Characterization of integrated waveguides by atomic-force-microscopy-assisted mid-infrared imaging and spectroscopy.

A novel spectroscopy technique to enable the rapid characterization of discrete mid-infrared integrated photonic waveguides is demonstrated. The technique utilizes lithography patterned polymer blocks that absorb light strongly within the molecular fingerprint region. These act as integrated waveguide detectors when combined with an atomic force microscope that measures the photothermal expansion when infrared light is guided to the block. As a proof of concept, the technique is used to experimentally characterize propagation loss and grating coupler response of Ge-on-Si waveguides at wavelengths from 6 to 10 µm. In addition, when the microscope is operated in scanning mode at fixed wavelength, the guided mode exiting the output facet is imaged with a lateral resolution better than 500 nm i.e. below the diffraction limit. The characterization technique can be applied to any mid-infrared waveguide platform and can provide non-destructive in-situ testing of discrete waveguide components.

Ge-on-Si waveguides for sensing in the molecular fingerprint regime.

Low loss, single mode, Ge-on-Si rib waveguides are used to demonstrated optical sensing in the molecular fingerprint region of the mid-infrared spectrum. Sensing is carried out using two spin-coated films, with strong absorption in the mid-infrared. These films are used to calibrate the modal overlap with an analyte, and therefore experimentally demonstrate the potential for Ge-on-Si waveguides for mid-infrared sensing applications. The results are compared to Fourier transform infrared spectroscopy measurements. The advantage of waveguide spectroscopy is demonstrated in terms of the increased optical interaction, and a new multi-path length approach is demonstrated to improve the dynamic range, which is not possible with conventional FTIR or attenuated total reflection (ATR) measurements. These results highlight the potential for Ge-on-Si as an integrated sensing platform for healthcare, pollution monitoring and defence applications.

1.4 million Q factor Si3N4 micro-ring resonator at 780 nm wavelength for chip-scale atomic systems.

A silicon nitride micro-ring resonator with a loaded Q factor of 1.4 × 106 at 780 nm wavelength is demonstrated on silicon substrates. This is due to the low propagation loss waveguides achieved by optimization of waveguide sidewall interactions and top cladding refractive index. Potential applications include laser frequency stabilization allowing for chip-scale atomic systems targeting the 87Rb atomic transition at 780.24 nm. The temperature dependent wavelength shift of the micro-ring was determined to be 13.1 pm/K indicating that a minimum temperature stability of less than ±15 mK is required for such devices for wavelength locking applications. If a polyurethane acrylate top cladding of an optimized thickness is used then the micro-ring could effectively be athermal, resulting in reduced footprint, power consumption, and cost of potential devices.

Plasmonic mid-infrared third harmonic generation in germanium nanoantennas.

We demonstrate third harmonic generation in plasmonic antennas consisting of highly doped germanium grown on silicon substrates and designed to be resonant in the mid-infrared frequency range that is inaccessible with conventional nonlinear plasmonic materials. Owing to the near-field enhancement, the result is an ultrafast, subdiffraction, coherent light source with a wavelength tunable between 3 and 5 µm, and ideally overlapping with the fingerprint region of molecular vibrations. To observe the nonlinearity in this challenging spectral window, a high-power femtosecond laser system equipped with parametric frequency conversion in combination with an all-reflective confocal microscope setup is employed. We demonstrate spatially resolved maps of the linear scattering cross section and the nonlinear emission of single isolated antenna structures. A clear third-order power dependence as well as mid-infrared emission spectra prove the nonlinear nature of the light emission. Simulations support the observed resonance length of the double-rod antenna and demonstrate that the field enhancement inside the antenna material is responsible for the nonlinear frequency mixing.

Midinfrared Plasmon-Enhanced Spectroscopy with Germanium Antennas on Silicon Substrates.

Midinfrared plasmonic sensing allows the direct targeting of unique vibrational fingerprints of molecules. While gold has been used almost exclusively so far, recent research has focused on semiconductors with the potential to revolutionize plasmonic devices. We fabricate antennas out of heavily doped Ge films epitaxially grown on Si wafers and demonstrate up to 2 orders of magnitude signal enhancement for the molecules located in the antenna hot spots compared to those located on a bare silicon substrate. Our results set a new path toward integration of plasmonic sensors with the ubiquitous CMOS platform.