising material for generating mode-locked pulses, suggesting new avenues in ultrafast laser development for diverse applications.

Efficient continuous-wave and passively Q-switched Nd:GSAG laser operating at 1.3 μm

We report on the laser performance of neodymium-doped gadolinium scandium aluminum garnet (Nd:GSAG) crystals at 1.3 μm for the first time, to the best of our knowledge. An efficient continuous-wave (CW) laser at 1336 nm and on output power of up to 2.01 W was obtained under an absorbed pump power of 4.89 W with slope and optical-to-optical efficiencies of 43.4 % and 41.1 %, respectively. Furthermore, a passively Q-switched (PQS) laser with a Co:MgAl2O4 crystal serving as a saturable absorber in a compact plano-plano resonator cavity was examined. A maximum average output power of 0.2 W was obtained with a pulse width, pulse energy, and peak power of 34.4 ns, 22.2 µJ, and 645 W, respectively. Our results indicated that Nd:GSAG crystals are promising for producing high-efficiency CW lasers and effective PQS lasers at 1336 nm.

Tunable continuous domain bound states based on Fabry-Perot cavities and their applications

Excellent optical absorbers are always characterized by high quality factors and perfect absorption; however, these absorbers usually encounter the ohmic losses due to traditional surface plasmon resonance, which limits their absorption performance in practical applications. To address the problem, a tunable bound state in the continuum (BIC) based on Fabry-Perot cavity is proposed in this work. Figure (a) shows the structural model of the designed Fabry-Perot cavity absorber, which consists of Ag as a substrate, a layer of the dielectric material Al<sub>2</sub>O<sub>3</sub> above the Ag substrate, and a high-refractive-index grating as the top dielectric layer Si ridge. By adjusting the thickness parameter <i>d</i> of Al<sub>2</sub>O<sub>3</sub>, the transformation of BIC into q-BIC is achieved. Specifically, when <i>d</i> is increased from 273 nm to 298 nm, the BIC can be transformed into quasi-BIC, and the perfect absorption of the absorber in the continuum spectrum can be increased to 100%. In this work, the factors affecting the perfect absorption are explored by using the interference theory; theoretical calculations of the quasi-BIC are carried out by using the coupled mode theory and impedance matching theory; the physical mechanism of the BIC is explained by using the electric and magnetic field theory. The BIC is caused by the electric and magnetic dipole modes as well as the mirror image of the base Ag, which causes the interferential phase cancellation effect. Compared with the conventional absorber, the proposed absorber has excellent structural parameter robustness and a wide range of BIC modulation. More importantly, the absorber has excellent sensing performance with a maximum sensitivity of up to 34 nm/RIU and a maximum quality factor of 9.5. Last but not least, the absorber also achieves dual-frequency open-light performance, the maximum modulation depth and the minimum insertion loss of the dual-frequency switch reach 99.4% and 0.0004 dB, respectively. These findings have significant implications in the fields of photonics, optical communication, and sensor technology.

Energy and exergy studies on the receiver models with materials and heat transfer fluids.

This work focuses on meeting the growing demand in solar energy conversion for small-scale applications. In this regard, experimental and CFD research has been done to examine the thermal performance (energy and exergy efficiencies) of a dish collector (reflector and receiver) system with different receiver models. In this work, receivers with uniform absorber cavity areas having cylindrical and hemispherical shapes were modeled for length-to-diameter ratios (L/D) of 1.5, 1, and 0.75. The modeled receivers having coil tube configurations concerning the geometrical shape of the models were tested with two different materials of aluminum and copper. The performance of the receiver models was compared by experimental and CFD methods for the average solar direct normal irradiations of 860 W/m2 by the dish reflector area of almost 12 m2. The supplied average heat flux by the dish reflector was 7kW/m2 at the absorbing area of the cavity receivers. The energy and exergy efficiencies from the experimental and CFD analyses on the models were determined based on the cavity surface temperature distribution of receiver walls, and heat gain for different mass flow rates by the heat transfer fluid water. The receiver with copper material and L/D ratio of 0.75 has been found as the optimized one among all other models with the maximum obtained energy and exergy efficiencies of 73.64 and 7.31% when water is used as the heat transfer fluid. The performance of the optimized receiver model was also validated with a few other heat transfer fluids such as SiC + water nanofluid and therminol VP1.

