Publications

2017
Michael Juhl, Carlos Mendoza, J. P. BALTHASAR MUELLER, Federico Capasso, and KRISTJAN LEOSSON. 2017. “Performance characteristics of 4-port in-plane and out-of-plane in-line metasurface polarimeters.” Optics Express, 25, 23, Pp. 28697-28709.Abstract
In-line polarimeters perform nonterminating measurements of the polarization state of light by sampling only a small part of the total light intensity. In-line polarimeters are used in applications such as polarization state generators and in optical communications. Current polarimeters use multiple optical components in sequence for polarization analysis and therefore often become bulky and expensive. Here, we experimentally demonstrate the operation of compact fiber-coupled polarimeters with high sampling rates, operating at telecom wavelengths, each polarimeter comprising a single ultra-thin metasurface aligned to four photodetectors. We compare two configurations of such metasurface polarimeters, with in-plane and out-of-plane detection, respectively. The metasurface polarimeters reported here show excellent agreement with commercial polarimeters and cover a bandwidth of at least 100 nm. (C) 2017 Optical Society of America
oe-25-23-28697.pdf
Patrice Genevet, Federico Capasso, Francesco Aieta, Mohammadreza Khorasaninejad, and Robert Devlin. 2017. “Recent advances in planar optics: from plasmonic to dielectric metasurfaces.” Optica, 4, 1, Pp. 139-152.Abstract
This article reviews recent progress leading to the realization of planar optical components made of a single layer of phase shifting nanostructures. After introducing the principles of planar optics and discussing earlier works on sub-wavelength diffractive optics, we introduce a classification of metasurfaces based on their different phase mechanisms and profiles and a comparison between plasmonic and dielectric metasurfaces. We place particular emphasis on the recent developments on electric and magnetic field control of light with dielectric nanostructures and highlight the physical mechanisms and designs required for efficient all-dielectric metasurfaces. Practical devices of general interest such as metalenses, beam deflectors, holograms, and polarizing interfaces are discussed, including high-performance metalenses at visible wavelengths. Successful strategies to achieve achromatic response at selected wavelengths and near unity transmission/reflection efficiency are discussed. Dielectric metasurfaces and dispersion management at interfaces open up technology opportunities for applications including wavefront control, lightweight imaging systems, displays, electronic consumer products, and conformable and wearable optics. (c) 2017 Optical Society of America.
optica-4-1-139.pdf
Henning Galinski, Gael Favraud, Hao Dong, Juan S. Totero Gongora, Gregory Favaro, Max Dobeli, Ralph Spolenak, Andrea Fratalocchi, and Federico Capasso. 2017. “Scalable, ultra-resistant structural colors based on network metamaterials.” LIGHT-SCIENCE & APPLICATIONS, 6.Abstract
Structural colors have drawn wide attention for their potential as a future printing technology for various applications, ranging from biomimetic tissues to adaptive camouflage materials. However, an efficient approach to realize robust colors with a scalable fabrication technique is still lacking, hampering the realization of practical applications with this platform. Here, we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless, ultra-thin dielectric coatings. By using theory and experiments, we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network, generating the formation of saturated structural colors that cover a wide portion of the spectrum. Ellipsometry measurements support the efficient observation of these colors, even at angles of 70 degrees. The network-like architecture of these nanomaterials allows for high mechanical resistance, which is quantified in a series of nano-scratch tests. With such remarkable properties, these metastructures represent a robust design technology for real-world, large-scale commercial applications.
lsa2016233.pdf
Dmitry Kazakov, Marco Piccardo, Yongrui Wang, Paul Chevalier, Tobias S. Mansuripur, Feng Xie, Chung-en Zah, Kevin Lascola, Alexey Belyanin, and Federico Capasso. 2017. “Self-starting harmonic frequency comb generation in a quantum cascade laser.” NATURE PHOTONICS, 11, 12, Pp. 789+.Abstract
Optical frequency combs(1,2) establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications(3). Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis(4,5) and for the generation of terahertz tones of high spectral purity in future wireless communication networks(6,7). Here, we demonstrate self–starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser(8,9). Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be-uniform to within 5 x 10(-12) parts of the central frequency. This new -harmonic comb state extends the range of applications of quantum cascade laser frequency combs(10-13).
