After three decades of developments, single particle tracking (SPT) has become a powerful tool to interrogate dynamics in a range of materials including live cells and novel catalytic supports because of its ability to reveal dynamics in the structure–function relationships underlying the heterogeneous nature of such systems In this review, we summarize the algorithms behind, and practical applications of, SPT We first cover the theoretical background including particle identification, localization, and trajectory reconstruction General instrumentation and recent developments to achieve two- and three-dimensional subdiffraction localization and SPT are discussed We then highlight some applications of SPT to study various biological and synthetic materials systems Finally, we provide our perspective regarding several directions for future advancements in the theory and application of SPT

Single Particle Tracking: From Theory to Biophysical Applications

The electro-optical Pockels effect is an essential nonlinear effect used in many applications. The ultrafast modulation of the refractive index is, for example, crucial to optical modulators in photonic circuits. Silicon has emerged as a platform for integrating such compact circuits, but a strong Pockels effect is not available on silicon platforms. Here, we demonstrate a large electro-optical response in silicon photonic devices using barium titanate. We verify the Pockels effect to be the physical origin of the response, with r42 = 923 pm V−1, by confirming key signatures of the Pockels effect in ferroelectrics: the electro-optic response exhibits a crystalline anisotropy, remains strong at high frequencies, and shows hysteresis on changing the electric field. We prove that the Pockels effect remains strong even in nanoscale devices, and show as a practical example data modulation up to 50 Gbit s−1. We foresee that our work will enable novel device concepts with an application area largely extending beyond communication technologies. Electro-optic modulators based on epitaxial barium titanate (BTO) integrated on silicon exhibit speeds up to 50 Gbit s–1 while the Pockels coefficient of the BTO film is found to be approaching the bulk value.

Large Pockels effect in micro- and nanostructured barium titanate integrated on silicon

As electronic device feature sizes scale-down, the power consumed due to onchip communications as compared to computations will increase dramatically; likewise, the available bandwidth per computational operation will continue to decrease. Integrated photonics can offer savings in power and potential increase in bandwidth for onchip networks. Classical diffraction-limited photonics currently utilized in photonic integrated circuits (PIC) is characterized by bulky and inefficient devices compared to their electronic counterparts due to weak light–matter interactions (LMI). Performance critical for the PIC is electro-optic modulators (EOM), whose performances depend inherently on enhancing LMIs. Current EOMs based on diffraction-limited optical modes often deploy ring resonators and are consequently bulky, photon-lifetime modulation limited, and power inefficient due to large electrical capacitances and thermal tuning requirements. In contrast, wavelength-scale EOMs are potentially able to surpass fundamental restrictions set by classical (i.e. diffraction-limited) devices via (a) high-index modulating materials, (b) nonresonant field and density-of-states enhancements such as found in metal optics, and (c) synergistic onchip integration schemes. This manuscript discusses challenges, opportunities, and early demonstrations of nanophotonic EOMs attempting to address this LMI challenge, and early benchmarks suggest that nanophotonic building blocks allow for densely integrated high-performance photonic integrated circuits.

Review and perspective on ultrafast wavelength-size electro-optic modulators

Integrated optical power splitters are one of the fundamental building blocks in photonic integrated circuits. Conventional multimode interferometer-based power splitters are widely used as they have reasonable footprints and are easy to fabricate. However, it is challenging to realize arbitrary split ratios, especially for multi-outputs. In this Letter, an ultra-compact power splitter with a QR code-like nanostructure is designed by a nonlinear fast search method. The highly functional structure is composed of a number of freely designed square pixels with the size of 120×120  nm which could be either dielectric or air. The light waves are scattered by a number of etched squares with optimized locations, and the scattered waves superimpose at the outputs with the desired power ratio. We demonstrate 1×2 splitters with 1:1, 1:2, and 1:3 split ratios, and a 1×3 splitter with the ratio of 1:2:1. The footprint for all the devices is only 3.6×3.6  μm. Well-controlled split ratios are measured for all the cases. The measured transmission efficiencies of all the splitters are close to 80% over 30 nm wavelength range.

Integrated photonic power divider with arbitrary power ratios

A Mach-Zehnder Interferometer (MZI) liquid sensor, employing ultra-compact double-slot hybrid plasmonic (DSHP) waveguide as active sensing arm, is developed. Numerical results show that extremely large optical confinement factor of the tested analytes (as high as 88%) can be obtained by DSHP waveguide with optimized geometrical parameters, which is larger than both, conventional SOI waveguides and plasmonic slot waveguides with same widths. As for MZI sensor with 40μm long DSHP active sensing area, the sensitivity can reach as high value as 1061nm/RIU (refractive index unit). The total loss, excluding the coupling loss of the grating coupler, is around 4.5dB.

