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.

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.

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.

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.

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.

Highly confined surface plasmon polaritons in the ultraviolet region

Surface plasmon polaritons are commonly believed to be a future basis for the next generation of optoelectronic and all-optical devices. To achieve this, it is critical that the surface plasmon polariton modes be strongly confined to the surface and have a sufficiently long propagation length and a nanosize wavelength. As of today, in the visible part of the spectrum, these conditions are not satisfied for any type of surface plasmon polaritons. In this paper, we demonstrate that in the ultraviolet range, surface plasmon polaritons propagating along a periodically nanostructured aluminum-dielectric interface have all these properties. Both the confinement length and the wavelength of the mode considered are smaller than the period of the structure, which can be as small as 10 nm. At the same time, the propagation length of new surface plasmon-polaritons can reach dozens of its wavelengths. These plasmon polaritons can be observed in materials that are uncommon in plasmonics such as aluminum. The suggested modes can be used for miniaturization of optical devices.

Superradiance of a subwavelength array of classical nonlinear emitters

We suggest a mechanism by which a superradiant burst emerges from a subwavelength array of nonlinear classical emitters that are not initially synchronized. The emitters interact via the field of their common radiative response. We show that only if the distribution of initial phases is not uniform does a non-zero field of radiative response arise, leading to a superradiant burst. Although this field cannot synchronize the emitters, it engenders long period envelopes for their fast oscillations. Constructive interference in the envelopes of several emitters creates a large fluctuation in dipole moments that results in a superradiant pulse. The intensity of this pulse is proportional to the square of the number of emitters participating in the fluctuation.

Spaser operation below threshold: autonomous vs. driven spasers

At the plasmon resonance, high Joule losses in a metal nanoparticle of a spaser result in its low Q-factor. Due to the latter, to achieve the spasing regime, in which the number of coherent plasmons exceeds the number of incoherent plasmons, unsustainably high pump rates may be required. We show that under the condition of loss compensation by a spaser driven by an external optical wave, the number of coherent plasmons increases dramatically, and the quantum noise is suppressed. Since the compensation of losses of the driving wave may occur even near the spasing threshold, the number of coherent plasmons may exceed the number of spontaneously excited plasmons at achievable pump rates.

Lasing induced by resonant absorption

We theoretically demonstrate that increase of absorption with constant gain in laser systems can lead to onset of laser generation. This counterintuitive absorption induced lasing (AIL) is explained by emergence of additional lasing modes created by an introduction of an absorbing medium with narrow linewidth. We show that this effect is universal and, in particular, can be encountered in simple Fabry-Perot-like systems and doped spherical dielectric nanoresonators. The predicted behavior is robust against detuning between the resonant frequencies of gain and absorbing medium.

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.

Loss compensation by spasers in plasmonic systems

We show that in plasmonic systems, exact loss compensation can be achieved with the help of spasers pumped over a wide range of pumping values both below and above the spasing threshold. We demonstrate that the difference between spaser operation below and above the spasing threshold vanishes, when the spaser is synchronized by an external field. As the spasing threshold loses its significance, a new pumping threshold, the threshold of loss compensation, arises. Below this threshold, which is smaller than the spasing threshold, compensation is impossible at any frequency of the external field.

Forced synchronization of spaser by an external optical wave

We demonstrate that when the frequency of the external field differs from the lasing frequency of an autonomous spaser, the spaser exhibits stochastic oscillations at low field intensity. The plasmon oscillations lock to the frequency of the external field only when the field amplitude exceeds a threshold value. We find a region of values of the external field amplitude and the frequency detuning (the Arnold tongue) for which the spaser synchronizes with the external wave.

Dipole response of spaser on an external optical wave

We find the conditions upon the amplitude and frequency of an external electromagnetic field at which the dipole moment of a Bergman-Stockman spaser oscillates in antiphase with the field. For these values of the amplitude and frequency the loss in metal nanoparticles is exactly compensated by the gain. This shows that spasers may be used as inclusions in designing lossless metamaterials.

Optical Tamm states in one-dimensional magnetophotonic structures

We demonstrate the existence of a spectrally narrow localized surface state, the so-called optical Tamm state, at the interface between one-dimensional magnetophotonic and nonmagnetic photonic crystals. The state is spectrally located inside the photonic band gaps of each of the photonic crystals comprising this magnetophotonic structure. This state is associated with a sharp transmission peak through the sample and is responsible for the substantial enhancement of the Faraday rotation for the corresponding wavelength. The experimental results are in excellent agreement with the theoretical predictions.