3451726.pdf

supplementary material

Supplementary document for Rewritable Photonic Integrated Circuits Using Dielectric-assisted Phase-change Material Waveguides - 6359778.pdf

Document containing details of the simulatio

Low-loss multilevel operation using lossy PCM-integrated silicon photonics

Chalcogenide phase-change materials (PCMs) offer new paradigms for programmable photonic integrated circuits (PICs) thanks to their zero static energy and significant refractive index contrast. However, prototypical PCMs, such as GeSbTe (GST), are lossy in their crystalline phase, albeit transparent in the amorphous state. Moreover, electrically switching PCMs to intermediate states is a stochastic process, limiting programming accuracy. As a result, achieving both low-loss and deterministic multi-level operation with GST remains challenging. Although low-loss PCMs, such as Sb2S3 and Sb2Se3, have been discovered in recent years, they are much less technologically mature. In this work, we propose a design with multiple GST segments to overcome the challenge of deterministic multilevel operation. GST segments are individually controlled by interleaved silicon PIN diode heaters in a binary but reliable fashion, and multiple levels are encoded in their phase sequence. A 1 x 1 programmable unit with two unequal GST segments is experimentally demonstrated, showcasing four distinct operation levels and negligible thermal crosstalk with only one pair of metal contacts. We then extend the design to 1 x 2 and 2 x 2 programmable units. For the 2 x 2 programmable unit design, we propose a phase-detuned three-waveguide directional coupler structure to mitigate the absorption and radiation loss, showing < -1.2 dB loss and three splitting ratios. Our work provides a new path toward low-loss and multi-level optical switches using lossy PCMs

Quantitative Phase Imaging with a Metalens

Quantitative phase imaging (QPI) recovers the exact wavefront of light from the intensity measured by a camera. Topographical maps of translucent microscopic bodies can be extracted from these quantified phase shifts. We demonstrate quantitative phase imaging at the tip of an optical fiber endoscope with a chromatic silicon nitride metalens. Our method leverages spectral multiplexing to recover phase from multiple defocus planes in a single capture. The half millimeter wide metalens shows phase imaging capability with a 280 field of view and 0.1{\lambda} sensitivity in experiments with an endoscopic fiber bundle. Since the spectral functionality is encoded directly in the imaging lens, no additional filters are needed. Key limitations in the scaling of a phase imaging system, such as multiple acquisition, interferometric alignment or mechanical scanning are completely mitigated in the proposed schem

Spectrally Encoded Nonscanning Imaging through a Fiber

Recent advancements in neuroimaging and microsurgery have sparked an increasing demand to capture images with miniaturized optical probes such as optical fibers. In this work, we present an approach to acquire images through a single fiber without the need for mechanical scanning. At the distal end of the fiber, a metasurface filter array encodes spatial information into a highly orthogonal spectrum. At the proximal end, the object can then be computationally recovered via the pseudo inverse of the encoding process. We demonstrate captures of a 4 × 4 binary object at the proximity of the spectral filter array using a 560–625 nm wavelength band. The recovered image maintains an error rate of <11% when measured using a spectrometer with a spectral resolution of 1.5 nm. Importantly, this modality remains unchanged as the fiber is bent or moved. Thus, our approach shows a robust way to image through a single optical fiber, with potential applications in compact endoscopes and angioscopes

Low-Contrast Dielectric Metasurface Optics

The miniaturization of current image sensors is largely limited by the volume of the optical elements. Using a subwavelength-patterned quasi-periodic structure, also known as a metasurface, one can build planar optical elements based on the principle of diffraction. Recent demonstrations of high-quality metasurface optical elements are mostly based on high-refractive-index materials. Here, we present a design of low-contrast metasurface-based optical elements. We fabricate and experimentally characterize several silicon nitride-based lenses and vortex beam generators. The fabricated lenses achieved beam spots of less than 1 μm with numerical apertures as high as ∼0.75. We observed a transmission efficiency of 90% and focusing efficiency of 40% in the visible regime. Our results pave the way toward building low-loss metasurface-based optical elements at visible frequencies using low-contrast materials and extend the range of prospective material systems for metasurface optics

Encapsulated Silicon Nitride Nanobeam Cavity for Hybrid Nanophotonics

Most existing implementations of silicon nitride photonic crystal cavities rely on suspended membranes due to their low refractive index. Such floating membranes are not mechanically robust, making them suboptimal for developing a hybrid optoelectronic platform where new materials, such as layered 2D materials, are transferred onto prefabricated optical cavities. To address this issue, we design and fabricate a silicon nitride nanobeam resonator where the silicon nitride membrane is encapsulated by material with a refractive index of ∼1.5, such as silicon dioxide or PMMA. The theoretically calculated quality factor of the cavities can be as large as 10⁵, with a mode-volume of ∼2.5­(λ/<i>n</i>)³. We fabricated the cavity and measured the transmission spectrum with the highest quality factor reaching 7000. We also successfully transferred monolayer tungsten diselenide on the encapsulated silicon nitride nanobeam and demonstrated coupling of the cavity with both the monolayer exciton and the defect emissions

Nanocavity Integrated van der Waals Heterostructure Light-Emitting Tunneling Diode

Developing a nanoscale, integrable, and electrically pumped single mode light source is an essential step toward on-chip optical information technologies and sensors. Here, we demonstrate nanocavity enhanced electroluminescence in van der Waals heterostructures (vdWhs) at room temperature. The vertically assembled light-emitting device uses graphene/boron nitride as top and bottom tunneling contacts and monolayer WSe₂ as an active light emitter. By integrating a photonic crystal cavity on top of the vdWh, we observe the electroluminescence is locally enhanced (>4 times) by the nanocavity. The emission at the cavity resonance is single mode and highly linearly polarized (84%) along the cavity mode. By applying voltage pulses, we demonstrate direct modulation of this single mode electroluminescence at a speed of ∼1 MHz, which is faster than most of the planar optoelectronics based on transition metal chalcogenides (TMDCs). Our work shows that cavity integrated vdWhs present a promising nanoscale optoelectronic platform

Supplementary document for Deeply Subwavelength Integrated Excitonic van der Waals Nanophotonics - 6601887.pdf

supplemental informatio

Beating bandwidth limits for large aperture broadband nano-optics

Flat optics have been proposed as an attractive approach for the implementation of new imaging and sensing modalities to replace and augment refractive optics. However, chromatic aberrations impose fundamental limitations on diffractive flat optics. As such, true broadband high-quality imaging has thus far been out of reach for low f-number, large aperture, flat optics. In this work, we overcome these intrinsic fundamental limitations, achieving broadband imaging in the visible wavelength range with a flat meta-optic, co-designed with computational reconstruction. We derive the necessary conditions for a broadband, 1 cm aperture, f/2 flat optic, with a diagonal field of view of 30{\deg} and an average system MTF contrast of 30% or larger for a spatial frequency of 100 lp/mm in the visible band (> 50 % for 70 lp/mm and below). Finally, we use a coaxial, dual-aperture system to train the broadband imaging meta-optic with a learned reconstruction method operating on pair-wise captured imaging data. Fundamentally, our work challenges the entrenched belief of the inability of capturing high-quality, full-color images using a single large aperture meta-optic