Generation of visible and mid-infrared radiation in periodically poled stoichiometric lithium tantalate

Transcription

1 PSI-SR-109 Generation of visible and mid-infrared radiation in periodically poled stoichiometric lithium tantalate Douglas J. Bamford David J. Cook Scott J. Sharpe Aaron Van Pelt Douglas J. Bamford, David J. Cook, Scott J. Sharpe, Aaron Van Pelt, "Generation of visible and mid-infrared radiation in periodically poled stoichiometric lithium tantalate," presented at Photonics West 005 (San Jose, CA), (-7 January005). Copyright 005 Society of Photo-Optical Instrumentation Engineers. This paper was published in Photonics West 005, and is made available as an electronic reprint (preprint) with permission of SPIE. One print or electronic copy may be made for personal use only. Systematic or multiple reproduction, distribution to multiple locations via electronic or other means, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibited. Downloaded from the Physical Sciences Inc. Library. Abstract available at

2 Generation of visible and mid-infrared radiation in periodicallypoled, nearly-stoichiometric lithium tantalate Douglas J. Bamford, * David J. Cook, Scott J. Sharpe, and Aaron Van Pelt Physical Sciences Inc., 110 Omega Road Suite D, San Ramon, CA, USA ABSTRACT Periodically poled, nearly-stoichiometric lithium tantalate has used to generate visible radiation (by second-harmonic generation using Nd:YAG laser) and mid-infrared radiation (by difference-frequency generation using a Nd:YAG laser and a tunable telecommunications-band laser). Phase-matching conditions have been measured for both interactions at temperatures between 5 degrees Centigrade and 131 degrees centigrade. The absolute conversion efficiency for SHG has been measured and used to derive an effective nonlinear optical coefficient for this process in the periodically poled material. These results can be used to guide the design of laser systems based on nonlinear optical frequency conversion in periodically poled nearly-stoichiometric lithium tantalate. Keywords: quasi-phase matching, periodic poling, stoichiometric lithium tantalate, nonlinear optics, visible. 1. INTRODUCTION Periodically poled, nearly stoichiometric lithium tantalate has the potential to be useful for the frequency conversion of near-infrared lasers into the visible and mid-infrared. 1-6 In a previous publication 7 we described the techniques used to perform periodic poling on nearly-stoichiometric lithium tantalate and presented the results of phase-matching experiments carried out at room temperature on the periodically poled material. To fully evaluate the potential of this material, it is also necessary to know its effective nonlinear optical coefficient. Moreover, because temperature tuning is needed to achieve phase-matching when fixed-frequency lasers are used, one also needs to know the phase-matching properties of the material at elevated temperatures. In this work, the effective nonlinear coefficient is measured using second-harmonic generation (SHG). In addition, the phase-matching properties for SHG and for another nonlinear optical interaction, difference-frequency generation (DFG) are measured as a function of temperature.. EXPERIMENTAL Two different wafers of nearly-stoichiometric lithium tantalate were used as substrates for electric-field poling. The procedures used to perform periodic poling have been described previously. 7 The first wafer, supplied by Deltronic Crystal Industries, was 76. mm in diameter and 0.5 mm thick. The coercive field for the material in this wafer, defined as the lowest field at which detectable poling current began to flow, was.5 kv/mm. The second wafer, supplied by Oxide Corporation, was 50 mm in diameter and 1.0 mm thick. The coercive field for the material in this wafer was 1. kv/mm. After periodic poling the wafers were etched in hydrofluoric acid to reveal the domain patterns. Several chips were diced from each wafer, and the end faces of the chips were polished. For the SHG experiments, a 9.7 mm long chip diced from the first wafer was used. This chip contained eight different quasi-phasematching (QPM) gratings with periods ranging from 7.3 µm to 8.0 µm in increments of 0.1 µm. For the DFG experiments, a 15.5 mm long chip diced from the second wafer was used. This chip contained eight different quasi-phasematching (QPM) gratings with periods ranging from 5.4 µm to 3.4 µm in increments of 1.0 µm. All of the gratings on both chips were designed for first-order QPM. A schematic diagram of the experimental apparatus is shown in Fig. 1. Two-different continuous-wave laser systems were used: a Coherent Compass Nd:YAG laser with a fixed wavelength of 1064 nm, and a Santec TSL-10 gratingstabilized diode laser tunable between 150 nm and 160 nm. The time-averaged output spectrum of the Nd:YAG laser, as measured with a Burleigh WA-4500 Wavemeter, consisted of three longitudinal modes with a mode spacing of * bamford@psicorp.com; phone ; fax ; psicorp.com

