We report on different refractive index change (RIC) behavior in borosilicate glasses induced by focused 1 kHz and 250 kHz femtosecond (fs) laser irradiation. The influence of fs laser irradiation condition and annealing temperature on RIC was examined. Absorption, electron spin resonance and Raman spectra, and transmission electron microscope were used to clarify the mechanisms of the RIC. Smaller RIC (up to 10−4) was observed after 1 kHz fs laser irradiation, while larger RIC (up to 10−1) was detected after 250 kHz fs laser irradiation, which were ascribed to the formation of color centers and precipitation of nanocrystals, respectively. The result highlights that the mechanisms of RIC induced by fs laser can be very different depending on the irradiation conditions.
©2011 Optical Society of America
Recently femtosecond (fs) laser materials micromachining has attracted considerable interest due to a wide range of applications including fabrication of optical waveguide , integrated optical component , optical data storage , and holographic grating etc [4,5]. It provides a unique and various possibilities to fabricate a myriad of three dimensional (3D) photonic components and devices. In addition, multi-functional devices  such as microelectronic components, microplasmonic elements, and electro-optics integration devices etc. can be fabricated in a single transparent matrix by fs laser micromachining, making it possible to manufacture hybrid devices composed of multi-functional elements for lab-on-a-chip applications. Among these unique advantages of fs laser micromachining, one of important applications is 3D refractive index modification (positive or negative index change with isotropic or anisotropic properties) depending on the laser parameters [7,8]. It was capable and suitable for formation of waveguides, gratings, splitters, couplers, and optical amplifiers etc [1,9–11] by using refractive index change inside glasses. However, the physical origin of the fs laser induced refractive index change (RIC) inside glass still has some controversies. Mechanisms of structural change, local densification and formation of defects e.g. color center have been suggested [1,3,12]. A recent study by Little et al.  revealed that the mechanism of RIC induced by fs laser is depended on the irradiation conditions. There have been many investigations which focused on low repetition-rate regime fs laser (generally 1 kHz) induced RIC [1,9–13]. However, the RIC induced by low repetition rate fs laser is small (generally about 10−3 or 10−4), optical elements e.g. diffraction grating have to be fabricated with a certain large volume for practical application. A good news is large RIC (up to 10−1) can be achieved using high repetition-rate (>100 kHz) fs laser [8,14,15]. The corresponding mechanisms were ascribed to elements redistribution or phase change [14,15].
In this study we describe the fabrication of diffraction gratings with different RICs by 1 kHz and 250 kHz fs laser micromachining borosilicate glasses. The influence of fs laser irradiation condition and annealing temperature on RIC was investigated. The mechanisms of the observed phenomena were also discussed.
2. Experiment details
Glasses with compositions (in mol %) of 10Na2O–35B2O3–40SiO2–10GeO2–5Al and 10Na2O–35B2O3–40SiO2–10SnO2–5Al were obtained by conventional melt-quenching technique using reagent grade Na2CO3, H3BO3, SiO2, GeO2, SnO2, and Al as raw materials. Glasses were melted in alumina crucibles with alumina caps in electric furnaces at 1450 °C for 1 h, cast into patties about 3 mm thick, and annealed at 400 °C for 2h. We denoted them as Ge and Sn glass for convenience. The obtained glasses were colorless and transparent. The refractive indices of the two glasses were 1.491 and 1.490, respectively. Two commercial 800 nm regenerative amplified Ti: sapphire fs (Spectra-Physics Ltd. and RegA 900, Coherent Inc.) were used to generate 120 fs, 1kHz or 120 fs, 250 kHz mode-locked pulses. The laser beam was focused via a microscope objective into the optically polished glass sample that was fixed on a computer controlled 3D XYZ stage. Grating structures were obtained by using direct laser writing technique. After irradiation, induced structure in the glass samples was observed with an optical microscope. The diffraction efficiencies of the grating structures were analyzed using a He-Ne laser beam. The images of the diffraction patterns were received on a white screen and were captured by a digital camera. Optical absorption spectra were recorded with a JASCO V-570 UV/VIS/NIR spectrophotometer. Electron spin resonance (ESR) spectra were obtained using a Bruker A30 ESR spectrometer (9.855 GHz, X-band). Structural changes of the modified glass regions were identified by a Micro-Raman spectrometer (Renishaw inVia) with a 514 nm laser excitation. Images of the crystalline microstructures were obtained with a JEOL 2010 transmission electron microscopy (TEM) instrument. All the measurements were carried out at room temperature.
