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We report on successful joining of a beta barium borate crystal by plasma-activated direct bonding. Based on this technology, a sandwich structure consisting of a beta barium borate crystal, joined with two sapphire heat spreaders has been fabricated. Due to the high thermal conductivity of sapphire, the sandwich structure possesses superior thermal properties compared to the single crystal. Simulations based on the finite element method indicate a significant reduction of thermal gradients and the resulting mechanical stresses. A proof of principle experiment demonstrates the high power capability of the fabricated structure. A pulsed fiber laser emitting up to 253 W average power has been frequency doubled with both a single BBO crystal and the fabricated sandwich structure. The bonded stack showed better heat dissipation and less thermo-optical beam distortion than the single crystal. The work demonstrates the huge potential of optical sandwich structures with enhanced functionality. In particular, frequency conversion at average powers in the kW range with excellent beam quality will be feasible in future.
© 2014 Optical Society of America
1. Introduction
Since their invention in 1960 [1], lasers found numerous applications in science, industry and our daily life. Efficient laser operation is usually limited to the infrared and visible spectral range [2]. Frequency conversion in nonlinear optical crystals allows transferring laser radiation from the infrared into the visible and ultra violet wavelength range. In addition, optical parametric amplifiers and oscillators generate tunable and ultra-short laser pulses [3] from the visible up to mid infrared wavelengths. Moreover, they allow for the amplification of ultra-broad optical spectra, hence supporting few-cycle laser pulses to very high peak [4] and average powers [5], enabling various applications in science and industry. In principle, no energy is stored in parametric processes, which does not result in additional thermal load. Although, the bulk and coating absorption coefficient of the interacting optical waves is very low in typical nonlinear crystals, thermo-optical problems may arise at very high average power operation.
Heating of the nonlinear crystal results in a temperature gradient, which can disturb phase-matching and lead to thermo-optical beam distortions. In addition, thermally induced tensile stress can result in crystal fracture. Indeed, such thermal effects have already been observed in optical parametric oscillators (OPOs) [6] and optical parametric amplifiers (OPAs) [7].
In this paper we propose improving the thermo-optical properties of birefringent nonlinear crystals by contacting them with a material with higher thermal conductivity. Our work is focused on beta barium borate (BBO) since it is widely used for harmonic generation and optical parametric devices [8]. It provides a high laser damage threshold, a large transparency range combined with high nonlinearity, small walk-off angle and a large temperature and spectral bandwidth [9]. Sapphire is chosen as the transparent heat spreader material with nearly two orders of magnitude higher thermal conductivity (κsapphire = 42 W/Km [10], κBBO = 0.08…0.8 W/Km depending on crystal orientation [11]).
For optical applications a joining technology working without any auxiliary materials like glue is favorable. Parasitic absorption caused by the joining material is avoided while stiff and permanent connections are formed. We choose to contact the materials by direct bonding, which is established within semiconductor industries for some decades [12], but is rarely used in optics despite its advantages. For direct bonding the even and smooth surfaces are plasma-treated prior to contacting, to create high surface activity. After contacting and a heat treatment, strong bonds are formed. This joining concept already proved successful for joining terbium gallium garnet and sapphire, to be used as magneto-optical element in a Faraday rotator or isolator [13].
Here we report on the successful transfer of the technology of direct bonding for joining BBO crystals with sapphire disks. Simulations of the heat and stress distribution in the sandwich structure and single crystal using the finite element method (FEM) are presented first. Afterwards, results of the surface characterization regarding flatness and roughness will be shown. Different cleaning methods were evaluated. Finally, results of bonding experiments of coated and uncoated samples are presented. A successfully bonded BBO-sapphire sample was utilized for second harmonic generation. Within this test, the surface temperature, the conversion efficiency and the beam profile at increasing incident laser power were characterized. The results are then compared to an unbonded single crystal, with comparable specifications, tested at the same conditions.