A tunable-wavelength Q-switched fiber laser based on organic metal 8-hydroxyquinoline chelate as a saturable absorber

This work demonstrates the potential of organic metal 8-hydroxyquinoline chelate, Znq2, as a thin film saturable absorber (SA) in a fiber laser cavity. The SA has a modulation depth and saturation intensity of 16.6 % and 95 MW/cm2, respectively. The Q-switched laser had a repetition rate increased from 45 - 85 kHz, while the pulse width shortened from 6.6 - 2.8 µs as the pump power increased from 38 - 198 mW. The laser has a high signal-to-noise ratio (SNR) of 57.4 dB at a central wavelength of 1560 nm. The laser produced a tunable-wavelength operation when a tunable bandpass filter was used in the cavity. The wavelength was continuously tuned between 1520 - 1563 nm with a very stable performance.

Experimental investigation of double slope solar still integrated with PCM nanoadditives microencapsulated thermal energy storage

Transparent covered slope solar stills are trending but characterised with low productivity, heat losses and high energy consumption, which are setbacks in practice. In this study, double slope solar still (DSSS) integrated with PCM-TES is presented. PCM was microencapsulated with epoxy resin composite using vacuum mould-filled techniques. Conventional DSSS and DSSS-TES data collected have been compared to establish the influence of TES on productivity. Daily average temperature of the glass cover, humid air, saline water, still basin absorber and TES cavity for the DSSS-TES attained are 65.2 °C, 77.5 °C, 82.4 °C, 79.5 °C and 68.4 °C, respectively. DSSS-TES has yielded higher production, with 7.5 Litres of potable water daily and extension in operation period by 3 h has been achieved. In addition, condensation and evaporation rates increased with increase in production by 105%. Integration of TES with the system has reduced the heat losses while leakages from PCM nanocomposite have been prevented by microencapsulated insulator. No trace of metals, bacteria and organic contaminants has been found in desalinated water. A payback period of 0.8 year has been recorded based on all-year-round operations. Findings are in good agreement with existing models. Moreover, sensorial characteristics obtained conform to WHO standards.

Numerical analysis of hybrid mode-locking stability in a Ho-doped fiber laser.

We numerically study a hybrid mode-locked holmium-doped fiber laser based on the coupled nonlinear Schrödinger equations, by introducing nonlinear polarization rotation and saturable absorber in a same cavity. Maps of mode-locking states related to the intracavity wave-plates are created to evaluate the stability of mode-locking. Nonlinear polarization rotation imposes negative effect of linear absorption and even reverse saturable absorption to the pulses in the hybrid cavity. Saturable absorber compensates the negative effect from nonlinear polarization rotation, then consequently contributes to the stability of single-pulse mode-locking states. The extension of pulse time duration indicates the dynamic synergy between nonlinear polarization rotation and saturable absorber rather than the static combination mechanism of them.

Q-switched and mode-locked fiber laser based on organic metal 8-hydroxyquinoline chelate as saturable absorber at 1.5 µm

This paper reports the demonstration of bis(8-hydroxyquinoline) calcium (CaQ2) as a passive saturable absorber (SA) in an erbium-doped fiber laser cavity. The SA has a saturation intensity of 511 MW/cm2 and a modulation depth of 14%. As the SA was incorporated into the cavity, a Q-switching was produced with a repetition rate and pulse duration ranging between 45.5 and 88.2 kHz and 8.76 to 2.9 µs, respectively. Then by increasing the cavity length, a soliton mode-locking operation was developed with a repetition rate and pulse duration of 1.82 MHz and 1.6 ps, respectively. Both lasers had a signal-to-noise ratio (SNR) higher than 63 dB and a central wavelength of 1561 nm. Our results suggest that CaQ2 has the potentials to act as a stable and good-performance SA. We believe that this work is the first to report CaQ2 as a functioning and promising SA.

