Dispersion Engineered Metasurfaces for Broadband, High-NA, High-Efficiency, Dual-Polarization Analog Image Processing

Analog computing and image processing with optical metasurfaces holds a great potential for increasing processing speeds and reducing power consumption. Among different functionalities, spatial differentiation and edge detection have recently attracted much interest in this context. While a few demonstrations have achieved analog edge detection with compact metasurfaces, current approaches often suffer from trade-offs in terms of spatial resolution, overall throughput, polarization asymmetry, operational bandwidth and isotropy. Here, we exploit dispersion engineering to design and realize metasurfaces capable of performing isotropic 2D edge detection over a broad operational bandwidth and for any input polarization, while simultaneously maintaining high numerical aperture and record efficiency. Remarkably, we show that this performance can be achieved within a single-layer metasurface consisting of a silicon photonic crystal on glass. We demonstrate metasurfaces performing isotropic dual-polarization edge-detection with numerical apertures larger than 0.35, and operating within a spectral bandwidth of 35 nm (5 THz) around 1500 nm. Moreover, we introduce quantitative metrics to properly assess the efficiency of the analog image processing. Thanks to the low insertion loss and the dual-polarization response, our metasurface provides edge-enhanced images with high efficiency and contrast across a broad operational bandwidth and for arbitrary input polarization. Remarkably, the experimentally measured efficiencies are very close to the ones of any ideal passive edge-detector device with a given NA, and they are in fact comparable to the efficiency obtained by performing analytically the Laplacian mathematical operation on the input images. Our results pave the way for the application of metasurfaces for low-loss, high-efficiency and broadband optical computing and image processing.Comment: 15 pages, 5 figures in main text. 9 pages, 7 figures in supplemental materia

Polarization Imaging and Edge Detection with Image-Processing Metasurfaces

Optical metasurfaces have been recently explored as ultrathin analog image differentiators. By tailoring the momentum transfer function, they can perform efficient Fourier-filtering - and thus potentially any linear mathematical operation - on an input image, replacing bulky 4f systems. While this approach has been investigated in different platforms, and several techniques have been explored to achieve the required angular response, little effort has been devoted so far to tailor and control also the polarization response of an image-processing metasurface. Here, we show that edge-detection metasurfaces can be designed with tailored polarization responses while simultaneously preserving an isotropic response. In particular, we demonstrate single-layer silicon metasurfaces yielding efficient Laplacian operation on a 2D image with either large polarization asymmetry, or nearly polarization-independent response. In the former case, we show that a strongly asymmetric polarization response can be used to unlock more sophisticated on-the-fly image processing functionalities, such as dynamically tunable direction-dependent edge detection. In parallel, metasurfaces with dual-polarized response are shown to enable efficient operation for unpolarized or arbitrarily polarized images, ensuring high efficiency. For both devices, we demonstrate edge detection within relatively large numerical apertures, with excellent isotropy and intensity throughput. Our study paves the way for the broad use of optical metasurfaces for sophisticated, massively parallel analog image processing with zero energy requirements.Comment: 8 pages, 4 figures in main text. 6 pages, 3 figures in supplemental materia

Reconfigurable Image Processing Metasurfaces with Phase-Change Materials

Optical metasurfaces have been enabling reduced footprint and power consumption, as well as faster speeds, in the context of analog computing and image processing. While various image processing and optical computing functionalities have been recently demonstrated using metasurfaces, most of the considered devices are static and lack reconfigurability. Yet, the ability to dynamically reconfigure processing operations is key for metasurfaces to be able to compete with practical computing systems. Here, we demonstrate a passive edge-detection metasurface operating in the near-infrared regime whose image processing response can be drastically modified by temperature variations smaller than 10{\deg} C around a CMOS-compatible temperature of 65{\deg} C. Such reconfigurability is achieved by leveraging the insulator-to-metal phase transition of a thin buried layer of vanadium dioxide which, in turn, strongly alters the nonlocal response of the metasurface. Importantly, this reconfigurability is accompanied by performance metrics - such as high numerical aperture, high efficiency, isotropy, and polarization-independence - close to optimal, and it is combined with a simple geometry compatible with large-scale manufacturing. Our work paves the way to a new generation of ultra-compact, tunable, passive devices for all-optical computation, with potential applications in augmented reality, remote sensing and bio-medical imaging.Comment: Main text (18 pages, 5 figures), followed by high-resolution vector-graphic versions of the figures and by the Supplementary Informatio

