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Abstract
An effective third harmonic generation (THG) 355 nm laser in a novel Ba3(ZnB5O10)PO4 (BZBP) crystal was demonstrated. Based on the phase matching (PM) calculation, the BZBP crystal has the type-I PM angle of θ = 90°, ϕ = 73.2° for generating the THG 355 nm laser, with a large acceptance angle of 66.3 mrad·mm and a small walk-off angle of 6.06 mrad. Using a nanosecond 1064 nm laser as the pump source, the THG 355 nm laser could achieve the maximum conversion efficiency of 28.4%, and the highest output power of 0.56 W. In addition, BZBP crystal also has a small weak absorption coefficient of 20-40 ppm/cm and a laser damage threshold of 1.04 GW/cm2. The achieved results indicate that the BZBP crystal is a promising UV nonlinear optical material due to its short absorption edge (180 nm), small walk-off effect, large acceptance angle, and non-hygroscopicity.
© 2023 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Ultraviolet (UV) band lasers have a wide range of applications in optical storage, lithography, high-resolution photoelectron spectroscopy, precision material, medical treatment and so on [1–3]. Nonlinear optical (NLO) frequency conversion has become one of the main method to realize the UV band lasers due to the advantages of high reliability, high beam quality and low maintenance cost [4–5]. UV lasers can be obtained by the third harmonic generation (THG) based on NLO crystals with the matured Nd3+ or Yb3+ doped pump lasers [6]. It should be noted that the THG here is realized through a second harmonic generation (SHG) cascaded with an sum frequency generation (SFG) process, not the directed third harmonic generation based on the third-order nonlinear effect. The output performance of UV laser depends greatly on the NLO crystals. Nowadays, many NLO crystals have been explored for achieving the THG lasers, such as LiB3O5 (LBO) [7–9], β-BaB2O4 (BBO) [10,11], CsB3O5 (CBO) [12,13], CsLiB6O10 (CLBO) [14,15], KD2PO4 (KD*P) [16] and so on. Among them, LBO has proven to be a good NLO crystal, on account of its high conversion efficiency, small walk-off angle and easy obtained large size crystal [17]. However, it is prone to deliquescence, and the same drawback that also plagues KD*P crystal. BBO has favorable condition in THG laser with a large NLO coefficient, but its application performance was impaired by the serious walk-off effect owing to a large birefringence [18], besides, the difficulty of obtaining large-size crystal growth of BBO [19]. CBO and CLBO also have been applied in THG conversion field, but their NLO coefficients are relatively small [20]. Furthermore, the softness and hygroscopicity of CLBO crystal make it difficult to cut and polish [6]. Based on the above analysis, advantageous NLO crystals for THG UV lasers should have the characteristics as following: a broad transparency range (short UV absorption edge), a large second-order NLO coefficient, a moderate birefringence for phase matching (PM), a high laser damage threshold, a small walk-off angle, a large angular acceptance, easy achieved a large size high optical single crystal and stable physical, chemical and mechanical performances [21].
Recent researches on borate-phosphate NLO crystals have been attracted the increasing interests due to their capability to exhibit very short absorption edge (180 nm) and large SHG response (4×KH2PO4) [22]. On the basis of these, the Ba3(ZnB5O10)PO4 (BZBP) crystal was designed to growth with the addition of acentric ZnO4 tetrahedra to enhance the SHG response, while the absorption edge without shifting [23]. The previous works have explored the basic NLO properties of this crystal [23–25]. BZBP crystal has a wide transmission range, a moderate nonlinear coefficient, a medium birefringence of 0.033 at 1064 nm and the thermal conductivity of 1.77-2.11 W/mK, a high damage threshold. Additionally, BZBP is non-hygroscopic and can be grown into high-optical-quality and large-size single crystal by top-seeded-solution-growth (TSSG) method. These characteristics make BZBP to be an attractive NLO crystal in the field of THG 355 nm ultraviolet laser [24].
In this work, we demonstrated an effective THG 355 nm laser with a novel BZBP crystal. The PM conditions and second-order NLO coefficients were investigated. The PM type-I THG could be achieved from 1.038 to 4.7 µm, with the 1064 nm fundamental wavelength, the THG PM angle of θ = 90°, ϕ = 73.2°. And the NLO coefficients d31 = 0.66 pm/V and d33 = 0.65 pm/V were measured. Using nanosecond 1064 nm laser as pumping source, type-I BZBP as THG crystal, the THG 355 nm laser could be achieved the maximum conversion efficiency of 28.4%. As a comparison, the type-I LBO THG laser was obtained the conversion efficiency of 30%. These results indicate that the BZBP crystal is a promising UV NLO material, and the scheme provides more material options for THG 355 nm lasers.
