Publications by Year: 2021

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.
Peng Lin, Wei Ting Chen, Kerolos M.A. Yousef, Justin Marchioni, Alexander Zhu, Federico Capasso, and Ji-Xin Cheng. 9/14/2021. “Coherent Raman scattering imaging with a near-infrared achromatic metalens.” APL Photonics, 6, 9, Pp. 096107. Publisher's Version 5.0059874.pdf
Noah A. Rubin*, Aun Zaidi*, Ahmed H. Dorrah, Zhujun Shi, and Federico Capasso. 8/13/2021. “Jones matrix holography with metasurfaces.” Science Advances, 7, 33, Pp. eabg7488. Publisher's Version eabg7488.full_.pdf
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.
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.
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
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.
Ileana-Cristina Benea-Chelmus, Sydney Mason, Maryna L. Meretska, Delwin L. Elder, Dmitry Kazakov, Amirhassan Shams-Ansari, Larry R. Dalton, and Federico Capasso. 2021. “Gigahertz free-space electro-optic modulators based on Mie resonances.” arXiv, 2108.03539. Publisher's Version gigahertz_free-space_electro-optic_modulators_base.pdf
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.