Galina Nemova

Fiber-Bragg-grating-assisted surface plasmon-polariton sensor

A theoretical scheme for a new surface plasmon-polariton (SPP) fiber sensor with a fiber Bragg grating imprinted into the fiber core for SPP excitation is presented for the first time to our knowledge. In our scheme the energy in the fiber core mode can be transferred to a SPP with high efficiency by means of a properly designed short-period fiber Bragg grating (SPG). Developed for the cylindrical (fiber) geometry, our scheme without loss of generality can be applied to a planar geometry. Our simulations are based on the coupled-mode method and are performed at telecommunications wavelengths.

Laser cooling of solids

Laser cooling of solids, sometimes also known as optical refrigeration, is a fast developing area of optical science, investigating the interaction of light with condensed matter. Apart from being of fundamental scientific interest, this topic addresses a very important practical issue: design and construction of laser pumped solid-state cryocoolers, which are compact, free from mechanical vibrations, moving parts, fluids and can cause only low electromagnetic interference in the cooled area. The optical cryocooler has a broad area of applications such as in the development of magnetometers for geophysical sensors, in biomedical sensing and can be beneficial for satellite instrumentations and small sensors, where compactness and the lack of vibrations are very important. Simply, a laser cooler works on the conversion of low energy pump photons into high-energy anti-Stokes fluorescence photons by extracting some of the phonons (heat energy) in a material. That is, the process of laser cooling of solids is based on anti-Stokes fluorescence also known as luminescence upconversion, when light quanta in the red tail of the absorption spectrum are absorbed from a pump laser, and blue-shifted photons are spontaneously emitted. The extra energy extracted from the solid-state lattice in the form of the phonons is the quanta of vibrational energy which generates heat. The idea to cool solids with anti-Stokes fluorescence was proposed in 1929 by Peter Pringsheim and first demonstrated experimentally by Epsteins research team in 1995. In 1999, Steven Bowman proposed to use the optical refrigeration by anti-Stokes fluorescence within the laser medium to balance the heat generated by the Stokes shifted stimulated emission in a high-power solid-state bulk laser. Such a laser without internal heating named radiation-balanced or athermal laser was experimentally demonstrated for the first time in 2002. At the present time laser cooling of solids can be largely divided into three main areas: laser cooling of rare-earth doped solids, laser cooling in semiconductors and radiation-balanced lasers. All three areas are very interesting and important and will be considered in this paper.

Surface plasmon-polariton Mach-Zehnder refractive index sensor

We present what we believe is a novel theoretical scheme for phase interrogation of a planar refractive index sensor based on a surface plasmon polariton (SPP) excited with a Bragg grating. The device is a Mach-Zehnder interferometer (MZI), which offers a simple integrated optical solution to monitor relative phase variations in waveguides. The principle of operation for this device is based on the significant phase change in the field of a waveguide mode transmitted through a grating. This phase change occurs during the SPP excitation and is caused by the change in the refractive index of the sensed layer in contact with the metal layer supporting the SPP, operating at commercialized telecommunications wavelengths.

Surface Plasmon Resonance-Based Fiber and Planar Waveguide Sensors

Bulk surface Plasmons resonance devices have been researched for several decades. These devices have found a special niche as high-sensitivity refractive index sensor in biomedical applications. Recent advances in guided wave devices are rapidly changing the capabilities of such sensors, not only increasing convenience of use but also opening opportunities due to their versatility. This paper reviews many of these devices and presents some of their salient features.

Theoretical model of a planar integrated refractive index sensor based on surface plasmon-polariton excitation with a long period grating

A theoretical model of a new integrated planar surface plasmon-polariton (SPP) refractive index sensor with a long period grating (LPG) is presented and comprehensively investigated. The main principle of operation of this device is based on high-efficiency energy transfer between a p-polarized guided mode propagating in a waveguide layer of the structure and copropagating SPP supported by a metal layer separated from the waveguide layer by a buffer. The high-efficiency energy transfer is realized by means of a properly designed LPG imprinted in the waveguide and buffer layers. This device is compact and free from any moving parts and can be easily integrated into any planar scheme. Our simulations are based on the coupled-mode theory and done at the well-developed and commercialized telecom wavelengths in the 1500 nm window.