Integrated nano-opto-electro-mechanical sensor for spectrometry and nanometrology

Spectrometry is widely used for the characterization of materials, tissues, and gases, and the need for size and cost scaling is driving the development of mini and microspectrometers. While nanophotonic devices provide narrowband filtering that can be used for spectrometry, their practical application has been hampered by the difficulty of integrating tuning and read-out structures. Here, a nano-opto-electro-mechanical system is presented where the three functionalities of transduction, actuation, and detection are integrated, resulting in a high-resolution spectrometer with a micrometer-scale footprint. The system consists of an electromechanically tunable double-membrane photonic crystal cavity with an integrated quantum dot photodiode. Using this structure, we demonstrate a resonance modulation spectroscopy technique that provides subpicometer wavelength resolution. We show its application in the measurement of narrow gas absorption lines and in the interrogation of fiber Bragg gratings. We also explore its operation as displacement-to-photocurrent transducer, demonstrating optomechanical displacement sensing with integrated photocurrent read-out

Dispersion Engineered Metasurfaces for Broadband, High-NA, High-Efficiency, Dual-Polarization Analog Image Processing

<p>This dataset contains tabulated versions of the experimental data shown in the pictures of the publication "Dispersion Engineered Metasurfaces for Broadband, High-NA, High-Efficiency, Dual-Polarization Analog Image Processing"</p><p><strong>Figure 2</strong></p><ul><li>The file Fig_2_tpp.xlsx contains the raw data of Fig. 2g. The first sheet contains a 31 x 121 matrix. The second sheet contains a column vector with 121 entries, corresponding to the wavelength axis. The third sheet contains a column vector with 31 entries, corresponding to the angle theta axis.</li><li>The file Fig_2_tss.xlsx contains the raw data of Fig. 2i. The first sheet contains a 31 x 121 matrix. The second sheet contains a column vector with 121 entries, corresponding to the wavelength axis. The third sheet contains a column vector with 31 entries, corresponding to the angle theta axis.</li><li>The data of the plots in Fig. 2h and 2j can be extracted from the corresponding row/columns in the files Fig_2_tpp.xlsx and  Fig_2_tss.xlsx</li></ul><p><strong>Figure 4</strong></p><ul><li>The file Fig_4b_MSoff.xlsx contains the raw data of the first panel ('NO Metasurface') of Fig. 4b</li><li>The files Fig_4b_MSon_X.xlsx, with X = [1435, 1445, 1450, 1452, 1455:5:1480, 1483, 1485, 1490, 1495], contain the raw data of the other panels of Fig. 4b.</li><li>For all panels of Fig. 4b, each pixel corresponds to 0.2864 microns.</li><li>The data of the plots in Fig. 4c can be extracted from the corresponding row/columns of the plots in Fig. 4b</li></ul><p><strong>Figure 5</strong></p><ul><li>The file Fig_5a.xlsx contains the spectra shown in Fig. 5a</li><li>The files Fig_5b_MSoff.xlsx, Fig_5b_MSon_xpol.xlsx, Fig_5b_MSon_ypol.xlsx, Fig_5b_MSon_+45.xlsx, Fig_5b_MSon_-45.xlsx, Fig_5b_MSon_Lpol.xlsx, Fig_5b_MSon_Rpol.xlsx contains the raw data of the panels b, c, d, e, f, g, h, respectively</li><li>For all 2D images in Fig. 5, each pixel corresponds to 0.2864 microns.</li><li>The data of the plots in Fig. 5i can be extracted from the corresponding row/columns of the plots in Figs. 5c-5h</li></ul&gt

Spontaneous emission from dipole-forbidden transitions in semiconductor quantum dots

We theoretically investigate the multipolar effects on the dipole-forbidden transitions of a semiconductor quantum dot. An approximated expression for the decay rate of these transitions is derived. Unlike the general theory of the spontaneous emission beyond the dipole approximation, the distinct roles of the emitter and the vacuum electric field in the transition rate are here clearly recognizable and can be separately optimized. We illustrate the potential of this formalism by calculating the spontaneous emission decay rate of an InAs/GaAs quantum dot embedded into two realistic nanostructures—an L3 photonic crystal cavity and a plasmonic dimer antenna. The obtained results show that, although the two structures provide an enhancement of the same order of magnitude, the plasmonic antenna constitutes a more promising candidate for the experimental observation of the dipole-forbidden transitions of a quantum dot