C. Pérez-Rodríguez

Whispering gallery modes in a glass microsphere as a function of temperature

Microspheres of Nd3+ doped barium titano silicate glass were prepared and the whispering gallery mode resonances were observed in a modified confocal microscope. A bulk sample of the same glass was calibrated as temperature sensor by the fluorescence intensity ratio technique. After that, the microsphere was heated by laser irradiation process technique in the microscope and the surface temperature was estimated using the fluorescence intensity ratio. This temperature is correlated with the displacement of the whispering gallery mode peaks, showing an average red-shift of 10 pm/K in a wide range of surface temperatures varying from 300 K to 950K. The limit of resolution in temperature was estimated for the fluorescence intensity ratio and the whispering gallery mode displacement, showing an improvement of an order of magnitude for the second method.

Whispering-gallery modes in glass microspheres: optimization of pumping in a modified confocal microscope

Whispering-gallery modes (WGMs) on Nd3+-doped glass microspheres with a radius of ∼15 μm were measured in a modified confocal microscope, where a dual spatial resolution in both excitation and detection zones was possible. As an alternative to the standard excitation mechanism by an evanescent wave, we used an efficient pumping/detecting scheme, focusing a laser in the microsphere and exciting the Nd3+ ions, whose fluorescent emission produces the WGMs. We have also measured the generated WGMs by changing the detection zone, where higher amplitude resonances were found when exciting in the center and detecting at the edge of the microsphere.

Stimulated emission in the red, green, and blue in a nanostructured glass ceramics

Red, green, and blue stimulated emissions have been achieved in Ho3+ doped oxyfluoride glass ceramic at room temperature. The material shows three emission bands at the red (650 nm), green (545 nm), and blue (488 nm) regions under infrared excitation at 750 nm. These emission bands are caused by a photon avalanche upconversion process previously reported. A pump and probe experimental setup has been designed to show stimulated emissions at the three bands. The pump power threshold for positive gain in the 490 nm band has been estimated around 2.7 kW/cm2. Higher thresholds are expected for the other bands.

Upconversion emission obtained in Yb 3+ -Er 3+ doped fluoroindate glasses using silica microspheres as focusing lens

An intensity enhancement of the green upconversion emission from a codoped Er(3+)-Yb(3+) fluoroindate glass has been obtained by coating the glass surface with silica microspheres (3.8 µm diameter). The microspheres focus an incoming beam (λ ≈ 950 nm) on the surface of the fluoroindate glass. The green emission (λ ≈ 545 nm) of the Er(3+) ions located in the microsphere focus was measured with a microscope in reflection mode, being the peak intensity 4.5 times the emission of the bare substrate. The transversal FWHM of the upconversion spot was experimentally determined by deconvolution with the experimental Point Spread Function of the system, obtaining a value of 309 nm. This value is in good agreement with Finite-Difference Time-Domain simulations taking into account the magnification of the image due to the microsphere.

Laser emission in Nd3+ doped barium-titanium-silicate microspheres under continuous and chopped wave pumping in a non-coupled pumping scheme

Laser action using non-coupled excitation and detection of microspheres made of Nd 3C doped barium‐titanium‐silicate glass has been demonstrated and measured. The microspheres have also been successfully deposited over Si3N4 strip waveguides with a SiO2 separation layer, thus enabling the laser emission extraction onto a CMOS compatible photonic circuit. The dynamics of the lasing wavelength and intensity has been studied as a function of the pump power and interpreted in terms of thermal effects generated through non-radiative recombination of the excited ions. (Some figures may appear in colour only in the online journal)

Temperature response of the whispering gallery mode resonances from the green upconversion emission of an Er3+–Yb3+ co-doped microsphere

Whispering gallery mode resonances lying in the green emission from an Er3+–Yb3+ co-doped glass microsphere have been analyzed under different temperatures in the range 290–380 K. Exciting the microsphere at 997 nm results in an upconversion emission with broad bands centered at 530 and 550 nm and overlapped resonances due to the spherical cavity. Both the band shape and the position of the resonances are temperature dependent which allow the correlation of the positions with the temperature inferred by the fluorescence intensity ratio technique. A resonances shift of 4.7 pm K−1 is achieved by laser heating in the power range 50–300 mW. The minimum temperature resolution has been found to be 0.7 K in the fluorescence intensity ratio technique and 100 times smaller for the whispering gallery mode shifts.

Optical study of the effect of the impurity content on the ferroelectric properties of Er3+ doped SBN glass-ceramic samples

Photoluminescence measurements have been performed on glass ceramic samples of strontium barium niobate doped with Er3+. The glass-ceramic samples were obtained from the precursor glass by thermal treatment and were doped with different contents of Er3+ ions. The average size of the nanocrystals in the samples was about 50 nm. The optical properties of the Er3+ ions were studied upon 532 nm excitation and the luminescence changes were measured as a function of temperature. The study of the luminescence spectra with increasing temperature has allowed us to ascertain the occurrence of the ferroelectric-paraelectric phase transition in the strontium barium niobate nanocrystals embedded in the glass matrix. We have particularly focused on the temperature range around the Curie temperature, when the system undergoes the transition from the ferroelectric phase to the paraelectric phase. Doping of the strontium barium niobate samples results in a change in the measured value of the Curie temperature which induce...

Local characterization of rare-earth-doped single microspheres by combined microtransmission and microphotoluminescence techniques

We present the optical characterization of single light emitting glass microspheres by means of an experimental setup that combines μ-transmission and μ-photoluminescence measurements without the need of optical fibers for excitation or detection purposes. We demonstrate that the results provided by both techniques are consistent among them and can provide complementary information regarding the active material properties (material losses and cross sections) and the passive resonator ones (radiative quality factors, group refractive indices, sphere radius, and pump power threshold for mode spectral shifting). This work addresses Nd3+ doped borate glass microspheres, but the reported studies could be realized in other rare-earth-doped glass microspheres as well.

Complete energy transfer due to rare-earth phase segregation in optical fiber preform glasses

An Yb3+ to Tm3+ energy-transfer quantum yield close to one has been found in phase-separated yttrium–alumina silicate optical fiber preform glasses. Optical absorption, luminescence, lifetime measurements, and rare-earth concentration dependence have been performed to investigate the feasibility of efficient blue upconversion fiber lasers through convenient Yb3+ sensitation. Luminescence decay measurements have demonstrated the co-existence of two phases. One of the phases is characterized by an yttrium-rich composition. Most of the RE ions partition into the yttrium-rich phase and produce the intense upconversion emission.

Enhanced energy upconversion and super-resolved focused spot generation in Tm 3+ -Yb 3+ codoped glass using silica microspheres

We report the enhancement of the blue upconversion emission in a Tm3+-Yb3+ codoped fluoroindate glass covered with silica microspheres. Each microsphere produces a photonic nanojet that concentrates the illuminating beam in a tiny region of the glass, increasing the intensity per unit area and so the upconversion emission by a factor of 3. Moreover, the mean size of the emission area has been reduced by a factor of 3 due to the three-photon process involved in the blue emission band. The experimental values of the full width at half-maximum of the emission spots have been found to be in agreement with the theoretical values obtained from finite-difference time-domain simulations.