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Transmission volume phase holographic gratings were fabricated in bulk photo-thermo-refractive glass using zero-order femtosecond laser Bessel beams and subsequent thermal treatment. Microstructures composed of nano-sized crystals were observed in the exposed regions. The concentration of nano-crystals depended on the writing power, but the size of the nano-crystals was found to be quasi power-independent. Low writing power led to sparse nano-crystals, while optimized writing power achieved dense nano-crystals distribution and high refractive index change. Effects of the gratings thickness, writing laser power and thermal treatment on the diffraction efficiency were investigated. A maximum diffraction efficiency of 94.73% was achieved (at 532 nm testing wavelength) with 1 mm grating thickness at period of 5 μm.
© 2016 Optical Society of America
1. Introduction
Volume diffractive optical elements are extensively applied for coupling elements in laser resonators, splitters, attenuators, holography, etc [1–4]. Thus, they have attracted considerable attention as promising solutions to decrease laser systems size and increase laser beams quality [5]. Photo-thermo-refractive (PTR) glass, which is a photosensitive silicate glass doped with silver, cerium, fluorine and other elements, owns a large number of advantages such as thermal and mechanical stability, high laser damage threshold, and wide transparency range [6,7]. It is a promising candidate for volume diffraction optical elements. PTR glass manifests photosensitivity resulting in the precipitation of nano-sized sodium fluoride (NaF) crystals within the glass matrix in typically UV-exposed regions and refractive index modulation after heat treatment [8]. The refractive index modulation is a significant factor to influence the diffraction property of volume holographic grating (VHGs). Specific efforts have been devoted to determining the origin of (local) refractive index modifications based on theoretical and experimental analyses [9–11]. The refractive index change may be caused by the difference of the refractive index between NaF crystalline phase (n ~1.33) and unexposed PTR glass area (n ~1.49), and also by residual stresses surrounding the NaF crystals in PTR glass after UV exposure and heat treatment. Based on the fact that refractive index of PTR glass can be modified, various high-efficiency phase gratings have been recorded by UV exposure and heat treatment [3,7].
Compared with UV exposure, near-infrared ultrashort laser pulses have gained more interest due to their ability to confine energy in the volume. Thus, the ultrafast regime can achieve more and more complex embedded photonic structures by a simple and effective process. The interaction between infrared femtosecond (fs) pulses and dielectric materials allows localized modifications of the material structural properties, depending on their specific characteristics and the possibility of nonlinear absorption [12–17]. For example, in the case of PTR, cerium ions (photo-sensitizer) can be photoionized under near-infrared ultrashort laser pulse exposure by nonlinear absorption inducing locally photochemical processes by electron release [18]. Such techniques could offer the advantages of rapid fabrication of three-dimensional (3D) structure in bulk materials relying on both structural and chemical effects. Inversely, UV exposure relying on linear absorption needs long time with very weak intensity to avoid over-dose absorption near the sample surface. The flexibility of ultrafast pulsed laser writing has been considered as a major feature of a promising tool to micro-fabricate optical elements in integrated optics inside bulk PTR glass.
In the present work, the phenomenon of ultrafast laser photoinscription using non-diffractive zero-order Bessel beams was observed. The aim was to gain high aspect ratio character in the fabricated traces. Refractive index changes before and after thermal development were compared and analyzed by phase contrast microscopy. After thermal development of the sample, photoinscribed microstructure composed of distributed nano-sized crystals in the fs laser exposed region was observed. The density of nano-sized crystals which played an important role in the index change could be tuned by the exposure dose. Besides, high diffraction efficiency VHGs were fabricated in PTR glass by zero-order chirp-controlled fs laser Bessel beams exposure and thermal treatment. Effects of writing power, Bessel beam transformation and heat treatment on the nano-crystals structure and diffraction efficiency (DE) were investigated in detail.