2. Experimental methods
Transmission Spectroscopy and Rocking Curve Measurements. The transmission spectrum was recorded with dimension of 3 × 3 × 5 mm3 along the type-I PM direction, used UV–VIS–NIR spectrophotometer (UH4150, HITACHI, Inc) and Fourier transform infrared spectrometer (Nicolet iS50 FT-IR, Thermo Fisher, Inc), wavelength range from 180 to 5000 nm. The rocking curve was made by the high-resolution X-ray diffraction measurement (SmartLab3KW, Rigaku, Inc.), through adjusting the angle between the X-ray light and the crystal surface around the Bragg diffraction peak θ, with a polished BZBP crystal oriented along the (101), and the dimensions of 5 × 6 × 2 mm3.
Bulk Weak Absorption. The bulk weak absorption was measured by the Photo-thermal Common-Path Interferometer system (SPTS, Inc) with the pump sources of 1064 nm and 532 nm lasers, respectively. The probe light (He-Ne) has a wavelength of 633 nm and the repetition rate of optical chopper is 1 kHz. The pump lasers were focused on the front surface of sample, and intersected with probe light via dispersion prisms and convergent. The angle of the two light sources was only 1.5°, the pump laser diameter of 100 µm, the probe beam diameter of 200 µm. The probe signals measured by a photo-detector translated to the amplifier and processed by the computer.
Maker Fringe Method. A Maker Fringe setup was implemented to determine the second-order electric susceptibility coefficient d31 and d33 of BZBP relatively to d36 of KDP crystal at the same wavelength. The light sources was a Q-switched Nd:YAG laser (NL305HT, EKSPLA, Inc) at the fundamental wavelength of 1064 nm with a 10 Hz repetition rate and 6 ns pulse width. The KDP dimensions of 10 × 10 mm2, the BZBP dimensions of 4 × 4 mm2, and polished to optical quality. These samples were stuck on a turntable which can ensure a continuous rotation of the crystals in the (z, y) and (x, y) planes. The power of the SHG light was measured as a function of the sample orientation, by using a photomultiplier tube (PMT, Hamamatsu, model R105). It was averaged by a fast-gated integrator combined with a boxcar (Stanford Research Systems), and recorded using a software.
Damage Threshold Measurement. A high energy Nd:YAG laser with the pulse energy of 40 mJ was used as the damage inducing source. The test sample was placed in a four-axis translation stage where the x, y and z axes were allowed to translate between the test points on a given sample and the surface of the test sample was always located in the plane of the focused beam waist.
Third Harmonic Generation. For the THG laser, the pump source was a high energy Nd:YAG laser. The input beam polarization was set ordinary, and the spot diameter of 2 mm. the LBO crystal was used for type-I SHG (θ = 90°, ϕ = 11.4°). A BZBP crystal with cutting dimensions of 5 × 5 × 17.6 mm3 of type-I (θ = 90°, ϕ = 73.2°) and an LBO crystal with dimensions of 3 × 3 × 15.8 mm3 type-I (θ = 90°, ϕ = 36.5°) were used as the THG materials to generate the 355 nm laser. The output power was measured by a power meter (OPHIR, P/N 7Z01560, VEGA, Inc), and the wavelength was measured by a spectrometer with the accuracy of 0.1 nm (Miniature Spectrometer, FLAME-S-VIS-NIR-ES, Ocean Optics, Inc).
3. Results and discussions
3.1 Transmission spectrum, rocking cure and bulk weak absorption of BZBP crystal
The BZBP sample was cut along z axis with the dimension of 3 × 3 × 5 mm3, the complete transmission curve of BZBP crystal with a wide range of 180 to 5000 nm is shown in the Fig. 1. The transitive surfaces of this sample were optically polished without any coatings. BZBP crystal had ∼20% transmittance at 190 nm in the DUV range, the transparency range of 320-3000 nm with the transmittance ≥70%. The corresponding transmittance was 83% at 355 nm, 85.2% at 532 nm and 85.5% at 1064 nm, respectively. In addition, the quality of the crystal is measured by the high resolution X-ray diffraction, as shows Fig. 1(c). The FWHM of the rocking cure of the crystal is 36″, which indicates the used crystal has the high optical quality. The weak absorption coefficients was also measured by the Photo-thermal Common-Path Interferometer system, and the relevant results were shown Fig. 2. The weak absorption coefficients of BZBP crystal at 1064 nm and 532 nm were about 20-40 ppm/cm, 130 ppm/cm, a slightly larger than that of LBO crystal 10 ppm/cm at 1064 nm [26], which implies that BZBP has the potential to be used in high-power frequency-converted lasers.