Modeling the external flow of a novel HorseShoe receiver and the evaluation of thermal performance

The linear receiver of a Parabolic Trough Collector is the most critical element in the entire system. The Universal Vacuum Air Collector concept is the most extended type of receiver in both experimental and industrial facilities. Besides their considerable cost, their efficiency usually drops as operation time passes. This is mainly due to a partial loss of vacuum in the evacuated annulus between the absorber and the glass cover. An alternative design called HorseShoe receiver is proposed in this work, whose main goal is to maintain the thermal performance throughout its entire lifespan. This innovative receiver is indicated for low-to-medium temperature ranges, which is particularly suitable for solar heat for industrial processes. It consists of a horseshoe-like cavity absorber having its upper border insulated. In addition, two main advantages can be taken by using two symmetric lenses as glass cover: reconcentrate solar radiation into the cavity (improvement of the intercept factor) and protect stratification conditions (reduction of thermal losses). A transient numerical model with customized boundary conditions has been implemented to evaluate both thermal performance and temperature difference in the absorber domain, which is critical for the thermal stress conditions. For that purpose and as a key contribution, not only the Heat Transfer Fluid (HTF) temperature but also the heat transfer coefficient in the duct are set. In particular, HTF temperature ranges from 80 °C to 220 °C and the inner heat transfer coefficient from 600 W/(m 2 ⋅ K) to 1800 W/(m 2 ⋅ K). Results show that numerical thermal performance is above 96%, which is mainly due to the reduction of thermal radiation losses, where the absorber active surface emittance is ϵ a b s = 0 . 3 . Since no inclination of the receiver has been considered in this initial study, it is shown that the air inside the cavity is stratified, which reduces convective losses. Moreover, temperature difference between the coldest and the hottest spot of the absorber is no higher than 30 K, which is affordable from a technical point of view. A polynomial heat losses correlation is also provided, whose parameters are on the same order of those by conventional receiver correlations available in the literature. • A new concept of solar linear cavity receiver (HorseShoe receiver) is proposed. • Thermal losses have been evaluated numerically. • Thermal performance is above 96% under the selected operation conditions. • The symmetric lenses enhance air stratification. • Most of the thermal losses are due to thermal radiation.

Fast Variable-Temperature Cryogenic Blackbody Sources for Calibration of THz Superconducting Receivers

An electrically heated blackbody radiation source comprising thin metal film on a dielectric substrate and an integrating cavity was designed, fabricated, and experimentally studied at frequencies from 75 to 500 GHz. Analytical and numerical modeling were performed to optimize the emissivity, spectral uniformity, and modulation frequency of the radiation source with the spherical integrating cavity and thin film absorber. The blackbody emissivity (absorptivity) increased from 0.3 to 0.5 for the bare thin film on dielectric substrate, and up to 0.95 when it was placed inside the integrating cavity. The fabricated source mounted at the 0.5 K stage was used to measure the response time of a few microseconds and for sensitivity measurement down to 10−18 W/Hz1/2 of the superconductor–insulator–normal metal–insulator–superconductor (SINIS) detector at 100 mK.