s41566-017-0026-y.pdf
C. A. Wang, B. Schwarz, D. F. Siriani, M. K. Connors, L. J. Missaggia, D. R. Calawa, D. McNulty, A. Akey, M. C. Zheng, J. P. Donnelly, T. S. Mansuripur, and F. Capasso. 2017. “Sensitivity of heterointerfaces on emission wavelength of quantum cascade lasers.” JOURNAL OF CRYSTAL GROWTH, 464, Pp. 215-220.Abstract
The measured emission wavelengths of AlInAs/GaInAs/InP quantum cascade lasers (QCLs) grown by metal organic vapor phase epitaxy (MOVPE) have been reported to be similar to 0.5-1 mu m longer than the designed QCL wavelength. This work clarifies the origin of the red-shifted wavelength. It was found that AlInAs/GaInAs heterointerfaces are compositionally graded over similar to 2.5-4.5 nm, and indium accumulates at the AlInAs-toGainAs interface. Thus, the as-grown QCLs are far from the ideal abrupt interfaces used in QCL modeling. When graded layers are incorporated in QCL band structure and wavefunction calculations, the emission wavelengths are red shifted. Furthermore, we demonstrate that QCLs with graded interfaces can be designed without compromising performance and show greatly improved correlation between designed and measured emission wavelength. QCLs were designed for emission between 7.5 and 8.5 mu m. These structures were grown and wet-etched ridge devices were fabricated. The QCLs exhibit room temperature peak powers exceeding 900 mW and pulsed efficiencies of similar to 8 to 10%.
2017_wang_et_al.pdf
Robert Charles Devlin, Antonio Ambrosio, Daniel Wintz, Stefano Luigi Oscurato, Alexander Yutong Zhu, Mohammadreza Khorasaninejad, Jaewon Oh, Pasqualino Maddalena, and Federico Capasso. 2017. “Spin-to-orbital angular momentum conversion in dielectric metasurfaces (vol 25, pg 377, 2017).” OPTICS EXPRESS, 25, 4, Pp. 4239.Abstract
We would like to clarify our paper [Opt. Express 25, 377 (2017)] abstract sentence ``These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront.'' (C) 2017 Optical Society of America
oe-25-4-4239.pdf
Robert Charles Devlin, Antonio Ambrosio, Daniel Wintz, Stefano Luigi Oscurato, Alexander Yutong Zhu, Mohammadreza Khorasaninejad, Jaewon Oh, Pasqualino Maddalena, and Federico Capasso. 2017. “Spin-to-orbital angular momentum conversion in dielectric metasurfaces.” OPTICS EXPRESS, 25, 1, Pp. 377-393.Abstract
Vortex beams are characterized by a helical wavefront and a phase singularity point on the propagation axis that results in a doughnut-like intensity profile. These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront. Vortex beams have many applications in optics, such as optical trapping, quantum optics and microscopy. Although beams with such characteristics can be generated holographically, spin-to-orbital angular momentum conversion has attracted considerable interest as a tool to create vortex beams. In this process, the geometrical phase is exploited to create helical beams whose handedness is determined by the circular polarization (left/right) of the incident light, that is by its spin. Here we demonstrate high-efficiency Spin-to- Orbital angular momentum-Converters (SOCs) at visible wavelengths based on dielectric metasurfaces. With these SOCs we generate vortex beams with high and fractional topological charge and show for the first time the simultaneous generation of collinear helical beams with different and arbitrary orbital angular momentum. This versatile method of creating vortex beams, which circumvents the limitations of liquid crystal SOCs and adds new functionalities, should significantly expand the applications of these beams. (C) 2017 Optical Society of America
oe-25-1-377.pdf
Alexander Y. Zhu, Wei-Ting Chen, Mohammadreza Khorasaninejad, Jaewon Oh, Aun Zaidi, Ishan Mishra, Robert C. Devlin, and Federico Capasso. 2017. “Ultra-compact visible chiral spectrometer with meta-lenses.” APL PHOTONICS, 2, 3.Abstract
Conventional compact spectrometers have a fixed spectral resolution and cannot resolve the polarization properties of light without additional optical elements, while their larger counterparts are bulky and costly. Here, we demonstrate multiple offaxis meta-lenses in the visible integrated on a single planar substrate. They possess both focusing and strongly dispersive properties and are designed to provide different spectral resolutions as well as working wavelength ranges on the same chip. We realize a compact spectrometer using only these meta-lenses and a CMOS camera and achieve detector-limited spectral resolutions as small as 0.3 nm and a total working wavelength range exceeding 170 nm for a beam propagation length of only a few cm. In addition, this spectrometer has the capability to resolve different helicities of light in a single measurement. This chip-camera setup represents the most compact configuration so far achieved for a spectrometer with similar performance and functionality, and its compatibility with large-scale fabrication processes makes it broadly applicable. (C) 2017 Author(s).
