Publications by Year: 2016

Jura Rensberg, Shuyan Zhang, You Zhou, Alexander S. McLeod, Christian Schwarz, Michael Goldflam, Mengkun Liu, Jochen Kerbusch, Ronny Nawrodt, Shriram Ramanathan, D. N. Basov, Federico Capasso, Carsten Ronning, and Mikhail A. Kats. 2016. “Active Optical Metasurfaces Based on Defect-Engineered Phase-Transition Materials.” NANO LETTERS, 16, 2, Pp. 1050-1055.Abstract
Active, widely tunable optical materials have enabled rapid advances in photonics and optoelectronics, especially in the emerging field of meta-devices. Here, we demonstrate that spatially selective defect engineering on the nanometer scale can transform phase transition materials into optical metasurfaces. Using ion irradiation through nanometer-scale masks, we selectively defect-engineered the insulator-metal transition of vanadium dioxide, a prototypical correlated phase-transition material whose optical properties change dramatically depending on its state. Using this robust technique, we demonstrated several optical metasurfaces, including tunable absorbers with artificially induced phase coexistence and tunable polarizers based on thermally triggered dichroism. Spatially selective nanoscale defect engineering represents a new paradigm for active photonic structures and devices.
Mohammadreza Khorasaninejad, Antonio Ambrosio, Pritpal Kanhaiya, and Federico Capasso. 2016. “Broadband and chiral binary dielectric meta-holograms.” SCIENCE ADVANCES, 2, 5.Abstract
Subwavelength structured surfaces, known as meta-surfaces, hold promise for future compact and optically thin devices with versatile functionalities. By revisiting the concept of detour phase, we demonstrate high-efficiency holograms with broadband and chiral imaging functionalities. In our devices, the apertures of binary holograms are replaced by subwavelength structured microgratings. We achieve broadband operation from the visible to the near infrared and efficiency as high as 75% in the 1.0 to 1.4 mm range by compensating for the inherent dispersion of the detour phase with that of the subwavelength structure. In addition, we demonstrate chiral holograms that project different images depending on the handedness of the reference beam by incorporating a geometric phase. Our devices' compactness, lightness, and ability to produce images even at large angles have significant potential for important emerging applications such as wearable optics.
Robert C. Devlin, Mohammadreza Khorasaninejad, Wei Ting Chen, Jaewon Oh, and Federico Capasso. 2016. “Broadband high-efficiency dielectric metasurfaces for the visible spectrum.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 113, 38, Pp. 10473-10478.Abstract
Metasurfaces are planar optical elements that hold promise for overcoming the limitations of refractive and conventional diffractive optics. Original dielectric metasurfaces are limited to transparency windows at infrared wavelengths because of significant optical absorption and loss at visible wavelengths. Thus, it is critical that new materials and nanofabrication techniques be developed to extend dielectric metasurfaces across the visible spectrum and to enable applications such as high numerical aperture lenses, color holograms, and wearable optics. Here, we demonstrate high performance dielectric metasurfaces in the form of holograms for red, green, and blue wavelengths with record absolute efficiency (> 78%). We use atomic layer deposition of amorphous titanium dioxide with surface roughness less than 1 nm and negligible optical loss. We use a process for fabricating dielectric metasurfaces that allows us to produce anisotropic, subwavelength-spaced dielectric nanostructures with shape birefringence. This process is capable of realizing any high-efficiency metasurface optical element, e.g., metalenses and axicons.
Gustav Nylund, Kristian Storm, Sebastian Lehmann, Federico Capasso, and Lars Samuelson. 2016. “Designed Quasi-1D Potential Structures Realized in Compositionally Graded InAs1-xPx Nanowires.” NANO LETTERS, 16, 2, Pp. 1017-1021.Abstract
III-V semiconductor heterostructures are important components of many solid-state optoelectronic devices, but the ability to control and tune the electrical and optical properties of these structures in conventional device geometries is fundamentally limited by the bulk dimensionality and the inability to accommodate lattice-mismatched material combinations. Here we demonstrate how semiconductor nanowires may enable the creation of arbitrarily shaped one-dimensional potential structures for new types of designed device functionality. We describe the controlled growth of stepwise compositionally graded InAs1-xPx heterostmctures defined along the axes of InAs nanowires, and we show that nanowires with sawtooth-shaped composition profiles behave as near-ideal unipolar diodes with ratchet-like rectification of the electron transport through the nanowires, in excellent agreement with simulations. This new type of designed quasi-1D potential structure represents a significant advance in band gap engineering and may enable fundamental studies of low-dimensional hot-carrier dynamics, in addition to constituting a platform for implementing novel electronic and optoelectronic device concepts.