3.2 Measurement of the NLO coefficients and laser damage threshold
BZBP is a negative biaxial optical crystal with space group Pmn21, by considering the Kleinman symmetry [27], which has three independent non-zero NLO coefficients, i.e. d31, d32, and d33. Here we measurement the two coefficients by the Maker fringe technique. The (010)-cut plane-parallel uncoated plate of BZBP (4 × 4 × 1.84 mm3) was used to measure d31and d33. A (110)-cut KH2PO4 (KDP) crystal with size of 10 × 10 × 2.30 mm3 was also prepared to be used as a reference sample. As an example, Fig. 3(b) shows the orientation of the BZBP sample and optical axis c is parallel to rotational axis to measure the Maker fringes of d31, fundamental laser perpendicular to the direction of SHG light. And optical axis a is parallel to rotational axis to measure the Maker fringes of d33, fundamental light parallel to the direction of SHG light, the Eω is the fundamental light and the P2ω is the SHG light. The measured Maker fringes of d36(KDP), d31 (BZBP) and d33 (BZBP) are in good agreement with the theoretical fringes, respectively (Fig. 3(a-c)). Based on the measurements and calculations, the NLO coefficients of BZBP crystal relative to d36(KDP) = 0.57 ± 0.02 pm/V have been determined, and the results were d31(BZBP) = 1.15 d36(KDP) = 0.66 ± 0.023 pm/V, d33(BZBP) = 1.140 d36(KDP) = 0.65 ± 0.022 pm/V. Compared with other commercial DUV NLO materials, BZBP has the moderate NLO coefficients. Additionally, the laser damage threshold of BZBP crystal was measured. The light source was a Q-switched Nd:YAG laser with a wavelength of 1064 nm, a pulse frequency of 1 Hz, a pulse width of 6 ns, and a laser beam diameter of 0.9 mm. At the same laser power, we randomly selected five positions on the sample, bombarded it with a pulse laser, and observed the damage under the microscope. When observed, all five positions were damaged, and then the laser power was reduced until no positions were damaged. Under the input energy of 40 mJ per pulse, there are no damaged positions, corresponding to a final laser damage threshold of 1.04 GW/cm2.
Fig. 3. Experimental fringes (solid curves) and theoretic Maker fringes and envelops (dashed curves) of (a) d36 (KDP), (b) d31 (BZBP) and (c) d33 (BZBP).
3.3 THG phase matching conditions of BZBP crystal
Based on the Sellmeier equations of our previous work [24], the PM conditions were calculated. Figure 4(a) shows the PM range of the x-y plane (θ = 90°), the parameter ϕ as a function of wavelength on difference PM types. For type-Ι, the effective THG PM range of BZBP crystal was from 1.038 to 4.7 µm. The waveband 1.7 to 2.8 µm can also achieve the THG PM in the x-z plane (ϕ = 0°) [24]. For type-ΙΙ, the PM wavelength was from 1.268 to 4.03 µm. Obviously, the BZBP crystal can achieve THG 355 nm laser only by adopting the type-Ι PM, and the angle of θ = 90°, ϕ = 73.2° on the x-y plane. Figure 4(b) shows the PM angle with the 1064 nm fundamental wavelength, the THG range from θ = 70.6° (ϕ = 90°) to 90° (ϕ = 73.2°). In addition, the effective NLO coefficient, the angular acceptance (Δϕ·L), walk-off angle and spectral acceptance (Δλ·L) of BZBP in the x-y plane were calculated, and shown in Fig. 4(c-f). Firstly, with the three independent non-zero NLO coefficients, i.e. d31, d32, and d33, the effective NLO coefficient (deff) of any plane in the BZBP crystal can be acquired with the difference fundamental wavelength, the corresponding expressions of the effective NLO coefficient (deff) in x-y plane and y-z plane were given by the following equations:
