Waveguide fabrication in KDP crystals with femtosecond laser pulses

Transcription

1 Appl. Phys. A (2015) 118: DOI /s Waveguide fabrication in KDP crystals with femtosecond laser pulses Leilei Huang Patrick Salter Michał Karpiński Brian Smith Frank Payne Martin Booth Received: 3 July 2014 / Accepted: 16 November 2014 / Published online: 30 November 2014 Ó Springer-Verlag Berlin Heidelberg 2014 Abstract Optical waveguides fabricated in potassium dihydrogen phosphate (KDP) by ultrafast laser pulses are demonstrated. Dependent on the incident pulse energy, two different types of refractive inde modification have been induced. For moderate laser powers, type I homogeneous waveguides are created. At higher pulse energies, type II waveguides are formed in the stressed area surrounding regions of laser-induced damage. Double-line and four-line structures are applied to the type II guides to increase mode confinement. Polariation sensitivity and transmission properties of the written waveguides are characteried and discussed. The results indicate that high-quality waveguides can be fabricated in KDP, which has potential for further applications in nonlinear integrated-optics. 1 Introduction Ultrashort pulse direct laser writing (DLW) is a powerful technique that can be used to fabricate optical waveguides and related components for three-dimensional optical L. Huang (&) P. Salter F. Payne M. Booth Department of Engineering Science, University of Oford, Parks Road, Oford OX1 3PJ, UK leilei.huang@eng.o.ac.uk M. Booth martin.booth@eng.o.ac.uk M. Karpiński B. Smith Clarendon Laboratory, Department of Physics, University of Oford, Parks Road, Oford OX1 3PU, UK M. Booth Centre for Neural Circuits and Behaviour, University of Oford, Mansfield Road, Oford OX1 3SR, UK devices in a single monolithic substrate, which eliminates the requirements for complicated manufacturing steps. Ever since the first demonstration of waveguides in glass in 1996 [1], considerable efforts have been made to eplore the potential of higher-quality waveguides with different structures and fabrication parameters in different materials [2 7]. Potassium dihydrogen phosphate (KDP) is a widely used nonlinear optical material with high damage threshold. It is also employed in a variety of frequency conversion applications. Considerable attention has been paid to the fabrication of waveguides in other nonlinear crystals [8], such as active-ion-doped YAG [9, 10], KTiOPO 4 [11, 12], and LiNbO 3 [13 17]. But successful demonstration has not been possible in KDP [18]. One issue affecting KDP in particular is the threshold for modification of refractive inde being close to the damage threshold, i.e., the laserwritten waveguide can be easily damaged during its fabrication. Two types of waveguide design are typically used for single-mode operation in crystalline materials [8]. At lower pulse energy when no visible material damage occurs, a refractive inde increase is created in the modification region at the focus. This type of DLW modification, similar to conventional DLW waveguide fabrication in amorphous materials, is typically called a type I waveguide. Type I waveguides have only previously been demonstrated in very few crystalline materials, notably including LiNbO 3, and have normally been observed to deteriorate with time and disappear after thermal annealing [14]. Type II waveguides, on the other hand, where a higher pulse energy is employed during fabrication, are permanent and thermally stable. Light guiding is observed adjacent to the laser damage tracks in the crystal, with the associated induced stress field believed to create a local refractive

