Ultra-low Absorption Research Articles

Terahertz laser-fabricated all-polymer hole arrays with high-quality quasi-bound states in the continuum

Abstract Dielectric metasurfaces promise to realize ultrahigh-quality (Q) resonances due to their ultralow material absorption. Most of them are silicon-based metasurfaces, requiring complex fabricated steps and thus suffering high costs. Laser etching processing has simple steps accompanied by low time consumption and exemplary processing efficiency. Here, an all-polymer metasurface based on hole arrays fabricated by laser processing has been proposed and investigated. Such metasurfaces achieve sharp quasi-bound states in the continuum (quasi-BICs) via breaking structural symmetry, form two annular circulation electric fields in different directions, and thus allow strong coupling between holes. Owing to the low refractive index of polymer, the calculated Q-factor reaches 9555 while the diameter discrepancy is 4 μm. Simulated results proved that the Q-factors of quasi-BICs can be further improved by reducing the film thickness and refractive indices of materials, which can be predicted by the fitting equation. Also, the fields in holes can be enhanced by reducing the film refractive index. These results in simulations and experiments provide an alternative method for designing high-Q resonators in terahertz regions.

The Design of Highly Reflective All-Dielectric Metasurfaces Based on Diamond Resonators

All-dielectric metasurfaces offer a low-loss alternative to plasmonic metasurfaces. We proposed the configuration for high-reflectivity all-dielectric metasurfaces based on single-crystal diamond (SCD) resonators on fused silica substrate and conducted simulations to optimize and analyze such a configuration via the FDTD solver. We utilized GMR as the design principle to select the configuration and the substrate material, and analyzed the scattering properties of a single SCD resonator by multipole decomposition. Then, we demonstrated that both the cylindrical resonators in square lattice and frustum-shaped resonators in hexagonal lattice can achieve near-unity reflectivity (>99.99%) and ultra-low absorption (<0.001%) at 795 nm, the typical alkali-metal laser wavelength. Additionally, we demonstrated that such a design is quite tolerant of fabrication errors and further supports its potential for realistic applications. To expand the functionality of such devices across multiple wavelengths, dual-band high-reflectivity metasurfaces at 744 nm and 828 nm were also designed. Our work is quite useful for designing diamond-based highly reflective mirrors, paving the way for low-loss all-dielectric reflective metasurfaces in high-power laser applications.

MgGa2O4-based thermal control coating: An ultra-low solar absorption coating with high irradiation stability

In this study, the viability of MgGa2O4 as a pigment for thermal control coatings is discussed. Notably, the MgGa2O4 based coating exhibited excellent initial optical properties and irradiation stability. After irradiation with 40 keV proton at a dose of 1.4 × 1014 P/cm2, the optical performance of the coating remained essentially unchanged, while the solar absorption of the coating is still maintained below 0.10 after irradiation with 40 keV electron at a dose of 1.6 × 1016 e/cm2. Further, the mechanism of the optical changes induced by electron irradiation was elucidated through experimental and computational methods. In MgGa2O4 lattice, two different types of oxygen vacancies and a transition process of them were found. This process was accompanied by the leakage of some oxygen atoms from the lattice, leading to an increased number of color centers and the change in optical properties. Despite the solar absorption raised after electron irradiation, the MgGa2O4 based thermal control coating showed minimal change and low end-of-irradiation-life values, which suggests that they have excellent stability and promising applications.

Linear Electro-Optic Effect in 2D Ferroelectric for Electrically Tunable Metalens.