STEVEN J. BYRNES, Alan Lenef, Francesco Aieta, and Federico Capasso. 2016. “Designing large, high-efficiency, high-numerical-aperture, transmissive meta-lenses for visible light.” OPTICS EXPRESS, 24, 5, Pp. 5110-5124.Abstract
A metasurface lens (meta-lens) bends light using nanostructures on a flat surface. Macroscopic meta-lenses (mm- to cm-scale diameter) have been quite difficult to simulate and optimize, due to the large area, the lack of periodicity, and the billions of adjustable parameters. We describe a method for designing a large-area meta-lens that allows not only prediction of the efficiency and far-field, but also optimization of the shape and position of each individual nanostructure, with a computational cost that is almost independent of the lens size. As examples, we design three large NA = 0.94 meta-lenses: One with 79% predicted efficiency for yellow light, one with dichroic properties, and one broadband lens. All have a minimum feature size of 100nm. (C) 2016 Optical Society of America
Shuyan Zhang, Myoung-Hwan Kim, Francesco Aieta, Alan She, Tobias Mansuripur, Ilan Gabay, Mohammadreza Khorasaninejad, David Rousso, Xiaojun Wang, Mariano Troccoli, Nanfang Yu, and Federico Capasso. 2016. “High efficiency near diffraction-limited mid-infrared flat lenses based on metasurface reflectarrays.” OPTICS EXPRESS, 24, 16, Pp. 18024-18034.Abstract
We report the first demonstration of a mid-IR reflection-based flat lens with high efficiency and near diffraction-limited focusing. Focusing efficiency as high as 80%, in good agreement with simulations (83%), has been achieved at 45 degrees incidence angle at lambda = 4.6 mu m. The off-axis geometry considerably simplifies the optical arrangement compared to the common geometry of normal incidence in reflection mode which requires beam splitters. Simulations show that the effects of incidence angle are small compared to parabolic mirrors with the same NA. The use of single-step photolithography allows large scale fabrication. Such a device is important in the development of compact telescopes, microscopes, and spectroscopic designs. (C) 2016 Optical Society of America
STEVEN J. BYRNES, Mohammadreza Khorasaninejad, and Federico Capasso. 2016. “High-quality-factor planar optical cavities with laterally stopped, slowed, or reversed light.” OPTICS EXPRESS, 24, 16, Pp. 18399-18407.Abstract
In a planar optical cavity, the resonance frequencies increase as a function of in-plane wavevector according to a standard textbook formula. This has well-known consequences in many different areas of optics, from the shifts of etalon peaks at non-normal angles, to the properties of transverse modes in laser diodes, to the effective mass of microcavity photons, and so on. However, this standard formula is valid only when the reflection phase of each cavity mirror is approximately independent of angle. There is a certain type of mirror-a subwavelength dielectric grating near a guided mode resonance-with not only a strongly angle-dependent reflection phase, but also very high reflectance and low losses. Simulations show that by using such mirrors, high-quality-factor planar cavities can be designed that break all these textbook rules, leading to resonant modes that are slow, stopped or even backward-propagating in the in-plane direction. In particular, we demonstrate experimentally high-Q planar cavities whose resonance frequency is independent of in-plane wavevector-i.e., the resonant modes have zero in-plane group velocity, for one polarization but both in-plane directions. We discuss potential applications in various fields including lasers, quantum optics, and exciton-polariton condensation. (C) 2016 Optical Society of America
Roman Gansch, Stefan Kalchmair, Patrice Genevet, Tobias Zederbauer, Hermann Detz, Aaron M. Andrews, Werner Schrenk, Federico Capasso, Marko Loncar, and Gottfried Strasser. 2016. “Measurement of bound states in the continuum by a detector embedded in a photonic crystal.” LIGHT-SCIENCE & APPLICATIONS, 5.Abstract
We directly measure optical bound states in the continuum (BICs) by embedding a photodetector into a photonic crystal slab. The BICs observed in our experiment are the result of accidental phase matching between incident, reflected and in-plane waves at seemingly random wave vectors in the photonic band structure. Our measurements were confirmed through a rigorously coupled-wave analysis simulation in conjunction with temporal coupled mode theory. Polarization mixing between photonic crystal slab modes was observed and described using a plane wave expansion simulation. The ability to probe the field intensity inside the photonic crystal and thereby to directly measure BICs represents a milestone in the development of integrated opto-electronic devices based on BICs.
