1. Introduction

Diamond has a number of significant optical material properties that make it desirable for a range of applications. For guided wave optics, these properties include low absorption and scattering over a broad spectrum, together with a high refractive index. For integrated optoelectronics, diamond’s high thermal conductivity, high density and large Young’s modulus are also of benefit [1]. These attributes make diamond an excellent material for applications involving high power and/or high frequencies [2]. Furthermore, the ability of diamond to produce single photons, when a lattice impurity is optically pumped [3], leads to applications in quantum key distribution (QKD) and linear optics quantum computing (LOQC) [4,5].

There has been much work done on the dry etching of diamond in recent years with work focussing mostly on reactive ion etching (RIE) [6–13] or inductively coupled plasma (ICP) [14–18] to produce a wide variety of structures in both polycrystalline and single crystal diamond. A multi-mode diamond waveguide was previously demonstrated in single crystal diamond [19], although its length was inherently limited to <100 µm by the focused ion beam (FIB) fabrication process which is consequently not scalable.

This work finalises the results of [19] by examining the modal properties of diamond waveguide ridge structures which permit single-moded operation with large cross-sectional mode area. These structures are fabricated using a scalable method consisting of photolithography, RIE, and the expansion of an ion implantation method to cover large regions of the crystal surface. Guidance is demonstrated and the modal patterns support the modelling suggesting a two-moded structure. This work demonstrates the feasibility of long (mm) single-mode diamond waveguides fabricated through scalable technologies and opens the door to the fabrication of a diamond-based optical chip integrating functional elements such as X-crossings, Y-junctions, evanescent couplers, Bragg reflectors/couplers and various interferometers.

2. Ridge waveguides in diamond

Diamond has a refractive index of approximately 2.39 for λ=532 nm which means that for a square core waveguide (surrounded by silica or air) to be single-moded the cross-section must have dimensions of roughly 200 nm×200 nm. Waveguides of these dimensions cannot be easily butt-coupled to standard optical fibre and are more difficult to fabricate. Ideally the development of diamond doping to give small (~0.01) changes in refractive index would allow larger waveguides of low index contrast to be fabricated in similar manner to that of silica waveguides. For the present, ridge waveguides permit guides in diamond with high contrast to be microns wide whilst maintaining single-mode operation. Shown in Fig. 1 is a ridge waveguide in diamond surrounded by air. The modes of this waveguide were simulated using the FIMMWAVE FMM real solver [20]. The waveguide has only one TE mode despite the ridge being 3.5 µm wide and having a height of 1.5 µm. The slab thickness beneath the ridge is 2 µm. These ridge dimensions are readily achievable by photolithography and RIE. The airgap shown beneath the waveguide can be created by ion implantation forming a sacrificial graphitic layer beneath the waveguide which can be removed later (see Section 3).

 

Fig. 1. Single TE mode of diamond ridge waveguide

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It can be shown that the substrate leakage loss depends exponentially on the gap thickness [21] through the functional dependence L∝e^-wd where $w=\sqrt{{\beta }^2-k^2{n}_{\mathrm{air}}^2}$, β is the propagation constant, k=2π/λ, and d is the thickness of the airgap.

Figure 2 illustrates the substrate loss dependence of the fundamental mode of the single mode ridge waveguide as calculated using FIMMWAVE [20] together with the analytical approach sketched above. As can be seen the substrate loss is tolerable even for airgaps as thin as 100 nm and drops off exponentially as the thickness increases. Given the airgap thickness produced by ion implantation is around 200–300 nm (which could be increased by multiple implants if necessary) the substrate loss for diamond waveguides fabricated by this method is expected to be negligible.

 

Fig. 2. Plot of substrate loss for varying airgap thickness in diamond ridge waveguide structure. a. Analytical treatment. b. Simulated using FIMMWAVE.

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3. Fabrication method

The samples used are type 1b high pressure high temperature (HPHT) single crystal diamonds purchased from Sumitomo. The samples are approximately 3.5 mm×3.5 mm×1.5 mm in dimension and appear yellow due to their nitrogen concentration (nominally 10–100 ppm).

Initially the sample is implanted with 2MeV He⁺ ions across the surface of the sample on the Blue microbeam line of a 5U NEC Pelletron accelerator. The whole sample was implanted at a fluence of 5×10¹⁷ ions·cm^-2 using a raster scanning ion beam which was focused to a spot size of approximately 5 µm. These ions penetrate the sample to a depth of approximately 3.5 µm resulting in the formation of a buried highly damaged sacrificial graphitic layer for subsequent etching. The depth and thickness of the damaged layer can be controlled with submicron spatial resolution with a proper choice of ion species and energy. The longitudinal straggling of the ions is related to the atomic number of the target element or compound. Materials like diamond with low Z number create sharp profiles. The nuclear collisions creating lattice defects occur mainly at the end of range of the implanted ions [22]. Figure 3 shows the ion-induced damage density profile calculated with “Transport of ions in matter” (TRIM 2006) Monte Carlo simulation code.

After implantation, the sample is annealed at 1100 °C which results in a sharp, well defined interface between the graphitic region and undamaged diamond, while the region between the surface and the graphitic end of range region exhibits a much lower damage density.

