Color Center Production by Femtosecond-Pulse Laser Irradiation in Fluoride Crystals

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1 ISSN X, Laser Physics, 26, Vol. 16, No. 2, pp MAIK Nauka /Interperiodica (Russia), 26. Original Text Astro, Ltd., 26. NONLINEAR OPTICS AND SPECTROSCOPY Color Center Production by Femtosecond-Pulse Laser Irradiation in Fluoride Crystals L. C. Courrol 1, R. E. Samad 2, L. Gomes 2, I. M. Ranieri 2, S. L. Baldochi 2, A. Z. Freitas 2, S. P. Morato 2, and N. D. Vieira, Jr. 2, * 1 Laboratório de Espectroscopia Óptica, CEETEPS, Fatec-SP, Praça Coronel Fernando Prestes 3, São Paulo, Brazil 2 Centro de Lasers e Aplicações, CNEN-IPEN-SP, Av. Prof. Lineu Prestes 2242, São Paulo, Brazil * nilsondv@ipen.br Received July 8, 25 Abstract Color centers are lattice vacancy defects trapping electrons or holes. They are easily created in single crystals by irradiation with ionizing radiation. We report the production of color centers in and LiYF 4 single crystals by ultrashort high-intensity laser pulses (6 fs, 12.5 GW). An intensity threshold for color center creation of 1.9 and 2 TW/cm 2 was determined in and, respectively, which is slightly smaller than the continuum generation threshold. Due to the high energy density of the coherent radiation of the focused laser beam, we were able to identify a large amount of F centers, which gave rise to aggregates such as,, and F 3. The proposed mechanism of formation is based on multiphoton excitation, which also produces short-lived centers. It is also shown that it is possible to write tracks in the crystals with dimensional control. DOI: /S15466X INTRODUCTION Color centers are lattice vacancy defects trapping electrons or holes. They are usually created in single crystals at room temperature by irradiation with ionizing radiation [1]. When LiYF 4 crystals are exposed to high intensities of pumping radiation in the UV/visible spectral regions or ionizing particles, degradation of the applicationrelated characteristics and performance results. The study of defects produced, such as color centers, is then a useful approach, which can be applied to understand the relevance of such degradation processes and their microscopic mechanisms. The threshold for color center creation was 1.9 TW/cm 2 for LiYF 4. Color center lasers have some important optical characteristics if created with particular spatial dimensions: they can modulate the material refraction index, creating waveguides or photonics devices. Recently, it was shown that it is possible to create color centers with interaction of ultrashort-pulse lasers in crystals [2, 3] and glasses [4]. In this case, the defects may be produced with dimensional control [5] by focusing high-intensity ultrashort laser pulses in the material. Due to their absorption, a modulation of the material s refraction index occurs, creating waveguides or photonics devices [6]. In this work, we show that it is possible to produce stable color centers in Y 4 () and in fluoride crystals with dimensional control. The optical properties of the color centers produced by irradiation with ultrashort laser pulses and by electron beam were investigated, allowing the determination of the center creation intensity threshold and therefore the discussion of the basic formation mechanisms of these centers. The optical absorption and emission properties of these color centers were measured. 2. MATERIALS AND METHODS In this paper, we study the creation of color centers in pure and irradiated by intense laser light. The crystals were grown by the Czochralski technique under Argon atmosphere. The absorption spectra of all samples were measured at room temperature in the 2- to 9-nm range, using a Varian Spectrometer Cary 17 D. The emission spectra were obtained by exciting the samples (1 mm in thickness) with a 15-W xenon lamp passing thought a monochromator. The emissions of the samples were analyzed with a second.5-m monochromator (Spex) and a PMT detector. The signal was analyzed with an EG&G 722 lock-in amplifier and processed by a computer. A time-resolved luminescence spectroscopy technique was employed to measure the luminescence at 71-nm, induced by resonant laser excitation. The excitation system consists of a short pulse tunable optical parametric oscillator (OPO from OPOTEK) pumped by the second harmonic of a Q-switched Quantel Nd-YAG laser. This laser system delivers pulses of 1 mj with a pulse duration of 4 ns and repetition rate of 1 Hz. The time-dependence luminescence of the europium was detected by a S-2 Hammamatsu PMT and analyzed using a 2-MHz Tektronix TDS 41 digital oscilloscope. The relative errors in the emission 331

