Marco Piccardo, Paul Chevalier, Benedikt Schwarz, Dmitry Kazakov, Yongrui Wang, Alexey Belyanin, and Federico Capasso. 2019. “Frequency-Modulated Combs Obey a Variational Principle.” PHYSICAL REVIEW LETTERS, 122, 25.Abstract
Laser dynamics encompasses universal phenomena that can be encountered in many areas of physics, such as bifurcation and chaos, mode competition, resonant nonlinearities, and synchronization-or locking- of oscillators. When a locking process occurs in a multimode laser, an optical frequency comb is produced, which is an optical spectrum consisting of equidistant modes with a fixed phase relationship. Describing the formation of self-starting frequency combs in terms of fundamental laser equations governing the field inside the cavity does not allow one, in general, to grasp how the laser synchronizes its modes. Our finding is that, in a particular class of lasers where the output is frequency modulated with small or negligible intensity modulation, a greatly simplified description of self-locking exists. We show that in quantum cascade lasers-solid-state representatives of these lasers characterized by an ultrashort carrier relaxation time-the frequency comb formation obeys a simple variational principle, which was postulated over 50 years ago and relies on the maximization of the laser output power. The conditions for the breakdown of this principle are also experimentally identified, shedding light on the behavior of many different types of lasers, such as dye, diode, and other cascade lasers. This discovery reveals that the formation of frequency-modulated combs is an elegant example of an optimization problem solved by a physical system.
Manuel Decker, Wei Ting Chen, Thomas Nobis, Alexander Y. Zhu, Mohammadreza Khorasaninejad, Zameer Bharwani, Federico Capasso, and Joerg Petschulat. 2019. “Imaging Performance of Polarization-Insensitive Metalenses.” ACS PHOTONICS, 6, 6, Pp. 1493-1499.Abstract
Metasurfaces have recently emerged as a promising technology to realize flat optical components with customized functionalities. In particular, their application to lenses in various imaging systems is of significant interest. However, a systematic and complete study of the focusing and imaging behavior of metalenses has not yet been conducted. In this work we analyze not only the on-axis focusing performance, but also the field-dependent wavefront aberrations via a phase-retrieval optimization method. We find that, particularly for high-NA metalenses, the field-dependent geometrical aberrations like coma are dominant at the design wavelength, while for longer and shorter operation wave- lengths, the effective numerical aperture is decreased and mainly spherical aberrations are dominant. Additionally, we investigate the spectral and angular bandwidth of a polarization-insensitive metalens by analyzing the metalens efficiencies as a function of numerical aperture, field angle, and wavelength. We then compare the metalens performance to its refractive and diffractive counterparts and show how the respective metalens properties affect the imaging performance. For this purpose, we perform an imaging simulation for these three cases based on their field- and wavelength-dependent absolute deflection efficiencies and analyze the imaging properties of an extended test object. Our calculations show that metalenses can outperform diffractive lenses in terms of their angle-dependent efficiency for large deflection angles.
Marco Piccardo, Dmitry Kazakov, Benedikt Schwarz, Paul Chevalier, Arman Amirzhan, Johannes Hillbrand, Sultan Z. AlMutairi, Yongrui Wang, Feng Xie, Kevin Lascola, Steffen Becker, Lars Hildebrandt, Robert Weih, Alexey Belyanin, and Federico Capasso. 2019. “Light and Microwaves in Laser Frequency Combs: An Interplay of Spatiotemporal Phenomena.” IEEE JOURNAL OF SELECTED TOPICS IN QUANTUM ELECTRONICS, 25, 6.Abstract
Interference of laser beams in a suitable medium creates dynamic optical gratings, which can serve for a wide variety of applications, ranging from real-time holography to ultrasound generation. Typically, the interference occurs in a sample material that is separated from the laser sources. Here, we explore a new aspect of laser-induced dynamic gratings: microwave generation occurring inside the cavity of semiconductor laser frequency comb generators, such as quantum cascade lasers and interband cascade lasers. The interplay between laser light and microwaves in these devices reveals intracavity spatiotemporal phenomena that are of great importance for the understanding of their physics and for their operation to be efficient. Grating effects related to laser locking dynamics, structured cavities, and microwave propagation are demonstrated. Applications in science and technology based on these phenomena, including the realization of novel hybrid electronic-photonic devices, will also be presented.