Realization of an efficient wide-angle solar selective absorber via the impedance matching

Constructing the solar absorber with high omnidirectional absorption has great potential for enhanced solar energy conversion. Although varieties of strategies have been carried out to enhance the solar absorption at the near-normal incidence and achieve the selective solar absorbers (SSAs) with high optical performance, the wide-angle absorber is rarely investigated. Here, we experimentally demonstrated a wide-angle high absorption solar selective absorber configurated with the W–SiO 2 cermet-based quasi-optical cavity structure for effective solar harvesting. It can realize a wideband absorption in the wavelength range of 300–1400 nm at the normal incidence, and maintain the average omnidirectional absorption of ∼95% over the incident angle ranges from 8 to 80°, which ascribes to the impedance matching between the absorber and the free space at a large incident angle. The SSA also demonstrates a large solar absorption of around 70% at the incident angle of 80°. A remarkable temperature increase of ∼40 °C relative to ambient temperature was achieved for the SSA directly exposed to the outdoor sunshine without solar concentration in the subtropical city of Shenzhen. • The W–SiO 2 based quasi-optical cavity absorber demonstrates an angle-insensitive absorption at up to 60° incident angle. • The wide-angle absorption is realized via matching the wave impedance between absorber and incidence medium. • The wide-angle solar absorber demonstrates an omnidirectional absorption of 95% among 8–85°. • This absorber indicates superior stability at 500 °C in vacuum and can maintain at ∼70 °C as directly exposed to outdoor sunlight.

Performance Investigation of Ethiopian Local Drinking Alcohol Distillation System Using Solar Dish Concentrator

In Ethiopia, in addition to the large quantity of biomass consumption per year for daily cooking, production of the traditional local “Areke” consumes large amounts of fire wood which further accelerates deforestation. This study introduces solar-based technology for distillation of the local “Areke” using an indirect heating system. A solar parabolic dish collector with an aperture diameter of 0.9 m and an improved truncated cone cavity absorber were installed. The heat transfer process is governed by the principle of natural circulation, boiling, and condensation between a receiver and a distillation column. The experiment was conducted in Debre Birhan city at 20°C ambient temperature and atmospheric pressure of 0.722 atm. The surface temperature of the truncated cone cavity absorber attained a maximum temperature of 300.3°C, and the thermal efficiency attained by the collector was 54.6%. The production efficiency of the solar thermal local alcohol “Areke” distillation system was found to increase by 1.67% compared to the traditional firewood distillation system.

Optical Performance Comparison of Different Shapes of Cavity Receiver in the Fixed Line-Focus Solar Concentrating System

To optimize the fixed-focus solar concentrating system (FLSCS) and linear cavity receiver of better optical performance, the effects of receiver parameters (geometric shape, receiver position f, receiver internal surface absorptivity αab, and end reflection plane reflectivity ρr) on the relative optical efficiency loss ηre-opt,loss, the maximum value of the local concentration ratio Xmax, and the non-uniformity factor σnon were studied in the present study. The results showed that the increases of sun declination angle δ in the range of 0–8° have a weak effect on the ηre-opt,loss. The ηre-opt,loss are 2.25%, 2.72%, 12.69% and 2.62%, 3.26%, 12.85%, respectively, when the solar hour angle ω is 0°, 30°, 60° as δ = 0° and 8° for linear rectangular cavity receiver. The Xmax mainly depends on the energy flux distribution of first intercepted sunlight on the cavity absorber inner wall. Increasing the distance between the cavity absorber inner wall and the focal line Δf can affect the Xmax. The smaller the Δf, the greater the Xmax, and vice versa. The changing trend of σnon is basically consistent with that of the Xmax. When the f is 600, 625, 650, 675, 700 mm and the ω = 0°, the σnon are 0.832, 0.828, 0.801, 0.747, and 0.671, respectively, for linear rectangular cavity receiver. This work could establish the foundation for further research on the optical to thermal energy conversion in the FLSCS.

Causally-guided acoustic optimization of single-layer rigidly-backed micro-perforated partitions: Theory

This theoretical study adapts results obtained in electromagnetism on the optimal performance of radar slab absorbers to the field of acoustics to enhance the broadband dissipation of single-layer rigidly-backed micro-perforated absorbers (MPAs) under normal incidence. Based on the causality principle, an integral identity is derived. It shows that the MPA ultimate wideband performance, that integrates contributions of the intensity reflection coefficient over all the positive wavelengths, is upper bounded by the absorber cavity depth. A sensitivity analysis led to the proposal of a causal-based optimization criterion in order to find optimal MPAs that achieve maximum wideband performance while reaching perfect absorption at their Helmholtz resonance frequency. This simple criterion maximizes the sensitivity of the total reflected intensity with respect to the micro-perforated panel constitutive parameters. It can be readily implemented using numerical quadrature and optimization solvers. It is shown to be a suitable alternative to maximization of the total absorption and inverse-frequency weighted absorption for single-layer MPA broadband optimization.

