Publications

2021
Soon Wei Daniel Lim, Joon-Suh Park, Maryna L. Meretska, Ahmed H. Dorrah, and Federico Capasso. 7/7/2021. “Engineering phase and polarization singularity sheets.” Nature Communications, 12, 1, Pp. 4190. Publisher's VersionAbstract
Optical phase singularities are zeros of a scalar light field. The most systematically studied class of singular fields is vortices: beams with helical wavefronts and a linear (1D) singularity along the optical axis. Beyond these common and stable 1D topologies, we show that a broader family of zero-dimensional (point) and two-dimensional (sheet) singularities can be engineered. We realize sheet singularities by maximizing the field phase gradient at the desired positions. These sheets, owning to their precise alignment requirements, would otherwise only be observed in rare scenarios with high symmetry. Furthermore, by applying an analogous procedure to the full vectorial electric field, we can engineer paraxial transverse polarization singularity sheets. As validation, we experimentally realize phase and polarization singularity sheets with heart-shaped cross-sections using metasurfaces. Singularity engineering of the dark enables new degrees of freedom for light-matter interaction and can inspire similar field topologies beyond optics, from electron beams to acoustics.
s41467-021-24493-y.pdf
Christina Spägele, Michele Tamagnone, Dmitry Kazakov, Marcus Ossiander, Marco Piccardo, and Federico Capasso. 6/18/2021. “Multifunctional wide-angle optics and lasing based on supercell metasurfaces.” Nature Communications, 12, 1, Pp. 3787. Publisher's VersionAbstract
Metasurfaces are arrays of subwavelength spaced nanostructures that can manipulate the amplitude, phase, and polarization of light to achieve a variety of optical functions beyond the capabilities of 3D bulk optics. However, they suffer from limited performance and efficiency when multiple functions with large deflection angles are required because the non-local interactions due to optical coupling between nanostructures are not fully considered. Here we introduce a method based on supercell metasurfaces to demonstrate multiple independent optical functions at arbitrary large deflection angles with high efficiency. In one implementation the incident laser is simultaneously diffracted into Gaussian, helical and Bessel beams over a large angular range. We then demonstrate a compact wavelength-tunable external cavity laser with arbitrary beam control capabilities – including beam shaping operations and the generation of freeform holograms. Our approach paves the way to novel methods to engineer the emission of optical sources.
sp-gele_et_al-2021-nature_communications.pdf
Arman Amirzhan, Paul Chevalier, Jeremy Rowlette, H. Ted Stinson, Michael Pushkarsky, Timothy Day, Henry O. Everitt, and Federico Capasso. 5/27/2021. “A quantum cascade laser-pumped molecular laser tunable over 1 THz.” ArXiv, 2105.13326. Publisher's Version 2105.13326.pdf
M. Ossiander, Y. W. Huang, W. T. Chen, Z. Wang, X. Yin, Y. A. Ibrahim, M. Schultze, and F. Capasso. 5/14/2021. “Slow Light Nanocoatings for Ultrashort Pulse Shaping.” arXiv, arXiv:2105.06805. Publisher's Version 2105.06805.pdf
L. Columbo, M. Piccardo, F. Prati, L. A. Lugiato, M. Brambilla, A. Gatti, C. Silvestri, M. Gioannini, N. Opačak, B. Schwarz, and F. Capasso. 4/30/2021. “Unifying Frequency Combs in Active and Passive Cavities: Temporal Solitons in Externally Driven Ring Lasers.” Phys. Rev. Lett., 126, Pp. 173903. Publisher's Version columbo_physrevlett.126.173903.pdf
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.
s41566-020-00750-2.pdf
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.
eabe4458.full_.pdf
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.
optica-8-3-409.pdf
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
2020
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.
20201113_Hillbrand_NatComm.pdf
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.
20201014_soliton_dynamics.pdf
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.
eaba3367.full_.pdf
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.
2020_sroor_et_al._nature_photonics.pdf

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