Multilayer optical coatings are used to control the reflection, transmission, and absorption of light in a variety of applications. However, the design of these coatings is a challenging and computationally expensive task. In this article, we present an inverse-design method that provides an efficient and automated way to design one-dimensional optical coatings with superior optical properties. Our approach involves starting with a desired optical response and working backward to determine the optimal coating structure that can achieve that response. To accomplish this, we define a figure of merit (FOM) that maximizes the reflection or transmission within a desired spectral range and polarization. We identify the design variables, calculate the derivatives of the FOM, and iteratively update the thickness of each layer until the desired FOM is achieved. Our method applies to a broad variety of one-dimensional optical coatings, and we demonstrate its effectiveness by designing broadband mirrors and anti-reflection coatings. Our results show that the optimized structures outperform the initial fixed-thickness structures, highlighting the potential of inverse design to simplify the design and fabrication of complex multilayer optical coatings with greater precision and efficiency.
The solar spectrum reaching Earth spans from ultraviolet to infrared wavelengths with a cutoff at around 2.5 μm. About 30% of the irradiance is absorbed by the atmosphere, and the rest is absorbed by the Earth's surface. The radiance available for generating renewable energy ranges from 400 nm to 2.0 μm. Silicon and III-V materials are used for photovoltaic (PV) cells. The PV cells reflect 30% of incoming radiation and to reduce this reflection, antireflection coating (ARC) is being used. Conventional techniques such as single or stacked multi-layer ARC or micro-nanostructures ARC are used. However, they lack in providing broadband and omnidirectional transmission, which limits its conversion efficiency in today’s PV cells. We present broadband ARC achieved with an inverse transfer design, a prospect towards significantly high conversion efficiency PV cells. The ARC exhibit significantly increased transmission more than 96% over the solar spectrum ranges up to 2.5 μm, even with wide angles of incidence from ~ 0° to >70°. This transmission over the same angles of incidence, is significantly higher than that of ARC based on quarter-wavelength optical thickness (QWOT), a state-of-art ARC. The results also rival the transmission performance of state-of-the-art nanostructure-based ARC even at large angles of incidence, but are significantly easier to fabricate using standard e-beam evaporation and/or sputtering. The results show over broadband spectrum ranges, radiation back due to reflection makes to zero, leading to significantly increased conversion efficiency of the PV cells.
The temperature difference between the earth (288K) and outer space (3K) can operate a heat engine and generate electricity. We can use an optical emitter to radiate strongly within the atmospheric transparency window (8-13µm) and cool the emitter below the ambient temperature. A thermoelectric generator can then harness the temperature difference between the emitter and ambient to generate electricity. However, parasitic conduction between air and the optical emitter, inefficient thermal contacts, and thermal conduction through the thermoelectric itself severely penalize the maximum electrical power generated. We minimize these three penalty terms by careful engineering while increasing the outgoing optical radiation and experimentally demonstrating a record >100mW/m2 for this technology. We can use this efficient, simple, and cheap electricity generation method to deliver power in remote places and the most efficient ambient energy harvesting scheme.
High performance broadband Distributed Bragg Reflectors (DBRs) have become instrumental in the design of high efficiency Thermophotovoltaic devices, improving efficiency by redirecting parasitic photon loss during conversion towards the device for regenerating power. Broadband Bragg Reflectors designed to reflect in the 3-5 micron, 3-12 micron, and 1-12 micron infrared ranges are of crucial importance in such applications to maximize regeneration efficiency in TPV devices, as most parasitic photons fall in these ranges under typical operating temperatures. Additionally, omnidirectional capabilities further improve regeneration efficiency by absorbing parasitic photons at large angles of incidence. In this work, we present a novel inverse design transfer matrix optimization algorithm for designing variable layer thickness broadband DBRs for arbitrary wavelength ranges and angles of incidence, and we showcase the resulting ultra-high reflection DBRs designed with our algorithm, specifically for the 1-12μm IR range. We demonstrate that our DBRs provide very significant increases in average reflection over the IR ranges of interest as compared to state-of-the-art reflective coatings, even at large angles of incidence, and are also optimized to be considerably thinner than quarter wavelength and other state of the art DBRs with comparable reflection spectra. As such, the DBRs designed with our inverse design algorithm have significant applications in the design of very high efficiency TPV systems, while also being thin enough to embed onto devices or windows, and are also easy to fabricate due to their simple multilayer 1-D structure.
