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Optical features of point defects photoluminescence in LiF crystals, irradiated by soft X-ray pulses of the Free Electron Laser with wavelengths of 17.2 – 61.5 nm, were measured. We found that peak of photoluminescence spectra lies near of 530 nm and are associated with emission of F3+ centers. Our results suggest that redistribution of photoluminescence peak intensity from the red to the green part of the spectra is associated with a shortening of the applied laser pulses down to pico - or femtosecond durations. Dependence of peak intensity of photoluminescence spectra from the soft X-ray irradiation fluence was measured and the absence of quenching phenomena, even at relatively high fluencies was found, which is very important for wide applications of LiF crystal X-ray imaging detectors.
©2012 Optical Society of America
1. Introduction
Point defects [1] in lithium fluoride (LiF) crystals have permanently attracted wide attention due to both fundamental and application interests, especially in such fields as electron and gamma – ray dosimetry [2], active medium in light emitting devices for optoelectronics and integrated optics [3–5]. Recently, a new field of application of optically stimulated luminescence of color centers (CCs) in lithium fluoride (LiF) crystals was proposed - using them for high-performance soft X-ray [6–15], or for hard X-ray [16], or for neutron [17] imaging - and promising results were obtained. It was found in all of the above-mentioned investigations that point defects or, as they are also called CCs, are produced sufficiently easily under interaction of particles or with photons with LiF crystal. Such CCs could be hosted in LiF at room temperature for a very long time and then under excitation by UV radiation the CCs would emit in the visible spectral range.
As it was demonstrated in [5, 9, 18] among various aggregates of CCs produced in LiF crystals, that only F2 and F3+ have practical relevance for imaging applications. They have almost overlapped broad absorption bands at about 450 nm (M band). It means that they can be simultaneously excited by a single pumping wavelength. Moreover, they mainly contribute to visible luminescence in two very well separated broad emission bands in the green (F3+) and in the red (F2) spectral ranges. For different imaging applications of LiF crystals, detailed investigations of their optical properties from irradiation parameters are needed. Particular interest for imaging application of LiF crystals is connected with measurements of its photoluminescence (PL) spectrum and dependence of PL intensity response on the fluence of LiF irradiation.
Measurements of PL spectra in the case of irradiation of LiF crystals by photons with different energy and pulse duration have also a fundamental interest in the study of color center physics. Indeed, as it could be seen from Fig. 1 , previous measurements [5, 9, 18] demonstrated that in the case of LiF crystal irradiation by X-ray sources with continuous (conventional X-ray tube) or nanosecond pulse durations (soft X-ray radiation of laser-produced plasma), mainly F2 CCs emit in the spectral range of around 680 nm (450 nm light was used for excitation of LiF crystals). At the same time, if LiF crystals were irradiated by high-intensive femtosecond 800 nm laser pulses with intensity of over of 2x1012 W/cm2 [3, 4, 18, 19], CCs in LiF crystal are produced due to multiphoton (in the case of Ti:Sa laser pulses by 9 photons) absorption and various ionization processes. In this case, emission of F3+ centers with wavelength of around 540 nm is mainly dominated in the PL spectra (the same 450 nm light was used for excitation of LiF crystals). So a natural question arises “Is the difference in the measured PL spectra connected with the energy of photon used for irradiation of LiF crystal or with the pulse duration of applied photon source?”
Fig. 1 PL spectra of LiF crystals irradiated by visible and X-ray photons with different pulse duration at room temperature (RT). 1 – X-ray tube with Cu anode (λ ~0.15 nm, τ - continuous), 2 – radiation of laser-produced plasma (λ ~0.8 - 60 nm, τ ~10 ns), 3 – intense Ti:Sa laser irradiation (λ ~800 nm, τ ~60 - 1000 fs, multiphoton colorization).
Another existing uncertainty in the optical properties of LiF luminescence is connected with the behavior of LiF crystal PL response to a soft X-ray fluence. Indeed, in the case of irradiation of LiF crystal by soft X-ray beams with of ~10th nanosecond pulse durations, it was measured [5, 9, 20] that growth of PL intensity follows a square root law with the increase of soft X-ray fluence. On the contrary [10, 13], when LiF crystals were irradiated by femtosecond or picosecond soft X-ray pulses, the measured PL intensity dependence from fluence was practically linear.
In order to resolve these above mentioned contradictions, it is necessary to measure PL intensity as well as spectra of CCs produced in LiF crystals by its irradiation with the source, which could exclude multiphoton ionization processes. This means that such photon source should deliver simultaneously ultra-short pulses with the energy of photons exceeding the 14 eV band-gap of LiF crystal and in a very wide range of soft X-ray fluence.