2. Simulation
In order to estimate the cooling efficiency of the aspired sandwich structure, FEM simulations have been performed with ANSYS software both for the single crystal and the sandwich structure. The sandwich consists of a BBO disk (10 x 10 x 2 mm3) with sapphire disks (10 x 10 x 1 mm3) connected to each optical surface. The dimensions of the crystals were chosen to facilitate their handling. A Gaussian beam with a waist of 500 µm and an absorbed power of 130 mW is applied on the model to simulate the thermal load, which corresponds to the absorbed power of a 250 W laser in the bulk, operating at λ = 1030 nm, transmitted through the crystal. The calculated temperature distribution for both structures is shown in Fig. 1. As expected due to the high thermal conductivity of the heat spreaders, the temperature distribution in the BBO sapphire sandwich shows significantly lower temperature gradients compared to the BBO single crystal (10 x 10 x 2 mm3; same orientation). The maximum temperature difference between the bulk material and the edge, where the temperature is set constant, within the bonded sandwich is 10.5 K compared to 25.0 K in the single crystal.
Fig. 1 Temperature difference in BBO single crystal (a) and BBO-sapphire sandwich (b), the black arrow indicates the incident beam.
Note that the temperature bandwidth (range over which the phase mismatch accumulated throughout the crystal length varies from –π to π) is as high as 377 K for a 1 mm thick BBO crystal [14]. Hence, the observed temperature gradients lead to negligible dephasing only.
The corresponding stress is displayed in Fig. 2. The maximum tensile stress is as high as 13 MPa at the surface of the single crystal, but significantly lower stress (4.9 MPa) is found on the sapphire surface of the sandwich structure. Note that the risk of surface damage is further reduced in this case, since the fracture toughness of sapphire (KIC = 4.5 MPa∙m1/2 [15]) is one order of magnitude larger than that of BBO (KIC = 0.2 MPa∙m1/2 [11]). In summary, the numerical simulations reveal that the temperature gradients and the surface tensile stresses can be significantly reduced by contacting sapphire heat spreaders for the chosen geometry or similar geometries [13].
Fig. 2 Main stresses in BBO single crystal (a) and BBO-sapphire sandwich (b), the black arrow indicates the incident beam.
3. Surface characterization
Plasma activated bonding relies on covalent bonds which are formed between the surfaces. From this it is obvious, that with decreasing roughness the contact area of the surfaces increases as well as the joining strength. Until now, no direct correlation between surface roughness and bond strength is established. Apart from this, from various experiments it is known, that the surfaces need to be smooth and flat, so that bonding can take place. For stable bonds a surface roughness of lower than 1 nm root-mean-square (rms) [12] and a high flatness (i.e. 160 nm peak-to-valley (PV), which corresponds to λ/4 for λ = 633 nm [16]) is necessary for bonding of any material. Therefore, samples are tested for roughness by means of atomic force microscopy (Veeco Dimension D3100 with measurement sizes of 1 x 1 µm2 and 10 x 10 µm2). The resulting images of an uncoated and a protection coated BBO sample are displayed in Fig. 3 and Fig. 4 respectively. The protection coating was chosen, to protect the surfaces from harmful water impact which is normally used for the cleaning steps. Figure 5 shows AFM measurements of a sapphire sample. The rms roughness values are summarized in Table 1.The protection coating is clearly visible as columnar surface structure in the left image of Fig. 4. Obviously, the coating leads to surface roughening, quantified by the increased surface rms roughness of the coated sample (1.5 nm rms of the coated BBO sample compared to 1.0 nm rms of the uncoated BBO surface at both measurement sizes). The sapphire surfaces fulfill the roughness criterion for successful bonding, having a rms roughness of 0.3 nm.
