Diamond encapsulated silicon optical fibers synthesized by chemical vapor deposition

Silica optical fibers are a mainstay in communications, aerospace, manufacturing, and medicine because of their high aspect ratio, high durability, and chemical resistance, as well as their minimal alignment requirements.^1–4 However, commercial fiber optics are based almost entirely on amorphous glasses encased in polymer coatings, severely restricting their operating wavelengths, temperatures, and thermal management properties. A diamond clad silicon core optical fiber would be transparent in the infrared and could be capable of guiding immense powers at high temperatures, especially when compared to germanium-doped silica optical fibers which are susceptible to chemical degradation at temperatures as low as 300 °C.⁵ This functionality may provide significant enhancements in remote sensing⁶ and structural monitoring in extreme environments^7,8 or when probing for chemical signatures at mid-infrared wavelengths.^9,10

Silicon and diamond are two of the most well-studied crystalline semiconductor materials. Synthesis of these materials is well established, and they have wide-ranging photonic and electro-optical applications.¹¹ Key features of silicon and diamond are their low absorption in the IR, low chromatic dispersion, high chemical inertness, and high thermal conductivities.^12–15 Diamond in particular possesses the highest known thermal conductivity of any material (2200 W m⁻¹ K⁻¹) and is often used as a protective coating in several applications.^16–19 These features make the two materials highly compatible for fabricating infrared optical fibers, where their high damage thresholds allow for massive power handling capabilities²⁰ and large thermal conductivities allow them to conduct away heat in a 360° cross section. Although the high shear modulus of diamond will limit the flexibility of such a fiber, short diamond-coated sections (e.g., cm or more in length) could prove useful for a variety of devices operating in extreme environments and could be coupled with a suitable gain material in optical pumping schemes, similar to inflexible planar waveguides.^21–23 

The largest obstacle to synthesizing crystalline fibers and claddings has been that they do not experience glass transition and softening, which prohibits typical fiber drawing approaches. Therefore, special techniques such as molten core drawing²⁴ or high pressure chemical vapor deposition (HPCVD) must be used. The chemical vapor deposition process operates by decomposing gas-phase precursors directly into hollow templates, producing semiconductor optical fibers with atomically smooth interfaces that may be used as cylindrical templates for diamond growth.^25,26 This technique is ideal because it does not require high processing temperatures and produces smooth fibers with low loss.

To coat these high-aspect ratio templates, microwave plasma-enhanced chemical vapor deposition (MPCVD) has emerged as an important method for producing carbon materials such as diamonds, carbon nanotubes, and graphene. The energetic electrons generated by the plasma boost the ionization, excitation, and dissociation of hydrocarbon precursors at relatively low temperatures, which is essential for this kind of carbon-based material growth.^27–29 One of the major challenges in plasma chemical vapor deposition on fibers is the ability to coat uniformly over a cylindrical surface. In addition, typically, the MPCVD reactor system is not compatible for a fiber because the system is designed for flat substrates instead of high aspect ratio cylindrical surfaces. Indeed, CVD techniques have been used previously to grow diamond on optical fibers, but only on the end faces for transmitting diamond N-V fluorescence through the fiber.³⁰ 

Here, we report MPCVD polycrystalline silicon optical fibers encapsulated in high structural quality diamond by microwave plasma chemical vapor deposition. Diamond coating entirely around the fibers was achieved by placing the fibers on a molybdenum holder within the deposition chamber. The material was characterized by using scanning electron microscopy as well as photoluminescence (PL), Raman spectroscopy, and optical microscopy. In addition, mid-infrared light (1908 nm) from a thulium fiber laser was waveguided to demonstrate the quality of the fiber.

Polycrystalline silicon optical fibers were synthesized by previously reported methods.³¹ Fused silica capillaries were infiltrated by thermally decomposing a gaseous silane and hydrogen mixture, resulting in silicon core optical fibers clad in silica. Temperature was controlled with a Stanford Research Systems PTC10 temperature controller and home-built furnace. This decomposition was performed under pressures of 300 atm at 475 °C to ensure that the fibers were amorphous and nonhydrogenated. The silicon optical fiber samples were then thermally annealed at temperatures between 700 °C and 800 °C for up to one week to induce crystallization, which was confirmed using Raman spectroscopy.

