Diamond diffractive optics—recent progress and perspectives

4.1 Far- and mid-infrared

The fabrication of long wavelength devices exploring the generalized manufacturing process presented in Figure 1, have been explored for optical elements for high-power CO₂ lasers. In this context, diffractive structures are particularly useful as antireflection layers in combination with other optical elements. A subwavelength periodic structure is formed, which acts as a modulation of refractive index, creating an index gradient between diamond and air, suppressing Fresnel reflections in a wavelength-dependent way. Karlsson et al. [48] fabricated antireflection structured surfaces for the infrared spectral region. Sputtered Al was patterned using e-beam lithography and etched using chlorine chemistry ICP RIE. The definition of the antireflection layer increases transmission from 71% to 97% when applied on both sides, for an illuminating wavelength of 10.6 μm (typical for CO₂ lasers).

Fu and Ngoi [49] fabricated diffractive elements in a polycrystalline 1.5-μm thick diamond film on fused silica, using a dedicated 50 keV Ga focused ion beam milling process. The authors report a diffraction efficiency of 73%, and remark that the fabrication of one DOE structure takes ∼20 min.

Figure 3:

Diffractive elements for the IR. (a) and (b) Antireflective microstructures (moth-eye) fabricated in polydiamond for far-IR (10.6 μm) [48]. (c) Circularly symmetric half-wave plates, fabricated via a hardmask resputtering-based process to maintain sidewall angle with high aspect ratios. Scale bars are 10 μm long [50].

Anti-Reflection (AR) layers were also fabricated by Martínez-Calderon et al. [51] using laser-induced periodic surface structuring via femtosecond laser pulses. Nanostructuring was carried out by pulsed laser light at different wavelengths, resulting in grooves of different morphologies. According to modeling carried out by the authors, these patterns can raise transmission by 12% compared to an unstructured surface.

Diffractive optical elements for shaping and transforming beams have been demonstrated by Karlsson et al. [52] in binary DOE fabricated in polycrystalline diamond, designed as a fan-out element (16-way beamsplitter). Sputtered Al was patterned via contact photolithography. The diamond etch was carried out in Ar/O₂ plasma, with an etch depth of 3.84 μm (for 10.6 μm operation) or 224 nm (for 633 nm operation). Uniformity error of 4% is reported for red light.

High aspect ratio gratings were fabricated by Forsberg and Karlsson [50]. Thick aluminum is used as hard mask that masks against strongly biased Ar/O₂ plasma. The process employs multiple hardmask layers and pattern transfers to create the desired Al hardmask. A critical problem addressed with this method is the erosion of the mask layer during the aggressive diamond etch step. The hardmask geometry undergoes evolution during etching that leads to faceting, which accelerates the erosion due to the increased sputtering yield of the lower angles, eventually receding from the edges of the diamond bar. To combat this effect, the mask layer thickness is increased to 1.7 μm, which enables 13.7 μm deep grooves in the diamond. The article also elaborates on the bottom surface roughness as a function of initial hardmask geometry: it is observed that vertical sidewalls prevent micromasking from the resputtering of the hardmask. When the hardmask has become faceted, the redeposition occurs, but the etch floor is no longer in line-of-sight for the sputtered atoms.

The grating fabrication technique was subsequently refined by Vargas Catalan et al. [53]. Fabrication was carried out in polycrystalline diamond, with a hardmask stack similar to previous work [50]. Solvent-assisted microcontact molding is used to define the grating patterns in the hardmask stack. A Si master is patterned using e-beam lithography, used to fabricate a PDMS stamp. Photoresist (PR) is coated onto the diamond with the hardmask, and the stamp is positioned on top of the resist. The substrate is placed into solvent-saturated atmosphere (ethanol), which permeates the PDMS stamp. The PR softens, and fills the patterns in the stamp due to capillary forces, which is then used to pattern the hardmask and subsequently the diamond substrate. Optimization of the etch parameters results in the control of the sidewall angle between 2.1° and 4.2°. The authors demonstrate greater control of the maximum etch depth by periodically resputtering the Al hardmask. Due to shadowing, the Al layer is thicker at the top of the groove, than at the bottom. A short etch is able to remove the mask material from the bottom of the groove, enabling the continuation of the etch process. An aspect ratio of 1:13.5 and a sidewall angle of 1.55° is demonstrated with this approach. Half-wave plates are also demonstrated for the mid-IR by Delacroix et al. [54]. Subwavelength gratings were optimized to work in the N-band (8–13 μm). The design employs gratings of trapezoidal profile, that were optimized to be tolerant of fabrication errors. An antireflective grating was fabricated on the backside to decrease transmission losses. The fabricated structures show increased transmission compared to the original plate (89–95%), along with a retardance of (180.08 ± 3.51)° over the N-band.