Mohammadreza Khorasaninejad, Wei Ting Chen, Robert C. Devlin, Jaewon Oh, Alexander Y. Zhu, and Federico Capasso. 2016. “Metalenses at visible wavelengths: Diffraction-limited focusing and subwavelength resolution imaging.” SCIENCE, 352, 6290, Pp. 1190-1194.Abstract
Subwavelength resolution imaging requires high numerical aperture (NA) lenses, which are bulky and expensive. Metasurfaces allow the miniaturization of conventional refractive optics into planar structures. We show that high-aspect-ratio titanium dioxide metasurfaces can be fabricated and designed as metalenses with NA = 0.8. Diffraction-limited focusing is demonstrated at wavelengths of 405, 532, and 660 nm with corresponding efficiencies of 86, 73, and 66%. The metalenses can resolve nanoscale features separated by subwavelength distances and provide magnification as high as 170x, with image qualities comparable to a state-of-the-art commercial objective. Our results firmly establish that metalenses can have widespread applications in laser-based microscopy, imaging, and spectroscopy.
M. Khorasaninejad, W. T. Chen, A. Y. Zhu, J. Oh, R. C. Devlin, D. Rousso, and F. Capasso. 2016. “Multispectral Chiral Imaging with a Metalens.” NANO LETTERS, 16, 7, Pp. 4595-4600.Abstract
The vast majority of biologically active compounds, ranging from amino acids to essential nutrients such as glucose, possess intrinsic handedness. This in turn gives rise to chiral optical properties that provide a basis for detecting and quantifying enantio-specific concentrations of these molecules. However, traditional chiroptical spectroscopy and imaging techniques require cascading of multiple optical components in sophisticated setups. Here, we present a planar lens with an engineered dispersive response, which simultaneously forms two images with opposite helicity of an object within the same field-of-view. In this way, chiroptical properties can be probed across the visible spectrum using only the lens and a camera without the addition of polarizers or dispersive optical devices. We map the circular dichroism of the exoskeleton of a chiral beetle, Chrysina gloriosa, which is known to exhibit high reflectivity of left-circularly polarized light, with high spatial resolution limited by the numerical aperture of the planar lens. Our results demonstrate the potential of metasurfaces in realizing a compact and multifunctional device with unprecedented imaging capabilities.
Mikhail A. Kats and Federico Capasso. 2016. “Optical absorbers based on strong interference in ultra-thin films.” LASER & PHOTONICS REVIEWS, 10, 5, Pp. 735-749.Abstract
Optical absorbers find uses in a wide array of applications across the electromagnetic spectrum, including photovoltaic and photochemical cells, photodetectors, optical filters, stealth technology, and thermal light sources. Recent efforts have sought to reduce the footprint of optical absorbers, conventionally based on graded structures or Fabry-Perot-type cavities, by using emerging concepts in plasmonics, metamaterials, and metasurfaces. Unfortunately, these new absorber designs require patterning on subwavelength length scales, and are therefore impractical for many large-scale optical and optoelectronic devices. In this article, we summarize recent progress in the development of optical absorbers based on lossy films with thicknesses significantly smaller than the incident optical wavelength. These structures have a small footprint and require no nanoscale patterning. We outline the theoretical foundation of these absorbers based on ``ultra-thin-film interference'', including the concepts of loss-induced phase shifts and critical coupling, and then review several applications, including ultra-thin color coatings, decorative photovoltaics, high-efficiency photochemical cells, and infrared scene generators.
