Haig A. Atikian, Neil Sinclair, Pawel Latawiec, Xiao Xiong, Srujan Meesala, Scarlett Gauthier, Daniel Wintz, Joseph Randi, David Bernot, Sage DeFrances, Jeffrey Thomas, Michael Roman, Sean Durrant, Federico Capasso, and Marko Lončar. 5/11/2022. “Diamond mirrors for high-power continuous-wave lasers.” Nature Communications, 13, Pp. 2610. Publisher's Version s41467-022-30335-2.pdf
Zhaoyi Li, Raphaël Pestourie, Joon-Suh Park, Yao-Wei Huang, Steven G. Johnson, and Federico Capasso. 5/3/2022. “Inverse design enables large-scale high-performance meta-optics reshaping virtual reality.” Nature Communications, 13, Pp. 2409. Publisher's Version s41467-022-29973-3.pdf
Ahmed H. Dorrah and Federico Capasso. 4/22/2022. “Tunable structured light with flat optics.” Science, 376, 6591, Pp. eabi6860. Publisher's VersionAbstract
Flat optics has emerged as a key player in the area of structured light and its applications, owing to its subwavelength resolution, ease of integration, and compact footprint. Although its first generation has revolutionized conventional lenses and enabled anomalous refraction, new classes of meta-optics can now shape light and dark features of an optical field with an unprecedented level of complexity and multifunctionality. Here, we review these efforts with a focus on metasurfaces that use different properties of input light—angle of incidence and direction, polarization, phase distribution, wavelength, and nonlinear behavior—as optical knobs for tuning the output response. We discuss ongoing advances in this area as well as future challenges and prospects. These recent developments indicate that optically tunable flat optics is poised to advance adaptive camera systems, microscopes, holograms, and portable and wearable devices and may suggest new possibilities in optical communications and sensing. The development of metasurfaces has provided a route to replacing bulk optical components with thin layers of engineered materials. In a review, Dorrah and Capasso highlight some of the recent advances in wavefront shaping using multifunctional meta-optics. They focus on the ability to tune the response of the metasurface by simply tuning one or more degrees of freedom of incident light, for example, by varying its angle of incidence, polarization, wavelength, or phase. The key feature of these metasurfaces is that although they are static, they can produce a tunable response without the need for complex switching. These developments enable multifunctional and lightweight components for technologies such as augmented and virtual reality displays, drone-based sensing, and endoscopy. —ISO A review discusses methods to control the functionality of optical metasurfaces by the incident light.
Noah A. Rubin, Paul Chevalier, Michael Juhl, Michele Tamagnone, Russell Chipman, and Federico Capasso. 3/8/2022. “Imaging polarimetry through metasurface polarization gratings.” Optics Express, 30, 6, Pp. 9389-9412. Publisher's Version oe-30-6-9389.pdf
Jaewon Oh, Kangmei Li, Jun Yang, Wei Ting Chen, Ming-Jun Li, Paulo Dainese, and Federico Capasso. 3/7/2022. “Adjoint-optimized metasurfaces for compact mode-division multiplexing.” ACS Photonics. Publisher's Version acsphotonics.1c01744.pdf
Paul Chevalier, Arman Amirzhan, Jeremy Rowlette, H. Ted Stinson, Michael Pushkarsky, Timothy Day, Federico Capasso, and Henry O. Everitt. 2/24/2022. “Multi-line lasing in the broadly tunable ammonia quantum cascade laser pumped molecular laser.” Applied Physics Letters, 120, 8, Pp. 081108. Publisher's Version 5_0079219.pdf
Masoud Pahlevaninezhad, Yao-Wei Huang, Majid Pahlevani, Brett Bouma, Melissa J Suter, Federico Capasso, and Hamid Pahlevaninezhad. 2/14/2022. “Metasurface-based bijective illumination collection imaging provides high-resolution tomography in three dimensions.” Nature Photonics. Publisher's Version naturephotonicfeb2022.pdf
Arman Amirzhan, Paul Chevalier, Jeremy Rowlette, H. Ted Stinson, Michael Pushkarsky, Timothy Day, Henry O. Everitt, and Federico Capasso. 1/25/2022. “A quantum cascade laser-pumped molecular laser tunable over 1 THz.” APL Photonics, 7, 1, Pp. 016107. Publisher's Version 5.0076310.pdf
Noah A. Rubin, Zhujun Shi, and Federico Capasso. 1/3/2022. “Polarization in diffractive optics and metasurfaces.” Advances in Optics and Photonics, 13, 4, Pp. 836-970. Publisher's VersionAbstract
Polarization, the path traced by light’s electric field vector, appears in all areas of optics. In recent decades, various technologies have enabled the precise control of light’s polarization state, even on a subwavelength scale, at optical frequencies. In this review, we provide a thorough, high-level review of the fundamentals of polarization optics and detail how the Jones calculus, alongside Fourier optics, can be used to analyze, classify, and compare these optical elements. We provide a review of work in this area across multiple technologies and research areas, including recent developments in optical metasurfaces. This review unifies a large body of work on spatially varying polarization optics and may be of interest to both researchers in optics and designers of optical systems more generally.
