Professional Education


  • Doctor of Philosophy, Georgia Institute of Technology (2016)

Stanford Advisors


All Publications


  • Nonreciprocal radiative heat transfer between two planar bodies PHYSICAL REVIEW B Fan, L., Guo, Y., Papadakis, G. T., Zhao, B., Zhao, Z., Buddhiraju, S., Orenstein, M., Fan, S. 2020; 101 (8)
  • Broadening Near-Field Emission for Performance Enhancement in Thermophotovoltaics. Nano letters Papadakis, G. T., Buddhiraju, S., Zhao, Z., Zhao, B., Fan, S. 2020

    Abstract

    The conventional notion for achieving high efficiency in thermophotovoltaics (TPVs) is to use a monochromatic emission at a photon energy corresponding to the band gap of the cell. Here, we prove theoretically that such a notion is only accurate under idealized conditions and further show that, when nonradiative recombination is taken into account, efficiency improvement can be achieved by broadening the emission spectrum, due to an enhancement in the open-circuit voltage. Broadening the emission spectrum also improves the electrical power density, by increasing the short-circuit current. Hence, broadening the emission spectrum can simultaneously improve the efficiency and power density of practical TPV systems. To illustrate these findings, we focus on surface polariton-mediated near-field TPVs. We propose a versatile design strategy for broadening the emission spectrum via stacking of multiple plasmonic thin film layers. As an example, we consider a realistic ITO/InAs TPV and predict a conversion efficiency of 50% simultaneously with a power density of nearly 80 W/cm2 at a 1300 K emitter temperature. The performance of our proposed system far exceeds previous works in similar systems using a single plasmonic layer emitter.

    View details for DOI 10.1021/acs.nanolett.9b04762

    View details for PubMedID 31978305

  • Integrated near-field thermo-photovoltaics for heat recycling. Nature communications Bhatt, G. R., Zhao, B., Roberts, S., Datta, I., Mohanty, A., Lin, T., Hartmann, J. M., St-Gelais, R., Fan, S., Lipson, M. 2020; 11 (1): 2545

    Abstract

    Energy transferred via thermal radiation between two surfaces separated by nanometer distances can be much larger than the blackbody limit. However, realizing a scalable platform that utilizes this near-field energy exchange mechanism to generate electricity remains a challenge. Here, we present a fully integrated, reconfigurable and scalable platform operating in the near-field regime that performs controlled heat extraction and energy recycling. Our platform relies on an integrated nano-electromechanical system that enables precise positioning of a thermal emitter within nanometer distances from a room-temperature germanium photodetector to form a thermo-photovoltaic cell. We demonstrate over an order of magnitude enhancement of power generation (Pgen ~ 1.25 μWcm-2) in our thermo-photovoltaic cell by actively tuning the gap between a hot-emitter (TE ~ 880 K) and the cold photodetector (TD ~ 300 K) from ~ 500 nm down to ~ 100 nm. Our nano-electromechanical system consumes negligible tuning power (Pgen/PNEMS ~ 104) and relies on scalable silicon-based process technologies.

    View details for DOI 10.1038/s41467-020-16197-6

    View details for PubMedID 32439917

  • Axion-Field-Enabled Nonreciprocal Thermal Radiation in Weyl Semimetals. Nano letters Zhao, B., Guo, C., Garcia, C. A., Narang, P., Fan, S. 2020

    Abstract

    Objects around us constantly emit and absorb thermal radiation. The emission and absorption processes are governed by two fundamental radiative properties: emissivity and absorptivity. For reciprocal systems, the emissivity and absorptivity are restricted to be equal by Kirchhoff's law of thermal radiation. This restriction limits the degree of freedom to control thermal radiation and contributes to an intrinsic loss mechanism in photonic energy harvesting systems. Existing approaches to violate Kirchhoff's law typically utilize magneto-optical effects with an external magnetic field. However, these approaches require either a strong magnetic field (∼3T) or narrow-band resonances under a moderate magnetic field (∼0.3T), because the nonreciprocity in conventional magneto-optical effects is weak in the thermal wavelength range. Here, we show that the axion electrodynamics in magnetic Weyl semimetals can be used to construct strongly nonreciprocal thermal emitters that nearly completely violate Kirchhoff's law over broad angular and frequency ranges without requiring any external magnetic field.

    View details for DOI 10.1021/acs.nanolett.9b05179

    View details for PubMedID 32073859

  • Near-complete violation of Kirchhoff's law of thermal radiation with a 0.3 T magnetic field OPTICS LETTERS ZHAO, B., Shi, Y., Wang, J., Zha, Z., Zhao, N., Fan, S. 2019; 44 (17): 4203–6

    Abstract

    The capability to overcome Kirchhoff's law of thermal radiation provides new opportunities in energy harvesting and thermal radiation control. Previously, design towards demonstrating such capability requires a magnetic field of 3 T, which is difficult to achieve in practice. In this work, we propose a nanophotonic design that can achieve such capability with a far more modest magnetic field of 0.3 Tesla, a level that can be achieved with permanent magnets. Our design uses guided resonance in low-loss dielectric gratings sitting on a magneto-optical material, which provides significant enhancement on the sensitivity to the external magnetic field.