Alan She and Federico Capasso. 2016. “Parallel Polarization State Generation.” SCIENTIFIC REPORTS, 6.Abstract
The control of polarization, an essential property of light, is of wide scientific and technological interest. The general problem of generating arbitrary time-varying states of polarization (SOP) has always been mathematically formulated by a series of linear transformations, i.e. a product of matrices, imposing a serial architecture. Here we show a parallel architecture described by a sum of matrices. The theory is experimentally demonstrated by modulating spatially-separated polarization components of a laser using a digital micromirror device that are subsequently beam combined. This method greatly expands the parameter space for engineering devices that control polarization. Consequently, performance characteristics, such as speed, stability, and spectral range, are entirely dictated by the technologies of optical intensity modulation, including absorption, reflection, emission, and scattering. This opens up important prospects for polarization state generation (PSG) with unique performance characteristics with applications in spectroscopic ellipsometry, spectropolarimetry, communications, imaging, and security.
M. Khorasaninejad, A. Y. Zhuit, C. Roques-Carmes, W. T. Chen, J. Oh, I. Mishra, R. C. Devlin, and F. Capasso. 2016. “Polarization-Insensitive Metalenses at Visible Wavelengths.” NANO LETTERS, 16, 11, Pp. 7229-7234.Abstract
In this Letter, we demonstrate highly efficient, polarization-insensitive planar lenses (metalenses) at red, green, and blue wavelengths (lambda = 660, 532, and 405 nm). Metalenses with numerical apertures (NA) of 0.85 and 0.6 and corresponding efficiencies as high as 60% and 90% are achieved. These metalenses are less than 600 nm-thick and can focus incident light down to diffraction-limited spots as small as similar to 0.64 lambda and provide high-resolution imaging. In addition, the focal spots are very symmetric with high Strehl ratios. The single step lithography and compatibility with large-scale fabrication processes make metalenses highly promising for widespread applications in imaging and spectroscopy.
D. F. Siriani, C. A. Wang, J. P. Donnelly, M. K. Connors, L. J. Missaggia, D. R. Calawa, D. McNulty, M. C. Zheng, T. S. Mansuripur, and F. Capasso. 2016. “Sensitivity of quantum cascade laser performance to thickness and doping variations.” JOURNAL OF CRYSTAL GROWTH, 452, Pp. 263-267.Abstract
We report on a study of the effects of intentional thickness and doping variations on QCL performance. The measured QCL data had very similar trends to those predicted by an in-house QCL model. It was found that absolute changes to the QCL period had a very small effect on emission wavelength (wave-length/period change < 10 nm/angstrom), whereas the complementary thickness changes between the wells and barriers had a large effect (wavelength/thickness change=550 nm/angstrom). The threshold voltage also changed with these variations and generally agreed well with the model. We show through modeling and experiments that intentional structure variations can have largely different magnitudes of effect on QCL performance. (C) 2015 Elsevier B.V. All rights reserved.