Marco Piccardo and Federico Capasso. 12/16/2021. “Laser Frequency Combs with Fast Gain Recovery: Physics and Applications.” Laser & Photonics Reviews, Pp. 2100403. Publisher's VersionAbstract
 The dynamics of semiconductor lasers with fast gain is generally considered to suppress amplitude modulation, resulting in a frequency-modulated output. Since their first demonstration of frequency comb operation, quantum cascade lasers—ideal representatives of this class of lasers with fast gain dynamics—were indeed shown to emit frequency-modulated combs. The understanding of these frequency comb regimes has progressively evolved, being refined from simpler arguments to more advanced theories, considering the role of Kerr nonlinearity and linewidth enhancement factor. By now, the traditional frequency-modulated combs are rather well understood in quantum cascade lasers, but new types of frequency combs have emerged, reopening interesting physics questions in these lasers. Recent advancements on two open frontiers will be discussed: harmonic frequency combs, characterized by mode skipping of many free spectral ranges, and spatiotemporal structures with solitonic features, recently reported in unidirectional ring cavities. These new types of frequency combs open technological and fundamental perspectives, sprouting new interest in quantum cascade lasers.
Marco Piccardo, Vincent Ginis, Andrew Forbes, Simon Mahler, Asher A Friesem, Nir Davidson, Haoran Ren, Ahmed H Dorrah, Federico Capasso, Firehun T Dullo, Balpreet S Ahluwalia, Antonio Ambrosio, Sylvain Gigan, Nicolas Treps, Markus Hiekkamäki, Robert Fickler, Michael Kues, David Moss, Roberto Morandotti, Johann Riemensberger, Tobias J Kippenberg, Jérôme Faist, Giacomo Scalari, Nathalie Picqué, Theodor W Hänsch, Giulio Cerullo, Cristian Manzoni, Luigi A Lugiato, Massimo Brambilla, Lorenzo Columbo, Alessandra Gatti, Franco Prati, Abbas Shiri, Ayman F Abouraddy, Andrea Alù, Emanuele Galiffi, JB Pendry, and Paloma A Huidobro. 12/16/2021. “Roadmap on multimode light shaping.” Journal of Optics, 24, 1, Pp. 013001. Publisher's VersionAbstract
Our ability to generate new distributions of light has been remarkably enhanced in recent years. At the most fundamental level, these light patterns are obtained by ingeniously combining different electromagnetic modes. Interestingly, the modal superposition occurs in the spatial, temporal as well as spatio-temporal domain. This generalized concept of structured light is being applied across the entire spectrum of optics: generating classical and quantum states of light, harnessing linear and nonlinear light-matter interactions, and advancing applications in microscopy, spectroscopy, holography, communication, and synchronization. This Roadmap highlights the common roots of these different techniques and thus establishes links between research areas that complement each other seamlessly. We provide an overview of all these areas, their backgrounds, current research, and future developments. We highlight the power of multimodal light manipulation and want to inspire new eclectic approaches in this vibrant research community.
Aun Zaidi, Noah A. Rubin, Ahmed H. Dorrah, Joon-Suh Park, and Federico Capasso. 11/22/2021. “Generalized polarization transformations with metasurfaces.” Opt. Express, 29, 24, Pp. 39065–39078. Publisher's VersionAbstract
Metasurfaces are arrays of sub-wavelength spaced nanostructures, which can be designed to control the many degrees-of-freedom of light on an unprecedented scale. In this work, we design meta-gratings where the diffraction orders can perform general, arbitrarily specified, polarization transformation without any reliance on conventional polarization components, such as waveplates and polarizers. We use matrix Fourier optics to design our devices and introduce a novel approach for their optimization. We implement the designs using form-birefringent metasurfaces and quantify their behavior &\#x2013; retardance and diattenuation. Our work is of importance in applications, such as polarization abberation correction in imaging systems, and in experiments requiring novel and compact polarization detection and control.
F. Prati, L. A. Lugiato, A. Gatti, L. Columbo, C. Silvestri, M. Gioannini, M. Brambilla, M. Piccardo, and F. Capasso. 11/20/2021. “Global and localised temporal structures in driven ring quantum cascade lasers.” Chaos, Solitons & Fractals, 153, Pp. 111537. Publisher's VersionAbstract
Starting from a full set of effective Maxwell-Bloch equations for a ring quantum cascade laser in the limit of fast material dynamics we derive a new set of equations which require a considerably lower numerical load because they evolve on the time scale of the electric field. With the further assumption of laser very close to threshold the equations take the form of the generalised Lugiato-Lefever equation. Using the latter, we study the formation and stability of multi-peaked localised structures which can be regarded as portions of a global pattern and exhibit a snaking structure.
