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This paper describes the first demonstration of ultraviolet (266nm) vortex generation using the combination of a frequency-doubled nanosecond green laser, a spiral phase plate, and a periodically bonded β-BaB2O4 device. For a laser pumping energy of 9.1 mJ, an ultraviolet vortex energy of 1.24 mJ was obtained, corresponding to a conversion efficiency of 13.7%.
© 2014 Optical Society of America
1. Introduction
An optical vortex with a phase singularity in a wavefront has a doughnut-shaped spatial profile, and it carries an orbital angular momentum characterized by an integer, m, termed the topological charge [1–5]. Optical vortices have attracted an intense amount of interest in a variety of fields [6–13]. In particular, it has been shown that optical vortex lasers allow for the fabrication of chiral metallic and organic nanostructures [14–17]. This opens the door to a wide range of new technologies, including nanoscale imaging systems with chiral selectivity, plasmonic metamaterials, and biomedical nano-electromechanical systems. However, to achieve this, there is a strong need to develop laser systems capable of producing ultraviolet (UV) vortices. This is because many materials have strong absorption bands in the UV region, and UV lasers are already widely used in research as well as industrial applications, such as spectroscopy, photochemistry, photolithography, and microfabrication.
To date, the only reported generation of ultraviolet vortices has been achieved in the extreme ultraviolet (13.5 nm) region using computer generated holograms fabricated by electron beam lithography [18]. It is difficult to produce optical vortices directly from ultraviolet laser beams because of the lack of efficient phase modulation devices, such as spatial light modulators, which operate in the ultraviolet region.
One promising approach is to generate optical vortices at longer laser wavelengths, and then to use nonlinear frequency conversion, such as second harmonic generation in a second-order nonlinear crystal, to achieve vortices at UV wavelengths. Efficient frequency-doubled vortex generation [19] has already been demonstrated in the visible and near-infrared regions using LiB3O5 [20] and periodically poled LiNbO3 [21] crystals. These crystals allow non-critical phase matching between the input vortex and the frequency-doubled output vortex. However, they cannot be used at UV wavelengths. A conventional nonlinear crystal β-BaB2O4 (BBO) in the UV region [22] exhibits strong birefringence, resulting in the spatial separation of any phase singularities due to walk-off effects [23, 24]. Thus, no effective frequency-doubling technique for the production of UV vortices has yet been established.
Recently, Hara et al. proposed a novel device structure to compensate for walk-off effects and to improve the conversion efficiency of UV generation [25]. This device consisted of a series of BBO crystals with alternating orientations bonded together at room temperature [26]. In the present study, a similar periodically bonded BBO device was used to generate UV (266 nm) vortices for the first time. A high-quality UV vortex output was generated at a moderate energy, without any spatial separation of the phase singularities, at a conversion efficiency of 13.7%.
2. Basic concept of periodically bonded BBO device
When walk-off effects in a nonlinear crystal are taken into consideration, the electric field E2ωbulk(x, y) of the frequency-doubled vortex output can be written in Cartesian coordinates as [27]
where Eω(x, y) is the electric field of the input vortex, ρ is the walk-off angle, ω0 is the beam waist, and L is the crystal length. To investigate the influence of the walk-off effect on the second harmonic generation of a vortex in nonlinear crystals, such a Cartesian formula for the vortex output is preferred rather than a conventional formula in cylindrical coordinates. The spatial intensity profile I2ωbulk(x, y) for the vortex output is then given byBBO crystals, which are widely used to generate UV output at a wavelength of 266 nm (fourth harmonic of 1064 nm), give rise to severe walk-off effects (ρ = 85.3 mrad [21]) owing to their large birefringence. Figures 1(a) and 1(b) show the simulated intensity profile I2ω(x, y) for 2ω0 = 450 μm and L = 0.5 and 2 mm, respectively, obtained by using Eq. (2). For L = 0.5 mm, the spatial separation of the phase singularities is negligible, whereas it is significant for L = 2 mm. The separation is in fact approximately proportional to the crystal length L.
Fig. 1 Simulated spatial profile for the frequency-doubled UV vortex output from (a) a 0.5-mm-long bulk BBO crystal, (b) a 2-mm-long bulk BBO crystal, and (c) 2-mm-long periodically bonded BBO device, shown schematically in (d).
