Supercontinuum generation in angle-etched diamond waveguides

TL;DR

This paper demonstrates on-chip supercontinuum generation in diamond waveguides, achieving a broad spectrum in the visible range, with potential for UV broadband sources due to diamond's properties.

Contribution

The study introduces a novel fabrication technique for angle-etched diamond waveguides enabling broadband supercontinuum generation in the visible spectrum.

Findings

Spectrum spans 670 nm to 920 nm

Uses 100 fs pulses with 187 pJ energy

Potential for UV supercontinuum generation

Abstract

We experimentally demonstrate on-chip supercontinuum generation in the visible region in angle etched diamond waveguides. We measure an output spectrum spanning 670 nm to 920 nm in a 5mm long waveguide using 100 fs pulses with 187 pJ of incident pulse energy. Our fabrication technique, combined with diamonds broad transparency window, offers a potential route toward broadband supercontinuum generation in the UV domain.

Figures

Full text

Supercontinuum generation (SCG) is a nonlinear optical process, where a short, high-intensity pulse of light experiences significant spectral broadening due to combined effect of self- and cross-phase modulation, soliton fission, Cherenkov radiation, modulation instability, and Raman scattering agrawal2000nonlinear . The development of supercontinuum (SC) sources has enabled numerous applications such as biomedical imaging, molecular detection, and high precision metrology dudley2006supercontinuum ; strogatz2018nonlinear . The generation of octave spanning SCG is of great importance, as it enables a fully stabilized frequency comb synthesizer through f-2f self-referencing diddams2000direct . Recently, there has been significant development of on-chip SC sources in various platforms such as silicon leo2015coherent ; leo2014dispersive ; lau2014octave , silica oh2014supercontinuum , silicon-germanium sinobad2018mid , aluminum nitride hickstein2017ultrabroadband ; liu2019beyond , silicon nitride lacava2017si ; porcel2017two ; johnson2015octave ; klenner2016gigahertz ; zhao2015visible ; halir2012ultrabroadband , chalcogenides lamont2008supercontinuum ; xie20142 , periodically poled lithium niobate phillips2011supercontinuum , and thin-film lithium niobate yu2019coherent ; lu2019octave ; Jankowski:19 , offering potential for compact devices that allow for low cost and large-scale fabrication.

There has been interest in extending the generated SC spectra into visible and ultra-violet (UV) wavelength range which can, for example, enhance the accuracy of optical coherence tomography fercher2003optical . However, observation of supercontinuum generation in UV and visible region in most materials are challenging due to the large normal material group-velocity dispersion (GVD). This dominates over efforts to engineer the dispersion via the waveguide geometry, making it challenging to achieve the anomalous GVD necessary for efficient phase-matched nonlinear optical processes. In addition, the intensified Rayleigh scattering at these wavelengths leads to challenges in making low-loss structures in the short wavelength regime.

Diamond is an emerging platform for integrated photonics, owing to its wide bandgap which significantly reduces multi-photon absorption processes, its relatively high refractive index (n=2.38), and its strong optical nonlinearities. Additionally, its structure can be engineered to have anomalous GVD in the telecom wavelength range feigel2017opportunities ; hausmann2014diamond . This has enabled realization of frequency combs hausmann2014diamond and Raman lasers latawiec2015chip ; latawiec2018integrated , for example. Importantly, these demonstrations mainly relied on diamond on insulator (DOI) platform faraon2011resonant ; hausmann2012integrated that consists of a sub-micron thick single crystalline diamond (SCD) film, that is prepared using combination of polishing and reactive ion etching, transferred on top of a low index substrate such as fused silica or SiO2/Si. While this platform enabled important advances in the field of diamond photonics, it suffers from a low device yield and significant thickness variations across the chip with characteristic “wedge”-like profile introduced during polishing step. This is particularly problematic for nonlinear optical devices that rely on precise dispersion engineering. An alternative platform that we developed leverages angle-etching of diamond to realize free standing structures with sufficient optical isolation and has been used in quantum photonics and opto-mechanics meesala2016enhanced ; burek2016diamond ; sipahigil2016integrated ; evans2018photon . Since the structures are made directly from unpolished bulk diamond there is no thickness variation across the entire structure, making this technique advantageous over DOI. Angled-etched diamond platform has been the workhorse of diamond-based quantum photonics efforts meesala2016enhanced ; sipahigil2016integrated ; sohn2018controlling ; sun2018cavity but has not been utilized for realization of nonlinear optical devices.

Here we leverage uniformity and scalability of reactive ion beam angled etching (RIBAE) atikian2017freestanding approach to realize, for the first time, SCG in SCD. By controlling the geometry of diamond waveguide with characteristic triangular cross-section, and in particular its apex angle ($\theta$ in Figure 1(a) – right inset), we could achieve anomalous GVD over wide wavelength range (Figure 1(d)). Our numerical modeling indicates that our devices should support 250 nm wide SC spectrum spanning 670 – 920 nm (Figure 2(a) and (b)).

The waveguide cross-section is that of an inverted triangle on a thin support pedestal [Figure 1(a)]. The pedestal provides mechanical support over the required waveguide length across the chip. The width of the support is 20 nm in order to prevent any mode leakage to the substrate. Figure 1(a) (left inset) shows the TE mode profile of a triangular diamond waveguide with an apex angle of 100 degree and width of 540 nm. Dispersion was calculated for different waveguide widths and apex angles, using a finite element mode solver, and results are summarized in Figure 1 (d) in the case of the fundamental TE mode. By varying the widths and apex angle, the anomalous dispersion region can be tuned over a broad region. Our design consists of two suspended tapered sections as in- and out-couplers surrounding the central fully supported dispersion-engineered region [Figure 1(a)- right inset]. The couplers (suspended regions) are designed to be 280 nm wide and 20 $\mu$m long to optimize the mode overlap with the input field.

