Effect of sample dimensions on observed photoluminescence from Er³⁺ ions in GeGaS and LaGaS glass hosts

The 4I9/2 - 4I13/2 emission band of trivalent Er3+ is potentially interesting light source for methane (CH4) detection because of its closeness to CH4 absorption band at 1.67 µm. In the present paper we report the influence of glass sample geometry on the shape of spectra and relative emission intensity of 4I9/2 - 4I13/2 band as well as three main 4I13/2 - 4I15/2 , 4I11/2 - 4I15/2 and 4I9/2 - 4I15/2 emission bands in sulfide glasses (GeGaS and LaGaS) doped with 0.5 at.% of Er. We show that the increase of sample size leads to a significant broadening of emission spectra as well as to the substantial suppression of 4I13/2 - 4I15/2 and 4I11/2 - 4I15/2 bands. The observed effects are explained by excitation diffusion or photon trapping (consecutive absorption and emission of light by Er3+ ions [1,2]) which turns out to be more effective in large samples. We present the results of Monte-Carlo simulations supporting our considerations and we discuss the possibility of increasing the 4I9/2 - 4I13/2 emission by controlling photon trapping

Amorphous and Polycrystalline Photoconductors for Direct Conversion Flat Panel X-Ray Image Sensors

In the last ten to fifteen years there has been much research in using amorphous and polycrystalline semiconductors as x-ray photoconductors in various x-ray image sensor applications, most notably in flat panel x-ray imagers (FPXIs). We first outline the essential requirements for an ideal large area photoconductor for use in a FPXI, and discuss how some of the current amorphous and polycrystalline semiconductors fulfill these requirements. At present, only stabilized amorphous selenium (doped and alloyed a-Se) has been commercialized, and FPXIs based on a-Se are particularly suitable for mammography, operating at the ideal limit of high detective quantum efficiency (DQE). Further, these FPXIs can also be used in real-time, and have already been used in such applications as tomosynthesis. We discuss some of the important attributes of amorphous and polycrystalline x-ray photoconductors such as their large area deposition ability, charge collection efficiency, x-ray sensitivity, DQE, modulation transfer function (MTF) and the importance of the dark current. We show the importance of charge trapping in limiting not only the sensitivity but also the resolution of these detectors. Limitations on the maximum acceptable dark current and the corresponding charge collection efficiency jointly impose a practical constraint that many photoconductors fail to satisfy. We discuss the case of a-Se in which the dark current was brought down by three orders of magnitude by the use of special blocking layers to satisfy the dark current constraint. There are also a number of polycrystalline photoconductors, HgI2 and PbO being good examples, that show potential for commercialization in the same way that multilayer stabilized a-Se x-ray photoconductors were developed for commercial applications. We highlight the unique nature of avalanche multiplication in a-Se and how it has led to the development of the commercial HARP video-tube. An all solid state version of the HARP has been recently demonstrated with excellent avalanche gains; the latter is expected to lead to a number of novel imaging device applications that would be quantum noise limited. While passive pixel sensors use one TFT (thin film transistor) as a switch at the pixel, active pixel sensors (APSs) have two or more transistors and provide gain at the pixel level. The advantages of APS based x-ray imagers are also discussed with examples

Tailoring the ⁴I9/2 -> ⁴I_13/2 emission in Er³⁺ ions

Many gas detection techniques rely on the absorption of light by specific absorption bands of the gas molecules. For example, methane (CH4) has two strong absorption bands around 3.3 and 1.67 µm. The latter band seems to overlap the 4I9/2 -> 4I13/2 emission band of trivalent Er3+, as shown in Figure 1; Er3+ is, of course, a popular rare-earth ion used various photonic device applications, including erbium doped optical fiber amplifier. In the present paper, we report that in some sulfide glasses (GeGaS and LaGaS) doped with 0.5 at.% of Er, the amplitude of 4I9/2 -> 4I13/2 emission band may reach up to 5% of the major 4I13/2 -> 4I15/2 emission band as shown in Figure 2. We investigate the possibility to "tailor" the 4I9/2 -> 4I13/2 emission band of trivalent Er3+ to better match the CH4 absorption band. In particular, we examined the possibility of spectral shift by using a nephelauxetic effect by substituting for sulfur with oxygen or selenium. Surprisingly, these substitutions suppress the 4I9/2 -> 4I13/2 emission band rather than shift it. The present paper also discusses the possible mechanisms for this suppression