High-Yield Exfoliation of Monolayer 1T'-MoTe2 as Saturable Absorber for Ultrafast Photonics.

Liquid-phase exfoliation can be developed for the large-scale production of two-dimensional materials for photonic applications. Although atomically thin 2D transition metal dichalcogenides (TMDs) show enhanced nonlinear optical properties or photoluminescence quantum yield relative to the bulk phase, these properties are weak in the absolute sense due to the ultrashort optical path, and they are also sensitive to layer-dependent symmetry properties. Another practical issue is that the chemical stability of some TMDs (e.g., Weyl semimetals) decreases dramatically as the thickness scales down to monolayer, precluding application as optical components in air. To address these issues, a way of exfoliating TMDs that ensures instantaneous passivation needs to be developed. Here, we employed a polymer-assisted electrochemical exfoliation strategy to synthesize PVP-passivated TMDs monolayers that could be spin coated and restacked into organic-inorganic superlattices with well-defined X-ray diffraction patterns. The segregation of restacked TMDs (e.g., MoS2) by PVP allows the inversion asymmetry of individual layers to be maintained in these superlattices, which allows second harmonic generation and photoluminescence to be linearly scaled with thickness. PVP-passivated monolayer 1T'-MoTe2 saturable absorber fabricated from these flakes exhibits fast response and recovery time (<150 fs) and pulse stability. Continuous-wave mode-locking based on 1T'-MoTe2 saturable absorber in a fiber ring laser cavity has been realized, attaining a fundamental repetition rate of 3.15 MHz and pulse duration as short as 867 fs at 1563 nm.

낮은 안테나 높이 및 균일한 이득 특성을 갖는 초광대역 네 개의 팔을 갖는 후방 공동 시뉴어스 안테나

In this study, an ultra-wideband cavity-backed four-arm sinuous antenna with low height and wide gain bandwidth is proposed. To reduce the antenna height, a feeding structure is applied with a coaxial bundle and two-port balun. In addition, a multi-layer radio absorber and metallic cavity are applied to obtain certain characteristics, such as a unidirectional radiation pattern, low profile, and stable reflection coefficient. Additionally, a radiator with low input impedance is applied for a wide gain bandwidth. The proposed antenna operates at 0.4~6.0 GHz and has a relatively low height and large gain bandwidth compared to the performance of previously studied antennas.

A Reversible Tuning of High Absorption in Chalcogenide–Metal Stacked‐Layer Structure and Its Application for Multichannel Biosensing

Perfect absorption ranging from visible, infrared, terahertz, to microwave is desirable for solar cell, photodetection, telecommunications, and molecular sensing. Recently, the air/dielectric–metal stacks/substrate‐based asymmetric Fabry–Pérot (FP) cavity has attracted much attention owing to lithography‐free design which is scalable and low cost. Herein, a reversibly tunable asymmetric FP cavity high absorber in the near‐infrared (NIR) region is experimentally demonstrated, relying on chalcogenide (Ge2Sb2Te5)–metal (Au) stacked layers. It exhibits an extremely high absorptance of 0.99 at a resonant wavelength of 1180 nm for amorphous (AM) state; yet, the peak absorptance redshifts to 1680 nm for crystalline (CR) state. Importantly, it takes about 5 ns to reversibly transit the peak absorptance by reamorphizing the GST225. It is also experimentally shown the Brewster modes can be excited in the cavity absorber, and a wide tuning of Brewster modes (from 820 to 1500 nm) is realized as switching the state between AM and CR. The Goos–Hänchen (GH) shift can be observed at the Brewster angle for each state. For the proof of concept multibiosensing application, the cavity to detect two different solutions of the copper sulfate and glucose in both AM and CR states is numerically functionalized, respectively.

