A differential two-photon spectroscopic technique for measurement of atomic temperatures in a working magneto-optical trap was demonstrated. We performed experimental and theoretical studies of two-photon resonances on Rydberg transitions 2S_1/2−2P_3/2−58S in cold Li7 atoms by using counter- and co-propagating laser beams. By analyzing the spectral shapes, a dependence of Rabi broadening and Doppler broadening on total intensity of cooling laser beams was obtained. We measured the temperature of Li7 atoms in the range from 0.25 to 0.75 mK. Our theoretical and experimental results are in reasonably good agreement. The developed approach for temperature measurements can be applied to cold atoms in different traps, including confined hydrogen and antihydrogen atoms.
Generation of third-harmonic radiation, visible to the naked eye as a collimated green beam, is obtained via femtosecond excitation of the optical surface modes (SMs) propagating along a one-dimensional (1D) photonic crystal (PC) for s- and p-polarizations separately. For both polarizations, the PC SMs exist at the fundamental and third-harmonic frequencies, which allows efficient nonlinear conversion at the phase-matching points. The pattern of the third-harmonic surface wave scattering is detected and modeled, and it is shown that this pattern reveals the mode structure of the PC. Applications of the studied 1D PC structure for the experimental testing of 2D nonlinear materials are discussed.
The use of improved fabrication technology, highly disordered NbN thin films, and intertwined section topology makes it possible to create high-performance photon-number-resolving superconducting single-photon detectors (PNR SSPDs) that are comparable to conventional single-element SSPDs at the telecom range. The developed four-section PNR SSPD has simultaneously an 86±3%86±3% system detection efficiency, 35 cps dark count rate, ∼2 ns∼2 ns dead time, and maximum 90 ps jitter. An investigation of the PNR SSPD’s detection efficiency for multiphoton events shows good uniformity across sections. As a result, such a PNR SSPD is a good candidate for retrieving the photon statistics for light sources and quantum key distribution systems.
Microdisk lasers having III-V core coated with TiO2 shell are studied under optical pumping. Initial core microdisk lasers with 5-10 µm diameter comprising five layers of InAs/In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics at the ground state optical transition in the 1.28-1.30 µm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering-gallery-modes of the microdisk resonator and, for thicker (> 100 nm) shells, in a blue shift of the lasing wavelength. Using numerical analysis we demonstrate that coating III-V core microdisk with high-index shell causes strong penetration of the 2nd and 3rd radial modes into the shell, whereas the 1 st radial mode remains nearly undisturbed. Though optical loss added by TiO2 dielectric shell are low (it does not exceed 2 cm-1 even for 250 nm-thick TiO2 layer), mode leakage to TiO2 results in decrease of the optical confinement factor and increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 µm). Thus, beside other applications, TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.
Microdisk lasers having a III–V core coated with a TiO2 shell are experimentally studied under optical pumping. Initial core microdisk lasers with a 5–10 μm diameter comprising five layers of InAs∕In0.15Ga0.85As quantum dots demonstrate room temperature lasing with excellent characteristics (threshold, quality factor) at the ground state optical transition in the 1.28–1.30 μm spectral range. Deposition of the TiO2 dielectric shell results in a decimation of the whispering gallery modes of the microdisk resonator and, for thicker (>100 nm) shells, in a blueshift of the lasing wavelength. Using numerical analysis, we demonstrate that coating a III–V core microdisk with a high-index shell causes strong penetration of the second and third radial modes into the shell, whereas the first radial mode remains nearly undisturbed. Though optical loss added by the TiO2 dielectric shell is low (it does not exceed 2 cm−1 even for a 250-nm-thick TiO2 layer), mode leakage to the TiO2 results in a decrease in the optical confinement factor and an increase in the threshold pump power. This effect is more pronounced in microlasers of the smallest diameter studied (5 μm). Thus, in addition to other applications, a TiO2 shell can be used for mode selection in microdisk lasers or for selective outcoupling of the emission to the waveguide structure, which requires proper adjustment of the TiO2 shell thickness and microdisk diameter.
The condensate density profile of trapped two-dimensional gas of photons in an optical microcavity, filled by a dye solution, is analyzed taking into account a coordinate-dependent effective mass of cavity photons and photon–photon coupling parameter. The profiles for the densities of the superfluid and normal phases of trapped photons in the different regions of the system at the fixed temperature are analyzed. The radial dependencies of local mean-field phase transition temperature T0cr and local Kosterlitz–Thouless transition temperature Tcr for trapped microcavity photons are obtained. The coordinate dependence of cavity photon effective mass and photon–photon coupling parameter is important for the mirrors of smaller radius with the high trapping frequency, which provides Bose–Einstein condensation and superfluidity for smaller critical number of photons at the same temperature. We discuss a possibility of an experimental study of the density profiles for the normal and superfluid components in the system under consideration.
A strip-loaded horizontal slot waveguide is used for routing the emission of a microdisk laser. The active region of the laser is InAs/InGaAs/GaAs quantum dots operating in continuous wave mode under optical pumping. Although the quantum dots emit in a broad range, the laser emission is single-mode with λ = 1290 nm. Atomic layer deposition is used for the fabrication of the TiO₂/SiO₂/TiO₂ slot waveguide covering the 10-μm-diameter disk. The horizontal slot waveguide covers the microdisk and traps the emission into the fundamental TM mode of the three-stack layer. Once light is localized inside this planar waveguide, a loading-strip creates an effective index difference, which is sufficient to confine the mode laterally and leads to the guiding of the lasing emission. An analytical model for the 5-layers horizontal planar slot waveguide was developed in order to optimize the slot waveguide according to the operation wavelength of the microlaser and the different materials involve. Scanning near-field optical microscopy is used to observe propagation of light within the waveguide and confirm the mode coupling into it and therefore the guiding along the loading strip. The device being inherently low loss, this first demonstration of a microdisk laser with a horizontal slot waveguide opens a path towards challenging application such as quantum computing integrated chips.