The results of an experimental investigation of a sensor intended for detection and measurement of concentration of nanoparticles in an aqueous medium, which is based on optical-dielectric whispering-gallery- mode microcavities, are presented. Variation of the frequency and Q-factor of the eigenmodes of the microcavity upon its interaction with silver nanoparticles is studied. It is demonstrated that this type of sensor can be used for measurement of infinitesimally low concentrations of nanoparticles.
We have studied high-resolution low-temperature IR luminescence and absorption spectra of undoped high-quality SiC single crystals of the 4H and 6H hexagonal modifications. Narrow lines with a width of smaller than 0.2 cm–1 have been revealed, with some of which being observed for the first time. We have found that some of the lines in the 4H and 6H modifications have similar structures; however, the lines in SiC-4H are shifted to the high-energy part of the spectrum by ~180 cm–1. For the most intense quartet in the range of 1.3 μm, we have succeeded in constructing the energy structure of levels for both the 4H modification and the 6H modification based on their luminescence and absorption spectra.
Experimental results on THz radiation emission by periodic arrays of semiconductor GaAs-based nanowires upon excitation by ultrashort optical pulses are presented. The efficiency of THz radiation emission is found to increase due to the resonant excitation of cylindrical modes in a nanocrystal. The maximum value of THz field can be achieved at distances between nanowires on the order of the excitation wavelength.
Upconversion luminescence of fluoride phosphors SrF2:Er,Yb co-precipitated from aqueous solutions is studied. Under excitation of Er3+ ions to the 4I13/2 level, upconversion luminescence spectra are recorded in the visible and near-IR spectral regions, which correspond to the 4G11/2 → 4I15/2, 2H9/2 → 4I15/2, 4F5/2 → 4I15/2, 4F7/2 → 4I15/2, 2H11/2 → 4I15/2, 4S3/2 → 4I15/2, 4F9/2 → 4I15/2, 4I9/2 → 4I15/2, and 4I11/2 → 4I15/2, electronic transitions of Er3+ ions and to the 2F5/2 → 2F7/2 transition of Yb3+. The absolute quantum yields of visible upconversion luminescence under excitation in the region of 1.5 μm are determined for SrF2:Er,Yb phosphors with different concentrations of Er3t+ and Yb3+ ions and analyzed comparatively with the corresponding values for SrF2:Er phosphor. A study of chromaticity coordinates showed that the yellow-green upconversion luminescence of SrF2:Er,Yb phosphors is characterized by color temperatures of 3037–3526 K at an incident power density of 850 W/cm2.
The results of a study of internal optical losses and current injection efficiency in vertical-emitting lasers of a spectral range of 1.55 µm obtained by sintering plates of high-q Bragg reflectors and the active region on the basis of thin strained InGaAs/InAlGaAs quantum wells have been presented. It has been shown that the proposed design of the laser provides a record low level of internal optical losses (less than 6.5 cm^–1) and high efficiency of current injection (more than 90%) at room temperature, which allows the realization of submilliampere threshold currents. As the temperature rises to 85°C, the current injection efficiency drops to 70% due to the thermal emission of charge carriers from the active region, accompanied by an increase in internal optical losses to 9.1 cm^–1 because of an increase in absorption on free carriers and/or intersubband absorption in the valence band.
Results of investigation of static characteristics of the vertical-cavity surface-emitting lasers (VCSEL) of the 850-nm spectral range based on strained InGaAs/AlGaAs quantum wells in a wide range of current-aperture sizes are presented. The reasons for their anomalous behavior at large design gain-to-cavity wavelength detuning are analyzed. Lateral spreading of carriers in the plane of quantum wells and specific profile of oxide aperture (leading to formation of an effective two-step waveguide) in the studied VCSELs makes possible the existence of higher-order modes localized at the current-aperture periphery. Inhomogeneity of carrier injection across the current aperture in wide-aperture lasers leads to initial onset lasing via the higher-order modes. Subsequent transition to classical lasing via the lower-order modes with increase in current is caused by changes in the gain-to-cavity detuning with increase in internal laser temperature. Anomalous lasing via higher-order modes in the case of narrow-aperture VCSELs becomes possible due to increase in the diffraction losses at the edge of the oxide current aperture for the fundamental mode. In the process, not only a decrease in the gain-to-cavity detuning but also the effect of thermal lens are responsible for subsequent laser hopping to the regime of co-lasing via two modes.
A microscope for studying internal inhomogeneities of the refractive index of optical dielectric microcavities by optical tomography is developed. The influence of these inhomogeneities on the Q factor of optical dielectric microcavities formed by thermal treatment is experimentally studied.
Luminescence spectra of single crystals of rare-earth gallium borates LnGa3(BO3)4 (Ln = Nd, Sm, Tb, Er, Dy, or Ho) at room (300 K) and cryogenic (10 K) temperatures are presented for the first time. Photoluminescence has been recorded in the wavelength range of 470–5000 nm (2000–21300 cm–1) with a high spectral resolution (down to 0.1 cm–1) upon excitation by different diode lasers. The spectra obtained cannot be unambiguously interpreted within one luminescent center, which can be due to the presence of defects and/or inclusions of other crystalline phases. The optical nonlinearity of rare-earth–gallium borates has been estimated using the Kurtz–Perry powder technique. The typical intensities of the second-harmonic generation in gallium borate powders are 30–40 (with respect to quartz), and the optical nonlinearity is as good as the nonlinearity of the efficient rare-earth aluminum borate YAl3(BO3)4.
The results of an experimental study of a sensor designed to detect and measure the concentration of nanoparticles in an aqueous medium based on optical dielectric microresonators with "whispering gallery" modes are presented. The change in the frequency and Q of the eigenmodes of the microresonator was studied in its interaction with silver nanoparticles. It is shown that this type of sensor can be used to measure extremely small concentrations of nanoparticles.
The results of the study of the degradation of thermoluminescent materials Li2B4O7:Be + Mn and Li2B4O7:Zn + Mn under the effect of radiation (pulsed electron beam) and laser radiation are presented. As a result of exposure to high doses of radiation, the structure of the samples under study partially acquires an amorphous character, while the effect of radiation exposure is manifested in the optical properties in the appearance of green luminescence due to manganese centers in the tetrahedral environment. Will subsequent irradiation with a laser at a wavelength of 350 nm, luminescence centers decay due to photochemical oxidation of manganese by the reaction of Mn2+ → Mn3+. It is shown that Li2B4O7:Be + Mn has a lower radiation resistance than Li2B4O7:Zn + Mn
A new optical element—a quadrolens that is used as part of a shearing speckle interferometer (shearograph) to provide the measurements of stress-strain states of objects simultaneously in two mutually perpendicular directions, has been proposed. The quadrolens consists of four identical sections cut from the original round lens and spaced from each other. It builds four object images that are shifted relative to the optical axis by a distance that depends on the size of the gaps between the sectors. The phase reconstruction using a single speckle interferogram has been performed by the spatial phase-shifting method based on the Fourier transform. An aperture diaphragm with four holes has been placed in front of the quadrolens sectors to increase the contrast of interference fringes and polarization channel isolation can be used to separate the channels and reduce the influence of cross interference. The experimental results of using a speckle interferometer with quadrolens for the study of microdeformation of a round membrane have been presented.
The linear magnetic dichroism in the crystallographically isotropic ab plane of a tetragonal CuB2O4 single crystal has been investigated by high-resolution Fourier spectroscopy at temperatures of 1.4–22 K. Magnetic phase transitions are found at T < 2.0 K and their nature is discussed.