Ahmed H. Dorrah, Michele Tamagnone, Noah A. Rubin, Aun Zaidi, and Federico Capasso. 11/19/2021. “Introducing Berry phase gradients along the optical path via propagation-dependent polarization transformations.” Nanophotonics. Publisher's Version 10.1515_nanoph-2021-0560.pdf
M. Ossiander, Y.-W. Huang, W. T. Chen, Z Wang, X Yin, YA Ibrahim, M Schultze, and F. Capasso. 11/11/2021. “Slow light nanocoatings for ultrashort pulse compression.” Nature Communications, 12, 1, Pp. 6518. Publisher's VersionAbstract
Transparent materials do not absorb light but have profound influence on the phase evolution of transmitted radiation. One consequence is chromatic dispersion, i.e., light of different frequencies travels at different velocities, causing ultrashort laser pulses to elongate in time while propagating. Here we experimentally demonstrate ultrathin nanostructured coatings that resolve this challenge: we tailor the dispersion of silicon nanopillar arrays such that they temporally reshape pulses upon transmission using slow light effects and act as ultrashort laser pulse compressors. The coatings induce anomalous group delay dispersion in the visible to near-infrared spectral region around 800 nm wavelength over an 80 nm bandwidth. We characterize the arrays' performance in the spectral domain via white light interferometry and directly demonstrate the temporal compression of femtosecond laser pulses. Applying these coatings to conventional optics renders them ultrashort pulse compatible and suitable for a wide range of applications.
Ahmed H Dorrah, Noah A Rubin, Michele Tamagnone, Aun Zaidi, and Federico Capasso. 10/29/2021. “Structuring total angular momentum of light along the propagation direction with polarization-controlled meta-optics.” Nature Communications, 12, Pp. 6249. Publisher's Version structuringtam_capassogroup.pdf
Ileana-Cristina Benea-Chelmus, Maryna L Meretska, Delwin L Elder, Michele Tamagnone, Larry R Dalton, and Federico Capasso. 10/11/2021. “Electro-optic spatial light modulator from an engineered organic layer.” Nature Communications, 12, 1, Pp. 5928. Publisher's Version s41467-021-26035-y.pdf
Soon Wei Daniel Lim, Maryna L Meretska, and Federico Capasso. 10/11/2021. “A High Aspect Ratio Inverse-Designed Holey Metalens.” Nano Letters. Publisher's Version acs.nanolett.1c02612.pdf
Dmitry Kazakov, Nikola Opačak, Maximilian Beiser, Alexey Belyanin, Benedikt Schwarz, Marco Piccardo, and Federico Capasso. 9/30/2021. “Defect-engineered ring laser harmonic frequency combs.” Optica, 8, 10, Pp. 1277–1280. Publisher's VersionAbstract
A monochromatic wave that circulates in a nonlinear and dispersive optical cavity can become unstable and form a structured waveform. This phenomenon, known as modulation instability, was encountered in fiber lasers, optically pumped Kerr microresonators and, most recently, in monolithic ring quantum cascade lasers (QCLs). In ring QCLs, the instability led to generation of fundamental frequency combs–-optical fields that repeat themselves once per cavity round trip. Here we show that the same instability may also result in self-starting harmonic frequency combs–-waveforms that repeat themselves multiple times per round trip, akin to perfect soliton crystals in ring Kerr microresonators. We can tailor the intermode spacing of harmonic frequency combs by placing two minute defects with a well-defined separation between them along the ring waveguide. On-demand excitation of frequency comb states with few powerful modes spaced by hundreds of gigahertz may find their use in future sub-terahertz generators.
Nikola Opačak, Florian Pilat, Dmitry Kazakov, Sandro Dal Cin, Georg Ramer, Bernhard Lendl, Federico Capasso, and Benedikt Schwarz. 9/17/2021. “Spectrally resolved linewidth enhancement factor of a semiconductor frequency comb.” Optica, 8, 9, Pp. 1227–1230. Publisher's VersionAbstract
The linewidth enhancement factor (LEF) has recently moved into the spotlight of research on frequency comb generation in semiconductor lasers. Here we present a novel modulation experiment that enables direct measurement of the spectrally resolved LEF in a laser frequency comb. By utilizing a phase-sensitive technique, we are able to extract the LEF for each individual comb mode in any laser type. We first investigate and verify this universally applicable technique using Maxwell–Bloch simulations. Following, we present the experimental demonstration on a quantum cascade laser frequency comb, confirming the predicted key role of the LEF in frequency comb dynamics.