Mean Fluorescence Wavelength Research Articles

Monte Carlo fluorescence ray tracing simulation for laser cooling of solids.

We propose an approach to evaluate solid-state media for laser cooling by anti-Stokes fluorescence employing a Monte Carlo-based simulation of fluorescence ray tracing. This approach prompted a revisit of the experimental method, laser-induced thermal modulation spectroscopy (LITMoS), showing that the external quantum efficiency and the background absorption coefficient can be retrieved solely from the two wavelengths where neither cooling nor heating is observed. Our simulation can accurately compute two experimentally inaccessible quantities essential to evaluate laser-cooling media: the mean fluorescence wavelength and the fluorescence escape efficiency. These computed quantities in combination with LITMoS results allow us to retrieve the internal quantum efficiency which is a performance indicator independent of various factors such as the sample size and doping level. Using the proposed approach, we thoroughly investigate the impact of doping level, sample geometry, and refractive index on the fluorescence escape efficiency and reveal its temperature dependency for the example of Yb:YLF. Through comprehensive numerical analysis, we demonstrate that the reduction of sample symmetry is crucial in achieving lower cooling temperatures.

Impact of site-selective spectroscopy on laser cooling parameter characterization.

From laser design to optical refrigeration, experimentally measured fluorescence spectra are often utilized to obtain input parameters for predictive models. However, in materials that exhibit site-selectivity, the fluorescence spectra depend on the excitation wavelength employed to take the measurement. This work explores different conclusions that predictive models reach after inputting such varied spectra. Here, temperature-dependent site-selective spectroscopy is carried out on an ultra-pure Yb, Al co-doped silica rod fabricated by the modified chemical vapor deposition technique. The results are discussed in the context of characterizing ytterbium doped silica for optical refrigeration. Measurements made between 80 K and 280 K at several different excitation wavelengths yield unique values and temperature dependencies of the mean fluorescence wavelength. For the excitation wavelengths studied here, the variation in emission lineshapes ultimately lead to calculated minimum achievable temperatures (MAT) ranging between 151 K and 169 K, with theoretical optimal pumping wavelengths between 1030 nm and 1037 nm. Direct evaluation of the temperature dependence of the fluorescence spectra band area associated with radiative transitions out of the thermally populated 2F5/2 sublevel may be a better approach to identifying the MAT of a glass where site-selective behavior precludes unique conclusions.

Radiation-Balanced Lasers: History, Status, Potential

The review of history and progress on radiation-balanced (athermal) lasers is presented with a special focus on rare earth (RE)-doped lasers. In the majority of lasers, heat generated inside the laser medium is an unavoidable product of the lasing process. Radiation-balanced lasers can provide lasing without detrimental heating of laser medium. This new approach to the design of optically pumped RE-doped solid-state lasers is provided by balancing the spontaneous and stimulated emission within the laser medium. It is based on the principle of anti-Stokes fluorescence cooling of RE-doped low-phonon solids. The theoretical description of the operation of radiation-balanced lasers based on the set of coupled rate equations is presented and discussed. It is shown that, for athermal operation, the value of the pump wavelength of the laser must exceed the value of the mean fluorescence wavelength of the RE laser active ions doped in the laser medium. The improved purity of host crystals and better control of the transverse intensity profile will result in improved performance of the radiation-balanced laser. Recent experimental achievements in the development of radiation-balanced RE-doped bulk lasers, fibre lasers, disk lasers, and microlasers are reviewed and discussed.