2 332 COURROL et al. Pump 1 khz@ 527 nm 13 W Seeder 83 nm, 6 fs 78 MHz, nj pulses Amplifier 83 nm, 75 µj, 6 fs, 1 khz M 2 = 1.6 f = 83 mm lens Fig. 1. Experimental setup. measurements are estimated to be <5%, while errors in the lifetime measurements are <1%. A Ti:Sapphire Chirped Pulse Amplified (CPA) laser system operating at 83 nm was used, producing a train of 64-µJ, 6-fs pulses at 1 khz, in a beam with a M 2 = 1.6 and a peak power of 12.5 GW. The beam was focused by an 83-mm lens to a radius of 12 µm. The beam waist was focused inside the crystal samples. The scheme of the experimental setup is shown in Fig. 1. The irradiations were performed at room temperature. 3. RESULTS Figure 2a shows a photograph of the effect of the focused laser beam impinging into a bulk crystal with the beam waist located inside it. A green emission and white light (continuum [7 9]) generation along the Spectral characteristics of color centers in Color center Absorption wavelength, nm Emission wavelength, nm 48 F 3 316, F beam path can be seen. Figure 2b shows the scheme of the shapes seen in the photograph, where the green light appears first and then the continuum, indicating that the green emission begins at a lower intensity than the continuum. After the laser irradiation, only the green emission geometry was preserved inside the crystal, forming a green track when viewed under white light. The crystal has a wide optical gap (~11.8 ev), which makes it a very good medium to study the formation of color centers. The simplest color center, formed by an electron trapped in an anion vacancy, is the F center. When two, three, or four F centers are aggregated,, F 3, and F 4 centers are formed, respectively. When ionized or when an electron is additionally trapped by these centers, positively or negatively charged color centers are formed ( or centers in the case of center), respectively. The table shows the known spectral characteristics of such color centers in crystals. In Fig. 3 the absorption spectra obtained for the crystals are shown. F 3 and centers are responsible for the blue absorption band, called the M band, due to the 448-nm ( F 3 ) and 444-nm ( ) absorption peaks. In addition to the main M absorption band, the red absorption band peaking at the 632-nm band is due to the center (ionized center). Correspondent absorption bands can be estimated by the Mollwo Ivey [1] relations for absorption bands in. They can be observed in the spectrum of Fig. 3: F = 296 nm, = 473 nm, F 3 = 343 nm, and = 65 nm, all shifted to the infrared.

3 COLOR CENTER PRODUCTION BY FEMTOSECOND-PULSE LASER 333 ( ) Laser beam crystal (b) Green emission region White light (c) Fig. 2. (a) Green emission and white light generation along the beam path while under irradiation by the femtosecond pulses (the pulses came from the left); (b) schematic representation of the light shapes of the preceding photograph: the color centers are created before the beam waist (focal point) and before the white light. There are no color centers after the beam waist, because crystal breakdown occurs at the waist position, scattering the laser beam; (c) emission of F 3 centers when excited by white light (the laser entered the sample from the top surface); note that these tracks start near the entrance surface of the sample, grow larger in radius, and then get smaller near the beam focus position. α, cm Wavelength, nm Fig. 3. Absorption spectra of the tracks created in and crystals by 64 µj, 6 fs laser pulses. The emission spectra obtained by exciting the samples at 63 nm are shown in Fig. 4 for both and crystals. Several differences are observed in the intensity of the bands. The emission band present in at 71 nm is very weak in. Fixing the emission at 71 nm, we obtained the excitation spectra and observed that the position, width, and quantity of the peaks changed, indicating the different kinds of color

4 334 COURROL et al. Signal, arb. units.35 λ exc = 63 nm fs λ em = 71 nm Wavelength, nm Fig. 4. Emission spectra obtained for and with excitation at 63 nm (upper graphic) and excitation spectra obtained fixing the emission at 71 nm (lower graphic). centers produced. In : (441 nm), (458 nm) and (6 nm) excitations are observed in this spectra. In, only two bands are observed at 43 and 65 nm, and, respectively. The pursuit for stable centers in crystals has been an important issue for building lasers. If both thermal and photo-stable centers are present, they can have high energy output and good efficiency [11]. Also, the spectral output is shown to be very wide. For crystals, the calculated emission cross section for was approximately cm 2, considering the measured lifetime of 22 ns (71 nm). We believe the stable centers produced in can be useful to make ultrafast color center lasers, as well. In order to determine the color center creation intensity threshold, the samples were placed in the F 3 focused laser beam before the waist, in a position where no color centers were formed, and were moved towards the waist until color centers were observed by naked eye to be formed by the laser pulses. The sample position relative to the lens was measured, and the pulse intensity was calculated at the sample position. Considering the laser spot size w given by the laser beam propagation law [12], M 2 λ( z z w w 1 ) 2 = , πw 2 (1) where λ = 83 nm is the laser wavelength, z = 19.8 cm is the beam waist position relative to the lens, w = 25 µm is the beam waist, M 2 = 1.6 is the beam quality factor, and z = 17.5 cm is the sample position where color centers were observed to be formed by the laser

