Strong coupling in metal-semiconductor microcavities featuring Ge quantum wells: a perspective study.

In this work we theoretically investigate the possibility of observing strong coupling at mid-infrared frequencies within the group-IV semiconductor material platform. Our results show that the strong coupling condition is attainable in Ge/SiGe quantum wells integrated in hybrid metal-semiconductor microcavities, featuring a highly n-doped SiGe layer as one of the mirrors.

Integrated PIN modulator and photodetector operating in the mid-infrared range from 5.5 μm to 10 μm.

This study reports the experimental demonstration of the first waveguide-integrated SiGe modulator using a PIN diode operating in a wide spectral range of the mid-infrared region. At the wavelength of 10 µm, an extinction ratio up to 10 dB is obtained in injection regime and 3.2 dB in depletion regime.

Waste Material Classification Based on a Wavelength-Sensitive Ge-on-Si Photodetector.

Waste material classification is critical for efficient recycling and waste management. This study proposes a novel, low-cost material classification system based on a single, voltage-tunable Ge-on-Si photodetector operating across the visible and short-wave infrared (SWIR) spectral regions. Thanks to its tunability, the sensor is able to extract spectral information, and the system effectively distinguishes between seven different materials, including plastics, aluminum, glass, and paper.

Modelling second harmonic generation at mid-infrared frequencies in waveguide integrated Ge/SiGe quantum wells.

A promising alternative to bulk materials for the nonlinear coupling of optical fields is provided by photonic integrated circuits based on heterostructures made of asymmetric-coupled quantum wells. These devices achieve a huge nonlinear susceptivity but are affected by strong absorption. Here, driven by the technological relevance of the SiGe material system, we focus on Second-Harmonic Generation in the mid-infrared spectral region, realized by means of Ge-rich waveguides hosting p-type Ge/SiGe asymmetric coupled quantum wells.

1 GHz electro-optical silicon-germanium modulator in the 5-9 µm wavelength range.

Spectroscopy in the mid-infrared (mid-IR) wavelength range is a key technique to detect and identify chemical and biological substances. In this context, the development of integrated optics systems paves the way for the realization of compact and cost-effective sensing systems. Among the required devices, an integrated electro-optical modulator (EOM) is a key element for advanced sensing circuits exploiting dual comb spectroscopy.

On-Chip Mid-Infrared Supercontinuum Generation from 3 to 13 μm Wavelength.

Midinfrared spectroscopy is a universal way to identify chemical and biological substances. Indeed, when interacting with a light beam, most molecules are responsible for absorption at specific wavelengths in the mid-IR spectrum, allowing to detect and quantify small traces of substances. On-chip broadband light sources in the mid-infrared are thus of significant interest for compact sensing devices.

Engineering of the spin on dopant process on silicon on insulator substrate.

We report on a systematic analysis of phosphorus diffusion in silicon on insulator thin film via spin-on-dopant process (SOD). This method is used to provide an impurity source for semiconductor junction fabrication. The dopant is first spread into the substrate via SOD and then diffused by a rapid thermal annealing process.

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.

Ge-rich graded SiGe waveguides and interferometers from 5 to 11 µm wavelength range.

The mid-infrared (mid-IR) wavelength range hosts unique vibrational and rotational resonances of a broad variety of substances that can be used to unambiguously detect the molecular composition in a non-intrusive way. Mid-IR photonic-integrated circuits (PICs) are thus expected to have a major impact in many applications. Still, new challenges are posed by the large spectral width required to simultaneously identify many substances using the same photonic circuit.

SiGe quantum well infrared photodetectors on strained-silicon-on-insulator.

We demonstrate p-type SiGe quantum well infrared photodetectors (QWIPs) on a strained-silicon-on-insulator (sSOI) substrate. The sSOI system allows strain-balancing between the QWIP heterostructure with an average composition of SiGe and the substrate, and therefore lifts restrictions to the active material thickness faced by SiGe growth on silicon or silicon-on-insulator substrates. The realized sSOI QWIPs feature a responsivity peak at detection wavelengths around 6 µm, based on a transition between heavy-hole states.

On-chip Fourier-transform spectrometer based on spatial heterodyning tuned by thermo-optic effect.

Miniaturized optical spectrometers providing broadband operation and fine resolution have an immense potential for applications in remote sensing, non-invasive medical diagnostics and astronomy. Indeed, optical spectrometers working in the mid-infrared spectral range have garnered a great interest for their singular capability to monitor the main absorption fingerprints of a wide range of chemical and biological substances. Fourier-transform spectrometers (FTS) are a particularly interesting solution for the on-chip integration due to their superior robustness against fabrication imperfections.

On-chip Bragg grating waveguides and Fabry-Perot resonators for long-wave infrared operation up to 8.4 µm.

Taking advantage of unique molecular absorption lines in the mid-infrared fingerprint region and of the atmosphere transparency window (3-5 µm and 8-14 µm), mid-infrared silicon photonics has attracted more research activities with a great potential for applications in different areas, including spectroscopy, remote sensing, free-space communication and many others. However, the demonstration of resonant structures operating at long-wave infrared wavelengths still remains challenging. Here, we demonstrate Bragg grating-based Fabry-Perot resonators based on Ge-rich SiGe waveguides with broadband operation in the mid-infrared.

