Optics and Spectroscopy最新文献

We consider the process of transition of a two-level system to the excited state with subsequent photon emission in the presence of a laser pulse with a high degree of unipolarity. Within the framework of quantum electrodynamics, we obtain analytical expressions for the differential probability of the process depending on the temporal scales of the problem: laser pulse duration, excited-state lifetime, inverse transition frequency, and inverse frequency of the photon emitted. Besides, we calculate the total absorption probability by integrating over the three-dimensional photon momentum and summing over polarizations. We compare the results obtained for unipolar and bipolar (many-cycle) pulses.

The most widely used and applied plasmonic materials, namely silver and gold, has limitations due to their high cost and restriction on the spectral position and shape of the plasmon resonance. This remains true for bimetallic silver–gold nanoparticles. Higher flexibility is required, in particular, for the design of broadband absorbers of light, and for this task the metals other than silver and gold are considered. In this paper we study the optical extinction spectra of alloy and composite nanoparticles containing magnesium and gold. The dielectric properties are calculated within the approximation of independent particles (IPA) based on the electronic structure obtained using density functional theory (DFT) with Hubbard correction (DFT+U). The obtained spectra of optical extinction of magnesium–gold alloy nanoparticles demonstrate that the most sensitive to the composition is the region of wavelengths below 500 nm. Simultaneously, the position of the plasmon resonance predicted by Vegard’s law is higher than obtained from accurate DFT+U based calculations. We managed to describe the experimental optical extinction spectra of the glass sample containing gold and magnesium atoms using the calculated spectra. The results points on the formation of composite nanoparticles with core of Au₃Mg alloy and shell of Au in the considered sample.

Raman spectroscopy (RS) is widely used in biomedicine to detect various chemical compounds in substrates, as well as to determine their concentration. In this work, RS was used to detect bilirubin in the hepatic bile of patients with obstructive jaundice (OJ) of different etiologies obtained via a cholangiostomy drainage catheter. The spectral bands of bilirubin (1258–1264 and 1615–1620 cm^–1) were identified using the excitation wavelength of 785 nm. It was found that the assessment of bilirubin in bile allows to predict the recovery of excretory liver function and the dynamics of convalescence of patients with OJ after biliary decompression.

A decrease in external quantum efficiency (EQE) of commercial LEDs based on MQW InGaN/GaN at wavelengths of 445, 530 and MQW AlGaN/GaN at 280 nm was experimentally studied in the standard aging mode at direct current. The decrease in EQE (regardless of the radiation wavelength) is found out to occur due to cooperative phenomena developing in 1–2 quantum wells (QWs) located in the space charge region (SCR) around the p–n junction, as well as in most of the QWs outside of SCR. It is shown that the inhomogeneous flow of current in these regions leads not only to the transformation of defects localized at heteroboundaries in the SCR and in lateral inhomogeneities in the alloy composition outside of SCR as well as in the extended defects, but also to a change in the alloy composition.

The effect of gold and silver nanoparticles with plasmon resonance on the chemiluminescence of luminol in an oxidizing environment was studied. It has been shown that when a colloidal solution of metal nanoparticles is added to luminol, the intensity of chemiluminescence caused by the presence of hydrogen peroxide and sodium hypochlorite increases and can be easily recorded at a pH level characteristic of biological media close to neutral, at which chemiluminescence is weak in the absence of metal nanoparticles.

A relatively simple physical method has been proposed for fabrication of stable oligomers of silver nanoparticles, preliminarily obtained by pulsed laser ablation of a metal target in a liquid. Oligomers of silver nanoparticles are formed in an aqueous solution after prolonged centrifugation at 18 000g and subsequent ultrasonication of the initial colloidal solution of spherical nanoparticles obtained by laser ablation. The plasmon resonance in oligomers is shifted relative to the plasmon resonance in spherical nanoparticles to the long wavelength region by 140 nm.

The interaction of NiO clusters with a graphene layer of a nanotube in NiO_x/CNT composites is simulated based on a theoretical analysis of Ni K-edge XANES spectra. Various cases of orientation of crystallographic planes relative to the nanotube wall are considered. It was determined that Ni in the composite is predominantly in the 2+ state, NiO particles are oriented towards the tube wall by the (200) plane. Accounting for carbon atoms even in the first sphere of the nickel environment makes it possible to achieve good agreement between the theoretical spectrum and experiment.

The results of experimental studies, theoretical analysis and numerical simulations of the features of the volume transmission holograms' formation by the interference pattern of laser beams with high contrast in the X-cut LiNbO₃:Cu, fabricated by diffusion from a metal film, are presented. Analytic formulas that take the inhomogeneities of the distribution of copper ions and the absorption index of recording beams over the thickness of the sample into account are derived to describe the time evolution of the amplitude of the first spatial harmonic of the field. The material parameters of the LiNbO₃:Cu structure are estimated. A comparative analysis of the spatial distribution of the amplitude of the first harmonic of the photorefractive hologram’s field in two types of LiNbO₃:Cu structures is performed. We concluded that it is necessary to use X‑cut LiNbO₃:Cu diffusion structures characterized by similar distributions of impurity centers, which maximums are localized near the boundary used to capture micro- and nanoparticles, for the implementation of photovoltaic tweezers.

Crystals of bismuth fluoride BiF₃ grown from a melt were studied for the first time by optical spectroscopy methods and calculated from first principles in the phonon excitations region. A study of IR reflectance spectra in polarized light was carried out. The parameters of optical phonons were obtained. In the IR reflectance spectra, a low-frequency rise in the range <500 cm^–1, characteristic of conductive materials, is observed. The reflection spectra were analyzed within the framework of the Drude–Lorentz model, taking into account the contribution of ionic conductivity. An ab initio calculation of the phonon spectrum of a BiF₃ crystal was carried out and the relationship between theoretical and experimental data was analyzed.

The results of numerical simulation of the optical properties of a one-dimensional (1D) photonic crystal with a defect based on semiconductor-dielectric layers in the near-IR range are presented. The simulations used layers of silicon and silicon dioxide with optical thicknesses 3λ/4, λ/4 and 10λ/4. The influence of radiation incident angle on the spectral position of the defect’s guidance band has been studied. It is shown that the sensitivity to the rotation angle lies within the limits of 6–20 nm/deg and 1.7–5.5 dB/deg, depending on the geometry of the sensor and the measurement method. This makes these photonic crystals promising for use in the rotation angle sensors as a sensitive element.