Direct-gap gain and optical absorption in germanium correlated to the density of photoexcited carriers, doping, and strain

High-quality microresonators in the longwave infrared based on native germanium

The longwave infrared (LWIR) region of the spectrum spans 8 to 14 μm and enables high-performance sensing and imaging for detection, ranging, and monitoring. Chip-scale LWIR photonics has enormous potential for real-time environmental monitoring, explosive detection, and biomedicine. However, realizing technologies such as precision sensors and broadband frequency combs requires ultra low-loss and low-dispersion components, which have so far remained elusive in this regime. Here, we use native germanium to demonstrate the first high-quality microresonators in the LWIR. These microresonators are coupled to partially-suspended Ge waveguides on a separate glass chip, allowing for the first unambiguous measurements of isolated linewidths. At 8 μm, we measured losses of 0.5 dB/cm and intrinsic quality (Q) factors of 2.5 × 10⁵, nearly two orders of magnitude higher than prior LWIR resonators. Our work portends the development of novel sensing and nonlinear photonics in the LWIR regime.

Developing longwave infrared technology hide intrinsic challenges but at the same time is important to develop sensing and imaging for detection, ranging, and monitoring systems. Here the authors demonstrate the fabrication of high-quality microresonators in the LWIR with the simple use of native germanium.

Analysis of stress distribution in microfabricated germanium with external stressors for enhancement of light emission

In the field of silicon photonics, germanium (Ge) is an attractive material for monolithic light sources. Tensile strain is a promising means for Ge based light sources due to enhancing direct band gap recombination. We investigated strain engineering in Ge using silicon nitride (SiN_x) stressors. We found that microfabricated Ge greatly improves the tensile strain because SiN_x on the Ge sidewalls causes a large tensile strain in the direction perpendicular to the substrate. Tensile strain equivalent to an in-plane biaxial tensile strain of 0.8% at maximum was applied, and the PL emission intensity was improved more than five times at the maximum.

Lasing in strained germanium microbridges

Germanium has long been regarded as a promising laser material for silicon based opto-electronics. It is CMOS-compatible and has a favourable band structure, which can be tuned by strain or alloying with Sn to become direct, as it was found to be required for interband semiconductor lasers. Here, we report lasing in the mid-infrared region (from λ = 3.20 μm up to λ = 3.66 μm) in tensile strained Ge microbridges uniaxially loaded above 5.4% up to 5.9% upon optical pumping, with a differential quantum efficiency close to 100% with a lower bound of 50% and a maximal operating temperature of 100 K. We also demonstrate the effect of a non-equilibrium electron distribution in k-space which reveals the importance of directness for lasing. With these achievements the strained Ge approach is shown to compare well to GeSn, in particular in terms of efficiency.

Germanium (based) lasers are a promising route towards a fully CMOS-compatible light source, key to the further development of silicon photonics. Here, the authors realize lasing from strained germanium microbridges up to 100 K, finding a quantum efficiency close to 100%.

Recent Progress on Ge/SiGe Quantum Well Optical Modulators, Detectors, and Emitters for Optical Interconnects

Germanium/Silicon-Germanium (Ge/SiGe) multiple quantum wells receive great attention for the realization of Si-based optical modulators, photodetectors, and light emitters for short distance optical interconnects on Si chips. Ge quantum wells incorporated between SiGe barriers, allowing a strong electro-absorption mechanism of the quantum-confined Stark effect (QCSE) within telecommunication wavelengths. In this review, we respectively discuss the current state of knowledge and progress of developing optical modulators, photodetectors, and emitters based on Ge/SiGe quantum wells. Key performance parameters, including extinction ratio, optical loss, swing bias voltages, and electric fields, and modulation bandwidth for optical modulators, dark currents, and optical responsivities for photodetectors, and emission characteristics of the structures will be presented.

Enhanced indirect-to-direct inter-valley scattering in germanium under tensile strain for improving the population of electrons in direct valley

A theoretical model is proposed to analyze the inter-valley electron transferring between direct Γ and indirect L valleys, which sheds light on the electron conduction dynamics in (0 0 1) tensile strained Ge. Inter-valley scattering is included to calculate average scattering time between Γ and L valleys based on a time-dependent Hamiltonian describing the electron-phonon interaction. Numerical results indicate that enhanced indirect-to-direct inter-valley scattering and reduced direct-to-indirect inter-valley scattering are reliable by introducing tensile strain in Ge material. The population ratio of electrons in Γ and L valleys in strained Ge will increase one to two orders of magnitude compared to the model without the inter-valley scattering. The results offer fundamental understanding of phonon engineering for further improvement of performance in strained germanium light sources.

Low-threshold optically pumped lasing in highly strained germanium nanowires

The integration of efficient, miniaturized group IV lasers into CMOS architecture holds the key to the realization of fully functional photonic-integrated circuits. Despite several years of progress, however, all group IV lasers reported to date exhibit impractically high thresholds owing to their unfavourable bandstructures. Highly strained germanium with its fundamentally altered bandstructure has emerged as a potential low-threshold gain medium, but there has yet to be a successful demonstration of lasing from this seemingly promising material system. Here we demonstrate a low-threshold, compact group IV laser that employs a germanium nanowire under a 1.6% uniaxial tensile strain as the gain medium. The amplified material gain in strained germanium can sufficiently overcome optical losses at 83 K, thus allowing the observation of multimode lasing with an optical pumping threshold density of ~3.0 kW cm^-2. Our demonstration opens new possibilities for group IV lasers for photonic-integrated circuits.