Table 1. Rms roughness revealed by AFM measurement of coated and uncoated BBO samples
The flatness of the surfaces is revealed with a Fizeau interferometer (ZYGO GPI Verifire with a reference flat of λ/30). Due to the high refractive indices of both sapphire (n = 1.75 [17]) and BBO (n = 1.67 [18]) the crystalline samples were immersed in diiodomethane liquid (n = 1.74 [19]) in order to reduce reflections from the backside of the sample at the transition to air. Note that these residual reflections at the backside are sufficiently suppressed for the sapphire sample. However, due to the slight difference in refractive index of BBO and diiodmethane weak interference fringes, caused by reflections from the backside, are visible in case of BBO. Images of both of the frontside and the backside of a representative BBO and sapphire sample are shown in Fig. 6 and Fig. 7, respectively. The peak-to-valley flatness values (averaged from three measurements), which are an indicator for stable bonds, are summarized in Table 2.The BBO surfaces exhibit low surface flatness, more than 160 nm PV, contrary to the sapphire surfaces with a flatness less than 160 nm PV. Low flatness may be compensated by using samples with lower stiffness. Hence, thin sapphire samples with only 1 or 2 mm thickness have been employed for bonding.
Fig. 6 Interferometric images of the BBO surface (image sizes: 10.1 x 7.9 mm2; (a): front; (b): back).
Fig. 7 Interferometric images of the sapphire surface (image sizes: 9.6 x 7.9 mm2; (a): front; (b): back).
Table 2. Peak-to-valley surface flatness values (average of three samples of one production lot)
4. Bonding process
For successful plasma-activated bonding, samples need to be cleaned intensively. Cleaning procedures are used to create a pure surface without any contaminating species. Therefore, the sapphire samples are cleaned in a bath cleaning procedure, consisting of several ultrasonic-assisted alkaline and water bath steps. Afterwards, samples are spin cleaned with chemicals similar to the RCA cleaning process, commonly used for semiconductors. Due to the hygroscopic character of the BBO crystals [20], another approach had to be used for this material. Therefore, several cleaning fluids were used to swipe the BBO surface clean. To examine the success of the particular cleaning process, the sample surfaces were inspected visually by light optical microscopy. The samples were also analyzed by means of AFM before (see Fig. 8 left) and after cleaning, in order to prove that the surface morphology did not change during the cleaning process. A mixture of anhydrous ethanol and ether (see Fig. 8 right), which is commonly used to clean BBO, as well as acetone (see Fig. 8 middle) were used as cleaning agents. From the AFM images it is obvious that both cleaning methods lead to a change of the surface morphology. The corresponding measured roughness values are summarized in Table 3.As visible from the images as well as from the rms values, the most effective cleaning agent is acetone, which reduces the roughness significantly to 0.6 nm compared to 1.0 nm before cleaning (1 x 1 µm2 measurement area). The use of the mixture of ether and ethanol leads to a slight increase of the roughness.
Fig. 8 AFM measurement image of BBO substrate as delivered (a), after wiping with acetone (b) and after cleaning with mixture of ether and ethanol (c).
Table 3. Surface roughness values before and after different cleaning procedures
To achieve a strong bond between the two surfaces, it is assumed that a covalent bond consisting of X-O-X’ connections is formed, where X and X’ are non-oxygen-atoms in the crystal lattice [12]. These bonds are formed by a condensation reaction. Therefore, a hydrophilic surface needs to be created, where the surface is covered by OH groups. This is achieved by low pressure nitrogen plasma activation of the oxide crystal, which forms dangling bonds at the oxygen atoms. Atmospheric humidity is sufficient to form a surface covered by OH groups. The activated surfaces are brought into close contact. Then, the samples are pressed together to compensate for the different surface morphologies and flatness, so that a weak connection via van der Waals forces is formed. A subsequent heat treatment at moderate temperatures around 100°C is used to enhance the reaction between adjacent OH groups of the opposing surfaces to form water. With this procedure bonding of an uncoated BBO crystal (10 x 10 x2 mm3; θ = 22°; φ = 90°) to sapphire (10 x 10 x 2 mm3) is possible. The result is visible in Fig. 9.The BBO crystals were cleaned with acetone in this case. Even though BBO showed surface flatness value larger than 160 nm PV, it was possible to bond a 2 mm thick sapphire disk to one side.
