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.

Mohammad Haghtalab, Michele Tamagnone, Alexander Y. Zhu, Safieddin Safavi-Naeini, and Federico Capasso. 2020. “Ultrahigh Angular Selectivity of Disorder-Engineered Metasurfaces.” ACS Photonics, 7, 4, Pp. 991-1000.Abstract
Metastructures hold promises for the realization of novel optical functions. Common topologies utilized in the form of metasurfaces featuring simple periodicities, however, are not exploiting the full potential such platform can offer. On the other hand, disordered metasurfaces consisting of strongly coupled elements can provide a versatile platform with large degrees of freedom to be exploited in an inverse design process. Here, we investigate a new class of disordered metasurfaces; a disordered network of scattering elements strongly connected through interelement free space coupling and excited guided waves. Such a highly connected network of scatterers exhibits very strong angular sensitivity for the plane waves illuminating the structure at arbitrary angles; a minute deviation in the angle of incidence results in a drastic change in the radiation profile both in near and far-field regions. This peculiar feature is harnessed by engineered disorder implemented through a rigorous near-optimal freeform inverse design algorithm to realize arbitrarily large metasurfaces with unexpected angular single/multifunctionality. Our proposed scheme is extendable to implement novel functions demanded by emerging applications such as LiDARs and highly secure communication systems.
Joon-Suh Park, Shuyan Zhang, Alan She, Wei Ting Chen, Peng Lin, Kerolos M.A. Yousef, Ji-Xin Cheng, and Federico Capasso. 2019. “All-Glass, Large Metalens at Visible Wavelength Using Deep-Ultraviolet Projection Lithography.” Nano Letters, 19, 12, Pp. 8673-8682.Abstract
Metalenses, planar lenses realized by placing subwavelength nanostructures that locally impart lenslike phase shifts to the incident light, are promising as a replacement for refractive optics for their ultrathin, lightweight, and tailorable characteristics, especially for applications where payload is of significant importance. However, the requirement of fabricating up to billions of subwavelength structures for centimeter-scale metalenses can constrain size-scalability and mass-production for large lenses. In this Letter, we demonstrate a centimeter-scale, all-glass metalens capable of focusing and imaging at visible wavelength, using deep-ultraviolet (DUV) projection stepper lithography. Here, we show size-scalability and potential for mass-production by fabricating 45 metalenses of 1 cm diameter on a 4 in. fused-silica wafer. The lenses show diffraction-limited focusing behavior for any homogeneously polarized incidence at visible wavelengths. The metalens performance is quantified by the Strehl ratio and the modulation transfer function (MTF), which are then compared with commercial refractive spherical and aspherical singlet lenses of similar size and focal length. We further explore the imaging capabilities of our metalens using a color-pixel sCMOS camera and scanning-imaging techniques, demonstrating potential applications for virtual reality (VR) devices or biological imaging techniques.
Wei Ting Chen, Alexander Y. Zhu, Jared Sisler, Zameer Bharwani, and Federico Capasso. 2019. “A broadband achromatic polarization-insensitive metalens consisting of anisotropic nanostructures.” Nature Communications, 10.Abstract
Metasurfaces have attracted widespread attention due to an increasing demand of compact and wearable optical devices. For many applications, polarization-insensitive metasurfaces are highly desirable, and appear to limit the choice of their constituent elements to isotropic nanostructures. This greatly restricts the number of geometric parameters available in design. Here, we demonstrate a polarization-insensitive metalens using otherwise anisotropic nanofins which offer additional control over the dispersion and phase of the output light. As a result, we can render a metalens achromatic and polarization-insensitive across nearly the entire visible spectrum from wavelength lambda = 460 nm to 700 nm, while maintaining diffraction-limited performance. The metalens is comprised of just a single layer of TiO2 nanofins and has a numerical aperture of 0.2 with a diameter of 26.4 mu m. The generality of our polarization-insensitive design allows it to be implemented in a plethora of other metasurface devices with applications ranging from imaging to virtual/augmented reality.
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.