Hybrid supervised and reinforcement learning for the design and optimization of nanophotonic structures.

From higher computational efficiency to enabling the discovery of novel and complex structures, deep learning has emerged as a powerful framework for the design and optimization of nanophotonic circuits and components. However, both data-driven and exploration-based machine learning strategies have limitations in their effectiveness for nanophotonic inverse design. Supervised machine learning approaches require large quantities of training data to produce high-performance models and have difficulty generalizing beyond training data given the complexity of the design space. Unsupervised and reinforcement learning-based approaches on the other hand can have very lengthy training or optimization times associated with them. Here we demonstrate a hybrid supervised learning and reinforcement learning approach to the inverse design of nanophotonic structures and show this approach can reduce training data dependence, improve the generalizability of model predictions, and significantly shorten exploratory training times. The presented strategy thus addresses several contemporary deep learning-based challenges, while opening the door for new design methodologies that leverage multiple classes of machine learning algorithms to produce more effective and practical solutions for photonic design.

Resonant Anti-Reflection Metasurfaces for Infrared Transmission Optics.

A fundamental capability needed for any transmissive optical component is anti-reflection, yet this capability can be challenging to achieve in a cost-effective manner over longer infrared wavelengths. We demonstrate that Mie-resonant photonic structures can enable high transmission through a high-index optical component, allowing it to function effectively over long-wavelength infrared wavelengths. Using silicon as a model system, we demonstrate a resonant metasurface that enables a window optic with transmission up to 40% greater than that of unpatterned Si. Imaging comparisons with unpatterned Si and off-the-shelf germanium optics are shown as well as modulation transfer function measurements, showing excellent performance and suitability for imaging applications. Our results show how resonant photonic structures can be used to improve optical transmission through high-index optical components and highlight their possible use in infrared imaging applications.

Relativistic Light Sails Need to Billow.

We argue that light sails with nanometer-scale thicknesses that are rapidly accelerated to relativistic velocities by lasers must be significantly curved in order to reduce their intrafilm mechanical stresses and avoid tears. Using an integrated opto-thermo-mechanical model, we show that the diameter and radius of curvature of a circular light sail should be comparable in magnitude, both on the order of a few meters, in optimal designs for gram-scale payloads. Moreover, we demonstrate that, when sufficient laser power is available, a sail's acceleration length decreases as its curvature increases. Our findings provide critical guidance for emerging light sail design programs, which herald a new era of interstellar space exploration to destinations such as the Oort cloud, the Alpha Centauri system, and beyond.

Multiscale Photonic Emissivity Engineering for Relativistic Lightsail Thermal Regulation.

The Breakthrough Starshot Initiative aims to send a gram-scale probe to our nearest extrasolar neighbors using a laser-accelerated lightsail traveling at relativistic speeds. Thermal management is a key lightsail design objective because of the intense laser powers required but has generally been considered secondary to accelerative performance. Here, we demonstrate nanophotonic photonic crystal slab reflectors composed of 2H-phase molybdenum disulfide and crystalline silicon nitride, highlight the inverse relationship between the thermal band extinction coefficient and the lightsail's maximum temperature, and examine the trade-off between minimizing acceleration distance and setting realistic sail thermal limits, ultimately realizing a thermally endurable acceleration minimum distance of 23.3 Gm. We additionally demonstrate multiscale photonic structures featuring thermal-wavelength-scale Mie resonant geometries and characterize their broadband Mie resonance-driven emissivity enhancement and acceleration distance reduction. More broadly, our results highlight new possibilities for simultaneously controlling optical and thermal response over broad wavelength ranges in ultralight nanophotonic structures.

Passive radiative cooling below ambient air temperature under direct sunlight.

Cooling is a significant end-use of energy globally and a major driver of peak electricity demand. Air conditioning, for example, accounts for nearly fifteen per cent of the primary energy used by buildings in the United States. A passive cooling strategy that cools without any electricity input could therefore have a significant impact on global energy consumption. To achieve cooling one needs to be able to reach and maintain a temperature below that of the ambient air. At night, passive cooling below ambient air temperature has been demonstrated using a technique known as radiative cooling, in which a device exposed to the sky is used to radiate heat to outer space through a transparency window in the atmosphere between 8 and 13 micrometres. Peak cooling demand, however, occurs during the daytime. Daytime radiative cooling to a temperature below ambient of a surface under direct sunlight has not been achieved because sky access during the day results in heating of the radiative cooler by the Sun. Here, we experimentally demonstrate radiative cooling to nearly 5 degrees Celsius below the ambient air temperature under direct sunlight. Using a thermal photonic approach, we introduce an integrated photonic solar reflector and thermal emitter consisting of seven layers of HfO2 and SiO2 that reflects 97 per cent of incident sunlight while emitting strongly and selectively in the atmospheric transparency window. When exposed to direct sunlight exceeding 850 watts per square metre on a rooftop, the photonic radiative cooler cools to 4.9 degrees Celsius below ambient air temperature, and has a cooling power of 40.1 watts per square metre at ambient air temperature. These results demonstrate that a tailored, photonic approach can fundamentally enable new technological possibilities for energy efficiency. Further, the cold darkness of the Universe can be used as a renewable thermodynamic resource, even during the hottest hours of the day.

