Optofluidic Accumulation of Silica Beads on Gel-Based Three-Dimensional SERS Substrate To Enhance Sensitivity Using Multiple Photonic Nanojets.

This paper presents a gel-based three-dimensional (3D) substrate for surface-enhanced Raman spectroscopy (SERS) mediated by photonic nanojets (PNJs) to enhance the sensitivity of SERS detection. The porous structure of the gel-based substrate allowed small molecules to diffuse into the substrate, while the placement of silica beads on the substrate surface resulted in the generation of photonic nanojets during SERS measurements. Because the gel-based SERS substrate had electromagnetic (EM) hot spots along the Z-direction for several tens of microns, the focuses of the PNJs, which were located a few microns away from the substrate surface, could excite the EM hot spots located within the substrate. Our objective was to maximize SERS signal intensity by coating the substrate with a close-packed array of silica beads to enable the generation of multiple PNJs. The bead array was formed using an optical fiber decorated with gold nanorods (AuNRs) to create a temperature gradient in a mixture containing silica beads, thereby enabling their arrangement and deposition in arbitrary locations across the substrate. In experiments, the Raman enhancement provided by multiple PNJs significantly exceeded that provided by single PNJs. The proposed PNJ-mediated SERS method reduced the limit of detection for malachite green by 100 times, compared to SERS results obtained using the same substrate without beads. The proposed enhancement scheme using a gel-based 3D SERS substrate with a close-packed array of silica beads could be utilized to achieve high-sensitivity SERS detection for a variety of molecules in a diverse range of applications.

Metasurfaces on silicon photonic waveguides for simultaneous emission phase and amplitude control.

Chip-scale photonic systems that manipulate free-space emission have recently attracted attention for applications such as free-space optical communications and solid-state LiDAR. Silicon photonics, as a leading platform for chip-scale integration, needs to offer more versatile control of free-space emission. Here we integrate metasurfaces on silicon photonic waveguides to generate free-space emission with controlled phase and amplitude profiles. We demonstrate experimentally structured beams, including a focused Gaussian beam and a Hermite-Gaussian TEM10 beam, as well as holographic image projections. Our approach is monolithic and CMOS-compatible. The simultaneous phase and amplitude control enable more faithful generation of structured beams and speckle-reduced projection of holographic images.

I-line photolithographic metalenses enabled by distributed optical proximity correction with a deep-learning model.

High pattern fidelity is paramount to the performance of metalenses and metasurfaces, but is difficult to achieve using economic photolithography technologies due to low resolutions and limited process windows of diverse subwavelength structures. These hurdles can be overcome by photomask sizing or reshaping, also known as optical proximity correction (OPC). However, the lithographic simulators critical to model-based OPC require precise calibration and have not yet been specifically developed for metasurface patterning. Here, we demonstrate an accurate lithographic model based on Hopkin's image formulation and fully convolutional networks (FCN) to control the critical dimension (CD) patterning of a near-infrared (NIR) metalens through a distributed OPC flow using i-line photolithography. The lithographic model achieves an average ΔCD/CD = 1.69% due to process variations. The model-based OPC successfully produces the 260 nm CD in a metalens layout, which corresponds to a lithographic constant k1 of 0.46 and is primarily limited by the resolution of the photoresist. Consequently, our fabricated NIR metalens with a diameter of 1.5 mm and numerical aperture (NA) of 0.45 achieves a measured focusing efficiency of 64%, which is close to the calculated value of 69% and among the highest reported values using i-line photolithography.

Avalanche photodiodes with multiple multiplication layers for coherent detection.

We demonstrate a novel avalanche photodiode (APD) design which fundamentally relaxes the trade-off between responsivity and saturation-current performance at receiver end in coherent system. Our triple In0.52Al0.48As based multiplication (M-) layers with a stepped electric (E-) field inside has more pronounced avalanche process with significantly less effective critical-field than the dual M-layer. Reduced E-field in active M-layers ensures stronger E-field allocation to the thick absorption-layer with a smaller breakdown voltage (Vbr) resulting in less serious space-charge screening effect, less device heating at high output photocurrent. Compared to the dual M-layer reference sample, the demonstrated APD exhibits lower punch-through (- 9 vs. - 24 V)/breakdown voltages (- 43 vs. - 51 V), higher responsivity (19.6 vs. 13.5 A/W), higher maximum gain (230 vs. 130), and higher 1-dB saturation-current (> 5.6 vs. 2.5 mA) under 0.95 Vbr operation. Extremely high saturation-current (> 14.6 mA), high responsivity (7.3 A/W), and decent O-E bandwidth (1.4 GHz) can be simultaneously achieved using the demonstrated APD with a 200 µm active window diameter. In coherent FMCW LiDAR test bed, this novel APD exhibits a larger signal-to-noise ratio and high-quality 3-D images than the reference dual M-layer and high-performance commercial p-i-n PD modules, while requiring significantly less optical local-oscillator (LO) power (0.5 vs 4 mW).

