Chip-scale Platform Research Articles

Laser cooling in a chip-scale platform

Chip-scale atomic devices built around micro-fabricated alkali vapor cells are at the forefront of compact metrology and atomic sensors. We demonstrate a micro-fabricated vapor cell that is actively pumped to ultra-high-vacuum (UHV) to achieve laser cooling. A grating magneto-optical trap (GMOT) is incorporated with a 4 mm-thick Si/glass vacuum cell to demonstrate the feasibility of a fully miniaturized laser cooling platform. A two-step optical excitation process in rubidium is used to overcome surface-scatter limitations to the GMOT imaging. The unambiguous miniaturization and form-customizability made available with micro-fabricated UHV cells provide a promising platform for future compact cold-atom sensors.

An in-situ TEM microreactor for real-time nanomorphology & physicochemical parameters interrelated characterization

In-situ quantitative structure-property relationship (QSPR) characterization between nanomorphology and physicochemical parameters is essential for revealing the growth mechanism and guiding the optimization of functional nanomaterials. However, extracting the interrelated physicochemical parameters in the process of nanomorphology observation is still challenging. Herein, this paper demonstrates an advanced chip-scale platform for in-situ QSPR characterization by integrating a gravimetric resonant microcantilever sensor within a transmission electron microscopy (TEM) microreactor. The microcantilever measures the thermodynamic and kinetic parameters, such as enthalpy (ΔH°), activation energy (Ea) and reaction rate (v), by monitoring the reaction-induced mass change of functional nanomaterials. Simultaneously, the low-stress SiNx films as electron transparent windows allow in-situ nanomorphology observation. Using such a QSPR characterization platform, the three-stage sulfuration mechanism based on nano Kirkendall effect for the gas-solid reaction of H2S and Cu nanowires is revealed, and the different molecule-interaction behaviors of SO2 on ZnO nanowires at various temperatures are distinguished for the first time. It is believed that the in-situ QSPR microreactor can be widely used for elucidating the underlying interfacial thermodynamic/kinetic reaction mechanism, and guiding the controllable synthesis and precise optimization of functional nanomaterials.

Chip-based self-referencing using integrated lithium niobate waveguides

The measurement and stabilization of the carrier–envelope offset frequencyfCEOvia self-referencing is paramount for optical frequency comb generation, which has revolutionized precision frequency metrology, spectroscopy, and optical clocks. Over the past decade, the development of chip-scale platforms has enabled compact integrated waveguides for supercontinuum generation. However, there is a critical need for an on-chip self-referencing system that is adaptive to different pump wavelengths, requires low pulse energy, and does not require complicated processing. Here, we demonstrate efficientfCEOstabilization of a modelocked laser with only 107 pJ of pulse energy via self-referencing in an integrated lithium niobate waveguide. We realize anf-2finterferometer through second-harmonic generation and subsequent supercontinuum generation in a single dispersion-engineered waveguide with a stabilization performance equivalent to a conventional off-chip module. ThefCEObeatnote is measured over a pump wavelength range of 70 nm. We theoretically investigate our system using a single nonlinear envelope equation with contributions from both second- and third-order nonlinearities. Our modeling reveals rich ultrabroadband nonlinear dynamics and confirms that the initial second-harmonic generation followed by supercontinuum generation with the remaining pump is responsible for the generation of a strongfCEOsignal as compared to a traditionalf-2finterferometer. Our technology provides a highly simplified system that is robust, low in cost, and adaptable for precision metrology for use outside a research laboratory.

A packaged, fiber-coupled waveguide-enhanced Raman spectroscopic sensor.

Waveguide-enhanced Raman spectroscopy (WERS) is a promising technique for sensitive and selective detection of chemicals in a compact chip-scale platform. Coupling light on and off the sensor chip with fibers however presents challenges because of the fluorescence and Raman background generated by the pump light in the fibers; as a result all WERS demonstrations to date have used free-space coupling via lenses. We report a packaged, fiber-bonded WERS chip that filters the background on-chip through collection of the backscattered Raman light. The packaged sensor is integrated in a ruggedized flow cell for reliable measurement over arbitrary time periods. We also derive the figures of merit for WERS sensing with the backscattered Raman signal and compare waveguide geometries with respect to their filtering performance and signal to noise ratio.

Classification of time-domain waveforms using a speckle-based optical reservoir computer.

