We have demonstrated volumetric Bragg gratings (VBGs) for mid-IR spectral range based on LiF color center (LiF:CC) crystals. γ-irradiated LiF:CC crystals feature strong absorption bands in the visible and near-IR spectral range, where selective color center photo-bleaching allows for the LiF refractive index modification. The absence of active absorption in LiF:CC crystals at wavelengths longer than 1.3 μm results in a VBG that is stable under mid-IR irradiation. Our calculations predict a ~60% diffraction efficiency over 1-6 μm spectral range, which could be realized in a ~1 cm long VBG. To verify this, we fabricated periodic structures in LiF:CC crystals using an amplified femtosecond Ti:sapphire laser. Diffraction at 1.56 μm is demonstration of a phase grating in LiF and is a proof of the feasibility of LiF:CC crystals for mid-IR VBG applications.
©2012 Optical Society of America
Development of new photorefractive materials operating in the mid-IR spectral range is essential for new compact mid-IR laser systems. These are required for many applications, including molecular spectroscopy, non-invasive medical diagnostics, industrial process control, environmental monitoring, atmospheric sensing, free space communication, oil prospecting, and numerous defense related applications such as infrared countermeasures, monitoring of munitions disposal, and stand-off detection of explosion hazards. VBGs are one of the key elements for development of compact narrow line laser systems. Currently, a majority of VBGs use photorefractive glasses with a transmission band between 0.3 and 2.7 μm. Recent progress in room temperature mid-IR lasers operating over 2-6 μm motivates development of new photorefractive materials for these lasers [1–3]. Alkali-halide crystals with color centers (CCs) have been known as an active media for tunable solid state lasers and as passive Q-switches for many years (see reviews [4–7] and references therein). Recent research includes photonics applications of these materials that are based on their photorefractive and photoluminescence properties [7–16]. In this paper, we have demonstrated LiF CC crystals for these applications. LiF has a wide transmission band and can potentially operate at wavelengths shorter than 6 μm. γ-irradiated LiF:CC crystals feature strong absorption bands in the visible and near-IR spectral range, where selective color center photo-bleaching allows for the LiF refractive index modification.
2. Experimental results
LiF crystals were γ-irradiated at 300 K with a dose of 2x108 rad using a 60Co source to produce CCs. After irradiation, the LiF sample was cleaved and polished to the size 5x5x5 mm3. After the absorption spectra measurements the crystal was mounted on a translational stage, which has computer controlled periodic spacing movement programmed through Thorlabs APT System Software.
One of the strongest absorption bands in LiF:CC crystal centered at 445 nm represents overlapped absorption bands of F2 and F3+ CCs. Excitation into this band causes not only transformation of F2 and F3+ CCs but also triggers a number of photochemical processes resulting in crystal bleaching. The basic initial photo induced processes under excitation into F2-F3+ absorption band include color center photo-ionization, dissociation and recharging:
The first process shows direct photo-ionization of the F2 CCs; the second describes capture of the electron released due to F2 CCs photo-ionization by F3+ CCs; and the third process illustrates photo dissociation of the F3+ CCs. The F2+ CCs obtained from the process (1) are not stable at room temperature (RT) and decay with several hours life-time. Irradiation with a shorter wavelength also includes processes of the photo ionization and photo dissociation of the F3 CCs:
Finally, irradiation into the absorption band of F CCs may result in CCs annihilation, via the return of interstitial electrically neutral fluoride ions (Hi) to the lattices nodes:
Therefore, irradiation by light with a wavelength shorter than F2-F3+ absorption band provides beaching of different types of CCs. However, high absorption coefficient in the visible spectral range limits the radiation penetration depth. For that reason in our experimental setup we used fs Ti:sapphire laser operating at 790 nm where absorption coefficient is more than two order of magnitude smaller. High peak power of the fs laser allows utilization of a wide range of photo induced processes due to multi-photon excitation mechanisms, including three-photon excitation of F CCs.