In this paper we are providing the first measurements of optical features of PL of CCs in LiF crystals, irradiated by monochromatic, femtosecond soft X-ray pulses in the spectral range of 17.2 – 61.5 nm. Flexibility and high repetition rate of self-amplified spontaneous emission-free electron laser (SASE-FEL) facility used in our experiments, allowed irradiating LiF crystals in a very wide range of soft X-ray fluencies from 10 μJ/cm2 up to 250 mJ/cm2. A new method is proposed for determination of the CCs PL response curve of the LiF - crystal imaging detector the soft X-ray fluence, based on the comparison of measured and calculated intensities of diffracted on the mesh SASE-FEL beam, while the measured data is compared with the data obtained with traditional methods.
2. Experimental setup
The experiment with the SASE-FEL facility was performed at the SPring-8 Compact SASE Source (SCSS). This system can provide laser pulses in the soft X-ray region (51-62 nm) [21, 22]. Using additional filtering it was also possible to select more short soft X-ray radiation generated by 3d harmonic of the main laser pulse [22]. In this case, the FEL energy reached ~1.5% value of the main pulse energy. In our experiments, FEL operated at wavelengths of 61.5 nm, 52 nm and their 3d harmonics (20.5 nm and ~17.3 nm, respectively). In the case of LiF crystal irradiation by 1 - 30 shots, we have used a single shot mode of SASE-FEL and in the case of accumulation, up to 24 000 shots repetition (10 Hz) mode was applied. In the case of single shot LiF crystal irradiation, the energy measurements were provided in each laser shot and for 10 Hz repetition case average laser energy measurements were done. SASE-FEL pulse energy was varied from 4 to 11 μJ in different shots for main laser pulses and from 60 to 165 nJ for 3d harmonic pulses, respectively. The duration of the pulse was estimated at ~(100 – 300) fs [21].
In our experiments we used commercially available LiF crystals with diameter of 20 mm and thickness of 2 mm. Crystals were placed at room temperature in a vacuum chamber at the
distance of 16.3 m from the SASE-FEL beam output (see experimental setup in Fig. 2(a) ).
Fig. 2 (a) Scheme of the experiments; (b) Typical image and trace of the 61.5 nm SASE-FEL beam intensity distributions obtained on the LiF crystal in one laser shot. Presented image was combined by tiling 3.5 x 3.5 mm luminescence images measured with luminescence microscope with 4X magnification; (c) Magnified (10x) image and trace of the laser beam diffraction on the nickel mesh with period of ~360 μm, placed at the distance of ~26 mm from the LiF crystal detector.
After irradiation, the LiF crystals were kept a few days in the dark at room temperature. Then the PL of irradiated crystals was observed with a confocal fluorescence laser microscope (OLYMPUS model FV300). The PL of stable CCs formed by soft X-ray radiation was used to measure the intensity/fluence distribution of SCSS-FEL laser beam at such distance and the typical image and trace are presented in Fig. 2(b). In the case of placing the mesh (Fig. 1(a)) in the downstream of propagated FEL beam at the distance of ~26 mm in front of the LiF crystal detector, patterns were observed, produced due to the coherent diffraction of beam radiation on the mesh (Fig. 2(c)). Diffraction images of the FEL beam were obtained in 1 shot for fundamental wavelength, and in several shots for the FEL harmonics pulses. Also diffraction patterns were obtained by accumulation of a large number of shots (up to 36 000). We did not observe changes in contrast of the diffraction patterns compared with the single shot measurement cases. This shows high stability of SASE-FEL beam parameters (spatial, spectral and coherence properties) from shot to shot. The spectroscopic characterization ofthe CCs PL spectra was done using F-4500 Hitachi Fluorescence Spectral Analyzer. LiF crystals were pumped by different radiation from 200 to 700 nm and photoluminescence spectra were measured in the spectral range of 200 – 700 nm. Typical photoluminescence spectra for blank LiF crystal and LiF crystal irradiated by 61.5 nm SASE-FEL beam are presented in Fig. 3 .
Fig. 3 (a) Typical plot of PL spectra dependence from pumping excitation wavelengths, obtained for blank and irradiated LiF crystals by 61.5 nm SASE-FEL beams. (b) Spectra of PL for blank and exposed LiF crystals, which were excited by λ1 = 270 and λ2 = 450 nm.