2. Experimental procedure
2.1 Preparation process
PTR glass with composition of 73SiO2–11Na2O–7(ZnO+Al2O3)–3(BaO+La2O3)–5NaF–1KBr (mol%) doped with 0.02SnO2–0.08Sb2O3–0.01AgNO3–0.02CeO2 (mol%) was used in this work. The PTR glass was polished and cut to dimensions of 10 × 10 × 2 mm3 and fixed on a 3D translation stage. A Ti: Sapphire regenerative amplifier fs laser system (Phidia, Upteksolutions) working at a wavelength of 800 nm and at a repetition rate of 50 kHz and pulse duration of 160 fs was used to fabricate gratings. The fs laser beam with a horizontal polarization (Y-axis) was focused into PTR glass through a 20 × microscope objective with numerical aperture of 0.42. By translating the sample (Z-axis), a range of parallel traces with interval of 5 μm were written 100 μm below the front surface. The writing velocity was fixed at 200 μm/s. It is worth noting that grating thickness directly influences the diffraction effect. The typical Gaussian beams suffer from high degree of spatial energy confinement for single-scan within the geometrical focal volume. Hence, the corresponding laser induced structure was observed to be small and limited by the confocal zone [19]. To speed up fabrication, zero-order Bessel beams, containing a narrow intense central core sustained over a long distance (called non diffracting length, Zmax), were used in this experiment [20,21]. The zero-order Bessel beams were generated using axicon lenses [22,23] and then imaged into the bulk glass. Compared with Gaussian filaments, Bessel filaments have a higher degree of nonlinear stability and higher aspect ratio [24]. Schematic drawing of the fs laser axicon-based Bessel beams employed to fabricate gratings is shown in Fig. 1. The Gaussian laser beams were converted into Bessel beams through an axicon (0.5°) and further demagnified using a convex lens (f = 30 cm) and a 20 × microscope objective lens. Since Bessel beam shaping techniques were employed, the grating thickness, namely, the length of Bessel filaments was about 1 mm in the direction of the propagation axis and was related to the non-diffractive distance. Along the scan direction, a grating length of 3 mm could be obtained. The grating period was 5 μm and the total grating size was 3 mm. Based on the grating geometry, a grating with area of 3 × 3 × 1 mm3 could be produced. After fs laser exposure, the PTR glass underwent heat treatment for nucleation of 5 h at 490 °C and crystallization of 3 h at 595 °C. The procedure was beneficial to generating a large refractive index variation on the tracks, which can improve the diffractive characteristics of phase grating.
2.2 Characterization
Optical transmission microscopy (OTM), phase contrast microscopy (PCM), and scanning electron microscopy (SEM, ZEISS EVO-18) were primarily employed to reveal the morphological characteristics. The diffraction efficiency of the VHGs was measured with continuous wavelength (CW) He-Ne laser (632.8 nm), laser diode (532 nm and 980 nm) and fiber lasers (1064 nm), respectively.
3. Results and discussion
3.1 Refractive index changes in PTR glass
The PCM images of PTR VHGs without annealing under different irradiating laser power levels (40, 100, 200 and 300 mW) are presented in Fig. 2. The insets are magnified views corresponding to the dotted boxes and the black curves show the corresponding grey level distribution normal to grating lines, indicating the relative phase modulation. For positive phase contrast microscopy, the darkened zones characterize the domains of relative refractive index increment. As shown in Fig. 2, when the shaping fs laser Bessel beams irradiate to PTR glass, the refractive index will increase in the focal regions. This could be attributed to the interaction between fs laser Bessel beams and the PTR glass, which leads to a series of complex changes in the glass, including nonlinear photoionization, structural transitions and fast thermal quenching. These interactions could lead to local structural modification and densification of the glass for fs laser Bessel beams. The phenomenon seems similar to the previous work about fused silica [21] and in general several glasses show a weak refractive index increase in irradiation regime based on a mix of electronic and structural factors. At the same time, nonlinear ionization effect results in the release of free electrons. Thereafter, Ag+ traps a free electron and forms Ag0, it is worth discussing the difference of fs pulse laser and UV radiation on PTR glass area. It is considered that for traditional UV radiation, all diffractive optical elements are recorded via linear excitation of Ce3+ (photo-sensitizer in PTR glass) within its spectrum of photosensitivity (280-350 nm), and UV exposure could not efficiently change the refractive index of the PTR glass in direct manner without annealing. On the contrary, fs laser pulses can trigger significant multiphoton ionization and generate free electrons. The strong photoionization can provide electrons to support both photochemical reactions and structural changes. The refractive index modulation achieved in the focal areas shows a visible influence of the photon dose as indicated in Fig. 2. In Fig. 2(a) and (b), the traces show a weak positive index contrast for lower power. As comparison, Fig. 2(c) and (d) present high contrast index traces at higher power.
