Optimal defect position in a DFB fiber laser.

Fiber lasers with compact cavity have numerous potential applications in sensing, communications, and medicine. Distributed feedback (DFB) rare-earth doped fiber lasers based on Bragg gratings with a phase shift are the most promising in this aspect. In this paper, we theoretically study such lasers and carry out a complex-frequency analysis of the DFB cavity modes. Our approach is based on the study of poles of open cavity response function and on the laser rate equations. An optimal defect position in the Bragg grating, which maximizes an output power towards one side, was found with this approach. We show that the optimal defect position depends on the pump power. At the pump level close to the lasing threshold, the defect should preferably appear in the middle of the grating to maximize the one-side output power. At higher pumping, the optimal position of the defect becomes asymmetric. We have found specific variables, which allow for determination of optimal defect position for a large variety of DFB laser configurations.

Evaluating the Response Time of an Optical Gas Sensor Based on Gasochromic Nanostructures.

We propose a method for determining complex dielectric permittivity dynamics in the gasochromic oxides in the course of their interaction with a gas as well as for estimating the diffusion coefficient into a gasochromic oxide layer. The method is based on analysis of a time evolution of reflection spectra measured in the Kretschmann configuration. The method is demonstrated with a hydrogen-sensitive trilayer including an Au plasmonic film, WO3 gasochromic oxide layer, and Pt catalyst. Angular dependences of the reflectance as well as transmission spectra of the trilayer were measured in series at a constant flow of gas mixtures with hydrogen concentrations in a range of 0-0.36%, and a detection limit below 40 ppm (0.004%) of H2 was demonstrated. Response times to hydrogen were found in different ways. We show that the dielectric permittivity dynamics of WO3 must be retrieved in order to correctly evaluate the response time, whereas a direct evaluation from intensity changes for chosen wavelengths may have a high discrepancy. The proposed method gives insight into the optical properties dynamics for sensing elements based on gasochromic nanostructures.

Optical properties of tungsten trioxide, palladium, and platinum thin films for functional nanostructures engineering.

In recent years, we have been witnessing the intensive development of optical gas sensors. Thin palladium and platinum films as well as tungsten trioxide films with palladium or platinum catalysts are widely used for hydrogen detection, and the optical constants of these materials are required for sensor development. We report the optical parameters retrieved from a set of ellipsometric and transmission spectra for electron-beam evaporated palladium, platinum, and tungsten trioxide films. The tungsten trioxide films were 81 nm, 162 nm, and 515 nm thick and the metal films were as thin as 5-7 nm. Ultrathin palladium and platinum films were shown to be successfully described by local and isotropic permittivity, which is quite different from known bulk values. However, this permittivity showed a strong dependence on adjacent materials, thus illustrating that the ultrathin metallic films can be considered composites characterized by effective permittivity. With the obtained refractive indices and permittivities, the optical spectra of fabricated WO3/Pd and WO3/Pt nanostructures incorporating 1D grating of Al2O3 were in an excellent agreement with the calculated ones without requiring any additional fitting procedures or inclusion of surface roughness layers in numerical models.

Optical properties of tungsten trioxide, palladium and platinum thin films for functional nanostructures engineering: erratum.

In our recent paper [D. P. Kulikova Opt. Express28(21), 32049 (2020).10.1364/OE.405403], an early version of Fig. 1 was published. This erratum corrects that error.

Steady state superradiance of a 2D-spaser array.

We show that due to near-field interaction of plasmonic particles via gain particles, a two-dimensional array of incoherently pumped spasers can be self-synchronized so that the dipole moments of all the plasmonic particles oscillate in phase and in parallel to the array plane. The synchronized state is established as a result of competition with the other possible modes having different wavenumbers and it is not destroyed by radiation of leaking waves, retardation effects, and small disorder. Such an array produces a narrow beam of coherent light due to continuous-wave superradiance. Thus, spasers, which mainly generate near-fields, become an efficient source of far-field radiation when the interaction between them is sufficiently strong.

Optical hydrogen sensing with high-Q guided-mode resonance of Al2O3/WO3/Pd nanostructure.

Nanostructure based on a dielectric grating (Al2O3), gasochromic oxide (WO3) and catalyst (Pd) is proposed as a hydrogen sensor working at the room temperature. In the fabricated structure, the Pd catalyst film was as thin as 1 nm that allowed a significant decrease in the optical absorption. A high-Q guided-mode resonance was observed in a transmission spectrum at normal incidence and was utilized for hydrogen detection. The spectra were measured at 0-0.12% of hydrogen in a synthetic air (≈ 80% [Formula: see text] and 20% [Formula: see text]). The detection limit below 100 ppm of hydrogen was demonstrated. Hydrogen was detected in the presence of oxygen, which provides the sensor recovery but suppresses the sensor response. Sensor response was treated by the principal component analysis (PCA), which effectively performs noise averaging. Influence of temperature and humidity was measured and processed by PCA, and elimination of the humidity and temperature effects was performed. Square root dependence of the sensor response on the hydrogen concentration (Sievert's law) was observed. Sensor calibration curve was built, and the sensor resolution of 40 ppm was found. Long term stability of the sensor was investigated. Particularly, it was shown that the sensor retains its functionality after 6 months and dozens of acts of response to gas.