where θ is the angle from the optic axis (z-axis), whereas ϕ is the angle in the x-y plane. Using the above equations, the effective nonlinear coefficient (deff) was calculated, and the results shown in Fig. 4(c). When θ = 90°, ϕ = 73.2°, the effective NLO coefficient deff = 0.632 pm/V, which is only a little smaller than the maximum value of 0.66 pm/V. Figure 4(d,e,f) illustrated the change of the angular acceptance, walk-off angle and spectral acceptance of THG phase matching with the fundamental wavelength in the x-y plane. For type-Ι PM (θ = 90°, ϕ = 73.2°), the BZBP crystal has an acceptance angle of 66.3 mrad·mm, walk-off angle of 6.06 mrad, and spectral acceptance of 2.59 × 10−3 µm·mm. In addition, we have compared some coefficients of other NLO crystals which used for generating THG laser, and the values obtained from the SNLO v5.8 software package, as shown in Table 1. From the Table 1, the walk-off angle of 6.06 mrad is smaller than that of LBO (18.3 mrad), and about one thirteenth of BBO (80 mrad), with the fundamental wavelength of 1064 nm. Besides, the acceptance angle of 66.3 mrad·mm, which is up to four times than that of LBO (17.1 mrad·mm), eighteen times as large as BBO (3.7 mrad·mm). Based on the above analysis, small walk-off angle and large angular acceptance enables BZBP to be better used in the field of nonlinear optics, and also indicates that BZBP crystal has a potential in THG 355 nm laser.Fig. 4. (a) The PM angle curves for type-I (red) and type-II (black) THG as a function of the fundamental wavelength; (b) The PM angle for THG of BZBP crystal at 1064 nm; (c) The effective NLO coefficient variation with fundamental wavelength; (d) The angular acceptance variation with fundamental wavelength; (e) The walk-off angle variation with fundamental wavelength; (f) the spectral acceptance variation with fundamental wavelength.
4. THG 355 nm laser with BZBP crystal
4.1 Experimental setup
The THG experimental scheme was shown in Fig. 5. A nanosecond pulsed laser (NL305HT, EKSPLA, Inc) as the fundamental laser source. The working wavelength of 1064 nm, the pulse width of 6 ns, spot diameter of 6 mm, 1/e2 spot diameter of 7.2 mm, beam divergence of < 0.6 mrad, M2 of 6 and the repetition rate of 10 Hz. The fundamental laser was converted into a vertically polarized laser through the laser free-space isolator (1045-1080 nm, 12 mm clear aperture, EOT, Inc) by a half-wave-plate P1. The spot coupling system consists of a focusing lens L1 (f = 150 mm) and a focusing lens L2 (f = 50 mm. An LBO crystal with dimensions of 10 × 10 × 15 mm3 was used for type-I SHG (θ = 90°, ϕ = 11.4°). In order to ensure with the same polarization at 1064 nm and 532 nm, a dual-wavelength half-wave plate P2 (λ/2@1064 nm, λ@532 nm, AR@1064 nm and 532 nm, CASTECH, Inc) was placed behind the LBO crystal. Then the 1064 nm laser was change to be horizontal polarization light which was alignment with the polarization direction of the SHG laser. A BZBP crystal and an LBO crystal were used as the THG materials to generate the 355 nm laser. The surfaces of the two THG crystals were optically polished without coating. The THG beam separate from the beam at the fundamental frequency and from the SHG beam by utilizing L3 mirrors (45°HR, Rs > 99.8% @355 nm & 45°HT, Ts > 99% @1064 & 532 nm, CASTECH, Inc). Finally, a Brewster prism was employed to separate the pump laser of 1064 nm and the SHG laser of 532 nm.
4.2 Output performances of THG 355 nm laser
Used the 1064 nm laser with pulse width of 6 ns as the pump source, type-Ι LBO as SHG crystal, the 532 nm laser can be obtained successfully. The Fig. 6(a) shows the relationship between the SHG output power at 532 nm and the 1064 nm pump power. The SHG 532 nm laser was achieved the highest output power of 1.09 W, the corresponding conversion efficiency of 43.1% under input power of 2.53 W. The efficient SHG and fundamental 1064 nm laser can ensure that the THG has sufficient laser to generate THG 355 nm laser. We noted that when the pump laser of 2.3 W, the power of 532 nm laser is same to the power of remaining 1064 nm laser (∼0.98 W), the effective utilization rate of pump laser is only 85.2% [(0.98 × 2)/2.3], which means the theoretical conversion efficiency should reach to 50%, but the actual measurement efficiency of 40%. This phenomenon indicates that the fundamental laser had lots of losses in the process of SHG. Especially, the loss caused by reflection and absorption of the fundamental laser.