3 1064 nm Pump Laser Lock-in Amplifier Chopper Filter 1550 nm Signal Laser Lens PPSLT Oven Lens Detector Temp Control Data Acquisition System G-988 Figure 1: Schematic diagram of the experimental setup used for SHG and DFG experiments. For SHG and DFG, filters transmitting 53-nm and mid-ir light were used, respectively. The 1550-nm laser was not used in the DFG experiments. 6 GHz. No rapid intensity fluctuations could be seen when the laser was observed using a photodiode with a response time of ~1 ns. The tunable diode laser was believed to operate on a single longitudinal mode. For DFG experiments the two lasers were combined using a dichroic beamsplitter and focused into the periodically poled crystal. For SHG experiments the diode laser was not used. In the DFG experiments the mid-infrared radiation generated within the crystal was collimated using a ZnSe lens, while in the SHG experiments no collimating lens was used. Radiation at the input wavelength (s) was filtered out using either a glass filter (in the SHG experiments) or an interference filter (in the DFG experiments). The radiation generated within the crystal was then focused onto a detector. For SHG experiments either a silicon photodiode or a calibrated silicon-based power meter was used. For the DFG experiments the radiation was detected using a liquid-nitrogen-cooled Judson indium antimonide detector. To improve the signal-to-noise ratio in the DFG experiments, the pump beam was chopped and lock-in detection was used. The crystal was housed in a temperature-controlled, resistively-heated copper oven. Phase-matching SHG experiments were performed by recording the relative second-harmonic power as a function of temperature. It was possible to achieve phase matching in three of the QPM gratings, with periods of 7.8 µm, 7.9 µm and 8.0 µm. To measure the effective nonlinear optical coefficient, the temperature was tuned to coincide with one of the phase-matching peaks. The fundamental input power was varied by changing the current passing through the diode lasers that pumped the Nd:YAG laser. The absolute second-harmonic power was then recorded as a function of the absolute fundamental power. The spatial profile of the fundamental beam was measured after the experiment was over by removing the crystal, placing a CCD camera in the location of the center of the crystal, recording the beam profile using a Spiricon beam analyzer, and fitting it to a Gaussian profile. Spatial profile measurements were also made at several other locations and used to estimate the M value of the beam. Phase-matching DFG experiments by recording the relative idler power as a function of signal wavelength at a fixed temperature. It was possible to achieve phase-matching in the QPM grating with period of 30.4 µm at a variety of temperatures. 3. PHASE-MATCHING RESULTS The phase-matching temperature for SHG is shown as a function of PM period in Fig.. Also shown are predictions based on the published Sellmeier equation for SLT. 8 The observed phase-matching temperature agrees well with the predicted phase-matching temperature at the highest temperatures, but diverges from the predicted temperature at lower phase-matching temperatures. It is possible that the optical properties of the material used in these experiments are different from the optical properties of the material used to derive the Sellmeier equation. As a consequence of this

4 Temperature (Centigrade) Experiment Theory QPM period (microns) Figure : Phase-matching temperature versus QPM grating period for SHG of a Nd:YAG laser operating at 1064 nm, along with predictions based on the published Sellmeier equation. 8 G-9356 Figure 3: Relative visible power as a function of temperature for SHG in PPSLT along with a sinc-squared curve whose peak and width were adjusted to give the best fit to the data. The fundamental wavelength was 1064 nm and the QPM grating period was 7.8 µm. discrepancy in the temperature dependence of the phase-matching condition, the observed widths of the phase-matching curves, shown in Figs. 3 through 5, were greater than the widths predicted by the Sellmeier equation. The discrepancy was larger for lower temperatures, ranging from a factor of 1.18 at 131 C to a factor of 1.45 at C. A simple correction procedure was used to explain the widths of the phase-matching curves. The data in Fig. 3 were fit to a second- order polynomial. Using this polynomial, the slope of this curve, ( T/ Λ) obs, was calculated at the temperatures of the three phase-matching peaks. The slope predicted by the Sellmeier equation, ( T/ Λ) calc, was also calculated at each temperature. The FWHM temperature acceptance predicted by the Sellmeier equation, δt calc, was then converted to a corrected temperature acceptance, δt corr, using the following equation:

5 T Λ obs δ Tcorr = δtcalc. (1) T Λ After this correction was made, the ratio between the observed and calculated temperature acceptances ranged from 1.01 at 131 C to 1.15 at C. More work is clearly needed to improve the accuracy of the temperature dependence, especially at lower temperatures. calc Figure 4: Relative visible power as a function of temperature for SHG in PPSLT along with a fitted sinc-squared curve. The QPM grating period was 7.9 µm. Figure 5: Relative visible power as a function of temperature for SHG in PPSLT along with a fitted sinc-squared curve. The QPM grating period was 8.0 µm. Only part of the phase-matching resonance is shown because the crystal could not be cooled below room temperature.

