Optical sensors typically provide compact, fast and precise means of performing quantitative measures for almost any kind of measurand that is usually probed electronically. High-contrast grating (HCG) resonators are known to manifest an extremely sharp and sensitive optical resonance and can constitute a highly suitable sensing platform. In this paper we present two advanced high-contrast grating designs improving the sensing performances of conventional implementations. These configurations, namely pedestal and half-buried HCGs, allow to enhance the shift of the photonic resonance while maintaining the spectral features of the standard configuration. First, the spectral feature of the HCGs was numerically optimized to express the sharpest possible resonance when the structure is immersed in serum. Second, the sensing properties of conventional and advanced HCG implementations were studied by modelling the biological entities to be sensed as a thin dielectric coating layer of increasing thickness. Pedestal HCGs were found to provide a ∼12% improvement in sensitivity and a six-fold improvement in resonance quality factor (Q-factor), while buried HCGs resulted in a ∼58% improvement in sensitivity at the expense of a slightly broader resonance. Such structures may serve as an improved sensitive biosensing platform for near-infrared spectroscopy.

We propose a method of spatial modulation of inhomogeneous broadening of a quantum-well excitonic resonance based on local generation of defects produced by a focused ion beam. The method is applied to fabrication of excitonic diffraction grating in a single quantum-well InGaAs/GaAs structure by irradiating the sample with a beam of 35-keV He⁺ ions of exposure doses <10¹² cm^-2. The spectrum of resonant diffraction on such a structure is narrower than that of reflectivity and decreases much faster with increasing temperature. A proposed model of formation of the diffractive response based on the single scattering approximation well describes the results of the spectral and temperature measurements.

High-contrast gratings (HCG) are an excellent candidate for label-free detection of various kinds of biomarkers because they exhibit sharp and sensitive optical resonances. In this work, we experimentally show the performance of pedestal HCG (PHCG), which is significantly enhanced in comparison with that of conventional HCG. PCHGs were found to provide a 11.2% improvement in bulk refractive index sensitivity, from 482 nm/RIU for the conventional design to 536 nm/RIU. The observed resonance was narrower, resulting in a higher Q-factor and figure of merit. By depositing Al2O3, HfO2, and TiO2 of different thicknesses as model analyte layers, surface sensitivity values were estimated to be 10.5% better for PHCG. To evaluate the operation of the sensor in solution, avidin was employed as a model analyte. For avidin detection, the surface of the HCG was first silanized and subsequently functionalized with biotin, which is well known for its ability to bind selectively to avidin. A consistent red shift was observed with the addition of each of the functional layers, and the analysis of the spectral shift for various concentrations of avidin made it possible to calculate the limit of detection (LoD) and limit of quantification (LoQ) for the structures. PHCG showed a LoD of 2.1 ng/mL and LoQ of 85 ng/mL, significantly better than the values 3.2 ng/mL and 213 ng/mL respectively, obtained with the conventional HCG. These results demonstrate that the proposed PHCG have great potential for biosensing applications, particularly for detecting and quantifying low analyte concentrations.

Photoluminescence including fluorescence plays a great role in a wide variety of applications from biomedical sensing and imaging to optoelectronics. Therefore, the enhancement and control of photoluminescence has immense impact on both fundamental scientific research and aforementioned applications. Among various nanophotonic schemes and nanostructures to enhance the photoluminescence, we focus on a certain type of nanostructures, hyperbolic metamaterials (HMMs). HMMs are highly anisotropic metamaterials, which produce intense localized electric fields. Therefore, HMMs naturally boost photoluminescence from dye molecules, quantum dots, nitrogen-vacancy centers in diamonds, perovskites and transition metal dichalcogenides. We provide an overview of various configurations of HMMs, including metal-dielectric multilayers, trenches, metallic nanowires, and cavity structures fabricated with the use of noble metals, transparent conductive oxides, and refractory metals as plasmonic elements. We also discuss lasing action realized with HMMs.

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The ultrafast photochemistry of the [Cr­(NCS)6]3– complex upon excitation to the 4T2 ligand-field (LF) state was studied in dimethyl sulfoxide (DMSO) and N,N-dimethylformamide (DMF) in a wide temporal range (100 fs to 9 ms) by a combination of femtosecond and nanosecond transient absorption spectroscopy techniques and supported by quantum-chemical DFT/TD-DFT calculations. The initially excited 4T2 state undergoes intersystem crossing to the vibrationally hot 2E state with time constants of 1.1 ± 0.2 and 1.8 ± 0.1 ps in DMSO and DMF, respectively. Vibrational relaxation occurs in the same time scale and takes 1–5 ps. A major part of the [Cr­(NCS)6]3– complex in the 2E state undergoes intersystem crossing to the ground state with time constants of 65 ± 5 and 85 ± 5 ns in DMSO and DMF, respectively. A minor part of electronically excited [Cr­(NCS)6]3– undergoes irreversible photochemical decomposition. In DMSO, the photolysis of the [Cr­(NCS)6]3– complex results in single or double isothiocyanate ion release followed by the coordination of the solvent molecules with a time constant of 1 ± 0.2 ms.

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