Fig. 9 Photograph of the bonded stack, consisting of a BBO disk (10x10x2 mm3; θ = 22°; φ = 90°) between two sapphire disks (10x10x2 mm3 on bonded to the top side of the BBO crystal and 10x10x1 mm3 on bonded on the bottom side of the BBO crystal)
Direct bonding of BBO to sapphire is difficult due to the large difference in coefficients of thermal expansion (CTE, CTE of BBO along the crystal a axis: 4∙10−6 K−1, along the crystal c axis: 36∙10−6 K−1 [11]; CTE of sapphire: 5∙10−6 K−1 [10]). Temperatures higher than 100°C during the bonding process lead to ripping of the samples with cracks normal to the direction of the highest CTE, due to thermally induced stresses. Bonding of protection coated BBO crystals to sapphire was not possible. Besides the poor roughness mentioned earlier, another reason could be the chemical composition of the coating, which consists of MgF2.
5. Laser testing
Having successfully produced high quality BBO sapphire compounds, a proof of principle experiment has been performed, in order to prove their optical functionality. From FEM simulations it was suspected, that stress frozen in the crystals during the bonding procedure, could disturb phase matching and the wave fronts of the laser beam. Furthermore, the durability under high power laser irradiation had to be tested. Therefore, a setup was created to investigate the samples via second harmonic generation of a high average power picosecond laser. The orientation of the BBO crystal has been initially chosen to enable Type 1 phase matching of second harmonic generation for radiation of λ = 1030 nm radiation. A high power fiber laser system operating at λ = 1030 nm wavelength is employed as driving laser [21]. The laser pulse duration is set to ~2 ps by restricting the spectral bandwidth to ~1 nm. The laser system delivered average output powers up to 253 W. For efficient second harmonic generation, the pulses were focused to ~100 GW/cm2 peak intensity. The phase matching angle was fine tuned to maximize the conversion efficiency for each crystal and laser repetition rate. Both the output power of the laser system at the fundamental and the frequency converted power of the second harmonic radiation were measured simultaneously by splitting them with two dichroic mirrors. During radiation the samples were also observed with an IR camera (FLIR T335) and the beam profile of the generated second harmonic radiation was recorded with a CCD camera. Under similar experimental conditions the bonded BBO stack was compared with a BBO single crystal with same length and orientation. The used BBO single crystal was anti-reflective coated on both sides.
5.1 Conversion efficiency
The conversion efficiency is defined by the ratio of generated second harmonic power and incident fundamental power onto the crystal. For comparison the input and output power were corrected for losses caused by reflections on the interfaces (sapphire-air, BBO-air or anti-reflective coatings).
The conversion efficiencies at different input powers for the BBO sapphire stack and the BBO single crystal each are shown in Table 4.There are no significant differences between the different samples. The slight decrease in conversion efficiency from 37 W to 126 W is caused by a slight reduction of the intensity of the fundamental laser at the frequency doubling crystal due to thermo-optical beam distortions induced by different lenses employed for collimation, changing the beam diameter via a telescope and focusing of the high power laser beam coming from the fiber amplifiers. In addition, the beam diameter and divergence of the radiation emitted from the amplifier fibers in the employed laser system are also slightly changing with average power [22]. All these changes are the same for the BBO single crystal and the stack, since they originate from other optical components located in the high power beam path before the SHG stage. In consequence, the same decrease is observed for the BBO single crystal and the stack, which also proves that these thermo-optical distortions are located outside the SHG crystal (stack).
Table 4. Conversion efficiency at different input powers
An additional test at higher average power was feasible by replacing a lens telescope in front of the pulse compressor of the laser system by a telescope composed of spherical mirrors. As a result an elliptical beam with slightly different beam parameters has been observed.
Nevertheless, efficient SHG was feasible also at 170 W and 253 W of infrared power without significant differences between stack and single crystal.