The basic concept of the periodically bonded BBO device is illustrated in Fig. 1(d). It consists of thin BBO crystals with alternating orientations, bonded together to form a device of the required length, thereby cancelling out the walk-out effect.
The spatial intensity profile I2ωdevice(x, y) for the vortex output in such a device is given by
where Λ is the length of the individual BBO crystals, and N is the number of crystals. The device length LD is then the product of N and Λ. Figure 1(c) shows the simulated output intensity profile for a device with ρ = 85.3 mrad, N = 4, Λ = 0.5 mm, and 2ω0 = 450 μm. The profile has a doughnut shape with a relatively large dark core. No spatial separation of the phase singularities is observed, indicating the generation of a second-order optical vortex. Thus, the simulation results imply that a high-quality UV vortex can be produced using such a device.3. Experiments
Figure 2 shows a schematic diagram of the experimental setup. A conventional frequency-doubled nanosecond Q-switched Nd:YAG laser (wavelength 532 nm) was used. It had a maximum pulse energy of 6.7 mJ and a pulse duration of 25 ns at a repetition rate of 50 Hz. A mode conversion technique using a polymer spiral phase plate (RPC Photonics, VPP-1c) with a 2π azimuthal phase shift was employed to produce an optical vortex with a topological charge of 1 in the vicinity of the focus (see the inset in Fig. 2).
The green optical vortex was weakly focused to a ∅600-μm spot on the surface of a periodically bonded BBO device. The device had an aperture of 3 mm × 3 mm, and a length of 2 mm, which consisted of four BBO crystals with lengths of 0.5 mm, cut at an angle of 47.7° relative to the c-axis. The crystals were bonded to each other in a vacuum, after first activating the bonding surfaces using argon atom beams. The generated UV vortex output was separated from the residual green beam using a quartz prism. No antireflection coating was used on either the input or output surface of the device.
Based on the simulation results, the device is expected to double both the frequency and the topological charge of the input vortex, and to produce the minimum spatial separation of the phase singularities. For a comparison, UV vortex generation was also performed using a 2-mm-long bulk BBO crystal for type-I phase matching between wavelengths of 532 and 266 nm.
Figure 3 shows near- and far-field spatial profiles for the original green vortex, and the outputs of the bulk BBO crystal and the periodically bonded device. Also shown are self-interference fringes in the near field formed by using a transmission grating with a low spatial frequency. As can be seen in Figs. 3(d) and 3(f) for the bulk BBO crystal, the UV output exhibits laterally displaced phase singularities in both the near- and far-field profiles, indicating a multiple vortex formed by the superposition of a Gaussian and second-order vortex modes. A quartet of forked fringes with two legs (Fig. 3(e)) can also be seen, indicating a spatial separation of the phase singularities.
Fig. 3 Experimental vortex outputs. (a), (b) Near- and far-field intensity profiles for a green vortex, and (c) self-interference fringes in the near field. (d), (e) Near- and far-field intensity profiles for the UV output from a 2-mm-long bulk BBO crystal, and (f) self-interference fringes. (g), (h) Near- and far-field intensity profiles for the UV output from a 2-mm-long periodically bonded BBO device, and (i) self-interference fringes in the near field.
The π/2 azimuthal rotation of the phase singularities between the near and far fields is due to a Gouy phase shift of (m + 1)tan−1(z/zR) between the Gaussian and the second-order vortex modes, where z is the propagation distance and zR is the Rayleigh length. In fact, two local minima in a line-intensity profile of the UV far-field can be seen (Fig. 4(a)). The UV vortex output energy was 1.1 mJ, corresponding to a conversion efficiency of 12.5%. In contrast, for the periodically-bonded BBO device, both the near- and far-field intensity profiles were annular, as shown in Figs. 3(g) and 3(i). In the self-interference pattern (Fig. 3(h)), a pair of forked fringes with three legs can be seen, indicating that the topological charge of the UV optical vortex output was 2. The output vortex energy for the maximum laser pumping energy of 9.1 mJ was 1.24 mJ, corresponding to an optical conversion efficiency of 13.7%. Note that the UV conversion efficiency was also slightly improved by enhancement of the effective interaction between the visible and violet vortices due to walk-off compensation, as shown in Fig. 5.