Reconfigurable scan lens based on an actively controlled optical phased array.

Integrated photonics provides a path for miniaturization of an optical system to a compact chip scale and offers reconfigurability by the integration of active components. Here we report a chip-scale reconfigurable scan lens based on an optical phased array, consisting of 30 actively controlled elements on the InP integrated photonic platform. By configuring the phase shifters, we show scanning of a nearly diffraction-limited focused spot with a full width at half maximum spot size down to 2.7 µm at the wavelength of 1550 nm. We demonstrate the key functions needed for a laser-scanning microscope, including light focusing, collection, and steering. We also perform confocal measurements to detect reflection at selective depths.

Free-Space Applications of Silicon Photonics: A Review.

Silicon photonics has recently expanded its applications to delivering free-space emissions for detecting or manipulating external objects. The most notable example is the silicon optical phased array, which can steer a free-space beam to achieve a chip-scale solid-state LiDAR. Other examples include free-space optical communication, quantum photonics, imaging systems, and optogenetic probes. In contrast to the conventional optical system consisting of bulk optics, silicon photonics miniaturizes an optical system into a photonic chip with many functional waveguiding components. By leveraging the mature and monolithic CMOS process, silicon photonics enables high-volume production, scalability, reconfigurability, and parallelism. In this paper, we review the recent advances in beam steering technologies based on silicon photonics, including optical phased arrays, focal plane arrays, and dispersive grating diffraction. Various beam-shaping technologies for generating collimated, focused, Bessel, and vortex beams are also discussed. We conclude with an outlook of the promises and challenges for the free-space applications of silicon photonics.

2D beam steerer based on metalens on silicon photonics.

Beam steering with solid-state devices represents the cutting-edge technology for next-generation LiDARs and free-space communication transceivers. Here we demonstrate a platform based on a metalens on a 2D array of switchable silicon microring emitters. This platform enables scalable, efficient, and compact devices that steer in two dimensions using a single wavelength. We show a field of view of 12.4° × 26.8° using an electrical power of less than 83 mW, offering a solution for practical miniature beam steerers.

Plug-and-play fiber to waveguide connector.

The mass production and commercialization of integrated photonics have been slowed down by the high cost of packaging its optical interfaces. We show a plug-and-play connector between a fiber and a nanophotonic waveguide consisting of a 3D polymer structure with a fiber entrance port that simultaneously achieves mechanical and optical passive alignment with tolerance beyond ±10 µm to the fiber input position. We take advantage of a mechanical and optical co-design, analogous to commercial fiber-to-fiber connectors. We fabricate the plug-and-play couplers using 3D nanoprinting directly on foundry fabricated diffraction grating couplers. We measure an average of only 0.05 dB excess coupling loss between a single mode fiber and a high confinement silicon waveguide in addition to the inherent grating coupler loss. Our coupling platform offers a passive plug-and-play solution for scalable integrated photonics fiber-chip packaging.

On-chip platform for a phased array with minimal beam divergence and wide field-of-view.

Current silicon photonics phased arrays based on waveguide gratings enable beam steering with no moving parts. However, they suffer from a trade-off between beam divergence and field of view. Here, we show a platform based on silicon-nitride/silicon that achieves simultaneously minimal beam divergence and maximum field of view while maintaining performance that is robust to fabrication variations. In addition, in order to maximize the emission from the entire length of the grating, we design the grating's strength by varying its duty cycle (apodization) to emit uniformly. We fabricate a millimeter long grating emitter with diffraction-limited beam divergence of 0.089°.

Optical Asymmetry and Nonlinear Light Scattering from Colloidal Gold Nanorods.

A systematic study is presented of the intensity-dependent nonlinear light scattering spectra of gold nanorods under resonant excitation of the longitudinal surface plasmon resonance (SPR). The spectra exhibit features due to coherent second and third harmonic generation as well as a broadband feature that has been previously attributed to multiphoton photoluminescence arising primarily from interband optical transitions in the gold. A detailed study of the spectral dependence of the scaling of the scattered light with excitation intensity shows unexpected scaling behavior of the coherent signals, which is quantitatively accounted for by optically induced damping of the SPR mode through a Fermi liquid model of the electronic scattering. The broadband feature is shown to arise not from luminescence, but from scattering of the second-order longitudinal SPR mode with the electron gas, where efficient excitation of the second order mode arises from an optical asymmetry of the nanorod. The electronic-temperature-dependent plasmon damping and the Fermi-Dirac distribution together determine the intensity dependence of the broadband emission, and the structure-dependent absorption spectrum determines the spectral shape through the fluctuation-dissipation theorem. Hence a complete self-consistent picture of both coherent and incoherent light scattering is obtained with a single set of physical parameters.

