Papers by John Joannopoulos
Science, 2022
Bombardment of materials by high-energy particles often leads to light emission in a process know... more Bombardment of materials by high-energy particles often leads to light emission in a process known as scintillation. Scintillation has widespread applications in medical imaging, x-ray nondestructive inspection, electron microscopy, and high-energy particle detectors. Most research focuses on finding materials with brighter, faster, and more controlled scintillation. We developed a unified theory of nanophotonic scintillators that accounts for the key aspects of scintillation: energy loss by high-energy particles, and light emission by non-equilibrium electrons in nanostructured optical systems. We then devised an approach based on integrating nanophotonic structures into scintillators to enhance their emission, obtaining nearly an order-of-magnitude enhancement in both electron-induced and x-ray–induced scintillation. Our framework should enable the development of a new class of brighter, faster, and higher-resolution scintillators with tailored and optimized performance.
Conference on Lasers and Electro-Optics, 2019
The inability of conventional electronic architectures to efficiently solve large combinatorial p... more The inability of conventional electronic architectures to efficiently solve large combinatorial problems motivates the development of novel computational hardware. There has been much effort recently toward developing novel, application-specific hardware, across many different fields of engineering, such as integrated circuits, memristors, and photonics. However, unleashing the true potential of such novel architectures requires the development of featured algorithms which optimally exploit their fundamental properties. We here present the Photonic Recurrent Ising Sampler (PRIS), a heuristic method tailored for parallel architectures that allows for fast and efficient sampling from distributions of combinatorially hard Ising problems. Since the PRIS relies essentially on vector-to-fixed matrix multiplications, we suggest the implementation of the PRIS in photonic parallel networks, which realize these operations at an unprecedented speed. The PRIS provides sample solutions to the ground state of arbitrary Ising models, by converging in probability to their associated Gibbs distribution. By running the PRIS at various noise levels, we probe the critical behavior of universality classes and their critical exponents. In addition to the attractive features of photonic networks, the PRIS relies on intrinsic dynamic noise and eigenvalue dropout to find ground states more efficiently. Our work suggests speedups in heuristic methods via photonic implementations of the PRIS. We also hint at a broader class of (meta)heuristic algorithms derived from the PRIS, such as combined simulated annealing on the noise and eigenvalue dropout levels. Our algorithm can also be implemented in a competitive manner on fast parallel electronic hardware, such as FPGAs and ASICs.
ACS Nano, 2021
The ability to control the propagation direction of light has long been a scientific goal. Howeve... more The ability to control the propagation direction of light has long been a scientific goal. However, the fabrication of large-scale optical angular-range selective films is still a challenge. This paper presents a polymer-enabled large-scale fabrication method for broadband angular-range selective films that perform over the entire visible spectrum. Our approach involves stacking together multiple one-dimensional photonic crystals with various engineered periodicities to enlarge the band gap across a wide spectral range based on theoretical predictions. Experimental results demonstrate that our method can achieve broadband transparency at a range of incident angles centered around normal incidence and reflectivity at larger viewing angles, doing so at large scale and low cost.