Design and analysis of a CMOS-compatible distributed Bragg reflector laser based on highly uniaxial tensile stressed germanium

We design a CMOS-compatible Distributed Bragg Reflector (DBR) laser based on highly uniaxial tensile stressed germanium. Our design first incorporates three critical elements including high uniaxial tensile stress, low loss optical resonator and heterojunction for electrical injection. A threshold current density of 80 kA/cm² and an internal quantum efficiency of 8.5% are estimated when the Shockley-Reed-Hall (SRH) lifetime is chosen to be 3 ns. Furthermore, the performance of the DBR laser can be enhanced by improving the crystal quality and carefully designing the p-n junction. The simulation results also indicate that the limitation of the improvement of threshold current density and internal quantum efficiency are 29 kA/cm² and 19.6%, resulting from the Auger recombination. The influences of strain and n-type doping on the threshold current density and the internal quantum efficiency are discussed. The proposed DBR laser offers a new approach to realize on-chip light source for silicon photonics.

Ultrafast carrier dynamics in Ge by ultra-broadband mid-infrared probe spectroscopy

In this study, we carried out 800-nm pump and ultra-broadband mid-infrared (MIR) probe spectroscopy with high time-resolution (70 fs) in bulk Ge. By fitting the time-resolved difference reflection spectra [ΔR(ω)/R(ω)] with the Drude model in the 200–5000 cm⁻¹ region, the time-dependent plasma frequency and scattering rate have been obtained. Through the calculation, we can further get the time-dependent photoexcited carrier concentration and carrier mobility. The Auger recombination essentially dominates the fast relaxation of photoexcited carriers within 100 ps followed by slow relaxation due to diffusion. Additionally, a novel oscillation feature is clearly found in time-resolved difference reflection spectra around 2000 cm⁻¹ especially for high pump fluence, which is the Lorentz oscillation lasting for about 20 ps due to the Coulomb force exerted just after the excitation.

Theoretical study of the effect of different n-doping elements on band structure and optical gain of GeSn alloys

Doping with Sb and Bi can assist in converting GeSn into a direct bandgap material and improve its optical gain.

Optical net gain measurement in n-type doped germanium waveguides under optical pumping for silicon monolithic laser

Silicon (Si) monolithic lasers are key devices in large-scale, high-density photonic integrated circuits. Germanium (Ge) is promising as an active layer due to the complementary metal-oxide semiconductor process compatibility with Si. A net optical gain from Ge is essential to demonstrate lasing operation. We fabricated Ge waveguides and investigated the n-type doping effect on the net optical gain. The estimated net gain of the n-Ge waveguide increased from -2200 to -500/cm, namely reducing loss, under optically pumped condition.

Direct Bandgap Light Emission from Strained Germanium Nanowires Coupled with High-Q Nanophotonic Cavities

A silicon-compatible light source is the final missing piece for completing high-speed, low-power on-chip optical interconnects. In this paper, we present a germanium nanowire light emitter that encompasses all the aspects of potential low-threshold lasers: highly strained germanium gain medium, strain-induced pseudoheterostructure, and high-Q nanophotonic cavity. Our nanowire structure presents greatly enhanced photoluminescence into cavity modes with measured quality factors of up to 2000. By varying the dimensions of the germanium nanowire, we tune the emission wavelength over more than 400 nm with a single lithography step. We find reduced optical loss in optical cavities formed with germanium under high (>2.3%) tensile strain. Our compact, high-strain cavities open up new possibilities for low-threshold germanium-based lasers for on-chip optical interconnects.

Strain distribution in single, suspended germanium nanowires studied using nanofocused x-rays

Within the quest for direct band-gap group IV materials, strain engineering in germanium is one promising route. We present a study of the strain distribution in single, suspended germanium nanowires using nanofocused synchrotron radiation. Evaluating the probed Bragg reflection for different illumination positions along the nanowire length results in corresponding strain components as well as the nanowire's tilting and bending. By using these findings we determined the complete strain state with the help of finite element modelling. The resulting information provides us with the possibility of evaluating the validity of the strain investigations following from Raman scattering experiments which are based on the assumption of purely uniaxial strain.

Electrically pumped lasing from Ge Fabry-Perot resonators on Si

Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices has been observed. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Perot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1660 nm and 1700 nm. This fits the theoretically predicted behavior for the n-type Ge material system. With further pulsed electrical injection of 500 kA/cm² it was possible to reach the lasing threshold for such edge emitters. Different lengths and widths of devices have been investigated in order to maintain best gain-absorption ratios.

Tensile Ge microstructures for lasing fabricated by means of a silicon complementary metal-oxide-semiconductor process

In this work we study, using experiments and theoretical modeling, the mechanical and optical properties of tensile strained Ge microstructures directly fabricated in a state-of-the art complementary metal-oxide-semiconductor fabrication line, using fully qualified materials and methods. We show that these microstructures can be used as active lasing materials in mm-long Fabry-Perot cavities, taking advantage of strain-enhanced direct band gap recombination. The results of our study can be realistically applied to the fabrication of a prototype platform for monolithic integration of near infrared laser sources for silicon photonics.