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Phase contrast microscopy has played a central role in the development of modern biology, geology, and nanotechnology. It can visualize the structure of translucent objects that remains hidden in regular optical microscopes. The optical layout of a phase contrast microscope is based on a 4  f image processing setup and has essentially remained unchanged since its invention by Zernike in the early 1930s. Here, we propose a conceptually new approach to phase contrast imaging that harnesses the non-local optical response of a guided-mode-resonator metasurface. We highlight its benefits and demonstrate the imaging of various phase objects, including biological cells, polymeric nanostructures, and transparent metasurfaces. Our results showcase that the addition of this non-local metasurface to a conventional microscope enables quantitative phase contrast imaging with a 0.02π phase accuracy. At a high level, this work adds to the growing body of research aimed at the use of metasurfaces for analog optical computing. The authors present an approach to phase imaging by using the non-local optical response of a guided-moderesonator metasurface. They demonstrate that this metasurface can be added to a conventional microscope to enable quantitative phase contrast imaging.

Optical phenomena always display some degree of partial coherence between their respective degrees of freedom. Partial coherence is of particular interest in multimodal systems, where classical and quantum correlations between spatial, polarization, and spectral degrees of freedom can lead to fascinating phenomena (e.g., entanglement) and be leveraged for advanced imaging and sensing modalities (e.g., in hyperspectral, polarization, and ghost imaging). Here, we present a universal method to analyze, process, and generate spatially partially coherent light in multimode systems by using self-configuring optical networks. Our method relies on cascaded self-configuring layers whose average power outputs are sequentially optimized. Once optimized, the network separates the input light into its mutually incoherent components, which is formally equivalent to a diagonalization of the input density matrix. We illustrate our method with numerical simulations of Mach-Zehnder interferometer arrays and show how this method can be used to perform partially coherent environmental light sensing, generation of multimode partially coherent light with arbitrary coherency matrices, and unscrambling of quantum optical mixtures. We provide guidelines for the experimental realization of this method, including the influence of losses, paving the way for self-configuring photonic devices that can automatically learn optimal modal representations of partially coherent light fields.The PCLA analyzes partially coherent light by optimizing output power through a series of self-configuring Mach-Zehnder interferometers, effectively decomposing the input light field into its incoherent components.

Free-space optics naturally offers multiple-channel communications and sensing exploitable in many applications. The different optical beams will, however, generally be overlapping at the receiver, and, especially with atmospheric turbulence or other scattering or aberrations, the arriving beam shapes may not even be known in advance. We show that such beams can be still separated in the optical domain, and simultaneously detected with negligible cross-talk, even if they share the same wavelength and polarization, and even with unknown arriving beam shapes. The kernel of the adaptive multibeam receiver presented in this work is a programmable integrated photonic processor that is coupled to free-space beams through a two-dimensional array of optical antennas. We demonstrate separation of beam pairs arriving from different directions, with overlapping spatial modes in the same direction, and even with mixing between the beams deliberately added in the path. With the circuit’s optical bandwidth of more than 40 nm, this approach offers an enabling technology for the evolution of FSO from single-beam to multibeam space-division multiplexed systems in a perturbed environment, which has been a game-changing transition in fiber-optic systems.We show that a programmable photonic integrated processor can separate, directly in the optical domain, spatially-overlapped free-space optical beams with unknown shapes, sharing the same wavelength and polarization.

Germanium is an attractive material for silicon-compatible optoelectronics, but in its bulk form it does not emit light efficiently because of its indirect bandgap. Applying tensile strain to germanium modifies its band structure such that radiative recombination is enhanced, leading to improved light emission. Here, we introduce the ‘suspension platform for optoelectronics under tension’, a micromachining-based technology that applies large, locally tunable tensile strains to suspended device layers. Using this approach, we demonstrate dramatically enhanced light emission from uniaxially and biaxially tensile-strained germanium-on-insulator device layers. Photoluminescence enhanced by a factor of 130 at a wavelength of 1,550 nm and integrated enhancement by greater than a factor of 260 over bulk germanium are described. The emission exhibits a superlinear dependence on optical pump power. We also report preliminary evidence for enhanced electroluminescence from suspended germanium-on-insulator light-emitting diodes. Scientists employ a micromachining-based technology to achieve significant enhancements in light emission from highly strained germanium-on-insulator samples.

