Silicon Nanophotonic Waveguides for the Mid-Infrared (0911.0949v1)

Abstract: It has recently been shown that silicon nanophotonic waveguides can be used to construct all of the components of a photonic data transmission system on a single chip. These components can be integrated together with CMOS electronics to create complex electronic-photonic integrated circuits. It has also emerged that the high field confinement of silicon nanoscale guides enables exciting new applications, from chipscale nonlinear optics to biosensors and light-force activated devices. To date, most of the experiments in silicon waveguides have been at wavelengths in the near-infrared, ranging from 1.1-2 microns. Here we show that single-mode silicon nano-waveguides can be used at mid-infrared wavelengths, in particular at 4.5 microns, or 2222.2 1/cm. This idea has appeared in theoretical literature, but experimental realization has been elusive. This result represents the first practical integrated waveguide system for the mid-infrared in silicon, and enables a range of new applications.

• The paper demonstrates the experimental realization of low-loss silicon waveguides at 4.5 μm using silicon-on-sapphire substrates.
• It employs a 1.8x0.6 μm ridge design validated by a Yee-grid eigensolver, achieving losses of 10.4±1.2 dB/cm at a 40 μm bend radius.
• The findings pave the way for integrated mid-IR photonic systems, opening opportunities for advanced sensing, imaging, and telecommunications applications.

Silicon Nanophotonic Waveguides for the Mid-Infrared

This paper addresses the experimental realization of silicon nanophotonic waveguides operating at mid-infrared (mid-IR) wavelengths, specifically focusing on a wavelength of 4.5 μm. The paper substantiates the theoretical proposition that silicon waveguides can extend their utility to mid-IR applications, marking a significant advancement from the conventional near-infrared operations. The research illustrates the construction and testing of these waveguides using the silicon-on-sapphire (SOS) materials system, which offers high field confinement and eliminates substrate leakage—an often encountered obstacle in silicon-on-insulator systems.

Historically, the mid-IR spectrum has posed challenges in photonics due to the necessity for bulky, expensive coherent sources, often requiring cryogenic cooling, and the absence of integrated optical waveguides. Recent technological developments, such as commercially available single-mode quantum cascade lasers and single-mode infrared fibers up to 6 μm wavelengths, have made mid-IR optical systems more feasible and affordable. This development broadens the potential for mid-IR applications, including thermal imaging, chemical spectroscopy, astronomy, and military uses, and signifies a shift in the landscape of integrated photonic systems.

Waveguide Design and Results

Mid-IR waveguides were engineered using a 1.8x0.6 μm ridge waveguide geometry. The choice of SOS material is particularly deliberate for its advantages in simplifying waveguide fabrication and maintaining electronics compatibility. The empirical results demonstrated a successful design of low-loss silicon waveguides, with measured waveguide losses of 10.4 ± 1.2 dB/cm at a 40 μm bend radius after testing. The design methodology employed a Yee-grid based eigensolver for simulations, confirming the guidance of the TE0 mode with low loss, effectively disregarding the TM0 mode near cutoff.

These waveguides are new to mid-IR photonics, with an ambitious mode confinement that encourages integration with CMOS electronics on a singular substrate. Fabrication utilized semiconductor processing techniques on epitaxial SOS wafers, leveraging CF4 plasma for pattern transfer and experimenting with polarizers for mode analysis. The waveguide operation aligns with theoretically predicted models, substantiating silicon's role in mid-IR photonics.

Implications and Prospects

The implications of this research are multifaceted. In the short term, this paper's strong numerical results suggest that improvements in fabrication could significantly reduce current waveguide losses, trending toward the theoretically low loss determined by bulk crystalline silicon properties. In the burgeoning field of mid-IR photonics, this experimental success propels the possibility of building integrated mid-IR lasers and detectors, utilizing techniques such as wafer bonding.

Additionally, the paper suggests the potential for silicon platforms to host advanced optical devices, including high-confinement nonlinear optics and fully integrated systems capable of applications from sensing to telecommunication. The SOS waveguides’ operational bandwidth covering over two octaves—including telecommunications wavelength regions—also opens new avenues for developing complex on-chip solutions previously constrained to free-space optics.

Conclusion

This research marks a pivotal step in advancing the practical application of mid-IR silicon nanophotonic waveguides, indicating a promising direction for on-chip photonic systems and mid-IR technology. Future developments are likely to focus on optimizing waveguide design, reducing insertion and propagation losses, and exploring broader mid-IR applications, potentially revolutionizing fields from precision sensing to integrated optical networks. This foundational work advocates for further exploration into the mid-IR domain, facilitated by the new capabilities demonstrated here.