We observed origin and evolution of optical anisotropy produced by self-organized nanogratings in silica glass. Its polarization dependence was also demonstrated.
©2011 Optical Society of America
When an intense femtosecond laser pulse is focused into transparent material, e.g. silica glass, high-order nonlinear absorption allows the energy to be deposited predominantly within the focal volume, producing a local permanent refractive-index modification . Although the process of energy absorption is now well understood, little is known about the actual formation of the directly written structures . Depending on the level of laser intensity, one can induce any of three qualitatively different types of modification or damage in silica glass: low intensity induces a smooth positive refractive-index change relative to the unprocessed material (type-I); intermediate intensity results in birefringent regions (type-II); and at high intensity the damage consists of voids embedded into the glass (type-III). When the induced refractive index change is positive, type-I modification has been used to achieve waveguides and couplers [1,3], resulting from the structural densification induced by fast cooling after the photoexcitation or shock wave generation [4,5]. In spite of the many previous efforts, the origin and the mechanism of the refractive index change have not been fully elucidated . Whereas type-III void like defects have been exploited for data storage and photonic crystals [7,8], which are derived from an electron-ion plasma triggered by avalanche ionization via inverse bremsstrahlung [9,10]. The intermediate type-II regime has received little attention until now but indeed seems to have intriguing properties. Type-II structures were first observed in Ge-doped silica where they show an anisotropic light scattering that is dependent on the plane of polarization of the light . Anisotropic scattering has been interpreted in terms of photoelectrons moving along the direction of the light’s polarization, inducing index inhomogeneities. Later uniaxial birefringence imprinted in structures written within fused-silica plates was observed [12,13]. Anisotropic reflection was then observed and explained in terms of the formation of a self-organized nanograting within the focal volume , and finally the existence of the self-induced nanograting was proved by direct imaging [15,16]. During the energy absorption process, tunneling, multiphoton and avalanche ionization produce free electrons within the focus of the ultrashort pulsed laser . The observations suggest that, in a certain intensity range, the interference between longitudinal electron plasma waves leads to the formation of nano-sized gratings with a pitch as small as 150 nm . These periodic nanostructures are ruled in the direction parallel to the polarization of the writing laser and consist of thin regions with a low refractive index characterized by a strong oxygen deficiency , surrounded by larger regions with a positive index change, similar to type-I modification . Such a periodic assembly behaves as a uniaxial negative birefringent material which is consistent with a planar form birefringence, whose optical axis is parallel to the direction of the polarization of the writing laser. It has been shown that these self-assembled nanostructures indicate the local refractive index change (~-0.1) with respect to the unprocessed material , change due to variation of the pulse duration . Embedded micro-reflectors , retardation plates , and micro-fluidic channels  based on these nanostructures have been reported; however the mechanism including dynamics of self-organized nanostructures formation is still not fully understood .
In this paper, we present two types of investigations: (1) the time-resolved observation of the structural modification of type-I by the detection of a laser induced pressure wave using a transient lens (TrL) method, and (2) evolution of form birefringence (type-II) produced by self-organized nanogratings as a function of inter pulse intervals (τ int) and number of pulses (N pulse). The polarization-dependent photosensitivity in type-II regions was also discussed.
2.1 Time-resolved observation of the structural modification of type-I by TrL method
Detailed experimental setup for the TrL observation has been described in elsewhere . A mode-locked, regeneratively amplified femtosecond laser pulse (Coherent Inc.; Mira9000-Legend), operating at 800 nm with 70 fs pulse and 250 kHz repetition rate was focused inside a commercially available synthetic fused silica (Shin-Etsu Quartz; VIOSIL-SQ), containing approximately 500 ppm OH, via a microscope objective (Nikon; LU Plan Fluor, 20 × , 0.45 N.A.). The photoexcited region was located at a depth of about 300 μm below the surface of glass sample. The optically delayed probe pulse (λ probe = 400 nm, 120 fs), which was frequency doubled replica of the pump pulse by a β-BaB2O4 (BBO) crystal, passed through the photoexcited region coaxially with the pump beam (Fig. 1 ). After the probe beam transmitted through the photoexcited region, the intensity distribution at ~30 mm apart from the photoexcited region was imaged on an ICCD camera (Hamamatsu Photonics; C10054-03). Before the detection, the pump beam was attenuated by a blue filter and completely eliminated from the probe beam by a prism. The TrL signal was obtained by plotting the intensities at the center of the probe beam as a function of the delay times of the probe pulses. The pulse energy was controlled by an optical neutral density filter. The glass sample was translated at about 10 mm/s during the data acquisition to avoid multi-excitation at the same area. The focal mismatch between the pump and probe beams was controlled by changing the divergence of the probe beam with a telescope (L1 and L2).
