Optical losses in pure crystalline silicon in the IR band measured using WGM microresonators

TL;DR

The study addresses optical losses in pure crystalline silicon within the mid-IR by using whispering gallery mode microresonators to extract Q-factors at 1.5, 2.6, 6.1, and 8.6 μm. It combines identical silicon resonators with varying resistivity and growth methods, employing multiple Q-measurement techniques to isolate loss channels such as Rayleigh scattering, bulk/multiphoton absorption, and surface effects. The key findings show that resistivity and growth method largely govern losses, with high-resistivity HFZ samples achieving record-like Q-values across wavelengths, while the conductance type is less influential; the Q trends across wavelengths reflect a competition between scattering reductions and absorption mechanisms (water-related at 6.1 μm and multiphonon at 8.6 μm). Overall, the work validates WGM microresonators as robust mid-IR loss metrology tools and informs the development of silicon-based mid-IR photonics.

Abstract

The need for semiconductor technology for crystalline silicon of the highest purity and homogeneity has provided samples exhibiting low optical absorption in the infrared range. Such silicon has become the basis for photonic elements in the telecommunication band, including high-Q microresonators, which are particularly important. However, at longer wavelengths, the loss mechanisms have not yet been sufficiently studied. At the same time, this range is extremely important, especially for biological and medical applications and for fundamental research. We used optical microresonators with whispering gallery modes made from various types of silicon crystals as a tool to study the loss mechanisms. The study involved the pump wavelengths 1.5, 2.6, 6.1, and 8.6 $μ$m and the maximum measured Q-factors were $1.5\cdot10^9$, $5\cdot10^8$, $1.6\cdot10^7$, and $5\cdot10^4$, respectively. We showed that the conductivity type does not noticeably influence the optical losses, while resistivity and the growing method are defining factors. Our study confirms the utility of WGM microresonators as loss measurement tools and provides significant potential for the development of silicon microresonator-based photonics in the mid-IR band.

Figures (6)

• Figure 1: Fig. 1. Experimental setup for Q-factor measurements. The pump wavelength was determined by the single-frequency laser used: fiber laser with isolator and polarization controller at 1.5 µ m, distributive feedback diode laser without isolator at 2.6 µ m, quantum-cascade laser without isolator at 6.1 µ m and 8.6 µ m), PD is the photodetector for the operating wavelength, $\Phi$ is the angle of incidence.
• Figure 2: Fig. 2. Ring-down measurements of samples 1-3 at 1.5 $\mu$m. In the left column transmission signals are presented. In the right column the exponential approximations are presented. The measurements are conducted in the undercoupled regime.
• Figure 3: Fig. 3. The full-width at half maximum measurements of the samples 4-7 at 1.5 $\mu$m. In the first row the transmission spectra of the forward and backward frequency scans are presented. In the middle and bottom row the Lorentzian approximations of the resonances are presented.
• Figure 4: Fig. 4. The transmission spectrum for high-power pumping of the 3d sample in the critical coupling regime. The distinct thermo-optic oscillations are observed.
• Figure 5: Fig. 5. The full-width at half maximum measurements for the 7th microresonator at 2.6 $\mu$m. In the first row the transmission spectra of the forward and backward frequency scans are presented. In the middle and bottom row the Lorentzian approximations of the resonances are presented.