    View details for DOI 10.1364/OL.44.004203

    View details for Web of Science ID 000483918900029

    View details for PubMedID 31465363

  • Self-sustaining thermophotonic circuits. Proceedings of the National Academy of Sciences of the United States of America Zhao, B., Buddhiraju, S., Santhanam, P., Chen, K., Fan, S. 2019

    Abstract

    Photons represent one of the most important heat carriers. The ability to convert photon heat flow to electricity is therefore of substantial importance for renewable energy applications. However, photon-based systems that convert heat to electricity, including thermophotovoltaic systems where photons are generated from passive thermal emitters, have long been limited by low power density. This limitation persists even with near-field enhancement techniques. Thermophotonic systems, which utilize active photon emitters such as light-emitting diodes, have the potential to significantly further enhance the power density. However, this potential has not been realized in practice, due in part to the fundamental difficulty in thermodynamics of designing a self-sustaining circuit that enables steady-state power generation. Here, we overcome such difficulty by introducing a configuration where the light-emitting diodes are connected in series, and thus multiple photons can be generated from a single injected electron. As a result we propose a self-sustaining thermophotonic circuit where the steady-state power density can exceed thermophotovoltaic systems by many orders of magnitude. This work points to possibilities for constructing heat engines with light as the working medium. The flexibility of controlling the relations between electron and photon flux, as we show in our design, may also be of general importance for optoelectronics-based energy technology.

    View details for DOI 10.1073/pnas.1904938116

    View details for PubMedID 31118287

  • Experimental demonstration of energy harvesting from the sky using the negative illumination effect of a semiconductor photodiode APPLIED PHYSICS LETTERS Ono, M., Santhanam, P., Li, W., Zhao, B., Fan, S. 2019; 114 (16)

    View details for DOI 10.1063/1.5089783

    View details for Web of Science ID 000466264600002

  • Gate-Tunable Near-Field Heat Transfer ACS PHOTONICS Papadakis, G. T., Zhao, B., Buddhiraju, S., Fan, S. 2019; 6 (3): 709–19
  • High Reflection from a One-Dimensional Array of Graphene Nanoribbons ACS PHOTONICS Zhao, N., Zhao, Z., Williamson, I. D., Boutami, S., Zhao, B., Fan, S. 2019; 6 (2): 339–44
  • High Reflection from a One-Dimensional Array of Graphene Nanoribbons Zhao, N., Zhao, Z., Williamson, I. D., Boutami, S., Zhao, B., Fan, S., IEEE IEEE. 2019
  • MESH: A free electromagnetic solver for far-field and near-field radiative heat transfer for layered periodic structures COMPUTER PHYSICS COMMUNICATIONS Chen, K., Zhao, B., Fan, S. 2018; 231: 163–72
  • Near-Field Thermophotonic Systems for Low-Grade Waste-Heat Recovery NANO LETTERS Zhao, B., Santhanam, P., Chen, K., Buddhiraju, S., Fan, S. 2018; 18 (8): 5224–30

    Abstract

    Low-grade waste heat contains an enormous amount of exergy that can be recovered for renewable-energy generation. Current solid-state techniques for recovering low-grade waste heat, such as thermoelectric generators and thermophotovoltaics, however, are limited by low conversion efficiencies or power densities. In this work, we propose a solid-state near-field thermophotonic system. The system consists of a light-emitting diode (LED) on the hot side and a photovoltaic (PV) cell on the cold side. Part of the generated power by the PV cell is used to positively bias the LED. When operating in the near-field regime, the system can have power density and conversion efficiency significantly exceeding the performance of current solid-state approaches for low-grade waste-heat recovery. For example, when the gap spacing is 10 nm and the hot side and cold side are, respectively, 600 and 300 K, we show that the generated electric power density and thermal-to-electrical conversion efficiency can reach 9.6 W/cm2 and 9.8%, respectively, significantly outperforming the current record-setting thermoelectric generators. We identify the alignment of the band gaps of the LED and the PV cell, the appropriate choice of thickness of the LED and PV cell to mitigate the effect of non-radiative recombination, and the use of highly reflective back mirrors as key factors that affect the performance of the system. Our work points to the significant potential of photonic systems for the recovery of low-grade waste heat.

    View details for DOI 10.1021/acs.nanolett.8b02184

    View details for Web of Science ID 000441478300084

    View details for PubMedID 30016115

  • High-performance near-field thermophotovoltaics for waste heat recovery NANO ENERGY Zhao, B., Chen, K., Buddhiraju, S., Bhatt, G., Lipson, M., Fan, S. 2017; 41: 344–50
  • Resonance perfect absorption by exciting hyperbolic phonon polaritons in 1D hBN gratings OPTICS EXPRESS Zhao, B., Zhang, Z. M. 2017; 25 (7): 7791-7796

    Abstract

    Natural materials with hyperbolic responses can confine light with well-defined propagation directions inside the micro/nanostructure. Here we theoretically demonstrate that strong resonance absorption can be achieved in one-dimensional gratings made of hexagonal boron nitride (hBN) due to hyperbolic phonon polaritons. The radiative properties of both trapezoidal and square resonators are calculated using anisotropic rigorous coupled-wave analysis. The resonance wavelengths can be theoretically predicted and are shown to follow the anomalous or traditional scaling laws depending on the hyperbolicity. These findings may benefit the applications including photodetection, color filters, and optomechanics.

    View details for DOI 10.1364/OE.25.007791

    View details for Web of Science ID 000398536000057

    View details for PubMedID 28380897