Noah A. Rubin, Gabriele D'Aversa, Paul Chevalier, Zhujun Shi, Wei Ting Chen, and Federico Capasso. 2019. “Matrix Fourier optics enables a compact full-Stokes polarization camera.” SCIENCE, 365, 6448, Pp. 43+.Abstract
Recent developments have enabled the practical realization of optical elements in which the polarization of light may vary spatially. We present an extension of Fourier optics-matrix Fourier optics-for understanding these devices and apply it to the design and realization of metasurface gratings implementing arbitrary, parallel polarization analysis. We show how these gratings enable a compact, full-Stokes polarization camera without standard polarization optics. Our single-shot polarization camera requires no moving parts, specially patterned pixels, or conventional polarization optics and may enable the widespread adoption of polarization imaging in machine vision, remote sensing, and other areas.
Rajath Sawant, Purva Bhumkar, Alexander Y. Zhu, Peinan Ni, Federico Capasso, and Patrice Genevet. 2019. “Mitigating Chromatic Dispersion with Hybrid Optical Metasurfaces.” ADVANCED MATERIALS, 31, 3.Abstract
Metasurfaces control various properties of light via scattering across a large number of subwavelength-spaced nanostructures. Although metasurfaces appear to be ideal photonic platforms for realizing and designing miniaturized devices, their chromatic aberrations have hindered the large-scale deployment of this technology in numerous applications. Wavelength-dependent diffraction and resonant scattering effects usually limit their working operation wavelengths. In refractive optics, chromatic dispersion is a significant problem and is generally treated by cascading multiple lenses into achromatic doublets, triplets, and so on. Recently, broadband achromatic metalenses in the visible have been proposed to circumvent chromatic aberration but their throughput efficiency is still limited. Here, the dispersion of refractive components is corrected by leveraging the inherent dispersion of metasurfaces. Hybrid refractive-metasurface devices, with nondispersive refraction in the visible, are experimentally demonstrated. The dispersion of this hybrid component, characterized by using a Fourier plane imaging microscopy setup, is essentially achromatic over about 150 nm in the visible. Broadband focusing with composite plano-convex metasurface lenses is also proposed. These devices could find applications in numerous consumer optics, augmented reality components, and all applications including imaging for which monochromatic performance is not sufficient.
Kundan Chaudhary, Michele Tamagnone, Xinghui Yin, Christina M. Spagele, Stefano L. Oscurato, Jiahan Li, Christoph Persch, Ruo Ping Li, Noah A. Rubin, Luis A. Jauregui, Kenji Watanabe, Takashi Taniguchi, Philip Kim, Matthias Wuttig, James H. Edgar, Antonio Ambrosio, and Federico Capasso. 2019. “Polariton nanophotonics using phase-change materials.” NATURE COMMUNICATIONS, 10.Abstract
Polaritons formed by the coupling of light and material excitations enable light-matter interactions at the nanoscale beyond what is currently possible with conventional optics. However, novel techniques are required to control the propagation of polaritons at the nanoscale and to implement the first practical devices. Here we report the experimental realization of polariton refractive and meta-optics in the mid-infrared by exploiting the properties of low-loss phonon polaritons in isotopically pure hexagonal boron nitride interacting with the surrounding dielectric environment comprising the low-loss phase change material Ge3Sb2Te6. We demonstrate rewritable waveguides, refractive optical elements such as lenses, prisms, and metalenses, which allow for polariton wavefront engineering and sub-wavelength focusing. This method will enable the realization of programmable miniaturized integrated optoelectronic devices and on-demand biosensors based on high quality phonon resonators.
Marco Piccardo, Michele Tamagnone, Benedikt Schwarz, Paul Chevalier, Noah A. Rubin, Yongrui Wang, Christine A. Wang, Michael K. Connors, Daniel McNulty, Alexey Belyanin, and Federico Capasso. 2019. “Radio frequency transmitter based on a laser frequency comb.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 116, 19, Pp. 9181-9185.Abstract
Since the days of Hertz, radio transmitters have evolved from rudimentary circuits emitting around 50 MHz to modern ubiquitous Wi-Fi devices operating at gigahertz radio bands. As wireless data traffic continues to increase, there is a need for new communication technologies capable of high-frequency operation for high-speed data transfer. Here, we give a proof of concept of a compact radio frequency transmitter based on a semiconductor laser frequency comb. In this laser, the beating among the coherent modes oscillating inside the cavity generates a radio frequency current, which couples to the electrodes of the device. We show that redesigning the top contact of the laser allows one to exploit the internal oscillatory current to drive a dipole antenna, which radiates into free space. In addition, direct modulation of the laser current permits encoding a signal in the radiated radio frequency carrier. Working in the opposite direction, the antenna can receive an external radio frequency signal, couple it to the active region, and injection lock the laser. These results pave the way for applications and functionality in optical frequency combs, such as wireless radio communication and wireless synchronization to a reference source.
