Nonlinear Materials: Diamond nonlinear-wave-mixing promises multiwavelength telecom source

Compared to traditional semiconductor materials used for parametric nonlinear frequency conversion such as silicon and group III-V compounds, diamond has ultrawide transparency from the ultraviolet (UV) through the infrared (IR) region, a high refractive index, extremely high thermal tolerance, and particular color centers that can act as quantum emitters.

Researchers at Harvard University, Massachusetts Institute of Technology (MIT), and the Schlumberger-Doll Research Center (all in Cambridge, MA) are exploiting these attributes of diamond and, for the first time, exploring the nonlinear optical properties of diamond in a ring-resonator configuration with high quality (Q) factor engineered to enable—through optical parametric oscillation (OPO)—a telecom multiwavelength source.¹

Diamond ring resonators

For the first time, researchers demonstrated OPO via four-wave mixing (FWM) in single-crystal diamond microring resonators integrated into waveguides. For FWM, two pump photons at frequency νP are converted to signal and idler photons at ν+ and ν− (denoted by signal and idler), with an OPO threshold power inversely proportional to Q².

Because the material dispersion of diamond is "normal" at telecom wavelengths, the diamond ring resonators were designed to be "anomalous" (in order to conserve energy between different resonator modes participating in FWM) for the transverse-electric (TE) optical mode in the 1300–1800 nm telecom window via nanoengineered waveguide dimensions of 800–900 nm width and 500–1000 nm height (see figure).

The cascade of 20 μm and 30 μm diameter ring resonators were fabricated in a single-crystal diamond film on a silicon dioxide/silicon (SiO₂/Si) substrate by first thinning down the 20 μm thick diamond slab to 1 μm using reactive ion etching (RIE), transferring it to the substrate, writing the ring/waveguide pattern using electron-beam lithography (EBL) and a second RIE step, and capping the structure with 3-μm-thick SiO₂. For the telecom wavelength range, record-highQ factors of approximately one million were measured.

In a demonstration of OPO operation, two different frequency-comb spectra are generated when ring resonators are pumped separately with a 1553 and 1599 nm continuous-wave laser source, respectively. If pumped with higher powers, a broad-spectrum frequency comb and multiwavelength telecom laser source could be developed.

"We hope that our results will motivate further research into diamond nonlinear photonics, which might enable novel technologies outside the scope of traditional semiconductor and nanophotonic material platforms, such as frequency combs in the visible spectrum, for example, that are important for precision astronomy,” says Harvard postdoctoral fellow Vivek Venkataraman. "Diamond chips hold promise for compact, robust (chemically inert and biologically compatible), temperature-insensitive photonic devices for diverse applications ranging from sensing to metrology."

REFERENCE
1. V. Venkataraman et al., SPIE Newsroom, June 26, 2014; see http://spie.org/x108872.xml?highlight=x2404&ArticleID=x108872.