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1. High-resolution and compact serpentine integrated grating spectrometer

   https://doi.org/10.1364/JOSAB.423968  

   Brand, Michael; Zhang, Bohan; Onural, Deniz; Al_Qubaisi, Kenaish; Popović, Miloš; Dostart, Nathan; Wagner, Kelvin (June 2021, Journal of the Optical Society of America B)

   Integrated astrophotonic spectrometers are integrated variants of conventional free-space spectrometers that offer significantly reduced size, weight, and cost and immunity to alignment errors, and can be readily integrated with other astrophotonic instruments such as nulling interferometers. Current integrated dispersive astrophotonic spectrometers are one-dimensional devices such as arrayed waveguide gratings or planar echelle gratings. These devices have been limited to ${10}^4$resolving powers and $<<\#comment/>1000$spectral bins due to having limited total optical delay paths and 1D detector array pixel densities. In this paper, we propose and demonstrate a high-resolution and compact astrophotonic serpentine integrated grating (SIG) spectrometer design based on a 2D dispersive serpentine optical phased array. The SIG device combines a 5.2 cm long folded delay line with grating couplers to create a large optical delay path along two dimensions in a compact integrated device footprint. Analogous to free-space crossed-dispersion high-resolution spectrometers, the SIG spectrometer maps spectral content to a 2D wavelength-beam-steered folded-raster emission pattern focused onto a 2D detector array. We demonstrate a SIG spectrometer with $\sim <\#comment/>100\mathrm{k}$resolving power and $\sim <\#comment/>6750$spectral bins, which are approximately an order of magnitude higher than previous integrated photonic designs that operate over a wide bandwidth, in a $0.4{\mathrm{m}\mathrm{m}}^2$footprint. We measure a Rayleigh resolution of $1.93\pm <\#comment/>0.07\mathrm{G}\mathrm{H}\mathrm{z}$and an operational bandwidth from 1540 nm to 1650 nm. Finally, we discuss refinements of the SIG spectrometer that improve its resolution, bandwidth, and throughput. These results show that SIG spectrometer technology provides a path towards miniaturized, high-resolution spectrometers for applications in astronomy and beyond. 

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2. High-speed multiview imaging approaching 4pi steradians using conic section mirrors: theoretical and practical considerations

   https://doi.org/10.1364/JOSAA.440592  

   Zhou, Kevin_C; Dhalla, Al-Hafeez; McNabb, Ryan_P; Qian, Ruobing; Farsiu, Sina; Izatt, Joseph_A (November 2021, Journal of the Optical Society of America A)

   Illuminating or imaging samples from a broad angular range is essential in a wide variety of computational 3D imaging and resolution-enhancement techniques, such as optical projection tomography, optical diffraction tomography, synthetic aperture microscopy, Fourier ptychographic microscopy, structured illumination microscopy, photogrammetry, and optical coherence refraction tomography. The wider the angular coverage, the better the resolution enhancement or 3D-resolving capabilities. However, achieving such angular ranges is a practical challenge, especially when approaching $\pm <\#comment/>{90}^{\circ <\#comment/>}$or beyond. Often, researchers resort to expensive, proprietary high numerical aperture (NA) objectives or to rotating the sample or source-detector pair, which sacrifices temporal resolution or perturbs the sample. Here, we propose several new strategies for multiangle imaging approaching 4pi steradians using concave parabolic or ellipsoidal mirrors and fast, low rotational inertia scanners, such as galvanometers. We derive theoretically and empirically relations between a variety of system parameters (e.g.,  NA, wavelength, focal length, telecentricity) and achievable fields of view (FOVs) and importantly show that intrinsic tilt aberrations donotrestrict FOV for many multiview imaging applications, contrary to conventional wisdom. Finally, we present strategies for avoiding spherical aberrations at obliquely illuminated flat boundaries. Our simple designs allow for high-speed multiangle imaging for microscopic, mesoscopic, and macroscopic applications. 

