# Near-field generalization of the

*blackbody*conceptHow 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.

*Shape-independent limits to near-field radiative heat transfer*[ doi | pdf ]

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 ]

- Demonstrated that the SQ limits are not
# 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.

*Fundamental limits to optical response in absorptive systems*[ doi | pdf ]

# Computational design of solar-cell surface textures

^{2}) absorption enhancement for sub-wavelength cells, where ray optics is not valid and random roughening is not sufficient. We (led by Vidya Ganapati):- 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

*Light Trapping Textures Designed by Electromagnetic Optimization...*[ doi | pdf ]

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.

*Photonic Design: From Fundamental Solar Cell Physics to Computational Inverse Design*[ arxiv | pdf ]

(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 ]

HAMR design:

*Inverse Design of Optical Antennas for Sub-Wavelength Energy Delivery*[ doi | pdf ]