Similar fabrication was shown for quarter-wave plates by Forsberg et al. [55]. In addition, free-hanging gratings are fabricated in a polycrystalline diamond thin film on silicon. After the patterning of the diamond layer using ICP RIE, the underlying Si bulk is etched in a modified anisotropic Si etch, that undercuts the grating, leaving it free-hanging in a silicon frame. Crossbars were added to the grating lines to enforce buckling in the same direction. The authors note that a free-hanging grating can be advantageous in reducing transmission losses (no bulk Fresnel losses/no AR layer needed), a less demanding fabrication (without a large index substrate, the grating lines can have a larger period while retaining subwavelength properties) and better production scalability but the resulting devices are more fragile and difficult to handle.

Similarly, microfabricated subwavelength gratings were demonstrated for coronagraphy imaging of objects close to a star [56]. Annular groove phase mask subwavelength gratings were fabricated for both near infrared (1–3 μm) and visible (400–700 nm), along with vortex phase masks with a topological charge of 4. An interesting technique shown in this article is the ability to tune the depth of gratings postfabrication. To decrease the depth of the fabricated gratings, photoresist is coated, filling the grooves. Subsequent etching decreases the groove depth, due to slower etch of the PR at the bottom of the groove. Increasing the groove depth is performed by Al resputtering, similarly to previous work [53].

Kononenko et al. [33] demonstrate far infrared (CO₂ wavelength) components by a novel combination of laser microstructuring and diamond growth. A silicon template is structured using pulsed laser machining to form the inverse of the desired surface, then polycrystalline diamond is grown over the template after the template is seeded with nanodiamonds. After the desired thickness is achieved (after 496 h of growth), the top surface is polished. The silicon template is etched away and the diamond surface is etched in air to remove residual nondiamond carbon phases. The resulting structured diamond surface is smooth (R_a = 21 nm), which results in low scattering and the fabricated components operate close to the theoretical maximum (77 ± 3%, maximum: 82%). Representative examples of demonstrated diamond diffractive optical elements are shown in Figure 3.

4.2 Near infrared

The interest in the near infrared wavelengths is mainly motivated by applications for telecommunications, typically in the C-band at 1.55 µm wavelength. Among the demonstrated devices are grating couplers for coupling to photonic integrated circuits and high contrast grating for high reflection devices.

Gao et al. [57] fabricate grating couplers for 1550 nm on diamond-on-insulator substrates. E-beam lithography with proximity correction is used to pattern the 950-nm pitch coupler, using a hydrogen silsesquioxane (HSQ) resist. The patterns are transferred into the diamond film using an O₂/Ar plasma. Characterization reveals a −6.3 dB grating coupler loss.