A light at the end of the chip Silicon chips dominate electronics while optical fibres dominate long-distance information transfer. Recent work, in search of the best of both worlds, has led to silicon devices capable of modulating light; these show promise but still rely on weak physical mechanisms found in silicon itself. Now a team working at Stanford University and at Hewlett-Packard's Palo Alto labs has developed thin germanium ‘quantum well’ nanostructures grown on silicon that generate a strong quantum-mechanical effect capable of turning light beams on and off. Their performance rivals the best seen in any material. This development may allow silicon/germanium chips to handle both electronics and optics, uniting computing and communications at the integrated chip level. Silicon is the dominant semiconductor for electronics, but there is now a growing need to integrate such components with optoelectronics for telecommunications and computer interconnections 1 . Silicon-based optical modulators have recently been successfully demonstrated 2 , 3 ; but because the light modulation mechanisms in silicon 4 are relatively weak, long (for example, several millimetres) devices 2 or sophisticated high-quality-factor resonators 3 have been necessary. Thin quantum-well structures made from III-V semiconductors such as GaAs, InP and their alloys exhibit the much stronger quantum-confined Stark effect (QCSE) mechanism 5 , which allows modulator structures with only micrometres of optical path length 6 , 7 . Such III-V materials are unfortunately difficult to integrate with silicon electronic devices. Germanium is routinely integrated with silicon in electronics 8 , but previous silicon–germanium structures have also not shown strong modulation effects 9 , 10 , 11 , 12 , 13 . Here we report the discovery of the QCSE, at room temperature, in thin germanium quantum-well structures grown on silicon. The QCSE here has strengths comparable to that in III-V materials. Its clarity and strength are particularly surprising because germanium is an indirect gap semiconductor; such semiconductors often display much weaker optical effects than direct gap materials (such as the III-V materials typically used for optoelectronics). This discovery is very promising for small, high-speed 14 , low-power 15 , 16 , 17 optical output devices fully compatible with silicon electronics manufacture.

There has been significant recent interest in synthetic dimensions, where internal degrees of freedom of a particle are coupled to form higher-dimensional lattices in lower-dimensional physical structures. For these systems, the concept of band structure along the synthetic dimension plays a central role in their theoretical description. Here we provide a direct experimental measurement of the band structure along the synthetic dimension. By dynamically modulating a resonator at frequencies commensurate with its mode spacing, we create a periodically driven lattice of coupled modes in the frequency dimension. The strength and range of couplings can be dynamically reconfigured by changing the modulation amplitude and frequency. We show theoretically and demonstrate experimentally that time-resolved transmission measurements of this system provide a direct readout of its band structure. We also realize long-range coupling, gauge potentials and nonreciprocal bands by simply incorporating additional frequency drives, enabling great flexibility in band structure engineering. Internal degrees of freedom allow to expand the effective dimensionality of a system along “synthetic” dimensions. Here, the authors demonstrate this by modulating a ring resonator at frequencies commensurate with its mode spacing, and are able to directly measure its synthetic band structure.