5 COLOR CENTER PRODUCTION BY FEMTOSECOND-PULSE LASER 335 pulses, resulting in w = 39 µm. In this measure, the pulse energy was 547 µj and the pulse width was 6 fs, resulting in a color center creation threshold of 1.9 TW/cm 2 for. By applying a method recently developed [2, 13] to estimate the threshold for color center formation in, its value was determined to be 2.4 TW/cm 2. In both cases, this is a first-approximation effect, because nonlinear effects were not considered. To explain the color center production by the ultrashort pulses, we propose a mechanism based on the electron avalanche that develops due to a strong initial nonlinear multiphoton absorption [14, 15]. This avalanche promotes many electrons to the conduction band, where they acquire kinetic energy from the laser field and create vacancies by impact with the fluorine ions. When this anion vacancy traps an electron (neutralizing the vacancy charge), an F center is created. The second step in the production of defects involves the migration of primary defects and the formation of complex defects. These secondary processes depend on temperature and intensity rate. The difference in using a focused laser beam is that the density of primary defects formed is much higher than the usual methods, and therefore the probability of aggregation is very much increased. This can be seen by the ratio of the F to the M bands in (Fig. 3). This ratio is much higher when the centers are produced by regular ionizing radiation [16]. In the region after the green emission track in Fig. 2, there is no significant color center formation. In this case, we propose that there is equilibrium between the creation and destruction processes that occur simultaneously during the ultrashort-pulse irradiation, since the fundamental and the harmonics [17] of the pumping laser can be absorbed by the defects as well as the white light generation. These centers are generally unstable with the combination of temperature and intense light [18]. The production of stable color centers in pure samples was obtained as a consequence of the interaction of an ultrashort laser pulse with the material. We think that a controllable refractive index change can be achieved by adjusting femtosecond laser irradiation parameters and subsequent annealing conditions. In this way, one can induce refractive index change for the crystals to fabricate internal diffraction gratings, optical waveguides, etc., useful for three-dimensional integrated optics devices. 4. CONCLUSIONS We created color centers in and crystals with high-intensity ultrashort laser pulses and measured their absorption and emission spectra. We propose that the mechanism responsible for the creation of centers is a multiphotonic process depending on the crystal energy gap as well as the higher color center density formation that induces their aggregation. Stable color centers were produced in and their emission cross section was calculated to be approximately cm 2. ACKNOWLEDGMENTS The authors thank the Fundação de Amparo à Pesquisa do Estado de São Paulo, FAPESP, for support under the grant / REFERENCES 1. A. M. Espirito, L. C. Courrol, I. M. Ranieri, et al., Opt. Mater. 27, 487 (24). 2. L. C. Courrol, R. E. Samad, L. Gomes, et al., Opt. Express 12 (2), 288 (24). 3. L. C. Courrol, R. E. Samad, I. M. Ranieri, et al., J. Opt. Soc. Am. B (in press). 4. S. M. Avanesyan, S. Orlando, S. C. Langford, and J. T. Dickinson, Appl. Surf. Sci. 248, 129 (25). 5. T. Kurobori, T. Kitao, Y. Hirose, et al., Radiat. Meas. 38, 759 (24). 6. G. Baldachini and R. M. Montereali, Opt. Mater. 16, 53 (21). 7. R. Alfano and S. L. Shapiro, Phys. Rev. Lett. 24, 584 (197). 8. R. L. Fork, C. V. Shank, C. Ifirlimann and R. Yen, Opt. Lett. 8, 1 (1983). 9. G. Yang, and. Y. Shen, Opt. Lett. 9, 51 (1984). 1. H. Yvey, Phys. Rev. 72, 341 (1947). 11. S. B. Mirov and A. Y. Dergachev, Proc. SPIE 2986, 162 (1997). 12. D. Hall and P. Jackson, The Physics and Technology of Laser Resonators, IOP Publ. (in press). 13. R. E. Samad, C. R. Todescan, A. Z. Freitas, and N. D. Vieira. Jr., Appl. Surf. Sci. (in press). 14. N. Bloembergen, IEEE J. Quantum Electron. 1, 375 (1974). 15. D. Liu Du, G. Korn, J. Squier, and G. Mourou, Appl. Phys. Lett. 64, 371 (1994). 16. L. C. Courrol, L. Gomes, and I. M. Ranieri, Phys. Rev. B 42, 7 (199). 17. A. Brodeur and S. L. Chin, Phys. Rev. Lett. 8, 446 (1998). 18. Y. A. Dergachev and S. B. Mirov, Opt. Commun. 147, 17 (1998).