2.2 Evolution of form birefringence (type-II) as a function of τint and Npulse
In the experiments, the femtosecond laser beam was focused via a microscope objective (Nikon; LU Plan Fluor, 100 × 0.90 N.A.) at a depth of about 100 μm below the surface of silica glass sample. The pulse energy was about 1.0 μJ and the beam power measured after microscope objective was independent on the orientation of light polarization. Repetition rate of the laser was set to 250 kHz corresponding to the interpulse interval τ int of 4 μs. The interpulse time τ int and the number of pulses N pulse were controlled by field programmable gate array (National Inst.; FPGA module 7811R) produced the trigger pulses. A series of dots were written in silica with different τ int and N pulse. After writing, the modified structures were inspected using optical and polarization (CRi Inc.; Polscope) microscopes. Previously we have observed the periodic nanostructure was induced in the orthogonal direction to the laser light polarization . Such nanograting structures, consisting of periodic oxygen defect regions, are responsible for form birefringence . The direction of slow axis of birefringence (a slow), which is always perpendicular to the writing light polarization (E), can be controlled by rotating polarization (Fig. 2 ) . Erasure of nanostructure requires sufficiently high temperatures as the induced oxygen defects are maintained up to the glass transition temperature T g of ~1200 °C .
To discuss the energy transfer from the laser radiation to the light-matter interaction during femtosecond laser irradiation, we performed pulse-by-pulse analysis of plasma emission during femtosecond laser irradiation. In this measurement, the trigger signal from FPGA is sent to a multichannel spectrometer (Hamamatsu Photonics; PMA-11) via a delay pulse generator (Stanford Research; DG-535). The emission spectra are taken by the detector at time that trigger signal rise with an acquisition gate width of 1 μs (Fig. 3 ). We have also observed transmission spectroscopy during laser beam irradiation using a multichannel spectrometer. In the characterization, a confocal Raman spectrometer (Tokyo Instruments; Nanofinder 30) was used for structural identification of the irradiated regions.
3. Results and Discussion
3.1 Excitation pulse energy dependence of TrL signals
The TrL signals at various pump pulse energies (I pump) are shown in Fig. 4(a) . In all the TrL signals, spike shaped signals at the delay time of 0 ps were observed as the decrease of the probe light intensity. This spike signal should be attributed to an optical Kerr effect and plasma formation, which has been reported by other researchers [21,23,24]. After this signal, the TrL signals were clearly oscillated at the high excitation pulse energy. The amplitude of the TrL oscillation became larger with increasing the pulse energy. Previously, we showed that this oscillation represents the pressure wave generated by the pump pulse . This oscillation disappeared at I pump < 90 nJ, indicating that the amplitude of a pressure wave is negligibly small at I pump < 90 nJ. To obtain the energy threshold of the generation of a pressure wave, the oscillation amplitudes were plotted against I pump in Fig. 4(b). From this plot, we determined the energy threshold of the pressure wave generation to be 90 nJ.
