Precision tuning of silicon photonic devices using multi-shot femtosecond laser irradiation and characterization of incubation effect in multi-shot femtosecond laser irradiated silicon
Creators: Zhang, Ruoheng
Publisher: name: University of Alberta Library
Year: 2024

<p>Silicon photonics has emerged as a promising platform for advanced on-chip integrated optical devices and advanced microelectronic circuits during the last decades.<br>
Due to its extreme sensitivity to fabrication precision and tolerance, a post-fabrication tuning method is typically required to correct phase errors in silicon photonic devices<br>
to achieve the resonant frequencies of interest. One of the most promising techniques is by applying the femtosecond (fs) laser to modify the refractive index of silicon, as it does not require additional material or processing steps. So far, in this method, only single-shot fs irradiation has been demonstrated, which still lacks precise control of the refractive index changes. A multi-shot approach, which takes advantage of the incubation effect, was investigated in this thesis to achieve precision tuning. By applying multi-shot 130 fs laser irradiation at 800 nm wavelength, permanent tuning of resonance wavelengths for silicon microring resonators (SiMRR) with both positive shift and negative shift are achieved. The resonance wavelength spectra were<br>
obtained by a characterization system using a tunable laser and a power meter. Three regions (“increasing,” “plateau,” and “decreasing”) exist in a fluence dependency tuning curve when the resonance wavelength shift is plotted as a function of laser<br>
fluences for a given number of pulses used. The formation of these regions depends on the relative contribution of the crystalline modification or ablation process.<br>
An improved laser tuning platform was developed to demonstrate the high precision permanent tuning of silicon photonic devices. The platform consists of an irradiation system utilizing 800 nm fs laser pulses, allowing the resonance wavelength of the Si photonic device to be tuned in real time to the desired value, and a characterization system utilizing a Superluminescent Diode (SLED) and an optical spectrum<br>
analyzer (OSA). The use of SLED and OSA led to an order of magnitude speed improvement in obtaining the resonance wavelength spectra. The platform was used to measure the shot-to-shot tuning response of SiMRRs. Five regions (“insignificant change”, “increasing”, “plateau”, “slow decreasing” and “fast decreasing”) exist in a shot dependency tuning curve, when the resonance wavelength shift is plotted as a function of pulse numbers at a given laser fluence. Precision permanent tuning of SiMRRs was demonstrated through a multi-shot approach using 50 fs laser pulses at 800<br>
nm wavelength with sub-microjoule pulse energies. Unprecedented fine-tuning of the microring resonance wavelength at a resolution of 2.5 pm per laser shot was achieved at a low laser fluence of 23.2 mJ/cm2 . This represents a 20 x finer resolution and 4 x lower fluence than previously demonstrated using single-shot femtosecond laser<br>
tuning. The low energy requirements allow for the use of compact, low-cost lasers for tuning.<br>
Femtosecond laser pulses at low fluences can modify materials even when there is no visible damage; thus, the processes are difficult to characterize. The accumulated modifications due to these low fluence fs pulses eventually resulted in visible damage to the materials. Such a multi-shot low fluence process is commonly called the incubation effect. The detailed processes of the incubation effect are not well understood. In<br>
this thesis, the details of the incubation effect in multi-shot irradiated Si were studied using surface characterization methods and the refractive index changes in SiMRRs.<br>
Multi-shot ablation thresholds, Fth(N), for single crystalline silicon irradiated with 800 nm 130 fs laser pulses were investigated for fluences above (F &gt; F1) and below (F &lt; F1) the single shot ablation threshold F1. It was observed that the<br>
ablation threshold for N shots Fth(N) in the case of (F &lt; F1) is significantly larger than the case of (F &gt; F1). The critical minimum fluence or sub-ablation threshold of Si, when no ablation would result even with an infinite number of laser shots, was estimated. Multi-shot ablation thresholds for 250 nm Si thin film irradiated with 800 nm or 400 nm 130 fs laser pulses were also investigated. For the 800 nm irradiation,<br>
it was observed that the F1 for the thin film Si is lower than the bulk Si, but their incubation coefficients are consistent with each other. It was also observed that the incubation effect is weak for the Si thin film under 400 nm irradiation.<br>
The silicon microring resonator was utilized as a novel tool to obtain information about the incubation effect dynamics at laser fluences well below thresholds for modifications that can be detected using conventional surface characterization methods. From the shot-to-shot refractive index changes measured by SiMRRs at a given irradiated laser fluence, three pulse-number-thresholds (Nth) can be determined from<br>
low to high pulse numbers as: “defect generation threshold,” “nano milling threshold,” and “ablation Threshold.” These Nth decrease with increasing laser fluence. The SiMRR results also confirm that the incubation effect is weak for the Si under 400<br>
nm irradiation.</p>