Further studies of radiation trapping in Er3+ doped chalcogenide glasses

Absorption and emission bands in trivalent erbium ions Er3+ are known to strongly overlap, which means that in heavily doped samples, it is possible to observe the effects of sequential radiation absorption and re-emission processes that involve the Er3+ ions. In particular, this effect of “radiation trapping” (RT) or “radiation/ excitation diffusion” is responsible for the dependence of the measured photoluminescence (PL) lifetime and PL spectrum shape in Er3+ chalcogenide glasses on the rare-earth doping concentration and sample size.[1,2]. It is worth mentioning here that the original name “excitation diffusion” originates from Milne’s paper where he pointed out the formal similarity of equations describing the effect with those for normal diffusion [3].In present paper we present the results of experiments which are design to maximize the effect of RT by passing the light along the bulk of a chalcogenide glass sample as shown in Figure 1 (b-d). We carry out experiments on samples having regular cylindrical shape, which allows us to do some simple calculations predicting the shape and lifetime of PL (see Equations (1-3) in Figure 1). The results of these predictions appear to be in a very good agreement with experimental data as shown in Figure 1(a). These results show that RT effectively “stirs” the excitation inside of sample, evenly distributing it throughout the whole volume of the sample.&more..

Tailoring the ⁴I_9/2 -> ⁴I_13/2 emission in Er³⁺ ions in different hosts media

The 4I9/2 -> 4I13/2 emission band of Er3+ is potentially interesting for CH4 detection because it can overlap with the optical absorption band of CH4. GaLaS and GeGaS glasses doped with Er3+ ions show the desirable emission under 808 nm excitation. An attempt of spectrally shifting 4I9/2 -> 4I13/2 emission band by substituting S by O or Se to obtain a perfect match to the absorption band of CH4 leads to the weakening and disappearance of the 4I9/2 -> 4I13/2 band. This phenomenon is tentatively related to the acceleration of non-radiative relaxation in the case of oxygen addition and to the suppression of up-conversion in the selenium substitution case. The relative intensity of the 4I9/2 -> 4I13/2 band (with respect to the 4I13/2 -> 4I15/2 band) varies significantly in powdered and bulk materials. This effect is discussed in terms of the radiation diffusion (i.e. radiation trapping) model and is confirmed by Monte-Carlo simulations

Radiation trapping in selected Er³⁺ doped chalcogenide glasses and the extraction of the nonradiative lifetime

Radiation trapping (RT) is a phenomenon wherein photons are emitted, absorbed and re-emitted many times before they leave the volume of the material. Trivalent Er3+ ions are particularly prone to RT because there is a whole set of strongly overlapping emission and absorption bands including  4I13/2−4I15/2 and 4I11/2−4I15/2 bands. The effect of RT on the PL decay time was investigated experimentally in this work in a variety of Er3+-doped GeGaS, GeGaSe, GaLaS(O) glasses. Sample geometry (powders, plates, disks, cylinders) and size were varied and the samples were also immersed in glycol, a liquid with high refractive index. PL decay times were measured and compared with the Judd-Ofelt results. A simple model of RT was developed and applied to the above mentioned bands. By comparing model conclusions with experimental data for different sample sizes, we were able to separate the direct relaxation of the 4I11/2 state to ground 4I15/2 state and relaxation via the intermediate 4I13/2 state; and hence obtain an approximate nonradiative lifetime