A recently developed thermal lens spectrometry configuration has been used to study CdSe/ZnS core-shell quantum dots ͑QDs͒ suspended in toluene and tetrahydrofuran ͑THF͒ solvents. The special features of this configuration make it very attractive to measure fluorescence quantum yield ͑͒ excitation spectrum since it simplifies the measurement procedure and consequently improve the accuracy. Furthermore, the precision reached is much higher than in conventional photoluminescence ͑PL͒ technique. Two methods, called reference sample and multiwavelength have been applied to determine , varying excitation wavelength in the UV-visible region ͑between 335-543 nm͒. The and PL spectra are practically independent of the excitation wavelength. For CdSe/ZnS QDs suspended in toluene we have obtained =76Ϯ 2%. In addition, the aging effect on and PL has been studied over a 200 h period for QDs suspended in THF.
A recently developed dual-beam configuration that optimizes the thermal lens technique has been used to obtain the absorption spectrum of pure water from 350 to 528 nm. Our results indicate the minimum linear absorption coefficient smaller than 2 x 10(-5) cm(-1) between 360 and 400 nm. This value is lower than previous literature data, and it is blueshifted. Absorption coefficients as small as 2 x 10(-7) cm(-1) can be measured for water using 1 W of excitation power. A detection limit of approximately 6 x 10(-9) cm(-1)(P=1 W) for CCl(4) was estimated, which represents, to the best of our knowledge, the highest sensitivity obtained in small absorption measurements in liquids.
Thermal lens (TL) is a key effect in laser engineering and photothermal spectroscopy. The amplitude of the TL signal or its dioptric power is proportional to the optical path difference (OPD) between the center and border of the beam, which is proportional to the heat power (Ph). Due to thermally induced mechanical stress and bulging of end faces of the sample, OPD depends critically on the geometry of the sample. In this investigation, TL measurements were performed as a function of the sample length keeping the same Ph. It is experimentally demonstrated that for materials with positive ∂n∕∂T OPD increases typically 30 to 50% with the decrease of sample length (from long rod to thin-disk geometry). For materials with negative ∂n∕∂T, this variation is much larger due to the cancelation of the different contributions to OPD with opposite signs. Furthermore, the experimental investigation presented here validates a recently proposed unified theoretical description of the TL effect.
The refractive index of most ion-doped materials increases with the excited state population. This effect was studied in many laser materials, particularly those doped with Cr 3 and rare earth ions, using several techniques, such as interferometry, wave mixing, and Z-scans. This refractive index variation is athermal (has an electronic origin) and is associated with the difference in the polarizabilites of the Cr 3 ion in its excited and ground states, Δα p . The Cr 3 optical transitions in the visible domain are electric-dipole forbidden, and they have low oscillator strengths. Therefore, the major contribution to Δα p has been assigned to allowed transitions to charge transfer bands (CTBs) in the UV with strengths ∼3 orders of magnitude higher. Although this CTB model qualitatively explains the main observations, it was never quantitatively tested. In order to further investigate the physical origin of Δα p in Cr 3 -doped crystals, excited state absorption (ESA) and Z-scan measurements were thus performed in Cr:Al 2 O 3 (ruby) and Cr:GSGG. Cr:GSGG was selected because of the proximity of its 2 E and 4 T 2 emitting levels, and thus the possibility to explore the role of the spin selection rule in the ESA spectra and the resulting variations in polarizability by comparing low and room temperature data, which were never reported before. On the other hand, Cr:Al 2 O 3 (ruby) was selected because it is the only crystal for which it is possible to obtain CTB absorption data from both ground and excited states, and thus for which it is possible to check the CTB model more accurately. Thanks to these more accurate and more complete data, we came to the first conclusion that the spin selection rule does not play any significant role in the variation of the polarizability with the 2 E-4 T 2 energy mismatch. We also discovered that using the CTB model in the case of ruby would lead to a negative Δα p value, which is contrary to all refractive index variation (including Z-scan) measurements.
The accurate knowledge of the water absorption spectrum is of vital importance for many branches of science and technology. Although it was investigated by different techniques, particularly between 300 -450 nm, there is still significant disagreement between various studies [1][2][3][4]. From 3 to 0.3 mm, the water absorption coefficient drops 8 orders of magnitude, reaching a minimum at -0.4. However, the wavelength and value of the minimum absorption is still unclear.The efficiency of conventional transmission methods used to measure absorption of high-transparent materials is usually limited by light scattering and surface reflections . Photothermal techniques, such as photoacoutic, photothermal deflection and Thermal Lens, are insensitive to scattering and have been used in ultra-sensitive spectroscopy [5][6][7]. Fry and collaborators obtained a minimum absorption of -6xlO-5 em", at420nm, using photothermal deflection and integration cavity spectroscopies [3,8]. However, hyperspectral irradiance measurements carried by Morel et al. indicates that the absorption in UV (350 -400) is lower than determined by group of Fry [4].In this study, a recently developed dual-beam Thermal Lens configuration [9], which optimizes the technique, is used to measure the absorption spectrum for pure water from 350 to 528 nm. The minimum linear absorption coefficient of -1.5xlO-5 em" was found around 380 nm. This value is the lowest ever obtained for water ( Figure I) and a good agreement was obtained with the literature in the blue-green range. At 380 nm, the absorption is about one order of magnitude smaller than the loss due to Rayleigh scattering and comparable to the Raman scattering.This "optimized" TL configuration has enough sensitivity to measure absorption coefficients as small as 2x10-7 em" for samples like water, with IW of excitation power; or -6xl 0-9 em" for samples like CCI 4 • 0.1 .----,--~----r-~_.___~----r~~.___~---, '5 0.01
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