To remove the silicon fibers from their cladding, 49% concentrated HF was used to etch away the fused silica cladding, leaving bare silicon fibers of 50 μm–75 μm in diameter and 2 cm–3 cm in length depending on the capillary template in which they were grown. The fibers were rinsed with deionized water and allowed to dry in air. White light profilometry was used to measure the rms surface roughness of etched silicon fibers and gave values on the order of 0.5 nm. This value is optically smooth and on the same order as other commercially available optical fibers. Figure 1 shows the cross section of the microwave plasma CVD reactor employed for the silicon optical fiber diamond synthesis, optical micrographs of the fibers before diamond growth, and schematic configuration of the sample placement during diamond deposition. The optical fibers were supported between two silicon wafers on top of a molybdenum holder before being placed above the substrate stage so that the plasma hovered above the samples. Prior to diamond deposition, quartz silica optical fibers were seeded by dip coating in an isopropanol solution mixed with nano-sized diamond powder in an ultrasonic bath. The sonication was performed for 30 min after which the samples were cleaned using de-ionized water and air-dried.

The samples were then loaded into the chamber, and the MPCVD reactor was pumped down to 5 mTorr. Diamond was grown on the fibers using gas mixtures containing H₂, CH₄, and N₂ at various ratios, with the total process pressure at 75 Torr. The nitrogen flow rate was set to 1 SCCM, and the total flow rate of all gases was 400 SCCM with various methane concentrations from 2% to 6%. During the deposition process, the MPCVD system was operated in a continuous mode and at a maximum power of 2.0 kW for up to 8 h. The temperature of the substrate measured during the deposition ranges between 600 °C and 750 °C.

A scanning electron microscope (SEM, JEOL) with 15 kV beam accelerating voltage with the SE-ETD detector working in a high vacuum mode was used to observe the structure of the optical fiber surface. The films were characterized by photoluminescence and Raman spectroscopy using a Renishaw InVia micro-Raman system. Spectra were recorded in a range of 500 cm⁻¹–1800 cm⁻¹ with an integration time of 1 s at various gratings using a 257 nm frequency doubled and the 514 nm fundamental of an Ar-ion laser, both magnified with a 100× microscope objective.

When polishing the fiber facet, standard abrasive polishing methods were ineffective due to diamond’s extreme hardness. A Hitachi IM4000plus ion mill was instead used to directly cleave the end facets of the fiber to provide a relatively smooth surface into which infrared light was coupled. An IPG thulium fiber laser emitting at 1908 nm was coupled into the fiber using a 5 cm Ar-coated CaF₂ focusing lens. The output was collimated using a second 0.5 cm Ar-coated CaF₂ lens. The output optical mode was imaged using a PV320V infrared camera.

Raman and PL spectra of fibers grown directly on silicon wires are shown in Fig. 2. The Raman spectra, measured with 257 nm excitation, reveal high-quality diamond coating, as indicated by the sharp diamond (D) peak with the weak G-band attributed to graphitic carbon [Fig. 2(a)]. The peak at 739 nm is due to silicon, whereas the weaker feature at 575 nm arising from nitrogen in the diamond is indicated in PL spectra obtained with 514.5 nm excitation [Fig. 2(b)]. Measurements at different locations along the diamond-coated fibers show similar Raman and PL spectra. The diamond films grown on optical fibers exhibit spectra similar to those reported for nanocrystalline diamond.³² 

Figure 3 shows the Raman spectra of the diamond-coated fibers at different methane concentrations. The two main bands correspond to diamond (D peak, sp³ carbon) and the weak, broad feature to graphitic sp² carbon (G band). The slight broadening of the diamond peak (and possibly the G band shift) may arise from small residual tensile stresses. The full width at half maximum (FWHM) of the peak was in the range that is typical for CVD diamond (10 cm⁻¹–12 cm⁻¹) in samples with low defect concentrations.