The advent of 2D ferroelectrics, characterized by their spontaneous polarization states in layer-by-layer domains without the limitation of a finite size effect, brings enormous promise for applications in integrated optoelectronic devices. Comparing with semiconductor/insulator devices, ferroelectric devices show natural advantages such as non-volatility, low energy consumption and high response speed. Several 2D ferroelectric materials have been reported, however, the device implementation particularly for optoelectronic application remains largely hypothetical. Here, the linear electro-optic effect in 2D ferroelectrics is discovered and electrically tunable 2D ferroelectric metalens is demonstrated. The linear electric-field modulation of light is verified in 2D ferroelectric CuInP2S6. The in-plane phase retardation can be continuously tuned by a transverse DC electric field, yielding an effective electro-optic coefficient rc of 20.28 pm V-1. The CuInP2S6 crystal exhibits birefringence with the fast axis oriented along its (010) plane. The 2D ferroelectric Fresnel metalens shows efficacious focusing ability with an electrical modulation efficiency of the focusing exceeding 34%. The theoretical analysis uncovers the origin of the birefringence and unveil its ultralow light absorption across a wide wavelength range in this non-excitonic system. The van der Waals ferroelectrics enable room-temperature electrical modulation of light and offer the freedom of heterogeneous integration with silicon and another material system for highly compact and tunable photonics andmetaoptics.

Enabling cryogenic gravitational wave detectors: growth of sapphire crystals with record low absorption in the near infrared

Sapphire was grown by Czochralski and ultra-low absorption (11 ppm cm−1) was achieved. The Fe2+–Fe3+ and Ti3+–Ti3+ pairs are crucial for the absorption at 1064 nm. The result is promised to grow large sapphire for the next generation of GW detector.

Problem of Measuring Absorption Using Time-Resolved Photothermal Common-Path Interferometry under Conditions of Developed Heat Diffusion

We present a study of the problem of measuring ultra-low absorption in quartz materials using the time-resolved photothermal common-path interferometry (TPCI) method, which we proposed and elaborated, with allowance for developed heat diffusion in the samples. This task is related to the implementation of one of the ways of increasing the sensitivity of measurements, namely, increasing the energy of the heating pulse by increasing its duration. Analytical formulas for the time dependence of the power of the time-varying component of the probe radiation are obtained in the Gaussian approximation for the laser beams. A correction factor that takes into account the heat diffusion effect is calculated theoretically and used during calibration. The rate at which the power of the time-varying component decreases when the sample is cooled after the end of the heating pulse is calculated. When measuring in crystalline quartz, quartz glass, and also in air, the calculated rate coincided with the experimental one, which is additional evidence for the reliability of the calculations of the correction factor for calibrating the measurements. When the duration of the heating pulse is increased to 5 ms, the calculated sensitivity of the scheme for measuring absorption in quartz glasses is 2 × 10−9 cm−1.

Hydrogen-Induced Ultralow Optical Absorption and Mechanical Loss in Amorphous Silicon for Gravitational-Wave Detectors.

The sensitivity of gravitational-wave detectors is limited by the mechanical loss associated with the amorphous coatings of the detectors' mirrors. Amorphous silicon has higher refraction index and lower mechanical loss than current high-index coatings, but its optical absorption at the wavelength used for the detectors is at present large. The addition of hydrogen to the amorphous silicon network reduces both optical absorption and mechanical loss for films prepared under a range of conditions at all measured wavelengths and temperatures, with a particularly large effect on films grown at room temperature. The uptake of hydrogen is greatest in the films grown at room temperature, but still below 1.5 at.% H, which show an ultralow optical absorption (below 10ppm) measured at 2000nm for 500-nm-thick films. These results show that hydrogenation is a promising strategy to reduce both optical absorption and mechanical loss in amorphous silicon, and may enable fabrication of mirror coatings for gravitational-wave detectors with improved sensitivity.

Hybrid nested negative curvature fiber with ultra-low-loss in the terahertz band

A novel hybrid nested negative curvature fiber (NCF) based on the three-layer anti-resonant effect is described. The elliptical tubes cladding structure exhibits the advantageofflexibledesign and can effectively reduce propagation losses. The influences of the nested methods and structural parameters on the propagation characteristic are analyzed numerically. Since high-resistance silicon (HRS) has an ultra-low absorption coefficient in the THz frequency range, the effective material loss is reduced. Meanwhile, the three-layer nested structure suppresses the confinement loss, the total loss is as low as 6.62 × 10−5 dB/m at 1.1 THz. The effects of the number of cladding tubes and dielectric layers on the propagation loss are compared and analyzed. The optimized NCF shows a loss of less than 10−4 dB/m between 1 and 1.06 THz and 1.1–1.24 THz. The result reveals a new method to design broadband ultra-low-loss hollow-core NCFs.