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3. Fast extended depth of focus meta-optics for varifocal functionality

   https://doi.org/10.1364/PRJ.434681  

   Whitehead, James E. M.; Zhan, Alan; Colburn, Shane; Huang, Luocheng; Majumdar, Arka (March 2022, Photonics Research)

   Extended depth of focus (EDOF) optics can enable lower complexity optical imaging systems when compared to active focusing solutions. With existing EDOF optics, however, it is difficult to achieve high resolution and high collection efficiency simultaneously. The subwavelength spacing of scatterers in a meta-optic enables the engineering of very steep phase gradients; thus, meta-optics can achieve both a large physical aperture and a high numerical aperture. Here, we demonstrate a fast $(f/1.75)$EDOF meta-optic operating at visible wavelengths, with an aperture of 2 mm and focal range from 3.5 mm to 14.5 mm (286 diopters to 69 diopters), which is a $250\times$elongation of the depth of focus relative to a standard lens. Depth-independent performance is shown by imaging at a range of finite conjugates, with a minimum spatial resolution of $\sim 9.84\mathrm{\mu m}$(50.8 cycles/mm). We also demonstrate operation of a directly integrated EDOF meta-optic camera module to evaluate imaging at multiple object distances, a functionality which would otherwise require a varifocal lens. 

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4. Modifications of ion beam sputtered tantala thin films by secondary argon and oxygen bombardment

   https://doi.org/10.1364/AO.59.00A150  

   Yang, Le; Randel, Emmett; Vajente, Gabriele; Ananyeva, Alena; Gustafson, Eric; Markosyan, Ashot; Bassiri, Riccardo; Fejer, Martin; Menoni, Carmen (January 2020, Applied Optics)

   Amorphous tantala ( ${\mathrm{T}\mathrm{a}}_2{\mathrm{O}}_5$) thin films were deposited by reactive ion beam sputtering with simultaneous low energy assist ${\mathrm{A}\mathrm{r}}^{+}$or ${\mathrm{A}\mathrm{r}}^{+}/{\mathrm{O}}_2^{+}$bombardment. Under the conditions of the experiment, the as-deposited thin films are amorphous and stoichiometric. The refractive index and optical band gap of thin films remain unchanged by ion bombardment. Around 20% improvement in room temperature mechanical loss and 60% decrease in absorption loss are found in samples bombarded with 100-eV ${\mathrm{A}\mathrm{r}}^{+}$. A detrimental influence from low energy ${\mathrm{O}}_2^{+}$bombardment on absorption loss and mechanical loss is observed. Low energy ${\mathrm{A}\mathrm{r}}^{+}$bombardment removes excess oxygen point defects, while ${\mathrm{O}}_2^{+}$bombardment introduces defects into the tantala films. 

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5. InGaP quantum nanophotonic integrated circuits with 1.5% nonlinearity-to-loss ratio

   https://doi.org/10.1364/OPTICA.440383  

   Zhao, Mengdi; Fang, Kejie (February 2022, Optica)

   Optical nonlinearity plays a pivotal role in quantum information processing using photons, from heralded single-photon sources and coherent wavelength conversion to long-sought quantum repeaters. Despite the availability of strong dipole coupling to quantum emitters, achieving strong bulk optical nonlinearity is highly desirable. Here, we realize quantum nanophotonic integrated circuits in thin-film InGaP with, to our knowledge, a record-high ratio of $1.5\mathrm{\%<\#comment/>}$between the single-photon nonlinear coupling rate ( $g/2\mathrm{\pi <\#comment/>}=11.2\mathrm{M}\mathrm{H}\mathrm{z}$) and cavity-photon loss rate. We demonstrate second-harmonic generation with an efficiency of $71200\pm <\#comment/>10300\mathrm{\%<\#comment/>}/\mathrm{W}$in the InGaP photonic circuit and photon-pair generation via degenerate spontaneous parametric downconversion with an ultrahigh rate exceeding 27.5 MHz/µW—an order of magnitude improvement of the state of the art—and a large coincidence-to-accidental ratio up to $1.4\times <\#comment/>{10}^4$. Our work shows InGaP as a potentially transcending platform for quantum nonlinear optics and quantum information applications. 

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