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Abstract
We report for the first time on optical waveguides in zinc oxide (ZnO) crystals fabricated by femtosecond laser direct writing. The confocal Raman microscopy under 488 nm laser excitation is used to investigate the micro-modifications of the laser irradiation, and guiding properties are studied via the end-face coupling at 632.8 nm. The mode modulation has been achieved by the adjustment of laser writing parameters. A minimum propagation loss of ∼6 dB/cm is obtained for the double-line waveguide structures. A Y-branch waveguide beam splitter is also fabricated, reaching a splitting ratio of nearly 1:1. The original optical properties in the guiding region have been well preserved, according to the confocal Raman investigation, which suggests potential applications of the ZnO waveguides for integrated photonics and nonlinear optics.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Zinc oxide (ZnO) is a wide-bandgap semiconductor material. Benefiting from its unique properties including good transparency, strong nonlinear optical coefficients, and large excitation binding energy (60 meV at ambient temperature) [1–3], ZnO has become one of the most favorite third-generation semiconductor materials with a wide range of promising applications [3–5], such as blue and UV light optoelectronic devices (e.g., UV light-emitting diodes and UV laser diodes) based on its direct and wide bandgap of 3.37 eV at room temperature [6–9]. It also plays an important role in the fields of biosensors because of the strong surface sensitivity to adsorbate [10–13], which can be used to detect infectious diseases and food freshness. Moreover, the strong nonlinear optical coefficients of ZnO show a potential avenue to construct nonlinear optics devices [14–16]. Jamadi et al. demonstrated an edge-emitting polariton laser operation from 5 to 300 K within ZnO waveguides [7]. Notably, stronger optical confinement can be provided by using optical waveguides, giving a possibility to achieve compact high pumping power densities on-chip light sources, that can be very useful to nonlinear optical devices. Several techniques have been utilized to fabricate waveguides in ZnO including ion implantation [17] and magnetron sputtering [18], but the fabricated ZnO plane waveguides only confine the light in one dimension. It was reported that electron-beam lithography can also produce ZnO channel waveguides with very precise scale control of widths 0.80–1.53 µm [15], but the relatively high propagation losses limit its applications for functional devices.
Femtosecond laser direct writing (FsLDW) is a flexible and cost-effective three-dimensional technique for materials modification and device fabrication [19–21]. By tightly focusing a femtosecond laser beam inside a transparent material to achieve modification of localized refractive index at micron-scale, it has been widely exploited to fabricate versatile photonic structures for diverse applications [22–24]. In 1996, the first optical waveguides produced by FsLDW were reported by Davis et al. [25] on platform of a few glass samples. So far, the FsLDW has been widely used to fabricate various photonic devices in transparent materials [26–30], including waveguide lasers [31–35], frequency converters [36–39], electro-optic modulators [40–42], and quantum chips [43–45], etc., without using any patterned masking. For waveguides fabricated in crystals, type-II configuration is more popular than type-I, because the well preservation of guiding region properties [42,46], and the easily controllable refractive index change of tracks. The waveguides in ZnO crystal may be a suitable candidate for nonlinear optical devices, and possess potential applications to construct low-threshold UV lasers. The fabrication of a low-loss ZnO waveguide is the first possible step to achieve these intriguing applications. The double-line waveguides in TeO2-ZnO glass have been fabricated by using FsLDW [47], while the properties of the glass matrix are significantly different from crystalline ZnO.
In this work, we fabricate the waveguides in ZnO crystals by using FsLDW. We induce tracks with multiple processing steps, and use a confocal Raman microscope to characterize the modifications. And then dual-line waveguides with different scan times and gap spacings are fabricated using the same parameters and characterized. The guiding properties of corresponding waveguides are investigated by the end-face coupling technique. In addition, a Y-branch beam splitter has been fabricated, displaying a nearly equal splitting ratio. The results are helpful for the construction of ZnO nonlinear optical devices and a better understanding of the mechanism of the formation of oxygen vacancy defects.
