Infrared broadband metasurface absorber for reducing the thermal mass of a microbolometer

High sensitivity bolometers based on metal nanoantenna dimers with a nanogap filled with vanadium dioxide

One critical factor for bolometer sensitivity is efficient electromagnetic heating of thermistor materials, which plasmonic nanogap structures can provide through the electric field enhancement. In this report, using finite element method simulation, electromagnetic heating of nanorod dimer antennas with a nanogap filled with vanadium dioxide (VO₂) was studied for long-wavelength infrared detection. Because VO₂ is a thermistor material, the electrical resistance between the two dimer ends depends on the dimer’s temperature. The simulation results show that, due to the high heating ability of the nanogap, the temperature rise is several times higher than expected from the areal coverage. This excellent performance is observed over various nanorod lengths and gap widths, ensuring wavelength tunability and ultrafast operating speed, thereby making the dimer structures a promising candidate for high sensitivity bolometers.

Ultrabroadband metal-black absorbers and the performance simulations based on a three-dimensional cluster-structure model

Broadband light absorbers are attractive for their applications in photodetection and thermo-photovoltaics. Metal-black porous coatings have been experimentally proven to have broadband light absorption. However, a theoretical model is of importance for the design and fabrication of metal-black absorbers. Here we propose a three-dimensional cluster-structure model to simulate the absorption of metal-black films. Based on experimental data, a model of uniform clusters formed by nanoparticles with Gaussian random distribution in position was constructed for the gold-black absorbers. The absorption spectra were simulated with this model by finite-difference time-domain method. The gold-black absorbers were fabricated by the one-step magnetron sputtering process. The average absorption of gold-black absorbers with sputtering pressure of 50, 65 and 80 Pa were 72.34%, 87.25% and 91.08% in the visible spectral range and 81.77% (80 Pa) in 3-12 µm infrared spectrum. The high broadband absorption was attributed to the multiple scattering of incident light inside the gold-black porous structure. The simulations showed good agreements with experimental results with an error of 2.35% in visible spectrum and 1.82% in 3-12 µm infrared spectrum. To verify the applicability of this model, aluminum-black absorbers with different thicknesses were fabricated, and the absorption error between simulation and experimental results was 3.96%. This cluster model can be a good tool to design ultrabroadband absorbers based on metal-black porous structures.

Subwavelength perforated absorbers for infrared detectors

This paper presents a detailed analysis examining the absorption performance of a metal-dielectric slab with subwavelength size periodic perforations exploiting quarter-wave impedance matching (QWIM) technique within long wave infrared (LWIR) regime (8-12µm). Integration of perforations to a simple stack with various period sizes and perforated area ratios are examined through theory, simulation, and measurements that are in great agreement. Advantages of perforated absorbers for thermal detectors are discussed in maximizing optical absorption and reducing thermal-mass point of view. Introducing perforation in umbrella type absorbers is mainly employed for reducing the thermal-mass while maintaining the high absorption performance. Within the scope, it is experimentally shown that a perforation ratio (width/period) of 50% with square holes for the umbrella layer is possible without degrading the maximum LWIR absorption performance of 96% when the sheet resistance of R_s=400Ω/□ is employed for the absorbing metal layer, which is close to free space impedance of 377Ω/□. Nevertheless, this ratio can be increased up to 77% by depositing a thicker absorber metal with smaller sheet resistance, such as R_s=100Ω/□ while still maintaining an average absorption performance of 93%, which are all predicted numerically by simulations and physically explained through effective medium approach (EMA).

Triple-Band Perfect Light Absorber Based on Hybrid Metasurface for Sensing Application

A simple design of triple-band perfect light absorber (PLA) based on hybrid metasurface in visible region has been presented in this work, which turns out to be applicable for refractive index (RI) sensing. Distinct from previous designs, the proposed hybrid metasurface for visible PLA is only consisted of periodic silicon cross nanostructure arrays and gold substrate. The periodic silicon cross arrays deposited on the gold substrate contribute to excite the guided modes under the normal incident light illumination. According to the simulation results, it can be found that three perfect absorption peaks of 98.1%, 98.7%, and 99.6% which are located at 402.5 THz, 429.5 THz, and 471.5 THz, respectively, have been clearly observed in PLA. This triple-band perfect absorption effect could be attributed to the intrinsic loss of silicon material originated from the guided mode excitations caused by the standing waves of different orders. It has been confirmed that the perfect absorption properties of the PLA can be easily regulated by changing the geometric parameters of the unit-cell nanostructure. Furthermore, the designed PLA served as a RI sensor can achieve sensitivity of about 25.3, 41.3, and 31.9 THz /refractive index unit (RIU). It can be believed that the proposed design of PLA for RI sensing would provide great potential applications in sensing, detecting, the enhanced visible spectroscopy, etc.

