Thermal lens microscopy (TLM) utilizes the effects, such as changes in the refractive index, caused by heat generated in the sample for the detection of nonfluorescent analytes with at least a hundred-fold enhancement in sensitivity compared with optical absorbance spectrometry. Micro-/nanofluidic devices can provide specificity and sample manipulation capabilities. The integration of these two technologies exposes the potential to attain the holy grail of continuous, real-time, label-free, specific, and ultrasensitive detection, which find applications in environmental monitoring, quality control of chemical manufacturing, single-cell analysis, and biomedicines. Here, we summarize the recent advances in the instrument development and innovative applications, and suggest future directions of research of TLM.

The measurement of microwave electric-field (E-field) exposure is an ever-evolving subject that has recently led the International Commission on Non-Ionizing Radiation Protection to change its recommendations. With frequencies increasing toward terahertz (THz), stimulated by 5G deployment, the measurement specifications reveal ever more demanding challenges in terms of bandwidth (BW) and miniaturization. We propose a focus on minimally invasive E-field sensors, which are crucial for the in situ and near-field characterization of E-fields both in harsh environments such as plasmas and in the vicinity of emitters. We browse the large varieties of measurement devices, among which the electro-optic (EO) probes stand out for their potential of high BW up to THz, minimal invasiveness, and ability of vector measurements. We describe and compare the three main categories of EO sensors, from bulk systems to nanoprobes. First, we show how bulk-sensors have evolved toward attractive fibered systems that are advantageously employed in plasmas, resonance magnetic imagings chambers or for radiation-pattern imaging up to THz frequencies. Then we describe how the integration of waveguides helps to gain robustness, lateral resolution, and sensitivity. The third part is dedicated to the ultra-miniaturization of components allowing ultimate steps toward electromagnetic invisibility. This review aims at pointing out the recent evolutions over the past 10 years, with a highlight on the specificities of each photonic architecture. It also shows the way to future multi-physics and multi-arrays smart sensing platforms.

We report two approaches of the fabrication techniques for three-dimensional patterning across vertical sidewalls based on the photolithography. One approach applies the techniques of spray coating the photoresist and the angled exposure. The supposed application is the microenergy source summing the voltage of silicon (Si) photo cells. Since Si photo cell islands on the buried oxide are isolated from each other, the output voltage is summed well without interacting with the common substrate. Wiring using the vertical sidewalls minimizes the shadow region caused by the metal electrode. Another approach introduces a new sheet, which consists of polyvinyl alcohol (PVA) and polyethylene terephthalate layers. The photoresist film is spin-coated and pasted on the substrate making the bridge over the trench. The pattern of the cantilever overhanging the trench is transferred. Then, PVA is dissolved in water and the resist is developed. When the rinse water dries, the cantilever bends and adheres on the vertical sidewall. The minimum pattern width designed is 10  μm and is obtained on the vertical sidewall.

We designed an on-chip transformative optic system with a broadband metalens coupler on a foundry compatible silicon photonic platform. By adjusting the on-chip metalens’ focusing length and mode dimension, the insertion loss between the metalens and the photonic crystal waveguide (PhC WG) structures is reduced to 2 dB by matching the mode on the metalens focal plane to the PhC WG mode. Alternatively, the integrated metalens allow for direct coupling from a multi-mode WG to the PhC cavity. The on-resonance transmission in a lens–cavity–lens microsystem achieves 60%. These micro-systems do not involve any single-mode silicon nanowire WG, and even a suspended PhC structure can be mechanically robust against vibrations. The proposed microsystem can be a new platform for miniaturized chemical and biosensor applications operating in air or solution environments.

We report on the design and measurement of tellurium oxide microcavity resonators coupled to silicon bus waveguides on silicon photonic chips. The resonators are fabricated using a standard silicon photonics foundry processing flow in which the SiO₂ top-cladding is etched in a ring shape and aligned next to a silicon bus waveguide. The resulting microtrench is coated in a tellurium oxide film by reactive sputtering in a post-processing step to form the waveguiding layer of the resonator. A 100-μm radius trench with a 1115-nm-thick TeO₂ film is measured to have an internal Q factor of 0.9  ×  10⁵. Smoothing the etch wall surface with a fluoropolymer coating is shown to enhance the Q factor of several devices, with a trench coated in a 630-nm-thick TeO₂ film demonstrating a Q factor of 2.1  ×  10⁵ corresponding to 1.7-dB/cm waveguide loss. These results demonstrate a potential pathway toward monolithic integration of tellurite glass-based nonlinear and rare-earth-doped devices compatible with silicon photonics platforms.

We report on a 32  ×  32 silicon photonic micro-electro-mechanical-system (MEMS) switch with gap-adjustable directional couplers. The switch is fabricated on 200-mm silicon-on-insulator wafers in a commercial complementary metal-oxide-semiconductor (CMOS) foundry. The fabricated device has a maximum on-chip loss of 7.7 dB and an extinction ratio of 50.8 dB. The switching voltage is 9.45 V and the 20-dB bandwidth is 28.7 nm. Our work shows a promising path for mass production of silicon photonic MEMS switches in commercial CMOS foundries.

Point-scanning-based microscopy systems require combination of axial and lateral scanning to obtain three-dimensional (3D) data. Axial scanning was commonly achieved by mechanical displacement of the objective or the sample. To improve, various adaptive lens-based solutions have been reported to circumvent the need for mechanically moving parts. The lateral scanning is predominantly implemented using galvanometric mirrors. Although the performance of such devices is flawless, they require bulky, folded beam-paths that make their incorporation in compact hand-held devices challenging. Recently, we introduced an adaptive prism as a transmissive device that enables lateral scanning. We demonstrate the first all-adaptive 3D scanning in laser scanning microscopes employing a compact in-line transmission geometry without mechanically moving parts and beam folding, combining an adaptive lens and a novel adaptive prism. Characterization of the all-adaptive microscope performance shows a lateral tuning range of approximately X  =  Y  =  130  μm and an axial tuning range of about Z  =  500  μm. We successfully demonstrate 3D raster scanning of the fluorescence of a thyroid of a zebrafish embryo.