Radiative cooling of solar absorbers using a visibly transparent photonic crystal thermal blackbody.

A solar absorber, under the sun, is heated up by sunlight. In many applications, including solar cells and outdoor structures, the absorption of sunlight is intrinsic for either operational or aesthetic considerations, but the resulting heating is undesirable. Because a solar absorber by necessity faces the sky, it also naturally has radiative access to the coldness of the universe. Therefore, in these applications it would be very attractive to directly use the sky as a heat sink while preserving solar absorption properties. Here we experimentally demonstrate a visibly transparent thermal blackbody, based on a silica photonic crystal. When placed on a silicon absorber under sunlight, such a blackbody preserves or even slightly enhances sunlight absorption, but reduces the temperature of the underlying silicon absorber by as much as 13 °C due to radiative cooling. Our work shows that the concept of radiative cooling can be used in combination with the utilization of sunlight, enabling new technological capabilities.

Broadband nonreciprocal thermal emissivity and absorptivity.

A body that violates Kirchhoff's law of thermal radiation exhibits an inequality in its spectral directional absorptivity and emissivity. Achieving such an inequality is of fundamental interest as well as a prerequisite for achieving thermodynamic limits in photonic energy conversion1 and radiative cooling2. Thus far, inequalities in the spectral directional emissivity and absorptivity have been limited to narrow spectral resonances3, or wavelengths well beyond the infrared regime4. Bridging the gap from basic demonstrations to practical applications requires control over a broad spectral range of the unequal spectral directional absorptivity and emissivity. In this work, we demonstrate broadband nonreciprocal thermal emissivity and absorptivity by measuring the thermal emissivity and absorptivity of gradient epsilon-near-zero InAs layers of subwavelength thicknesses (50 nm and 150 nm) with an external magnetic field. The effect occurs in a spectral range (12.5-16 µm) that overlaps with the infrared transparency window and is observed at moderate (1 T) magnetic fields.

Simultaneous Control of Spectral And Directional Emissivity with Gradient Epsilon-Near-Zero InAs Photonic Structures.

Controlling both the spectral bandwidth and directionality of emitted thermal radiation is a fundamental challenge in contemporary photonics. Recent work has shown that materials with a spatial gradient in the frequency range of their epsilon-near-zero (ENZ) response can support broad spectrum directionality in their emissivity, enabling high total radiance to specific angles of incidence. However, this capability is limited spectrally and directionally by the availability of materials with phonon-polariton resonances over long-wave infrared wavelengths. Here, an approach is designed and experimentally demonstrated using doped III-V semiconductors that can simultaneously tailor spectral peak, bandwidth, and directionality of infrared emissivity. InAs-based gradient ENZ photonic structures that exhibit broadband directional emission with varying spectral bandwidths and directional ranges as a function of their doping concentration profile and thickness are epitaxially grown and characterized. Due to its easy-to-fabricate geometry, it is believed that this approach provides a versatile photonic platform to dynamically control broadband spectral and directional emissivity for a range of emerging applications in heat transfer and infrared sensing.

Controlling radiative heat flows in interior spaces to improve heating and cooling efficiency.

Heating and cooling in buildings account for nearly 20% of energy use globally. The goal of heating and cooling systems is to maintain the thermal comfort of a building's human occupants, typically by keeping the interior air temperature at a setpoint. However, if one could maintain the occupant's thermal comfort while changing the setpoint, large energy savings are possible. Here we propose a mechanism to achieve these savings by dynamically tuning the thermal emissivity of interior building surfaces, thereby decoupling the mean radiant temperature from actual temperatures of interior surfaces. We show that, in cold weather, setting the emissivity of interior surfaces to a low value (0.1) can decrease the setpoint as much as 6.5°C from a baseline of 23°C. Conversely, in warm weather, low-emissivity interior surfaces result in a 4.5°C cooling setpoint decrease relative to high emissivity (0.9) surfaces, highlighting the need for tunable emissivity for maximal year-round efficiency.

Broadband directional control of thermal emission.

Controlling the directionality of emitted far-field thermal radiation is a fundamental challenge. Photonic strategies enable angular selectivity of thermal emission over narrow bandwidths, but thermal radiation is a broadband phenomenon. The ability to constrain emitted thermal radiation to fixed narrow angular ranges over broad bandwidths is an important, but lacking, capability. We introduce gradient epsilon-near-zero (ENZ) materials that enable broad-spectrum directional control of thermal emission. We demonstrate two emitters consisting of multiple oxides that exhibit high (>0.7, >0.6) directional emissivity (60° to 75°, 70° to 85°) in the p-polarization for a range of wavelengths (10.0 to 14.3 micrometers, 7.7 to 11.5 micrometers). This broadband directional emission enables meaningful radiative heat transfer primarily in the high emissivity directions. Decoupling the conventional limitations on angular and spectral response improves performance for applications such as thermal camouflaging, solar heating, radiative cooling, and waste heat recovery.