6 The phase-matching wavelength for DFG is shown as a function of temperature in Fig. 6. There is a systematic discrepancy between the observed and calculated phase-matching wavelengths. As was the case in the SFG experiments, the material used in these experiments appears to differ slightly in optical properties from the material used to derive the Sellmeier equation. However, the slopes of the experimental and theoretical curves in Fig. 6 are in good agreement. Therefore, the published Sellmeier equation can be used to predict the widths of the phase-matching curves. At 10 C, the observed wavelength acceptance was 4. nm, slightly smaller than the wavelength acceptance of 4.4 nm predicted by the Sellmeier equation. Similar agreement between observed and calculated wavelength acceptances was obtained at the other temperatures Phase-matching wavelength (nm) Theory Experiment Temperature (Centigrade) Figure 6: Phase-matching wavelengths vs. temperature for DFG, along with theoretical predictions based on the published Sellmeier equation. The pump wavelength was 1064 nm, the QPM period was 30.4 µm. G-9360 Figure 7: Relative mid-ir power as a function of signal laser wavelength for DFG at 10 C along with a sinc-squared curve whose peak and width were adjusted to give the best fit to the data. The pump wavelength was 1064 nm, the QPM period was 30.4 µm, and the crystal length was 15.5 mm.

7 4. CONVERSION EFFICIENCY RESULTS To interpret the SHG conversion efficiency experiments, the equations normally used to describe SHG must be modified to account for the presence of multiple longitudinal modes in the output of the Nd:YAG laser. Mode-beating can cause intensity fluctuations on a sub-nanosecond timescale which are too fast to be observed by the power meter. The efficiency of SHG is proportional to the mean square intensity, while the power meter (in combination with the CCD camera) measures the square mean intensity. A similar problem arises in the interpretation of quantitative twophoton absorption experiments carried out using multiple-longitudinal mode lasers. 9 The unresolved temporal fluctuations can be characterized a parameter called G () (0), the second-order intensity autocorrelation function at time zero, which is the ratio between the mean-squared and square-mean intensities. According to one theoretical model of the photon-statistical properties of lasers, 10 that the value of G () (0) for a laser with N longitudinal modes depends on the phase relationship between the modes. If the modes have random phases, the value of G () (0) is equal to [-(1/N)]. If the modes have fixed phases (i.e. if the laser is mode-locked), the value of G () (0) is equal to (1/3)[N + (1/N)]. However, the absence of detectable intensity fluctuations when the laser was observed with a fast photodiode suggests that the temporal fluctuations may not be as large as the fluctuations predicted by this model. Two simplifying assumptions are made to interpret the SHG results. First, the fundamental beam is assumed to be undepleted by the interaction. This assumption is justified since the maximum conversion efficiency, as will be shown below, was less than 0.03%. Second, the fundamental beam is assumed to have a Gaussian shape with a spot size that remains constant as the beam propagates through the crystal. This assumption is justified since the Rayleigh range for the fundamental beam calculated from the measured spot sizes at positions near the beam waist (.4 cm) is significantly longer than the crystal length (1.0 cm). Under these assumptions, the average second-harmonic power at the crystal exit <P h >, at the peak of the phase-matching curve, is given by the following equation: 11 ω deff L < P1 h > < P h >=, () 3 π n n ε c W h 0 where ω h is the angular frequency for the fundamental beam, d eff is the effective nonlinear optical coefficient, L is the crystal length, <P > is the mean-square power in the fundamental beam, n is the index of refraction at the fundamental wavelength, n h is the index of refraction at the second-harmonic wavelength, ε 0 is the permittivity of free space, c is the speed of light, and W is the spot size for the fundamental beam. The mean-squared and square-mean powers are related by the following equation: () < 1 h > By combining Eqs. () and (3), we arrive at the following expression: P >= G (0) < P. (3) d eff G () (0) = π n n 3 hε 0c < ω L W P < P > h >. (4) A plot of the quantity <P h >/<P > as a function of <P > for one of the QPM gratings is shown in Fig. 8. By combining this data with the data from the beam profile measurement, the quantity on the left-hand side of Eq. (4) can be calculated. The results of this experiment are summarized in Table 1.

8 0.030% 0.05% 0.00% <Ph > /<P > 0.015% 0.010% 0.005% 0.000% <P >(W) G-9454 Figure 8: Average second-harmonic power divided by average fundamental power as a function of average fundamental power for SHG at 131 C in a QPM grating with a period of 7.8 µm. Also shown is a least-squares fit to a straight line, constrained to pass through the origin. The slope of this line is 0.07%/W. Table 1: Measured values of the parameters in Eq. (4) Quantity Value λ 1064 nm Λ 7.9 µm T 80 C n.134 n h.01 W 7 µm < P h > < P1 h > W -1 L 9.7 mm () G d eff (0) 6.9 pm/v 5. DISCUSSION These experiments have provided useful new information about the properties of periodically-poled, nearly stoichiometric lithium tantalate. For both SHG and DFG interactions, the phase-matching properties for the material that we have used are slightly different from the properties predicted by the published Sellmeier equation. For the QPM grating used in the SHG experiment, the most important deviation from an ideal QPM structure is the presence of domain merges. The effective nonlinear coefficient for such a structure is related to the d 33 coefficient by ( 1 f ) the following equation: 1 d eff d33 =, (5) π

