Material Investigation for Laser Amplification (MILA)

In this project, based on previous as well as future experimental studies, we propose to investigate a new class of materials i.e. silicon nitride (SiN) and silicon oxynitride (SiON) for their integration as non-linear elements in a recently developed mode-locked laser [1]. This project is a collaboration between the UOP group, and the Fondazione Bruno Kessler (Trento, Italy, reference person: G. Pucker, M. Ghulinyan) and the University of Trento (Trento, Italy, reference person: L. Pavesi).

One specific objectives is the characterization of SiN and SiON micro-resonators and low loss waveguides previously designed and fabricated at the Micro-fabrication and Technologic Laboratory (MTLab) of the Fondazione Bruno Kessler (FBK). Those two materials or their combination i.e. SiN ring resonators and SiON bus waveguides, are actually tested for their implementation in the FD-FWM laser operating scheme, thus providing a feedback for the simulation of the laser structure and the optimization of the specific waveguides and micro-ring resonators geometry. Furthermore thanks to the good stability of the studied mode-locked laser, frequency combs which are of increasing interest for generation of radio-frequency reference signals, could be envisioned. In the first part of the present study, SiN and SiON thin films with different refractive index using various conditions of PECVD atmosphere with the purpose of obtaining high quality near stoichiometric have been prepared. Different deposition routines have been employed, including variable flow ratio of silane (SiH4) ammonia (NH3) and nitrous oxide (N2O) and deposited on to a silicon substrate. Films have been characterized in term of deposition rate, thickness, refractive index, surface morphology and composition. Waveguides made of SiN and SiON, have then been fabricated by PECVD process. The optimum geometry for light propagation and coupling between the waveguiding structure, and the ring resonators has been determined by simulation tools thus defining the optimum micro-fabrication parameters i.e. deposited thickness, etching depth and waveguide width. In figure 1 is presented the Mode profile at 1550 nm and losses calculation from 1480 to 1550 nm for a SiON waveguide. 

 
 
 
Figure 1: Mode profile (left) at 1550 nm and losses calculation (right) from 1480 to 1550 nm for a SiON based waveguide.
 
 
 
A particular attention has been paid to the process control for the minimization of absorption losses and then for successful operation of the devices. The microresonators were characterized in typical waveguide transmission experiments in a broad near-infrared wavelength range between 1350 nm and 1600 nm. Figure 2a shows a series of sharp and broad resonances corresponding to first- (fundamental) and second-order radial mode families of the SiN based resonator. A blow- up of the spectrum around a fundamental mode is shown in the top panel of Figure 2b.
 
 
 
 
Figure 2: Left, the measured broad range spectrum of the wedge resonator shows a series of 1st and 2nd order radial family modes of TE-polarization. Right, the high-resolution spectra taken around a wavelength of 1554 nm is shown for the SiN resonators.
 

References
[1] M. Peccianti, A. Pasquazi, Y. Park, B.E. Little, S.T. Chu, D.J. Moss, R. Morandotti, Demonstration of a stable ultrafast laser based on a nonlinear microcavity, Nature Communication, 3:765 (2012) pp. 1-6.
[2]  M. Mbonye et al., Applied Physics Letters, 95, 233506 (2009).