Ytterbium-doped oxyfluoride nano-glass-ceramic fibers for laser cooling

Ytterbium (Yb3+)-doped oxyfluoride SiO2-Al2O3-CdF2-PbF2-YF3 glass and nano-glass-ceramic (i.e. glass-ceramics containing nanocrystals) single-index optical fibers were fabricated using two methods: by traditional glass preform drawing and by the crucible technique (‘direct-melt process’). The latter technique permitted the fabrication of perfectly vitreous optical fibers (of about 200 µm diameter), leading subsequently to the fabrication of nano glass-ceramic fibers by a well-controlled heat-treatment process above the glass transition temperature. Structural characterizations have (i) confirmed the vitreous state (absence of crystals) of the glass preforms and the glass fiber obtained from the ‘direct-melt process’ and, (ii) evidenced the formation of Pb1-x-y-zCdxYyYbzF2 (x + y + z ≈0.3-0.4) fluorite nanocrystals in the final glass-ceramic fibers. In particular, the nanocrystal size was found to be rather homogenous and smaller than 10 nm from TEM measurements for the nano-glass-ceramic fibers produced by controlled crystallization. The absolute photoluminescence quantum efficiency, mean fluorescence wavelength and anti-Stokes fluorescence of the Yb3+-doped fibers were measured upon laser excitation at wavelengths of 940 nm, 975 nm and 1030 nm, respectively. As expected, higher photoluminescence quantum yield in the near infrared (0.95, close to unity) was obtained for the nano-glass-ceramic fiber when compared with the glass-fiber. Theoretical calculations were also carried out, showing that optical refrigeration would be achievable from these chemically stable and durable nano-glass-ceramic fibers provided that 95% (segregation ratio) of Yb3+ ions are incorporated into the fluorite nanocrystals.

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.

Ultrafast excited state dynamics of the green fluorescent protein chromophore and its kindling fluorescent protein analogue

Fluorescent proteins exhibit a very diverse range of photochemical behaviour, from efficient fluorescence through photochromism to photochemical reactivity. Remarkably this diverse behaviour arises from chromophores which have very similar structures. Here we describe measurements and modelling of the excited state dynamics in the chromophores of GFP (HBDI) and the kindling fluorescent protein, KFP (FHBMI). The methods are ultrafast fluorescence spectroscopy with sub 50 fs time resolution and the modelling is based on the Smoluchowski equation. The excited state decays of both chromophores are very fast, longer for their anions than for the neutral form and independent of wavelength. Detailed studies show the mean fluorescence wavelength to be independent of time. The excited state decay times are also observed to be a very weak function of solvent polarity and viscosity. These results are modelled utilising recently calculated potential energy surfaces for the ground and excited states as a function of the twist coordinates about the two bridging bonds of the chromophore. For FHBMI and the scarce data on the neutral HBDI the calculations are not successful suggesting the need for refinement of these potential energy surfaces. For HBDI in methanol the simulation is successful provided a strong dependence of the radiationless decay rate on the coordinate is assumed. Such dependence should be included in future calculations of excited state dynamics. When the simulations are extended to more viscous solvents they fail to reproduce the observed weak viscosity dependence. The implications of these results for the nature of the coordinate leading to radiationless decay in the chromophore and for the photodynamics of fluorescent proteins are discussed.

Fluorescence reabsorption analysis on laser cooling of Tm3+ doped ZrF4 -BaF2 -LaF3 -AlF3 -NaF-PbF2 glass

Laser cooling of solid material has become a new developing research area in recent years. Tm3+ doped ZrF4-BaF2-LaF3-AlF3-NaF-PbF2 glass is one of the hot materials in this field. Compared with Yb3+, Tm3+ has better cooling potential. Up to date, one of the main factors restricting the cooling effect is fluorescent reabsorption. In this paper, firstly, using several spectral parameters of Tm3+, the reabsorption effect is calculated by stochastic model which is a semianalytical approach to this problem. The average number of absorption events is obtained. Afterwards, the effect of fluorescence trapping due to total internal reflection is analyzed. The results show that the quantum efficiency will be lowed by 0.5%1% due to reabsorption, that the redshift of the mean fluorescence wavelength is in the range of 210 nm, and that the cooling efficiency and the cooling power decrease. Finally, after discussion, we find that the use of a small size and a long thin geometry will benefit to the fluorescence emission and cooling effect.

Optical cooling in Er^3+:KPb_2Cl_5

For the first time, optical cooling has been observed in the (4)I(13/2) excited state of erbium(III), using the low phonon energy host materal, potassium lead chloride (KPb(2)Cl(2)). Cooling was observed when samples were pumped at wavelengths longer than 1557 nm, 17 nm longer than the mean fluorescence wavelength of 1540 nm, which implies a nonradiative heat load of 1.1% for the (4)I(13/2)-->(4)I(15/2) transition. When pumped at 1568 nm, the total cooling efficiency was 0.38% of the absorbed power. These results highlight the potential of Er(3+):KPb(2)Cl(5) as a material for lasers operating in an eye safe spectral region.