 

Fig. 3. TRIM Monte Carlo simulation of the damage density profile induced by 2 MeV He +ions.

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The waveguide structures are then etched into the diamond using the fabrication process described more thoroughly in [13]. A silica mask deposited by PECVD is patterned using photolithography and RIE. The mask pattern is transferred into the diamond through further RIE in a predominantly oxygen plasma and the mask is removed by an HF (hydrofluoric acid) dip. Figure 4 shows an optical microscope reflection image of the surface of the diamond sample once it has gone through the RIE process. High magnification SEM images of similar ridges may be seen in [13].

 

Fig. 4. Optical microscope reflection image of the surface of reactive ion etched sample. 7 ridge waveguide structures can be seen running horizontally across the sample. The waveguides range in length from 1 mm up to 2.7 mm. RIE depth is 1.5 µm.

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Finally the sample is processed using the Focused Ion Beam (FIB) with 30 keV Ga⁺ ions to make 45° cuts into the waveguides to act as total internal reflection mirrors as well as drilling holes through the upper diamond layer to gain access to the 3.5 µm deep layer of graphite for subsequent etching. A 50 nm gold layer was sputter deposited prior to FIB milling to prevent any ion damage to the surface as well as to prevent charging during milling. Figure 5 shows a scanning electron microscope (SEM) image of one of the 45° cuts at one end of the waveguide.

 

Fig. 5. SEM image of total internal reflection mirror cut into one end of the waveguide.

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An electrochemical etch is used to etch the graphite from beneath the waveguides leaving an airgap to provide vertical confinement for the waveguiding structure. A graphite rod and a copper rod were used as electrodes and diluted boric acid was used as electrolyte. Figures 6(a) and (b) show a transmission and reflection optical microscope image of the electrochemically etched sample, respectively. The images clearly show a series of holes that have been milled into the sample with the FIB in order to allow a free flow of acid to remove the graphitic region beneath the waveguide. A diamond waveguide is also shown with a total internal reflection mirror at each end.

 

Fig. 6. Optical microscope images of the waveguide with graphite etched from beneath structure. (a) depicts a transmission image and (b) shows a reflection image. Both image widths are ~1mm.

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4. Results and discussion

The waveguide fabricated has a ridge width of approximately 4.5 µm, roughly 1 µm wider than that of the single-mode waveguide simulated, due to the usage of pressure-contact as opposed to vacuum-contact photolithography. This wider ridge results in a second TE mode for the structure which is shown in Fig. 7 as fabricated with small airgap above the rest of the diamond.

 

Fig. 7. Intensity plot for second order mode of fabricated ridge waveguide.

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In order to optically characterize the structures, an input waveguide coupling and output light-microscope collection system was utilized [19]. Light was coupled into and out of the waveguide via reflection off total internal reflection (TIR) mirrors inclined at 45° with respect to the sample surface. A schematic representation of the setup can be observed in Fig. 8 (a). The mirrors exhibit TIR due to the difference in the refractive index of diamond with respect to air, with the critical angle being ≈25°. When using a high power microscope objective such as 50X, the field of view of the coupling/collection setup was limited to around 100 µm. Hence, in order to validate whether optical guidance could be achieved in RIE-diamond structures, two 45° mirrors were milled either end of a waveguide with a length of around 80 µm.

The propagation of light in the RIE waveguide device was investigated by focusing λ=532 nm laser light, with a long-distance optical objective, to a micrometer-sized spot at the input mirror. The collection of the transmitted light from the output mirror was performed with the same objective, as shown schematically in Fig. 8(a). The waveguiding results can clearly be observed in Fig. 8(b) and (c) by the multi-moded intensity patterns observed at the output mirror. In both Figs. 8(b) and (c), light was coupled in through a mirror in the lower right corner of the figure and the output of the waveguide can be observed at the top left of each image. The outputs for Fig. 8(b) and (c) differ due to a variation in input beam position leading to changed modal excitations. Figure 8(d) (Media 1) shows a magnified image of the output of the diamond waveguide. The video available online shows the same magnified image of the waveguide output and the variation in the modal pattern due to different input launch conditions.

 

Fig. 8. (a) Schematic of the input waveguide coupling and output light-collection system. (b) and (c) show 80 µm section of waveguide with light coupling in through mirror in bottom right and the multimode output from the mirror in top left corner. (d) shows a close up of mode pattern on output mirror, the variation in the waveguide output modal pattern due to differing input launch conditions can be seen online (Media 1).

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5. Conclusion

This paper discusses how a single-mode waveguide can be formed in single crystal diamond and models the substrate loss associated with this method. A diamond waveguide is fabricated which supports two modes and demonstrates the first case of optical guidance in diamond waveguides which have been fabricated using a scalable technique (photolithography and RIE). Ion implantation, and subsequent chemical etching, has been successfully used to provide vertical optical confinement within the diamond ridge waveguide structures. The techniques presented in this paper have the potential to produce large scale diamond waveguides and integrated optical devices in diamond. Future work will focus on the further characterization of these waveguides and the fabrication of other integrated optic structures in diamond.