We report the optical constants of graphene oxide and reduced graphene oxide determined by spectroscopic ellipsometry. The dynamic changes in optical properties and thickness of a drop-cast graphene oxide layer during reduction by long-term exposure to focused broad-band white light are monitored in situ. The anisotropic optical constants of the graphene oxide layer and the isotropically averaged optical constants of the reduced layer are precisely determined from a multiple-location analysis of spatially resolved data across the exposed location and a multiple-time-step analysis of the dynamic data, respectively. Observed inter-band transitions in the graphene oxide layer are discussed in relation to theoretical predictions for different coverage levels of the graphene oxide sheets with oxygen containing functional groups. The derived optical constants of the reduced graphene oxide layer are compared to reported values of graphene and thermally reduced graphene oxide. (C) 2017 Elsevier B.V. All rights reserved.

In-line polarimeters perform nonterminating measurements of the polarization state of light by sampling only a small part of the total light intensity. In-line polarimeters are used in applications such as polarization state generators and in optical communications. Current polarimeters use multiple optical components in sequence for polarization analysis and therefore often become bulky and expensive. Here, we experimentally demonstrate the operation of compact fiber-coupled polarimeters with high sampling rates, operating at telecom wavelengths, each polarimeter comprising a single ultra-thin metasurface aligned to four photodetectors. We compare two configurations of such metasurface polarimeters, with in-plane and out-of-plane detection, respectively. The metasurface polarimeters reported here show excellent agreement with commercial polarimeters and cover a bandwidth of at least 100 nm. (C) 2017 Optical Society of America

This article reviews recent progress leading to the realization of planar optical components made of a single layer of phase shifting nanostructures. After introducing the principles of planar optics and discussing earlier works on sub-wavelength diffractive optics, we introduce a classification of metasurfaces based on their different phase mechanisms and profiles and a comparison between plasmonic and dielectric metasurfaces. We place particular emphasis on the recent developments on electric and magnetic field control of light with dielectric nanostructures and highlight the physical mechanisms and designs required for efficient all-dielectric metasurfaces. Practical devices of general interest such as metalenses, beam deflectors, holograms, and polarizing interfaces are discussed, including high-performance metalenses at visible wavelengths. Successful strategies to achieve achromatic response at selected wavelengths and near unity transmission/reflection efficiency are discussed. Dielectric metasurfaces and dispersion management at interfaces open up technology opportunities for applications including wavefront control, lightweight imaging systems, displays, electronic consumer products, and conformable and wearable optics. (c) 2017 Optical Society of America.

Structural colors have drawn wide attention for their potential as a future printing technology for various applications, ranging from biomimetic tissues to adaptive camouflage materials. However, an efficient approach to realize robust colors with a scalable fabrication technique is still lacking, hampering the realization of practical applications with this platform. Here, we develop a new approach based on large-scale network metamaterials that combine dealloyed subwavelength structures at the nanoscale with lossless, ultra-thin dielectric coatings. By using theory and experiments, we show how subwavelength dielectric coatings control a mechanism of resonant light coupling with epsilon-near-zero regions generated in the metallic network, generating the formation of saturated structural colors that cover a wide portion of the spectrum. Ellipsometry measurements support the efficient observation of these colors, even at angles of 70 degrees. The network-like architecture of these nanomaterials allows for high mechanical resistance, which is quantified in a series of nano-scratch tests. With such remarkable properties, these metastructures represent a robust design technology for real-world, large-scale commercial applications.

Optical frequency combs(1,2) establish a rigid phase-coherent link between microwave and optical domains and are emerging as high-precision tools in an increasing number of applications(3). Frequency combs with large intermodal spacing are employed in the field of microwave photonics for radiofrequency arbitrary waveform synthesis(4,5) and for the generation of terahertz tones of high spectral purity in future wireless communication networks(6,7). Here, we demonstrate self–starting harmonic frequency comb generation with a terahertz repetition rate in a quantum cascade laser. The large intermodal spacing caused by the suppression of tens of adjacent cavity modes originates from a parametric contribution to the gain due to temporal modulations of population inversion in the laser(8,9). Using multiheterodyne self-detection, the mode spacing of the harmonic comb is shown to be-uniform to within 5 x 10(-12) parts of the central frequency. This new -harmonic comb state extends the range of applications of quantum cascade laser frequency combs(10-13).

