The authors report what is, to our best knowledge, the first observation of second-harmonic generation (SHG) directly from the crystalline particles in Ge-doped SiO2 (Ge:SiO2) glass films. Ge:SiO2 glass films with approximately 5 µm thickness were fabricated by chemical vapor-phase deposition. X-ray diffraction (XRD) peaks at around 2θ=22° in thermally crystallized Ge:SiO2 films were observed, and obtained XRD patterns are exactly the same as those in ultraviolet-laser-poled Ge:SiO2 glasses. Using SHG microscopic technique with a Nd:YAG laser, it has been successfully found that SH emitting with 532 nm wavelength is observed directly from the crystalline particles induced in the crystallized films.
© 2003 Optical Society of America
SiO2 and SiO2-related glasses are one of the most promising materials for advanced photonic applications. Conventional optical fibers made of Ge-doped SiO2 glass, for instance, have been exclusively used for optical information network systems at the moment.
Since glass is the material that has the inversion symmetry, glass should not have, in principle, the coefficient of second-order optical nonlinearity, χ(2). This has brought the glass materials only to passive usages like fibers and couplers in optical transmission networks, while second-order optical nonlinearity is the property absolutely required to active, switching by electro-optic (EO) effect and wavelength conversion by second-harmonic generation (SHG) etc, applications in light-wave signal processing of photonic information technology. So far, active devices, which require the second-order optical nonlinearity, have been realized by the use of organic and inorganic crystal materials.
Ultraviolet (UV)  and thermal  poling are known to be one of the most efficient method to induce the second-order optical nonlinearity in SiO2-related glasses and films. It has been reported that crystallization with nano- and/or micro-scale particles in the glasses due to the UV and thermal poling was found [3,4], and becomes understanding of its important role for obtaining a large induced second-order nonlinearity in the glass. Matsumoto et al.  reported on the convoluted X-ray diffraction (XRD) peaks at around 2θ=22°, consisting of three or more components of crystalline phase, in UV-poled silica glasses, and suggested that one of the XRD peaks at around 2θ=22°, that is positioned at the lowest 2θ angle between those components, should directly decide an amplitude of the induced third-order optical nonlinearity, χ(3), so that induced χ(2) may increase with increasing of χ(3), through the relation of χ(2)=3E scχ(3), where E sc is a space-charge electric field built-in glass . Recently, crystallization behaviors in a fiber preform of Ge-doped SiO2 glasses have been investigated, and found that modifications of induced crystalline phases by changing the initial condition of defects with pre-treatments of thermal and UV-laser irradiations were successfully achieved for larger induced second-order nonlinearity .
However, there has ever been no reports of direct evidence to show that induced crystalline particles are the origin of second-order nonlinearity in the Ge-doped SiO2 glasses, so that nature of such crystalline phases and mechanism of the induced nonlinearity are still kept in the stage of not fully understanding.
In this paper, using a SHG microscopy, we report the first observation of SHG emission obtained directly from the crystalline particles induced by heat-treatments in Ge-doped SiO2 glass films, which are much more suitable for photonic application use such as optical switches and modulators in integrated waveguides, compared with bulk-form glasses.
Ge-doped SiO2 glass films with composition of xGeO2-(1-x)SiO2, x=15, 20, 29, 30, and 33 in mol% in this study were fabricated by chemical vapor-phase deposition (CVD) method. For all samples, glass films with thickness of approximately 2–5 µm were deposited on SiO2 substrates with no-contents of GeO2. CVD film samples with about 1 mm total thickness were cut to the dimension of 10×10 mm.
Heat-treatments for crystallization were performed at 1150–1200°C for 1–3 hours in air to obtain transparent crystallized glass films, in contrast with the thermal condition at around 1150°C for 5 hours in the crystallized bulk glasses with 15GeO2-85SiO2 produced by VAD method . XRD patterns were measured in the crystallized glass films to confirm induced crystalline phases, and optical measurements, using both polarization optical microscope and SHG microscope, were investigated for direct observation of SHG origin. A SHG microscopy is reported as an interesting technique to obtain periodic structures of nonlinearity, such as domain inversion structure in nonlinear single-crystal materials . SHG microscopic measurements were performed by the use of a fundamental laser light of 1.06 µm wavelength from Q-switched Nd:YAG laser source. We tried to have a setup of SHG microscopy be suitable for observation of crystallized glasses, which are, in general, not uniform and quite space localized appearances of crystalline particles. A YAG laser beam with diameter about 10 mm was introduced to illuminated samples, then transparent fundamental and induced SH lights (λ=532 nm) were measured, although fundamental one must be cut off by filtering before reaching to CCD image detection. A typical condition of pulsed YAG laser is 125 mJ/cm2/pulse of laser energy, 10 pps of repetition, and 10 nsec of pulse width.
