Laser‐based mid‐infrared (mid‐IR) photothermal spectroscopy (PTS) represents a selective, fast, and sensitive analytical technique. Recent developments in laser design permits the coverage of wider spectral regions in combination with higher power, enabling for qualitative reconstruction of broadband absorption features, typical of liquid or solid samples. In this work, we use an external cavity quantum cascade laser (EC‐QCL) that emits in pulsed mode in the region between 5.7 and 6.4 µm (1770–1560 cm ⁻¹ ), to measure the absorption spectrum of a thin film of polymethyl methacrylate (PMMA) spin‐coated on top of a silicon nitride (Si ₃ N ₄ ) micro‐ring resonator (MRR). Being the PTS signal inversely proportional to the volume of interaction, in the classical probe–pump dual beam detection scheme, we exploit a Si ₃ N ₄ transducer coated with PMMA, as a proof‐of‐principle for an on‐chip photothermal sensor. By tuning the probe laser at the inflection point of one resonance, aiming for highest sensitivity, we align the mid‐IR beam on top of the ring’s area, in a transversal configuration. To maximize the amplitude of the photoinduced thermal change, we focus the mid‐IR light on top of the ring using a Cassegrain reflector enabling for an optimal match between ring size and beam waist of the excitation source. We briefly describe the transducer design and fabrication process, present the experimental setup, and perform an analysis for optimal operational parameters. We comment on the obtained results showing that PTS allows for miniaturized robust sensors opening the path for on‐line/in‐line monitoring in several industrial processes.

We demonstrated, for the first time, micro-ring resonator assisted photothermal spectroscopy measurement of a gas phase sample. The experiment used a telecoms wavelength probe laser that was coupled to a silicon nitride photonic integrated circuit using a fibre array. We excited the photothermal effect in the water vapor above the micro-ring using a 1395 nm diode laser. We measured the 1f and 2f wavelength modulation response versus excitation laser wavelength and verified the power scaling behaviour of the signal.

Using electronic beam lithography and reactive ion beam etching, a metalens is manufactured in a thin layer of amorphous silicon of a 130-nm depth, a 30-µm diameter, and a 633-nm focal length (equal to the illumination wavelength). The metalens is composed of 16 sectored subwavelength binary gratings with a 220-nm period. The uniqueness of this metalens is that when illuminated by left-handed circularly polarized light, it is capable of generating a left-handed circularly polarized vortex beam with a topological charge of 2, generating a second-order cylindrical vector beam when illuminated by linearly polarized light. Both for linear and circular incident polarization, an energy backflow is found to be generated in the vicinity of the tight focus. Transverse intensity distributions measured with a scanning near-field optical microscope near the focus of the metalens are in qualitative agreement with the intensity distributions calculated by the FDTD method. This confirms that a backward energy flow takes place at the focus of the metalens. A metalens generating an energy backflow near its focus is fabricated and characterized for the first time.

We designed, numerically simulated, fabricated and characterized a thin metalens in an amorphous silicon film. The lens has a diameter of 30 µm, a 633-nm focal length equal to the incident wavelength, a 220-nm period of the subwavelength features, and a 110-nm relief depth. The lens simultaneously controls the phase and polarization of the incident laser light, transforming linear polarization into the radial one and generating a sharp focus. The metalens was fabricated using electron lithography and reactive ion etching. The focal spot obtained from an incident linearly polarized Gaussian beam of wavelength λ measured 0.49λ and 0.55λ on the x- and y-axes. This is a record-breaking focal spot ever obtained using metalenses. Experimental results agree well with the numerically simulated focal spot size of 0.46λ and 0.52λ.

This work reports on the implementation of a wafer-bonded Fabry–Perot interferometer (FPI) as the transducer of a photothermal spectroscopy system. A near-infrared laser is collimated and propagated through an approximately 1-mm-spaced Fabry-Pérot (FP) cavity to probe the photo-thermal effect induced by the pump beam. The sensor’s metrological figures of merit were evaluated by detecting methane (CH4) gas molecules at various concentrations. For the absorption band centered at 6047 cm⁻¹, a minimum detection limit (MDL) of 42 ppm was achieved with a 1 s integration time, showing that the sensor system has significant potential for the design of compact, stable, and versatile gas detectors with excellent sensitivity. From Allan deviation, we estimate an MDL of 5.1 ppm for an optimum integration time of 100 s.