We used Ti:sapphire laser radiation with an average power 400 mW, operating at 1 kHz repetition rate, and 35fs pulse duration. Laser radiation with 1 cm beam diameter was focused by cylindrical lens with a 15 mm focal distance on LiF crystal surface. The beam width on the crystal surface was ~1.53 µm. Some of the photo-induced processes enable migration of the excited CCs in the crystalline lattice. This migration could decrease the contrast of the photo-induced grating. Therefore, to demonstrate feasibility of the proposed approach we fabricate grating with a relatively large period comparing to the writing beam width. The crystal was irradiated for 5 seconds to produce a single groove (bleached stripe), then the sample was mechanically shifted 12 µm or 24 µm by using a computer controlled system and re-exposed to laser radiation. The procedure was repeated for 100 times, to produce 100 grooves. Two series of periodic gratings with 12 µm and 24 µm periods were fabricated. Optical microscopy images of the gratings are shown in the Fig. 1 . These images were obtained using transmitted light through the sample. We have not observed any changes using reflected light, i.e. due to optical damage of the crystal.
Photoluminescence spectra of a LiF CC crystal before and after Ti:sapphire laser irradiation are depicted in Fig. 2 . The photoluminescence (PL) spectra were measured under excitation by Argon laser at 514 nm. The PL spectrum of the sample before fs-irradiation consists of F2 and F3+ bands with maxima at 670 nm and 530 nm, correspondingly. The exposure to fs laser radiation produced bleached areas (stripes or grooves of diffraction grating) that were clearly visible to the eye. The color of bleached area changed from brown to light green indicating ionization of F2 and formation of F2+color centers. The PL spectra of the bleached area are shown in Fig. 2c and demonstrate substantial decrease of signal intensity in 600-800 nm spectral range corresponding to PL of F2 CCs and appearance of a new band around 900 nm corresponding to PL of F2+ CCs [4,5]. The F2+ CCs are unstable at room temperature and disappear after a 12-24 hour time period [4,5]. Figure 2d shows that after 12 hours the intensity of PL band at 900 nm decreases and PL band intensity of F2 CC slightly recovers. The photoluminescence imaging of 84 grooves/mm and 42 grooves/mm diffraction gratings is shown in Figs. 3 and 4 . The scanning was done by using XY translation stage with 100 nm precision and 1 µm lateral resolution of the Confocal Micro-Raman System.
The photoluminescence imaging of 84 grooves/mm and 42 grooves/mm diffraction gratings is shown in Figs. 3 and 4. The scanning was done by using XY translation stage with 100 nm precision and 1 µm lateral resolution of the Confocal Micro-Raman System.
Figure 3 shows that, for the 84 grooves/mm (12 µm period) grating, the caustic of the Ti:sapphire laser beam after focusing by 15 mm cylindrical lens was sufficient to produce grating with a good contrast. An intensity decrease can be seen in Figs. 3a and 3c and is due to poor parallelism of the crystal surfaces that results in the microscope defocusing during scanning over the lateral 1.2 mm distance. A similar defocusing was observed for the 42 grooves/mm grating, as shown in Figs. 4a and 4c, with an improved grating contrast, as expected due to a smaller overlap of the bleached and unbleached zones of the crystal. The PL signal intensity slightly increased after 12 hours in both cases, as shown in Figs. 4c and 5c , compared to initial observations shown in Fig. 3.
Diffraction grating efficiencies were characterized at normal incidence using three different CW lasers. In these experiments, we measured the efficiency of Raman-Nath diffraction to the first order at normal incidence. The laser beams were slightly focused on the grating surfaces to insure the beam size smaller than the size of the grating. The sets of calibrated neutral filters were used to increase a dynamic range of the photo-detectors. Figure 6 shows image of the diffraction pattern of the radiation of He-Ne (0.632 μm) as well as second harmonic radiation of the Nd:YAG (0.532μm) lasers.
We observed up to 3 diffraction orders at normal incidence. The positions of the diffraction orders were in agreement with diffraction grating equation and grating periods measured from the previous experiments. The diffraction efficiencies at 0.532 μm were equal to 2-3% for both gratings with periods of 12 and 24 μm. At 0.632 μm wavelength, the measured efficiencies for the 24 and 12 μm gratings were equal to 5% and 1%, respectively. It is noteworthy that, for visible spectral range, the induced amplitude grating will prevail over a phase grating. To demonstrate feasibility of the mid-IR applications of the phase grating in the LiF CC crystals we measured the diffraction efficiency using Er- fiber laser operating at 1.56 μm. The measured efficiencies were approximately two-orders of magnitude smaller than in visible spectral range and were equal to 2 × 10−4 and 5 × 10−4 for the 24 and 12 μm gratings, respectively.
