We report the fabrication of internal diffraction gratings in calcium fluoride crystals by a focused near-IR 800 nm femtosecond laser. The diffraction efficiency and refractive index change were evaluated after femtosecond laser irradiation and subsequent annealing. The maximum refractive index change was estimated to be 3.57×10-4. Optical absorption spectra, measured for the crystals before and after the laser irradiation and subsequent annealing, indicate that the absorbance increase after femtosecond laser irradiation and decrease with increasing annealing temperature. The mechanisms of refractive index change are proposed. The results may be useful for fabrication of three-dimensional integrated optics devices in the crystals.
©2004 Optical Society of America
Calcium Fluoride (CaF2) crystals are used for optical windows, lenses and prisms in the 0.15 µm~9 µm range. This material has found wide uses in high power laser optics due to its low absorption. Particularly, great attention has been paid for the crystals used in photolithographic exposure systems [1,2] in the deep ultraviolet (DUV) at 193 nm and 157 nm, the vacuum emission wavelength of ArF and F2 excimer lasers. Previous research on CaF2 crystals mainly has been concerned with color centers induced by X and γ rays radiation [3, 4], as well as on the evaluation for this crystals as suitable windows, prisms and lenses materials. There are few reports for the crystals suitable for micro-optics devices.
Of late, focused femtosecond laser pulses induce various localized microstructures near the focal point of the laser beam inside transparent materials has opened up a new approach to directly fabricate three-dimensional microstructures with various optical functions. Several kinds of photonic structures or devices, such as waveguide, grating, coupler, photonic crystals, etc., have been fabricated during the past decades [5–10].
Recently, we have fabricated microstructure and induced color centers in LiF crystals . In this paper, we study the fabrication of internal diffraction gratings in CaF2 crystals, which can be used in three-dimensional integrated optics. Also, the annealing temperature-dependence of the refractive index change has been investigated.
Samples used in the experiment were taken from a high purity single crystals of CaF2, with typical dimensions of 10×10×5 mm3. All the six surfaces of the sample were optically polished.
Grating structures with a pitch of 10 µm were inscribed in the crystals by a regeneratively amplified 800 nm Ti: Sapphire laser (Spectra-Physics Ltd.) that emits 120 fs, 1 kHz modelocked pulses. The laser pulses with output power of 50 mW was focused by a 10× objective lens with a numerical aperture of 0.3, and the position of the focal point was 500 µm below the sample surface with the help of a computer controlled three-dimensional translation stage at the scanning rate of 1000 µm/s.
After exposure to the femtosecond laser, the sample was subjected to annealing with different temperatures of 100°C, 200°C, 300°C and 400°C for 1h, respectively. We then examined the diffraction efficiencies using a He-Ne laser beam. Also, the absorption spectra of the crystals sample before and after femtosecond laser irradiation and subsequent annealing were measured by a spectrophotometer (JASCO V-570).
3. Results and discussion
Figure 1 shows the internal diffraction grating structures fabricated by scanning the CaF2 crystals sample. Apparent structural change is observed after femtosecond laser irradiation. The induced grating structures have parameters with a pitch of 10 µm and a thickness of 145 µm observed by optical microscope.
In order to clarify the influence of femtosecond laser irradiation and annealing temperature on diffraction efficiency and refractive index change, we checked the diffraction efficiencies for the sample. We coupled a He-Ne laser (632.8 nm) beam into the fabricated internal grating and clearly observed ~±4 order diffraction patterns. The room temperature diffraction efficiency as function of diffraction spot orders is shown in Fig. 2. The maximum diffraction efficiency 8.5% was measured.
The refractive index changes were calculated by the assumption that the fabricated internal gratings with a pure sinusoidal phase modulation, in which the maximum phase change is Δϕ=2πΔnd/λ. The mth order diffraction efficiency (η m) can be written as η m=[J m(2πdΔn/λcosθ)]2 with the Bessel function . In this equation J m(δ) is a Bessel function, d is the thickness of the grating, Δn is the refractive index change, λ is the wavelength of the incident beam, and θ is the angle between surface normal and the incident beam inside the grating, respectively.