5.2 Beam profile
The beam profile was detected with a CCD camera to observe heat-induced beam distortions, caused by thermal lensing, birefringence, thermally induced stress or disturbed phase-matching (see Fig. 10). Up to an input power of PIR = 126 W the symmetrical beam profiles of the single BBO and the bonded stack are identical. Hence, the sandwich structure does not show any stress induced beam distortions. Analyzing the cross-sections of the beam profiles, reveals a deviation of the beam profile of the single crystal from the beam profile of the stack, starting to evolve at PIR = 170 W. This deviation can be described as an additional bump at both slopes, starting 800 µm from the maximum in the horizontal direction and 600 µm in the vertical direction of the cross-sections at a power of PIR = 253 W (see Fig. 11 (c) and 11(d)), but not in the beam profile of the stack. In the beam profiles of Fig. 10, this deviation is visible as partial ring, which is a hint for thermo-optical effects taking place.
Fig. 10 Beam profiles after passing the single crystal BBO (left images) or the sapphire BBO stack (right images) at different power; (a) and (b): P = 37 W; (c) and (d): P = 253 W.
Fig. 11 Beam profile sections (left images: vertical cross-section, right images: horizontal cross-sections); (a) and (b) at P = 37 W and (c) and (d) at P = 253 W.
5.3 Temperature measurement
The temperature was detected by a FLIR T335 camera. The camera was focused on the BBO surface of the single crystal and the sapphire surface of the BBO sapphire stack. The data were corrected for the distinct emissivities of BBO (εBBO = 0.78 [23]) and sapphire (εsapphire = 0.95 [24]). From the FEM simulation it is assumed, that the BBO single crystal will exhibit a parabolic shaped temperature distribution on its surface due to the Gaussian beam profile and the low thermal conductivity. In contrast, a huge fraction of the heat developed within the BBO crystal of the bonded stack is dissipated due to the thermal contact between BBO and sapphire. This results in a nearly flat temperature distribution. Experimentally a significant difference between the temperature distributions could be measured at laser input powers PIR in excess of 80 W. Figure 12 displays the temperature difference between sample holder and the measured temperature at the front surface of the single crystal and the stack respectively as symbols. The continuous lines in the diagram of 253 W incident power represent the results from the FEM simulation (temperature difference between the surface and the sample holder). While at low average power (PIR = 37 W) there is no significant temperature gradient, at PIR = 253 W, the difference is significant.
Fig. 12 Surface temperature difference of the BBO single crystal and the bonded stack at different incident powers; (a) P = 37 W, (b) P = 253 W. At a power of 253 W the results of the simulation are inserted into the graph as a continuous line.
In addition, the BBO single crystal has a significantly higher maximum temperature of 43.5°C compared to the sandwich structure (T = 31.5°C) as displayed in Fig. 13.
Although, the experimental conditions are not precisely known (e.g. the heat dissipation via the sample holder), the results of the FEM simulation (see Fig. 1) reproduce the experiment quite well. The conditions of the simulation correspond to the experimental situation at PIR = 253 W. From the FEM simulation we estimated a maximum temperature difference at the surface of the BBO single crystal of 24.9 K and 0.4 K at the surface of the bonded stack. We measured a maximum temperature difference at the surface of the single crystal of 14 K and at the bonded stack of 0.4 K. The differences can be explained by the different heat transfer between the sample and its holder, as well as between the sample surface and the surrounding atmosphere and the spatial resolution of the IR camera on the samples surface (0.5 to 1 mm, depending on the camera position). Nevertheless, the simulation allows accessing the temperature distribution (and stress distribution) within the sandwich structure. Taking into account the results of the simulation, the temperature difference of the BBO within the sandwich structure is reduced by a factor of 2.4 and that of the stresses by a factor of 2.8 compared to the single BBO crystal.
6. Conclusions and outlook
We reported for the first time to our knowledge on a new technology to improve cooling of nonlinear optical crystals (BBO) used for high power laser applications. The technique is based on joining a sapphire disk with high thermal conductivity to a BBO crystal, to effectively dissipate the heat, which is generated due to absorption within the nonlinear crystal. As joining technology plasma-activated direct bonding was chosen, due to its ability to form stiff and durable bonds, without using any auxiliary material. Before bonding, sample surfaces were examined regarding roughness and flatness, due to strict requirements for bonding. Cleaning with acetone showed the lowest impact on the surface roughness of the BBO surface. Although the BBO surface flatness is higher than the criterion for successful bonding, it was possible to join BBO samples with sapphire disks on both BBO faces. Bonding of protection coated BBO samples to sapphire disks was not possible, due to high surface roughness of the coated surface and the non-oxide composition of the coating.