Modeling of plasmon-polariton refractive-index hollow core fiber sensors assisted by a fiber Bragg grating

The theoretical model of a new hollow core fiber sensor based on the specific properties of the surface plasmon polariton (SPP) excited with a fiber Bragg grating is proposed and comprehensively investigated. The main principle of operation of this new device is based on the efficient energy transfer between the fiber waveguide mode (FWM) and the SPP provided by a properly designed short-period fiber Bragg grating imprinted into a waveguide fiber layer of a specially designed hollow core optical fiber. The waveguide fiber layer is the dielectric layer of the fiber with the highest refractive index. The FWM is a fiber mode oscillating in this layer and exponentially decaying in all other fiber layers. Presented for the cylindrical (fiber) geometry, the scheme without loss of generality can be applied to the planar geometry. The simulations are based on the coupled mode theory and performed for well-developed telecom wavelength ranges

Laser-induced cooling of a Yb:YAG crystal in air at atmospheric pressure.

We report for the first time the experimental demonstration of optical cooling of a bulk crystal at atmospheric pressure. The use of a fiber Bragg grating (FBG) sensor to measure laser-induced cooling in real time is also demonstrated for the first time. A temperature drop of 8.8 K from the chamber temperature was observed in a Yb:YAG crystal in air when pumped with 4.2 W at 1029 nm. A background absorption of 2.9 × 10⁻⁴ cm⁻¹ was estimated with a pump wavelength at 1550 nm. Simulations predict further cooling if the pump power is optimized for the samples dimensions.

Ytterbium-doped glass-ceramics for optical refrigeration

We report for the first time the characterization of glass-ceramics for optical refrigeration. Ytterbium-doped nanocrystallites were grown in an oxyfluoride glass matrix of composition 2YbF(3):30SiO(2)-15Al(2)O(3)-25CdF(2)-22PbF(2)-4YF(3), forming bulk glass-ceramics at three different crystalisation levels. The samples are compared with a corresponding uncrystalised (glass) sample, as well as a Yb:YAG sample which has presented optical cooling. The measured X-ray diffraction spectra, and thermal capacities of the samples are reported. We also report for the first time the use of Yb:YAG as a reference for absolute photometric quantum efficiency measurement, and use the same setup to characterize the glass and glass-ceramic samples. The cooling figure-of-merit was measured by optical calorimetry using a fiber Bragg grating and found to depend on the level of crystallization of the sample, and that samples with nanocrystallites result in higher quantum efficiency and lower background absorption than the pure-glass sample. In addition to laser-induced cooling, the glass-ceramics have the potential to serve as a reference for quantum efficiency measurements.

A Compact Integrated Planar-Waveguide Refractive-Index Sensor Based on a Corrugated Metal Grating

In this paper, a theoretical model of a new planar integrated surface plasmon-polariton (SPP)-excitation-based refractive-index sensor is presented and comprehensively investigated. The main principle of operation of this device is based on energy transfer by means of a corrugated metal grating between a p-polarized guided mode propagating in a waveguide layer and the SPP propagating in the opposite direction in a metal layer. The corrugated grating is engraved in the metal layer in contact with the sensed medium. This device is free from any moving parts and can be simply integrated into any planar-waveguide system. Our sensor simulations are based on the transfer-matrix method with the mode-matching technique and have been performed at commercialized telecom wavelengths.

Development of ytterbium-doped oxyfluoride glasses for laser cooling applications.

Oxyfluoride glasses doped with 2, 5, 8, 12, 16 and 20 mol% of ytterbium (Yb3+) ions have been prepared by the conventional melt-quenching technique. Their optical, thermal and thermo-mechanical properties were characterized. Luminescence intensity at 1020 nm under laser excitation at 920 nm decreases with increasing Yb3+ concentration, suggesting a decrease in the photoluminescence quantum yield (PLQY). The PLQY of the samples was measured with an integrating sphere using an absolute method. The highest PLQY was found to be 0.99(11) for the 2 mol% Yb3+: glass and decreases with increasing Yb3+ concentration. The mean fluorescence wavelength and background absorption of the samples were also evaluated. Upconversion luminescence under 975 nm laser excitation was observed and attributed to the presence of Tm3+ and Er3+ ions which exist as impurity traces with YbF3 starting powder. Decay curves for the Yb3+: 2F5/2 → 2F7/2 transition exhibit single exponential behavior for all the samples, although lifetime decrease was observed for the excited level of Yb3+ with increasing Yb3+ concentration. Also observed are an increase in the PLQY and a slight decrease in lifetime with increasing the pump power. Finally, the potential of these oxyfluoride glasses with high PLQY and low background absorption for laser cooling applications is discussed.