Electrical programmable multilevel nonvolatile photonic random-access memory

Photonic Random-Access Memories (P-RAM) are an essential component for the on-chip non-von Neumann photonic computing by eliminating optoelectronic conversion losses in data links. Emerging Phase-Change Materials (PCMs) have been showed multilevel memory capability, but demonstrations still yield relatively high optical loss and require cumbersome WRITE-ERASE approaches increasing power consumption and system package challenges. Here we demonstrate a multistate electrically programmed low-loss nonvolatile photonic memory based on a broadband transparent phase-change material (Ge2Sb2Se5, GSSe) with ultralow absorption in the amorphous state. A zero-static-power and electrically programmed multi-bit P-RAM is demonstrated on a silicon-on-insulator platform, featuring efficient amplitude modulation up to 0.2 dB/μm and an ultralow insertion loss of total 0.12 dB for a 4-bit memory showing a 100× improved signal to loss ratio compared to other phase-change-materials based photonic memories. We further optimize the positioning of dual microheaters validating performance tradeoffs. Experimentally we demonstrate a half-a-million cyclability test showcasing the robust approach of this material and device. Low-loss photonic retention-of-state adds a key feature for photonic functional and programmable circuits impacting many applications including neural networks, LiDAR, and sensors for example.

A wideband absorber working in the L- and S-bands based on magnetic materials

In this paper, a multi-layered microwave absorber that can work in the L- and S-bands is proposed. The absorber is composed of a traditional medium and magnetic material coatings. A genetic algorithm is used to achieve rapid design of a multi-layered absorber. The absorptivity of the absorber is more than 90% in 1.13–4.36 GHz under normal incidence, i.e., 117.7% fractional bandwidth. Moreover, the total thickness is 13 mm, corresponding to 0.049λ0 at the lowest operating frequency. Meanwhile, the absorber also has good wide-angle and polarization-independent characteristics. When the incident angle increases to 40°, it still can keep stable absorption in both TE and TM polarization modes. The working mechanism of the absorber is analyzed from the perspective of equivalent complex impedance, transmission line theory, and power loss density. Finally, a sample was fabricated and measured. The experimental results agree well with numerical simulations, achieving ultra-broadband and ultra-low frequency absorption.

Effect of ionic oxygen concentration on properties of SiO2 and Ta2O5 monolayers deposited by ion beam sputtering

Optical thin films with ultralow absorption loss are crucial for high-precision optical systems. The ionic oxygen concentration and oxygen flow rate play decisive roles in the absorption loss of the film. We prepared SiO2 and Ta2O5 monolayers using a double-ion beam sputtering (DIBS) deposition system and studied the effects of different ionic oxygen concentrations and oxygen flow rates on the optical constants, weak absorption, –OH defects, and dispersion energy parameters. The results show that the oxygen flow slightly affects the weak absorption of SiO2 films, while a higher ionic oxygen concentration facilitates the reduction of the absorption loss of SiO2 films, resulting in fewer –OH defects. A higher ionic oxygen concentration decreases the refractive index of the Ta2O5 films. The relatively low lattice permittivity of the Ta2O5 films may be attributed to the argon bubbles wrapped in the film. Annealing can reduce the absorption of the film but cannot improve its surface roughness. A Ta2O5/SiO2 high-reflection film was produced with an optimal ionic oxygen concentration and oxygen flow rate, with the lowest absorption loss for both materials. The weak absorption attained 1.4 ppm, playing a significant role in improving the performance of the high-power and high-precision optical systems.