The measured emission wavelengths of AlInAs/GaInAs/InP quantum cascade lasers (QCLs) grown by metal organic vapor phase epitaxy (MOVPE) have been reported to be similar to 0.5-1 mu m longer than the designed QCL wavelength. This work clarifies the origin of the red-shifted wavelength. It was found that AlInAs/GaInAs heterointerfaces are compositionally graded over similar to 2.5-4.5 nm, and indium accumulates at the AlInAs-toGainAs interface. Thus, the as-grown QCLs are far from the ideal abrupt interfaces used in QCL modeling. When graded layers are incorporated in QCL band structure and wavefunction calculations, the emission wavelengths are red shifted. Furthermore, we demonstrate that QCLs with graded interfaces can be designed without compromising performance and show greatly improved correlation between designed and measured emission wavelength. QCLs were designed for emission between 7.5 and 8.5 mu m. These structures were grown and wet-etched ridge devices were fabricated. The QCLs exhibit room temperature peak powers exceeding 900 mW and pulsed efficiencies of similar to 8 to 10%.

We would like to clarify our paper [Opt. Express 25, 377 (2017)] abstract sentence ``These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront.'' (C) 2017 Optical Society of America

Vortex beams are characterized by a helical wavefront and a phase singularity point on the propagation axis that results in a doughnut-like intensity profile. These beams carry orbital angular momentum proportional to the number of intertwined helices constituting the wavefront. Vortex beams have many applications in optics, such as optical trapping, quantum optics and microscopy. Although beams with such characteristics can be generated holographically, spin-to-orbital angular momentum conversion has attracted considerable interest as a tool to create vortex beams. In this process, the geometrical phase is exploited to create helical beams whose handedness is determined by the circular polarization (left/right) of the incident light, that is by its spin. Here we demonstrate high-efficiency Spin-to- Orbital angular momentum-Converters (SOCs) at visible wavelengths based on dielectric metasurfaces. With these SOCs we generate vortex beams with high and fractional topological charge and show for the first time the simultaneous generation of collinear helical beams with different and arbitrary orbital angular momentum. This versatile method of creating vortex beams, which circumvents the limitations of liquid crystal SOCs and adds new functionalities, should significantly expand the applications of these beams. (C) 2017 Optical Society of America

Conventional compact spectrometers have a fixed spectral resolution and cannot resolve the polarization properties of light without additional optical elements, while their larger counterparts are bulky and costly. Here, we demonstrate multiple offaxis meta-lenses in the visible integrated on a single planar substrate. They possess both focusing and strongly dispersive properties and are designed to provide different spectral resolutions as well as working wavelength ranges on the same chip. We realize a compact spectrometer using only these meta-lenses and a CMOS camera and achieve detector-limited spectral resolutions as small as 0.3 nm and a total working wavelength range exceeding 170 nm for a beam propagation length of only a few cm. In addition, this spectrometer has the capability to resolve different helicities of light in a single measurement. This chip-camera setup represents the most compact configuration so far achieved for a spectrometer with similar performance and functionality, and its compatibility with large-scale fabrication processes makes it broadly applicable. (C) 2017 Author(s).

We present recent advances in metasurface-based photonics, which enables the realization of high performance planar lenses (metalenses) in the visible spectrum. They are enabled by a technique based on atomic layer deposition of titanium dioxide allowing for the fabrication of nanostructures with high fidelity. First, we demonstrate highly efficient metalenses with numerical aperture NA = 0.8 using the Pancharatnam-Berry phase approach. These metalenses can focus light into a diffraction-limited spot. They have efficiencies as high as 86% and provide high imaging resolution. Furthermore, by judicious design of the phase-shifting elements, we achieve a multispectral chiral metalens realized with a single metasurface layer. This chiral metalens can resolve both the chiral and spectral information of an object without the requirement of any additional optical components. Finally, we discuss the experimental realization of polarization-insensitive metalenses with NAs as high as 0.85. They are able to focus incident light to a spot as small as similar to 0.64 lambda with efficiencies up to 60%. Due to its straightforward and CMOS-compatible fabrication, this platform is promising for a wide range of applications ranging from camera modules, displays, laser-based imaging, microscopy, and spectroscopy to laser fabrication and lithography.

Bifunctional active regions, capable of light generation and detection at the same wavelength, allow a straightforward realization of the integrated mid-infrared photonics for sensing applications. Here, we present a high performance bifunctional device for 8 pm capable of 1 W single facet continuous wave emission at 15 degrees C. Apart from the general performance benefits, this enables sensing techniques which rely on continuous wave operation, for example, heterodyne detection, to be realized within a monolithic platform and demonstrates that bifunctional operation can be realized at longer wavelength, where wavelength matching becomes increasingly difficult and that the price to be paid in terms of performance is negligible. In laser operation, the device has the same or higher efficiency compared to the best lattice-matched QCLs without same wavelength detection capability, which is only 30% below the record achieved with strained material at this wavelength.