Fig. 2 The phase contrast images of VHGs under different writing power: (a) 40 mW, (b) 100 mW, (c) 200 mW and (d) 300 mW.
Figure 3 shows the PCM images of PTR VHGs writing with laser power of 40, 100, 200 and 300 mW after annealing. Compared with Fig. 2, it is obvious that the refractive index decreases in PTR glass after fs exposure followed by thermal development. Two phases can be clearly distinguished, which are attributed to the aggregation of Thereafter, Ag+ traps a free electron and forms Ag0 producing atomic silver nucleation centers resulting in the accelerated growth of nano-sized fluoride crystals in fs laser exposure regions [25]. Because the refractive index of the crystalline phase of fluoride crystals is lower than that of the glass matrix, nano-crystals are considered to be responsible for producing a greater but negative refractive index difference after annealing. It is noted that the structure evolves through two different types of morphology depending on the laser power. One type emerges at the low laser power, which has a remarkable spatial uniformity. Further increasing laser power, the radiation areas with high writing power produce discontinuous and thinner tracks, which could deteriorate the quality of the machined structure as shown in Figs. 3(c) and 3(d). Such unique structure is the combined effect of high power fs laser Bessel beam and consequences of the heat treatment. The exact mechanism responsible for the formation of unstable non-uniform regimes is still under investigation [24,26]. The discontinuous structure could affect the DE due to the strong light scattering. In such circumstances, the fs laser writing power should be controlled below 200 mW in the present geometry to obtain optimal trace uniformity.
Fig. 3 The PCM images of VHGs under different writing power: (a) 40 mW, (b) 100 mW, (c) 200 mW and (d) 300 mW after thermal annealing.
3.2 Origin of crystallization in VHGs
As shown in Fig. 4, the SEM images demonstrate that the fs laser exposure regions followed by thermal development are composed of nano-crystalline particles embedded in the PTR glass matrix. The micron sized structure of the traces containing nano-sized crystals demonstrates that the size of nano-crystals is independent of the writing power. At 40 mW (Fig. 4(a)) after annealing, the microstructure of PTR VHGs is relatively smooth, and a small number of sparse nano-crystalline particles appear in the laser exposure regions. The distinct crystal boundaries and the densely distributed nano-crystals domains can be observed at 100 mW (Fig. 4(b)), illustrating a full growth of the nano-crystals. However, for higher writing power of 300 mW, the fs Bessel laser will produce small-scale distortions in the uniformity of energy deposition in the PTR glass. These non-uniformities (self-focusing or other axial disturbances) can lead to changes in the photo-chemistry dynamics, leading eventually to compact nano-crystal clusters, which could destroy the filament and make the tracks thinner and discontinuous (Fig. 4(c)). The experimental phenomenon is consistent with the Fig. 3(d). The magnified morphology of nano-crystals in Fig. 4(b) is displayed in Fig. 4(d). From Fig. 4(a) and 4(b), it is clear that in the appropriate power range, different writing power determines uniform particle size. By statistical calculation (Fig. 4(d)), the mean crystal size is about 200-300 nm.
Fig. 4 SEM images of nano-crystal formation in PTR glass under different writing power: (a) 40 mW, (b) 100 mW, (c) 300 mW, (d) Magnified view of (b).
3.3 Diffraction properties of VHGs
Figure 5 displays the contrast diagrams of DE for the grating thickness, thermal treatment and different writing power for a grating period of 5 μm and in the writing power range of 40-150 mW, respectively. Compared with Gaussian beams, the Bessel beams show the longer trace thickness namely longer grating thickness. Based on Kogelnik’s theory of coupled waves [3,27], a solution of the scalar wave equation for VHGs gives the following formula for DE:
where, ξ is dephasing parameter that describes deviation from the Bragg condition, Φ is phase incursion that determines the maximum DE of VHGs. When the VHGs are in Bragg condition, ξ = 0 and Φ is written as:where, t is the thickness of the traces, namely grating thickness, δn is an amplitude of refractive index modulation, λ0 is free space wavelength, parameter Fφ is an inclination factor:Fig. 5 DE contrasts for (a) Bessel beams, (b) before and after heat treatment and (c) under different test wavelengths.