Surface-enhanced photoacoustic wave generation from light absorbers located in the gap of high-refractive-index dielectric nanoparticles

Plasmonic nanoparticles, such as Au nanoparticles, have been used as effective exogeneous contrast agents in photoacoustic imaging. However, using these particles to enhance photoacoustic wave generation from other light absorbers is difficult because the signal is overwhelmed by the signal from plasmonic nanoparticles themselves. In this study, we realized the surface-enhanced photoacoustic effect by using high-refractive-index dielectric nanoparticles in their ultralow light absorption Ohmic-loss wavelength band. We performed finite element method simulation on a model with gallium phosphide nanorod dimmer and light absorbers in the dimmer gap. The Au nanowire, carbon nanotube, and nanotube filled with melanin molecules were sequentially considered as the light absorber. It is found that a photoacoustic wave surface enhancement factor of approximately 10 was achieved and the enhanced photoacoustic wave ensures the photoacoustic spectral signature of the light absorber. The potential of optimization for boosting enhancement factors was further analyzed. These results can provide considerable insight into molecular photoacoustic sensing and imaging.

Optical properties and radiation stability of SiO2/ZnO composite pigment prepared by co-sintering method

Thermal control coatings (TCCs) are an essential part of the thermal control systems in the spacecraft. Solar absorptance and emittance are the key performance parameters of TCCs. To develop an ultra-low solar absorption and stable inorganic TCCs for surface radiator, different TCCs were prepared by co-sintering ZnO and SiO2 nanoparticles to form Zn2SiO4/SiO2 pigment in this work, and the optical properties and radiation stability were systematically studied. It is found that the coating based on composite pigment has high reflectivity in the ultraviolet band and excellent optical performance possessing the low solar absorption of 0.06. In addition, the Zn2SiO4/SiO2 coating demonstrates the highest proton and electron radiation stability because that SiO2 between Zn2SiO4 particles acts as the relaxation center of the defects caused by radiation.

Fabrication of Mid-Infrared ZnGeP2 Crystals and Devices with Large Apertures and Ultra-Low Absorption Coefficients

Fabrication of Mid-Infrared ZnGeP2 Crystals and Devices with Large Apertures and Ultra-Low Absorption Coefficients

Sustainable Development of Particulate Reinforced Composites by Recycling Marble Waste for Advanced Construction Applications: Ultra-low Water Absorption, Remarkable Thermal and Mechanical Behaviour

Sustainable Development of Particulate Reinforced Composites by Recycling Marble Waste for Advanced Construction Applications: Ultra-low Water Absorption, Remarkable Thermal and Mechanical Behaviour

Hybrid metamaterial absorber for ultra-low and dual-broadband absorption.

Developing high-efficiency microwave absorbers remains challenging in the broadband range, particularly in the low-frequency range containing the L band and even lower. To overcome this challenge, a hybrid metamaterial absorber comprising a conventional magnetic absorbing material and a multi-layered meta-structure predesigned with graphene films is proposed to realize wideband absorption performance starting from ultra-low frequencies (0.79-20.9 GHz and 25.1-40.0 GHz). The high absorption ability of the proposed device originates from fundamental resonance modes and their coupling. The experimental results agree well with the simulated ones, proving the effectiveness of our design method. In addition, owing to the use of low-density polymethylacrylimide foam and graphene films with outstanding mechanical properties, our design is lightweight and environmentally adaptable, which reflects its engineering value.

Local-field-enhanced and polarisation-sensitive graphene/MoS2 film on side-polished fibre with coated Au film

The performances of devices based on two-dimensional (2D) materials are limited by their atomic thickness, which leads to ultraweak interactions between light and 2D materials and ultralow optical absorption. We propose an integrated all-fibre platform based on a side-polished fibre (SPF) and graphene/MoS2 (GM) film. A coated Au film can enhance the interaction between the transverse-magnetic-like (TM) mode and GM film owing to the light field confinement effect of the Au film. By coating the Au film, the light absorption by the GM film can be enhanced 4.25 times for the TM mode. Surprisingly, the GM film absorption in the transverse-electric-like (TE) mode is suppressed by the Au film. The coated Au film makes the all-fibre device, based on the GM film, a TE-passing polariser with an extinction ratio of 19.7 dB at 1550 nm, compared to an extinction ratio of 15.9 dB for the Au-film-coated SPF without GM film. The extinction ratio of the all-fibre device can be further increased by reducing the thickness of the Au film. These findings improve our understanding of the interaction between light and 2D materials and provide an effective method towards the development of polarisation-sensitive multi-functional all-fibre devices based on 2D materials.