3.2 Characterization of Type-I modification with Raman spectroscopy
After writing the modified region by translating the glass sample with scanning speed of 100 μm/s, the Raman spectra were measured with a confocal laser scanning microscope (Tokyo Inst.; Nanofinder 30). The excitation laser beam from a laser diode-pumped Nd:YAG laser with a wavelength of 532 nm was focused in the modified region with a 40 × objective and the spectra of the Raman scattering were analyzed with a spectrometer. The sequence of Raman spectra of the modified region, taken as a function of femtosecond laser pulse energies are shown in Fig. 5(a) . To improve the data quality, each Raman spectra has been subtracted a luminescence background corresponding to a non-bridging oxygen hole center (NBOHC) at 650 nm , and corrected for temperature and frequency dependence of Raman scattering . All the Raman bands in Fig. 5(a) have been assigned [28,29]. For example, the dominant broadband around 450 cm−1 has been attributed to symmetric oxygen vibration of the bent Si−O−Si linkages. The sharp bands at ~490 cm−1 (D1) and ~600 cm−1 (D2) have been attributed to the oxygen breathing modes in four- and three-membered rings of SiO4 tetrahedra, respectively. These bands are important for obtaining the structural information in the structural modification inside a silica glass, because these bands come from the compaction of silica networks . Here, we focused the intensity change in the D2 band, because this band can be separated more easily from the other bands. The intensity of the D2 band increased with increasing laser pulse energy (Fig. 5(b)). The peak intensity of this band started to increase at about 90 nJ. Compared to the pulse energy dependences of the TrL oscillation (Fig. 4(b)), the threshold of a pressure wave generation is nearly equal to the threshold of an increase in three-membered rings of SiO4 tetrahedra (Fig. 5(b)), suggesting that the three-membered ring structure in silica networks, i.e., structural densification, should be induced by a pressure wave generation. Although the laser energy dependence of the D1 band was much smaller, the details were discussed elsewhere .
According to the simulation of elastic deformation, the pressure wave is generated as the result of the stress relaxation in the photoexcited region . After the photoexcitation, the temperature is elevated due to the energy transfer from the electrons to the lattice in a glass, and the temperature elevation induces thermoelastic stress. Because the temperature elevation by femtosecond laser irradiation occurs much faster than the elastic deformation of the material , the thermoelastic stress reaches the maximum just after the photoexcitation. The subsequent stress relaxation induces the generation of a pressure wave. Finally the structural densification could be generated by the shock due to the instantly generated thermoelastic stress in the photoexcitation. Indeed, it has been reported that the shock pressure induces the apparent increase of D2 band .
3.3 Structural characterization of Type-II modification
Figure 6 indicates optical micrograph of femtosecond laser induced structure in silica glass with different number of pulses at a repetition rate of 250 kHz (τ int = 4 μs). The pulse energy was 1.0 μJ. The length of the modified region was saturated to about 20 μm at 100 pulses irradiation. We have also observed that the size changes of the modified region with increasing number of pulses were independent of the interpulse interval. Even though it is expected to be stronger heat accumulation produced by higher repetition rate femtosecond laser irradiation, the size of the modified region was smaller than that of other multicomponent glass . This phenomenon could be caused by a low thermal expansion coefficient of silica glass (~5 × 10−7 K−1) .
Figure 7(a) indicates pulse-by-pulse plasma emission spectra by changing number of trigger pulse (P trig). The acquisition gate width is 1 μs which is longer than a plasma lifetime and shorter than an interpulse interval (τ int). The broad plasma emission ranging from 350 nm to 750 nm was observed regardless of the number of trigger pulse . The drop of emission intensity around 450 nm was due to the short-pass filter characteristics. The peak intensity profiles of plasma emission ranging from 350 nm to 750 nm as a function of the number of trigger pulse for various interpulse intervals (tint) were also shown in Fig. 7(b). It was also observed that the plasma emission intensity steeply increased with increasing number of pulse and reached maximum for exposition time below 50 pulses. Subsequent pulses led to a steep decrease of the emission intensity below 100 pulses followed by a slight increase thereafter (Fig. 7(b)). It is possible that the observed phenomenon is related to a change of the physical-chemical properties of the modified region after the initial laser irradiation that leads to an alteration involving structural densification, refractive index changes, and/or defect formation.
