1. Introduction

Compact, continuous wave ultraviolet (UV) lasers have been proposed for different applications, as optical data storage, biomedical applications, fundamental spectroscopic research, and laser lithography. Since there are no diode-lasers emitting in the deep UV, the only way to produce coherent radiation in this wavelength range consists in the optical frequency conversion of solid-state lasers in nonlinear optical crystals which are transparent in the UV, as β-BaB₂O₄ (BBO) or other borates. Frequency conversion in resonant cavities represents the most common method for achieving the UV wavelength range, but such laser devices are bulky and expensive. In contrast, channel waveguides, maintaining high power density over long interaction length, offer the possibility of achieving high conversion efficiency in a single-pass scheme, and constitute the fundamental premise for the realization of cw compact-all-solid-state lasers emitting in the deep UV. Among the borates, BBO possesses the highest nonlinearity (d₂₂ = 2.3 pm/V) [1, 2] combined with a wide transparency range (190–3300 nm) and a high damage threshold (> 5 GW/cm² at 532 nm for ns pulses). Despite the advantages of the waveguide approach, the realization of integrated frequency converters based on BBO waveguides has been hindered so far by fabrication issues, mostly related to the hygroscopic nature of the borate crystals.

Recently, we have realized optical waveguides in BBO combining different techniques. Planar waveguides were produced by He⁺ ion implantation while the structuring of channel waveguides has been realized by photolithography followed by ion-etching. Furthermore, we have demonstrated the suitability of these waveguides for second harmonic generation in the UV [3] and electro-optic modulation [4]. In our previous work, the maximum output power (∼ 30 μW) in the BBO waveguides was achieved at 278 nm. Here, we present the results of optical frequency conversion in BBO waveguides designed to achieve maximum frequency conversion at 266 nm, with an UV output power an order of magnitude higher. Compact, stable and powerful laser sources at 532 nm are commercially available and could be efficiently combined with BBO waveguides to obtain the UV cw output power of the order of mW, suitable for many aforementioned applications.

This paper is structured as follows. Section 2 describes the fabrication and characterization of planar and channel BBO waveguides with a particular attention on the annealing process. The nonlinear optical properties of the He⁺ implanted waveguides are presented in Section 3, showing the influence of the ion implantation on the depth-profile of the nonlinear optical coefficient d₂₂. Section 4 presents the results of the second harmonic generation of 266-nm laser light in the fabricated ridge waveguides.

2. Fabrication of planar and ridge optical waveguides

Ion implantation is a well known technique for the fabrication of planar optical waveguides [5, 6]. We irradiated the BBO crystals with He⁺ions in order to produce a partially amorphous layer buried a few micrometers below the crystal surface [3]. This layer has a decreased refractive index and acts as an optical barrier. In the past, planar optical waveguides in BBO crystals have been successfully realized implanting different ions, such as He⁺ [7, 8], O⁺ [9], or Si⁺ [10]. However, in none of these studies the nonlinear optical properties of such waveguides have been investigated. Also, channel waveguiding structures were not reported because of the difficulties connected with the lithographic structuring of hygroscopic BBO crystals. We decided to irradiate the BBO crystals with He⁺ ions, since He⁺ implantation has been reported to produce a stable optical barrier in other non linear crystals, as KNbO₃ [11] or Ca₄GdO(BO₃)₃ [12, 13], preserving the nonlinear properties in the guiding region. All BBO crystals were implanted at room temperature at an angle of 10° to the normal direction to the x polished surface to avoid channelling effects. We irradiated several BBO crystals with He⁺ ions varying the ion energy and the ion fluence, which represent the principal process parameters determining the position of the optical barrier and the width of the barrier, respectively. The samples were irradiated with fluences varying from 5 to 15 ∙ 10¹⁵ ions/cm². Implantation with fluences lower than 5 ∙ 10¹⁵ ions/cm² would lead to an insufficient optical confinement, mostly for the fundamental wave, due to the low refractive index contrast (δn/n < 1.5 ∙ 10^-2). On the other hand, implantation with fluences higher than 15 ∙ 10¹⁵ ions/cm², would increase the density of lattice defects and color centers in the guiding region, which are responsible for the absorption losses. We performed several implantations varying the energy of the He⁺ ions from 1.6 MeV to 2.2 MeV, corresponding to a depth of the optical barrier between 3 μm to 5 μm, respectively. The optimum thickness of the BBO waveguides is a trade-off between the propagation losses and the cross-sections of the interacting modes. Waveguides with a depth lower than 3 μm do not provide sufficient optical confinement for the pump modes, which undergo high tunneling losses. In contrast, the interacting modes in thicker (> 5 μm) waveguides have larger cross sections and thus lower intensities, leading to low conversion efficiency.