1.4974259.pdf
Mohammadreza Khorasaninejad, Wei Ting Chen, Alexander Y. Zhu, Jaewon Oh, Robert C. Devlin, Charles Roques-Carmes, Ishan Mishra, and Federico Capasso. 2017. “Visible Wavelength Planar Metalenses Based on Titanium Dioxide.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 23, 3.Abstract
We present recent advances in metasurface-based photonics, which enables the realization of high performance planar lenses (metalenses) in the visible spectrum. They are enabled by a technique based on atomic layer deposition of titanium dioxide allowing for the fabrication of nanostructures with high fidelity. First, we demonstrate highly efficient metalenses with numerical aperture NA = 0.8 using the Pancharatnam-Berry phase approach. These metalenses can focus light into a diffraction-limited spot. They have efficiencies as high as 86% and provide high imaging resolution. Furthermore, by judicious design of the phase-shifting elements, we achieve a multispectral chiral metalens realized with a single metasurface layer. This chiral metalens can resolve both the chiral and spectral information of an object without the requirement of any additional optical components. Finally, we discuss the experimental realization of polarization-insensitive metalenses with NAs as high as 0.85. They are able to focus incident light to a spot as small as similar to 0.64 lambda with efficiencies up to 60%. Due to its straightforward and CMOS-compatible fabrication, this platform is promising for a wide range of applications ranging from camera modules, displays, laser-based imaging, microscopy, and spectroscopy to laser fabrication and lithography.
07588110.pdf
Benedikt Schwarz, Christine A. Wang, Leo Missaggia, Tobias S. Mansuripur, Paul Chevalier, Michael K. Connors, Daniel McNulty, Jeffrey Cederberg, Gottfried Strasser, and Federico Capasso. 2017. “Watt-Level Continuous-Wave Emission from a Bifunctional Quantum Cascade Laser/Detector.” ACS PHOTONICS, 4, 5, Pp. 1225-1231.Abstract
Bifunctional active regions, capable of light generation and detection at the same wavelength, allow a straightforward realization of the integrated mid-infrared photonics for sensing applications. Here, we present a high performance bifunctional device for 8 pm capable of 1 W single facet continuous wave emission at 15 degrees C. Apart from the general performance benefits, this enables sensing techniques which rely on continuous wave operation, for example, heterodyne detection, to be realized within a monolithic platform and demonstrates that bifunctional operation can be realized at longer wavelength, where wavelength matching becomes increasingly difficult and that the price to be paid in terms of performance is negligible. In laser operation, the device has the same or higher efficiency compared to the best lattice-matched QCLs without same wavelength detection capability, which is only 30% below the record achieved with strained material at this wavelength.
acsphotonics.7b00133.pdf
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.
2016_rensberg_et_al.pdf
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.
2016_khorasaninejad_et_al_4.pdf
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.
2016_devlin_et_al.pdf
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.
2016_nylund_et_al.pdf
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
2016_byrnes_et_al_1.pdf
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
2016_zhang_et_al.pdf
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
2016_byrnes_et_al.pdf
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.
2016_gansch_et_al.pdf
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.
2016_khorasaninejad_et_al_2.pdf

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