Photoluminescence excitation of lithium fluoride films by surface plasmon resonance in Kretschmann configuration
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We report on excitation of the photoluminescence of lithium fluoride by means of the surface plasmon resonance of Al layer. Advantage of this method is high efficiency of the excitation, which is applicable to ultra-thin films. P-polarized UV diode laser light is coupled to the surface plasmon resonance using a fused silica prism in Kretschmann configuration. The angular dependence of the reflected intensity is measured using a theta–2theta goniometer. The surface plasmon at resonance condition induces photoluminescence in the adjacent lithium fluoride layer. The fluoride layers were deposited on Al-coated fused silica substrates by electron beam evaporation. For the experiment, we prepared several samples with thickness ranging from 20 to 71 nm. We studied the effect of the luminescence enhancement by the surface plasmon resonance effect. Strong quenching effect was observed in the thinnest LiF layer. Influence of X-ray irradiation on the photoluminescence was studied.
KeywordsSurface Plasmon Resonance Surface Plasmon Polariton Color Center Surface Plasmon Resonance Peak Lithium Fluoride
Alkali halides are a group of material with unique properties. Among them, the lithium fluoride (LiF) occupies a special place because of its unique properties such as low refractive index and very large bandgap of 13.6 eV allowing most extreme UV transmission beyond wavelength of 120 nm. Lithium fluoride can be used as a phosphor, which is highly sensitive for ionizing radiation, especially when it is doped with a proper activator. It is known for phenomenon of coloring of compounds due to color centers (CC). The color center is a crystallographic defect, in which one or more electrons are trapped in an anionic vacancy . The primary color center (F) has strong absorption at 250 nm. Till now, the photoluminescence was not detected originating from this CC. The individual color centers can be aggregated together forming aggregate of two (F2) or three (F3) CCs. Moreover, the CC can be ionized negatively or positively if one electron is attached to or removed from the CC, respectively. The absorption of F2 and F3 + CC is almost overlapping around 450 nm. This absorption varies in a large range depending on the CC concentration [2, 3]. Under the optical pumping at this wavelength, they exhibit efficient emissions peaking around 670 and 540 nm for F2 and F3 +, respectively [1, 4]. These defects can be produced by the irradiation of the LiF material with ionizing radiations such as electrons, ions, extreme ultraviolet, X-rays, and gamma rays. The coloration of the LiF occurs in the surface layer, which is limited by the penetration depth of the radiation in the LiF material . There is a high interest about the color centers because of their potential applications . They can be used for optical fluorescence scanning microscopies, optical microcavities, X-ray imaging, proton beam diagnostics and dose mapping, and laser active electronic states, which are characterized by wide tunability and good stability at room temperature.
The surface plasmon resonance (SPR) has attracted substantial interest in many applications such as biosensors  and bioimaging , surface-enhanced Raman scattering, solar cells, and others. SPR is characterized as collective oscillations of free carriers of charge coupled to electromagnetic waves as surface plasmon polariton (SPP). This coupling can be realized as an interface between dielectrics and metal layer in Kretschmann  or Otto configuration  or via metallic grating. At the resonant condition, there is a significant enhancement of the electric field of the incident light at the metal surface by several factors of ten. This effect can be used for significant enhancement of photoluminescence in the adjacent fluorescent layer due to the surface plasmon-coupled emission (SPCE) [7, 10, 11]. This effect enables efficient photoluminescence excitation in very thin layers. Moreover, coupling of the surface plasmon to Fabry–Perot resonance was investigated [12, 13]. In that case, directionality of the photoluminescence emission can be tailored as it is shown in  where the Fabry–Perot resonator is accomplished using metal–dielectric–metal structure. Selection of suitable plasmonic metal plays an important role. In the ultraviolet region, aluminum is a good choice because its dielectric constant possesses negative real part and small imaginary part. Effect of the Al layer thickness and the adjacent fluorescent layer thickness on SPR was studied in Refs. [7, 10, 14]. In these works, it is shown that shape and amplitude of the electric field intensity peak correlate with the SPR dip in the reflectance spectrum. In this manner, it corresponds to the fluorescence intensity enhancement.
We report on study of the photoluminescence excitation using the surface plasmon resonance coupling on Al layer in the Kretschmann configuration. The aim of this study was to enhance efficiency of the luminescence excitation allowing to study very thin luminescent layers.
2 Reflectance calculation
Transfer matrix method generalized for absorbing materials was used for the reflectance calculation. The reflectance of m plan-parallel layers is calculated using the characteristic matrix of the system and generalized Fresnell coefficients .
The Al layers were deposited in a vacuum chamber by a radiofrequency (13.56 MHz) magnetron sputtering of an Al target. The r.f. power was set to 400 W. The vacuum chamber was evacuated to ultimate pressure of 2 × 10−5 Pa before each deposition. The Ar gas pressure was adjusted to 0.5 Pa by selecting the Ar gas flow to 20 sccm and by subsequent tuning of the throttle valve between the deposition chamber and the vacuum turbomolecular pump. The Al target was pre-sputtered for 120 s prior the deposition in order to clean the target surface from native oxide. The Al layers were grown on fused silica substrates with dimensions of 12 × 14 mm2. The substrates were carefully cleaned and annealed at 200 °C for 2 h prior the deposition.