Ultrafast pyroelectric photodetection with on-chip spectral filters

Thermal detectors, such as bolometric, pyroelectric and thermoelectric devices, are uniquely capable of sensing incident radiation for any electromagnetic frequency; however, the response times of practical devices are typically on the millisecond scale^1-7. By integrating a plasmonic metasurface with an aluminium nitride pyroelectric thin film, we demonstrate spectrally selective, room-temperature pyroelectric detectors from 660-2,000 nm with an instrument-limited 1.7 ns full width at half maximum and 700 ps rise time. Heat generated from light absorption diffuses through the subwavelength absorber into the pyroelectric film producing responsivities up to 0.18 V W^-1 due to the temperature-dependent spontaneous polarization of the pyroelectric films. Moreover, finite-element simulations reveal the possibility of reaching a 25 ps full width at half maximum and 6 ps rise time rivalling that of semiconductor photodiodes⁸. This design approach has the potential to realize large-area, inexpensive gigahertz pyroelectric detectors for wavelength-specific detection from the ultraviolet to short-wave infrared or beyond for, for example, high-speed hyperspectral imaging.

Small-sized long wavelength infrared absorber with perfect ultra-broadband absorptivity

Two types of ultra-broadband long wavelength infrared (LWIR) absorbers with small period and super thin thickness are designed. The absorption with high absorptivity and large bandwidth is achieved through combined propagating and localized surfaced plasmon resonances. We first design a three-layer absorber with a Ti-Ge-Ti configuration, the period of the structure is only 1.4 µm (nearly 1/8 of the center wavelength), the thickness of its dielectric is only 0.5 µm (1/22 of the center wavelength), and the average absorption is 87.9% under normal incident from 8µm to 14µm. Furthermore, the four-layer absorber with a Ti-Ge-Si₃N₄-Ti configuration is designed to obtain more average absorption increasing to 94.5% from 8 µm to 14µm under normal incident, the period of the structure increases to 1.6 µm and the total thickness of dielectric increases to 0.6µm. The proposed absorber is polarization-independent and possesses a good tolerance of incident angle. We calculate that the average absorption of the four-layer absorber for both TE- and TM-modes still exceeds 90% up to an incident angle of θ = 40° (90.7% for TE-mode, 91.9% for TM-mode), and exceed 80% up to an incident angle of θ = 60° (80.2% for TE-mode, 82.1% for TM-mode).

Advanced numerical investigation of the heat flux in an array of microbolometers

The investigation of the thermal properties of an array of microbolometers has been carried out by mean of two independent numerical analysis, respectively the Direct-Simulation Monte Carlo (DSMC) and the classic diffusive approach of the Fourier’s equation. In particular, the thermal dissipation of a hot membrane placed in a low-pressure cavity has been studied for different values of the temperature of the hot body and for different values of the pressure of the environment. The results for the heat flux derived from the two approaches have then been compared and discussed.

Material-Versatile Ultrabroadband Light Absorber with Self-Aggregated Multiscale Funnel Structures

Broadband light absorbers are essential components for a variety of applications, including energy harvesting and optoelectronic devices. Thus, the development of a versatile absorbing structure that is applicable in various operating environments is required. In this study, a material-versatile ultrabroadband absorber consisting of metal-coated self-aggregated Al₂O₃ nanowire bundles with multiscale funnel structures is fabricated. A high absorptance of ∼0.9 over the AM 1.5G spectrum (300-2500 nm) is realized for absorbers with a range of metal coatings, including Al, W, and titanium nitride (TiN). We demonstrate that the plasmonic nanofocusing and index-matching effects of the funnel structure result in strong ultrabroadband absorption for the various metal coatings, even though the coating materials have different optical properties. As an example of applicability in an operating environment, in the evaluation of the thermal-oxidation resistance, the Al-coated solar absorber exhibits superior performance to those coated with refractory materials such as W and TiN because of the protective alumina layer formed on the Al surface.