Charles Roques-Carmes1,‡,∗ Nicholas Rivera2,‡,† Ali Ghorashi, Steven E. Kooi, Yi Yang, Zin Lin, J... more Charles Roques-Carmes1,‡,∗ Nicholas Rivera2,‡,† Ali Ghorashi, Steven E. Kooi, Yi Yang, Zin Lin, Justin Beroz, Aviram Massuda, Jamison Sloan, Nicolas Romeo, Yang Yu, John D. Joannopoulos, Ido Kaminer, Steven G. Johnson, and Marin Soljačić ‡ denotes equal contribution. 1 Research Laboratory of Electronics, MIT, Cambridge, MA 02139, USA 2 Department of Physics, MIT, Cambridge, MA 02139, USA 3 Institute for Soldier Nanotechnologies, MIT, Cambridge, MA, 02139, USA 4 Department of Mathematics, MIT, Cambridge, MA, 02139, USA 5 Microsystems Technology Laboratories, MIT, Cambridge, MA 02139 USA, 6 Raith America, Inc., USA, and 7 Department of Electrical and Computer Engineering, Technion Haifa 32000, Israel
Light: Science & Applications, 2020
The Hofstadter model, well known for its fractal butterfly spectrum, describes two-dimensional el... more The Hofstadter model, well known for its fractal butterfly spectrum, describes two-dimensional electrons under a perpendicular magnetic field, which gives rise to the integer quantum Hall effect. Inspired by the real-space building blocks of non-Abelian gauge fields from a recent experiment, we introduce and theoretically study two non-Abelian generalizations of the Hofstadter model. Each model describes two pairs of Hofstadter butterflies that are spin–orbit coupled. In contrast to the original Hofstadter model that can be equivalently studied in the Landau and symmetric gauges, the corresponding non-Abelian generalizations exhibit distinct spectra due to the non-commutativity of the gauge fields. We derive the genuine (necessary and sufficient) non-Abelian condition for the two models from the commutativity of their arbitrary loop operators. At zero energy, the models are gapless and host Weyl and Dirac points protected by internal and crystalline symmetries. Double (8-fold), trip...
Optics Express, 2020
We demonstrate new axisymmetric inverse-design techniques that can solve problems radically diffe... more We demonstrate new axisymmetric inverse-design techniques that can solve problems radically different from traditional lenses, including reconfigurable lenses (that shift a multi-frequency focal spot in response to refractive-index changes) and widely separated multi-wavelength lenses (λ = 1 µm and 10 µm). We also present experimental validation for an axisymmetric inverse-designed monochrome lens in the near-infrared fabricated via two-photon polymerization. Axisymmetry allows fullwave Maxwell solvers to be scaled up to structures hundreds or even thousands of wavelengths in diameter before requiring domain-decomposition approximations, while multilayer topology optimization with ∼105 degrees of freedom can tackle challenging design problems even when restricted to axisymmetric structures.
Optica, 2020
Conventional computing architectures have no known efficient algorithms for combinatorial optimiz... more Conventional computing architectures have no known efficient algorithms for combinatorial optimization tasks such as the Ising problem, which requires finding the ground state spin configuration of an arbitrary Ising graph. Physical Ising machines have recently been developed as an alternative to conventional exact and heuristic solvers; however, these machines typically suffer from decreased ground state convergence probability or universality for high edge-density graphs or arbitrary graph weights, respectively. We experimentally demonstrate a proof-of-principle integrated nanophotonic recurrent Ising sampler (INPRIS), using a hybrid scheme combining electronics and silicon-on-insulator photonics, that is capable of converging to the ground state of various four-spin graphs with high probability. The INPRIS results indicate that noise may be used as a resource to speed up the ground state search and to explore larger regions of the phase space, thus allowing one to probe noise-dep...
Nature, 2019
Local, bulk response functions, e.g. permittivity, and the macroscopic Maxwell equations complete... more Local, bulk response functions, e.g. permittivity, and the macroscopic Maxwell equations completely specify the classical electromagnetic problem, which features only wavelength λ and geometric scales. The above neglect of intrinsic electronic length scales L e leads to an eventual breakdown in the nanoscopic limit. Here, we present a general theoretical and experimental framework for treating nanoscale electromagnetic phenomena. The framework features surface-response functions-known as the Feibelman d-parameters-which reintroduce the missing electronic length scales. As a part of our framework, we establish an experimental procedure to measure these complex, dispersive surface response functions, enabled by quasi-normal-mode perturbation theory and observations of pronounced nonclassical effects-spectral shifts in excess of 30% and the breakdown of Kreibig-like broadening-in a quintessential multiscale architecture: film-coupled nanoresonators, with feature-sizes comparable to both L e and λ.