It was considered and checked that in the case of heat-treated SiO2 substrate without CVD films, any signal and sign of crystallization in both XRD and optical measurements were not observed. Elevated temperature at least up to around 1200°C for several hours is high sufficient for crystallization in these Ge-doped SiO2 films, but not for that in pure SiO2 glasses.
3. Results and discussion
3.1 Crystallization in Ge-doped SiO2 films
A typical XRD pattern of transparent crystallized glass films with composition of 33GeO2-67SiO2 in mol% obtained by heat-treatment at 1200°C for 2 hours in air is presented in Fig. 1. Positions and intensities of powder XRD peaks for α-cristobalite (SiO2) phase are also exhibited as reference. Measured XRD pattern for Ge-doped SiO2 films is found to be exactly the same as those previously reported for UV-poled 15GeO2-85SiO2 glass  and thermally crystallized Ge-doped SiO2 glass (VAD) . In addition, this XRD pattern much more resembles the one of α-cristobalite in SiO2 than any other SiO2 crystalline phases and GeO2 phases. The relative intensity of XRD peak at around 22° in Fig. 1 corresponding to (101) plane of α-crstobalite is remarkably large compared to the other XRD peaks, and in comparison with powder XRD patterns in α-crstobalite, higher order peaks, i.e. (202) and (303) for (101), are also emphasized in measured XRD patterns. This means that the orientation of crystalline particles is more or less aligned to a unique direction, probably that normal to the surface.
Expanded XRD pattern from 2θ=21.0° to 23.0° of crystallized glass films is shown in Fig. 2. The XRD peak has a complicated shape consisting of at least three components between 21.5° and 22.2°, and the peak was deconvoluted into three Gaussian components as shown in the figure. Peak components were assigned as 2θ=21.6°, 21.8°, and 22.0°, for “peak 1”, “2”, and “3”, respectively.
These fine structures of XRD peaks at around 22° are also properties measured in UV-poled glasses and VAD bulk crystallized glasses. In UV-poled glasses, the intensity of XRD peak positioned at the lowest 2θ angle, i.e., “peak 1” in this paper, was pointed out to be corresponding directly to an amplitude of SHG caused through the third-order optical nonlinearity5) as stated previously.
Figure 3 shows photographs of the typical crystallized glass films obtained under a polarization optical microscope (crossed-polarizers), and also differential interference microscope. For both photos, crystalline particles with approximately 20–30 µm in diameter were observed in Ge-doped SiO2 films after heart-treatments for crystallization. The diameter size, particle form, and appearance of the optical retardation for these crystalline particles are quite look-alike as those of crystalline particles exhibited in Refs. [3, 5, 7]. Especially for the case of UV-poling, only non-heating process among these, this obtained result here, XRD and optical microscope measurements in glass films, strongly suggests that crystallization obtained in UV-poled glasses should be originated from a thermal effect almost entirely.
3.2 GeO2–concentration dependence of XRD patterns
For considerations of the induced crystalline phases in Ge-doped SiO2 glass films, it is interesting to investigate more details of the XRD peaks depending on GeO2 concentration. CVD glass films with composition of xGeO2–(1-x)SiO2, x=15, 20, 29, and 33 in mol% were heat-treated for crystallization at 1150–1200°C for 1–3 hours in air. Optimum conditions of heat-treatments for each Ge-contents in glass films were decided by conditions for the largest peak intensity at 2θ=22.0° of XRD patterns in crystallized films. It was not obvious of dependence of crystalline particle size against the GeO2 concentration.
Measured XRD peaks at around 22.0° for CVD glass films were deconvoluted to three-Gaussian components, “peak 1”, “2”, and “3”, as the same manner stated previously. Intensity of three components, both peak-intensities and integral-intensities of Gaussian peaks, presented no systematic dependence on GeO2–concentration in this investigation, because such intensity must be strongly depending on volume fractions of crystallization. In this case of CVD glass films, volume for crystalline phases should be affected by thickness (few µm) of films as well as surface area.