Figure 4 shows the scanning electron micrographs of the diamond-coated fibers. As can be seen, the silica-clad silicon-core optical fibers are coated entirely around the fiber surface with well-crystallized, faceted diamond (Fig. 4). The size of the diamond crystals ranges from hundreds of nanometers to several micrometers. Tetrahedral and square-shaped facets are shown. The grown diamond films exhibit the homogeneous morphology over the entire length of the fiber. The octahedral and plate-like diamond grains dominate the film because the deposition took place at low temperatures, as reported previously.^33,34 Both the plates and the octahedral grains are faceted, revealing their crystalline nature. Some of the diamond-coated fibers grown under different conditions reveal cracking along the fibers (cross sectional SEM images not shown here). This could be attributed to the thermal expansion mismatch between diamond and silicon when cooling from high deposition temperature or from structural defects prior to diamond growth. The high stiffness of diamond and the associated brittleness especially when polycrystalline contribute to this challenge. Thicker coatings or different growth conditions may be necessary to prevent cracking, and further studies are underway to investigate this phenomenon.

Waveguiding was demonstrated on silicon samples clad in a thin layer of silica and diamond, as shown in Fig. 5. The silica layer was ∼3 μm to 5 μm in thickness and was slightly nonuniform due to the fast etching procedure. These samples tended to have small pores in the silicon, an evidence that the MPCVD process had not proceeded to completion in this section of fiber. The diamond coating extended around the entire fiber and had a thickness of 9 μm–24 μm. The fiber facets were polished to a smooth finish using a Hitachi IM4000plus ion mill to allow for efficient free-space light coupling. A thulium-doped silica fiber laser was used to generate 1908 nm light that was coupled into the fiber using an antireflection coated 50 mm CaF₂ focusing lens and collimated out of the fiber using a 10-mm uncoated CaF₂ lens. The resulting waveguide mode structure was imaged using an Electrophysics PV320 infrared camera.

Figure 5(b) shows the resulting mode structure from waveguiding 1908 nm light, with the color scheme representing the light intensity: red indicates high optical intensity and purple indicates no optical intensity. This structure shows strong optical confinement to the core. Optical losses were on the order of 10 dB/cm, which is unexpected due to the low surface roughness of the fiber template. Finite element analysis predicts that the minimum loss is on the order of 10⁻⁴ dB/m, and many semiconductor-core optical fibers possess losses below 2 dB/cm, which is a reasonable goal if the synthesis procedures are improved for diamond-coated fibers.^35,36 The high value of the current measurements is thought to be a result of uneven polishing with the ion mill, which leads to high coupling losses, as well as from defects introduced during the fiber etching and MPCVD process. Optical losses are expected to dramatically decrease as improvements are made to the facet polishing and as etching and MPCVD processes are optimized to leave a smooth interfacial surface.

The losses of standard silicon-core optical fibers deposited by HPCVD with oxide-passivated surfaces can be under 1 dB/cm.²⁶ Despite the higher losses found here, waveguiding was strongly confined to the silicon core, which indicates that the waveguide structure is preserved, and loss can be improved if the MPCVD process is optimized. Losses are thought to predominantly stem from structural defects introduced during the MPCVD process, such as transverse cracks, which act as scattering sites. To reduce the number of structural defects and improve waveguiding properties, more gentle diamond growth parameters may be used including utilizing lower temperatures or slower temperature ramp rates. In addition, larger-diameter silicon fibers may be more robust for high-temperature diamond growth processes. Finally, other material systems with gentler growth procedures can be explored, such as coatings of diamond-like carbon.³⁷ 

In conclusion, diamond-clad silicon optical fibers have the potential to guide multi-watt levels of optical power without degradation due to the refractory nature of silicon and the unmatched thermal conductivity of diamond. Here, we demonstrate that silicon optical fibers can be encapsulated by bulk diamond using chemical vapor deposition techniques. The diamond was shown to have high quality comparable to the material grown on flat substrates under similar conditions, and the encapsulated fibers were shown to guide 1908 nm light. By refining the growth conditions of these fibers and improving polishing techniques, it is expected that light may be guided with low loss at much higher powers than has been documented in silica optical fibers.