Alexander Y. Zhu, Wei Ting Chen, Jared Sisler, Kerolos M.A. Yousef, Eric Lee, Yao-Wei Huang, Cheng-Wei Qiu, and Federico Capasso. 2019. “Compact Aberration-Corrected Spectrometers in the Visible Using Dispersion-Tailored Metasurfaces.” Advanced Optical Materials, 7, 14, SI.Abstract
The spectral resolution and range of conventional spectrometers are typically limited by optical aberrations of their focusing elements, mainly due to chromatically induced astigmatism and an intrinsically curved focal plane. Traditional approaches to overcome this challenge require additional optical components which introduce significant bulk and design complexity to the system and prevent easy integration with portable devices. Here a single planar off-axis focusing metalens consisting of subwavelength TiO2 nanofins whose focal spots lie along a plane and undergo minimal focal spot broadening for almost 200 nm across the visible spectrum is demonstrated. This allows us to achieve a miniature aberration-corrected spectrometer with nanometer spectral resolution, while having a beam propagation distance of only 4 cm to the camera plane. This is achieved by dispersion engineering: tailoring the phase, group delay (GD) and GD dispersion of the metalens. This approach is general and can also be used to introduce customized functionalities to the metalens such as a linear dispersion in the frequency domain with minimal additional overhead.
Qi Guo, Zhujun Shi, Yao-Wei Huang, Emma Alexander, Cheng-Wei Qiu, Federico Capasso, and Todd Zickler. 2019. “Compact single-shot metalens depth sensors inspired by eyes of jumping spiders.” Proceedings of the National Academy of Sciences of the United States, 116, 46, Pp. 22959-22965.Abstract
Jumping spiders (Salticidae) rely on accurate depth perception for predation and navigation. They accomplish depth perception, despite their tiny brains, by using specialized optics. Each principal eye includes a multitiered retina that simultaneously receives multiple images with different amounts of defocus, and from these images, distance is decoded with relatively little computation. We introduce a compact depth sensor that is inspired by the jumping spider. It combines metalens optics, which modifies the phase of incident light at a subwavelength scale, with efficient computations to measure depth from image defocus. Instead of using a multitiered retina to transduce multiple simultaneous images, the sensor uses a metalens to split the light that passes through an aperture and concurrently form 2 differently defocused images at distinct regions of a single planar photosensor. We demonstrate a system that deploys a 3-mm-diameter metalens to measure depth over a 10-cm distance range, using fewer than 700 floating point operations per output pixel. Compared with previous passive depth sensors, our metalens depth sensor is compact, single-shot, and requires a small amount of computation. This integration of nanophotonics and efficient computation brings artificial depth sensing closer to being feasible on millimeter-scale, microwatts platforms such as microrobots and microsensor networks.
Ermes Toninelli, Bienvenu Ndagano, Adam Valles, Bereneice Sephton, Isaac Nape, Antonio Ambrosio, Federico Capasso, Miles J. Padgett, and Andrew Forbes. 2019. “Concepts in quantum state tomography and classical implementation with intense light: a tutorial.” Advances in Optics and Photonics, 11, 1, Pp. 67-134.Abstract
A tomographic measurement is a ubiquitous tool for estimating the properties of quantum states, and its application is known as quantum state tomography (QST). The process involves manipulating single photons in a sequence of projective measurements, often to construct a density matrix from which other information can be inferred, and is as laborious as it is complex. Here we unravel the steps of a QST and outline how it may be demonstrated in a fast and simple manner with intense (classical) light. We use scalar beams in a time reversal approach to simulate the outcome of a QST and exploit non-separability in classical vector beams as a means to treat the latter as a ``classically entangled'' state for illustrating QSTs directly. We provide a complete do-it-yourself resource for the practical implementation of this approach, complete with tutorial video, which we hope will facilitate the introduction of this core quantum tool into teaching and research laboratories alike. Our work highlights the value of using intense classical light as a means to study quantum systems and in the process provides a tutorial on the fundamentals of QSTs. (C) 2019 Optical Society of America
Lei Jin, Yao-Wei Huang, Zhongwei Jin, Robert C. Devlin, Zhaogang Dong, Shengtao Mei, Menghua Jiang, Wei Ting Chen, Zhun Wei, Hong Liu, Jinghua Teng, Aaron Danner, Xiangping Li, Shumin Xiao, Shuang Zhang, Changyuan Yu, Joel K. W. Yang, Federico Capasso, and Cheng-Wei Qiu. 2019. “Dielectric multi-momentum meta-transformer in the visible.” Nature Communications, 10.Abstract
Metasurfaces as artificially nanostructured interfaces hold significant potential for multifunctionality, which may play a pivotal role in the next-generation compact nano-devices. The majority of multi-tasked metasurfaces encode or encrypt multi-information either into the carefully tailored metasurfaces or in pre-set complex incident beam arrays. Here, we propose and demonstrate a multi-momentum transformation metasurface (i.e., meta-transformer), by fully synergizing intrinsic properties of light, e.g., orbital angular momentum (OAM) and linear momentum (LM), with a fixed phase profile imparted by a metasurface. The OAM meta-transformer reconstructs different topologically charged beams into on-axis distinct patterns in the same plane. The LM meta-transformer converts red, green and blue illuminations to the on-axis images of ``R'', ``G'' and ``B'' as well as vivid color holograms, respectively. Thanks to the infinite states of light-metasurface phase combinations, such ultra-compact meta-transformer has potential in information storage, nanophotonics, optical integration and optical encryption.