In order to characterize the planar waveguides fabricated with the implantation process, the effective indices of both TE and TM guided modes were measured at a wavelength of 633 nm with the prism coupling technique [14]. We reconstructed the profiles of both ordinary and extraordinary refractive indices in the guiding layer using an iterative numerical algorithm which minimized the differences between the measured and calculated effective mode indices [5, 15]. An example of a reconstructed refractive index profile for He⁺ implanted BBO crystal is presented in Ref. [3]. The reconstruction of the refractive index profile is also fundamental for the simulation of the mode profile. A comparison between simulated and measured mode profiles obtained in similar waveguides is presented in Ref. [4].

Ridge-type optical waveguides were fabricated by photolithography and ion etching. Smooth waveguides with ridges as high as 2 μm and widths varying from 2 μm to 5 μm have been successfully produced by using SU8 negative epoxy photoresist in the lithography process, and subsequently etching the samples in a plasma chamber with Ar⁺ ions. A microscope front view picture of a BBO crystal implanted with an ion energy of 2 MeV and a fluence of 1 ∙ 10¹⁶ ions/cm² is presented in Fig. 1 and shows the input polished facet: the ridge structures and the optical barrier are clearly distinguishable. The optical barrier is positioned ∼ 4 μm below the top surface, in agreement with our reconstruction of the refractive index profile.

Although the bulk material is optically transparent at the wavelength range of interest, the waveguide fabrication itself introduces several sources of loss. These can be divided into tunneling losses, absorption center losses, and scattering losses. Tunneling losses in these waveguides arise from the fact that the light is confined by an optical refractive index barrier of a finite width. Absorption losses are caused by color centers, ion dislocations in the crystal lattice, produced in the guiding region during the implantation process. After irradiation, the fabricated planar waveguides present high transmission losses, which increase with the implantation fluence. Crystal lattice defects and color centers are particularly relevant in the UV range, and can be partially reduced by annealing. In order to characterize the effect of annealing on the implanted crystals we performed a series of transmission measurements varying the annealing temperature. The transmission losses for TM polarized light of a waveguide (a ridge waveguide with height of 1.3 μm and width of 5 μm, implanted with He⁺ ions with energy of 2.2 MeV and fluence 1∙10¹⁶ ions/cm²) have been measured at 532 nm via end fire coupling. The incoming light at 532 nm was coupled in and out of the waveguide using microscope objectives with 10.5 mm focal length and assuming a coupling efficiency of 0.7. For each measurement point the crystal was annealed for 8 hours at temperatures between 150° C and 350° C, and then slowly cooled down to room temperature. The results of the transmission losses measured after each step are presented in Fig. 2. The propagation losses decreased from an initial value of 8 cm ^-1 (no annealing) to a minimum value of 1.3 cm^-1 after annealing at 300° C. This can be explained by the fact that annealing partially repairs the lattice damage produced in the guiding region by the implantation process. A further increase of the annealing temperature does not improve the transmission in the waveguide and after having annealed the crystal at 350° C the propagation losses started to increase again. This effect was attributed to the partial annealing of the optical barrier, and thus to the increase of the losses by tunneling in the bulk crystal. A similar effect was already observed in He⁺implanted LBO crystals [16] and it can be explained by the fact that the the refractive indices in the barrier region do not reach a complete amorphization after the implantation.