Modeling of second harmonic generation in hole-doped silicon-germanium quantum wells for mid-infrared sensing.

The development of Ge and SiGe chemical vapor deposition techniques on silicon wafers has enabled the integration of multi-quantum well structures in silicon photonics chips for nonlinear optics with potential applications to integrated nonlinear optics, however research has focused up to now on undoped quantum wells and interband optical excitations. In this work, we present model calculations for the giant nonlinear coefficients provided by intersubband transitions in hole-doped Ge/SiGe and Si/SiGe multi-quantum wells. We employ a valence band-structure model for SiGe to calculate the confined hole states of asymmetric-coupled quantum wells for second-harmonic generation in the mid-infrared.

Broadband integrated racetrack ring resonators for long-wave infrared photonics.

Long-wave infrared photonics is an exciting research field meant to revolutionize our daily life by means of key advances in several domains including communications, imaging systems, medical care, environmental monitoring, or multispectral chemical sensing, among others. For this purpose, integrated photonics is particularly promising owing to its compactness, mass fabrication, and energy-efficient characteristics. We present in this Letter, for the first time to the best of our knowledge, broadband integrated racetrack ring resonators operating within the crucial molecular fingerprint region.

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.

Integrated broadband dual-polarization Ge-rich SiGe mid-infrared Fourier-transform spectrometer.

Miniaturized on-chip spectrometers covering a wide band of the mid-infrared spectrum have an immense potential for multi-target detection in high-impact applications, such as chemical sensing or environmental monitoring. Specifically, multi-aperture spatial heterodyne Fourier-transform spectrometers (SHFTS) provide high throughput and improved tolerance against fabrication errors, compared to conventional counterparts. Still, state-of-the-art implementations have only shown single-polarization operation in narrow bandwidths within the near and short infrared.

Nonlinear Properties of Ge-rich SiGe Materials with Different Ge Concentrations.

Silicon photonics is a large volume and large scale integration platform for applications from long-haul optical telecommunications to intra-chip interconnects. Extension to the mid-IR wavelength range is now largely investigated, mainly driven by absorption spectroscopy applications. Germanium (Ge) is particularly compelling as it has a broad transparency window up to 15 µm and a much higher third-order nonlinear coefficient than silicon which is very promising for the demonstration of efficient non-linear optics based active devices.

Ultra-wideband Ge-rich silicon germanium integrated Mach-Zehnder interferometer for mid-infrared spectroscopy.

This Letter explores the use of Ge-rich SiGe waveguides on graded SiGe substrate for the demonstration of ultra-wideband photonic integrated circuits in the mid-infrared (mid-IR) wavelength range. We designed, fabricated, and characterized broadband Mach-Zehnder interferometers fully covering a range of 3 μm in the mid-IR band. The fabricated devices operate indistinctly in quasi-TE and quasi-TM polarizations, and have an extinction ratio higher than 10 dB over the entire operating wavelength range.

Low-loss Ge-rich SiGe waveguides for mid-infrared photonics.

We demonstrate low-loss Ge-rich SiGe waveguides on SiGe (x from 0 to 0.79) graded substrates operating in the mid-infrared wavelength range at λ=4.6  μm.

Optical Activation of Germanium Plasmonic Antennas in the Mid-Infrared.

Impulsive interband excitation with femtosecond near-infrared pulses establishes a plasma response in intrinsic germanium structures fabricated on a silicon substrate. This direct approach activates the plasmonic resonance of the Ge structures and enables their use as optical antennas up to the mid-infrared spectral range. The optical switching lasts for hundreds of picoseconds until charge recombination redshifts the plasma frequency.

Sharp bends and Mach-Zehnder interferometer based on Ge-rich-SiGe waveguides on SiGe graded buffer.

The integration of germanium (Ge)-rich active devices in photonic integrated circuits is challenging due to the lattice mismatch between silicon (Si) and Ge. A new Ge-rich silicon-germanium (SiGe) waveguide on graded buffer was investigated as a platform for integrated photonic circuits. At a wavelength of 1550 nm, low loss bends with radii as low as 12 µm and Multimode Interferometer beam splitter based on Ge-rich SiGe waveguide on graded buffer were designed, fabricated and characterized.

Giant electro-optic effect in Ge/SiGe coupled quantum wells.

Silicon-based photonics is now considered as the photonic platform for the next generation of on-chip communications. However, the development of compact and low power consumption optical modulators is still challenging. Here we report a giant electro-optic effect in Ge/SiGe coupled quantum wells.

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.

Recent progress in GeSi electro-absorption modulators.

Electro-absorption from GeSi heterostructures is receiving growing attention as a high performance optical modulator for short distance optical interconnects. Ge incorporation with Si allows strong modulation mechanism using the Franz-Keldysh effect and the quantum-confined Stark effect from bulk and quantum well structures at telecommunication wavelengths. In this review, we discuss the current state of knowledge and the on-going challenges concerning the development of high performance GeSi electro-absorption modulators.

Quantum-confined Stark effect at 1.3 μm in Ge/Si(0.35)Ge(0.65) quantum-well structure.

Room-temperature quantum-confined Stark effect in a Ge/SiGe quantum-well structure is reported at the wavelength of 1.3 μm. The operating wavelength is tuned by the use of strain engineering.