3. Results and discussion
Figure 1 shows the front view of microscopic images of the fabricated gratings in the transmitted mode. The writing parameters for Figs. 1(a) and 1(c) are as follows, repetition rate: 1 kHz, laser power: 40 mW, objective: 10 × (NA = 0.30), focused depth: 1 mm below the glass surface, writing speed: 500 µm/s. Clear induced structures were observed in the transmitted images. The induced grating structures have a period of 30 µm and a thickness of approximately 1 mm for Sn glass and a period of 20 µm and a thickness of 1 mm for Ge glass. Gratings were classified based on the grating thickness parameter Q , defined as Q = 2πλd/(nΛ2), where n is the refractive index, Λ is the grating period and d is the thickness of grating, respectively. The calculated parameters Q are 2.9 and 6.6 for grating in Sn and Ge glasses, respectively, and made them reasonable to investigate the characteristics of the gratings by using diffraction theory of thin grating. One also can observe that the laser unmodified area was colorless, while the laser modified region was chocolate for Sn glass and dark-orange for Ge glass, respectively. For comparison, grating structures were also written in both glasses by a high repetition rate laser (250 kHz). The fabrication conditions were as follows, laser power: 600-800 mW, objective: 50 × (NA = 0.80), focused depth: 200 µm below the glass surface, writing speed: 60 µm/s. Apparent grating structures were also obtained. The calculated parameters Q are 0.004 and 0.005 for grating in Sn and Ge glasses, respectively, based on the grating parameters (period 80µm, thickness 10 and 12µm). The color of the modified regions was changed to black, which could be due to the formation of Ge and Sn crystals since the crystals have broad absorption in the visible range.
We further examined the grating structures by using the reflected mode of the optical microscope. The glass was polished away to expose the femtosecond laser induced crystalline line array written within the glass for index analysis. We cannot observe the clear grating structures fabricated by 1 kHz fs laser, while high bright gratings are observed in the 250 kHz fs laser machined glasses. In this mode, a bright area indicated a high reflectivity region. The interface is possible to use the one between air and glass. Although there is much deviation, the Fresnel formula is applicable for an approximate calculation . According to the relation between reflectivity and refractive index of the material under normal incidence, the expressions were as follows:Fig. 2 ). The subscripts mod and unmod express laser unmodified and modified region. The maximum RICs were estimated to be about 12% and 11% for Ge and Sn gratings fabricated by 250 kHz fs laser, respectively. Since no clear bright regions can be observed in the 1 kHz fabricated gratings, it can imply that the RIC was very small.
According to the diffraction theory for a thin grating , we can calculate the RIC by measuring the diffraction efficiency. The images and profiles of the diffraction patterns are shown in Fig. 3 . Only low order diffraction spots were founded from the 1 kHz fs laser fabricated gratings, while high orders diffraction spots could also be observed from 250 kHz fs laser fabricated gratings. In addition, the maximum diffraction efficiencies were measured to be about 4.2%, 40%, 4.6%, and 51% for gratings fabricated in Sn and Ge glasses by 1 kHz and 250 kHz fs laser, respectively. Then the corresponding RICs were calculated to be about 4.1 × 10−4, 0.12, 4.3 × 10−4, and 0.12, respectively. The refractive index changes obtained from the diffraction pattern was close to the results obtained from the reflectivity measurements.
In order to clarify the mechanisms for the different RIC, we further carried out absorption, ESR, and micro-Raman spectrum measurements. We have compared the results by using same parameters for the irradiation with 1 kHz and 250 kHz lasers. We used the parameters, as follows, 3.0 µJ pulse energy, 0.6 NA, 200 µm depth and 60 µm/s writing speed. In case of 1 kHz, the structure was changed only in the laser focus region (about 1~2 µm), and crystallization did not occur even the writing speed was ranging from 2000 to 10 µm/s. While, in the case of 250 kHz, crystallization occurred within the writing speed ranging from 200 to 20 µm/s. In fact, crystallization does not depend on pulse numbers. When the laser was focused on one spot with 200 µm depth from the surface, 250kHz laser could induce crystallization after 10s irradiation, while 1kHz laser could not induce crystallization even after 2500s irradiation (i.e. the same pulse numbers). Thus, it is possible to link repetition rate to the mechanism of structural change. Figure 4 shows the absorption spectra of the fs laser unmodified and modified glasses. After 1 kHz laser irradiation, both Sn and Ge glasses show a broad absorption band from 400 to 700 nm, which might be due to color centers . And the intensity of this absorption band increased with the increase of laser power. After 250 kHz fs laser irradiation, the absorption edge of both glasses shifts to longer wavelength. And with the increase of laser power, the absorption edge keeps on red-shifting. This might be ascribed to heat accumulation effect, resulting in precipitation of crystals around the focal volume [5,15,18].