Broadband and omnidirectional anti-reflection coatings, Distributed Bragg Reflectors (DBR), and optical bandpass filter coatings in the 3-5 μm and 1-12 μm IR ranges are of crucial importance in the design of high-efficiency MWIR / LWIR detectors, Thermophotovoltaic devices, and IR imaging systems. We discuss a novel inverse design approach to multilayer optical coatings by a Transfer Matrix Method simulation and optimization algorithm. This algorithm iteratively simulates a multilayer structure's reflection and transmission characteristics by the Transfer Matrix Method and updates layer thicknesses to optimize a performance metric – whether reflection, transmission, or filter performance – over a given band and range of angles of incidence. The optimization procedure provides a systematic, computational approach to designing optical coatings with desired reflection, transmission, and absorption characteristics over wavelength bands and angles of incidence of interest in an application. We show that Distributed Bragg Reflectors and Anti-reflection coatings generated by the algorithm outperform conventional quarter-wavelength layer coatings over broadband MWIR and LWIR ranges and angles of incidence as high as 70 degrees, while reducing total component thickness significantly – a major benefit when fabricating optical coatings on semiconducting substrates directly. We also discuss how the approach can be extended to sharp-cutoff broadband bandpass filter design.
Broadband and omnidirectional antireflection coatings in the 3-5 μm and 3-12 μm infrared ranges are of crucial importance in maximizing quantum efficiency in MWIR and LWIR photodetectors, as well as in improving the efficiency of LWIR thermophotovoltaic devices. Conventional approaches to AR coatings on semiconductors, such as quarter-wavelength optical thickness layers, fail to provide consistently high transmission over mid-IR bandwidths and acute angles of incidence. Unconventional approaches such as metasurface and nanostructure AR coatings are difficult and expensive to fabricate on a large scale. We discuss ultra-broadband antireflection coating designed through an inverse design procedure driven by a differential evolution optimization algorithm. Our approach iteratively simulates the transmission characteristics of a multilayer structure using the Transfer-Matrix Method and optimizes layer parameters to maximize the average transmission within a given wavelength band and range of angles of incidence. We present AR coatings which exhibit 98% and 96% average transmission over the 3 μm - 5 μm and 3 μm - 12 μm ranges respectively, over angles of incidence between 0° and 70°. Compared to quarter optical wavelength antireflection coatings over the same ranges, our structures exhibit significantly higher broadband transmission – quarter-wavelength structures saturate near 80% transmission over the 3-5 μm or 3-12 μm ranges, with transmission significantly lowered at large angles. As such, our results rival the transmission performance of state-of-the-art nanostructure AR coatings even at large angles of incidence but are significantly easier to fabricate using standard e-beam evaporation and/or sputtering.
Thermophotovoltaic power conversion relies on a highly reflective mirror to reduce parasitic power consumption and increase power conversion efficiency. Here, we show the designs of broadband thermophotovoltaic mirrors, that can achieve >99% reflectivity, over 3-octaves of bandwidth.
Thermophotovoltaic power conversion utilizes thermal radiation from a local heat source to generate electricity in a photovoltaic cell. It was shown in recent years that the addition of a highly reflective rear mirror to a solar cell maximizes the extraction of luminescence. This, in turn, boosts the voltage, enabling the creation of record-breaking devices. Now we report that the rear mirror can be used to create thermophotovoltaic systems with unprecedented high efficiency. This mirror reflects low-energy infrared photons back into the heat source,
recovering their energy. This radically improves thermophotovoltaic efficiency. Therefore, the rear mirror serves a dual function; boosting the voltage and re-using infrared thermal photons. This allows the possibility of a practical >50% efficient thermophotovoltaic system. Based on this reflective rear mirror concept, we recently experimentally
demonstrated a thermophotovoltaic efficiency of 29.1%, a new efficiency record. In this work,
A highly reflective rear mirror can increase the efficiency of thermophotovoltaics through regeneration of unused photons. Based on this concept, we recently demonstrated a record 29.1% thermophotovoltaic device efficiency. We have also identified the challenges as we aim towards 50% thermophotovoltaic efficiency.
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