2 832 L. Huang et al. inde increase. A double-line structure is usually used in type II waveguides to obtain more symmetric and wellconfined guiding [13 17]. In this paper, for the first time to our knowledge, we demonstrate both types of waveguides in the KDP crystal. Room-temperature stable type I waveguides and temperature stable type II waveguides have been obtained and studied. Analysis and characteriation of the written waveguides are presented, including refractive inde modifications, transmission losses, polariation-dependent guiding properties, and thermal annealing effects. 2 Eperimental setup Figure 1 shows the schematic diagram of the DLW fabrication system. The femtosecond laser was a regeneratively amplified Ti-sapphire laser (Solstice-100F), which operates at a repetition rate of 1 kh with 100 fs pulses centered at a wavelength of 790 nm. The power of the laser pulses was reduced by a rotatable half-wave plate and a subsequent polarier. The beam was then epanded and directed to a spatial light modulator (SLM: Hamamatsu Photonics X ). The output of the SLM was imaged to the pupil plane of the microscope objective (209, Zeiss Plan Neofluar with a numerical aperture (NA) of 0.5). The specimen was placed and translated on a computer-controlled three ais translation stage (Aerotech ABL10100 and ANT95-3-V). A pinhole was placed in the Fourier plane of the SLM. It was used together with a blaed grating on the SLM to block the ero order while using only the first diffracted order for fabrication. This eliminated interference with unmodulated light from the SLM allowing effective control Fig. 1 Schematic diagram of the ultrafast laser fabrication system. Lens focal lengths are shown in mm. Some intermediate optics have been omitted for clarity of the intensity and phase distribution of light in the pupil plane of the objective lens [19]. In this configuration, the SLM could be used for adaptive aberration correction and beam shaping [20, 21], greatly improving the quality of the fabrication focus. The laser pulses were focused lm below the surface of a 10-mm-thick KDP specimen ( mm), which was cut with the optic ais at 67.5 to the ais, providing the desired phase-matching conditions for type II spontaneous parametric downconversion along the direction [22]. All waveguides were fabricated at a speed of 10 lm/s along the direction (perpendicular to the laser optic ais) at a length of 3 mm. For directly writing type I waveguides, slit beam shaping was applied on the SLM to achieve a symmetric focal disk for fabrication [19]. The cross section for structural modifications in type II waveguides was strongly asymmetric due to the relatively low NA of the objective lens. For characteriation, a laser beam at a chosen wavelength was fiber-coupled into the KDP waveguides and the end facet was imaged to a CCD camera. Although most of the waveguides were characteried at a wavelength of 535 nm, guiding properties were also investigated at 415 and 630 nm with similar results. 3 Results and discussion The properties of the written waveguides were related to numerous fabrication parameters, such as pulse energy, translation speed, and focus shaping. By varying just the pulse energy, two different types of waveguiding structure were first obtained in KDP. 3.1 Type I waveguide Figure 2 shows a single-line type I waveguide fabricated with low pulse energy of 1.05 lj (measured at the back aperture of the objective) at a speed of 10 lm/s, with slit beam shaping. Without slit beam shaping, although the refractive inde was modified, little guiding could be observed due to the highly asymmetric cross section. With slit beam shaping, as shown in Fig. 2b, circular and homogenous waveguide was created by a smooth refractive inde modification in the focal region. It was noticed that careful control of the pulse energy was necessary as the threshold for observed structural modification was very close to the damage threshold. If the pulse energy was too small (lower than 0.97 lj), insufficient refractive inde modification was induced, while too high pulse energy would damage the material (higher than 1.11 lj). In both cases, a negligible transmitted optical mode was observed at the end facet, and waveguide fabrication was unsuccessful.