Tobias S. Mansuripur, Camille Vernet, Paul Chevalier, Guillaume Aoust, Benedikt Schwarz, Feng Xie, Catherine Caneau, Kevin Lascola, Chung-en Zah, David P. Caffey, Timothy Day, Leo J. Missaggia, Michael K. Connors, Christine A. Wang, Alexey Belyanin, and Federico Capasso. 2016. “Single-mode instability in standing-wave lasers: The quantum cascade laser as a self-pumped parametric oscillator.” PHYSICAL REVIEW A, 94, 6.Abstract
We report the observation of a clear single-mode instability threshold in continuous-wave Fabry-Perot quantum cascade lasers (QCLs). The instability is characterized by the appearance of sidebands separated by tens of free spectral ranges (FSR) from the first lasing mode, at a pump current not much higher than the lasing threshold. As the current is increased, higher-order sidebands appear that preserve the initial spacing, and the spectra are suggestive of harmonically phase-locked waveforms. We present a theory of the instability that applies to all homogeneously broadened standing-wave lasers. The low instability threshold and the large sideband spacing can be explained by the combination of an unclamped, incoherent Lorentzian gain due to the population grating, and a coherent parametric gain caused by temporal population pulsations that changes the spectral gain line shape. The parametric term suppresses the gain of sidebands whose separation is much smaller than the reciprocal gain recovery time, while enhancing the gain of more distant sidebands. The large gain recovery frequency of the QCL compared to the FSR is essential to observe this parametric effect, which is responsible for the multiple-FSR sideband separation. We predict that by tuning the strength of the incoherent gain contribution, for example by engineering the modal overlap factors and the carrier diffusion, both amplitude-modulated (AM) or frequency-modulated emission can be achieved from QCLs. We provide initial evidence of an AM waveform emitted by a QCL with highly asymmetric facet reflectivities, thereby opening a promising route to ultrashort pulse generation in the mid-infrared. Together, the experiments and theory clarify a deep connection between parametric oscillation in optically pumped microresonators and the single-mode instability of lasers, tying together literature from the last 60 years.
Lulu Liu, Simon Kheifets, Vincent Ginis, and Federico Capasso. 2016. “Subfemtonewton Force Spectroscopy at the Thermal Limit in Liquids.” PHYSICAL REVIEW LETTERS, 116, 22.Abstract
We demonstrate thermally limited force spectroscopy using a probe formed by a dielectric microsphere optically trapped in water near a dielectric surface. We achieve force resolution below 1 fN in 100 s, corresponding to a 2 angstrom rms displacement of the probe. Our measurement combines a calibrated evanescent wave particle tracking technique and a lock-in detection method. We demonstrate the accuracy of our method by measurement of the height-dependent force exerted on the probe by an evanescent wave, the results of which are in agreement with Mie theory calculations.
M. Khorasaninejad, W. T. Chen, J. Oh, and F. Capasso. 2016. “Super-Dispersive Off-Axis Meta-Lenses for Compact High Resolution Spectroscopy.” NANO LETTERS, 16, 6, Pp. 3732-3737.Abstract
Metasurfaces have opened a new frontier in the miniaturization of optical technology by allowing exceptional control over the wavefront. Here, we demonstrate off-axis meta-lenses that simultaneously focus and disperse light of different wavelengths with unprecedented spectral resolution. They are designed based on the geometric phase via rotated silicon nanofins and can focus light at angles as large as 80. Due to the large angle focusing, these meta-lenses have superdispersive characteristics (0.27 nin/mrad) that make them capable of resolving wavelength differences as small as 200 pm in the telecom region. In addition; by stitching several meta-lenses together, we maintain a high spectral resolution for a wider wavelength range. The meta-lenses have measured efficiencies as high as 90% in the wavelength range of 1.1 to 1.6 mu m. The planar and compact configuration together with high spectral resolution of these meta-lenses has significant potential for emerging portable/wearable optics technology.
J. P. BALTHASAR MUELLER, KRISTJAN LEOSSON, and Federico Capasso. 2016. “Ultracompact metasurface in-line polarimeter.” OPTICA, 3, 1, Pp. 42-47.Abstract
In-line polarimeters perform nondestructive polarization measurements of optical signals, and play a critical role in monitoring and controlling the polarization environment in, for example, optical networks. While current in-line polarimeters are constructed with multiple optical components, either fabricated into an optical fiber or using free-space optics, we present here a novel architecture conducive to monolithic on-chip integration. This enables the scalable fabrication of high-performance polarization sensors with exceptional stability, compactness, and speed. The method relies on the detection of the highly polarization-dependent scattered field of a subwavelength antenna array known as a metasurface, and is shown here to provide polarization state measurements matching those of a state-of-the-art commercial polarimeter. (C) 2016 Optical Society of America