Jichi Ma

Heterogeneous lithium niobate photonics on silicon substrates

A platform for the realization of tightly-confined lithium niobate photonic devices and circuits on silicon substrates is reported based on wafer bonding and selective oxidation of refractory metals. The heterogeneous photonic platform is employed to demonstrate high-performance lithium niobate microring optical resonators and Mach-Zehnder optical modulators. A quality factor of ~7.2 × 10⁴ is measured in the microresonators, and a half-wave voltage-length product of 4 V.cm and an extinction ratio of 20 dB is measured in the modulators.

Silicon-on-nitride waveguides for mid- and near-infrared integrated photonics

Silicon-on-nitride ridge waveguides are demonstrated and characterized at mid- and near-infrared optical wavelengths. Silicon-on-nitride thin films were achieved by bonding a silicon handling die to a silicon-on-insulator die coated with a low-stress silicon nitride layer. Subsequent removal of the silicon-on-insulator substrate results in a thin film of silicon on a nitride bottom cladding, readily available for waveguide fabrication. At the mid-infrared wavelength of 3.39 μm, the fabricated waveguides have a propagation loss of 5.2 ± 0.6 dB/cm and 5.1 ± 0.6 dB/cm for the transverse-electric and transverse-magnetic modes, respectively.

High-contrast, all-silicon waveguiding platform for ultra-broadband mid-infrared photonics

Suspended silicon-membrane ridge waveguides are fabricated and characterized on a single-material photonic device platform. By using direct bonding, a thin layer of silicon is fused to a bulk silicon substrate, which is patterned with narrow trenches. Waveguides are etched on the resulting suspended membranes and are characterized at mid- and near-infrared wavelengths. Transverse magnetic-mode propagation losses of 2.8 ± 0.5 and 5.6 ± 0.3 dB/cm at 3.39 and 1.53 μm wavelengths are measured, respectively. This all-silicon optical platform is capable of continuous low-loss waveguiding from wavelengths of 1.2–8.5 μm, enabling numerous applications in frequency conversion and spectral analysis.

Submicron optical waveguides and microring resonators fabricated by selective oxidation of tantalum

Submicron tantalum pentoxide ridge and channel optical waveguides and microring resonators are demonstrated on silicon substrates by selective oxidation of the refractory metal, tantalum. The novel method eliminates the surface roughness problem normally introduced during dry etching of waveguide sidewalls and also simplifies fabrication of directional couplers. It is shown that the measured propagation loss is independent of the waveguide structure and thereby limited by the material loss of tantalum pentoxide in waveguides core regions. The achieved microring resonators have cross-sectional dimensions of ~600 nm × ~500 nm, diameters as small as 80 µm with a quality, Q, factor of 4.5 × 10(4), and a finesse of 120.

Low-loss and high index-contrast tantalum pentoxide microring resonators and grating couplers on silicon substrates

A platform for high index-contrast integrated photonics based on tantalum pentoxide submicrometer waveguides on silicon substrates is introduced. The platform allows demonstration of microring resonators with loaded quality factor, Q, of 67,000 and waveguides with a propagation loss of 4.9 dB/cm. Grating couplers, with an insertion loss of ~6 dB per coupler and 3 dB bandwidth of ~50 nm, are also demonstrated and integrated with microring resonators.

Pump-to-Stokes relative intensity noise transfer and analytical modeling of mid-infrared silicon Raman lasers

An analytical model for mid-infrared (mid-IR) silicon Raman lasers (SRLs) is developed. The relative intensity noise (RIN) transfer from the pump to the Stokes in the lasers is also investigated. The analytical model can be used as a versatile and efficient tool for analysis, design and optimization of mid-IR SRLs. It is shown that conversion efficiency of 70% is attainable and the low-frequency RIN transfer may be suppressed to below 1 dB by pumping low-loss waveguides at high intensities.

Optical transmission properties of C-shaped subwavelength waveguides on silicon

Optical properties of C-shaped subwavelength waveguides in metallic (silver) films on silicon substrates are studied in the range of 0.6–6 μm. Power throughput and resonant wavelengths of several transmission modes are studied by varying the waveguide length (or metal thickness). Among three types of transmission modes, the fundamental order of the Fabry–Perot-type mode was shown to attain remarkably high power throughputs (as high as 12). With optimized design of the aperture, the resonant wavelength of this mode occurs in the 1–2 μm wavelength range, suggesting that such apertures can be utilized to achieve plasmonic-enhanced silicon photonic devices at telecommunication wavelengths.

Noise Figure in Near-Infrared Amorphous and Mid-Infrared Crystalline Silicon Optical Parametric Amplifiers

The noise figures (NF) of near-infrared (near-IR) amorphous silicon (a-Si) and mid-infrared (mid-IR) crystalline silicon (c-Si) optical parametric amplifiers (OPA) are numerically investigated. The impact of nonlinear losses, i.e., two-photon absorption (TPA) and TPA-induced free carrier absorption (FCA), as well as Raman-effect-induced complex nonlinear coefficient are taken into account in a-Si OPAs. The amplified spontaneous emission (ASE) of Erbium-doped fiber amplifiers (EDFA) and the relative intensity noise (RIN) of the pump laser are considered as the dominant pump noises when simulating the pump transferred noise (PTN) of near-IR a-Si and mid-IR c-Si OPAs, respectively. It is shown that in typical near-IR a-Si OPAs, the NF is ~5 dB on the Stokes side but increases sharply to above 10 dB at the gain edge on the anti-Stokes side. In high-gain mid-IR c-Si OPAs, the NF is dominated by the PTN and is well above 10 dB at the gain edge. These results indicate that both near-IR a-Si OPAs and mid-IR c-Si OPAs are promising alternatives to near-IR c-Si OPAs, but they both have limitations in broadband operation.

Two-photon photovoltaic effect in gallium arsenide

The two-photon photovoltaic effect is demonstrated in gallium arsenide at 976 and 1550 nm wavelengths. A waveguide photodiode biased in its fourth quadrant is used to harvest electrical power from photons lost to two-photon absorption.

Analytical modeling of mid-infrared silicon Raman lasers

Silicon photonics has significantly matured in the near-infrared (telecommunication) wavelength range with several commercial products already in the market. More recently, the technology has been extended into the mid-infrared (mid- IR) regime with potential applications in biochemical sensing, tissue photoablation, environmental monitoring and freespace communications. The key advantage of silicon in the mid-IR, as compared with near-IR, is the absence of twophoton absorption (TPA) and free-carrier absorption (FCA). The absence of these nonlinear losses would potentially lead to high-performance nonlinear devices based on Raman and Kerr effects. Also, with the absence of TPA and FCA, the coupled-wave equations that are usually numerically solved to model these nonlinear devices lend themselves to analytical solutions in the mid-IR. In this paper, an analytical model for mid-IR silicon Raman lasers is developed. The validity of the model is confirmed by comparing it with numerical solutions of the coupled-wave equations. The developed model can be used as a versatile and efficient tool for analysis, design and optimization of mid-IR silicon Raman lasers, or to find good initial guesses for numerical methods. The effects of cavity parameters, such as cavity length and facet reflectivities, on the lasing threshold and input-output characteristics of the Raman laser are studied. For instance, for a propagation loss of 0.5 dB/cm, conversion efficiencies as high as 56% is predicted. The predicted optimum cavity (waveguide) length at 2.0 dB/cm propagation loss is ~ 3.4 mm. The results of this study predict strong prospects for mid-IR silicon Raman lasers for the mentioned applications.