To investigate the formation of color centers due to fs laser irradiation, we performed ESR spectrum measurements for the laser unmodified and modified Sn glasses, which is shown in Fig. 5 . No ESR signal was observed before laser exposure, while, after laser irradiation, ESR signals with g factors of 1.996, and 2.003 appeared, which can be assigned to SiE´ centers . Similar ESR signals were also observed in Ge glass after the fs laser irradiation. Such SiE´ centers are produced by the formation of a so-called oxygen-deficiency center by capturing electrons . It is also noticed that the concentration of SiE´ centers produced by 250 kHz fs laser was much lower than that generated by 1 kHz fs laser, which might be due to the annealing effect based on heat accumulation in the case of 250 kHz laser.
Micro-Raman spectra of laser unmodified and modified Sn glass are shown in Fig. 6 , which was used to reveal the structural changes due to the fs laser irradiation. Little structure change was observed after 1 kHz fs laser irradiation (Fig. 6(a)). However, dramatic structure change appeared after 250 kHz laser irradiation. A characteristic Raman peak due to Sn crystal (197 cm−1)  was observed (Fig. 6(b)). Figure 6(c) shows the Raman mapping picture at 197 cm−1, indicating that a part of the laser modified region was changed from glass to crystal. Although the exact mechanism for the femtosecond laser induced crystallization is still unclear, it is believed that high temperature and localized heating generated by the heat accumulation effect from the high-repetition rate fs laser irradiation plays a significant role [5,15,18]. The laser pluses deposit large amounts of energy into the glass through nonlinear multiphoton absorption, and heat accumulation occurs at high repetition rates as new pluses arrive before the energy of previous pulses fully dissipate, causing the increase in temperature around the focal point. Then the crystallization would happen in the region where the temperature exceeded the glass crystallization temperature of the glass. Similar Raman results were observed in Ge glass after the fs laser irradiation. Glass structure changes a little after 1 kHz laser irradiation, while Ge crystals were produced after 250 kHz laser irradiation.
TEM measurement also confirms the precipitation of crystals after 250 kHz fs laser irradiation. Figure 7 shows the TEM images of the Sn glasses before and after 250 kHz fs laser irradiation. No crystal was found before laser irradiation, while after the fs laser irradiation, Sn nanocrystals with the average size about 4 nm were mono-dispersed in the glass matrix. Based on the volume fraction of crystals from TEM image (5.64%, and 4.12% for Sn and Ge crystals, respectively), the refractive index is calculated to be about 1.60 and 1.62 for Sn and Ge glass, respectively, which was close to the results estimated based on the refraction analysis.
Furthermore, the influence of fs laser irradiation and annealing temperature on diffraction efficiency (i.e. RIC) was examined. Both dependences of ESR signal intensity on 1 kHz laser power and annealing temperature are in good agreement with those of the measured diffraction efficiency as shown in Figs. 8(a) and 8(b), respectively. It appears an increasing tendency for ESR signal intensity and diffraction efficiency with increasing the laser power, while a decrease tendency for ESR signal intensity and diffraction efficiency with increasing annealing temperature. When the color centers disappear after annealing, the diffraction efficiency decreases to zero. It indicates that the RIC induced by 1 kHz fs laser irradiation is primarily due to the formation of color centers. Figures 8(c) and 8(d) show both the Raman peak intensity of Sn crystals and diffraction efficiency as function of 250 kHz laser power and annealing temperature. It reveals an increasing tendency for Raman peak intensity and diffraction efficiency with increasing the laser power, while nearly unchanged with increasing annealing temperature. We can conclude that precipitation of metal nanoparticles induced by 250 kHz fs laser is responsible for the large RIC.
The fabrication of diffraction gratings in borosilicate glasses by focused 1 kHz and 250 kHz fs laser was demonstrated. The influence of fs laser irradiation and annealing temperature on RIC was examined. Smaller RIC (10−4) was observed after 1 kHz laser irradiation, which was primarily due to the formation of color centers. Larger RIC (10−1) was obtained after 250 kHz fs laser irradiation, which was ascribed to the precipitation of nanocrystals. The result highlights that the mechanisms of RIC induced by fs laser can be very different depending on the laser irradiation conditions. These observations may be useful for fabrication of three-dimensional optical components in glasses by using direct fs laser writing technique..
This work was financially supported by the National Natural Science Foundation of China (Grant Nos.51072054, 50872123 and 50802083) and partially supported by the National Basic Research Program of China (2011CB808103 and 2010CB923203).
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