Fig. 6. (a) The SHG output performances of LBO crystal with type-Ι; (b) The BZBP THG conversion efficiency and the output power of 355 nm, and remaining 532 nm and 1064 nm lasers; (c) The SHG and THG conversion efficiency versus input power ratio of 532/1064 nm; (d) The LBO THG output power and conversion efficiency versus input power of 532 nm and 1064 nm.
In order to accurately reflect the THG output, using the SHG green laser and remaining 1064 nm laser as pump light. Figure 6(b) shows the output power of 355 nm, and remaining 532 nm and 1064 nm lasers. From the figure, it can be seen that the power of 355 nm laser was increased with the increasing the pump power. When the pump power of 2.11 W, 355 nm laser was acquired the highest output power of 0.56 W, the remaining 532 and 1064 nm power were 0.53 W and 0.73 W. Under pump of 1.39 W, the THG laser had the maximum conversion efficiency of 28.4%, and the results pink curve shown in Fig. 6(b). The conversion efficiency increases versus the input pump power at first, then shows a small decreasing trend, and finally stabilized at 26.6%. When the conversion efficiency reaches the highest of 28.4%, the corresponding power of 355 nm, 532 nm,1064 nm were 0.4 W, 0.21 W, and 0.63 W respectively, and the growth rates were both maximum at this time. In the THG experiment, the output efficiency of the THG can be optimized by balancing the fundamental wave and the SHG power. With the increase input power the ratio of 532 nm and 1064 nm lasers, the output power of THG gradually increases. When the ratio reaches 0.7, the output efficiency of the THG reaches the maximum of 28.4%, and then shows a decreasing trend as shown in Fig. 6(c). According to Ref. [31], the maximum THG conversion efficiency with ratio of 0.54∼0.82 of Ca5(BO3)3F crystal. In the BZBP THG experiment, the optimal ratio of 532 nm to 1064 nm of 0.7, which is just in this range. Using a type I LBO crystal as comparison, the LBO THG laser obtained the highest output power of 0.6 W with a conversion efficiency of 28.6%. The maximum conversion efficiency was 30% under a pump power of 1.2 W, and the results were illustrated in Fig. 6(d). Besides, the maximum output power shown in Fig. 7, the UV laser has good stability without any heat sink under the output power of 560 mW, and the peak-to-valley power deviation jitter was 1.3%. The spectrum of 355 nm, 532 nm and 1064 nm lasers shown inset Fig. 7. The FWHM of the spectrum was 0.8 nm, which is slightly smaller than that of 1064 nm (1.1 nm) and 532 nm (0.9 nm) lasers.
5. Conclusions
In summary, an effective third harmonic generation operation, produced by type-I phase-match in a 17.6 mm long BZBP nonlinear optical crystal was demonstrated. The BZBP has the NLO coefficients d31 = 0.66 pm/V, d33 = 0.65 pm/V, the type I PM of θ = 90°, ϕ = 73.2° for THG 355 nm laser with the walk-off angle of 6.06 mrad, the angular acceptance of 66.3 mrad·mm and spectral acceptance of 2.59 × 10−3 µm·mm. Using nanosecond pulsed 1064 nm laser as pump source, the BZBP THG laser could be achieved the maximum conversion efficiency of 28.4%. As a comparison, the LBO THG laser was obtained the conversion efficiency of 30%. The achieved results indicates that the BZBP crystal is a promising UV nonlinear optical material.
Funding
National Natural Science Foundation of China (51890860, 51890864, 51890865, 52002272, 61835014).
Acknowledgement
Y. C. and X. Z. contributed equally to this work, conceived the experiments, Y. C performed the experiments, collected and analyzed the data, H. Y, S. M and Y. C wrote the paper; X. Z prepared the crystal materials; H. W, J. W, Z. H. and Y. W. helped with the data analysis, theoretical calculation and paper writing.
Disclosures
The authors declare no conflicts of interest.
Data availability
No data were generated or analyzed in the presented research.
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