1. Introduction

Integrated optical systems allow combining highly optimized photonic building blocks into functional circuits with small footprints and reproducible characteristics [1]. Their inherent stability and scalability make them particularly attractive for applications in telecommunications [2,3], metrology [4,5] and sensing [6,7]. Besides wide-spread use in classical optics, nanophotonic circuits are also gaining increased relevance for quantum photonic applications due to their potential of operating in the single photon regime [8–10]. Building on established fabrication routines transferred from microelectronics, photonic circuits are nowadays realized with high yield and reproducibility. Because sub-wavelength devices are often required, electron beam lithography with subsequent dry etching is a frequently used approach for realizing such photonic devices [11]. Yet these fabrication procedures lead to quasi-planar geometries and prevent access to the third dimension which is required for advanced optical components such as broadband polarizers, three-dimensional photonic crystals or functional metamaterials. New lithography techniques which enable arbitrary geometries to be devised in three-dimensions are increasingly sought after. Direct laser writing (DLW) is among the most promising approach for generating three-dimensional photonic structures. Voxel-wise addressing provides the means to create photonic devices which contain both in-plane and out-of-plane components and are therefore out of reach for traditional planar fabrication techniques. While DLW removes restrictions on the desired geometry, the combination with functional materials is still challenging because to date DLW is carried out predominantly in organic polymers, glass or crystalline materials [12–17]. This in general limits the operational wavelength range of the devices based on DLW, and also may lead to reduced mechanical stability in the case of polymeric structures. In order to extend material options, inversion-based strategies have been developed to transfer organic resist structures to other materials using atomic layer deposition and infiltration techniques. Alternatively, DLW can be carried out directly in transformable materials which inherently provide the desired material characteristics.

Here, we break new grounds in a different direction: we implement DLW in large-scale polycrystalline diamond samples and demonstrate functional waveguides allowing single mode guiding in the near-infrared wavelength regime at the telecommunication C-band. Previous studies on waveguide fabrication with DLW were limited to single crystal diamond [18–21]. Even though single crystal substrates offer a high purity, the fabrication of large samples is challenging and limits its applications. Polycrystalline diamond in contrast can be fabricated in wafer-like sizes with high optical quality and reproducibility at low cost compared to single diamond substrates. Furthermore, the ability to grow large-area polycrystalline diamond thin films on different insulating substrates as for instance SiO₂, makes it attractive for on-chip photonic circuits [22,23]. To date diamond photonic devices have exclusively been fabricated using planar nanofabrication to realize a range of photonic components including optical cavities, photonic crystals, optomechanical resonators and frequency combs. However, the recent interest in three-dimensional quantum photonic structures such as quantum simulators exploiting Boson sampling, random walk structures and coupled waveguide devices requires appropriate fabrication tools that enable true three-dimensional integration. Our approach allows combining the geometric flexibility of DLW with the material advantages of polycrystalline diamond to yield a rich platform for broadband operational photonic circuitry. The use of polycrystalline diamond for DLW holds promise for both classical and quantum photonic systems to be realized in a flexible and scalable fashion.

2. Methods

Diamond substrate fabrication: Direct laser writing was carried out on polycrystalline diamond substrates prepared by microwave plasma enhanced chemical vapor deposition (PECVD) in ellipsoidal cavity reactors with a microwave frequency of 2.45 GHz. CVD diamond layers with thicknesses of 300 µm were grown on silicon templates and afterwards surface treated by chemomechanical polishing. After dissolving the silicon template, the diamond layers were laser-diced into 3×3 mm² pieces.

Waveguide fabrication in polycrystalline diamond: Type III depressed cladding waveguides were fabricated using a femtosecond Ti-sapphire laser system (Coherent Legend) with a central wavelength of 800 nm, a pulse duration of 115 fs and a repetition rate of 1 kHz. Focusing the laser pulse into the diamond substrate by using a high numerical aperture (NA) microscope objective (Nikon 100x, 0.8 NA) exceeds the field for non-linear absorption and results in permanent structural modification [24]. Three-dimensional permanent structures are induced by moving the sample with a high-precision translation stage (Aerotech ALS 130H) relative to the beam focus (c.f. Figs. 1(b) and (c)). A sketch of the induction scheme is displayed in Fig. 1(a). Previous studies which have identified the structural modification by Raman-spectroscopy arising from diamond-graphite phase transitions leads to a highly localized tensile stress in the surrounding area [25]. This enables the fabrication of Type III depressed cladding waveguides, allowing both, s- and p-polarized light guiding [19,26,27]. Note that due to so called filamentation [28] and spherical aberration, caused by a refractive index mismatch of air and the diamond substrate we obtain axially elongated filaments. This in general can be overcome by applying costly adaptive optics techniques to compensate for the arising aberrations. Here, to induce the desired waveguide profile, the geometrical dimensions of the elongated filaments are analyzed with respect to the writing pulse energy and the nominally addressed depth. Single filaments are then placed on an ellipse forming the desired circular waveguide core. Each waveguide consists of approximately 20 individual filaments. The waveguide structures are induced 90 µm beneath the substrate surface. We have successfully achieved optimal induction conditions of our waveguide structures by performing an extensive parameter scan with respect to the propagation properties for pulse energies between 20 nJ to 30 nJ at a scanning speed of 30 µm/s.

 

Fig. 1. Three-dimensional diamond photonic waveguide fabrication: (a) sketch of the scheme, (b) photography of the processed sample exhibiting numerous waveguide structures, (c) bright-field transmission image of an exemplary laser-induced material modification inside the diamond substrate.