2. Experimental
The ZnO crystal samples we use in this work are cut to the size of 5×10×2 mm3 and optical polished, the c-axis of ZnO is perpendicular to the 5×2 mm2 surface. A femtosecond laser system (FemtoYL TM-25, operating at a central wavelength of 1030 nm, 400 fs pulse width, and 25 kHz repetition rate) is employed to writing the ZnO sample with speed of 0.1 mm/s (therefore multiphoton absorption instead of two-photon absorption worked in FsLDW process caused by the wide-bandgap of ZnO). A schematic diagram of modification in ZnO crystal by using FsLDW is shown in Fig. 1. The laser beam is focused to the depth of ∼100 µm below the sample surface by a 50× objective lens (Sigma, N.A. 0.45), of which sample is placed on a computer-controlled XYZ stage. Then a half-wave plate and a Glan-Taylor prism are combined to control the output power and polarization of the laser beam. The pulse energy of femtosecond laser reaching sample surface is estimated to be ∼28 µJ. The laser polarization is of the y-direction, which is parallel to the direction of sample translation. Thus, single tracks have been induced by using multiple process steps as shown in Fig. 2. The corresponding double-line waveguides are fabricated by utilizing the same writing configuration (WGs 1-5 as shown in Fig. 3). The femtosecond laser pulses lead to a negative refractive index change in the irradiated area of ZnO (type-II modification), and the guiding zone between two irradiated tracks is with relatively refractive index increment due to the stress-induced effects [48]. To study the mode modulation properties, the double-line waveguides with different gap spacings (WGs 6–8 as depicted in Fig. 5) have been also fabricated. The samples are optically polished after the irradiation process to reduce unnecessary scattering loss of two end-faces. Then the cross-section geometries are measured by using the metalloscope (Axio Imager, Carl Zeiss), and more detailed images are obtained by the microscope objective (EC Epiplan-Neofluar DIC, 100×, N.A. 0.90, Carl Zeiss).
Fig. 2. Optical microscope cross-section images of single tracks subjected to different numbers of scans are shown in (a). The spatial distribution images of Raman peak intensity at 576 cm−1 for 1–5 scans are demonstrated in (b). Raman spectra of irradiated ZnO by FsLDW with different scan times are illustrated in (c), the corresponding data points are marked with the cross symbols of the same color as depicted in (a).
Fig. 3. (a) End-face microscope images of WGs 1–5. (b) Corresponding near-field mode distribution images are obtained under TM-polarization at 632.8 nm, the dashed lines next to the guiding area represent the spatial locations of tracks induced by FsLDW. Scale bars, 10 µm.
To characterize the irradiated ZnO sample, the confocal Raman microscope (alpha300_R, WITec) is employed at room temperature. The 488 nm continuous-wave excitation laser is focused on a spot in the cross-section by the microscope objective (EC Epiplan-Neofluar DIC, 100×, N.A. 0.90, Carl Zeiss), of which the direction of the excitation laser is along the c-axis of the ZnO crystal. The scattered Raman light from this spot is collected with the spectrometer (UHTS300S VIS, WITec) with 600 lines/mm grating after passing through a pinhole aperture. Using this experimental setup, the Raman spectra of each pixel on the image in the scanned area, including bulk, irradiated area, and guiding zone, are obtained. The obtained Raman data are further processed with software Project FIVE 5.2, allowing a 2D spatial mapping display of peak intensity and frequency shift.
The end-face coupling system is arranged at wavelength of 632.8 nm from a He-Ne laser to explore the guiding properties of the waveguides. Two microscope objectives (Daheng Optics, 20×, N.A. 0.40) are equipped to couple the incident light into the waveguides and collect the output light into the photodetector. The powermeter (S121C, Thorlabs) and CCD camera (RayCi) are employed to measure the output power and record near-field modal distribution, respectively. In addition, a half-wave plate situated right after the laser is utilized to control the polarization of incident laser beam, so that the relationship between the output power and input light polarization of the waveguides can be acquired.
3. Results and discussion
Modifications in ZnO are induced by scanning multiple times (multiple pass operations). As shown in Fig. 2(a), the irradiated area expands as the scan times increase. It can be seen that the induced tracks on the cross-section are ellipse-like shape lines for ∼30 µm along the laser incident direction, which is caused by the asymmetry of focal volume. Figure 2(c) shows the confocal Raman spectra obtained in the irradiated area with 1–5 scans. There are two main eigenmodes of bulk ZnO crystal which located at 101 cm−1 [E2(low) vibration mode] and 437 cm−1 [E2(high) vibration mode], respectively [49]. After the laser irradiation, broad vibration absorption bands of 80–200 cm−1 and 526–600 cm−1 appear, indicating that defects have been created as a result of FsLDW operation. The band from 80–200 cm−1 is caused by interstitial zinc defects (Zni), while the band from 526–600 cm−1 is caused by oxygen vacancy defects (Vo) [50]. As one can see in Fig. 2(c), the Raman intensity of bands 80–200 cm−1 and 526–600 cm−1 rises as scan times increase, while the peak 437 cm−1 decreases, and then reaches saturation after 3 scans. This result is obvious that the original bulk area is damaged by laser irradiation, and a portion of the ZnO transforms into defects. The peak E2(high) is associated with the original property of ZnO crystal, therefore the peak E2(high) decreases, while the band 526–600 cm−1 increases. Consequently, the amount of defects could be controlled by changing the scan times. One may expect that more precise defects acquirement can be achieved by changing FsLDW parameters (e.g., pulse energy, pulse width, and repetition rate). Moreover, the Raman mapping spatial distributions of intensity at peak E2(high) are depicted in Fig. 2(b), suggesting the area where Vo are generated in ZnO is as same as the track area observed via the optical microscope.