Nature Communications, 2020
The inability of conventional electronic architectures to efficiently solve large combinatorial p... more The inability of conventional electronic architectures to efficiently solve large combinatorial problems motivates the development of novel computational hardware. There has been much effort toward developing application-specific hardware across many different fields of engineering, such as integrated circuits, memristors, and photonics. However, unleashing the potential of such architectures requires the development of algorithms which optimally exploit their fundamental properties. Here, we present the Photonic Recurrent Ising Sampler (PRIS), a heuristic method tailored for parallel architectures allowing fast and efficient sampling from distributions of arbitrary Ising problems. Since the PRIS relies on vector-to-fixed matrix multiplications, we suggest the implementation of the PRIS in photonic parallel networks, which realize these operations at an unprecedented speed. The PRIS provides sample solutions to the ground state of Ising models, by converging in probability to their ...
Journal of Physics: Conference Series, 2018
Hafnia-filled, two dimensional (2D) tantalum (Ta) photonic crystals (PhCs) are promising emitters... more Hafnia-filled, two dimensional (2D) tantalum (Ta) photonic crystals (PhCs) are promising emitters for high performance thermophotovoltaic (TPV) systems because they enable, for a wide range of incidence angles, efficient spectral tailoring of thermal radiation. However, fabricating these PhCs to the required tolerances has proven to be a challenging task. In this paper, we use both focused ion beam (FIB) imaging and simulations to investigate the effects of fabrication imperfections on the emittance of a fabricated hafnia-filled PhC and to identify critical geometric features that drive the overall PhC performance. We demonstrate that, more so than uniform cavity filling, the key to the best filled PhC performance is the precise cavity period and radius values and thickness of the top hafnia layer.
Nature Communications, 2019
Extracting light from silicon is a longstanding challenge in modern engineering and physics. Whil... more Extracting light from silicon is a longstanding challenge in modern engineering and physics. While silicon has underpinned the past 70 years of electronics advancement, a facile tunable and efficient silicon-based light source remains elusive. Here, we experimentally demonstrate the generation of tunable radiation from a one-dimensional, all-silicon nanograting. Light is generated by the spontaneous emission from the interaction of these nanogratings with low-energy free electrons (2–20 keV) and is recorded in the wavelength range of 800–1600 nm, which includes the silicon transparency window. Tunable free-electron-based light generation from nanoscale silicon gratings with efficiencies approaching those from metallic gratings is demonstrated. We theoretically investigate the feasibility of a scalable, compact, all-silicon tunable light source comprised of a silicon Field Emitter Array integrated with a silicon nanograting that emits at telecommunication wavelengths. Our results rev...
Nature Physics, 2018
Free electron radiation such as Cerenkov [1], Smith-Purcell [2], and transition radiation [3, 4] ... more Free electron radiation such as Cerenkov [1], Smith-Purcell [2], and transition radiation [3, 4] can be greatly affected by structured optical environments, as has been demonstrated in a variety of polaritonic [5, 6], photoniccrystal [7], and metamaterial [8-10] systems. However, the amount of radiation that can ultimately be extracted from free electrons near an arbitrary material structure has remained elusive. Here we derive a fundamental upper limit to the spontaneous photon emission and energy loss of free electrons, regardless of geometry, which illuminates the effects of material properties and electron velocities. We obtain experimental evidence for our theory with quantitative measurements of Smith-Purcell radiation. Our framework allows us to make two predictions. One is a new regime of radiation operation-at subwavelength separations, slower (nonrelativistic) electrons can achieve stronger radiation than fast (relativistic) electrons. The second is a divergence of the emission probability in the limit of lossless materials. We further reveal that such divergences can be approached by coupling free electrons to photonic bound states in the continuum (BICs) [11-13]. Our findings suggest that compact and efficient free-electron radiation sources from microwaves to the soft X-ray regime may be achievable without requiring ultrahigh accelerating voltages. The Smith-Purcell effect epitomizes the potential of freeelectron radiation. Consider an electron at velocity β = v/c traversing a structure with periodicity a; it generates far-field radiation at wavelength λ and polar angle θ, dictated by [2]
Nature Physics, 2018
There is a century-old tenet [1, 2] that the inverse Doppler frequency shift of light [3-13] is i... more There is a century-old tenet [1, 2] that the inverse Doppler frequency shift of light [3-13] is impossible in homogeneous systems with a positive refractive index. Here we break this longheld tenet by predicting a new kind of Doppler effect of light inside the Cherenkov cone. Ever since the classic work of Ginzburg and Frank, it has been known that a superlight (i.e., superluminal) normal Doppler effect [14-18] appears inside the Cherenkov cone when the velocity of the source is larger than the phase velocity of light. By further developing their theory we discover that an inverse Doppler frequency shift will arise when >. We denote this as the superlight inverse Doppler effect. Moreover, we show that the superlight inverse Doppler effect can be spatially separated from the other Doppler effects by using highly squeezed polaritons (such as graphene plasmons), which may facilitate the experimental observation.
