Publications by Year: 2015

Mohammadreza Khorasaninejad, Francesco Aieta, Pritpal Kanhaiya, Mikhail A. Kats, Patrice Genevet, David Rousso, and Federico Capasso. 2015. “Achromatic Metasurface Lens at Telecommunication Wavelengths.” NANO LETTERS, 15, 8, Pp. 5358-5362. Publisher's VersionAbstract
Nanoscale optical resonators enable a new class of flat optical components called metasurfaces. This approach has been used to demonstrate functionalities such as focusing free of monochromatic aberrations (i.e., spherical and coma), anomalous reflection, and large circular dichroism. Recently, dielectric metasurfaces that compensate the phase dispersion responsible for chromatic aberrations have been demonstrated. Here, we utilize an aperiodic array of coupled dielectric nanoresonators to demonstrate a multiwavelength achromatic lens. The focal length remains unchanged for three wavelengths in the near-infrared region (1300, 1550, and 1800 nm). Experimental results are in agreement with full-wave simulations. Our findings are an essential step toward a realization of broadband flat optical elements.
Mohammadreza Khorasaninejad and Federico Capasso. 2015. “Broadband Multifunctional Efficient Meta-Gratings Based on Dielectric Waveguide Phase Shifters.” NANO LETTERS, 15, 10, Pp. 6709-6715. Publisher's VersionAbstract
Molding the wavefront of light is a basic principle of any optical design. In conventional optical components such as lenses and waveplates, the wavefront is controlled via propagation phases in a medium much thicker than the wavelength. Metasurfaces instead typically produce the required phase changes using subwavelength-sized resonators as phase shift elements patterned across a surface. This ``flat optics'' approach promises miniaturization and improved performance. Here we introduce metasurfaces which use dielectric ridge waveguides (DRWs) as phase shift elements in which the required phase accumulation is achieved via propagation over a subwavelength distance. By engineering the dispersive response of DRWs, we experimentally realize high resolving power meta-gratings with broadband (lambda = 1.2-1.7 mu m) and efficient routing (splitting and bending) into a single diffraction order, thus overcoming the limits of blazed gratings. In addition, we demonstrate polarization beam splitting capabilities with large suppression ratios.
Alejandro W. Rodriguez, Pui-Chuen Hui, David P. Woolf, Steven G. Johnson, Marko Loncar, and Federico Capasso. 2015. “Classical and fluctuation-induced electromagnetic interactions in micron-scale systems: designer bonding, antibonding, and Casimir forces.” ANNALEN DER PHYSIK, 527, 1-2, SI, Pp. 45-80. Publisher's VersionAbstract
Whether intentionally introduced to exert control over particles and macroscopic objects, such as for trapping or cooling, or whether arising from the quantum and thermal fluctuations of charges in otherwise neutral bodies, leading to unwanted stiction between nearby mechanical parts, electromagnetic interactions play a fundamental role in many naturally occurring processes and technologies. In this review, we survey recent progress in the understanding and experimental observation of optomechanical and quantum-fluctuation forces. Although both of these effects arise from exchange of electromagnetic momentum, their dramatically different origins, involving either real or virtual photons, lead to different physical manifestations and design principles. Specifically, we describe recent predictions and measurements of attractive and repulsive optomechanical forces, based on the bonding and antibonding interactions of evanescent waves, as well as predictions of modified and even repulsive Casimir forces between nanostructured bodies. Finally, we discuss the potential impact and interplay of these forces in emerging experimental regimes of micromechanical devices.
Patrice Genevet, Daniel Wintz, Antonio Ambrosio, Alan She, Romain Blanchard, and Federico Capasso. 2015. “Controlled steering of Cherenkov surface plasmon wakes with a one-dimensional metamaterial.” NATURE NANOTECHNOLOGY, 10, 9, Pp. 804-809. Publisher's VersionAbstract
In the Cherenkov effect a charged particle moving with a velocity faster than the phase velocity of light in the medium radiates light that forms a cone with a half angle determined by the ratio of the two speeds. Here, we show that by creating a running wave of polarization along a one-dimensional metallic nanostructure consisting of subwavelength-spaced rotated apertures that propagates faster than the surface plasmon polariton phase velocity, we can generate surface plasmon wakes, a two-dimensional analogue of Cherenkov radiation. The running wave of polarization travels with a speed determined by the angle of incidence and the photon spin angular momentum of the incident radiation. By changing either one of these properties we demonstrate controlled steering of the Cherenkov surface plasmon wakes.