3D characterization of low optical absorption structures in large crystalline sapphire substrates for gravitational wave detectors

Very high-quality sapphire substrates are key elements of the cryogenic Japanese gravitational interferometer KAGRA, in which they are used to build the main mirrors, working as the test masses to sense the gravitational waves. To meet the extreme requirements of this system, the sapphire test masses must possess an extremely low optical absorption, which makes their study challenging using standard methods. In this paper, we illustrate the results obtained on two typical samples using a specialized absorption setup based on the technique of Photo-thermal Common-path Interferometry (PCI). Our system combines a very high sensitivity to small absorption features with the possibility to perform a full three-dimensional mapping of the sample volume. Our results elucidate how the ultra-low absorption variations inside the sample possess a structure that is probably inherited from the growth history of the sample. Some conclusions on the role of structural defects as preferential sites for the inclusion of absorbing centers are drawn.

Ultra-high quality graphene perfect absorbers for high performance switching manipulation.

Wavelength-selective light absorption and the related switching operations are highly desired in optical devices. Herein, we report the approach for ultra-high quality (Q) graphene perfect optical absorption, which possesses impressive performance in switching manipulation. A record-breaking Q-factor (up to 105) is observed, suggesting one or two orders of magnitude larger than that of the conventional graphene absorbers. The ultra-low external leakage loss rate of quasi-bound states in the continuum (BIC) resonator and the ultra-low intrinsic absorption loss rate in the resonant mode volume are the main contributions for the ultra-high Q perfect absorption. By introducing a Kerr nonlinear medium, spectral relative intensity can be changed from 0 to 100% when an ultra-low pump light with the intensity of only 5 kW cm-2 is used. After a rather slight tuning of the refractive index (Δn = 5×10-4) for the resonators, the absorption contrast ratio reaches 31 dB. The switching related spectral wavelength shift sensitivity is up to 915 nm/RIU and the figure of merit (FOM) is 50 833. These features confirm the ultra-high tunability and switching manipulation. It is believed that the ultra-high Q-factor absorption offered by all-dielectric configuration provides plentiful potential applications for graphene-based devices in the all-optical switch, modulator, notch filter, etc.

Galactosamine-Conjugating Zwitterionic Block Copolymer for Reduction-Responsive Release and Active Targeted Delivery of Doxorubicin to Hepatic Carcinoma Cells

Nanocarriers with integrated advantage, such as excellent stealth property, active targeting function, and rapid intracellular drug release, are significant for cancer treatment. Herein, a biodegradable zwitterionic triblock copolymer containing disulfide-linked poly-ε-caprolactone and polycarboxybetaine methacrylate (PCB-SS-PCL-SS-PCB) was first synthesized and then partly modified with galactosamine (GAL) for constructing polymeric micelle drug carrier with multifunctionality. Polymeric micelles showed ultralow protein absorption in serum and obvious reduction-responsiveness in the presence of glutathione, provided by the zwitterionic polymer shell and the disulfide bond, respectively. Furthermore, active targeting of the carrier to hepatic carcinoma cells was achieved via GAL ligands on PCB shells due to their specific binding to asialoglycoprotein receptors on the cell surface. As expected, in vivo competition studies demonstrated that doxorubicin- (DOX-) loaded GAL-modified micelles have better anticancer effect in hepatic tumor-bearing mice than free DOX and nontargetable micelles. As a result, this novel multifunctional carrier provides a valuable strategy to design promising anticancer drug delivery systems for liver cancer treatment.