Volume 1: Micro/Nanofluidics and Lab-on-a-Chip; Nanofluids; Micro/Nanoscale Interfacial Transport Phenomena; Micro/Nanoscale Boiling and Condensation Heat Transfer; Micro/Nanoscale Thermal Radiation; Micro/Nanoscale Energy Devices and Systems, 2016
The increasing power demands of portable electronics and micro robotics has driven recent interes... more The increasing power demands of portable electronics and micro robotics has driven recent interest in millimeter-scale microgenerators. Many technologies (fuel cells, Stirling, thermoelectric, etc.) that potentially enable a portable hydrocarbon microgenerator are under active investigation. Hydrocarbon fuels have specific energies fifty times those of batteries, thus even a relatively inefficient generator can exceed the specific energy of batteries. We proposed, designed, and demonstrated a first-of-a-kind millimeter-scale thermophotovoltaic (TPV) system with a photonic crystal emitter. In a TPV system, combustion heats an emitter to incandescence and the resulting thermal radiation is converted to electricity by photovoltaic cells. Our approach uses a moderate temperature (1000–1200°C) metallic microburner coupled to a high emissivity, high selectivity photonic crystal selective emitter and low bandgap PV cells. This approach is predicted to be capable of up to 30% efficient fuel...
Journal of Vacuum Science & Technology A: Vacuum, Surfaces, and Films, 2015
Thick sputtered tantalum (Ta) coatings on polished Inconel were investigated as a potential repla... more Thick sputtered tantalum (Ta) coatings on polished Inconel were investigated as a potential replacement for bulk refractory metal substrates used for high-temperature emitters and absorbers in thermophotovoltaic energy conversion applications. In these applications, high-temperature stability and high reflectance of the surface in the infrared wavelength range are critical in order to sustain operational temperatures and reduce losses due to waste heat. The reflectance of the coatings (8 and 30 μm) was characterized with a conformal protective hafnia layer as-deposited and after one hour anneals at 700, 900, and 1100 °C. To further understand the high-temperature performance of the coatings, the microstructural evolution was investigated as a function of annealing temperature. X-ray diffraction was used to analyze the texture and residual stress in the coatings at four reflections (220, 310, 222, and 321), as-deposited and after anneal. No significant changes in roughness, reflectan...
Physical Review B, 1993
Two improvements for the solution of Maxwell s equations in periodic dielectric media are introdu... more Two improvements for the solution of Maxwell s equations in periodic dielectric media are introduced, abandoning the plane-wave cutoff and interpolating the dielectric function. These improvements permit the accurate study of previously inaccessible systems. Example calculations are discussed, employing a basis of-10 plane waves for which these two improvements reduce both the memory and central processing unit requirements by-10 .
Proceedings of SPIE - The International Society for Optical Engineering, 2006
Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontin... more Finite-difference time-domain (FDTD) methods suffer from reduced accuracy when modeling discontinuous dielectric materials, due to the inhererent discretization ("pixellization"). We show that accuracy can be significantly improved by using a sub-pixel smoothing of the dielectric function, but only if the smoothing scheme is properly designed. We develop such a scheme based on a simple criterion taken from perturbation theory, and compare it to other published FDTD smoothing methods. In addition to consistently achieving the smallest errors, our scheme is the only one that attains quadratic convergence with resolution for arbitrarily sloped interfaces. Finally, we discuss additional difficulties that arise for sharp dielectric corners.
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Papers by John Joannopoulos