Ahmed H. Dorrah, Noah A. Rubin, Aun Zaidi, Michele Tamagnone, and Federico Capasso. 1/28/2021. “Metasurface optics for on-demand polarization transformations along the optical path.” Nature Photonics. Publisher's VersionAbstract
Polarization plays a key role in science; hence its versatile manipulation is crucial. Existing polarization optics, however, can only manipulate polarization in a single transverse plane. Here we demonstrate a new class of polarizers and wave plates—based on metasurfaces—that can impart an arbitrarily chosen polarization response along the propagation direction, regardless of the incident polarization. The underlying mechanism relies on transforming an incident waveform into an ensemble of pencil-like beams with different polarization states that beat along the optical axis thereby changing the resulting polarization at will, locally, as light propagates. Remarkably, using form-birefringent metasurfaces in combination with matrix-based holography enables the desired propagation-dependent polarization response to be enacted without a priori knowledge of the incident polarization—a behaviour that would require three polarization-sensitive holograms if implemented otherwise. Our work expands the use of polarization in the design of multifunctional metasurfaces and may find application in tunable structured light, optically switchable devices and versatile light–matter interactions.
Noah A. Rubin, Aun Zaidi, Ahmed Dorrah, Zhujun Shi, and Federico Capasso. 1/1/2021. “Jones matrix holography with metasurfaces.” arXiv, 2012.14874v1. Publisher's Version
Zhaoyi Li, Peng Lin, Yao-Wei Huang, Joon-Suh Park, Wei Ting Chen, Zhujun Shi, Cheng-Wei Qiu, Ji-Xin Cheng, and Federico Capasso. 2021. “Meta-optics achieves RGB-achromatic focusing for virtual reality.” Science Advances, 7, 5. Publisher's VersionAbstract
Virtual and augmented realities are rapidly developing technologies, but their large-scale penetration will require lightweight optical components with small aberrations. We demonstrate millimeter-scale diameter, high-NA, submicron-thin, metasurface-based lenses that achieve diffraction-limited achromatic focusing of the primary colors by exploiting constructive interference of light from multiple zones and dispersion engineering. To illustrate the potential of this approach, we demonstrate a virtual reality system based on a home-built fiber scanning near-eye display.
Shaoliang Yu, Jinsheng Lu, Vincent Ginis, Simon Kheifets, Soon Wei Daniel Lim, Min Qiu, Tian Gu, Juejun Hu, and Federico Capasso. 2021. “On-chip optical tweezers based on freeform optics.” Optica, 8, 3, Pp. 409–414. Publisher's VersionAbstract
Since its advent in the 1970s, optical tweezers have been widely deployed as a preferred non-contact technique for manipulating microscale objects. On-chip integrated optical tweezers, which afford significant size, weight, and cost benefits, have been implemented, relying upon near-field evanescent waves. As a result, these tweezers are only capable of manipulation in near-surface regions and often demand high power since the evanescent interactions are relatively weak. We introduce on-chip optical tweezers based on freeform micro-optics, which comprise optical reflectors or refractive lenses integrated on waveguide end facets via two-photon polymerization. The freeform optical design offers unprecedented degrees of freedom to design optical fields with strong three-dimensional intensity gradients, useful for trapping and manipulating suspended particles in an integrated chip-scale platform. We demonstrate the design, fabrication, and measurement of both reflective and refractive micro-optical tweezers. The reflective tweezers feature a remarkably low trapping threshold power, and the refractive tweezers are particularly useful for multiparticle trapping and interparticle interaction analysis. Our integrated micro-optical tweezers uniquely combine a compact footprint, broadband operation, high trapping efficiency, and scalable integration with planar photonic circuits. This class of tweezers is promising for on-chip sensing, cell assembly, particle dynamics analysis, and ion trapping.