Fig. 4 Line-intensity profiles (along a broken line shown in Figs. 3(f) and 3(i)) of the UV outputs obtained by using (a) the bulk BBO crystal, and the periodically bonded BBO device.
Fig. 5 (a) UV vortex output energy as a function of the green vortex energy. (b) UV vortex conversion efficiency as a function of the green vortex energy.
The above results are consistent with those obtained from the simulations, indicating that a periodically bonded BBO device can effectively compensate walk-off effects and efficiently produce a high-quality UV vortex output. Quantitative evaluation methods concerning the purity of the vortex output, e.g. orbital angular momentum analysis based on a spatial light modulator, have been proposed [28]. However, they are not attempted to the UV vortex, because of the lack of efficient spatial light modulators in the UV region. Thus, the purity of the vortex output will be quantitatively discussed in the future.
5. Conclusion
High-quality UV vortex generation has been demonstrated for the first time using a device in which BBO crystals with alternating orientations are bonded together in order to compensate for any walk-out effects. Using this device, the spatial separation of phase singularities was suppressed, and an output energy of 1.24 mJ was obtained, corresponding to a frequency conversion efficiency of 13.7%. Additional energy scaling will be possible by increasing the device length, i.e., the number of bonded crystals, and by using appropriate antireflection coatings on the input and output surfaces of the device. The ability to efficiently produce optical vortices in the UV region, in which a variety of materials, such as semiconductors, have strong absorption bands, is expected to allow new fields of materials science to be explored.
Acknowledgments
The authors acknowledge support from a Grant-in-Aid for Scientific Research (No. 24360022) from the Japan Society for the Promotion of Science.
References and links
1. L. Allen, M. W. Beijersbergen, R. J. C. Spreeuw, and J. P. Woerdman, “Orbital angular momentum of light and the transformation of Laguerre-Gaussian laser modes,” Phys. Rev. A 45(11), 8185–8189 (1992). [CrossRef] [PubMed]
2. G. Indebetouw, “Optical vortices and their propagation,” J. Mod. Opt. 40(1), 73–87 (1993). [CrossRef]
3. M. Padgett, J. Courtial, and L. Allen, “Light’s orbital angular momentum,” Phys. Today 57(5), 35–40 (2004). [CrossRef]
4. M. S. Soskin and M. V. Vasnetsov, “Singular optics,” in Progress in Optics42, E. Wolf, ed. (Elsevier, 2001), pp. 219–176.
5. A. M. Yao and M. J. Padgett, “Orbital angular momentum: origins, behavior and applications,” Adv. Opt. Photon 3(2), 161–204 (2011). [CrossRef]
6. D. G. Grier, “A revolution in optical manipulation,” Nature 424(6950), 810–816 (2003). [CrossRef] [PubMed]
7. J. E. Curtis, B. A. Koss, and D. G. Grier, “Dynamic holographic optical tweezers,” Opt. Commun. 207(1–6), 169–175 (2002). [CrossRef]
8. M. Chen, M. Mazilu, Y. Arita, E. M. Wright, and K. Dholakia, “Dynamics of microparticles trapped in a perfect vortex beam,” Opt. Lett. 38(22), 4919–4922 (2013). [CrossRef] [PubMed]
9. B. Harke, J. Keller, C. K. Ullal, V. Westphal, A. Schönle, and S. W. Hell, “Resolution scaling in STED microscopy,” Opt. Express 16(6), 4154–4162 (2008), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-16-6-4154. [CrossRef] [PubMed]
10. Y. Tokizane, K. Shimatake, Y. Toda, K. Oka, M. Tsubota, S. Tanda, and R. Morita, “Global evaluation of closed-loop electron dynamics in quasi-one-dimensional conductors using polarization vortices,” Opt. Express 17(26), 24198–24207 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-26-24198. [CrossRef] [PubMed]