3. Experimental results and discussion
3.1 Measurements of LiF crystal PL spectra
The normalized PL spectra of blank and exposed LiF crystals, which were exited by λ1 = 270 nm and λ2 = 450 nm, are presented in Fig. 3(b). Such excitation wavelengths correspond to the two main absorption bands, which are host to the F centers for the first excitation wavelength and to the both F2 and F3+ centers for the second one. From this figure, it is clearly seen, thattwo peaks around 340 nm and 530 nm are existed in the spectra. Under excitation by λ1=270 nm PL peak, around 340 nm are presented for exposed LiF crystal, and absent for the blank one. In our opinion, such PL peak was not observed in previous investigations and needs to be explained in the future. Under excitation by λ2=450 nm, PL peak around 530 nm is the most pronounced feature for exposed LiF crystal, while it is absent for the blank one. At the same time, typical for F2 centers PL peak around 650 nm, which is a typical dominated peak observed for PL of LiF crystals irradiated by soft X-ray conventional or 10th ns pulses durations beams, is absent. We also measured the LiF crystal PL spectra dependence from irradiation wavelength of soft X-ray FEL beams (See Fig. 4(a) ). The fundamental wavelengths of 61.5 nm and of 51.5 nm and its 3rd harmonics were used. In all cases, PL with peak at ~530 nm (green component) is dominated compared with peak at ~650 nm (red component peak).
Fig. 4 (a) Normalized PL spectra of LiF irradiated by SASE – FEL beam with different wavelengths. (b) Comparison of normalized PL spectra of LiF crystals irradiated by 61.5 nm SASE – FEL and by optical laser pulses with different durations. In all cases the LiF crystals were excited by 450 nm pulses.
As it was shown in Fig. 1, such PL spectra behavior is typical for ultra-short visible laser beam multiphoton interaction with LiF crystals. Moreover, if we compare PL spectra of LiF crystals irradiated by optical laser beams with different pulse durations, and with the ones by SASE-FEL soft X-ray pulses (Fig. 4(b)), we could see that the PL spectrum features in the last case are closer to the spectra features, obtained by irradiation of LiF crystals by the shortest (100 fs) optical laser pulses. It means that our measurements of the PL spectrum of LiF crystals give an indirect confirmation of the fact that the duration of the SASE-FEL beam pulse is of ~100 fs [21]. Thus, our results suggest that redistribution of intensity in the PLspectrum of LiF crystal from the red part of spectra to the green one is associated not with the wavelengths of laser beam irradiation (visible compare with XUV), but with a shortening of the applied laser pulse duration down to pico - or femtosecond range. This result confirms the proposed [3] statement, that in the case of LiF crystal irradiation by fs optical pulses, energy deposition into the material can occur before any energy transfer to the layer and in such case, mainly F3+ centers are created.
3.2 Measurements of LiF crystal PL response from the fluence of soft X-ray irradiation
Dependence of peak intensity of 530 nm PL spectra from the soft X-ray fluence was measured by irradiance of LiF crystals from 1 up to 24000 shots of 61.5 nm SASE-FEL pulses. Such variation of FEL shots numbers corresponds to the wide accumulated soft X-ray fluence changes from ~10 μJ/cm2 up to ~260 mJ/cm2. Obtained results are presented in Fig. 5 showing that at low soft X-ray fluence, intensity of PL spectra increases practically linearly with increasing of the fluence. With additional increasing of soft X-ray fluence higher than ~3 mJ/cm2 this dependence changes and starts to follow a square root law.
Fig. 5 Dependence of PL peak intensities (λ = 530 nm) from the accumulated fluence of SASE-FEL beam irradiation of LiF crystal at the wavelengths of 61.5 nm.
To increase the number of measured points and thus to improve the accuracy of our measurements, we proposed to also use another approach. This approach is connected with the fact that the soft X-ray laser beam diffraction [23] on the mesh could be modeled nowadays with a very high accuracy. It means that we could compare experimental intensities of the SASE-FEL beam after diffraction on the mesh with modeled one. Suppose that, in the case of a linear response of the detector, the measured intensity distribution in diffraction patterns should coincide with modeled one, we consider that any discrepancy between normalized experimental and modeled intensities of diffraction peaks is mainly caused by nonlinearity of detector response. In our experimental case the discrepancy between experimental and modeled intensities of diffraction patterns increased with an increasing of SXRL fluence. This fact proves the nonlinearity of detector response. In order to define detector response function we should transform experimental diffraction intensities to fit modeled one.