Stefan Kalchmair, Romain Blanchard, Tobias S. Mansuripur, Guy-Mael de Naurois, Christian Pfluegl, Mark F. Witinski, Laurent Diehl, Federico Capasso, and Marko Loncar. 2015. “High tuning stability of sampled grating quantum cascade lasers.” OPTICS EXPRESS, 23, 12, Pp. 15734-15747. Publisher's VersionAbstract
Predictable tuning behavior and stable laser operation are both crucial for laser spectroscopy measurements. We report a sampled grating quantum cascade laser (QCL) with high spectral tuning stability over the entire tuning range. We have determined the minimum loss margin required to suppress undesired lasing modes in order to ensure predictable tuning behavior. We have quantified power fluctuations and drift of our devices by measuring the Allan deviation. To demonstrate the feasibility of sampled grating QCLs for high-precision molecular spectroscopy, we have built a simple transmission spectroscopy setup. Our results prove that sampled grating QCLs are suitable light sources for highly sensitive spectroscopy measurements. (C) 2015 Optical Society of America
Daniel Wintz, Patrice Genevet, Antonio Ambrosio, Alex Woolf, and Federico Capasso. 2015. “Holographic Metalens for Switchable Focusing of Surface Plasmons.” NANO LETTERS, 15, 5, Pp. 3585-3589. Publisher's VersionAbstract
Surface plasmons polaritons (SPPs) are light like waves confined to the interface between a metal and a dielectric. Excitation and control of these modes requires components such as couplers and lenses. We present the design of a new lens based on holographic principles. The key feature is the ability to switchably control SPP focusing by changing either the incident wavelength or polarization. Using phase-sensitive near-field imaging of the surface plasmon wavefronts, we have observed their switchable focusing and steering as the wavelength or polarization is changed.
Patrice Genevet and Federico Capasso. 2015. “Holographic optical metasurfaces: a review of current progress.” REPORTS ON PROGRESS IN PHYSICS, 78, 2.Abstract
In this article, we review recent developments in the field of surface electromagnetic wave holography. The holography principle is used as a tool to solve an inverse engineering problem consisting of designing novel plasmonic interfaces to excite either surface waves or free-space beams with any desirable field distributions. Leveraging on the new nanotechnologies to carve subwavelength features within the large diffracting apertures of conventional holograms, it is now possible to create binary holographic interfaces to shape both amplitude phase and polarization of light. The ability of the new generation of ultrathin and compact holographic optical devices to fully address light properties could find widespread applications in photonics.
Tobias S. Mansuripur, Guy-Mael de Naurois, Alexey Belyanin, and Federico Capasso. 2015. “Lasers with distributed loss have a sublinear output power characteristic.” OPTICA, 2, 1, Pp. 48-55. Publisher's VersionAbstract
It is a generally accepted fact of laser physics that in a homogeneously broadened gain medium, above threshold the output power of the laser grows linearly with the pump power. The derivation requires only a few simple lines in laser textbooks, and the linear growth is a direct result of the fact that above threshold, the intracavity optical intensity will increase to the point that the gain is saturated to the level of the net loss-so-called gain pinning or clamping. Such a derivation, however, assumes that the mirror loss is distributed (the approximation of uniform gain saturation) which is only a good assumption for cavities whose end mirrors have reflectivities close to one. Furthermore, in gain media with a distributed loss there is a maximum achievable intracavity intensity that in turn limits the output power. We show that the approximation of uniform gain saturation leads to output powers that violate this limit. More generally, for lasers with low mirror reflectivities that also have distributed loss, we prove that the output power grows sub-linearly with the pump power close to threshold. Furthermore, after threshold the output grows linearly, but with a slope efficiency that can be substantially smaller than predicted by the uniform gain saturation theory, with the largest deviation occurring for traveling-wave lasers and asymmetric Fabry-Perot lasers. These results are particularly applicable to semiconductor lasers, and specific applications to quantum cascade lasers are discussed. (C) 2015 Optical Society of America
Amaury Hayat, J. P. BALTHASAR MUELLER, and Federico Capasso. 2015. “Lateral chirality-sorting optical forces.” PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA, 112, 43, Pp. 13190-13194. Publisher's VersionAbstract
The transverse component of the spin angular momentum of evanescent waves gives rise to lateral optical forces on chiral particles, which have the unusual property of acting in a direction in which there is neither a field gradient nor wave propagation. Because their direction and strength depends on the chiral polarizability of the particle, they act as chirality-sorting and may offer a mechanism for passive chirality spectroscopy. The absolute strength of the forces also substantially exceeds that of other recently predicted sideways optical forces.
Mark F. Witinski, Romain Blanchard, Christian Pfluegl, Laurent Diehl, Biao Li, Benjamin Pancy, Daryoosh Vakhshoori, and Federico Capasso. 2015. “Monolithic DFB QCL array aims at handheld IR spectral analysis.” LASER FOCUS WORLD, 51, 11, Pp. 36-39. Publisher's VersionAbstract
Many QCLs combined on a single chip demonstrate fully electronic wavelength tuning for stand-off IR spectroscopy of explosives and other materials.