The performance of the bonded stack for second harmonic generation was tested and the conversion efficiency, the beam profile and the temperature distribution were evaluated and compared to that of a BBO single crystal of same thickness at input powers up to 253 W. The conversion efficiency achieved with both samples is very similar (~70%). The temperature distributions are as expected from the FEM simulations. The bonded stack has a significantly lower maximum temperature compared to the single crystal BBO. The simulation reveals a reduction of the temperature in the bonded BBO stack by a factor of 2.4 compared to a BBO single crystal for our experimental conditions. Concluding from the FEM simulation, the heat induced stress is significantly reduced. In addition, thermo-optical beam distortions are reduced by the sandwich structure, demonstrating the improved heat dissipation. The temperature gradient could be further reduced by subdividing the nonlinear crystal into several thin slices and subsequent fabrication of a stack consisting of alternating heat spreader and nonlinear crystal disks. Of course, the thickness of the heat spreader disks has to be precisely controlled to maintain phase matching.
The presented approach can be employed to cool BBO crystals for various applications. In future bonding of coated samples and extension to different nonlinear crystals will be examined. Due to significantly improved thermal properties, efficient frequency conversion to the visible, the UV and the mid infrared spectral range will be feasible at kW average powers. Furthermore, optical parametric amplifiers may deliver few-cycle laser pulses with average powers approaching the kW level [23], which is envisioned for example by the European Extreme Light Infrastructure project (ELI) [25].
Acknowledgments
We greatly acknowledge work of Sandra Müller and Karina Johrke on sample cleaning and Dr. Stefan Risse and Dr. Gerhard Kalkowski of Fraunhofer IOF for fruitful discussions.
References and links
1. T. Maiman, “Stimulated optical radiation in ruby,” Nature 187(4736), 493–494 (1960). [CrossRef]
2. A. E. Siegman, Lasers (University Science Books, 1986).
3. G. Cerullo and S. De Silvestri, “Ultrafast optical parametric amplifiers,” Rev. Sci. Instrum. 74(1), 1 (2003). [CrossRef]
4. A. Dubietis, R. Butkus, and A. P. Piskarskas, “Trends in chirped pulse optical parametric amplification,” IEEE J. Sel. Top. Quantum Electron. 12(2), 163–172 (2006). [CrossRef]
5. J. Rothhardt, S. Demmler, S. Hädrich, J. Limpert, and A. Tünnermann, “Octave-spanning OPCPA system delivering CEP-stable few-cycle pulses and 22 W of average power at 1 MHz repetition rate,” Opt. Express 20(10), 10870–10878 (2012). [CrossRef] [PubMed]
6. S. T. Lin, Y. Y. Lin, Y. C. Huang, A. C. Chiang, and J. T. Shy, “Observation of thermal-induced optical guiding and bistability in a mid-IR continuous-wave, singly resonant optical parametric oscillator,” Opt. Lett. 33(20), 2338–2340 (2008). [CrossRef] [PubMed]
7. J. Rothhardt, S. Demmler, S. Hädrich, T. Peschel, J. Limpert, and A. Tünnermann, “Thermal effects in high average power optical parametric amplifiers,” Opt. Lett. 38(5), 763–765 (2013). [CrossRef] [PubMed]
8. D. N. Nikogosyan, “Beta Barium Borate (BBO): a review of its properties and applications,” Appl. Phys. A 368, 359-368 (1991).