Kogelnik  classify gratings based on the grating thickness parameter Q, which is defined as Q=2πλd/(n Λ 2), where n is the refractive index, Λ is the grating pitch and d is the thickness of grating, respectively. When Q<1, the grating is considered thin, whereas when Q>10, the grating is considered thick. For our grating structures, the calculated parameter Q is 4.0 based on the grating parameters.
Figure 3 shows both the diffraction efficiency and the refractive index change as functions of annealing temperature for the grating, it appears a decrease tendency for both diffraction efficiency and refractive index change with increasing the annealing temperature. After the crystals sample was annealed at 300°C, with increasing annealing temperature, no apparent changes were measured for the diffraction efficiency change and the refractive index change until the temperature up to the melting point 1360°C of the crystals.
Figure 4 shows the absorption spectra of the sample before (a) and after femtosecond laser irradiation (b) and subsequent annealing at 200°C (c) and 400°C (d) for 1h. There was an apparent increase for the absorbance in femtosecond laser irradiated area in comparison with the unirradiated area. In addition, with increasing the annealing temperature, the absorbance has a little decrease. An absorption peak at about 306 nm was caused by cerium present in the starting material .
We have fabricated an internal diffraction grating in CaF2 crystals with photo-induced refractive index modification. The photo-induced refractive index change should be a nonlinear optical process, as CaF2 crystals have no intrinsic absorption in the wavelength regions around 800nm. We deduce that absorbance and refractive index decrease with increasing annealing temperature for the crystals sample can be attributed to the process of formation and decoloration of color centers. Firstly, free electrons are generated by the multiphoton absorption (about 8 photon absorption should occur as ~12eV band gap in CaF2 crystals) of the incident photon and consequent avalanche ionization when the sample irradiated by femtosecond laser with power density up to 5.0×1015 W/cm2. Such avalanche ionization produces high absorptive and dense plasma, facilitating the transfer of energy from the laser to the crystals. The adjacent F vacancies trap free electrons to form color centers. In the meantime, plasma expansion induces the local melting and material displacement, resulting in the permanent structural changes. Secondly, with increasing the annealing temperature, the electrons are thermally excited and combined with the trapped holes, leading to decrease of refractive index, however there are some permanent structural changes exist in the irradiated area, so the refractive index change decrease to constant value after annealing at 400oC until the temperature reaches melting point of the crystals. From above analysis, the color centers and the permanent structural changes induced by a femtosecond laser are responsible for the refractive index change. When color centers are bleached, the surplus refractive index change is decided by the structural changes induced by the femtosecond laser.
It is noticeable that the sign of the refractive index change induced by the femtosecond pulses is possible positive or negative. In a crystal one would expect that melting and subsequent resolidification would bring the material to a less dense state, thus decreasing refractive index. On the other hand, color centers should increase the absorption and by the Kramers-Kroening relationship cause an increase in refractive index. In our case, we expect there is a positive refractive index change for the fabrication of waveguide-type devices. We write a waveguide structure inside the crystals at same laser parameters and ensured an increase in refractive index.
Finally, we would like to point out that large refractive index changes (~10-3) are induced in glasses like fused silica by a femtosecond laser . In our case, the maximum refractive index change was estimated to be 3.57×10-4, which is one order of magnitude lower than the change in glass materials. We deduce that the possible reasons for the lower refractive index change observed in the crystals are due to the band gap of the crystals is wider than that of most of glasses. For same laser parameters, it is difficult for femtosecond laser interaction with the crystals compared to glasses.
The fabrication of internal diffraction gratings in CaF2 crystals by a focused femtosecond laser was demonstrated. The temperature dependence of femtosecond laser induced refractive index change in CaF2 crystals indicates that the formation of color centers and the permanent structural changes contribute to the refractive index change. When color centers are bleached, the structural changes play a major role for the refractive index change in our crystals. Controllable refractive index change can be achieved by adjusting femtosecond laser irradiation parameters and subsequent annealing conditions. By this way, one can induce refractive index change for the crystals to fabricate internal diffraction grating or optical waveguide, etc. for three-dimensional integrated optics devices.
The authors would like to acknowledge the financial support provided by the National Natural Science Foundation of China under the grant number of 50125208 and the author Zhao gratefully acknowledges the support of K. C. Wong Education Foundation, Hong Kong.
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