Huszka et al. [58] demonstrate a high contrast grating (HCG) design that was configured to achieve higher than 95% reflectance at operating wavelength of 1550 nm into the zeroth order, which is an essential requirement for integration in vertical external cavity surface emitting laser structures. Numerical optimization was carried out to produce a design with >99.99% reflection for the target wavelength. The single crystal diamond plates (3 × 3 × 0.25 mm³ Element Six) were polished as described in the study by Mi et al. [32]. A dual-purpose metal mask was deposited on the diamond surface in order to provide a conductive layer for e-beam lithography and a hard mask for diamond etching, patterned subsequently via an e-beam exposed HSQ resist. Using the HSQ as mask, the titanium layer was then plasma etched using Cl₂/BCl₃ chemistry. The subsequent diamond etch was carried out using solely O₂ plasma, with an etch rate of 90–100 nm/min. The mask was removed in 49% HF acid. The fabricated gratings were subsequently measured by SEM and AFM to confirm the geometrical shapes and dimensions. The SEM images showed well-defined structures of the extruded bars with smooth and near-vertical sidewalls, and a complete absence of micromasking, frequently encountered in diamond processing [59]. AFM measurements confirmed good agreement with target dimensions. The fabricated HCG reached 95.85% at 1550 nm wavelength, which is ∼3% lower than designed. This discrepancy is attributed to a combination of imperfections resulting from the fabrication process, such as surface roughness leading to scattering losses, nonverticality of the grating bars or the residual trenches observed at the bottom edges of the grating bars, and tolerances resulting from the measurement method, such as imperfect alignment of the grating with respect of the collimating lens.

4.3 Visible and ultraviolet

In the visible, the high refractive index and the transparency window that extends into the deep UV makes it stand out from fused silica and sapphire, enabling applications such as welding and spectrometry. Kiss et al. [38] demonstrate a novel method for fabricating single-crystal diamond diffraction gratings based on crystallographic etching. Commercially available <100> diamond plates were patterned using contact lithography, with grooves aligned to both <100> and <110> directions, resulting in V-groove and rectangular gratings, due to the selectivity of the etch to crystallographic planes. The resulting gratings were characterized using scanning electron microscopy and atomic force microscopy, revealing angles of 57° and 87° depending on the crystal orientation, with mean roughness below R_a = 5 nm on the sidewalls. The optical performance of the gratings was simulated using RCWA [60] and compared to with measured values at 633 nm, showing good agreement. This work follows previous experiments into triangular and trapezoidal profile gratings [61], [62].

Toros et al. the fabricate high-precision 3D micro-optical components in single-crystal diamond using 3D laser lithography [40]. Fresnel microlens arrays and blazed diffraction gratings are microfabricated and characterized. The fabrication process is carried out on commercially available single-crystal diamond plates. The photoresist structures are created using layer-by-layer writing via 3D dip-in laser lithography (NanoScribe). This allows high-resolution 3D structures, which are then transferred into the substrate by a chlorine-based reactive ion etching with a low diamond:photoresist selectivity (1:15). The resulting surfaces are characterized using SEM and AFM, revealing smooth etched surfaces with a mean roughness (R_a) smaller than 2 nm. Furthermore, this pattern transfer process faithfully reproduces the features written into the photoresist, scaled by the selectivity. The Fresnel microlenses were optically characterized using a Mach–Zehnder interferometer, showing a focal length deviation of 4% from the target value. The gratings show a relative efficiency of 66% in the first order.

An example of exploiting the unique color centers along with diffractive optics is shown by Li et al. [63]. A circular grating is patterned into the diamond surface, enclosing a nitrogen-vacancy center. The grating is used to enhance the collection of photons from the center. Prepatterned silicon membranes were transferred to the diamond surface to use as mask during the subsequent etching, then removed. The collection efficiency is increased to 30% due to the grating.

Wildi et al. [39] demonstrate the design, fabrication and experimental characterization of near-field binary-phase transmission Diffractive Optical Elements (DOEs) in single-crystal diamond. Top-hat and arbitrary pattern DOE beam shapers were numerically optimized using an iterative Fourier transform algorithm. Commercially available single-crystal diamond plates (3 × 3 × 0.3 mm³) were polished using ion beam etching [32]. Amorphous silicon hardmask was deposited and e-beam lithography was carried out using an HSQ mask. Chlorine-based plasma etching was used to transfer the e-beam written patterns into the Si hardmask layer, followed by O₂ plasma-based diamond reactive ion etching for transferring the patterns into diamond. The hardmask was then stripped in poly-Si etch. The resulting binary phase relief patterns were characterized using SEM and atomic force microscopy (AFM), showing smooth etch floors and vertical sidewalls. Subsequently, the DOEs were experimentally in transmission at λ = 532 nm, confirming excellent uniformity of the resulting top-hat beam profile as required in copper welding applications.