The LiF films were deposited by an e-flux Mini Electron Beam Evaporator (Tectra GmbH) equipped with a molybdenum crucible with the internal diameter of 6 mm. The crucible was filled by sintered LiF pellets of 99.95 % purity. The electron accelerating voltage was set to 1 kV. The emission current was stabilized at 10 mA by a feedback control of a filament current between 6.8 and 7.0 Å. The deposition rate was 0.16 nm/s. The ultimate pressure of the vacuum chamber was 5 × 10−6 Pa, but during the deposition process, it increased to about 3 × 10−5 Pa. The substrate was placed at a distance of 12 cm from the crucible. The substrate was heated by a bulb reflector heater to temperature about 150 and 300 °C during the deposition process. The LiF layers were deposited on Al-coated fused silica, bare fused silica, and Si substrates. The LiF-coated Si substrates were used for X-ray diffraction (XRD), scanning electron microscopy (SEM), and optical measurements. The LiF-coated bare fused silica substrate was used as a reference sample for the photoluminescence measurement.
Optical constants of the deposited Al and LiF layers were analyzed by means of spectral ellipsometer (J.A. Woollam Co., M2000) in the wavelength range from 245 to 1000 nm. The optical constants were obtained from the ellipsometric data by fitting with an appropriate model. For LiF, we used the Cauchy dispersion model; for the Al layer, we used a dispersion model composed of several Lorentz oscillators. The Al layer thickness was obtained by a profilometer (Tencor P-06). The samples were characterized by X-ray diffractometer X’Pert with Co Kα radiation. The crystallite size was estimated using Scherrer’s equation from the measured diffraction peaks after subtraction of instrumental broadening. The same X-ray source was used for sample’s irradiation (photon energy 6.9 keV, wavelength 0.1789 nm) for 60 min. Composition of the LiF layer was characterized by X-ray photoemission spectroscopy (XPS)—using NanoESCA instrument. The spectra were acquired by Al Kα excitation.
For the excitation of the photoluminescence, we used both: the above-mentioned laser with the output power increased to 50 mW and the diode laser with the constant output power of 500 mW working at the wavelength of 445 nm. The photoluminescence was detected using a miniature spectrophotometer, Ocean Optics, USB4000, which was attached to the experimental setup using an optical fiber (core diameter 600 µm) and a focusing lens (focus length 8 mm) as a light collector. The light collector was fixed to the rotating table in front of the measured sample. Edge long-pass optical filter (optical density, OD4) with cut-on wavelength of 425 or 475 nm was inserted between the collecting lens and the optical fiber for blocking the excitation wavelength of the 403 or 445 nm laser, respectively. The measured data were corrected using a spectral-dependent, sensitivity curve of the spectrophotometer. This sensitivity curve was obtained from measurement of a calibrated tungsten lamp (20 W) with known spectral irradiance at the distance of 500 mm.
Comparing the results of the two samples with 38- and 71-nm-thick LiF layer, we can relate the reflectance measurement with the photoluminescence intensity. In accordance with the Ref. , the increase in the adjacent photoluminescent layer on the Al layer leads to the decrease in electric field enhancement in the Al layer vicinity. This is associated with broadening of the reflectance dip at the SPR condition as it can be observed from Fig. 7. This effect leads to the decrease in photoluminescence that is consistent with our observation. Accordingly, we can expect the highest photoluminescent signal for the 20-nm-thick LiF layer. In the contrary to our expectation, the photoluminescence of this sample is very weak. Obviously, the strong quenching effect leads to non-radiative decay due to energy transfer from excited luminophore to the metallic Al under plasmon excitation . We can expect that this quenching occurs in the vicinity of the metallic layer of the thicker samples as well. Consequently, mainly the top layer and the layer surface are involved in the luminescence emission of those samples. With respect to the enhanced surface roughness (see Fig. 6), we can conclude that the luminescence emission originates mainly from CCs aggregated at the surface and the grain boundaries of the LiF layers. The surface CC of different alkali halides was already studied [22, 23]. Their optical characteristics differ from that of the bulk. Increase in the refractive index due to enhanced CC concentration was studied in . The slightly enhanced refractive index of our samples is in accordance with that observation confirming the CC defects in our LiF layers.
Our experiments show that there is a significant photoluminescence of CCs revealing high concentration of the aggregated anion vacancy defects in “as deposited” LiF layers. The composition analysis confirms that there is lack of fluorine in the LiF layers. The missing fluorine atoms are prerequisite for the formation of anionic vacancies and their aggregation in the LiF structure. The strong photoluminescence of “as deposited” LiF layers can be explained by high concentration of CCs due to escape of volatile fluorine during the deposition process. In different publications, there are shown the similar photoluminescence spectrum with the dominant F2 and the minor F3 + CC after irradiation by gamma rays  or X-rays . We did not observe an increase in the photoluminescence of our samples upon the X-ray irradiation showing that the concentration of CCs of as-deposited samples is saturated.
We proved that the surface plasmon resonance can be used for efficient excitation of the photoluminescence of very thin LiF layers. Aluminum is one of the most appropriate metals for this method, but its optical constants are critical for the efficient SPR coupling leading to high photoluminescence emission enhancement. The evaporated thin LiF layers exhibit polycrystalline nature. The high concentration of defects in non-irradiated LiF layers is revealed by intensive F2 and F3 + color centers’ emissions. Strong quenching was observed for the thinnest LiF layer due to non-radiative decay at vicinity of Al surface.
This research was supported by Czech Science Foundation, project GAP108/11/1312 and by Ministry of Education Youth and Sports, project LM2011029.
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