Light bulbs emit white light, consisting of a full spectrum of visible light from 380 to 740 nm. Lasers, on the other hand, emit only one wavelength (a very narrow spectrum) of light but with much higher intensity. Frequency comb is an emerging laser source that emits discrete and equally-spaced frequencies of laser light that can cover a wide range of the spectrum, which can be crucial for future metrology, precision spectroscopy, astronomical observations, ultrafast optics, and quantum information technology.
We will utilize non-linearity engineering to control the laser light propagation inside the Ge waveguide/cavity. The laser light will be generated from a quantum cascade laser or a semiconductor diode laser, and then guided into the Ge waveguide/cavity. We will optimize the waveguide/cavity to achieve low loss and will engineer the aspect ratio to enhance the so-called Kerr nonlinearity, and this will lead to the creation of frequency combs at MIR and LWIR wavelength. In the last year of the project, we will use frequency combs to perform dual-comb spectroscopy for real-time gas analysis.
Now, Ge microresonators have been successfully made by a series of self-developed fabrication processes, and the microresonator shows a record-high quality (Q) factor for the long-wave infrared spectrum. The improved Q factor is two orders of magnitude higher than previous reports, which is highly beneficial for generating frequency comb at long-wave infrared, since the threshold of the frequency comb generation is inversely related to Q squared.
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.
