Near-field generalization of the blackbody concept
How much radiant heat can two bodies exchange? For 150 years, the Stefan–Boltzmann Limit — achievable only with blackbodies — has provided an answer at macroscopic distances. At nanoscale separations, it has long been known that greater-than-blackbody transfer is possible, but general limits to this process have not been known, and the computational complexity of fluctuational electrodynamics has prevented systematic study.
In this work we:
- Derived fundamental, shape-independent limits to near-field radiative heat transfer. The limits depend only on the material parameters and separation distance of the bodies.
- Provided examples of structures that approach the limits in idealized cases.
- Demonstrated that the simple and high-symmetry systems studied to date fall far short of our bounds.
- Showed that achieving the bounds would transform the experimental landscape of radiative heat transfer, with significant potential for thermal extraction applications, particularly thermophotovoltaics.
Further coverage of this work: APS Physics Viewpoint, MIT News article
Design principles for solar cells
- Demonstrated that the SQ limits are not robust: small imperfections, in material quality or optical design, lead to significant voltage and efficiency penalties.
- Showed the importance of light extraction to achieving a high voltage, such that a good solar cell must be a good LED.
- Developed efficiency limits that incorporate internal imperfections, building upon the SQ limits and increasing their utility.
Paper: Strong Internal and External Luminescence as Solar Cells Approach the Shockley-Queisser Limit [ doi | pdf ]
Research Highlight: The Opto-Electronics of Solar Cells [ pdf ]
Fundamental limits to optical response in absorptive systems
Metals interact strongly with electromagnetic radiation. They support subwavelength resonances that have generated interest in the field of "plasmonics" for more than a decade, but which are also damped by material loss. A long-standing question has been the extent to which resonant enhancement can overcome material loss. Here we:
- Develop conservation-of-energy arguments that yield new, fundamental limits to the optical response of absorptive systems (including but not limited to metals).
- We derive limits to absorption, scattering, and local density of states enhancements. The limits depend on |χ|2 / Im χ, where χ is the material susceptibility (generalized to include anisotropic/bi-anisotropic materials)
- We show that there are stuctures that approach the limits, and also frequency ranges at which common structures fall orders of magnitude short.
- We compare common metals by the material metric |χ|2 / Im χ across visible and infrared wavelengths.
Computational design of solar-cell surface textures
- Computationally designed surface textures (with ~30 parameters) with angle- and frequency-averaged enhancements of 25, the largest such high-index enhancements achieved to date.
- Showed that at subwavelength scales, designed structures can be better than random ones
- Demonstrated a cutting-edge approach to designing realistic solar cells with many potential degrees of freedom
For the physics of solar cells in the near field, see (led by Avi Niv):
Near-Field Electromagnetic Theory for Solar Cells [ doi | pdf ]
Nanoparticle design: large, tunable response
- Computationlly designed maximum-extinction nanoparticles (over ~1000 degrees of freedom), arriving at new, unique shapes.
- Used quasistatic sum rules to develop fundamental bounds on extinction by nanoparticles.
- Leveraged the bounds to design tailored distributions of particles that can achieve nearly ideal extinction over broad and tunable bandwidths, experimentally demonstrated with Ag nanodisks over three visible-wavelength windows. (Led by Emma Anquillare.)
- Experimental and analytical design of coherent plasmon-exciton coupling in J-aggregate-on-metal systems, demonstrating dark-state formation. (Led by Brendan DeLacy.)
Fundamental Limits to Extinction by Metallic Nanoparticles [ doi | pdf ]
Computational design and experimental demonstration of tunable extinction (led by Emma Anquillare)
Efficient broad- and tunable-bandwidth optical extinction via ... (Submitted) [ arxiv | pdf ]
Expt. + computation for plasmon-exciton coupling (led by Brendan DeLacy)
Coherent Plasmon-Exciton Coupling in Silver Platelet-J-aggregate Nanocomposites [ doi | open-access link ]
All-dielectric cloaking structures
- Performed three-dimensional large-scale topology optimization (over hundreds of degrees of freedom) to design non-magnetic cloaking structures.
- Leveraged practical (application-oriented) tradeoffs—frequency bandwidth, angular bandwith, etc. vs. material complexity—to achieve simpler ``cloaking'' designs that are readily achievable experimentally.
- Demonstrated very low scattering cross-sections even with highly lossy metals, with designs very different from those used for perfect metals.
(See also the paper by Sigmund et. al., similar work that appeared in print at roughly the same time that this work was first presented.)
Computational design for silicon photonics, HAMR
- Design of a waveguide splitter—the most compact designed to date—to split energy at 1550nm wavelength with minimal (less than 0.07 dB) insertion loss. (Led by Chris Keraly.)
- Design of an optimal-delivery structure for maximum absorption in a small spot-size, critical for heat-assisted magnetic recording (HAMR), the next-generation hard-drive technology. (Led by Samarth Bhargava.)
Adjoint shape optimization applied to Electromagnetic design [ doi | pdf ]
Inverse Design of Optical Antennas for Sub-Wavelength Energy Delivery [ doi | pdf ]