Optical nonlinearity has been widely used to try to produce optical isolators. However, this is very difficult to achieve due to dynamical reciprocity. Here, we show the use of the chiral cross-Kerr nonlinearity of atoms at room temperature to realize optical isolation, circumventing dynamical reciprocity. In our approach, the chiral cross-Kerr nonlinearity is induced by the thermal motion of $N$-type atoms. The resulting cross phase shift and absorption of a weak probe field are dependent on its propagation direction. This proposed optical isolator can achieve more than 30 dB of isolation ratio, with a low loss of less than 1 dB. By inserting this atomic medium in a Mach-Zehnder interferometer, we further propose a four-port optical circulator with a fidelity larger than 0.9 and an average insertion loss less than 1.6 dB. Using atomic vapor embedded in an on-chip waveguide, our method may provide chip-compatible optical isolation at the single-photon level of a probe field.

https://link.aps.org/accepted/10.1103/PhysRevLett.121.203602

Cavity-Free Optical Isolators and Circulators Using a Chiral Cross-Kerr Nonlinearity

The ability to slow down the propagation of light touches both fundamental aspects of light–matter interactions and practical applications in photonic communication and computation1,2,3. Optical quantum interference can substantially reduce the speed of light while offering additional dramatic optical effects based on the ability to control electronic quantum states4,5. Recent efforts are increasingly being directed towards harnessing these effects in integrated photonic structures6,7. Here, we report the first demonstration of slow light and electromagnetically induced transparency in a self-contained, planar atomic spectroscopy chip. Using hot rubidium atoms in hollow-core waveguides, we demonstrate 44% optical transparency with a group index of 1,200, or more than sevenfold slower light than in photonic-crystal waveguides8. Optical pulse delays of 16 ns with a delay-bandwidth product of 0.8 are observed. This implementation of atomic quantum state control in integrated photonic structures will enable coherent photonics at ultralow power levels. Researchers exploit atomic quantum state control in a fully integrated photonic atomic spectroscopy chip to reduce the group velocity of light by a factor of 1,200 — the lowest group velocity ever reported for a solid-state material. The findings will enable the creation of on-chip nonlinear optical devices with enhanced quantum coherence operating at ultralow power levels.

Slow light on a chip via atomic quantum state control

FPGA application size is rapidly growing by reuse and replication. Achieved quality of results (QoR) of these large designs is often much lower than what could be realized with localized circuits at a modular level. One underlying reason for QoR loss is that back-end implementation tools compile the designs as one large circuit entry. Is there a way to bring innovation to the implementation stage of FPGA compilation that can improve QoR? This work proposes a pre-implemented methodology for FPGAs to achieve higher performance or productivity and introduces RapidWright, an open-source platform to enable this new approach. We aim to enhance either QoR or productivity through the reuse of modular implementations and present examples that improve QoR up to 50% or accelerate compilation time and debug by more than an order of magnitude. Finally, we demonstrate how RapidWright enables custom crafted implementations with near spec performance.

RapidWright: Enabling Custom Crafted Implementations for FPGAs

Complete integration of microfluidic and optical functions in a single lab-on-chip device is one goal of optofluidics. Here, we demonstrate the hybrid integration of a PDMS-based fluid handling layer with a silicon-based optical detection layer in a single optofluidic system. The optical layer consists of a liquid-core antiresonant reflecting optical waveguide (ARROW) chip that is capable of single particle detection and interfacing with optical fiber. Integrated devices are reconfigurable and able to sustain high pressures despite the small dimensions of the liquid-core waveguide channels. We show the combination of salient sample preparation capabilities—particle mixing, distribution, and filtering—with single particle fluorescence detection. Specifically, we demonstrate fluorescent labelling of λ-DNA, followed by flow-based single-molecule detection on a single device. This points the way towards amplification-free detection of nucleic acids with low-complexity biological sample preparation on a chip.