9. D. N. Nikogosyan, Properties of Optical and Laser-Related Materials: A Handbook (Wiley, 1997).
10. Kyocera Headquarters, Single Crystal Sapphire (2010), pp. 1–8.
11. D. Eimerl, L. Davis, S. Velsko, E. K. Graham, and A. Zalkin, “Optical, mechanical, and thermal properties of barium borate,” J. Appl. Phys. 62(5), 1968 (1987). [CrossRef]
12. U. Gösele and Q. Tong, “Semiconductor wafer bonding,” Annu. Rev. Mater. Sci. 28(1), 215–241 (1998). [CrossRef]
13. C. Rothhardt, M. Rekas, G. Kalkowski, N. Haarlammert, R. Eberhardt, and A. Tünnermann, “Fabrication of a high power Faraday isolator by direct bonding,” in Proc. SPIE 8601, Fiber Lasers X: Technology, Systems, and Applications, S. T. Hendow, ed. (2013), Vol. 8601, p. 86010T–86010T–7.
14. A. Smith, “SNLO,” http://www.as-photonics.com/snlo.
15. M. Iwasa, T. Ueno, and R. C. Bradt, “Fracture Toughness of Quartz and Sapphire Single Crystals at Room Temperature,” Zairyo 30(337), 1001–1004 (1981). [CrossRef]
16. G. Kalkowski, S. Risse, C. Rothhardt, M. Rohde, and R. Eberhardt, “Optical contacting of low-expansion materials,” in Optical Manufacturing and Testing IX, J. H. Burge, O. W. Fähnle, and R. Williamson, eds. (SPIE, 2011), Vol. 8126, pp. 1–7.
17. M. Bass, C. DeCusatis, J. Enoch, V. Lakshminarayanan, G. Li, C. MacDonald, V. Mahajan, and E. Van Stryland, Handbook of Optics, Third Edition Volume IV: Optical Properties of Materials, Nonlinear Optics, Quantum Optics (set), Handbook of Optics (McGraw-Hill Education, 2009).
18. D. Eimerl, S. Velsko, L. Davis, and F. Wang, “Progress in nonlinear optical materials for high power lasers,” Prog. Cryst. Growth Charact. Mater. 20(1-2), 59–113 (1990). [CrossRef]
19. Z. Rappoport, CRC Handbook of Tables for Organic Compound Identification (CRC Press, 1985).
20. L. Bromley, A. Guy, and D. Hanna, “Synchronously pumped optical parametric oscillation in beta-barium borate,” Opt. Commun. 67(4), 3–7 (1988). [CrossRef]
21. A. Klenke, S. Breitkopf, M. Kienel, T. Gottschall, T. Eidam, S. Hädrich, J. Rothhardt, J. Limpert, and A. Tünnermann, “530 W, 1.3 mJ, four-channel coherently combined femtosecond fiber chirped-pulse amplification system,” Opt. Lett. 38(13), 2283–2285 (2013). [CrossRef] [PubMed]
22. F. Jansen, F. Stutzki, H. Otto, T. Eidam, A. Liem, C. Jauregui, J. Limpert, and A. Tünnermann, “Thermally induced waveguide changes in active fibers,” Opt. Express 20, 3997–4008 (2012).
23. R. Riedel, J. Rothhardt, K. Beil, B. Gronloh, A. Klenke, H. Höppner, M. Schulz, U. Teubner, C. Kränkel, J. Limpert, A. Tünnermann, M. J. Prandolini, and F. Tavella, “Thermal properties of borate crystals for high power optical parametric chirped-pulse amplification,” Opt. Express (submitted to).
24. S. Kaplan and L. Hanssen, “Normal infrared spectral emittance of Al203,” in Optical Diagnostic Methods for Inorganic Transmissive Materials (SPIE, 1998), Vol. 3425, pp. 120–125.
25. P. Baum, D. Charalambidis, J. A. Fülöp, U. Kleineberg, F. Krausz, G. Szabó, G. Tsakiris, K. Varjú, L. Veisz, and M. Vrakking, “The Attosecond Light Pulse Source (ALPS) of the Extreme Light Infrastructure (ELI),” http://www.eli-hu.hu/uploads/File/Ceges_doksik_2013/Science/ELI_ALPS_Science_and_Technology.pdf.