Realization of mid-infrared graphene hyperbolic metamaterials.

While metal is the most common conducting constituent element in the fabrication of metamaterials, graphene provides another useful building block, that is, a truly two-dimensional conducting sheet whose conductivity can be controlled by doping. Here we report the experimental realization of a multilayer structure of alternating graphene and Al2O3 layers, a structure similar to the metal-dielectric multilayers commonly used in creating visible wavelength hyperbolic metamaterials. Chemical vapour deposited graphene rather than exfoliated or epitaxial graphene is used, because layer transfer methods are easily applied in fabrication. We employ a method of doping to increase the layer conductivity, and our analysis shows that the doped chemical vapour deposited graphene has good optical properties in the mid-infrared range. We therefore design the metamaterial for mid-infrared operation; our characterization with an infrared ellipsometer demonstrates that the metamaterial experiences an optical topological transition from elliptic to hyperbolic dispersion at a wavelength of 4.5 µm.

Ultrafast Lateral Photo-Dember Effect in Graphene Induced by Nonequilibrium Hot Carrier Dynamics.

The photo-Dember effect arises from the asymmetric diffusivity of photoexcited electrons and holes, which creates a transient spatial charge distribution and hence the buildup of a voltage. Conventionally, a strong photo-Dember effect is only observed in semiconductors with a large asymmetry between the electron and hole mobilities, such as in GaAs or InAs, and is considered negligible in graphene due to its electron-hole symmetry. Here, we report the observation of a strong lateral photo-Dember effect induced by nonequilibrium hot carrier dynamics when exciting a graphene-metal interface with a femtosecond laser. Scanning photocurrent measurements reveal the extraction of photoexcited hot carriers is driven by the transient photo-Dember field, and the polarity of the photocurrent is determined by the device's mobility asymmetry. Furthermore, ultrafast pump-probe measurements indicate the magnitude of photocurrent is related to the hot carrier cooling rate. Our simulations also suggest that the lateral photo-Dember effect originates from graphene's 2D nature combined with its unique electrical and optical properties. Taken together, these results not only reveal a new ultrafast photocurrent generation mechanism in graphene but also suggest new types of terahertz sources based on 2D nanomaterials.

Graphene photodetectors with ultra-broadband and high responsivity at room temperature.

The ability to detect light over a broad spectral range is central to several technological applications in imaging, sensing, spectroscopy and communication. Graphene is a promising candidate material for ultra-broadband photodetectors, as its absorption spectrum covers the entire ultraviolet to far-infrared range. However, the responsivity of graphene-based photodetectors has so far been limited to tens of mA W(-1) (refs 5-10) due to the small optical absorption of a monolayer of carbon atoms. Integration of colloidal quantum dots in the light absorption layer can improve the responsivity of graphene photodetectors to ∼ 1 × 10(7) A W(-1) (ref. 11), but the spectral range of photodetection is reduced because light absorption occurs in the quantum dots. Here, we report an ultra-broadband photodetector design based on a graphene double-layer heterostructure. The detector is a phototransistor consisting of a pair of stacked graphene monolayers (top layer, gate; bottom layer, channel) separated by a thin tunnel barrier. Under optical illumination, photoexcited hot carriers generated in the top layer tunnel into the bottom layer, leading to a charge build-up on the gate and a strong photogating effect on the channel conductance. The devices demonstrated room-temperature photodetection from the visible to the mid-infrared range, with mid-infrared responsivity higher than 1 A W(-1), as required by most applications. These results address key challenges for broadband infrared detectors, and are promising for the development of graphene-based hot-carrier optoelectronic applications.

Multi-wavelength heterodyne-detected scattering-type scanning near-field optical microscopy.

We report on the implementation of a scattering-type scanning near-field optical microscope based on a heterodyne detection scheme, which has the ability to record near-field optical images at multiple wavelengths simultaneously. It is used to map out local field distribution and to investigate the dispersion behavior of plasmon created by nanometer-scale metallic structures. It opens up an unprecedented opportunity to study nano-photonics.

Design and construction of linear laser encoders that possess high tolerance of mechanical runout.

A linearly diffracted laser encoder that has high tolerance of head-to-scale misalignment and a high signal-to-noise ratio is described. The preservation of parallelism between the incident and the diffracted beams, which can be attributed to a built-in folded 1x telescope, allows for the high alignment tolerance. It can be shown that, by coupling this newly developed circular polarization interferometer configuration with grating scale geometry optimization, one can eliminate the problems associated with signal distortion that arise from various efficiencies of the p- and the s-polarized light beams and obtain a high signal-to-noise ratio. Both theoretical and experimental results are presented to confirm the improved results and performance.