3 Waveguide fabrication in KDP crystals 833 The type I waveguide in KDP, in contrast to those in LiNbO 3, was observed to be stable at room temperature for a period of over 4 months without any change to either the coupled optical mode or the physical appearance of the waveguide when viewed by a transmission microscope. However, after thermal annealing at 150 C for 2 h, the inde contrast of the type I waveguides deteriorated and light guiding was not possible, as shown in Fig. 2d f The polariation guiding properties of the type I waveguide (before annealing) were investigated by rotating a linear polarier in front of the CCD camera. Results revealed a highly polariation-dependent guiding, with only TM mode (E-field polaried along the y ais) being observed (Fig. 3). This indicates that the laser-induced structural modification constitutes a degree of amorphiation of the crystal lattice, raising the refractive inde relative to the surrounding material for only the TM mode. This further indicates that there may be some reduction of the nonlinear properties of the crystal within these type I waveguides. A total loss of 5.8 db at 535 nm was measured for the type I structural modification guided light of TM mode. Significant losses are mainly attributed to butt-coupling into the waveguides with fiber, since we were not able to polish the KDP crystal following fabrication, due to a lack of suitable equipment. As a consequence, the waveguides terminated 30 um from the end facet, and hence, there were large losses incurred when coupling light into the guide from a fiber. The refractive inde profile for TM light was estimated based on the propagation mode near-field method [23]. The calculated refractive inde profile is shown in Fig. 4a, with a refractive inde difference of 2: at the peak. The field distribution of the fundamental mode for the calculated refractive inde profile is shown in Fig. 4b, with a mean square error of from the measured field profile. These results are in good agreement with measurements based upon quantitative phase microscopy using a polaried source of illumination [24]. 3.2 Type II waveguide (d) (e) (c) Fig. 2 Type I single-line waveguide fabricated at a pulse energy of 1.05 lj and a speed of 10 lm/s, with the slit beam shaping before (a c) and after (d f) annealing. a, d Fabricated waveguide top-down view under transmission microscope; b, e fabricated waveguide cross section under transmission microscope; c, f the near-field intensity distribution for a wavelength of 535 nm at the end of the waveguide TE mode diagonal (f) (c) TM mode Fig. 3 Optical mode of type I single-line waveguide at different polariation. a Horiontal polariation (TE mode); b diagonal polariation; c vertical polariation (TM mode) For pulse energies above 0.17 lj, without any beam shaping (uniformly illuminated objective pupil), tracks were induced that are clearly visible as faults in the KDP crystal lattice. Such tracks were found to guide light adjacent to the region of laser damage in analogy with type II waveguides observed in other crystalline materials [8]. Normally, a double-line structure is applied for type II waveguides to improve mode confinement. Proper separation distance (in the y direction) between two adjacent lines was chosen to achieve optimal guiding, as shown in Fig. 5. If the two lines were too close, a non-uniform of refractive inde distribution would occur between the adjacent stress fields of the two damaged tracks. If the two lines were too far apart, the behavior was similar to a single-line damaged track, whose mode could not be well confined. For our system, 15 lm was found to be the optimal separation distance. It could be clearly seen that the confinement of the transmitted mode of the type II waveguide (Fig. 5c) was not as good as the type I waveguide (Fig. 2c). Three different structures of type II waveguides were therefore investigated in an attempt to improve the mode confinement, as shown in Fig. 6. Based on the result from Fig. 5, the intuitive structures we first tried in order to better confine the mode were four-line structure as shown in Fig. 6a, b. Their resulting optical modes were shown in Fig. 6d, e, which were better confined compared with Fig. 5c. A double-line structure with greater aial elongation in