Devices are fabricated using mechanically polished $\sim$ 5$\times$5 mm – 500 $\mu$m thick electronic-grade SCD with a [001]-oriented surface (Element Six). After cleaning in a boiling mixture of HClO4:H2SO4:HNO3 (1:1:1) followed by solvent cleaning, a layer of Nb is deposited as hard-mask using magnetron sputtering (AJA-ATC). Waveguides of a length of 5mm are written using E-beam lithography with multi-pass exposure (Elionix –F125) with negative-tone e-beam resist (FOx-16, Dow Corning). This pattern is then transferred first to Nb using an Ar/Cl2 etch chemistry, and then to diamond using an inductively coupled plasma-reactive ion etcher (ICP-RIE, Unaxis Plasma-Therm) with O2. This results in 5 mm long and 2 $\mu$m thick diamond ridges. After this top-down etch, the sample is placed in a reactive ion-beam etcher and stage tilt was selected to result in the desired apex angle. Oxygen ion beam was used to perform the etch. Finally, the Nb mask was removed in H3PO4:H2SO4: HNO3 (1:1:1) and device is cleaned in H2SO4:H2O2 (3:1) followed by solvent clean and dried in a critical point drier to prevent damaging the suspended in-out couplers. Because the diamond cannot be cleaved or polished after the fabrication is complete, extra steps are performed to ensure the waveguides are written as close to the edge as possible (5-10 $\mu$m) to allow for efficient in-coupling of light using a lens. These steps consist of oxygen-plasma surface treatments and the use of adhesion promoters before spinning. Scanning electron micrographs (SEM) of the coupling section and cross section of a fabricated waveguide are shown in the inset of Figure1(b) and Figure 1(c), respectively.

We model the pulse propagation dynamics in the diamond waveguide using the split-step Fourier technique to solve the generalized nonlinear envelope equation. We consider the contributions from higher-order dispersion, third-order nonlinearity, and self-steepening in the model. The dispersion operator, $D=\sum_{n=2,3,\ldots}\frac{\beta_{n}(\omega_{0})}{n!}(\omega-\omega_{0})^{n}$ shown in figure 2(a), predicts the onset of a dispersive wave near 850 nm okawachi2017coherent . Figure 2(b) shows the simulated spectrum for a 5-mm-length diamond waveguide with a 540 nm width and a 100∘ apex angle. In our simulation we assumed a 100-fs pump with a pulse energy of 21 pJ in the waveguide. However, we observe that the spectral broadening is largely due to self-phase modulation agrawal2000nonlinear .

In our experiment, the angle-etched waveguides are pumped with a Ti: Sapphire laser centered at 810 nm with a pulse duration of 100 fs and a repetition rate of 80 MHz. A variable neutral density filter is used to control the pump power. We use an aspheric lens for coupling into the waveguide and collect the output using a lensed fiber and send it to an optical spectrum analyzer (OSA). Figure 2(c) shows the generated SC spectrum. For 187 pJ of incident pulse energy, we observe a broadband spectrum spanning 250 nm from 670– 920 nm. Our measured spectrum shows good agreement with the simulated spectrum. The discrepancy in spectral position is attributed to fabrication tolerances in the waveguide width.

The low transmission of the device can be attributed to imperfections in the coupling facet and the sidewall roughness of the waveguides. By comparing the simulation and the experiment, a coupling loss of  10 dB/ facet is calculated. In addition, we observe damage to the waveguides facet for high pump powers. We attribute this to excessive peak- intensity leading to femto-second ablation of diamond which can happen with photons with energies below diamonds bandgap in the presence of ultra-short pulses dumitru2002femtosecond ; wu2003femtosecond ; shinoda2009femtosecond . This results in graphitization phenomenon and incubation effects starting from the coupling region and extending toward the rest of the waveguide [Figure 3].

As it was discussed before, diamond allows for dispersion engineering in the visible and UV region. To confirm this, we numerically design angle-etched structures which would extend the SC spectra toward the UV range. To shift the region of anomalous GVD to even lower wavelengths, we consider the 80∘ apex angle with narrower widths. Figure 4(a) shows the GVD profile for an angle-etched waveguide with apex angle of 50∘ and a 250 nm width and 4(b) shows the corresponding dispersion operator for a pump wavelength of 520 nm. The dispersion operator indicates that two dispersive waves can be generated, one near 260 nm and the other near 650 nm. Figure 4(c) shows the simulated SC spectrum. We neglect the Raman effect for TE polarization assuming propagation along the (100) axis mildren2013optical . Assisted by the dispersive wave, the SC spectrum covers over an octave of bandwidth, well into the UV. However, in our experiment we were not able to see any broadening because of the fast degradation of the waveguides exposed to 520nm laser light. We project that optimization of the waveguide design and length, as well as coupler design, will allow for lower required pulse energies, preventing taper damage.

In conclusion, by leveraging fabrication advances in angled-etching toward a novel integrated optics device architecture, we have demonstrated the first SCG in SC-diamond in the visible region spanning 670-920 nm. Owing to diamond’s material properties, the proposed design has the potential of generating a SC spectrum extending into the UV. Even though, achieving a broad spectrum in diamond is challenging, but our angle-etched design provides a new way for dispersion engineering that can be used for other integrated platforms.