Wei Ting Chen and Federico Capasso. 2021. “Will flat optics appear in everyday life anytime soon?” Applied Physics Letters, 118, 10, Pp. 100503. Publisher's Version 5.0039885.pdf
Johannes Hillbrand, Nikola Opačak, Marco Piccardo, Harald Schneider, Gottfried Strasser, Federico Capasso, and Benedikt Schwarz. 11/13/2020. “Mode-locked short pulses from an 8 um wavelength semiconductor laser.” Nature Communications, 11, 1, Pp. 5788. Publisher's VersionAbstract
Quantum cascade lasers (QCL) have revolutionized the generation of mid-infrared light. Yet, the ultrafast carrier transport in mid-infrared QCLs has so far constituted a seemingly insurmountable obstacle for the formation of ultrashort light pulses. Here, we demonstrate that careful quantum design of the gain medium and control over the intermode beat synchronization enable transform-limited picosecond pulses from QCL frequency combs. Both an interferometric radio-frequency technique and second-order autocorrelation shed light on the pulse dynamics and confirm that mode-locked operation is achieved from threshold to rollover current. Furthermore, we show that both anti-phase and in-phase synchronized states exist in QCLs. Being electrically pumped and compact, mode-locked QCLs pave the way towards monolithically integrated non-linear photonics in the molecular fingerprint region beyond 6 $μ$m wavelength.
Franco Prati, Massimo Brambilla, Marco Piccardo, Lorenzo Luigi Columbo, Carlo Silvestri, Mariangela Gioannini, Alessandra Gatti, Luigi A. Lugiato, and Federico Capasso. 10/14/2020. “Soliton dynamics of ring quantum cascade lasers with injected signal.” Nanophotonics, 10, 1, Pp. 195–207. Publisher's VersionAbstract
Nonlinear interactions in many physical systems lead to symmetry breaking phenomena in which an initial spatially homogeneous stationary solution becomes modulated. Modulation instabilities have been widely studied since the 1960s in different branches of nonlinear physics. In optics, they may result in the formation of optical solitons, localized structures that maintain their shape as they propagate, which have been investigated in systems ranging from optical fibres to passive microresonators. Recently, a generalized version of the Lugiato–Lefever equation predicted their existence in ring quantum cascade lasers with an external driving field, a configuration that enables the bistability mechanism at the basis of the formation of optical solitons. Here, we consider this driven emitter and extensively study the structures emerging therein. The most promising regimes for localized structure formation are assessed by means of a linear stability analysis of the homogeneous stationary solution (or continuous-wave solution). In particular, we show the existence of phase solitons – chiral structures excited by phase jumps in the cavity – and cavity solitons. The latter can be deterministically excited by means of writing pulses and manipulated by the application of intensity gradients, making them promising as frequency combs (in the spectral domain) or reconfigurable bit sequences that can encode information inside the ring cavity.
Andrew Lininger, Alexander Y. Zhu, Joon-Suh Park, Giovanna Palermo, Sharmistha Chatterjee, Jonathan Boyd, Federico Capasso, and Giuseppe Strangi. 8/10/2020. “Optical properties of metasurfaces infiltrated with liquid crystals.” Proceedings of the National Academy of Sciences, 117, 34, Pp. 20390-20396. Publisher's Version 2006336117.full_.pdf
Vincent Ginis, Marco Piccardo, Michele Tamagnone, Jinsheng Lu, Min Qiu, Simon Kheifets, and Federico Capasso. 7/24/2020. “Remote structuring of near-field landscapes.” Science, 369, 6502, Pp. 436–440. Publisher's Version
Wei Ting Chen, Alexander Y. Zhu, and Federico Capasso. 6/19/2020. “Flat optics with dispersion-engineered metasurfaces.” Nature Reviews Materials, 5, Pp. 604-620. Publisher's Version
Marco Piccardo, Benedikt Schwarz, Dmitry Kazakov, Maximilian Beiser, Nikola Opačak, Yongrui Wang, Shantanu Jha, Johannes Hillbrand, Michele Tamagnone, Wei Ting Chen, Alexander Y. Zhu, Lorenzo L. Columbo, Alexey Belyanin, and Federico Capasso. 6/1/2020. “Frequency combs induced by phase turbulence.” Nature, 582, 7812, Pp. 360-364. Publisher's Version s41586-020-2386-6.pdf
Zhujun Shi, Alexander Y. Zhu, Zhaoyi Li, Yao-Wei Huang, Wei Ting Chen, Cheng-Wei Qiu, and Federico Capasso. 2020. “Continuous angle-tunable birefringence with freeform metasurfaces for arbitrary polarization conversion.” Science Advances, 6, 23. Publisher's VersionAbstract
Birefringence occurs when light with different polarizations sees different refractive indices during propagation. It plays an important role in optics and has enabled essential polarization elements such as wave plates. In bulk crystals, it is typically constrained to linear birefringence. In metamaterials with freeform meta-atoms, however, one can engineer the optical anisotropy such that light sees different indices for arbitrary—linear, circular, or elliptical—orthogonal eigen-polarization states. Using topology-optimized metasurfaces, we demonstrate this arbitrary birefringence. It has the unique feature that it can be continuously tuned from linear to elliptical birefringence, by changing the angle of incidence. In this way, a single metasurface can operate as many wave plates in parallel, implementing different polarization transformations. Angle-tunable arbitrary birefringence expands the scope of polarization optics, enables compact and versatile polarization operations that would otherwise require cascading multiple elements, and may find applications in polarization imaging, quantum optics, and other areas.