Vincent Ginis, Lulu Liu, Alan She, and Federico Capasso. 2019. “Using the Belinfante momentum to retrieve the polarization state of light inside waveguides.” SCIENTIFIC REPORTS, 9.Abstract
Current day high speed optical communication systems employ photonic circuits using platforms such as silicon photonics. In these systems, the polarization state of light drifts due to effects such as polarization mode dispersion and nonlinear phenomena generated by photonic circuit building blocks. As the complexity, the number, and the variety of these building blocks grows, the demand increases for an in-situ polarization determination strategy. Here, we show that the transfer of the Belinfante momentum to particles in the evanescent field of waveguides depends in a non-trivial way on the polarization state of light within that waveguide. Surprisingly, we find that the maxima and minima of the lateral force are not produced with circularly polarized light, corresponding to the north and south poles of the Poincare sphere. Instead, the maxima are shifted along the great circle of the sphere due to the phase differences between the scattered TE and TM components of light. This effect allows for an unambiguous reconstruction of the local polarization state of light inside a waveguide. Importantly, this technique depends on interaction with only the evanescent tails of the fields, allowing for a minimally invasive method to probe the polarization within a photonic chip.
Yao-Wei Huang, Noah A. Rubin, Antonio Ambrosio, Zhujun Shi, Robert C. Devlin, Cheng-Wei Qiu, and Federico Capasso. 2019. “Versatile total angular momentum generation using cascaded J-plates.” OPTICS EXPRESS, 27, 5, Pp. 7469-7484.Abstract
Optical elements coupling the spin and orbital angular momentum (SAM/OAM) of light have found a range of applications in classical and quantum optics. The J-plate, with J referring to the photon's total angular momentum (TAM), is a metasurface device that imparts two arbitrary OAM states on an arbitrary orthogonal basis of spin states. We demonstrate that when these J-plates are cascaded in series, they can generate several single quantum number beams and versatile superpositions thereof. Moreover, in contrast to previous spin-orbit-converters, the output polarization states of cascaded J-plates are not constrained to be the conjugate of the input states. Cascaded J-plates are also demonstrated to produce vector vortex beams and complex structured light, providing new ways to control TAM states of light. (C) 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Paul Chevalier, Arman Amirzhan, Fan Wang, Marco Piccardo, Steven G. Johnson, Federico Capasso, and Henry O. Everitt. 2019. “Widely tunable compact terahertz gas lasers.” SCIENCE, 366, 6467, SI, Pp. 856+. Publisher's VersionAbstract
The terahertz region of the electromagnetic spectrum has been the least utilized owing to inadequacies of available sources. We introduce a compact, widely frequency-tunable, extremely bright source of terahertz radiation: a gas-phase molecular laser based on rotational population inversions optically pumped by a quantum cascade laser. By identifying the essential parameters that determine the suitability of a molecule for a terahertz laser, almost any rotational transition of almost any molecular gas can be made to lase. Nitrous oxide is used to illustrate the broad tunability over 37 lines spanning 0.251 to 0.955 terahertz, each with kilohertz linewidths. Our analysis shows that laser lines spanning more than 1 terahertz with powers greater than 1 milliwatt are possible from many molecular gases pumped by quantum cascade lasers.
Alan She, Shuyan Zhang, Samuel Shian, David R. Clarke, and Federico Capasso. 2018. “Adaptive metalenses with simultaneous electrical control of focal length, astigmatism, and shift.” SCIENCE ADVANCES, 4, 2.Abstract
Focal adjustment and zooming are universal features of cameras and advanced optical systems. Such tuning is usually performed longitudinally along the optical axis by mechanical or electrical control of focal length. However, the recent advent of ultrathin planar lenses based on metasurfaces (metalenses), which opens the door to future drastic miniaturization of mobile devices such as cell phones and wearable displays, mandates fundamentally different forms of tuning based on lateral motion rather than longitudinal motion. Theory shows that the strain field of a metalens substrate can be directly mapped into the outgoing optical wavefront to achieve large diffractionlimited focal length tuning and control of aberrations. We demonstrate electrically tunable large-area metalenses controlled by artificial muscles capable of simultaneously performing focal length tuning (> 100%) as well as onthe-fly astigmatism and image shift corrections, which until now were only possible in electron optics. The device thickness is only 30 mm. Our results demonstrate the possibility of future optical microscopes that fully operate electronically, as well as compact optical systems that use the principles of adaptive optics to correct many orders of aberrations simultaneously.