The use of optical interconnects has burgeoned as a promising technology that can address the limits of data transfer for future high-performance silicon chips. Recent pushes to enhance optical communication have focused on developing wavelength-division multiplexing technology, and new dimensions of data transfer will be paramount to fulfill the ever-growing need for speed. Here we demonstrate an integrated multi-dimensional communication scheme that combines wavelength- and mode- multiplexing on a silicon photonic circuit. Using foundry-compatible photonic inverse design and spectrally flattened microcombs, we demonstrate a 1.12-Tb/s natively error-free data transmission throughout a silicon nanophotonic waveguide. Furthermore, we implement inverse-designed surface-normal couplers to enable multimode optical transmission between separate silicon chips throughout a multimode-matched fibre. All the inverse-designed devices comply with the process design rules for standard silicon photonic foundries. Our approach is inherently scalable to a multiplicative enhancement over the state of the art silicon photonic transmitters. The authors demonstrate a multi-dimensional communication scheme that combines wavelength- and mode- multiplexing on photonic integrated circuits using foundry-compatible photonic inverse design and spectrally flattened microcombs

A critical challenge for the convergence of optics and electronics is that the micrometre scale of optics is significantly larger than the nanometre scale of modern electronic devices. In the conversion from photons to electrons by photodetectors, this size incompatibility often leads to substantial penalties in power dissipation, area, latency and noise 1 , 2 , 3 , 4 . A photodetector can be made smaller by using a subwavelength active region; however, this can result in very low responsivity because of the diffraction limit of the light. Here we exploit the idea of a half-wave Hertz dipole antenna (length ∼ 380 nm) from radio waves, but at near-infrared wavelengths (length ∼ 1.3 µm), to concentrate radiation into a nanometre-scale germanium photodetector. This gives a polarization contrast of a factor of 20 in the resulting photocurrent in the subwavelength germanium element, which has an active volume of 0.00072 µm 3 , a size that is two orders of magnitude smaller than previously demonstrated detectors at such wavelengths. By scaling down device size, the principles of radio antennas can be used in the optical regime. These optical antennas act as a bridge between optics and electronics, collecting and enhancing light to enable the creation of tiny semiconductor photodetectors.

Controlled interference can separate overlapping light beams for device functionality When light beams become mixed up, can we sort them out again? Some cases are easy. The light from two flashlights on the other side of a room overlaps when it reaches us, but the lens in our eye separates them again as it constructs an image. By turning the flashlights on and off, we could also communicate two independent, spatial channels of information to different detectors at the back of our eyes.

Propagation of light beams through scattering or multimode systems may lead to the randomization of the spatial coherence of the light. Although information is not lost, its recovery requires a coherent interferometric reconstruction of the original signals, which have been scrambled into the modes of the scattering system. Here we show that we can automatically unscramble optical beams that have been arbitrarily mixed in a multimode waveguide, undoing the scattering and mixing between the spatial modes through a mesh of silicon photonics tuneable beam splitters. Transparent light detectors integrated in a photonic chip are used to directly monitor the evolution of each mode along the mesh, allowing sequential tuning and adaptive individual feedback control of each beam splitter. The entire mesh self-configures automatically through a progressive tuning algorithm and resets itself after significantly perturbing the mixing, without turning off the beams. We demonstrate information recovery by the simultaneous unscrambling, sorting and tracking of four mixed modes, with residual cross-talk of −20 dB between the beams. Circuit partitioning assisted by transparent detectors enables scalability to meshes with a higher port count and to a higher number of modes without a proportionate increase in the control complexity. The principle of self-configuring and self-resetting in optical systems should be applicable in a wide range of optical applications. Silicon photonics: unscrambling scrambled light A silicon photonics chip featuring a mesh of tunable beam splitters can unscramble mode mixing that occurs in multimode waveguides. Scattering or multimode systems can randomize the spatial coherence of light beams. Francesco Morichetti and co-workers from Politecnico di Milano, Italy, and Stanford University, USA, have fabricated a chip-based descrambler that can automatically unscramble optical beams. A progressive tuning algorithm that monitors the output of the chip enables the mesh to self-configure so that it can unscramble and sort different spatial modes. In a demonstration of the device, four optical beams containing mixed modes were unmixed and separated into outputs with a residual crosstalk of less than −20 dB between the modes. The approach is scalable to a higher number of modes and is promising for optical communication systems employing mode division multiplexing.