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Waveguide characterization: The induced waveguide structures are analyzed in the near-infrared C-band by a superluminescent diode (SLED). The laser light is guided by a SMF28 fiber from the source and fiber-butt coupled to the laser written diamond photonic waveguides (c.f. Fig. 2(b)). A fiber squeezer controls the polarization of the laser light which is set to vertical polarization. The processed sample is mounted on a 4-Axis stage enabling translation in the x- and y-directions and the correction of the polar and azimuthal tilts. The guided near field profile is imaged by a microscope objective (Melles Griot 25x, 0.5 NA) onto a CCD camera (c.f. Fig. 2(c)). After replacing the camera with a photodiode (PD) the transmitted intensity is measured and the insertion losses are characterized. A pinhole in front of the PD prevents unguided light hitting the detector. A sketch of the full characterization set up is depicted in Fig. 2(a).

 

Fig. 2. Three-dimensional diamond photonic waveguide characterization: (a) schematic of the characterization setup (for simplicity the microscope at the sample position is not illustrated), (b) top view microscope image at the front-facet depicting the cleaved SMF28 fiber and the processed diamond sample, (c) end-facet of the type III depressed cladding waveguides, over layered with the measured guided near-field mode profile for a core diameter of 20 µm and 25 µm, (d) top view microscope image of a fabricated waveguide.

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3. Results and discussion

We have investigated the guided mode profiles and insertion losses of laser-induced diamond waveguides for several fabrication parameters including induction pulse energies and waveguide core diameters. To calibrate the system with respect to the spatial dimensions and to control the quality of the excitation laser beam, we measured the mode profile guided by the SMF28 fiber (c.f. Fig. 3(c1)). The observed mode profile is Gaussian shaped and suitable to excite the fabricated diamond waveguides. For a waveguide having a core diameter of 20 µm induced with a pulse energy of 20 nJ, we observed single mode guiding with an overall insertion loss of -16.4 dB (c.f. Fig. 3(a1)). Increasing the core diameter to 25 µm leads to multimode guiding (c.f. Fig. 3(b1)). Furthermore, we note a decrease in the insertion loss of 30%. Keeping the core diameter constant at 20 µm and increasing the induction pulse energy to 25 nJ we do not find a significant change in insertion losses but multimode guiding with two hot spots (c.f. Fig. 3(a2)). Increasing the pulse energy further to 30 nJ results in an increased loss of 12% retaining a multimode guiding behavior (c.f. Fig. 3(a3)). Keeping the core diameter constant at 25 µm and increasing the induction pulse energy to 25 nJ reveals dramatically higher insertion losses of 80% (c.f. Fig. 3(b2)). When increasing the pulse energy to 30 nJ, the insertion losses are 20% higher compared to an induction energy of 20 nJ. For a larger core diameter, lower coupling losses are expected, resulting in lower insertion losses which is in good accordance with the experimental results. We found waveguides with a core diameter of less than 20 µm not being able to guide light. This can be ascribed to the cutoff wavelength for the given modulation strength. Furthermore, we note a dependence of the guiding properties on the induction energy. On the one hand the insertion losses increase for higher induction energies. On the other hand a change from single mode guiding to multi-mode guiding is observed. An increased pulse energy is expected to induce a higher tensile stress field which leads to an increased effective refractive index modulation. This in general decreases the propagation losses due to a higher mode confinement. Moreover, stronger material modifications originating from higher induction pulse energies in the waveguide core cause higher absorption and scattering which reduces the transmittance. In our realizations, we observe the latter case to be predominant. An exception is found for a waveguide induced with a pulse energy of 25 nJ and a core diameter of 25 µm. This may be caused by spatial inhomogeneities in the polycrystalline diamond substrate resulting in a non-uniform threshold characteristic along the waveguide as seen in Fig. 2(d). Additionally, the polished front facet shows absorbing tracks (c.f. Fig. 2(c)) which leads to a decreased coupling efficiency. This is not related to the material quality nor the fabrication method but can be simply improved by an additional polishing of the front facet.

 

Fig. 3. Measured near-field mode profile for vertically polarized laser light at 1550 nm (a) for a waveguide core diameter of 20 µm and (b) 25 µm for induction energies reaching from (a1,b1) 20 nJ to (a3,b3) 30 nJ, (c1) near-field mode profile of the SMF28 fiber acting as a reference to calibrate the system.

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4. Conclusions

In conclusion we showed polycrystalline diamond to be a versatile material for full three-dimensional integrated optical circuitry. It can be effectively modified by femtosecond DLW to yield photonic waveguides. Compared to DLW in single crystal diamond our approach allows the fabrication of large-scale diamond photonic circuits. Using DLW enables arbitrary optical components to be realized in thick diamond samples. Besides full flexibility in designing optical circuits, the use of DLW enables structuring large-scale samples made from bulk CVD diamond, thus avoiding nucleation-related defects and surface contamination. Buried waveguides also allow for incorporating deep color defects directly into waveguiding networks which will be attractive for diamond quantum photonics and the realization of high-quality single photon emitters on a chip [29,30]. Individual color defects could be identified prior to DLW and then being interconnected with customized waveguides into single photon networks. Furthermore, the wide spectral transparency window of diamond enables ultra-broadband waveguide devices to be realized within a single material. As diamond provides optical transparency into the far infrared region DLW paves this way to access the unexplored wavelength regimes and make it become accessible for photonics computing chips [20].