The irradiation of femtosecond laser pulses enables the formation of localized Zn2+ and O2- plasma instantaneously, numerous bonds break. After the laser pulse irradiation, the plasma quickly cools down and the reorganization occurs. Consequently, Zn2+ and O2- ions combine to form nanocrystals in the irradiated area, but not all the ions are reformed, some of them present as Zni and Vo defects which lead to the refractive index decrease in the irradiated area. These defects contribute to the expansion, resulting in the refractive index increment in surrounding area by stress-induced effects.
By using the same parameters of single tracks, the double-line waveguides (based on type-II configuration, labelled by WG 1 to WG 5) are fabricated. The optical microscope images of the double-line waveguides on cross-section are shown in Fig. 3(a), the gaps between the centers of two lines are 30 µm. Most of the double-line waveguides can only support TM-polarization guidance [48,51,52], mainly depending on the system of the crystals [26], which is same for the double-line waveguide fabricated in ZnO crystal. The near-field mode profiles through 1–5 scans illustrated in Fig. 3(b) are obtained, under the same intensity of the incident laser at 632.8 nm. As can be observed, the light confinement of the waveguide becomes more obvious as the scan times increases from WG 1 to WG 3, since the gain stress between the two lines gives rise to the increase of refractive index in the guiding area. Furthermore, the mode area shrinks from WG 3 to WG 5, which is attributed to the guiding area decrease induced by the growth of two tracks.
We characterize the waveguides by the confocal Raman microscope. The characteristic peak of 437 cm−1 in spectra is selected for Raman mapping of WG 3, and the spatial distribution images of Raman peak intensity and frequency shift are shown in Fig. 4. Obviuos differences are appreciated between the irradiated area and unirradiated area (i.e., guiding zone and bulk area). In the irradiated area, the Raman intensity is reduced by 72.5% where the Raman peak experiences the redshift of 0.73 cm−1, corresponding to the refractive index decrease. Moreover, comparing with the bulk area, the Raman peak intensity and frequency shift in the guiding area barely change. This result reveals that the original bulk properties have been well preserved after the FsLDW process, which is of great significance for the construction of efficient ZnO integrated photonic circuits and optical quantum devices.
Fig. 4. The spatial distribution images of Raman peak intensity and frequency shift for WG 3 are obtained at 437 cm−1 in (a) and (b), respectively. Scale bars, 10 µm.
Fig. 5. Double-line waveguides fabricated by FsLDW are shown with different gap spacings. Optical microscope cross-section images of WGs 6–8 are demonstrated in (a), (e), and (i). Optical microscope images from the top of the sample are shown in (b), (f), and (j). Corresponding experimentally measured and simulated near-field TM-mode profiles at 632.8 nm are illustrated in (c) and (d), (g) and (h), and (k) and (l), respectively. Scale bars, 10 µm.