Jared Sisler, Wei Ting Chen, Alexander Y. Zhu, and Federico Capasso. 2020. “Controlling dispersion in multifunctional metasurfaces.” APL Photonics, 5, 5, Pp. 056107. Publisher's Version 1.5142637.pdf
Isaac Nape, Bereneice Sephton, Yao-Wei Huang, Adam Vallés, Cheng-Wei Qiu, Antonio Ambrosio, Federico Capasso, and Andrew Forbes. 2020. “Enhancing the modal purity of orbital angular momentum photons.” APL Photonics, 5, 7, Pp. 070802. Publisher's Version 5.0005597.pdf
Hend Sroor, Yao-Wei Huang, Bereneice Sephton, Darryl Naidoo, Adam Valles, Vincent Ginis, Cheng-Wei Qiu, Antonio Ambrosio, Federico Capasso, and Andrew Forbes. 2020. “High-purity orbital angular momentum states from a visible metasurface laser.” Nature Photonics, 14, Pp. 498–503.Abstract
A metasurface laser generates orbital angular momentum states with quantum numbers reaching l = 100. Simultaneous output vortex beams, with Delta l as great as 90, are demonstrated in the visible regime. Orbital angular momentum (OAM) from lasers holds promise for compact, at-source solutions for applications ranging from imaging to communications. However, conjugate symmetry between circular spin and opposite helicity OAM states (+/- l) from conventional spin-orbit approaches has meant that complete control of light's angular momentum from lasers has remained elusive. Here, we report a metasurface-enhanced laser that overcomes this limitation. We demonstrate new high-purity OAM states with quantum numbers reaching l = 100 and non-symmetric vector vortex beams that lase simultaneously on independent OAM states as much as Delta l = 90 apart, an extreme violation of previous symmetric spin-orbit lasing devices. Our laser conveniently outputs in the visible, producing new OAM states of light as well as all previously reported OAM modes from lasers, offering a compact and power-scalable source that harnesses intracavity structured matter for the creation of arbitrary chiral states of structured light.
I-Ju Chen, Steven Limpert, Wondwosen Metaferia, Claes Thelander, Lars Samuelson, Federico Capasso, Adam M. Burke, and Heiner Linke. 2020. “Hot-carrier extraction in nanowire-nanoantenna photovoltaic devices.” Nano Letters, 20, 6, Pp. 4064–4072. Publisher's VersionAbstract
Nanowires bring new possibilities to the field of hot-carrier photovoltaics by providing flexibility in combining materials for band engineering and using nanophotonic effects to control light absorption. Previously, an open-circuit voltage beyond the Shockley-Queisser limit was demonstrated in hot-carrier devices based on InAs-InP-InAs nanowire heterostructures. However, in these first experiments, the location of light absorption, and therefore the precise mechanism of hot-carrier extraction, was uncontrolled. In this letter, we combine plasmonic nanoantennas with InAs-InP-InAs nanowire devices to enhance light absorption within a subwavelength region near an InP energy barrier that serves as an energy filter. From photon energy- and irradiance-dependent photocurrent and photovoltage measurements, we find that photocurrent generation is dominated by internal photoemission of non-thermalized hot electrons when the photoexcited electron energy is above the barrier, and by photo-thermionic emission when the energy is below the barrier. We estimate that an internal quantum efficiency up to 0.5–1.2% is achieved. Insights from this study provide guidelines to improve internal quantum efficiencies based on nanowire heterostructures.