9 where f is the number of domain merges divided by the total number of QPM periods. This fraction is approximately equal to 6% for the structure used in the SHG experiment, based on microscopic inspection of the etched surfaces. One can safely assume that G () (0) lies somewhere between 1 and. Therefore, the values of d 33 implied by the experimental result shown in Table 1 are in the range 9 to 1 pm/v. This range of values can be contrasted with the d 33 coefficient for congruent lithium niobate, stated to be 7 pm/v according to one commonly-accepted tabulation. 13 Nearly-stoichiometric lithium tantalate clearly has a lower d 33 coefficient than congruent lithium niobate. Therefore, its advantages lie in applications for which other properties (such as resistance to optical damage, amenability to the formation of thicker periodically-poled crystals, and greater ultraviolet transparency) are more important than raw conversion efficiency. The d 33 value implied by our measurement is slightly lower than the results of another measurement, carried out using a wedge method, 14 which found d 33 values in the range pm/v for un-doped lithium tantalate crystals with various compositions close to stoichiometry. The results presented here can be used in the design of laser systems based on nonlinear optical frequency conversion in periodically poled nearly-stoichiometric lithium tantalate. In particular, the conditions needed to achieve phasematching, and the output powers which can be expected, can now be calculated more accurately. ACKNOWLEDGMENTS This work was supported by the U.S. Air Force under Contract No. F C REFERENCES 1. K. Kitamura, Y. Furukawa, K. Niwa, V. Gopalan, and T.E. Mitchell, Crystal growth and low coercive field 180 domain switching characteristics of stoichiometric LiTaO 3, Appl. Phys. Lett. 73, , I.G. Kim, S. Tekakawa, Y. Furukawa, M. Lee, and K Kitamura, Growth of Li x Ta 1-x O 3 single crystals and their optical properties, J. Cryst. Growth 9, 43-47, K. Kitamura, Y. Furukawa, S. Takekawa, T. Hatanaka, H. Ito, and V. Gopalan, Non-stoichiometric control of LiNbO 3 and LiTaO 3 in ferroelectric domain engineering for optical devices, Ferroelectrics 57, 35-43, M. Katz, R. Route, D. Hum, G. D. Miller, and M. Fejer, Vapor-transport equilibrated near-stoichiometric lithium tantalate for frequency conversion applications, Opt. Lett. 9, , T. Hatanaka, K. Nakamura, T. Taniuchi, H. Ito, Y. Furukawa, and K. Kitamura, Quasi-phase-matched optical parametric oscillation with periodically poled stoichiometric LiTaO 3, Opt. Lett. 5, , A.L. Alexandrovski, G. Foulon, L.E. Myers, R.K. Route, and M.M. Fejer, UV and visible absorption in LiTaO 3, SPIE 3610, 44-51, D.J. Bamford, D.J. Cook, and S.J. Sharpe, Periodic poling of stoichiometric lithium tantalite, in Nonlinear Frequency Generation and Conversion: Materials, Devices, and Applications III, edited by Kenneth L. Schepler, Dennis D. Lowenthal, Proceedings of SPIE Vol (SPIE, Bellingham, WA, 004), A. Bruner, D. Eger, M.B. Oron, P. Blau, M. Katz, and S. Ruschin, Temperature-dependent Sellmeier equation for the refractive index of stoichiometric lithium tantalate, Opt. Lett. 8, , D.J. Bamford, A.P. Hickman, M.J. Dyer, and W.K. Bischel, Comparative photon statistics of several ultraviolet laser systems determined by transient two-photon absorption, J. Opt. Soc. Am B 5, , H. Mahr, in Quantum Electronics: A Treatise, H. Rabin and C.L. Tang, eds. (Academic, New York, 1975), Vol. I, Part A, Chap. 4, pp R.L. Byer, Parametric oscillators and nonlinear materials, in Nonlinear Optics, P.G. Harper and B.S. Wherrett, Editors (Academic, New York, 1977) pp M.M. Fejer, G.A. Magel, D.H. Jundt, and R.L. Byer, Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, IEEE J. Quantum Elect. 8, , D.A. Roberts, Simplified characterization of uniaxial and biaxial nonlinear optical crystals: a plea for standardization of nomenclature and conventions, IEEE J. Quantum Elect. 8, , K. Kitamura, Y. Furukawa, S. Takekawa, S. Kimura, Single crystal of lithium niobate or tantalate and its optical element, and process and apparatus for producing an oxide single crystal, U.S. Patent #6,464,777 (00).