Wei Ting Chen, Alexander Y. Zhu, Vyshakh Sanjeev, Mohammadreza Khorasaninejad, Zhujun Shi, Eric Lee, and Federico Capasso. 2018. “A broadband achromatic metalens for focusing and imaging in the visible.” NATURE NANOTECHNOLOGY, 13, 3, Pp. 220+.Abstract
A key goal of metalens research is to achieve wavefront shaping of light using optical elements with thicknesses on the order of the wavelength. Such miniaturization is expected to lead to compact, nanoscale optical devices with applications in cameras, lighting, displays and wearable optics. However, retaining functionality while reducing device size has proven particularly challenging. For example, so far there has been no demonstration of broadband achromatic metalenses covering the entire visible spectrum. Here, we show that by judicious design of nanofins on a surface, it is possible to simultaneously control the phase, group delay and group delay dispersion of light, thereby achieving a transmissive achromatic metalens with large bandwidth. We demonstrate diffraction-limited achromatic focusing and achromatic imaging from 470 to 670 nm. Our metalens comprises only a single layer of nanostructures whose thickness is on the order of the wavelength, and does not involve spatial multiplexing or cascading. While this initial design (numerical aperture of 0.2) has an efficiency of about 20% at 500 nm, we discuss ways in which our approach may be further optimized to meet the demand of future applications.
Wei Ting Chen, Alexander Y. Zhu, Jared Sisler, Yao-Wei Huang, Kerolos M.A. Yousef, Eric Lee, Cheng-Wei Qiu, and Federico Capasso. 2018. “Broadband Achromatic Metasurface-Refractive Optics.” NANO LETTERS, 18, 12, Pp. 7801-7808.Abstract
Existing methods of correcting for chromatic aberrations in optical systems are limited to two approaches: varying the material dispersion in refractive lenses or incorporating grating dispersion via diffractive optical elements. Recently, single-layer broadband achromatic meta surface lenses have been demonstrated but are limited to diameters on the order of 100 mu cm due to the large required group delays. Here, we circumvent this limitation and design a metacorrector by combining a tunable phase and artificial dispersion to correct spherical and chromatic aberrations in a large spherical plano-convex lens. The tunability results from a variation in light confinement in sub-wavelength waveguides by locally tailoring the effective refractive index. The effectiveness of this approach is further validated by designing a metacorrector, which greatly increases the of a state-of-the-art immersion objective (composed of 14 lenses and 7 types of glasses) from violet to near-infrared wavelengths. This concept of hybrid metasurface-refractive optics combines the advantages of both technologies in terms of size, scalability, complexity, and functionality.
L. Koehler, P. Chevalier, E. Shim, B. Desiatov, A. Shams-Ansari, M. Piccardo, Y. Okawachi, M. Yu, M. Loncar, M. Lipson, A. L. Gaeta, and F. Capasso. 2018. “Direct thermo-optical tuning of silicon microresonators for the mid-infrared.” OPTICS EXPRESS, 26, 26, Pp. 34965-34976.Abstract
We use light from a visible laser diode to directly tune silicon-on-chip microresonators by thermo-optical effect. We show that this direct tuning is local, non invasive and has a much smaller time constant than global temperature tuning methods. Such an approach could prove to be highly effective for Kerr comb generation in microresonators pumped by quantum cascade lasers, which cannot be easily tuned to achieve comb generation and soliton-modelocked states. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Tapashree Roy, Shuyan Zhang, Il Woong Jung, Mariano Troccoli, Federico Capasso, and Daniel Lopez. 2018. “Dynamic metasurface lens based on MEMS technology.” APL PHOTONICS, 3, 2.Abstract
In the recent years, metasurfaces, being flat and lightweight, have been designed to replace bulky optical components with various functions. We demonstrate a monolithic Micro-Electro-Mechanical System (MEMS) integrated with a metasurface-based flat lens that focuses light in the mid-infrared spectrum. A two-dimensional scanning MEMS platform controls the angle of the lens along two orthogonal axes by +/- 9 degrees, thus enabling dynamic beam steering. The device could be used to compensate for off-axis incident light and thus correct for aberrations such as coma. We show that for low angular displacements, the integrated lens-on-MEMS system does not affect the mechanical performance of the MEMS actuators and preserves the focused beam profile as well as the measured full width at half maximum. We envision a new class of flat optical devices with active control provided by the combination of metasurfaces and MEMS for a wide range of applications, such as miniaturized MEMS-based microscope systems, LIDAR scanners, and projection systems. (C) 2018 Author(s).
