Electrically driven reprogrammable phase-change metasurface reaching 80% efficiency.

Phase-change materials (PCMs) offer a compelling platform for active metaoptics, owing to their large index contrast and fast yet stable phase transition attributes. Despite recent advances in phase-change metasurfaces, a fully integrable solution that combines pronounced tuning measures, i.e., efficiency, dynamic range, speed, and power consumption, is still elusive. Here, we demonstrate an in situ electrically driven tunable metasurface by harnessing the full potential of a PCM alloy, Ge2Sb2Te5 (GST), to realize non-volatile, reversible, multilevel, fast, and remarkable optical modulation in the near-infrared spectral range. Such a reprogrammable platform presents a record eleven-fold change in the reflectance (absolute reflectance contrast reaching 80%), unprecedented quasi-continuous spectral tuning over 250 nm, and switching speed that can potentially reach a few kHz. Our scalable heterostructure architecture capitalizes on the integration of a robust resistive microheater decoupled from an optically smart metasurface enabling good modal overlap with an ultrathin layer of the largest index contrast PCM to sustain high scattering efficiency even after several reversible phase transitions. We further experimentally demonstrate an electrically reconfigurable phase-change gradient metasurface capable of steering an incident light beam into different diffraction orders. This work represents a critical advance towards the development of fully integrable dynamic metasurfaces and their potential for beamforming applications.

High-Q ultrasensitive integrated photonic sensors based on slot-ring resonator on a 3C-SiC-on-insulator platform.

We demonstrate, to the best of our knowledge, the first high-Q silicon carbide (SiC) integrated photonic sensor based on slot-ring resonators on a 3C-SiC-on-insulator (SiCOI) platform. We experimentally demonstrate an intrinsic Q of 17,400 at around 1310 nm wavelength for a slot-ring resonator with 40 µm radius with water cladding. By applying different concentrations of a sodium chloride (NaCl) solution that covers the devices, measured bulk sensitivities of 264-300 nm/RIU (refractive index unit) are achieved in the slot-ring resonator with a 400-450 nm rail width and a 100-200 nm slot width. The device performance for biomolecular layer sensing (BMLS) is proved by the detection of the cardiac biomarker troponin with 248-322 pm/nm surface sensitivity. The reported slot-ring resonators can be of great interest for diverse sensing applications from visible to infrared wavelengths.

ITO-based microheaters for reversible multi-stage switching of phase-change materials: towards miniaturized beyond-binary reconfigurable integrated photonics.

Inducing a large refractive-index change is the holy grail of reconfigurable photonic structures, a goal that has long been the driving force behind the discovery of new optical material platforms. Recently, the unprecedentedly large refractive-index contrast between the amorphous and crystalline states of Ge-Sb-Te (GST)-based phase-change materials (PCMs) has attracted tremendous attention for reconfigurable integrated nanophotonics. Here, we introduce a microheater platform that employs optically transparent and electrically conductive indium-tin-oxide (ITO) bridges for the fast and reversible electrical switching of the GST phase between crystalline and amorphous states. By the proper assignment of electrical pulses applied to the ITO microheater, we show that our platform allows for the registration of virtually any intermediate crystalline state into the GST film integrated on the top of the designed microheaters. More importantly, we demonstrate the full reversibility of the GST phase between amorphous and crystalline states. To show the feasibility of using this hybrid GST/ITO platform for miniaturized integrated nanophotonic structures, we integrate our designed microheaters into the arms of a Mach-Zehnder interferometer to realize electrically reconfigurable optical phase shifters with orders of magnitude smaller footprints compared to existing integrated photonic architectures. We show that the phase of optical signals can be gradually shifted in multiple intermediate states using a structure that can potentially be smaller than a single wavelength. We believe that our study showcases the possibility of forming a whole new class of miniaturized reconfigurable integrated nanophotonics using beyond-binary reconfiguration of optical functionalities in hybrid PCM-photonic devices.

Racetrack microresonator based electro-optic phase shifters on a 3C silicon-carbide-on-insulator platform.