Designing Optically &amp; Utilization of Thermopile Chip with Resonant Cavity Absorber Structure as IR Absorber

This paper presents a novel thermopile chip in which the resonant cavity structure was fully utilized as an absorber by an optical design. The resonant cavity absorber structure was designed using Al as anthe bottom reflective metal layer, air as the intermediate dielectric layer, and SiO2/TiN/Si3N4 sandwich layers as the top absorption layer, while the bottom reflective metal (Al) was deposited on the cold junctions of the thermopile. The simulation and calculation results show that the thermopile chip with resonant cavity absorber structure not only has great infrared absorption in the wide infrared absorption range but also can effectively prevent the cold junctions from absorbing infrared radiation and inhibit the rise of temperature. As a result, the temperature difference between the hot junctions and the cold junctions is increased, and the responsivity of the thermopile chip is further improved. Moreover, the duty cycle of the thermopile chip is greatly improved due to the double-layer suspension structure. Compared with the traditional thermopile chip structure, the sizes of the thermopile chip with the resonant cavity absorber structure can be further reduced while maintaining responsivity and specific detectivity.

Recent Advance in Tunable Single-Frequency Fiber Laser Based on Two-Dimensional Materials

Two-dimensional (2D) materials possess optoelectronic and nonlinear optical properties make them used in many fields such as optical modulator, optical switch, and single frequency (SF) fiber lasers as a saturable absorber (SA) in the laser cavity. This review deals with recent advances in wavelength tunable single frequency fiber laser based on these properties. It extends the contents from previous reviews on pulsed fiber lasers to SF fiber lasers which have emerged in recent years. First, the research status of 2D materials, including the structural characteristics and properties of some typical cases and their saturable absorption characteristics are introduced. After that, the principle and some applicable technologies with these 2D materials to achieve the wavelength tunable SF operation are discussed. Further, the latest research progress on such fiber lasers are summarized. Finally, a discussion on future prospects and challenges are included.

Terahertz Sensor With Resonance Enhancement Based on Square Split-Ring Resonators

A novel design of a nearly perfect metamaterial absorber based on square split-ring resonators for terahertz sensing applications is proposed and analyzed. The design in this report is simulated and analyzed by using standard numerical simulation software. Magnetic and electric resonant field enhancement in the impedance matched absorber cavity enables stronger interaction with the dielectric analyte. The proposed structure is based on the simultaneous increase in the electromagnetic field and the surface current distribution at the resonance frequency. An absorptivity of 99% is achieved at 0.53 THz with a narrow resonance peak and a Q-factor of 44.17. At a fixed analyte thickness, the resonance frequency is sensitive to the refractive index of the surrounding medium. The influence of the thickness of the covering sample on the sensitivity and absorption coefficient of the absorber is comprehensively analyzed, and the reported design can be used as a refractive index sensor with a high sensitivity of 126.0 GHz/RIU and a figure of merit of 10.5 in the refractive index range from 1.0 to 2.0 at an analyte thickness of $15.0~\mu \text{m}$ . The results show that the sensor has high sensitivity to the analyte covering it. The sensor not only exhibits good sensitivity to thin analytes but also shows high sensitivity to analytes more than $10~\mu \text{m}$ thick in the terahertz low frequency band. Specifically, the sensitivity changes rapidly when the thickness of the sample changes in the range of 0- $6~\mu \text{m}$ , but slowly in the range of 6- $16~\mu \text{m}$ . In general, the response of the resonance frequency to changes in the refractive index of the sample becomes more sensitive as the thickness of the sample is increased from 0 to $16~\mu \text{m}$ . The reported terahertz sensor of a metamaterial absorber has potential applications in biomedical sensing and trace detection of substances.