Gao et al. [64] report on the design, fabrication and experimental demonstration of SCD waveguide devices with integrated grating couplers optimized for 850, 635 and 405 nm wavelengths, respectively. The devices are fabricated on SCD thin films using electron-beam lithography and reactive-ion etching technologies. A tilted etching technique is used to produce flat diamond films suitable for guided wave fabrication. Tapered fibers are used to probe the 850 and 635 nm integrated devices, showing coupling losses of 20.6 and 22.7 dB. The waveguide losses are measured as of 11 dB mm⁻¹ and 20.5 dB mm⁻¹. The 405 nm devices are probed instead with a lensed fiber, and show a coupling loss of 33.1 dB and a larger 46.7 dB mm⁻¹ waveguide loss.

Blazed diffraction gratings and diffractive optical elements were fabricated in polycrystalline diamond by Karlsson et al. [65] for operation at 400 nm wavelength. Continuous relief structures were exposed by electron beam lithography, which were transferred into diamond using Ar/O₂ ICP RIE, with a selectivity of 1:10. The authors report a smooth transferred surface, but attribute most of the deviation from the theoretical diffraction efficiency to deviation from the ideal resist profile. Experimentally demonstrated diamond diffractive optical elements for the visible and ultra-violet are summarized in Figure 4.

Figure 4:

Devices operating in the visible. (a) V-groove and vertical gratings fabricated using crystallographic etching [38]. (b) High reflectance binary grating for VCSELs [58]. (c) Top-hat forming DOE designed for 532 nm welding [39]. (d) Grating couplers for UV guided-wave optics [64].

4.4 X-ray

Microfabricated X-ray gratings have been demonstrated by Makita et al. [66]. A 500 nm HSQ layer is used with a chromium adhesion/conduction layer of 30 nm to create the grating patterns. The grating pattern is exposed via electron-beam lithography resulting in grating lines with 30 nm to 4 μm pitches. The etch process first transfers the electron-beam written pattern into the chromium using chlorine/oxygen chemistry, then transfer the pattern into diamond using an oxygen-based recipe. The HSQ layer is cured before diamond etching to increase recipe selectivity (300 °C, 40 min). The diamond etching is continuously tuned every 1–2 μm to maintain sidewall angle. The resulting structures have an aspect ratio between 10 and 20. Continuing this line of research, suspended gratings were fabricated by Kujala et al. [67], somewhat similarly to the free-hanging gratings previously demonstrated by Forsberg et al. [55], on a suspended CVD diamond membrane attached to a Si frame. A diffraction efficiency map of the first order was measured, indicating an efficiency variation of 20% across the whole grating. The variation is attributed to a nonflat diamond substrate, resulting in variation in the e-beam-written patterns due to focus error.

An alternative fabrication process was carried out using ion implantation as a way of graphitizing selected areas in diamond by Stepanov et al. [68]. Boron ions with an energy of 40 keV were implanted with a high fluence of 1.43 × 10¹⁸ ion/cm², masked by nickel grid with 40 μm square openings. SRIM simulation [69] indicates that the boron atoms are deposited in a ∼100-nm-thick layer resulting in graphitization due to the high fluence. The graphitized layer expands due to lower density of amorphous carbon (g_a−C = 2.09–2.23 g/cm³) compared to diamond (g_Diamond = 3.47–3.55 g/cm³) and the refractive index changes to n_a−C = 2.1–2.223, thereby creating the optical path difference. An investigation into tungsten-relief and diamond-relief gratings for high energy hard X-ray diffractive optics revealed that the diamond gratings withstand a fluence of 59000 mJ/cm² at 8.2 keV, 118 × the amount measured for tungsten gratings before damage occurred [70]. The damage manifested in the diamond gratings as unintended graphitization. An innovative way of improving diffraction efficiency for diamond nanofabricated Fresnel zone plates were presented by David et al. [71]. The electron-beam lithography-structured diamond structure was filled with Ir, deposited conformally via atomic layer deposition. The high density of iridium confers an increase in phase shifting, while the diamond fins serve as an effective way of cooling the metal to prevent damage. A representative selection of demonstrated diamond diffractive optical elements for X-ray operation is shown in Figure 5.