4 834 L. Huang et al. Fig. 4 Refractive inde modification of type I singleline waveguide. a Calculated refractive inde profile using propagation mode near-field method; b measured and calculated field distribution of the fundamental mode 8μm 10μm (c) 8μm 10μm (d) (e) (f) 8μm 10μm (c) Fig. 5 Type II double-line waveguides written at a pulse energy of 0.25 lj and a speed of 10 lm/s, without slit pupil beam shaping. a Fabricated waveguide top-down view under microscope; b fabricated waveguide side on view; c the near-field intensity distribution for a wavelength of 535 nm at the end of the waveguide Fig. 6c was also investigated. The aial elongation of the laser-induced features was increased by reducing the effective NA of the objective lens using the SLM [19]. Fig. 6 Type II waveguides with different structures. a c Cross section under microscope of different waveguide structures; d f the near-field intensity distribution for a wavelength of 535 nm at the end of respective waveguide structures (a c) Results showed that the three structures ended up with similar guided modes. Offering faster fabrication, a more circular mode, and simpler structure, the elongated doubleline structure (Fig. 6c) was therefore chosen for further investigation. Attention should be paid to choose appropriate dimensions for the elongated focus. The optimal sie applied in our eperiment was approimately 2 lm 9 15 lm, corresponding to an effective NA of 0.3. A less elongated focus would result in worse mode confinement, while a more elongated one would dramatically increase the pulse energy necessary for fabrication. In contrast to the type I waveguide, the properties of the type II waveguide were much less sensitive to the fabrication pulse energy. The type II waveguides were stable at room temperature for over 4 months without any change in the coupled optical mode. After thermal annealing at 150 C for 2 h,

5 Waveguide fabrication in KDP crystals 835 Fig. 7 Type II elongated double-line waveguide after annealing. a Fabricated waveguide top-down view under microscope; b the nearfield intensity distribution at the end of the waveguide TE,before (d) TE,after diag,before (e) diag,after (c) TM,before (f) TM,after Fig. 8 Optical mode of type II elongated double-line waveguide at different polariation before (a c) and after (d f) annealing. a, d Horiontal polariation (TE mode); b, e diagonal polariation; c, f vertical polariation (TM mode). Note that the power coupled into the guide is increased by a factor of 1.85 (d f) unlike the type I waveguide, the type II waveguide remained, but with changes in the intensity distribution of the guided mode Fig. 7b, compared with Fig. 6f. Under inspection with a transmission microscope, it was found that the laser-induced damaged track (Fig. 7a) changed under annealing, revealing much greater non-uniformity along its length. This likely affects the refractive inde profile of the guide, and therefore the guiding mode. The polariation-guiding properties of the type II waveguide (before and after annealing) were also investigated. The type II waveguides were much less polariation dependent relative to the type I guides discussed previously. It is shown in Fig. 8 that before annealing, the type II waveguides guided in both polariation states, but with a propagation loss that was still polariation dependent. After annealing, the waveguide became almost polariation independent, which shows a potential for the application such as the generation of pure heralded single photons [25]. A total loss of 7.8 db at 535 nm was measured for the elongated double-line type II waveguide for circular polariation. Again, this is mainly due to the high coupling loss. An additional 1.3 db loss was observed for guiding after annealing. The refractive inde profiles of the type II waveguide after annealing were also estimated based on the propagation mode near-field method. The calculated refractive inde profile of the type II waveguide is shown in Fig. 9a, with a refractive inde difference of 1: at the peak center. The field distribution of the fundamental mode for the calculated refractive inde profile had a mean square error of from the measured field profile. The error was mainly contributed by the measurement noise from the background around the boundary. 