Lorenzo Pattelli, Diederik Wiersma, Mark Brongersma, and Federico Capasso. 2018. “Focus Point on Complex Photonics Measurements, Theory and Simulations for Extreme Light-Matter Interactions.” EUROPEAN PHYSICAL JOURNAL PLUS, 133, 3. pattelli2018.pdf
Federico Capasso. 2018. “The future and promise of flat optics: a personal perspective.” NANOPHOTONICS, 7, 6, SI, Pp. 953-957.Abstract
Metasurfaces enable the redesign of optical components into thin, planar and multifunctional elements, promising a major reduction in footprint and system complexity as well as the introduction of new optical functions. The planarity of flat optics will lead to the unification of semiconductor manufacturing and lens-making, where the planar technology to manufacture computer chips will be adapted to make CMOS-compatible metasurface-based optical components, ranging from metalenses to novel polarization optics, areas where I foresee the greatest technological and scientific impact.
Alexander Y. Zhu, Wei Ting Chen, Aun Zaidi, Yao-Wei Huang, Mohammadreza Khorasaninejad, Vyshakh Sanjeev, Cheng-Wei Qiu, and Federico Capasso. 2018. “Giant intrinsic chiro-optical activity in planar dielectric nanostructures.” LIGHT-SCIENCE & APPLICATIONS, 7.Abstract
The strong optical chirality arising from certain synthetic metamaterials has important and widespread applications in polarization optics, stereochemistry and spintronics. However, these intrinsically chiral metamaterials are restricted to a complicated three-dimensional (3D) geometry, which leads to significant fabrication challenges, particularly at visible wavelengths. Their planar two-dimensional (2D) counterparts are limited by symmetry considerations to operation at oblique angles (extrinsic chirality) and possess significantly weaker chiro-optical responses close to normal incidence. Here, we address the challenge of realizing strong intrinsic chirality from thin, planar dielectric nanostructures. Most notably, we experimentally achieve near-unity circular dichroism with similar to 90% of the light with the chosen helicity being transmitted at a wavelength of 540 nm. This is the highest value demonstrated to date for any geometry in the visible spectrum. We interpret this result within the charge-current multipole expansion framework and show that the excitation of higher-order multipoles is responsible for the giant circular dichroism. These experimental results enable the realization of high-performance miniaturized chiro-optical components in a scalable manner at optical frequencies.
Daniel Wintz, Kundan Chaudhary, Ke Wang, Luis A. Jauregui, Antonio Ambrosio, Michele Tamagnone, Alexander Y. Zhu, Robert C. Devlin, Jesse D. Crossno, Kateryna Pistunova, Kenji Watanabe, Takashi Taniguchi, Philip Kim, and Federico Capasso. 2018. “Guided Modes of Anisotropic van der Waals Materials Investigated by near-Field Scanning Optical Microscopy.” ACS PHOTONICS, 5, 4, Pp. 1196-1201.Abstract
Guided modes in nanometer thick anisotropic van der Waals materials are experimentally investigated and their refractive indices in visible wavelengths are extracted. Our method involves near-field scanning optical microscopy of waveguide (transverse electric) and surface plasmon polariton (transverse magnetic) modes in h-BN/SiO2/Si and Ag/h-BN stacks, respectively. We determine the dispersion of these modes and use this relationship to extract anisotropic refractive indices of h-BN flakes. In the wavelength interval 550-700 nm, the in-plane and out-of-plane refractive indices are in the range 1.98-2.12 and 1.45-2.12, respectively. Our approach of using near-field scanning optical microscopy allows for the direct study of the interaction between light and two-dimensional van der Waals materials and heterostructures.
Marco Piccardo, Paul Chevalier, Tobias S. Mansuripur, Dmitry Kazakov, Yongrui Wang, Noah A. Rubin, Lauren Meadowcroft, Alexey Belyanin, and Federico Capasso. 2018. “The harmonic state of quantum cascade lasers: origin, control, and prospective applications [Invited].” OPTICS EXPRESS, 26, 8, Pp. 9464-9483.Abstract
The recently discovered ability of the quantum cascade laser to produce a harmonic frequency comb has attracted new interest in these devices for both applications and fundamental laser physics. In this review we present an extensive experimental phenomenology of the harmonic state, including its appearance in mid-infrared and terahertz quantum cascade lasers, studies of its destabilization induced by delayed optical feedback, and the assessment of its frequency comb nature. A theoretical model explaining its origin as due to the mutual interaction of population gratings and population pulsations inside the laser cavity will he described. We explore different approaches to control the spacing of the harmonic state, such as optical injection seeding and variation of the device temperature. Prospective applications of the harmonic state include microwave and terahertz generation, picosecond pulse generation in the mid-infrared, and broadband spectroscopy. (C) 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement