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Picosecond stimulated Raman scattering of BaWO4 crystal
ARTICLE IN PRESS
Optics & Laser Technology 39 (2007) 1239–1242 www.elsevier.com/locate/optlastec
Picosecond stimulated Raman scattering of BaWO4 crystal Haohai Yu, Dawei Hu, Huaijin Zhang, Zhengping Wang, Wenwei Ge, Xinguang Xu, Jiyang Wang, Zongshu Shao, Minhua Jiang National Key Lab of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China Received 11 July 2005; received in revised form 29 July 2005; accepted 3 August 2006 Available online 20 September 2006
Abstract In single-pass configuration, a picosecond pulse laser was used to investigate the stimulated Raman scattering in different BaWO4 crystals. Using the thresholds for the first Stokes line along the a- and c-axis, the Raman gain coefficients of the BaWO4 crystals were calculated along their respective direction. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. Compared with other well-known Raman crystals, such as KGd(WO4)2 and Ba(NO3)2, BaWO4 has favorable properties for scattering the pump radiation with picosecond pulse duration. The Raman gain coefficients of BaWO4 are different, but do not vary for different transmission directions, which means that this Raman material can be selected more freely. r 2006 Elsevier Ltd. All rights reserved. Keywords: BaWO4; Threshold; Raman gain coefficient
1. Introduction Stimulated Raman scattering (SRS) is an inelastic scattering process, whereby light pumped at one wavelength is converted to another wavelength, accompanied by the excitation of an internal mode of the Raman medium. Therefore, the solid-state Raman laser could extend the spectral region of solid-state laser sources. Due to the interest in their use in areas such as medical applications, spectroscopy, ecological monitoring techniques and laser guidestars, much attention has been focused on SRS [1–8]. In the selection of Raman laser materials, the Raman medium should have a high Raman gain coefficient, low SRS threshold and large optical damage threshold. The Raman gain coefficient is the key parameter determining the threshold for SRS and the growth rate of the intensity of the Stokes line. Therefore, much attention was focused on Raman crystals that have large Raman gain, including some tungstates (BaWO4 [2,6], PbWO4 [7], Nd:KGd(WO4)2 [9] and KY(WO4)2 [10]) and nitrates (Ba(NO3)2 [8,11–13]). Compared with other tungstates
Corresponding author. Tel.: +86 0531 8364848.
E-mail address: [email protected] (Z. Wang). 0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.08.006
and nitrates, BaWO4 has better properties for scattering the pump radiation with picosecond pulse duration [2]. In the past, due to a lack of high optical quality and large-dimensional BaWO4 crystals, the experiments that have been undertaken only used one sample length (30 mm) [2] and the measurement of the threshold and Raman gain was only in one direction (K?C4, EJC4) [2,6]. In this paper, we report that the BaWO4 single crystals may be successfully grown using the Czochralski method, and the thresholds of BaWO4 crystals which are cut along different axes (a and c) with different lengths are determined. Using the thresholds, the Raman gain coefficients are calculated. Furthermore, higher SRS generations including Stokes and anti-Stokes lines in this experiment are also obtained. 2. Prepared crystal samples Fig. 1 shows the as-grown BaWO4 crystals. BaWO4 single crystals were successfully grown along a- and c-axis by the Czochralski method. The crystals are colorless with dimensions of about 22 80 mm2, weighing more than 213 g. There are no cracks or low-angle boundaries in the as-grown crystals. The cell parameters of BaWO4 crystal ˚ ˚ are a ¼ b ¼ 5:6041 A, c ¼ 12:7080 A. Before cutting into crystal samples, the crystal boules were annealed.
ARTICLE IN PRESS H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
1240
Fig. 1. As-grown BaWO4 crystal boule [14].
Three kinds of crystal samples with respective dimensions of 6 6 30 mm3, 6 6 50 mm3 and 6 20 6 mm3 (a a c) were chosen.
Fig. 2. Experimental setup.
3. Experimental setup The experimental setup to measure the thresholds of BaWO4 crystal is shown in Fig. 2. A PY61 mode-locked Q-switched Nd:YAG laser system generated single pulse radiation at the pump wavelength (1064 nm) with 40 ps duration. KTP crystal was used as second harmonic generation (SHG) crystal to produce green output at 532 nm. The second harmonic output was obtained by doubling in SHG and consisted of TEM00 mode pulses of duration 30 ps. The power was constant with 10 Hz repetition. The diameter and divergence of the beam are about 1 mm and 0.4 mrad with a spectral width of approximately 0.5 nm. Mirror (M1) has high reflectivity (499%) at 1064 nm and high transmission (490%) at 532 nm. PP1 and PP2 are two polarizing prisms. The polarizers provided smooth variation of pump energy into the Raman crystal. M2 reflected the laser (532 nm) to the system of two positive lenses (T) that increased the intensity of the laser beam by decreasing its diameter. The focal lengths of the positive lenses were 8 and 2 cm. D is a diaphragm with a diameter of 1 mm. The second harmonic laser beam transmitted though the Raman medium (BaWO4 crystal) and was reflected with the Raman laser onto a white paper screen (A). The Stokes images were obtained by the spectrum analyzer (USB200VIS-NIR) (B). The energy meter was used to measure the pump power when the Stokes lines appeared. 4. Results and discussion By using the experimental setup (Fig. 2), the thresholds for first Stokes line of different BaWO4 crystals (2 cm along a-axis, 3 and 5 cm along c-axis) were obtained.
Fig. 3. Image that determined the thresholds.
When the image such as that shown in Fig. 3 was observed, the intensity of the pump laser was considered to be the threshold of the first Stokes line. Here we considered SRS in the forward direction in the steady-state regime, which is the situation where practical, efficient frequency conversion is most likely to occur. Using a plane-wave approximation and neglecting dispersion, the equations that describe the model of SRS were shown in Refs. [15,16]. Near threshold, the growth of the Stokes wave yields the familiar expression: I s ðlÞ ¼ I s ð0Þ expðgss I p lÞ. Here, Is(0), Is(l) and Ip are the intensity from which the Stokes wave grows, at Stokes wavelength and pump wavelength, respectively, gss is the Raman gain coefficient and l is the interaction length of the laser and Raman medium. The pump intensity necessary to reach the steady-state SRS threshold leads to the conversion efficiency of about 1% and the value of the exponent G ¼ gss I t l ¼ 25. Here, It could be considered as the threshold.
ARTICLE IN PRESS H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
1241
Table 1 Thresholds and Raman gain of BaWO4 in different axes Transmission direction
Crystal length (cm)
Intensity of pump (mW)
Threshold (MW/cm2)
Raman gain (cm/GW)
First Stokes wavelength (nm)
a c
2 3 5
1.57 1.13 0.59
666.3 479.6 250.4
18.7 17.4 19.9
560
Fig. 4. High Stokes and anti-Stokes lines.
The thresholds and the Raman gains are shown in Table 1. It has been shown from Table 1 and references that: (1) the Raman gains of BaWO4 are much larger than that of KGd(WO4)2 (11.5 cm/GW [2]) and BaNO3 (4.7 cm/ GW [2]) for scattering the pump radiation with picosecond pulse duration. (2) the crystal has almost the same Raman gains and shift along different directions (a- and c-axis). It can be explained from the spontaneous Raman spectra [17]: the intensity at Stokes wavelength was born from the interaction of light and the internal vibration mode n1(Ag) [17] of BaWO4 molecule and the molecule has isotropic property.
be obtained conveniently, which could attribute to its high Raman gain coefficient. 5. Conclusion The BaWO4 crystal was grown by Czochralski method. Using the single-pass experimental setup, the thresholds were measured along a- and c-axis for the first Stokes line by which the Raman gain coefficients were calculated. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. The experiments showed that BaWO4 crystal is a potential material for the development of SRS solid-state laser and people can select this Raman material with more degrees of freedom. Acknowledgments
Compared to Ref. [2], the threshold was determined by observing the image and the conversion efficiency may not be 1%, which leads to the fact that threshold (as well as the Raman gain) in this paper may have some discrepancy with traditional definition [2]. Cˇerny´ et al. [2] also showed that the error would be less than 10%. Increasing the pump power, the higher Stokes and antiStokes lines (Fig. 4) were obtained. In Fig. 4, the highest Stokes and anti-Stokes lines are the fourth (665 nm) and the third (462 nm), respectively. It has been shown that, with BaWO4 crystal, different Raman frequency shifts can
This work is supported by the National Natural Science Foundations of China (no. 50590401 and 60508010). References [1] Pask HM. Prog Quantum Electron 2003;27:1. [2] Cˇerny´ P, Zcerev PG, Jelı´ nlova´ H, Basiev TT. Opt Commun 2000;177:397. [3] Eremmenko AS, Kaprukin SN, Stepanov AI. Sov J Quantum Electron 1980;10:113. [4] Pask HM, Piper JA. Opt Commun 1998;148:285.
ARTICLE IN PRESS 1242
H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
[5] Findeisen J, Eichler HJ, Peuser P, Kaminskii AA, Hulliger J. Appl Phys B 2000;70:159. ˙ endzian W, Janbczyn´ski J, Jelı´ nkova´ H, Sˇulc J, [6] Cˇerny´ P, Z Kopczyn´ski K. Opt Commun 2002;209:403. [7] Kaminskii AA, McCray CL, Lee HR, Lee SW, Temple DA, Chyba TH, et al. Opt Commun 2000;183:277. [8] Zverev PG, Basiev TT, Osiko VV, Kulkov AM, Voitsehovskii VN, Yakobson VE. Opt Mater 1999;11:315. [9] Findeisen J, Eichler HJ, Peuser P. Opt Commun 2000;181:129. [10] Omatsu T, Ojima Y, Pask HM, Piper JA, Dekker P. Opt Commun 2004;232:327. [11] He C, Chyba TH. Opt Commun 1997;135:273.
[12] Findeisen J, Eichler HJ, Peuser P, Kaminskii AA, Hulliger J. Appl Phys B 2000;70:159. [13] Cˇerny´ P, Jelı´ nlova´ H, Zcerev PG, et al. Prog Quantum Electron 2004;28:113. [14] Ge W, Zhang H, Wang J, Liu J, Xu X, Hu X, et al. J Crystal Growth 2004;270:582. [15] Kaiser W, Maier M. In: Arecchi FT, Schultz-Dubois EO, editors. Laser Handbook, vol. 2. Amsterdam: North Holland; 1972. p. 1077. [16] von der Londe D, Maier M, Kaiser W. Phys Rev A 1969;178(1):11. [17] Basiev TT, Sobol AA, Voronko YuK, Zverev PG. Opt Mater 2000;15:205.
Optics & Laser Technology 39 (2007) 1239–1242 www.elsevier.com/locate/optlastec
Picosecond stimulated Raman scattering of BaWO4 crystal Haohai Yu, Dawei Hu, Huaijin Zhang, Zhengping Wang, Wenwei Ge, Xinguang Xu, Jiyang Wang, Zongshu Shao, Minhua Jiang National Key Lab of Crystal Materials, Institute of Crystal Materials, Shandong University, Jinan 250100, China Received 11 July 2005; received in revised form 29 July 2005; accepted 3 August 2006 Available online 20 September 2006
Abstract In single-pass configuration, a picosecond pulse laser was used to investigate the stimulated Raman scattering in different BaWO4 crystals. Using the thresholds for the first Stokes line along the a- and c-axis, the Raman gain coefficients of the BaWO4 crystals were calculated along their respective direction. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. Compared with other well-known Raman crystals, such as KGd(WO4)2 and Ba(NO3)2, BaWO4 has favorable properties for scattering the pump radiation with picosecond pulse duration. The Raman gain coefficients of BaWO4 are different, but do not vary for different transmission directions, which means that this Raman material can be selected more freely. r 2006 Elsevier Ltd. All rights reserved. Keywords: BaWO4; Threshold; Raman gain coefficient
1. Introduction Stimulated Raman scattering (SRS) is an inelastic scattering process, whereby light pumped at one wavelength is converted to another wavelength, accompanied by the excitation of an internal mode of the Raman medium. Therefore, the solid-state Raman laser could extend the spectral region of solid-state laser sources. Due to the interest in their use in areas such as medical applications, spectroscopy, ecological monitoring techniques and laser guidestars, much attention has been focused on SRS [1–8]. In the selection of Raman laser materials, the Raman medium should have a high Raman gain coefficient, low SRS threshold and large optical damage threshold. The Raman gain coefficient is the key parameter determining the threshold for SRS and the growth rate of the intensity of the Stokes line. Therefore, much attention was focused on Raman crystals that have large Raman gain, including some tungstates (BaWO4 [2,6], PbWO4 [7], Nd:KGd(WO4)2 [9] and KY(WO4)2 [10]) and nitrates (Ba(NO3)2 [8,11–13]). Compared with other tungstates
Corresponding author. Tel.: +86 0531 8364848.
E-mail address: [email protected] (Z. Wang). 0030-3992/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.optlastec.2006.08.006
and nitrates, BaWO4 has better properties for scattering the pump radiation with picosecond pulse duration [2]. In the past, due to a lack of high optical quality and large-dimensional BaWO4 crystals, the experiments that have been undertaken only used one sample length (30 mm) [2] and the measurement of the threshold and Raman gain was only in one direction (K?C4, EJC4) [2,6]. In this paper, we report that the BaWO4 single crystals may be successfully grown using the Czochralski method, and the thresholds of BaWO4 crystals which are cut along different axes (a and c) with different lengths are determined. Using the thresholds, the Raman gain coefficients are calculated. Furthermore, higher SRS generations including Stokes and anti-Stokes lines in this experiment are also obtained. 2. Prepared crystal samples Fig. 1 shows the as-grown BaWO4 crystals. BaWO4 single crystals were successfully grown along a- and c-axis by the Czochralski method. The crystals are colorless with dimensions of about 22 80 mm2, weighing more than 213 g. There are no cracks or low-angle boundaries in the as-grown crystals. The cell parameters of BaWO4 crystal ˚ ˚ are a ¼ b ¼ 5:6041 A, c ¼ 12:7080 A. Before cutting into crystal samples, the crystal boules were annealed.
ARTICLE IN PRESS H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
1240
Fig. 1. As-grown BaWO4 crystal boule [14].
Three kinds of crystal samples with respective dimensions of 6 6 30 mm3, 6 6 50 mm3 and 6 20 6 mm3 (a a c) were chosen.
Fig. 2. Experimental setup.
3. Experimental setup The experimental setup to measure the thresholds of BaWO4 crystal is shown in Fig. 2. A PY61 mode-locked Q-switched Nd:YAG laser system generated single pulse radiation at the pump wavelength (1064 nm) with 40 ps duration. KTP crystal was used as second harmonic generation (SHG) crystal to produce green output at 532 nm. The second harmonic output was obtained by doubling in SHG and consisted of TEM00 mode pulses of duration 30 ps. The power was constant with 10 Hz repetition. The diameter and divergence of the beam are about 1 mm and 0.4 mrad with a spectral width of approximately 0.5 nm. Mirror (M1) has high reflectivity (499%) at 1064 nm and high transmission (490%) at 532 nm. PP1 and PP2 are two polarizing prisms. The polarizers provided smooth variation of pump energy into the Raman crystal. M2 reflected the laser (532 nm) to the system of two positive lenses (T) that increased the intensity of the laser beam by decreasing its diameter. The focal lengths of the positive lenses were 8 and 2 cm. D is a diaphragm with a diameter of 1 mm. The second harmonic laser beam transmitted though the Raman medium (BaWO4 crystal) and was reflected with the Raman laser onto a white paper screen (A). The Stokes images were obtained by the spectrum analyzer (USB200VIS-NIR) (B). The energy meter was used to measure the pump power when the Stokes lines appeared. 4. Results and discussion By using the experimental setup (Fig. 2), the thresholds for first Stokes line of different BaWO4 crystals (2 cm along a-axis, 3 and 5 cm along c-axis) were obtained.
Fig. 3. Image that determined the thresholds.
When the image such as that shown in Fig. 3 was observed, the intensity of the pump laser was considered to be the threshold of the first Stokes line. Here we considered SRS in the forward direction in the steady-state regime, which is the situation where practical, efficient frequency conversion is most likely to occur. Using a plane-wave approximation and neglecting dispersion, the equations that describe the model of SRS were shown in Refs. [15,16]. Near threshold, the growth of the Stokes wave yields the familiar expression: I s ðlÞ ¼ I s ð0Þ expðgss I p lÞ. Here, Is(0), Is(l) and Ip are the intensity from which the Stokes wave grows, at Stokes wavelength and pump wavelength, respectively, gss is the Raman gain coefficient and l is the interaction length of the laser and Raman medium. The pump intensity necessary to reach the steady-state SRS threshold leads to the conversion efficiency of about 1% and the value of the exponent G ¼ gss I t l ¼ 25. Here, It could be considered as the threshold.
ARTICLE IN PRESS H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
1241
Table 1 Thresholds and Raman gain of BaWO4 in different axes Transmission direction
Crystal length (cm)
Intensity of pump (mW)
Threshold (MW/cm2)
Raman gain (cm/GW)
First Stokes wavelength (nm)
a c
2 3 5
1.57 1.13 0.59
666.3 479.6 250.4
18.7 17.4 19.9
560
Fig. 4. High Stokes and anti-Stokes lines.
The thresholds and the Raman gains are shown in Table 1. It has been shown from Table 1 and references that: (1) the Raman gains of BaWO4 are much larger than that of KGd(WO4)2 (11.5 cm/GW [2]) and BaNO3 (4.7 cm/ GW [2]) for scattering the pump radiation with picosecond pulse duration. (2) the crystal has almost the same Raman gains and shift along different directions (a- and c-axis). It can be explained from the spontaneous Raman spectra [17]: the intensity at Stokes wavelength was born from the interaction of light and the internal vibration mode n1(Ag) [17] of BaWO4 molecule and the molecule has isotropic property.
be obtained conveniently, which could attribute to its high Raman gain coefficient. 5. Conclusion The BaWO4 crystal was grown by Czochralski method. Using the single-pass experimental setup, the thresholds were measured along a- and c-axis for the first Stokes line by which the Raman gain coefficients were calculated. The higher Stokes (fourth) and anti-Stokes (third) lines were observed. The experiments showed that BaWO4 crystal is a potential material for the development of SRS solid-state laser and people can select this Raman material with more degrees of freedom. Acknowledgments
Compared to Ref. [2], the threshold was determined by observing the image and the conversion efficiency may not be 1%, which leads to the fact that threshold (as well as the Raman gain) in this paper may have some discrepancy with traditional definition [2]. Cˇerny´ et al. [2] also showed that the error would be less than 10%. Increasing the pump power, the higher Stokes and antiStokes lines (Fig. 4) were obtained. In Fig. 4, the highest Stokes and anti-Stokes lines are the fourth (665 nm) and the third (462 nm), respectively. It has been shown that, with BaWO4 crystal, different Raman frequency shifts can
This work is supported by the National Natural Science Foundations of China (no. 50590401 and 60508010). References [1] Pask HM. Prog Quantum Electron 2003;27:1. [2] Cˇerny´ P, Zcerev PG, Jelı´ nlova´ H, Basiev TT. Opt Commun 2000;177:397. [3] Eremmenko AS, Kaprukin SN, Stepanov AI. Sov J Quantum Electron 1980;10:113. [4] Pask HM, Piper JA. Opt Commun 1998;148:285.
ARTICLE IN PRESS 1242
H. Yu et al. / Optics & Laser Technology 39 (2007) 1239–1242
[5] Findeisen J, Eichler HJ, Peuser P, Kaminskii AA, Hulliger J. Appl Phys B 2000;70:159. ˙ endzian W, Janbczyn´ski J, Jelı´ nkova´ H, Sˇulc J, [6] Cˇerny´ P, Z Kopczyn´ski K. Opt Commun 2002;209:403. [7] Kaminskii AA, McCray CL, Lee HR, Lee SW, Temple DA, Chyba TH, et al. Opt Commun 2000;183:277. [8] Zverev PG, Basiev TT, Osiko VV, Kulkov AM, Voitsehovskii VN, Yakobson VE. Opt Mater 1999;11:315. [9] Findeisen J, Eichler HJ, Peuser P. Opt Commun 2000;181:129. [10] Omatsu T, Ojima Y, Pask HM, Piper JA, Dekker P. Opt Commun 2004;232:327. [11] He C, Chyba TH. Opt Commun 1997;135:273.
[12] Findeisen J, Eichler HJ, Peuser P, Kaminskii AA, Hulliger J. Appl Phys B 2000;70:159. [13] Cˇerny´ P, Jelı´ nlova´ H, Zcerev PG, et al. Prog Quantum Electron 2004;28:113. [14] Ge W, Zhang H, Wang J, Liu J, Xu X, Hu X, et al. J Crystal Growth 2004;270:582. [15] Kaiser W, Maier M. In: Arecchi FT, Schultz-Dubois EO, editors. Laser Handbook, vol. 2. Amsterdam: North Holland; 1972. p. 1077. [16] von der Londe D, Maier M, Kaiser W. Phys Rev A 1969;178(1):11. [17] Basiev TT, Sobol AA, Voronko YuK, Zverev PG. Opt Mater 2000;15:205.