Mena N. Gadalla, Kundan Chaudhary, Christine M. Zgrabik, Federico Capasso, and Evelyn L. Hu. 2020. “Imaging of surface plasmon polaritons in low-loss highly metallic titanium nitride thin films in visible and infrared regimes.” Opt. Express, 28, 10, Pp. 14536–14546. Publisher's VersionAbstract
Titanium nitride (TiN) has been identified as a promising refractory material for high temperature plasmonic applications such as surface plasmon polaritons (SPPs) waveguides, lasers and light sources, and near field optics. Such SPPs are sensitive not only to the highly metallic nature of the TiN, but also to its low loss. We have formed highly metallic, low-loss TiN thin films on MgO substrates to create SPPs with resonances between 775-825&\#x2005;nm. Scanning near-field optical microscopy (SNOM) allowed imaging of the SPP fringes, the accurate determination of the effective wavelength of the SPP modes, and propagation lengths greater than 10 microns. Further, we show the engineering of the band structure of the plasmonic modes in TiN in the mid-IR regime and experimentally demonstrate, for the first time, the ability of TiN to support Spoof Surface Plasmon Polaritons in the mid-IR (6 microns wavelength).
A.A. Loya Villalpando, J. Mart\'ın-Albo, W. T. Chen, R. Guenette, C. Lego, J.S. Park, and F. Capasso. 2020. “Improving the light collection efficiency of silicon photomultipliers through the use of metalenses.” Journal of Instrumentation, 15, 11, Pp. P11021–P11021. Publisher's VersionAbstract
Metalenses are optical devices that implement nanostructures as phase shifters to focus incident light. Their compactness and simple fabrication make them a potential cost-effective solution for increasing light collection efficiency in particle detectors with limited photosensitive area coverage. Here we report on the characterization and performance of metalenses in increasing the light collection efficiency of silicon photomultipliers (SiPM) of various sizes using an LED of 630 nm, and find a six to seven-fold increase in signal for a 1.3×1.3 mm2 SiPM when coupled with a 10-mm-diameter metalens manufactured using deep ultraviolet stepper lithography. Such improvements could be valuable for future generations of particle detectors, particularly those employed in rare-event searches such as dark matter and neutrinoless double beta decay.
Johannes Hillbrand, Dominik Auth, Marco Piccardo, Nikola Opacak, Erich Gornik, Gottfried Strasser, Federico Capasso, Stefan Breuer, and Benedikt Schwarz. 2020. “In-Phase and Anti Phase Synchronization in a Laser Frequency Comb.” Physical Review Letters, 124, 2, Pp. 023901.Abstract
Coupled clocks are a classic example of a synchronization system leading to periodic collective oscillations. Already in 1665, Christiaan Huygens described this phenomenon as a kind of ``sympathy'' among oscillators. In this work, we describe the formation of two types of laser frequency combs as a system of oscillators coupled through the beating of the lasing modes. We experimentally show two completely different types of synchronization in a quantum dot laser-in-phase and splay-phase states. Both states can be generated in the same device, just by varying the damping losses of the system. This modifies the coupling among the oscillators. The temporal laser output is characterized using both linear and quadratic autocorrelation techniques. Our results show that both pulses and frequency-modulated states can be generated on demand within the same device. These findings allow us to connect laser frequency combs produced by amplitude-modulated and frequency-modulated lasers and link these to pattern formation in coupled systems such as Josephson junction arrays.
Bereneice Sephton, Yao-Wei Huang, Antonio Ambrosio, Cheng-Wei Qiu, Adam Vallés, Takashige Omatsu, Federico Capasso, and Andrew Forbes. 2020. “Purity and efficiency of hybrid orbital angular momentum-generating metasurfaces.” Journal of Nanophotonics, 14, 1, Pp. 1 – 15. Publisher's VersionAbstract

Controlling light with subwavelength-designed metasurfaces (MSs) has allowed for the arbitrary creation of structured light by precisely engineered matter. We report on the purity and conversion efficiency of hybrid orbital angular momentum (OAM)-generating MSs. We use a recently reported method to design and fabricate meta-surfaces that exploit generalized spin-orbit coupling to produce vector OAM states with asymmetric OAM superpositions, e.g., 1 and 5, coupled to linear and circular polarization states, fractional vector OAM states with OAM values of 3.5 and 6.5, and also the common conjugate spin and OAM of ±1 as reported in previous spin-orbit coupling devices. The OAM and radial modes in the resulting beams are quantitatively studied by implementing a modal decomposition approach, establishing both purity and conversion efficiency. We find conversion efficiencies exceeding 75% and purities in excess of 95%. A phase-flattening approach reveals that the OAM purity can be very low due to the presence of undesired radial components. We characterize the effect and illustrate how to suppress the undesired radial modes.