For normal VHGs with φ = ± π/2, the expression for Fφ is:
where, Λ is the period.Derived from the above formulas when VHGs meet the Bragg condition (ξ = 0) and they are normal (φ = ± π/2), the formula for DE of PTR VHGs is:
This formula shows that the longer grating thickness, the higher DE value. It coincides with the experimental results (Fig. 5(a)) in which the maximum DE increases from 65.35% to 92.00% at 632.8 nm testing laser by beam shaping technique changing Gaussian beams to Bessel beams, which will increase grating thickness from about 100 μm to 1 mm. Figure 5 (b) demonstrates that the maximum DE value of VHGs at grating thickness of 1 mm increases from 2% to 92% after thermal treatment. The main reason is that the fs laser exposure regions will produce enough nano-crystals during annealing in a proper writing power range, which causes large refractive index difference. According to formula (5), the increased refractive index difference will result in the increase of DE value. The relationships between DE value and writing laser power tested with continuous laser at different wavelength are shown in Fig. 5(c). The DE value firstly increases with the increase of writing laser power and reaches maximum value when the laser power is about 100 mW. Then the DE value monotonously decreases with the increase of writing laser power. The highest DE value recorded with Bessel beam is 94.73% at 100 mW writing power and 532 nm testing wavelength. Based on the above experimental results and theoretical analyses, the diffraction properties of VHGs in PTR glass can be tailored by controlling writing power, grating thickness and thermal treatment serving as spectral or angular selectors in integrated optics. In addition, the gratings are fabricated inside the PTR glass with good thermal and mechanical stability, avoiding contact with air and dust. Thus, the gratings have the potential advantages of wear-free, refractive index change stability and a long grating lifetime. Moreover, relying on the flexibility of fs laser writing, gratings with different sizes and functions can be fabricated, which demonstrates a simple, efficient, reliable and flexible method.
4. Conclusions
In this work, the VHGs with different writing power were fabricated in PTR glass by fs laser Bessel beams. The phase variation, microstructure and diffraction characteristics were investigated. The microstructure of nano-sized crystals reveals that the size of nano-crystals is about 200-300 nm and that is independent of the writing power. With the increase of the writing power, the overall microstructure of the nano-crystals agglomeration evolves from sparse distributions to dense ones, and further forms compact nano-crystal clusters. Higher density of nano-sized crystal shows stronger index modification. Using Bessel beams shaping technique, the maximum DE could increase 26.65% due to the increase of the grating thickness. Moreover, owing to the increasing of refractive index difference by thermal treatment, the maximum DE could increase 90.00%. The achieved maximum DE is 94.73% in PTR glass with 1 mm grating thickness at 100 mW writing power and period of 5 μm. These results demonstrate that laser writing condition and annealing play a crucial role in determining the DE of the PTR VHGs. The superior diffraction property of PTR VHGs indicates that they can be attractive candidates for the spatial filtering elements. In our future work, the growth mechanism and components of nano-crystals in PTR glass prepared with fs laser Bessel beams and thermal development will be discussed. By using SEM, energy dispersive spectrometer and wave dispersive spectrometer the detailed crystallization and quantitative analyse of microcrystalline particles composition in fs-exposed PTR glass will be studied.
Funding
National Natural Science Foundation of China (61378019, 61223007, 61471301, 61078057); the Specialized Research Fund for the Doctoral Program of Higher Education (Grant Nos. 20126102110045); National Natural Science Foundation of Zhejiang (No. LY14F050002).
Acknowledgments
The corresponding author thanks Prof. Kuaisheng Zou in SooChow University for sample thermal treatment, and Prof. Hong Chang and Dr. Maojie Yang in Institute of Earth Environment, CAS for SEM imaging.
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