Microfluidic mid-infrared spectroscopy via microresonator-based dual-comb source

Recent advances in droplet sequential monitoring methods for droplet sorting

Droplet microfluidics is an attractive technology to run parallel experiments with high throughput and scalability while maintaining the heterogeneous features of individual samples or reactions. Droplet sorting is utilized to collect the desired droplets based on droplet characterization and in-droplet content evaluation. A proper monitoring method is critical in this process, which governs the accuracy and maximum frequency of droplet handling. Until now, numerous monitoring methods have been integrated in the microfluidic devices for identifying droplets, such as optical spectroscopy, mass spectroscopy, electrochemical monitoring, and nuclear magnetic resonance spectroscopy. In this review, we summarize the features of various monitoring methods integrated into droplet sorting workflow and discuss their suitable condition and potential obstacles in use. We aim to provide a systematic introduction and an application guide for choosing and building a droplet monitoring platform.

Modelling second harmonic generation at mid-infrared frequencies in waveguide integrated Ge/SiGe quantum wells

A promising alternative to bulk materials for the nonlinear coupling of optical fields is provided by photonic integrated circuits based on heterostructures made of asymmetric-coupled quantum wells. These devices achieve a huge nonlinear susceptivity but are affected by strong absorption. Here, driven by the technological relevance of the SiGe material system, we focus on Second-Harmonic Generation in the mid-infrared spectral region, realized by means of Ge-rich waveguides hosting p-type Ge/SiGe asymmetric coupled quantum wells. We present a theoretical investigation of the generation efficiency in terms of phase mismatch effects and trade-off between nonlinear coupling and absorption. To maximize the SHG efficiency at feasible propagation distances, we also individuate the optimal density of quantum wells. Our results indicate that conversion efficiencies of ≈ 0.6%/W can be achieved in WGs featuring lengths of few hundreds µm only.

Upconversion time-stretch infrared spectroscopy

High-speed measurement confronts the extreme speed limit when the signal becomes comparable to the noise level. In the context of broadband mid-infrared spectroscopy, state-of-the-art ultrafast Fourier-transform infrared spectrometers, in particular dual-comb spectrometers, have improved the measurement rate up to a few MSpectra s-1, which is limited by the signal-to-noise ratio. Time-stretch infrared spectroscopy, an emerging ultrafast frequency-swept mid-infrared spectroscopy technique, has shown a record-high rate of 80 MSpectra s-1 with an intrinsically higher signal-to-noise ratio than Fourier-transform spectroscopy by more than the square-root of the number of spectral elements. However, it can measure no more than ~30 spectral elements with a low resolution of several cm-1. Here, we significantly increase the measurable number of spectral elements to more than 1000 by incorporating a nonlinear upconversion process. The one-to-one mapping of a broadband spectrum from the mid-infrared to the near-infrared telecommunication region enables low-loss time-stretching with a single-mode optical fiber and low-noise signal detection with a high-bandwidth photoreceiver. We demonstrate high-resolution mid-infrared spectroscopy of gas-phase methane molecules with a high resolution of 0.017 cm-1. This unprecedentedly high-speed vibrational spectroscopy technique would satisfy various unmet needs in experimental molecular science, e.g., measuring ultrafast dynamics of irreversible phenomena, statistically analyzing a large amount of heterogeneous spectral data, or taking broadband hyperspectral images at a high frame rate.

Modelling of an intersubband quantum confined Stark effect in Ge quantum wells for mid-infrared photonics

In this work we theoretically investigate quantum confined Stark effect of intersubband transitions in asymmetric Ge/SiGe quantum wells for intensity modulation in the mid-infrared. Our calculations show that extinction ratios up to 1 dB and modulation speeds of several tens of GHz could be obtained in 100 µm long waveguides.

Study of microcomb threshold power with coupling scaling

We model the generation threshold and conversion efficiency of microcombs by scaling the cavity coupling. With the Lugiato–Lefever equation (LLE), quantitative analysis of threshold is established in the parameter space of pump power and coupling. Considering the large detuning and Kerr-induced phase shift, the threshold power is numerically solved with the minimum at over-coupling, in agreement with that from the traveling wave theory. Furthermore, the coupling dependence on microcomb generation is discussed, providing the accessibility of high-efficient, stable combs (≥ 40%) around the threshold. This work offers universal guidelines for the design of microcombs with low-power and high-efficient operation.