4 Conclusion We have demonstrated femtosecond written waveguides in KDP for the first time. Dependent on the incident fabrication pulse energy, structures supporting both type I (guided mode contained within the fabricated region) and type II (guided mode adjacent to the fabricated region) Fig. 9 Refractive inde modification of type II elongated double-line waveguide after annealing. a Calculated refractive inde profile using propagation mode near-field method; b measured and calculated field distribution of the fundamental mode

6 836 L. Huang et al. waveguides were formed. We note that, to the best of our knowledge, this is the first demonstration of waveguides in the KDP crystal. For type I waveguide, with appropriate pulse energy, smooth and room-temperature stable singleline waveguides have been created. However, the type I waveguides are highly polariation dependent with only the TM mode transmitted. In addition, type I waveguides were found to disappear after annealing at 150 C for 2 h. On the other hand, type II waveguides with an elongated double-line structure persist after annealing and demonstrate polariation-independent waveguiding. They also have the advantage of easy fabrication, compared with type I waveguide which requires careful control over the pulse energy. References 1. K.M. Davis, K. Miura, N. Sugimoto, K. Hirao, Opt. Lett. 21(21), 1729 (1996) 2. G. Della Valle, R. Osellame, P. Laporta, J. Opti. A Pure Appl. Opt. 11(1), (2009) 3. A.M. Streltsov, N.F. Borrelli, JOSA B 19(10), 2496 (2002) 4. S. Nolte, M. Will, J. Burghoff, A. Tünnermann, Appl. Phys. A Mater. Sci. Process 77(1), 109 (2003) 5. A. Ferrer, V. Die-Blanco, A. Rui, J. Siegel, J. Solis, Appl. Surf. Sci. 254, 1121 (2007) 6. M. Ams, G.D. Marshall, P. Dekker, J.A. Piper, M.J. Withford, Laser Photon. Rev. 3, 535 (2009) 7. L.A. Fernandes, J.R. Grenier, P.R. Herman, J.S. Aitchison, P.V.S. Marques, Opt. Epress 20(22), (2012) 8. F. Chen, J.R. Váque de Aldana, Laser Photonics Rev. 8(2), 251 (2014) 9. A.G. Okhrimchuk, A.V. Shestakov, I. Khrushchev, J. Mitchell, Opt. Lett. 37, 2248 (2005) 10. A. Benayas, W.F. Silva, C. Jacinto, E. Cantelar, J. Lamela, F. Jaque, J.R. Váque de Aldana, G.A. Torchia, L. Roso, A.A. Kaminskii, D. Jaque, Opt. Lett. 35(3), 330 (2010) 11. S. Campbell, R.R. Thomson, D.P. Hand, A.K. Kar, D.T. Reid, C. Canalias, V. Pasiskevicius, F. Laurell, Opt. Epress 15(25), (2007) 12. S. Zhang, J. Yao, W. Liu, Z. Huang, J. Wang, Y. Li, C. Tu, F. Lu, Opt. Epress 16(18), (2008) 13. R.R. Thomson, S. Campbell, I.J. Blewett, A.K. Kar, D.T. Reid, Appl. Phys. Lett. 88, (2006) 14. J. Burghoff, S. Nolte, A. Tünnermann, Appl. Phys. A Mater. Sci. Process 89(1), 127 (2007) 15. R. Osellame, M. Lobino, N. Chiodo, M. Marangoni, G. Cerullo, R. Ramponi, H.T. Bookey, R.R. Thomson, N.D. Psaila, A.K. Kar, Appl. Phys. Lett. 90, (2007) 16. J. Thomas, M. Heinrich, P. Zeil, V. Hilbert, K. Rademaker, R. Riedel, S. Ringleb, C. Dubs, J.-P. Ruske, S. Nolte, A. Tunnermann, Phys. Status Solidi A 208, 276 (2011) 17. E. Neyra, S. Suare, G.A. Torchia, Opt. Lett. 39(5), 1125 (2014) 18. D. Paipulas, V. Kudriašov, M. Malinauskas, V. Smilgevičius, V. Sirutkaitis, Appl. Phys. A Mater. Sci. Process 104(3), 769 (2011) 19. P.S. Salter, A. Jesacher, J.B. Spring, B.J. Metcalf, N. Thomas- Peter, R.D. Simmonds, N.K. Langford, I.A. Walmsley, M.J. Booth, Opt. Lett. 37(4), 470 (2012) 20. A. Jesacher, M.J. Booth, Opt. Epress 18(20), (2010) 21. E.H. Waller, M. Renner, G. von Freymann, Opt. Epress 20(22), (2012) 22. P.J. Mosley, J.S. Lundeen, B.J. Smith, I.A. Walmsley, New J. Phys. 10, (2008) 23. K. Morishita, J. Lightwave Technol. LT 1(3), 445 (1983) 24. A. Jesacher, P.S. Salter, M.J. Booth, Opt. Mat. Epress 3(9), 1223 (2013) 25. P.J. Mosley, J.S. Lundeen, B.J. Smith, P. Wasylcyk, A.B. U Ren, C. Silberhorn, I.A. Walmsley, Phys. Rev. Lett. 100, (2008)