Reservoir computing is a recurrent machine learning framework that expands the dimensionality of a problem by mapping an input signal into a higher-dimension reservoir space that can capture and predict features of complex, non-linear temporal dynamics. Here, we report on a bulk electro-optical demonstration of a reservoir computer using speckles generated by propagating a laser beam modulated with a spatial light modulator through a multimode waveguide. We demonstrate that the hardware can successfully perform a multivariate audio classification task performed using the Japanese vowel speakers public data set. We perform full wave optical calculations of this architecture implemented in a chip-scale platform using an SiO2 waveguide and demonstrate that it performs as well as a fully numerical implementation of reservoir computing. As all the optical components used in the experiment can be fabricated using a commercial photonic integrated circuit foundry, our result demonstrates a framework for building a scalable, chip-scale, reservoir computer capable of performing optical signal processing.

Enhancing greywater treatment via MHz-Order surface acoustic waves

There is a pressing need for efficient biological treatment systems for the removal of organic compounds in greywater given the rapid increase in household wastewater produced as a consequence of rapid urbanisation. Moreover, proper treatment of greywater allows its reuse that can significantly reduce the demand for freshwater supplies. Herein, we demonstrate the possibility of enhancing the removal efficiency of solid contaminants from greywater using MHz-order surface acoustic waves (SAWs). A key distinction of the use of these high frequency surface acoustic waves, compared to previous work on its lower frequency (kHz order) bulk ultrasound counterpart for wastewater treatment, is the absence of cavitation, which can inflict considerable damage on bacteria, thus limiting the intensity and duration, and hence the efficiency enhancement, associated with the acoustic exposure. In particular, we show that up to fivefold improvement in the removal efficiency can be obtained, primarily due to the ability of the acoustic pressure field in homogenizing and reducing the size of bacterial clusters in the sample, therefore providing a larger surface area that promotes greater bacteria digestion. Alternatively, the SAW exposure allows the reduction in the treatment duration to achieve a given level of removal efficiency, thus facilitating higher treatment rates and hence processing throughput. Given the low-cost of the miniature chipscale platform, these promising results highlight its possibility for portable greywater treatment for domestic use or for large-scale industrial wastewater processing through massive parallelization.

Acoustopipetting: Tunable Nanoliter Sample Dispensing Using Surface Acoustic Waves.

We seek to demonstrate a robust, low-cost, and user-friendly acoustomicrofluidic platform that facilitates rapid, reproducible, and precise nanoliter sample dispensing. The solid-state chipscale platform exploits the unprecedented acceleration arising from high-frequency nanoelectromechanical vibrations, on the order of 10 million g, to jet the sample and hence generate a liquid bridge that spans across the substrate, on which the vibrations are generated and from which the sample originates, to a top target plate before rapidly pinching off to deposit the sample on the target with precise and reproducible volumes that can be tuned down to 0.22 μL with a standard error of 6.5% and coefficient of variation of 11.3%. The entire process occurs within approximately 10 ms. In addition to explicating the fundamental physical mechanism that underpins the technology, we demonstrate its use for serial dilution and concentration and, in particular, a cell-based drug toxicology assay. Moreover, we also show that multiple drop dispensing in an array, without requiring repositioning of the chip between dispensing steps, can be achieved through a simple but yet effective sequential directional jetting strategy, therefore allowing significant reduction in the total dispensing time in the case of massive-scale microarray operation. Given its low cost and compact size, the platform can easily be automated and parallelized, thus offering the prospect for introducing large-scale efficiencies in the laboratory workflow.

Waveguide-integrated superconducting nanowire single-photon detectors

Abstract Integration of superconducting nanowire single-photon detectors with nanophotonic waveguides is a key technological step that enables a broad range of classical and quantum technologies on chip-scale platforms. The excellent detection efficiency, timing and noise performance of these detectors have sparked growing interest over the last decade and have found use in diverse applications. Almost 10 years after the first waveguide-coupled superconducting detectors were proposed, here, we review the performance metrics of these devices, compare both superconducting and dielectric waveguide material systems and present prominent emerging applications.

Integrated visible light phased array system for autostereoscopic image projection.

We demonstrate a chip-scale autostereoscopic image projection system that utilizes a system of multiple integrated visible light optical phased arrays to reconstruct virtual light fields. Each phased array in this system serves as a micro-projector that illuminates the desired virtual object from a different angle. This recreates the virtual object in space with continuous parallax observable by the human visual system. In this work, a static virtual image with horizontal parallax and a viewing angle of 5° was generated with an array of 16 integrated silicon nitride phased arrays with a 635nm operating wavelength. Each phased array is comprised of 32×32 optical antennas with passively encoded relative phases. The presented device demonstrates the promise of integrated visible light phased array platforms for implementing projection-based autostereoscopic displays in compact chip-scale platforms suitable for mobile devices.