Kundan Chaudhary, Michele Tamagnone, Mehdi Rezaee, D. Kwabena Bediako, Antonio Ambrosio, Philip Kim, and Federico Capasso. 2019. “Engineering phonon polaritons in van der Waals heterostructures to enhance in-plane optical anisotropy.” SCIENCE ADVANCES, 5, 4.Abstract
Van der Waals (vdW) heterostructures assembled from layers of two-dimensional materials have attracted considerable interest due to their novel optical and electrical properties. Here, we report a scattering-type scanning near-field opticalmicroscopy study of hexagonal boron nitride on black phosphorus (h-BN/BP) heterostructures, demonstrating the first direct observation of in-plane anisotropic phonon polariton modes in vdW heterostructures. Notably, the measured in-plane optical anisotropy along the armchair and zigzag crystal axes exceeds the ratio of refractive indices of BP in the x-y plane. We explain that this enhancement is due to the high confinement of the phonon polaritons in h-BN. We observe a maximum in-plane optical anisotropy of alpha(max) = 1.25 in the frequency spectrum at 1405 to 1440 cm(-1). These results provide new insights into the behavior of polaritons in vdW heterostructures, and the observed anisotropy enhancement paves the way to novel nanophotonic devices and to a new way to characterize optical anisotropy in thin films.
Mena N. Gadalla, Andrew S. Greenspon, Michele Tamagnone, Federico Capasso, and Evelyn L. Hu. 2019. “Excitation of Strong Localized Surface Plasmon Resonances in Highly Metallic Titanium Nitride Nano-Antennas for Stable Performance at Elevated Temperatures.” ACS APPLIED NANO MATERIALS, 2, 6, Pp. 3444-3452.Abstract
New opportunities for plasmonic applications at high temperatures have stimulated interest in refractory plasmonic materials that show greater stability at elevated temperatures than the more commonly used silver and gold (Au). Titanium nitride (TiN) has been identified as a promising refractory material, with deposition of TiN thin films through techniques ranging from plasma-enhanced atomic laser deposition to sputter deposition to pulsed laser deposition, on a variety of substrates, including MgO, polymer, SiO2 and sapphire. A variety of plasmonic devices have been evaluated, including gratings, nanorods, and nanodisks. An implicit metric for TiN behavior has been the comparison of its plasmonic performance to that of Au, in particular at various elevated temperatures. This paper carries out a one-to-one comparison of bowtie nanoantennas formed of TiN and Au MgO substrates), examining the far-field characteristics, related to the measured near-field resonances. In both cases, the optical constants of the TiN films were used to simulate the expected plasmonic responses and enjoyed excellent agreement with the experimental measurements. Furthermore, we examined the consistency of the plasmonic response and the morphological changes in the TiN and Au nanoantennas at different temperatures up to 800 degrees C in the atmosphere. This comparison of the measured plasmonic response from nanoscale resonances to the far-field response allows for anomalies or imperfections that may be introduced by the nanofabrication processes and provides a more accurate comparison of TiN plasmonic behavior relative to the Au standard.
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