Hybrid optofluidic integration

Micrometer-sized hollow antiresonant reflecting optical waveguides on silicon substrates have been previously demonstrated with liquid and gas-filled cores. Previous designs have nonideal geometries, with nonuniform lateral layers around the hollow core, resulting in higher loss than could potentially be achieved. A new design and fabrication process has been developed involving hollow waveguide fabrication on a self-aligned pedestal (SAP) using anisotropic plasma etching. With the SAP structure, the hollow core is surrounded by uniform layers and a terminal layer of air on three sides, resulting in air-core waveguide loss of 1.54 cm-1 at 785 nm and high fabrication yield.

Hollow ARROW Waveguides on Self-Aligned Pedestals for Improved Geometry and Transmission

We demonstrate the optimization of the concentration, temperature and cycling of a piranha (H2O2:H2SO4) mixture that produces high yields while quickly etching hollow structures made using a highly crosslinked SU8 polymer sacrificial core. The effects of the piranha mixture on the thickness, refractive index and roughness of common micro-electromechanical systems and micro-opto-electromechanical systems fabrication materials (SiN, SiO2 and Si) were determined. The effectiveness of the optimal piranha mixture was demonstrated in the construction of hollow anti-resonant reflecting optical waveguides.

/pdf/optimized-piranha-etching-process-for-su8-based-mems-and-37k4mmw5s8.pdf

Optimized piranha etching process for SU8-based MEMS and MOEMS construction.

A rapid post-map insertion of an embedded logic analyzer is discussed. The proposed technique makes use of otherwise unused resources in an already-mapped circuit and does not disturb the original placement and routing of the circuit. Using this technique, designers can add debugging circuitry to existing circuits and quickly modify the set of observed signals in just a few minutes instead of waiting for a recompile of their circuit. All tests were performed on a Xilinx Virtex-5 FPGA.

Rapid Post-Map Insertion of Embedded Logic Analyzers for Xilinx FPGAs

Optofluidic waveguides have been integrated with solid core waveguides on silicon using an antiresonant reflecting optical waveguide (ARROW) design. Interface transmission between solid and liquid core waveguides is one of the most important factors for overall optical throughput. The optimization of interface transmission by adjusting the thickness of top waveguide cladding layers was demonstrated experimentally and theoretically. The measured coupling efficiency increases from 18% to 67% and the overall throughput was improved 17× due to improved mode matching while liquid core waveguides maintain a low average loss coefficient.

Optimization of Interface Transmission Between Integrated Solid Core and Optofluidic Waveguides

Antiresonant reflecting optical waveguides (ARROWs) provide a promising approach to realizing high-sensitivity
sensing platforms on planar substrates. We have previously developed ARROW platforms that guide light in
hollow cores filled with liquid and gas media. These platforms include integrated traditional solid waveguides
to direct light into and out of sensing media. To improve the sensitivity of these platforms for optical sensing,
hollow waveguide loss must be reduced. We are working towards this by using anisotropic plasma etching to
create near-ideal hollow ARROW geometries. These structures rely on an etching mask that also serves as the
sacrificial core for the waveguide. This self-aligned process creates a hollow waveguide on a pedestal which is
surrounded by a terminal layer of air in three directions. We previously produced ARROWs by pre-etching the
silicon substrate and aligning the sacrificial core to the pedestal. However, this necessitates using a pedestal
which is wider than the core, leading to higher loss and poor reproducibility. We have also increased the hollow
to solid waveguide transmission efficiency by using a design that coats the sides and top of the hollow core with
a single layer of silicon dioxide. Using this design, we have demonstrated an interface transmission improvement
of more than two times. A much improved optical sensor platform will incorporate both of these features, using
the self-aligned pedestal process for most of the length of the hollow waveguides to decrease loss, and employing
the single layer design only at the interfaces to improve hollow-solid waveguide coupling.

Jared Keeley

Papers

Hollow ARROW Waveguides on Self-Aligned Pedestals for Improved Geometry and Transmission

Optimized piranha etching process for SU8-based MEMS and MOEMS construction.

Rapid Post-Map Insertion of Embedded Logic Analyzers for Xilinx FPGAs

Optimization of Interface Transmission Between Integrated Solid Core and Optofluidic Waveguides

Hollow ARROW waveguides on self-aligned pedestals for high-sensitivity optical sensing