For modeling diffracted by mesh the SASE-FEL beam the parabolic wave equation
was applied. Using Eq. (1) for 2D case the diffracted field was found in the formon the assumptionwhere u0(ξ) is the initial distribution of the field, z is the distance along the laser beam propagation and d is the diameter of wire in the mesh.Simulated intensity distribution in diffraction pattern on the plane of detector, traced perpendicular to the mesh wire, is shown in Figs. 6(a) and 6(b) in red. For comparison in the same plots experimentally measured intensity distribution Iexp(x) is shown in black. From thefigures it follows that periods of diffraction patterns are in good coincidence for both plots, but the lowest and peak intensities are not in agreement. To reach the appropriate agreement in intensities we compared experimental intensities Iexp(x) (in black) with calculated Im(x) ones (in red) in 4 points (see Figs. 6(a) and 6(b)): at I(x1) = Imax; I(x2) = 1; I(x3) = Imin and in the center of geometrical shadow I(x = 0) = I0. By means of linear interpolation the dependence of Iexp(Im) was obtained and inverse function was used for correction of experimental diffraction of intensities. Results of carried out procedure are presented in Fig. 6(a), 6(b) in blue line with open circles. A very good coincidence of corrected experimental and modeled diffraction patterns is demonstrated, both for their period and intensities.
Fig. 6 The soft X-ray coherent diffraction patterns, measured at ~26 mm distance from the Ni mesh. The square mesh with 36.5 μm size of wire and period of 360 μm was irradiated by SASE-FEL beam with wavelength of 61.5 nm by one (a) and by 30 laser shots (b). Good reproducibility of SASE-FEL beam optical properties from shot to shot is clearly seen from the high contrast of diffraction patterns in the last case. (c) The PL spectra peak intensities (λ = 530 nm) dependences on the fluence of SASE-FEL beam irradiation of LiF crystal at the wavelengths of 61.5 nm were obtained by diffraction and by direct measurements.
We applied the above mentioned procedure of LiF crystal response determination for wide range of fluence, when 1, 10, 30, 300 and 600 shots of SASE-FEL laser pulses diffracted on the mesh and accumulated by the LiF crystal detector. The results of the reconstruction of LiF crystal PL response to the soft X-ray beam fluence obtained using diffraction method procedure are presented in Fig. 6(c) and are compared with the results measured by using the direct method of measurements of the curve described above. Solid agreement between the two methods of measurements is clearly seen.
It is also interesting to compare the results of our measurements for irradiation of LiF crystals by monochromatic, femtosecond soft X-ray FEL pulses with the previous results, obtained by using nanosecond soft X-ray laser-produced plasma source. In Fig. 5 such data from [20] are presented. We could see that for relatively large fluencies both data are in good enough coincidence and demonstrate the PL peak intensity growth according with a squared root law with the increase of soft X-ray fluence. Also, the quenching phenomena for PL intensity similar to [20] are not evident despite the high CCs densities reached in LiF crystal by high irradiation fluence. This is a very important point for application of LiF crystals and films for quantitative imaging in different high-resolution measurements. It is also necessary to mention that our results obtained for low X-ray fluencies are in an agreement with the results obtained in [10, 12], where the linear dependence of luminescence peak intensity from the low X-ray fluencies was observed. Thus our measurements remove the contradiction in different previous measurements of PL peak intensities dependence on soft X-ray fluence, which has been mentioned in the Introduction.
4. Conclusion
Using a high-intensity, monochromatic, femtosecond soft X-ray irradiation of LiF crystals by SASE-FEL beams allowed producing high - performance optical characterization of colorization processes of such crystals, which is a fundamental metrological aspect for their application as the sensitive soft X-ray imaging detectors with submicron spatial resolution. It is necessary to stress the fact that our measurements of the PL spectrum of LiF crystals give indirect evidence that the duration of the SASE-FEL beam pulse is of ~100 fs. Another very important result found was the absence of quenching phenomena even at relatively high soft X-ray fluence, which is of particular relevance not only for the use of LiF as an imaging detector based on optical simulated luminescence of active CCs, but also for the fundamental aspects of other applications. This is also very important as it will allow us to define accurate local values of the propagated SASE - FEL soft X-ray beam intensity, to measure the quality of its profile along beam propagation, and to find exact values of the real local fluence at a target surface after focusing of the FEL beam. Characterization of LiF – based detector for harder X-ray emission and improvements of its performance are currently under further development.
Acknowledgments
This work is supported by the “X-ray Free Electron Laser utilization research project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan and partly by the Presidium of the Russian Academy of Sciences programs 2 and 22.
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