High-sensitivity liquid refractive-index sensor based on a Mach-Zehnder interferometer with a double-slot hybrid plasmonic waveguide

We propose a silicon-compatible plasmonic electro-optic modulator employing a silicon racetrack ring resonator coupled to a bus waveguide. A silicon-plasmonic hybrid phase shifter clad with electro-optic polymer is introduced to achieve high-speed performance and low energy consumption. Simulations show that the proposed modulator can achieve an extinction ratio of more than 15 dB at 1550-nm wavelength under a 1.2-V bias voltage. The misalignment tolerance and fabrication feasibility of the modulator are also discussed.

Design of an Electro-Optic Modulator Based on a Silicon-Plasmonic Hybrid Phase Shifter

A broad-band sum frequency generation microscope has been developed for the study of molecular monolayers on surfaces. Because sum frequency generation is a vibrational spectroscopy based on a second-order optical process, it is uniquely sensitive to detecting a molecule’s vibrational fingerprints specifically at interfaces. In this microscope, a structured illumination beam generated by a spatial light modulator is used to irradiate the sample with a series of sparsifying pseudorandom patterns. The spectra associated with each pattern are then input into a reconstruction algorithm to compressively recover the full hyperspectral image cube. As a proof-of-principle, this system performed molecule-specific imaging of a microcontact-printed self-assembled monolayer of alkanethiolate on copper. This hyperspectral compressive imaging effectively recovered both spatial and spectral surface features with compression greater than 80%, meaning more than a 5-fold decrease in acquisition time compared to traditional...

Compressive Broad-Band Hyperspectral Sum Frequency Generation Microscopy to Study Functionalized Surfaces.

Modern Automatic Speech Recognition (ASR) systems can achieve high performance in terms of recognition accuracy. However, a perfectly accurate transcript still can be challenging to read due to grammatical errors, disfluency, and other errata common in spoken communication. Many downstream tasks and human readers rely on the output of the ASR system; therefore, errors introduced by the speaker and ASR system alike will be propagated to the next task in the pipeline. In this work, we propose a novel NLP task called ASR post-processing for readability (APR) that aims to transform the noisy ASR output into a readable text for humans and downstream tasks while maintaining the semantic meaning of the speaker. In addition, we describe a method to address the lack of task-specific data by synthesizing examples for the APR task using the datasets collected for Grammatical Error Correction (GEC) followed by text-to-speech (TTS) and ASR. Furthermore, we propose metrics borrowed from similar tasks to evaluate performance on the APR task. We compare fine-tuned models based on several open-sourced and adapted pre-trained models with the traditional pipeline method. Our results suggest that finetuned models improve the performance on the APR task significantly, hinting at the potential benefits of using APR systems. We hope that the read, understand, and rewrite approach of our work can serve as a basis that many NLP tasks and human readers can benefit from.

Improving Readability for Automatic Speech Recognition Transcription

A mode-selective grating reflector based on a hybrid plasmonic waveguide, together with a two-to-one coupling structure, is proposed and analyzed. The grating structure is designed to reflect only the fundamental even mode existing in the hybrid plasmonic waveguide, and to pass through the fundamental odd mode at a 1550-nm wavelength with an extinction ratio of ~16.7 dB. The simulated transmission spectra verify our analysis and the expected features of the mode-selective hybrid plasmonic Bragg grating.

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Mode-Selective Hybrid Plasmonic Bragg Grating Reflector

Sum frequency generation microscopy is a label-free optical imaging technique with intrinsic molecular vibrational contrast for surface studies. Recent developments of compressive sensing broad-band hyperspectral SFG microscopy have demonstrated the potential application for imaging monolayer at metal surfaces with micrometer spatial resolution. Here is presented the capability of chemical imaging of spatially patterned monolayers of 1-octadecanethiol (ODT) and 16-methoxy-1-hexadecanethiol (MeOHT) molecules assembled on a copper surface. The spatial distributions of the monolayer with vibrational-spectral contrast are well-demonstrated at different frequency regions through reconstruction of the hypercube using a 3-dimensional total variation regularization algorithm (3DTV). Spatial-chemical distributions of each component are also reconstructed directly from the compressive measurements by endmember unmixing (CEU) scheme. Compared to 3DTV algorithm, the reconstruction from CEU shows spatial distribution ...

Liyang Lu

Papers

Design of an Electro-Optic Modulator Based on a Silicon-Plasmonic Hybrid Phase Shifter

Compressive Broad-Band Hyperspectral Sum Frequency Generation Microscopy to Study Functionalized Surfaces.

Improving Readability for Automatic Speech Recognition Transcription

Mode-Selective Hybrid Plasmonic Bragg Grating Reflector

Chemical Imaging of Self-Assembled Monolayers on Copper Using Compressive Hyperspectral Sum Frequency Generation Microscopy