In order to estimate the structural change in silica glass with different OH concentration induced by femtosecond irradiation, we compared the microscopic fluorescence spectra of femtosecond laser modified regions in a synthetic fused silica (Shin-Etsu Quartz; VIOSIL-SX), containing below 100 ppm OH. In the experiments, microscopic photoluminescence spectra were measured using a CW DPSS laser (532 nm) as an excitation source at room temperature. The broad photoluminescence peaks located around 1.9 eV were observed from the laser processed region in the both glass samples (Fig. 8 ). This peak matches the characteristics of non-bridging oxygen hole center (NBOHC) defects . Despite of the fact that no apparent difference in the size of the modified region was observed between the different OH concentrations, this broad PL intensity becomes remarkably higher with decreasing OH concentration. This phenomenon could be interpreted in terms of the dependence of fictive temperature on OH concentration in silica glass. When femtosecond laser pulses are focused inside the silica glass, most of the pulse energy is deposited into the silica glass sample at the focal volume either through nonlinear absorption or subsequent linear absorption by the plasma, resulting in rapid increase in temperature at the focus. Then many kinds of defects including NBOHC are produced. After irradiation, as the accumulated heat caused by successive pulses diffuses away, the temperature is quickly decreased during several micro seconds . It is known that since the silica glass with a higher OH concentration exhibits a lower fictive temperature, the silica glass containing a lower OH concentration requires longer structural relaxation time . In our case, the degree of the structural relaxation of the defects generated in VIOSIL-SX is less compared to that of VIOSIL-SQ. In the case of VIOSIL-SX, the glass structure, including the unrelaxed NBOHC defects, at the higher temperature could be frozen at the cooling process.To understand the effect of the thermal accumulation on the generation of NBOHC defects in VIOSIL-SX glass sample, we have observed the evolution of photoluminescence intensity of NBOHC defects varying the interpulse time τ int and the number of pulse N pulse at the pulse energy of 1.0 μJ (Fig. 9 ). The PL intensity attributed to NBOHC defects initially increases strongly with increasing exposures below 100 pulses and begins to be saturated. Induced NBOHC defects showed tendency to saturate after several hundreds of pulses that could be explained by the competition between creation and annihilation of such defects. In spite of the same pulse energy, the evolution of PL intensity is obviously different depending on the polarization direction, especially for the shorter interpulse time corresponding to the higher thermal accumulation effect. For interpulse time τ int of 4 μs and number of pulse above 100 pulses the saturated PL intensity produced by the vertical polarization (E = 90°) was 10 times lower compared with the horizontal polarization (E = 0°), suggesting that horizontal and vertical polarization produces different heating. Let us assume that horizontal polarization produces less heating than vertical. The thermal accumulation effect for horizontal polarization is also less than vertical. In this case the generated NBOHC defects by vertically polarized pulses could be partially annihilated by the recombination of NBOHC and E’ centers. Deeper investigation of this phenomenon will be required. The oxygen mobility in glass matrix is generally very slow compared with the optical phenomena. Assuming the temperature of the focal spot reaches T = 3000°C, the oxygen diffusion length was calculated by Fick’s law , where D0 ( = 2.6 × 10−4 m2/s) is maximum diffusion coefficient (at infinite temperature), Q is the activation energy ( = 454 kJ/mol) and R is gas constant. Since the width of the oxygen defect regions in nanograting structure was ~20 nm, the diffusion time was estimated to be at least 40 μs .
3.4 Growth mechanism of the optical anisotropy
In order to reveal the growth mechanism of the anisotropic nanostructure (type-II modification), we have investigated the evolution of birefringence varying repetition rate (or the interpulse time τ int) and exposition (or the number of pulse N pulse) at the pulse energy of 1.0 μJ. The evolution of retardance δ and intensity of the D2 Raman peak with exposition time was then measured at different interpulse intervals τ int (4, 40, 400, and 4000 μs) (Fig. 10 ). Micrographs taken with polarization microscope indicating the orientation of slow axis (a slow) and the retardance value (δ) induced by various numbers of pulses at τ int of 40 μs also shown in Fig. 11 . In these experiments, we used VIOSIL-SQ glass sample. Only stress birefringence caused by local heating was observed for exposures below 100 pulses. While form birefringence induced by sub-wavelength periodic structures was detectable only after more than 100 pulses. Induced form birefringence showed tendency to saturate after several hundreds of pulses that could be explained by total depletion of oxygen inside nanograting corrugations. The optimal interpulse time τ int to achieve highest retardance value was of 40 μs, which also corresponds to oxygen diffusion time mentioned above. Alternatively, lower retardance value for τ int < 40 μs, could be explained by thermal accumulation  which deteriorates quality of the self-assembled nanostructure. These results obviously indicate that the evolution of retardance can be controlled as a function of interpulse time. Such dependence between the retardance, the interpulse time, and the number of pulse can also be seen in the intensity of the D2 Raman peak. Similar changes of the increase in D2 Raman intensity with increasing irradiated pulse energy have been observed, suggesting that an increase in the relative number of the smaller membered silica ring in the glass network leads to an increase in the refractive index .