Corresponding maximum refractive index contrasts of waveguides (WGs 1–5) Δn can be approximately estimated by using the following equation [53]:
where θm is the maximum incident angle of the waveguide, any incident light beyond this angle cannot propagate in the waveguide, and n is the refractive index of bulk ZnO. According to the measurement results of θm, the maximum refractive index contrast Δn under different scan times can be determined as listed in Table 1. Also, we investigate the guiding properties of WGs 1–5, the corresponding propagation loss αprop is listed in the table for reference, which can be calculated by subtracting the coupling loss and Fresnel reflection loss from the insert loss (i.e., total loss). The equation can be described as following [54]:
Table 1. Refractive index contrasts and relative optical propagation losses (dB/cm) of WGs 1–5
To further investigate the influence of different gap spacings on mode modulation, 3 times scan is employed to fabricate the following WGs 6–8. The optical microscope images of the cross-section of the WGs 6–8 are shown in Figs. 5(a), 5(e), and 5(i), the gap spacings are 20 µm, 30 µm, and 40 µm, respectively. Optical microscope images taken from the top of the sample are exhibited in Figs. 5(b), 5(f), and 5(j), of which straight lines can be seen clearly without break. Near-field mode profiles of WGs 6–8 are obtained under TM-polarization as illustrated in Figs. 5(c), 5(g), and 5(k), respectively. The light propagates in a single-mode state in WG 6 and WG 7, and a multi-mode state in WG 8. That is to say, the propagation mode of the double-line waveguide can be tailored by simply changing the gap spacing between the two tracks. As for guiding properties, the equations above [see Eqs. (1) and (2)] have been used to calculate the refractive index contrasts and propagation losses, the calculation results are listed in Table 2 for reference. This table shows that the refractive index contrast Δn decreases as the gap spacing rises, since the stress introduced by the expansion of the two tracks is reduced simultaneously. The propagation loss first decreases and then stabilizes from WG 6 to WG 8, which is quite reasonable because a large part of the propagation loss comes from the rough side-walls of the waveguide given by FsLDW. Such a non-perfect boundary serves as a scattering region, which has a stronger side effect on optical guiding properties for more compact waveguides (e.g., WG 6), in comparison with larger-size waveguides (e.g., WG 7 and WG 8). According to the maximum refractive index contrasts as listed in Table 2, and using the finite-difference beam propagation method (FD-BPM), the simulated near-field profile images along the TM-polarization are obtained in Figs. 5(d), 5(h), and 5(l), indicating that the simulated profiles are in good accordance with the experimental results. Additionally, using the parameters of 3 scans and 30 µm gap spacing, the double-line waveguides in ZnO can be fabricated with an optimized propagation loss of ∼6.5 dB/cm. This is mainly attributed to the rough guiding boundaries, therefore, it is difficult to eliminate the FsLDW-produced scattering loss by simply adjusting FsLDW parameters (e.g., pulse width, repetition rate, wavelength, focusing depth, and writing speed, etc.). Thermal annealing may be helpful to further reduce the propagation loss to optimize the guiding performance of the ZnO waveguides.
Table 2. Refractive index contrasts and relative optical propagation losses (dB/cm) of WGs 6–8.
By using the same FsLDW parameters of WG 7, a Y-branch waveguide beam splitter has been fabricated as shown in Fig. 6(c). Figure 6(a) demonstrates the design of the splitter structure, which the length of each straight waveguide segment is set to 3 mm, the length of the two-beam splitting waveguide segment is 4 mm. And the splitting angle (i.e., the angle between the two arms of the beam splitter) is selected to be 0.033°, so the centers of two output beams will be exactly 100 µm apart. Figure 6(b) depicts the experimental near-field 2D profile measured at 632.8 nm, where the guiding mode remains a single-mode feature, the measured output splitting ratio is about 1:1.01 (which is close to the ideal ratio of 1:1). This is because the symmetry structure of the splitter, which shows a good splitting performance of the Y-branch beam splitter. One may expect that arbitrary output splitting ratio can be achieved by further design of the splitter structure (e.g., changing the splitting angle and using an asymmetric splitting structure).
Fig. 6. (a) Schematic plot of the design of the beam splitter, where θ means the splitting angle between the two arms of the splitter. (b) 2D spatial light intensity distribution at the output face. (c) The partial optical microscope image of the Y-branch splitter from the top view.
4. Conclusions
In summary, we have fabricated the optical waveguides for the first time in ZnO crystal by FsLDW. According to the Raman spectra results, the densities of oxygen vacancy defects in ZnO crystal can be roughly controlled by changing the scan times of FsLDW. Based on the Raman mapping and mode profile performance, the original optical properties in the guiding area are well preserved, and the mode profiles can be finely controlled by adjusting the gap spacing between the tracks. A Y-branch beam splitter has been fabricated with a splitting ratio of nearly 1:1. We believe this study throws new light into efficient ZnO nonlinear integrated photonic circuits and optical quantum devices in the future.
Funding
National Natural Science Foundation of China (12174222); Natural Science Foundation of Shandong Province (ZR2021ZD02); Taishan Scholar Project of Shandong Province (tspd20210303).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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