Integrated III-V Semiconductor Frequency Comb on a Chip

Light bulbs emit white light, encompassing the entire visible spectrum from 380 to 740 nanometers. In contrast, lasers emit a single wavelength, resulting in a narrow spectrum of light. However, a new type of laser source called a frequency comb has emerged, emitting multiple frequencies of light that are evenly spaced. This property allows frequency combs to cover a broad range of the spectrum, making them highly valuable in fields such as metrology, precision spectroscopy, astronomical observations, ultrafast optics, and quantum information technology. In the realm of photonic sensing, the weight of a molecular compound affects its intramolecular vibration frequency. Heavy compounds vibrate at lower frequencies. The longwave infrared (LWIR) spectrum, which spans from 6 to 14 micrometers, contains distinctive molecular fingerprints. These fingerprints have found applications in molecular sensing for health monitoring, production control, and environmental observation. If we could access a broadband frequency comb in the LWIR, we could identify a multitude of molecules in real-time. Such breakthrough development would open the door to a range of possibilities for molecular analysis. Our project has achieved a remarkable breakthrough in the LWIR by developing an optical microresonator with the lowest optical loss 0.5 dB/cm and the highest quality factor 250,000. We have achieved an improvement of two orders of magnitude compared to previous studies. The quality factor is a measure of the resonator's ability to efficiently store and sustain light energy. A higher quality factor corresponds to a lower threshold for frequency comb generation. In fact, the threshold for frequency comb generation is inversely proportional to the square of the quality factor (Q squared). By attaining such an incredibly high-quality factor, we have drastically reduced the threshold for generating a broadband frequency comb. Our accomplishment in this regard has been complemented by the successful simulation of the frequency comb generation process using the Lugiato-Lefever equation. To further enhance the stability and reliability of our optical platform and achieve robust frequency comb generation, we have utilized heterogenous integration techniques. By integrating different materials, we have successfully created a native Germanium (Ge) waveguide on a Zinc Selenide (ZnSe) substrate. This Ge waveguide showcases an impressively low optical loss, measuring as low as 1/cm at 8 micrometers. Additionally, it offers a broadband transparency window from 2 to 14 micrometers. This innovative Ge on ZnSe platform holds immense promise for integrated LWIR photonics, analogous to the widely utilized Silicon (Si) on insulator platform employed in the near infrared. We foresee significant contributions and advancements through the application of this platform. By utilizing the exceptional properties of frequency combs and leveraging our achievements in the LWIR optical microresonator and Ge on ZnSe platform, we are propelling forward the frontiers of scientific research and technological applications in various domains, ultimately leading to a more comprehensive understanding of the world around us.

We have achieved the below milestones for longwave infrared (LWIR) photonics: 1. developed an ultralow loss microresonator in the LWIR using native Ge. The microresonator shows two orders of magnitude improvement in the quality factor, which corresponds to an optical loss of 0.5dB/cm. 2. we have successfully simulated the frequency comb generation from our Ge microresonator using Lugiato–Lefever equation. 3. we have developed a novel Ge on ZnSe platform for fully integrated LWIR photonics. This will provide a robust platform to achieve LWIR frequency comb.

We propose to realize the generation of frequency combs using diode lasers and quantum cascade lasers (QCLs) at mid-infrared (MIR) and long-wavelength infrared (LWIR) wavelengths, respectively. Since Ge is transparent at both MIR and LWIR wavelengths, we will utilize a heterogeneous wafer bonding technique to monolithically integrate Ge on top of laser structures. The etching process will be optimized to minimize the waveguide loss, and group velocity engineering will be applied to achieve zero group velocity dispersion at both MIR and LWIR by adjusting the aspect ratio and geometry of the waveguide. For the most challenging LWIR wavelengths, we will maximize the Kerr nonlinearity of the waveguide and utilize coupled-waveguides for dispersion compensation to achieve zero group velocity dispersion. By achieving the frequency comb generation using diode lasers and QCLs, demonstration experiments will be carried out to quantify the composition of a gas mixture at MIR wavelength and to detect DMMP molecules with its concentration under 0.5 ppb in air. These will allow us to achieve a frequency comb platform at both MIR and LWIR that ushers in an era of on-chip applications in ultra-sensitive sensing technologies, which will benefit a variety of subjects. These include bio-medical and environmental sensing, screening, imaging, astronomical exploration, and more.