11. A. Mair, A. Vaziri, G. Weihs, and A. Zeilinger, “Entanglement of the orbital angular momentum states of photons,” Nature 412(6844), 313–316 (2001). [CrossRef] [PubMed]
12. N. Bozinovic, Y. Yue, Y. Ren, M. Tur, P. Kristensen, H. Huang, A. E. Willner, and S. Ramachandran, “Terabit-scale orbital angular momentum mode division multiplexing in fibers,” Science 340(6140), 1545–1548 (2013). [CrossRef] [PubMed]
13. Y. Ueno, Y. Toda, S. Adachi, R. Morita, and T. Tawara, “Coherent transfer of orbital angular momentum to excitons by optical four-wave mixing,” Opt. Express 17(22), 20567–20574 (2009), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-17-22-20567. [CrossRef] [PubMed]
14. T. Omatsu, K. Chujo, K. Miyamoto, M. Okida, K. Nakamura, N. Aoki, and R. Morita, “Metal microneedle fabrication using twisted light with spin,” Opt. Express 18(17), 17967–17973 (2010), http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-18-17-17967. [CrossRef] [PubMed]
15. K. Toyoda, K. Miyamoto, N. Aoki, R. Morita, and T. Omatsu, “Using optical vortex to control the chirality of twisted metal nanostructures,” Nano Lett. 12(7), 3645–3649 (2012), doi:. [CrossRef] [PubMed]
16. K. Toyoda, F. Takahashi, S. Takizawa, Y. Tokizane, K. Miyamoto, R. Morita, and T. Omatsu, “Transfer of Light Helicity to Nanostructures,” Phys. Rev. Lett. 110(14), 143603 (2013). [CrossRef]
17. M. Watabe, G. Juman, K. Miyamoto, and T. Omatsu, “Light induced conch-shaped relief in an azo-polymer film,” Sci Rep 4, 4281 (2014). [CrossRef] [PubMed]
18. B. Terhalle, A. Langner, B. Päivänranta, V. A. Guzenko, C. David, and Y. Ekinci, “Generation of extreme ultraviolet vortex beams using computer generated holograms,” Opt. Lett. 36(21), 4143–4145 (2011). [CrossRef] [PubMed]
19. S. M. Li, L. J. Kong, Z. C. Ren, Y. Li, C. Tu, and H. T. Wang, “Managing orbital angular momentum in second-harmonic generation,” Phys. Rev. A 88(3), 035801 (2013). [CrossRef]
20. C. Chen, Y. Wu, A. Jiang, B. Wu, G. You, R. Li, and S. Lin, “New nonlinear-optical crystal: LiB3O5,” J. Opt. Soc. Am. B 6(4), 616–621 (1989). [CrossRef]
21. 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 Electron. 28(11), 2631–2654 (1992). [CrossRef]
22. C. Chen, Z. Lin, and Z. Wang, “The development of new borate-based UV nonlinear optical crystals,” Appl. Phys. B 80(1), 1–25 (2005). [CrossRef]
23. M. Koyama, T. Hirose, M. Okida, K. Miyamoto, and T. Omatsu, “Power scaling of a picosecond vortex laser based on a stressed Yb-doped fiber amplifier,” Opt. Express 19(2), 994–999 (2011). [CrossRef] [PubMed]
24. K. Dholakia, N. B. Simpson, M. J. Padgett, and L. Allen, “Second-harmonic generation and the orbital angular momentum of light,” Phys. Rev. A 54(5), R3742–R3745 (1996). [CrossRef] [PubMed]
25. K. Hara, S. Matsumoto, T. Onda, W. Nagashima, and I. Shoji, “Efficient ultraviolet second-harmonic generation from a walk-off-compensating β-BaB2O4 device with a new structure fabricated by room-temperature bonding,” Appl. Phys. Express 5(5), 052201 (2012). [CrossRef]
26. T. Suga, Y. Takahashi, H. Takagi, B. Gibbesch, and G. Elssner, “Structure of Al-Al and Al-Si3N4 interfaces bonded at room temperature by means of the surface activation method,” Acta Metall. Mater. 40, S133–S137 (1992). [CrossRef]
27. A. Berzanskis, A. Matijosius, A. Piskarskas, V. Smilgevicius, and A. Stabinis, “Sum-frequency mixing of optical vortices in nonlinear crystals,” Opt. Commun. 150(1-6), 372–380 (1998). [CrossRef]
28. G. Molina-Terriza, J. P. Torres, and L. Torner, “Twisted photons,” Nat. Phys. 3(5), 305–310 (2007). [CrossRef]