GeO2 concentration dependence of peak positions for three Gaussian components are shown in Fig. 4. Error bar for each data point decided by peak fitting is represented in the figure. Two different sorts of trend are clearly seen in this result; 1) positions of “peak 1” and “peak 2” are depending on GeO2–concentration, i.e., decrease of position angles with increasing GeO2, which means increase of d-spacing (lattice constant) with increasing GeO2, 2) “peak 3” seems to be almost independent against GeO2–concentration, averaged 2θ~21.97° for all CVD films. This obtained 2θ value for “peak 3” is exactly the same as that of (101) in α-crystobalite in SiO2, 2θ=21.98°, thus independent phenomenon of “peak 3” to GeO2–concentration is consistent with that due to pure SiO2 crystalline phase.
Although other two components, “peak 1” and “peak 2”, are found to be crystalline phases related to GeO2–concentration, in contrast with the case of “peak 3”, assignation for those two crystalline phases are not clarified at present. Several crystalline phases positioned at these 2θ angles, for example (100) of α-quartz (2θ=21.19°) and (111) of β-crystobalite (2θ=21.62°), may be possible candidates for “peak 1” and “peak 2”, however, to obtain conclusive result, precise GeO2 solubility to SiO2 phases must be considered by taking account of phase separation between GeO2 and SiO2.
3.3 Observation of SHG microscope
Photograph of SHG microscope for a glass film with 30GeO2–70SiO2 heat-treated at 1130°C for 8 hours in air is presented in Fig. 5(a), in comparison with photographs taken under polarization microscope in Fig. 5 for, (b) crossed-polarizers and (c) opened-polarizers configurations. Since heat-treatment for this case was intentionally performed to be longer than those for other cases to have clear SHG emissions from well-growth crystalline particles, observed crystalline particles as shown in Fig. 5 are relatively larger than those for other experiments. Diameter for crystalline particles is approximately 40–60 µm, almost two to three times larger than those in Fig. 3 with heat-treatment at 1150°C for 2 hour. In addition, crystalline particles with 3- and/or 4-fold symmetry can be clearly seen in Fig. 5 by enhancement of crystal growth.
Positions and relative intensities for XRD peaks in crystallized films with larger crystalline particles are the same as those for smaller particles, which was shown in Fig. 1. In Fig. 5(a) of a photo by SHG microscope, clear SHG emissions (λ=532 nm) are appeared from some crystalline particles, seven particles in the area, and it can be found in comparison with Fig. 5(b) (cross-polarizers configuration of polarization microscope) that just the crystalline particles with a strong phase retardation contribute to SHG emission. Other crystalline particles with neither SHG nor phase retardation are still bright in cross-polarizers configuration. It is possible to observe SHG emission from smaller crystalline particles (~20 µm), but not obvious for positioning of particles rather than the case of larger particles. Intensity of the fundamental laser beam was not averaged in our experiment of SHG microscope, so that, close to the center region in a view of photo may be the most emphasized because of distribution on fundamental beam intensity.
Several snake lines are noticed in Figs. 5(b) and (c), also can slightly observed in Fig. 5(a). Those are cracking introduced after heat-treatment for crystallization with larger particle size. Such lines were never seen in the case of smaller particle size induced by relatively shorter period of time for heat-treatments. In Fig. 5(b), phase retardation is not clear rather than those in Fig. 3(a). One possible reason for this can be explained by release of retardations due to the introduction of cracking lines. In Fig. 5(c), SHG-active particles are quite transparent in contrast with non SHG-active particles, which are clearly visible. Non SHG-active particles may not be oriented and/or may have poor crystallinity because of irregular shapes of crystalline particles compared to those of SHG-active ones, therefore, they are leading to visible observation (not good transparent) in non SHG-active particles.
Possible anisotropy of SHG emission was checked for a plane parallel to the surface. No clear evidence of polarization-angle dependence of SHG was observed in this case. Preferred orientation of crystalline direction, (101), in crystallized films should be normal to the surface as stated previously. Although it may not be easy to measure, an observation of anisotropy of SHG to the normal direction is interesting.
