Mid-infrared Silicon Research Articles

Mid-infrared silicon photonics for sensing applications

ABSTRACTThe mid-infrared wavelength region offers a plethora of possible applications ranging from sensing, medical diagnostics and free space communications, to thermal imaging and IR countermeasures. Hence group IV mid-infrared photonics is attracting more research interest lately. Sensing is an especially attractive area as fundamental vibrations of many important gases are found in the 3 to 14 μm spectral region. To realise group IV photonic mid-infrared sensors several serious challenges need to be overcome. The first challenge is to find suitable material platforms for the mid-infrared. In this paper we present experimental results for passive mid-infrared photonic devices realised in silicon-on-insulator (SOI), silicon-on-sapphire (SOS), and silicon on porous silicon (SiPSi). Although silicon dioxide is lossy in most parts of the mid-infrared, we have shown that it has potential to be used in the 3-4 μm region. We have characterized SOI waveguides with < 1 dB/cm propagation loss. We have also designed and fabricated SOI passive devices such as MMIs and ring resonators. For longer wavelengths SOS or SiPSi structures could be used. An important active device for long wavelength group IV photonics will be an optical modulator. We present relationships for the free-carrier induced electro-refraction and electro-absorption in silicon in the mid-infrared wavelength range. Electro-absorption modulation is calculated from impurity-doping spectra taken from the literature, and a Kramers-Kronig analysis of these spectra is used to predict electro-refraction modulation. We have examined the wavelength dependence of electro-refraction and electro-absorption, and found that the predictions suggest longer-wave modulator designs will in many cases be different than those used in the telecom range.

Device engineering for silicon photonics

Silicon photonics has attracted increasing attention and research effort in recent years because of its potential for low-cost integration using existing complementary metal–oxide–semiconductor technology. One motivation for research on silicon photonics has been its potential application in energy-efficient high-speed optical interconnects for computing systems, particularly in circumstances where large power dissipation from electrical interconnects will limit performance. The high-density integration of photonic devices, the wide availability of fabrication facilities and the potential for monolithic integration with electronic circuits offer the prospect for future low-cost, reliable and energy-efficient silicon photonic devices suitable for applications in computing, communications and sensing. Herein we review recent progress in the engineering of new devices and functional elements in silicon photonics, including low-loss waveguides, passive integrated devices, integrated lasers, modulators, photodetectors and fiber–chip coupling techniques. We also identify some emerging opportunities with potential for future development, such as mid-infrared silicon photonics and optomechanical devices, and summarize some current trends, including the development of photonic devices based on amorphous silicon and the use of foundries for silicon-photonic device fabrication.

Mid-infrared optical parametric amplifier using silicon nanophotonic waveguides

All-optical signal processing is an approach used to dramatically decrease power consumption and speed up the performance of next-generation optical telecommunications networks1,2,3. Nonlinear optical effects such as four-wave mixing and parametric gain have been explored to realize all-optical functions in glass fibres4. An alternative approach is to use nanoscale engineering of silicon waveguides to enhance optical nonlinearities by up to five orders of magnitude5, enabling integrated chip-scale all-optical signal processing. Four-wave mixing within silicon nanophotonic waveguides has been used to demonstrate telecom-band (λ ≈ 1,550 nm) all-optical functions including wavelength conversion6,7,8,9, signal regeneration10 and tunable optical delay11. Despite these important advances, strong two-photon absorption12 of the telecom-band pump presents a fundamental obstacle, limiting parametric gain to values of several decibels13. Here, we demonstrate a silicon nanophotonic optical parametric amplifier exhibiting broadband gain as high as 25.4 dB, using a mid-infrared pump near one-half the bandgap energy (E ≈ 0.55 eV, λ ≈ 2,200 nm), where parasitic two-photon absorption-related absorption vanishes12,14,15. This gain is high enough to compensate all insertion losses, resulting in 13-dB net off-chip amplification, using only an ultra-compact 4-mm silicon chip. Furthermore, engineering of higher-order waveguide dispersion16 can potentially enable mid-infrared-pumped silicon parametric oscillators17,18,19 and amplifiers for telecom-band optical signals. By taking advantage of the absorption reduction at wavelengths approaching the two-photon absorption bandedge of 2,200 nm, scientists demonstrate a mid-infrared silicon optical parametric amplifier that exhibits broadband gain as large as 25.4 dB and a net off-chip amplification of 13 dB using only an ultra-compact 4-mm silicon chip.

Rib waveguides for mid-infrared silicon photonics

Design rules for both single-mode and polarization-independent strained silicon-on-insulator rib waveguides at the wavelength of 3.39 μm are presented for the first time to our knowledge. Waveguide geometries with different parameters, such as waveguide height, rib width, etch depth, top oxide cover thickness and sidewall angle, have been studied in order to investigate and define design rules that will make devices suitable for mid-IR applications. Chebyshev bivariate interpolation with a standard deviation of less than 1% has been used to represent the zero-birefringence surface. Experimental results for the upper cladding stress level have been used to determine the influence of top oxide cover thickness and different levels of upper cladding stress on waveguide characteristics. Finally, the polarization-insensitive and single-mode locus is presented for different waveguide heights.

Demonstration of a Mid-infrared silicon Raman amplifier

We demonstrate, for the first time, a mid infrared silicon Raman amplifier. Amplification of 12 dB is reported for a signal at 3.39 micron wavelength. The active medium was a 2.5 cm long silicon sample that was pumped with 5ns pulses at 2.88 micron. Such a technology can potentially extend silicon photonics' application beyond data communication in the near IR and into the mid-IR world of remote sensing, biochemical detection and laser medicine. Challenges faced in the mid-IR regime such as a higher free carrier scattering rate longer lifetime in mid-IR waveguides are also discussed.

Nonlinear absorption in silicon and the prospects of mid‐infrared silicon Raman lasers

AbstractWe present the first experimental results of nonlinear absorption in silicon at the mid infrared wavelengths. Nonlinear losses due to two‐photon and free‐carrier absorption that are found to degrade near infrared silicon Raman devices become negligible at photon energies less than half the bandgap (i.e., λ > 2.2 µm in wavelength). Moreover, the low loss window for linear absorption in silicon extends from 1.2 to 6.5 µm. These factors along with the excellent thermal conductivity and high optical damage threshold renders silicon an ideal material for building Raman lasers and amplifiers that operate in the mid infrared wavelengths. This new technology will expand the application space of silicon photonics beyond data communication and into biochemical sensing, laser medicine, and LIDAR. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)