 

Fig. 1. Typical front view picture of the waveguides produced by lithography and etching, having widths varying from 3 to 5 μm, and a height of 1.7 μm. The guiding region and the implanted barrier are clearly distinguishable.

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Fig. 2. Measured transmission losses for TM polarized light at 532 nm in a channel waveguide with a height of 1.3 μm and a width of 5 μm, upon thermal annealing. The error bars originate from the indetermination of the optical elements in the end-fire coupling set-up. The BBO crystal was implanted with an energy of 2.2 MeV and a fluence of 1∙10¹⁶ ions/cm²

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Fig. 3. Total propagation losses for TE₀₀ mode at 532 nm in a waveguide with a height of 1.3 μm, as a function of the ridge width. The losses increase when the ridge width decreases because of the sidewall scattering.

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Finally, the last source of losses are the scattering losses, which arise from imperfections at the waveguide boundaries, especially at the air/guide boundary. We attributed the scattering to the sidewall roughness of the ridge waveguides, which was measured with the atomic force microscope ≤ 15 nm (rms). In order to identify in more details the influence of the scattering losses, several ridges with different widths (1 μm to 5 μm) and same height (1.3 μm) were structured by photolithography on top of a BBO crystal implanted with an ion energy of 2.2 MeV and a fluence of 5∙10¹⁵ ions/cm². After having performed annealing we measured the propagation losses for TE modes, since the effective index of the first TE mode is higher than the bulk refractive index, which implies no tunneling losses. The total propagation losses can thus be attributed to the absorption caused by lattice defects and color centers, not reduced by the annealing process, and to the scattering from the ridge sidewalls. While the first contribution is independent from the channel width, the second one depends on the roughness and on the channel width, according to the theory described in Refs. [17, 18]. Fig. 3 shows the transmission losses for TE polarized light measured at 532 nm with the end-fire coupling technique. Transmission losses lower than 4 dBcm^-1 can be achieved for waveguides wider than 5 μm, or with side walls smoother than 5 nm rms. By further increasing the ridge width, the transmission losses are mainly determined by the absorption centers, which increase for higher implantation fluence. It is important to mention that in these waveguides the optical confinement for TE modes is produced by the combined contribution of two effects: an increase in the refractive index in the guiding region, and the refractive index barrier. TM modes, for such low fluence, experienced transmission losses as high as 24 dBcm^-1, because of the poor optical confinement (δn/n = 1.5 ∙ 10^-2) provided only by the refractive index barrier positioned at the end of the guide.