Dispersion engineered high quality lithium niobate microring resonators.

Lithium niobate (LN) exhibits outstanding material properties with great potential for many applications. Recent advance in LN integrated photonics on chip-scale platforms has shown significant advantages in device engineering and functionality innovation. Precise engineering of group-velocity dispersion (GVD) is crucial for many important nonlinear photonic applications. In this paper, we demonstrate high-Q LN microring resonators, with optical Q above 1 million, whose GVD can be flexibly controlled in both normal and anomalous dispersion regimes, with a value between -0.128 ps2/m and 0.043 ps2/m in the telecom band, by controlling the device cross section and by utilizing the birefringence. We are able to achieve a small anomalous GVD of -0.015 ps2/m that is even smaller than that of a silica optical fiber. The flexible engineering of GVD paves a critical step towards broad nonlinear photonic applications in high-Q LN microring resonators.

Loss induced coherent combining in InP-Si3N4 hybrid platform

Loss, as a time-reversed counterpart of gain, can also be used to control lasing in an optical system with coupled cavities. In this study, by manipulating mirror losses at different output ports of coupled Fabry-Perot cavities, an integrated coherently combined laser system is proposed and experimentally demonstrated in the InP-Si3N4 hybrid platform. Two InP-based reflective semiconductor amplifiers are coherently combined through an adiabatic 50:50 directional coupler in silicon nitride. The combining efficiency is ~92% at ~2× threshold. The novel system not only realizes the miniaturization of coherent laser beam combining but also provides a chip-scale platform to study the coherent coupling between coupled laser cavities.

Continuous tuneable droplet ejection via pulsed surface acoustic wave jetting.

We report a miniaturised platform for continuous production of single or multiple liquid droplets with diameters between 60 and 500 μm by interfacing a capillary-driven self-replenishing liquid feed with pulsed excitation of focussed surface acoustic waves (SAWs). The orifice-free operation circumvents the disadvantages of conventional jetting systems, which are often prone to clogging that eventuates in rapid degradation of the operational performance. Additionally, we show the possibility for flexibly tuning the ejected droplet size through the pulse width duration, thus avoiding the need for a separate device for every different droplet size required, as is the case for systems in which the droplet size is set by nozzles and orifices, as well as preceding ultrasonic jetting platforms where the droplet size is controlled by the operating frequency. Further, we demonstrate that cells can be jetted and hence printed onto substrates with control over the cell density within the droplets down to single cells. Given that the jetting does not lead to significant loss to the cell's viability or ability to proliferate, we envisage that this versatile jetting method can potentially be exploited with further development for cell encapsulation, dispensing and 3D bioprinting applications.

Ultra-low-loss on-chip resonators with sub-milliwatt parametric oscillation threshold

On-chip optical resonators have the promise of revolutionizing numerous fields including metrology and sensing; however, their optical losses have always lagged behind their larger discrete resonator counterparts based on crystalline materials and flowable glass. Silicon nitride (Si3N4) ring resonators open up capabilities for optical routing, frequency comb generation, optical clocks and high precision sensing on an integrated platform. However, simultaneously achieving high quality factor and high confinement in Si3N4 (critical for nonlinear processes for example) remains a challenge. Here, we show that addressing surface roughness enables us to overcome the loss limitations and achieve high-confinement, on-chip ring resonators with a quality factor (Q) of 37 million for a ring with 2.5 {\mu}m width and 67 million for a ring with 10 {\mu}m width. We show a clear systematic path for achieving these high quality factors. Furthermore, we extract the loss limited by the material absorption in our films to be 0.13 dB/m, which corresponds to an absorption limited Q of at least 170 million by comparing two resonators with different degrees of confinement. Our work provides a chip-scale platform for applications such as ultra-low power frequency comb generation, high precision sensing, laser stabilization and sideband resolved optomechanics.

Acousto-optic modulation and opto-acoustic gating in piezo-optomechanical circuits.

Acoustic wave devices provide a promising chip-scale platform for efficiently coupling radio frequency (RF) and optical fields. Here, we use an integrated piezo-optomechanical circuit platform that exploits both the piezoelectric and photoelastic coupling mechanisms to link 2.4 GHz RF waves to 194 THz (1550 nm) optical waves, through coupling to propagating and localized 2.4 GHz acoustic waves. We demonstrate acousto-optic modulation, resonant in both the optical and mechanical domains, in which waveforms encoded on the RF carrier are mapped to the optical field. We also show opto-acoustic gating, in which the application of modulated optical pulses interferometrically gates the transmission of propagating acoustic pulses. The time-domain characteristics of this system under both pulsed RF and pulsed optical excitation are considered in the context of the different physical pathways involved in driving the acoustic waves, and modelled through the coupled mode equations of cavity optomechanics.