3. Discussion18–21]. We have numerically calculated refractive index change induced by CCs absorption bands. The strongest absorption line of the CCs crystal belongs to the F- centers. An F- center is an anionic vacancy trapped a single-electron. It is the simplest CC in the crystals with the highest concentration compared to other possible CCs. In LiF crystals, the absorption band of the F- CCs is located at 248 nm, the absorption coefficient can reach 1000 cm−1 in highly irradiated crystal . Using this value for α(ω) and the Kramers-Kronig relations, Eq. (6) estimates the refractive index change using:
We note that the CCs absorption bands are better approximated by Gaussian shape due to a strong electron-phonon coupling. This requires a numerical calculation of Cauchy's integral in Eq. (6). For these calculations, we measured absorption coefficients of the prepared samples with different thicknesses (from hundreds of micron to several mm) to increase accuracy of measurement of absorption coefficients of different CCs. The experimental data of the absorption spectra are shown in Fig. 6a. Due to a high absorption coefficient, we could not use the direct measurements of the maximum of the F-band, however, it can also be estimated from the band shape and position of the maximum measured from the low irradiated samples. For refractive index calculations, we fit the measured absorption spectra in the frequency domain using Gaussian absorption bands of the F center and 10 other aggregate CCs. The fitting results are shown using gray curves in Fig. 6b and summarized in the Table 1 . The bands positions and their bandwidths used in the fitting are in a good agreement with previously reported data [4–7]. The most dominant bands are F band at 248 nm with absorption coefficient 675 cm−1 and band at 450 nm which results from overlapping of the F2 and F3+ bands with a total absorption coefficient equal to 314 cm−1.
Calculation of the refractive index change using Eq. (6) was performed with MAPLE 4 software and compared to the analytical solution for the Lorentz bands. The absorption index changes induced by color centers are shown in the Fig. 7 . The black curve shows the results for Δn induced by all color centers in the sample and the red curve shows the results only for the absorption by two bands (F and F2 & F3+). As one can see from the calculation, Δn≥10−4 can be obtained in the near-mid-IR spectral range (1300-3000 nm). Consideration of only two major absorptions bands at 248 and 450 nm (F and F2 &F3+) reduces the Δn calculated value only on 30%. This analysis of the LiF CC crystal absorption spectra demonstrated that one can obtain Δn> 10−4 in the near-mid-IR spectral range (1.3-6.0 µm).
The measurements of the diffraction efficiencies allow us to estimate the induced Δn at 1.56 μm. The first-order Raman-Nath diffraction could be calculated using equation :23]:
Using Δn~10−4, the required length of the diffraction grating is L = 0.5-1 cm for λ0 = 1.5-6 μm. In current experimental setup, the beam divergence is the limiting factor for the grating thickness. This limitation could be overcome by using interference method for grating writing. However, it will require careful optimization of exposition time to maximize grating efficiency. Current technology enables fabrication of homogeneously colored LiF crystals with typical sizes larger than 10 cm. The grating 1.5-6 μm period and 1 cm length could be fabricated using standard holographic method or direct e-beam writing.
We have demonstrated gamma irradiated LiF Color Center Crystals as a material for Volumetric Bragg Grating operating in the mid-IR spectral range. Our calculations have demonstrated that photorefractive effect based on color center bleaching could provide VBG efficiency ~60% over 1-6 μm spectral range. Periodic structures with 24 and 12μm periods in LiF:CC crystals were fabricated by fs radiation of Ti:sapphire laser. The fabricated structures have been characterized using Raman-Nath diffraction at 0.532, 0.632, and 1.56 μm. Experimentally obtained diffraction at 1.56 μm is a clear evidence of phase grating fabrication and feasibility of these materials for mid-IR VBG applications.
The authors would like to acknowledge funding support from National Science Foundation (NSF) EPS-0814103, 1158862 and ECCS-0901376. The work reported here partially involves intellectual property developed at the University of Alabama at Birmingham.
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