3. Nonlinear properties of ion implanted BBO waveguides.

In order to assess the influence of He⁺ implantation on the nonlinear optical properties of BBO waveguides, we performed a direct measurement of the nonlinear optical coefficient d₂₂ using a technique already presented in [19, 20, 21, 11, 22]. The experimental arrangement is shown in Fig. 4. An x-cut BBO crystal was implanted with He⁺ ions with an energy of 2 MeV and a fluence of 2∙10¹⁶ ions/cm². The resulting barrier was centered at a depth of 4.1 μm. The sample was polished at an angle of 1° to enhance the resolution depth of the measurements. The 175-fs long light pulses emitted at 1176 nm from a laser system operating at 1 KHz repetition rate (a travelling-wave optical parametric amplifier of superfluorescence TOPAS pumped by a Ti:sapphire laser Clark MXR CPA-2001) were focused with a 5X microscope objective onto the sample and the resulting reflected second harmonic signal was detected by a silicon avalanche photodiode. Since the reflected fundamental and second harmonic signals had the same direction and polarization (parallel to the y axis), the reflected fundamental wave was filtered out with band pass filters and an interference filter at 588 nm. The lower surface of the crystals was polished at an angle of 7° to avoid a second harmonic signal reflected back from the bottom. We performed several scans by moving the sample along the polished surface, perpendicular to the incident beam, while simultaneously detecting the generated second harmonic signal, which is proportional to d² ₂₂. In this way it was possible to reconstruct the relative decrease of the nonlinear optical coefficient d ₂₂ in the implanted region of the crystal. Figure 5 presents a typical result of such a scan. As expected, the maximum second harmonic signal is observed in the unperturbed bulk material as shown in the left part of the graph (area 1). The minimum of the second harmonic signal clearly appears in the barrier at ∼ 4 μm depth (area 2), where it decreases to ∼ 65% with respect to the value of the bulk material underneath. The adjacent guiding region (area 3) is marginally affected by the implantation process and ∼ 90% of the bulk nonlinearity (d₂₂) is preserved. This can be explained by the fact that the crystal lattice is predominantly damaged by the nuclear energy loss of the impinging light ions, which mainly occurs at the end of the ion track. Consequently, the linear and nonlinear optical properties of the implanted crystal are perturbed much more in the barrier than in the waveguiding region. The spatial resolution in the implantation direction (x) of these measurements is ∼ 0.25 μm, given by the diameter of the focused laser spot (15 μm) and by the wedge angle (1°). In order to increase the contrast between implanted and non-implanted regions in the crystal, these measurements were performed on a BBO crystal irradiated with high fluence (2∙10¹⁶ ions/cm²). As a consequence, the measurements presented in Fig. 5 are referred to a strongly implanted, not annealed sample, and yield rather an upper limit for the reduction of the nonlinear optical coefficient d₂₂ in the BBO waveguides used for SHG. An attempt to perform the annealing process on this sample ended up with a damaged crystal surface preventing further measurements of the d₂₂ profile.

 

Fig. 4. Experimental arrangement for the measurement of the nonlinear optical coefficient d₂₂ in He⁺ implanted waveguides

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Fig. 5. Reflected second harmonic signal from the wedged He⁺ implanted BBO crystal. Different regions of the implanted crystal are denoted as: 1-bulk material, 2-barrier region, 3-waveguiding region.

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4. Second Harmonic Generation at 266 nm

To generate efficient second harmonic power at a particular wavelength in waveguides, it is fundamental to compensate the mismatch between effective indices in the guide and refractive indices in the bulk material. Since temperature tuning is not effective, because frequency doubling is relatively insensitive to temperature variations in BBO compared to angle tuning, we compensated this shift in the phase matching conditions by varying the θ angle.

In fact, for temperatures T close to the phase matching temperature, the vector mismatch δk can be expanded in power series of T. In the case of frequency doubling this is given by:

where n^ω₀ and n ^2ωpm _e are the ordinary and extraordinary refractive indices at the fundamental and second harmonic wavelength, respectively, and n_e(θ) is:

The acceptance temperature (FWHM) for a crystal with length L, ΔT ∙ L, is then, according to Ref. [1]:

Analogously, it is possible to derive ∂Δk/∂θ and consequently the acceptance angle. The ratio between acceptance angle and acceptance temperature has been measured in [1] to be 43.7±0.1 μrad/°C for type I second harmonic generation at 266 nm, which means that to compensate a mismatch of 0.2° in the θ angle, the temperature should be changed by ∼ 80 °C.