High quality factor nanophotonic resonators in bulk rare-earth doped crystals.

Numerous bulk crystalline materials exhibit attractive nonlinear and luminescent properties for classical and quantum optical applications. A chip-scale platform for high quality factor optical nanocavities in these materials will enable new optoelectronic devices and quantum light-matter interfaces. In this article, photonic crystal nanobeam resonators fabricated using focused ion beam milling in bulk insulators, such as rare-earth doped yttrium orthosilicate and yttrium vanadate, are demonstrated. Operation in the visible, near infrared, and telecom wavelengths with quality factors up to 27,000 and optical mode volumes close to one cubic wavelength is measured. These devices enable new nanolasers, on-chip quantum optical memories, single photon sources, and non-linear devices at low photon numbers based on rare-earth ions. The techniques are also applicable to other luminescent centers and crystal.

Elliptical quantum dots as on-demand single photons sources with deterministic polarization states

In quantum information, control of the single photon's polarization is essential. Here, we demonstrate single photon generation in a pre-programmed and deterministic polarization state, on a chip-scale platform, utilizing site-controlled elliptical quantum dots (QDs) synthesized by a top-down approach. The polarization from the QD emission is found to be linear with a high degree of linear polarization and parallel to the long axis of the ellipse. Single photon emission with orthogonal polarizations is achieved, and the dependence of the degree of linear polarization on the QD geometry is analyzed.

Tb/s Coherent Optical OFDM Systems Enabled by Optical Frequency Combs

This paper discusses the realization of terabit per second high speed and high spectral-efficiency optical transmissions using much lower speed electronics and optoelectronics through parallel processing of coherent optical frequency combs at both the transmitter and receiver. The coherent and parallel processing enables electrical-to-optical and optical-to-electrical (E/O and O/E) conversion of wide-bandwidth optical signals which would otherwise exceeds the capability of conventional optoelectronics. In the first experiment, an optical frequency comb (OFC) generator provides 32 comb lines with less than 5-dB power variation. Subsequently, 1.008-Tb/s modulation capability is realized on 32 × 106 OFDM subcarriers with 16-QAM modulation in a 318-GHz seamless optical bandwidth. It demonstrates an effective way to generate an optical OFDM signal with tens of times wider optical bandwidth than that of analog-to-digital converters and digital-to-analog converters (ADC/DAC). The second experiment demonstrates simultaneous detection of multiple OFDM bands from a 32-band coherent optical OFDM signal using another optical frequency comb, a silica planar lightwave circuit (PLC) that implemented the major optical devices, and two pairs of balanced photodiodes. The experimental results indicate prospects for an optically integrated coherent optical OFDM system on a chip-scale platform.

Design, Characterization, and Integration of Nanometric Objects with Chip-scale Platforms for Disease Diagnosis

Nanomaterials (e.g., metal nanoparticles) are playing increasingly important roles in disease detection. This emergence arises from the need to detect markers and pathogens at ever-lower levels in human and veterinary diagnostics, homeland security, and food and water safely. This paper reviews our recent work using surface enhanced Raman scattering for detection of proteins, viruses, and microorganisms in heterogeneous immunoassays. It describes the assay platform, which consists of an antibody-modified capture substrate and gold nanoparticle-based label. The latter draws on the ability to reproducibly construct gold nanoparticles modified with a monolayer of an intrinsically strong Raman scatterer that is coated with a layer of antibodies. This construct, referred to as an extrinsic Raman label, exploits both the signal enhancement of scatterers when coated on nanometer-sized gold particles and the antigenic binding specificity of the immobilized antibody layer. Issues related to nonspecific adsorption, particle stability, and measurement reproducibility are also discussed.

Design, Characterization, and Integration of Nanometric Objects with Chip-scale Platforms for Disease Diagnosis

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Guiding, modulating, and emitting light on Silicon-challenges and opportunities

Silicon photonics could enable a chip-scale platform for monolithic integration of optics and microelectronics for applications of optical interconnects in which high data streams are required in a small footprint. This paper discusses mechanisms in silicon photonics for waveguiding, modulating, light amplification, and emission. These mechanisms, together with recent advances of fabrication techniques, have enabled the demonstration of ultracompact passive and active silicon photonic components with very low loss.