Ultrathin SOI strip-loaded resonators: permanent mitigation of losses using UV light
We report on the design, fabrication and characterization of Silicon Nitride strip-loaded guiding optical components realized on a sub-30 nm ultra-thin SOI platform [1]. Omitting physically etched boundaries within the guiding core is known to suppress significantly the scattering loss, as shown by us previously for Si3N4-based devices. Contrary to expectations, the freshly fabricated SOI devices suffer large losses of 5 dB/cm, which we related to the absorption by free carriers, accumulated under the positively charged Si3N4 loading layer. Successively, we demonstrate that exposures to UV light neutralize progressively and permanently Si3N4’s bulk charge, associated with diamagnetic K+ defects. As a result, a net decrease of electron concentration in the SOI layer reduces the propagation losses down to 0.9 dB/cm. We performed accurate cavity linewidth measurements showing how the intrinsic cavity Q’s boost from 70,0000 up to 500,000 after UV illumination. Our results may open routes towards engineering of new functionalities in photonic devices, unveil the origin of induced optical nonlinearities in Si3N4/Si micro-photonic systems, as well as envisage possible integration of these with standard as well as ultrathin SOI electronics.
Circular microresonators, in which the electromagnetic radiation is confined via resonant circulation, are essential building blocks for planar integrated photonics. The spectral width of resonances in these devices is inversely proportional to the amount of optical power which is lost per round trip. Generally, the power is lost due to material absorption, αm, scattering on boundary imperfections, αsc, and out-radiation, αrad, due to the curved geometry. Reducing the intrinsic loss enables several applications from passive filtering in optical communication networks to quantum optics, space and sensing. Thus, in order to push the device characteristics to an ultimate limit, the challenge is to limit the intrinsic losses to only that of the material. Towards this goal, approaches, such as strong modal confinement and realization of smooth device boundaries are necessary to achieve minute contributions from radiative and scattering loss.
We realized the samples on 27 nm thick SOI platform depositing a 145 nm thick LPCVD Si3N4 film (loading layer). The wafer was patterned lithographically and transferred to Si3N4 using a combination of dry and wet etch (Fig. 1). The fabricated chips, containing ring resonators and long spiral waveguides, were diced and characterized in optical transmission experiments.
The freshly fabricated waveguides showed a propagation loss of about 3.83(0.26) dB ∕cm according to the Beer–Lambert law (Fig. 2(a), red squares), which, if attributed to sidewall scattering, are unexpectedly high for the adopted processing technology [2]. A hypothesis of charge-related losses was suggested and successively confirmed through capacitance measurements of the Si3N4 films. We found that the as-deposited Si3N4 layer contains a large amount of net positive electrical charge related to diamagnetic charge centers, which, successively, when exposed the film to UV light (254 nm), can be permanently neutralized. In fact, the waveguides losses were significantly improved (Fig. 2(a), blue diamonds) when the devices were exposed to UV light for a couple of hours.
The freshly fabricated waveguides showed a propagation loss of about 3.83(0.26) dB ∕cm according to the Beer–Lambert law (Fig. 2(a), red squares), which, if attributed to sidewall scattering, are unexpectedly high for the adopted processing technology [2]. A hypothesis of charge-related losses was suggested and successively confirmed through capacitance measurements of the Si3N4 films. We found that the as-deposited Si3N4 layer contains a large amount of net positive electrical charge related to diamagnetic charge centers, which, successively, when exposed the film to UV light (254 nm), can be permanently neutralized. In fact, the waveguides losses were significantly improved (Fig. 2(a), blue diamonds) when the devices were exposed to UV light for a couple of hours.
Figure 2(b) summarizes the measurements of ring resonators – from freshly prepared devices to UV exposures of different duration. The corresponding loss values, based on the results from the ring resonators, and the calculated curve for p-type Si in the situation of residual loss are shown in Fig. 2(c). Thus, we revealed a net improvement of losses down to 0.9 dB/cm at the longest exposures, improving the rings’ intrinsic Q-factors from 70,000 to 500,000.
Our results may open the door to the implementation of UV-induced charge modification for the design and study of new photonic devices in which the space-charge-related static electric fields can be engineered to modulate the linear and nonlinear refractive indices of materials. In addition, our findings are general and may be implemented also in other geometries of guiding devices and material systems where silicon nitride is present, such as widely used standard SOI waveguides. We also envisage the possibility of compact integration of micro-photonic components with ultra-thin SOI electronics in the future.
REFERENCES
[1]. G. Piccoli, M. Bernard, and M. Ghulinyan, Permanent mitigation of loss in ultrathin silicon-on-insulator high-Q resonators using ultraviolet light. Optica 5, 1271 (2018); 10.1364/OPTICA.5.001271
[2]. L. Stefan et al., Ultra-high-Q thin-silicon nitride strip-loaded ring resonators. Opt. Lett. 40, 3316 (2015).