Furthermore, in spite of the same pulse energy, the evolution of retardance is obviously different depending on the polarization direction, especially for the longer interpulse time corresponding to the lower thermal accumulation effect. For interpulse time τ int of 4 μs and exposition times bellow 100 pulses the growth speed of the retardance for two perpendicular polarizations E = 0° and 90° was estimated to be 3.6 × 10−2 and 1.5 × 10−2 nm/μs, respectively. The lower saturation value of retardance and D2 Raman band intensity produced by the vertically polarized pulses compared with the horizontal polarization could be interpreted in terms of the difference in the internal stress distribution due to local heating. These phenomena indicate that the induced birefringent structure depends on the light polarization and could be related to pulse front tilt . Since the anisotropic sensitivity of an isotropic medium cannot be defined by material structure or its interfaces, however, the experimental results can be derived from intrinsic anisotropy of light-matter interaction involving intense ultrashort light pulses with tilted intensity front. Such heating anisotropy in homogeneous medium could be interpreted in terms of the plasma inhomogeneity originating from the mutual orientation between a light polarization and a pulse front tilt. Indeed we have also observed a weak dependence of the absorbed light power and strong dependence of the hot plasma emission on the light polarization. This reveals that the same amount of absorbed energy can result in different excitations and material modifications.
We propose the following explanation of the dynamics of anisotropic nanostructure self-organization. Once a high free electron density is produced by multiphoton and avalanche ionizations, periodic modulation of electron concentration appears within the pulse width, resulting from the plasma waves interference. Most electrons are then trapped into deep levels and form the well-known E’ centers within a time scale of ~15 ps . The electron density modulation is then frozen into permanent material modification. Multiple pulses gradually improve this modulation until total reduction of the oxygen due to diffusion is reached.
3.5 Polarization dependent absorption and plasma emission
We performed transmission spectroscopy during laser beam irradiation to characterize the polarization dependent energy transfer in laser produced plasma. By making such absorption measurements as a function of the laser polarization direction, while all other conditions are the same, it would likely be possible to build up a set of data that can be used to constrain and inform a quantitative model of the polarization-dependent local heating process. From the observed transmitted spectra, we calculated the peak intensity profiles of the broad plasma emission ranging from 360 to 720 nm and the laser energy attenuation (Fig. 12 ). This result clearly indicates that the energy transfer from laser pulse to plasma emission depends on the polarization. Furthermore, these experimental results confirmed our suggestion that the consumed laser energy may not depend significantly on the polarization direction compared to the polarization-dependent plasma emission. The absorption of light could be the same for different polarization plane orientations, but the same amount of absorbed energy can produce two different types of excitations depending on the laser polarization direction: one is associated with isotropic heating of glass matrix by electrons accelerated via inverse bremsstrahlung involving vertically polarized light in our experiment and another one is associated with the anisotropic glass heating, cavitation and strong coloration produced by the electron plasma waves excited by the light with polarization direction parallel to the pulse front tilt.
In summary, we found the threshold of the femtosecond laser induced type-I modification inside fused silica based on the observation of refractive index dynamics by the TrL method. The type-I modification was accompanied with the generation of a pressure wave after the photoexcitation. We consider that the pressure wave is the main origin of the structural change in the glass. Furthermore, we have experimentally observed evolution of form birefringence (type-II modification). Self-organization of defect structures gradually evolves during multiple pulses irradiation. We anticipate that these results will motivate development of a rewritable 5D optical storage.
The authors thank Professor Qiu from Zhejiang University, Professor Kazansky from University of Southampton, and Professor Hirao from Kyoto University for good suggestion and discussions. This work is partially supported by Grant-in-Aid for Scientific Research (B).
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