Francesco Aieta, Mikhail A. Kats, Patrice Genevet, and Federico Capasso. 2015. “Multiwavelength achromatic metasurfaces by dispersive phase compensation.” SCIENCE, 347, 6228, Pp. 1342-1345. Publisher's VersionAbstract
The replacement of bulk refractive optical elements with diffractive planar components enables the miniaturization of optical systems. However, diffractive optics suffers from large chromatic aberrations due to the dispersion of the phase accumulated by light during propagation. We show that this limitation can be overcome with an engineered wavelength-dependent phase shift imparted by a metasurface, and we demonstrate a design that deflects three wavelengths by the same angle. A planar lens without chromatic aberrations at three wavelengths is also presented. Our designs are based on low-loss dielectric resonators, which introduce a dense spectrum of optical modes to enable dispersive phase compensation. The suppression of chromatic aberrations in metasurface-based planar photonics will find applications in lightweight collimators for displays, as well as chromatically corrected imaging systems.
Patrick Rauter and Federico Capasso. 2015. “Multi-wavelength quantum cascade laser arrays.” LASER & PHOTONICS REVIEWS, 9, 5, Pp. 452-477.Abstract
The progress on multi-wavelength quantum cascade laser arrays in the mid-infrared is reviewed, which are a powerful, robust and versatile source for next-generation spectroscopy and stand-off detection systems. Various approaches for the array elements are discussed, from conventional distributed-feedback lasers over master-oscillator power-amplifier devices to tapered oscillators, and the performances of the different array types are compared. The challenges associated with reliably achieving single-mode operation at deterministic wavelengths for each laser element in combination with a uniform distribution of high output power across the array are discussed. An overview of the range of applications benefiting from the quantum cascade laser approach is given. The distinct and crucial advantages of arrays over external cavity quantum cascade lasers as tunable single-mode sources in the mid-infrared are discussed. Spectroscopy and hyperspectral imaging demonstrations by quantum cascade laser arrays are reviewed.
Bernhard J. Bohn, Martin Schnell, Mikhail A. Kats, Francesco Aieta, Rainer Hillenbrand, and Federico Capasso. 2015. “Near-Field Imaging of Phased Array Metasurfaces.” NANO LETTERS, 15, 6, Pp. 3851-3858. Publisher's VersionAbstract
Phased-antenna metasurfaces can impart abrupt, spatially dependent changes to the amplitude, phase, and polarization of light and thus mold wavefronts in a desired fashion. Here we present an experimental and computational near-field study of metasurfaces based on near-resonant V-shaped antennas and connect their near- and far-field optical responses. We show that far fields can be obtained from limited, experimentally obtained knowledge of the near fields, paving the way for experimental near-field characterization of metasurfaces and other optical nanostructures and prediction of their far fields from the near-field measurements.
Mikhail A. Belkin and Federico Capasso. 2015. “New frontiers in quantum cascade lasers: high performance room temperature terahertz sources.” PHYSICA SCRIPTA, 90, 11. Publisher's VersionAbstract
In the last decade quantum cascade lasers (QCLs) have become the most widely used source of mid-infrared radiation, finding large scale applications because of their wide tunability and overall high performance. However far-infrared (terahertz) QCLs have lagged behind in terms of performance and impact due to the inability so far of achieving room temperature operation. Here we review recent research that has led to a new class of QCL light sources that has overcome these limitations leading to room temperature operation in the terahertz spectral range, with nearly 2 mW of optical power and significant tunability, opening up also this region of the spectrum to a wide range of applications.
Nanfang Yu and Federico Capasso. 2015. “Optical Metasurfaces and Prospect of Their Applications Including Fiber Optics.” JOURNAL OF LIGHTWAVE TECHNOLOGY, 33, 12, SI, Pp. 2344-2358. Publisher's VersionAbstract
Metasurfaces have emerged in the recent years as a platform to design subwavelength-thick optical components (''flat optics''), which can be used to implement any optical function (beam deflection, focusing, waveplates, etc). These flat optical components can be fabricated using a single lithographic step. The approach is particularly suited for patterning nonconventional substrates, such as semiconductor laser facets and optical fiber facets. In this paper, we review recent applications of metasurfaces to flat optical devices, including their use in semiconductor lasers and fiber optics. Metasurfaces make it possible to design all properties of light (amplitude, phase, and polarization), which enable us to build a large variety of flat optical components, including planar lenses, quarter-wave plates, optical vortex plates, holograms for vector beam generation, and ultrathin perfect absorbers and color coatings. We also review flat collimating lenses integrated on the facets of mid-infrared and far-infrared (terahertz) quantum cascade lasers, and novel techniques to create large arrays of nanostructures on fiber facets.