Diamond: a new platform for nonlinear photonics

Parametric (that is, without transfer of energy or momentum between light and its propagating medium) frequency conversion via optical nonlinearities in an on-chip platform is of great interest for developing compact, robust, low-power, broadband, coherent light sources. Such laser-like light sources, based on optical gain from parametric amplification rather than stimulated emission, have applications in spectroscopy, metrology, sensing, and all-optical information processing.^{1, 2} So far, nonlinear nanophotonic systems have been realized in traditional materials used in the semiconductor industry, including silica (SiO₂), silicon (Si), silicon nitride (Si₃N₄), and group III–V compounds. However, these traditional materials suffer from linear and nonlinear loss mechanisms, especially at shorter wavelengths of light (e.g., the visible spectrum).

In comparison, diamond has an extremely wide transparency window (spanning IR to UV wavelengths), a relatively large refractive index, and various color-centers acting as quantum emitters, which make it a promising material for photonic applications.^{3, 4} In addition, the superior thermal properties of diamond enable it to handle large optical powers in a robust, temperature-insensitive manner. Despite these features, optical nonlinearities in diamond (apart from Raman scattering—a non-parametric process—in bulk samples) have not been explored much until now. We have realized nonlinear photonics in diamond for the first time by demonstrating optical parametric oscillation (OPO) via four-wave mixing (FWM) in single-crystal diamond (SCD) micro-ring resonators that are integrated into waveguides and offer a high quality (Q) factor.⁵

FWM is a third-order nonlinear parametric process whereby two pump photons at frequency ν_P are converted to two different photons at ν₊ and ν₋ (denoted by signal and idler), such that energy is conserved (2ν_P=ν₊+ν₋). OPO is achieved when the round-trip FWM gain exceeds the loss, a process analogous to a laser above threshold, and bright coherent light is generated at the signal and idler wavelengths. The OPO threshold power is inversely proportional to Q².

‘Anomalous’ dispersion (that is, when higher-frequency components of light propagate faster through a medium than lower-frequency components) is required for energy conservation between different modes of the optical resonator that participate in the FWM process.^{6, 7} Although the intrinsic material dispersion of diamond is ‘normal’ at telecom wavelengths, the net resonator dispersion can be engineered to be anomalous through geometrical dispersion by appropriately designing the nano-waveguide cross-sectional dimensions.

Figure 1(a) shows waveguide-coupled ring resonators of radii 20 and 30μm fabricated in a SCD film on a SiO₂/Si substrate using a procedure we developed recently.⁸ In brief, a 20μm thick SCD slab is thinned down to 1μm by reactive ion etching (RIE) and then transferred to a SiO₂/Si substrate. The ring/waveguide pattern is written using electron-beam lithography (EBL) and a second RIE step. Polymer coupling pads with a 3 × 3μm cross-section are defined in a second EBL step to extend the diamond waveguides to the ends of the chip. Finally, a SiO₂ layer (3μm thick) is deposited using chemical vapor deposition to cap the devices. We have measured record-high loaded Q-factors of ∼1 million for these SCD devices in the telecom wavelength range: see Figure 1(b). The waveguide dimensions (width 800–900nm and height 500–1000nm) are designed such that anomalous resonator dispersion for the transverse-electric (TE) optical mode can be achieved in the 1300–1800nm range.

Figure 1. Integrated single-crystal diamond (SCD) ring resonators for nonlinear optics. (a) Scanning electron micrograph of an array of bus-waveguide-coupled SCD ring resonators on a silica/silicon (SiO₂/Si) chip. Inset shows magnified view of the ring-bus-waveguide coupling section. The devices are later capped with a layer of deposited SiO₂. (b) Normalized transmission spectrum of a ring resonator reveals high quality (Q)-factor modes. A loaded Q-factor of Q_L∼1×10⁶ is inferred from a Lorentzian fit for the mode at 1545.1nm. a.u.: Arbitrary units.

A continuous-wave input laser is sent through an erbium-doped fiber amplifier and then coupled into the on-chip device to observe OPO operation. Tuning the pump into a cavity resonance generates several new modes away from the pump, resulting in a spectrum of multiple lines with a frequency spacing given by the resonator free-spectral range.

Figure 2shows that a ‘frequency comb’ of 20 new wavelengths is generated, spanning a range of >150nm with pump power

Figure 2. An integrated multiple-wavelength source based on a diamond microresonator. The generated optical parametric oscillation spectrum from the same ring resonator (20μm radius) is shown for two different pump positions, 1553nm and 1599nm, respectively. The pump power is the same in each case (∼80mW in the bus-waveguide). A total of 20 new lines are observed when pumping at 1599nm, whereas 10 new lines are generated when pumping at 1553nm, the difference most likely arising from higher coupling efficiency between the bus-waveguide and ring resonator for longer wavelengths.

Coupling higher pump powers should enable the generation of broadband, high-repetition-rate optical frequency combs that are desirable for numerous applications, and this technology in diamond can be readily extended to new wavelength ranges inaccessible so far (such as the visible spectrum). In addition to pursuing these goals, we are also working towards expanding the scope of diamond nonlinear photonics by building continuous-wave, low-threshold, on-chip Raman lasers at exotic wavelengths, taking advantage of the giant Raman frequency shift and large Raman gain in diamond. In the long term, diamond nonlinearities might enable in-situ frequency translation and pulse-shaping of single photons emitted by its various color centers, which is promising for quantum information processing and future quantum networks.