We report, to the best of our knowledge, the first demonstration of integrated electro-optic (EO) phase shifters based on racetrack microresonators on a 3C silicon-carbide-on-insulator (SiCOI) platform working at near-infrared wavelengths. By applying DC voltage in the crystalline axis perpendicular to the waveguide plane, we have observed optical phase shifts from the racetrack microresonators whose loaded quality ($ Q $) factors are $\sim\! {30,\!000}$. We show voltage-length product (${{V}_{\pi}} \cdot {{L}_{ \pi}}$) of ${118}\;{{\rm V}\cdot{\rm cm}}$, which corresponds to an EO coefficient ${{r}_{41}}$ of 2.6 pm/V. The SiCOI platform can be used to realize tunable silicon carbide integrated photonic devices that are desirable for applications in nonlinear and quantum photonics over a wide bandwidth that covers visible and infrared wavelengths.

Dynamic Hybrid Metasurfaces.

Efficient hybrid plasmonic-photonic metasurfaces that simultaneously take advantage of the potential of both pure metallic and all-dielectric nanoantennas are identified as an emerging technology in flat optics. Nevertheless, postfabrication tunable hybrid metasurfaces are still elusive. Here, we present a reconfigurable hybrid metasurface platform by incorporating the phase-change material Ge2Sb2Te5 (GST) into metal-dielectric meta-atoms for active and nonvolatile tuning of properties of light. We systematically design a reduced-dimension meta-atom, which selectively controls the hybrid plasmonic-photonic resonances of the metasurface via the dynamic change of optical constants of GST without compromising the scattering efficiency. As a proof-of-concept, we experimentally demonstrate two tunable metasurfaces that control the amplitude (with relative modulation depth as high as ≈80%) or phase (with tunability >230°) of incident light promising for high-contrast optical switching and efficient anomalous to specular beam deflection, respectively. Our findings further substantiate dynamic hybrid metasurfaces as compelling candidates for next-generation reprogrammable meta-optics.

Synthetic Engineering of Morphology and Electronic Band Gap in Lateral Heterostructures of Monolayer Transition Metal Dichalcogenides.

Heterostructures of two-dimensional transition metal dichalcogenides (TMDs) can offer a plethora of opportunities in condensed matter physics, materials science, and device engineering. However, despite state-of-the-art demonstrations, most current methods lack enough degrees of freedom for the synthesis of heterostructures with engineerable properties. Here, we demonstrate that combining a postgrowth chalcogen-swapping procedure with the standard lithography enables the realization of lateral TMD heterostructures with controllable dimensions and spatial profiles in predefined locations on a substrate. Indeed, our protocol receives a monolithic TMD monolayer (e.g., MoSe2) as the input and delivers lateral heterostructures (e.g., MoSe2-MoS2) with fully engineerable morphologies. In addition, through establishing MoS2xSe2(1-x)-MoS2ySe2(1-y) lateral junctions, our synthesis protocol offers an extra degree of freedom for engineering the band gap energies up to ∼320 meV on each side of the heterostructure junction via changing x and y independently. Our electron microscopy analysis reveals that such continuous tuning stems from the random intermixing of sulfur and selenium atoms following the chalcogen swapping. We believe that, by adding an engineering flavor to the synthesis of TMD heterostructures, our study lowers the barrier for the integration of two-dimensional materials into practical optoelectronic platforms.

High-Q microresonators integrated with microheaters on a 3C-SiC-on-insulator platform.

We demonstrate, to the best of our knowledge, the first thermally reconfigurable high-Q silicon carbide (SiC) microring resonators with integrated microheaters on a 3C-SiC-on-insulator platform. We extract a thermo-optic coefficient of around 2.67×10-5/K for 3C-SiC from wavelength shift of a resonator heated by a hot plate. Finally, we fabricate a 40-µm-radius microring resonator with intrinsic Q of 139,000 at infrared wavelengths (∼1550 nm) after integration with a NiCr microheater. By applying current through the microheater, a resonance shift of 30 pm/mW is achieved in the microring, corresponding to ∼50 mW per π phase shift. This platform offers an easy and reliable way for integration with electronic devices as well as great potential for diverse integrated optics applications.

High-Q integrated photonic microresonators on 3C-SiC-on-insulator (SiCOI) platform.