As it can be seen in Fig. 5, crystallization is occurred in the surface of CVD films. We have traced the experiment on change of XRD patterns depending on chemically surface etching, which was reported by Matsumoto et al. . In our experiment, after the polishing of surface less than approximately 1 µm, the original complex shape of XRD patterns at around 22° changes to a single peak just with the highest 2θ angle of “peak 3”, and at that situation SHG emission also disappeared entirely with vanishing of “peak 1” by surface removal. In addition, phase retardation observed in Fig. 5(b) was fade-out with increasing surface polishing. After the polishing, enhancement of polishing in boundary area, grooves with narrow width, between crystalline particles and glassy surroundings was confirmed, the same aspect in Fig. 4 of Ref. .
For overall discussion of these results, a residual stress between crystalline and glassy regions should be considered because of phase retardation stated originally in crystalline particles and enhancement of etching in boundary regions after surface polishing. The centrosymmetric α- and β-cristobalites are not SHG-active, therefore, it is strongly suggested that SHG-active crystalline particles have stress-distorted cristobalite phase (“peak 1”) as thermally crystallized. On this assumption, the most plausible explanation of SHG disappearance from the particles by surface polishing is that release of residual stress by means of surface etching is the main origin for both changes of XRD patterns and disappearance of SHG. As reported previously5), it has already found that amplitude of the XRD peak at the lowest 2θ angle, i.e. “peak 1”, reflects the amount of third-order optical nonlinearity, χ(3). In this case, space-charge electrical field caused by the poling can give the effective (extrinsic) second-order optical nonlinearity, χ(2), through the relation of χ(2)=3E scχ(3). But here, SHG emission can be obtained directly from the crystalline particles without electric poling of glass films, thus there should be certainly non negligible SHG that is intrinsically originating from the crystalline nature of the particles in crystallized glass films.
Although precise crystalline phase of “peak 1” with SHG-active is not clarified yet at present, our experimental results presented in this paper strongly suggest that crystalline particles with stress-induced second-order optical nonlinearity is the main origin of the nonlinearity in crystallized Ge-doped SiO2 glass films. Because the second-order nonlinearity induced by the poling shows the degradation at room temperature , such an intrinsic nonlinearity is quite important in particular for long-term stability in application fields.
For optical waveguides application, the crystalline particles with a few 10 µm diameter, which will cause strong propagation loss, are not appropriate. Formation of single-domain crystalline phase, i.e., single crystal, with line patterns is one of the best solutions for waveguides application in crystallized glass. Recently, Honma et al.  reported on novel technique of YAG laser writing for single crystal waveguides in Sm-doped glasses. In this method, highly effective “atomic-scale heating” through the optical absorption of Sm ions can be used, and challenges of single-crystal waveguides patterning in silica-based glasses are now in progress by taking account of thermally crystallization behaviors in silica glass as stated in this paper.
We have, for the first time in our knowledge, found SHG emission directly from the crystalline particles induced by thermally crystallization in Ge-doped SiO2 glass films. XRD peaks at around 2θ=22° in the crystallized Ge-doped SiO2 films were observed, and the obtained XRD patterns in the CVD glass films are exactly the same as those in UV-poled Ge:SiO2 glasses. GeO2–concentration dependence of three XRD peaks positioned at around 2θ=22° were investigated, and the peak at the highest angle of 2θ was found to be almost independent against GeO2-concentration, in contrast with those of other two peaks strongly depending on GeO2–concentration.
By the use of a SHG microscopic technique, it has been successfully measured that SHG signal with 532 nm wavelength, corresponding to a fundamental Q-switched Nd:YAG laser with 1.06 µm, is clearly measured directly from the crystalline particles induced in thermally crystallized Ge-doped SiO2 glass films. Although conclusive assignments of a crystalline phase for SHG-active particles in the films are not defined yet at the moment, possible crystalline phases, for example stress-induced SHG-active phase, were discussed here. These results obtained in this study indicate the origin of second-order optical nonlinearity induced by the crystallization, and suggest a new photonic application of space-localized crystallization in silica-based glass materials.
The authors would like to thank supports from Applied Laser Engineering Center, Asahi Glass Company, Mitsubishi Electric Corporation, and the Program of “Center of Excellence (COE) for 21st century” in Nagaoka University of Technology.
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