We irradiated BBO crystals with a different off-axis cut (θ = 49.5°, ϕ = 90°) respect to the standard cut for type I second harmonic generation at 266 nm in the bulk material (θ = 47.6°, ϕ = 90°), according to the previous measurements of the effective mode indices in BBO waveguides, already presented in [3]. The crystal orientation for for type I second harmonic generation in BBO waveguides is reported in the inset in Fig. 6. In addition, in order to have some flexibility, we decided to finely adjust the phase matching conditions, by varying the orientation of the ridges on top of the crystal. Thus, the waveguides were structured at different angles in the interval θ=48.5° to 0=50.5°, instep of 0.2°. The BBO crystal was implanted with two different energies, 2.1 MeV and 2.05 MeV each with a fluence of 7∙10¹⁵ ions/cm², in order to produce a double barrier positioned 4.2 μm below the crystal surface, and, consequently, to reduce the tunneling transmission losses of the guided light. The final ridges fabricated have a width of 2 μm and a height of 1.5 μm. After annealing, their optical losses for TM polarization at 532 nm and TE polarization at 266 nm were reduced to less than 5 dBcm^-1 and ∼ 8 dBcm^-1, respectively. The second harmonic experiments were carried out using a solid state cw laser emitting at 532 nm (Laser Quantum, model Ventus) with more than 1 W output power and spectral bandwidth of 0.07 nm. Additionally, a continuous wave dye laser with a spectral bandwidth of 0.04 nm (Coherent, model CR-599-21), was employed as a tunable source from 526 nm to 570 nm, to select the waveguides with maximum phase matching at 532 nm at room temperature. We used microscope objectives with focal lengths of 4.5 mm, and 12 mm to couple the light in and out of the waveguides assuming a coupling efficiency of 0.7. We measured the second harmonic power after filtering out the fundamental wave transmitted through the waveguide. The second harmonic power at 266 nm as a function of the fundamental input power is shown in Fig. 6, together with the best quadratic fit. We measured a maximum UV output power of 0.32 ± 0.01 mW with a fundamental input power of 670 ± 5 mW. This corresponds to an internal conversion efficiency η = P _2ω/P ² _ω of 0.07 % W^-1, similar to our results presented in [3] for the second harmonic generation at 555 nm. CTI 7523.3 NMPP-NM. It is important to mention that in Fig. 6 there is no deviation from the theoretical linear trend expected in the double logarithmic plot. Furthermore, the maximum output power of 0.32 mW at 266 nm was limited in these measurements by the input power of the fundamental source. A compact 1 mW power light source emitting at 266 nm could be realized if the internal pump power would be increased to 1.2 W.

 

Fig. 6. Second harmonic power at 266 nm generated in a BBO waveguide, as a function of the fundamental internal input power at 532 nm. Crystal temperature T = 32.1 °C. The dashed line represents the best curve which approximate the quadratic dependence of the second harmonic power data as a function of the internal fundamental wave power. In the inset is shown the crystal orientation for type I second harmonic generation in BBO waveguides: the propagation direction is along β = 2πN_eff/λ where N_eff is the effective index of the mode, and a, b, c are the main crystallographic axes.

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The conversion efficiency could be further improved. Because of the asymmetry in the profiles of the ordinary and extraordinary refractive index, already presented in [3], the TE modes are more shifted towards the implanted barrier than the TM ones. Therefore, this asymmetry results in a reduced overlap between the two modes, and, in turns, it affects the second harmonic output power. This effect can be reduced by structuring higher ridges to obtain a more efficient lateral confinement, while maintaining the optical barrier always at the same depth ∼ 4 μm below the crystal surface. According to our calculations, the fabrication of ridges higher than 2 μm would yield an increase in the UV output by a factor higher than three. In addition, the fabrication of higher ridges would allow to use larger implantation depths, and to irradiate the crystals with lower fluences without increasing the tunneling losses. We estimate that combining all these factors, it could be possible to achieve an output power larger than 1 mW with a fundamental input power lower than 1 W.

5. Conclusion

In conclusion, we have demonstrated the second harmonic generation at 266 nm in waveguides with a length of 9 mm, a width of 2 μm and a height of 1.5 μm, with a maximum UV output power of 0.32 mW at 266 nm, which represents an increase of an order of magnitude compared to our previous results. Furthermore we have reported also on the linear and nonlinear properties of BBO channel waveguides fabricated by He⁺ implantation, photolithography and Ar⁺ sputtering. A thorough study of the transmission losses has been carried out, and the different sources of losses have been determined. Particularly, the effect of annealing on these waveguides was found to be very important for achieving efficient optical conversion.