We report a high-quality 3C-silicon carbide (SiC)-on-insulator (SiCOI) integrated photonic material platform formed by wafer bonding of crystalline 3C-SiC to a silicon oxide (SiO2)-on-silicon (Si) substrate. This material platform enables to develop integrated photonic devices in SiC without the need for undercutting the Si substrate, in contrast to the structures formed on conventional 3C-SiC-on-Si platforms. In addition, we show a unique process in the SiCOI platform for minimizing the effect of lattice mismatch during the growth of SiC on Si through polishing after bonding. This results in a high-quality SiCOI platform that enables record high Qs of 142,000 in 40 µm radius SiC microring resonators. The resulting SiCOI platform has a great potential for a wide range of applications in integrated optics, including nonlinear optical devices, quantum optical devices, and high-power optical devices.

Defect-Mediated Alloying of Monolayer Transition-Metal Dichalcogenides.

Alloying plays a central role in tailoring the material properties of 2D transition-metal dichalcogenides (TMDs). However, despite widespread reports, the details of the alloying mechanism in 2D TMDs have remained largely unknown and are yet to be further explored. Here, we combine a set of systematic experiments with ab initio density functional theory (DFT) calculations to unravel a defect-mediated mechanism for the alloying of monolayer TMD crystals. In our alloying approach, a monolayer MoSe2 film serves as a host crystal in which exchanging selenium (Se) atoms with sulfur (S) atoms yields a MoS2 xSe2(1- x) alloy. Our study reveals that the driving force required for the alloying of CVD-grown films with abundant vacancy-type defects is significantly lower than that required for the alloying of exfoliated films with fewer vacancies. Indeed, we show that pre-existing Se vacancies in the host MoSe2 lattice mediate the replacement of chalcogen atoms and facilitate the synthesis of MoS2 xSe2(1- x) alloys. Our DFT calculations suggest that S atoms can bind to Se vacancies and then diffuse throughout the host MoSe2 lattice via exchanging the position with Se vacancies, further supporting our proposed defect-mediated alloying mechanism. Beside native vacancy defects, we show that the existence of large-scale defects in CVD-grown MoSe2 films causes the fracture of alloys under the alloying-induced strain, while no such effect is observed in exfoliated MoSe2 films. Our study provides a deep insight into the details of the alloying mechanism and enables the synthesis of 2D alloys with tunable properties.

Highly-uniform resonator-based visible spectrometer on a Si3N4 platform with robust and accurate post-fabrication trimming.

A resonator array-based spectrometer for visible/near-infrared (NIR) wavelengths is fabricated on a low-loss silicon nitride (Si3N4) material platform. Ideally, a spectrometer should uniformly sample the input spectrum. However, resonator-based spectrometers, in which each spectral sample corresponds to resonance wavelength of one of the resonators in the array, suffer from wavelength sampling non-uniformity caused by the high sensitivity of the resonant wavelengths of different resonators to the dimensional variations caused by fabrication imperfections. Using an alignment-insensitive post-fabrication trimming technique, we reduce the standard deviation (STD) of resonance wavelength of a 60-channel integrated photonic spectrometer in Si3N4 to a record-low value of 5 pm in the visible wavelength range. This approach can be used to realize wideband and uniform visible spectrometers that are desirable for applications such as optical signal processing and biological sensing.

Multiplexed detection of lectins using integrated glycan-coated microring resonators.

We present the systematic design, fabrication, and characterization of a multiplexed label-free lab-on-a-chip biosensor using silicon nitride (SiN) microring resonators. Sensor design is addressed through a systematic approach that enables optimizing the sensor according to the specific noise characteristics of the setup. We find that an optimal 6 dB undercoupled resonator consumes 40% less power in our platform to achieve the same limit-of-detection as the conventional designs using critically coupled resonators that have the maximum light-matter interaction. We lay out an optimization framework that enables the generalization of our method for any type of optical resonator and noise characteristics. The device is fabricated using a CMOS-compatible process, and an efficient swabbing lift-off technique is introduced for the deposition of the protective oxide layer. This technique increases the lift-off quality and yield compared to common lift-off methods based on agitation. The complete sensor system, including microfluidic flow cell and surface functionalization with glycan receptors, is tested for the multiplexed detection of Aleuria Aurantia Lectin (AAL) and Sambucus Nigra Lectin (SNA). Further analysis shows that the sensor limit of detection is 2 × 10(-6) RIU for bulk refractive index, 1 pg/mm(2) for surface-adsorbed mass, and ∼ 10 pM for the glycan/lectins studied here.

High-quality silicon on silicon nitride integrated optical platform with an octave-spanning adiabatic interlayer coupler.

Hybrid nanophotonic platforms based on three-dimensional integration of different photonic materials are emerging as promising ecosystems for the optoelectronic device fabrication. In order to benefit from key features of both silicon (Si) and silicon nitride (SiN) on a single chip, we have developed a wafer-scale hybrid photonic platform based on the integration of a thin crystalline Si layer on top of a thin SiN layer with an ultra-thin oxide buffer layer. A complete optical path in the hybrid platform is demonstrated by coupling light back and forth between nanophotonic devices in Si and SiN layers. Using an adiabatic tapered coupling method, a record-low interlayer coupling-loss of 0.02 dB is achieved at 1550 nm telecommunication wavelength window. We also demonstrate high-Q resonators on the hybrid material platform with intrinsic Q's as high as 3 × 10(6) for a 60 µm-radius microring resonator, which is (to the best of our knowledge) the highest Q observed for a micro-resonator on a hybrid Si/SiN platform.

Compact, 15 Gb/s electro-optic modulator through carrier accumulation in a hybrid Si/SiO(2)/Si microdisk.

High-speed electro-optic modulators are among the key elements in any optical interconnect system. In this work we design and demonstrate an electro-optic modulator based on carrier accumulation on a multilayer integrated photonic platform comprising a stack of high quality Si, SiO(2), and Si layers. The device consists of a 3-µm radius microdisk with an embedded capacitor. Characterization results reveal an operation bandwidth of exceeding 10 GHz. The device is capable of transmitting 15 Gb/s with the on/off keying format in a single polarization. The proposed structure can be self-trimmed by up to 1 nm in wavelength by applying a dc bias voltage without any power consumption. This feature eliminates the need for power-hungry thermal-based compensation methods to address the resonance wavelength mismatch due to fabrication imperfections.

Robust postfabrication trimming of ultracompact resonators on silicon on insulator with relaxed requirements on resolution and alignment.

One of the main drawbacks of the high-index-contrast silicon-on-insulator platform in integrated photonics is the high sensitivity of the resonance wavelength of resonators to dimensional variations caused by fabrication imperfection. In this work, we experimentally demonstrate an accurate postfabrication trimming technique for compensating the fabrication-induced variations in the resonance properties of nanophotonic devices. Using this technique, we reduce the variation of the resonance wavelength of 4 µm diameter microdonut resonators by more than 1 order of magnitude to about 25 pm, which is adequate for most interconnect, optical signal processing, and sensing applications. In addition, our proposed technique has improved misalignment toleration and throughput compared to previous reports.

Optical bistability in a one-dimensional photonic crystal resonator using a reverse-biased pn-junction.

Optical bistability provides a simple way to control light with light. We demonstrate low-power thermo-optical bistability caused by the Joule heating mechanism in a one-dimensional photonic crystal (PC) nanobeam resonator with a moderate quality factor (Q ~8900) with an embedded reverse-biased pn-junction. We show that the photocurrent induced by the linear absorption in this compact resonator considerably reduces the threshold optical power. The proposed approach substantially relaxes the requirements on the input optical power for achieving optical bistability and provides a reliable way to stabilize the bistable features of the device.

An efficient technique for the reduction of wavelength noise in resonance-based integrated photonic sensors.

A systematic study of the limit of detection (LOD) in resonance-based silicon photonic lab-on-chip sensors is presented. The effects of the noise, temperature fluctuations, and the fundamental thermodynamic limit of the resonator are studied. Wavelength noise is identified as the dominant source of noise, and an efficient technique for suppressing this noise is presented. A large ensemble of statistical data from the transmission measurements in a laser-scanning configuration on five silicon nitride (SiN) microrings is collected to discuss and identify the sources of noise. The experimental results show that the LOD is limited by a 3σ wavelength noise of ∼1.8 pm. We present a sub-periodic interferometric technique, relying on an inverse algorithm, to suppress this noise. Our technique reduces the wavelength noise by more than one order of magnitude to an ensemble average of 3σ = 120 fm, for a resonator quality factor (Q) of about 5 × 10(4) without any temperature stabilization or cooling. This technique is readily amenable to on-chip integration to realize highly accurate and low-cost lab-on-chip sensors.

High-efficiency and wideband interlayer grating couplers in multilayer Si/SiO2/SiN platform for 3D integration of optical functionalities.

We have designed interlayer grating couplers with single/double metallic reflectors for Si/SiO(2)/SiN multilayer material platform. Out-of-plane diffractive grating couplers separated by 1.6 µm thick buffer SiO(2) layer are vertically stacked against each other in Si and SiN layers. Geometrical optimization using genetic algorithm coupled with electromagnetic simulations using two-dimensional (2D) finite element method (FEM) results in coupler designs with high peak coupling efficiency of up to 89% for double- mirror and 64% for single-mirror structures at telecom wavelength. Also, 3-dB bandwidths of 40 nm and 50 nm are theoretically predicted for the two designs, respectively. We have fabricated the grating coupler structure with single mirror. Measured values for insertion loss and 3-dB bandwidth in the fabricated single-mirror coupler confirms the theoretical results. This opens up the possibility of low-loss 3D dense integration of optical functionalities in hybrid material platforms.

Field-programmable optical devices based on resonance elimination.

Optical switches are among the essential building blocks in optical networks due to their unique role in routing data. In this Letter, for the first time to our knowledge, we have exploited a high-quality factor (Q) optical microresonator combined with the well-known irreversible dielectric breakdown phenomenon to introduce a simple field-programmable on/off optical switch. This simple unit can be thought of as a building block for more complex optical systems with different functionalities. By using this simple unit we have demonstrated an optical field-programmable 2×2 switch. After the device is programmed by the user, no external electrical signal is needed to maintain the state of the device. The same approach can readily be adopted to design a field-programmable arbitrary N×N optical switch.

Hadamard multiplexed fluorescence tomography.

Depth-resolved three-dimensional (3D) reconstruction of fluorophore-tagged inclusions in fluorescence tomography (FT) poses a highly ill-conditioned problem as depth information must be extracted from boundary data. Due to the ill-posed nature of the FT inverse problem, noise and errors in the data can severely impair the accuracy of the 3D reconstructions. The signal-to-noise ratio (SNR) of the FT data strongly affects the quality of the reconstructions. Additionally, in FT scenarios where the fluorescent signal is weak, data acquisition requires lengthy integration times that result in excessive FT scan periods. Enhancing the SNR of FT data contributes to the robustness of the 3D reconstructions as well as the speed of FT scans. A major deciding factor in the SNR of the FT data is the power of the radiation illuminating the subject to excite the administered fluorescent reagents. In existing single-point illumination FT systems, the source power level is limited by the skin maximum radiation exposure levels. In this paper, we introduce and study the performance of a multiplexed fluorescence tomography system with orders-of-magnitude enhanced data SNR over existing systems. The proposed system allows for multi-point illumination of the subject without jeopardizing the information content of the FT measurements and results in highly robust reconstructions of fluorescent inclusions from noisy FT data. Improvements offered by the proposed system are validated by numerical and experimental studies.

Vertical integration of high-Q silicon nitride microresonators into silicon-on-insulator platform.

We demonstrate a vertical integration of high-Q silicon nitride microresonators into the silicon-on-insulator platform for applications at the telecommunication wavelengths. Low-loss silicon nitride films with a thickness of 400 nm are successfully grown, enabling compact silicon nitride microresonators with ultra-high intrinsic Qs (~ 6 × 10(6) for 60 µm radius and ~ 2 × 10(7) for 240 µm radius). The coupling between the silicon nitride microresonator and the underneath silicon waveguide is based on evanescent coupling with silicon dioxide as buffer. Selective coupling to a desired radial mode of the silicon nitride microresonator is also achievable using a pulley coupling scheme. In this work, a 60-µm-radius silicon nitride microresonator has been successfully integrated into the silicon-on-insulator platform, showing a single-mode operation with an intrinsic Q of 2 × 10(6).