Similar to the dielectric nanoparticles, the metallic nanoparticles will make the resonance wavelength shift and broadening of the splitting bandwidth. As illustrated in
, the main difference of the Au nanoparticle is its smaller real refractive index than air and very large imaginary part (absorption part), which leads to the blue shift of the resonance mode wavelength position. As the nanoparticle numbers reach a certain number, the splitting of the resonance began to appear, within our simulation resolution. For the 80nm Au nanoparticle on the micro ring resonator with the nanoparticle number increasing from one to two, it is observed that both splitting modes are blue shifted. This phenomenon is unique as it provides us a very convenient approach to distinguish the dielectric nanoparticles and the Au nanoparticles, both are used extensively for sensing and nano medicine field. Furthermore, the intensity of Drop port is reduced rapidly with the increasing number of Au nanoparticles (thirty in this case), which represents its large absorption characteristics at this wavelength. For sensing applications using Au nanoparticles or other metallic nanoparticles, it is inferred from this work that there is a limit for the number of metallic nanoparticles adsorbed on the ring resonator, as there is normally large absorption for metallic nanoparticles. When the number reaches a certain point – critical number (30 Au nanoparticles in this case), the interaction between metallic nanoparticles and micro ring resonator is becoming so strong that they completely degrade the resonance – the Q is strongly degraded and the intensity at Drop port is approaching zero.
(a) The Drop port intensity with different number of 80nm size Au nanoparticles adsorbed on the micro ring resonator. (b) the example of multiple Au nanoparticles randomly distributed on the micro ring resonator.
For using Au nanoparticle and Drop port as a detection mechanism, the dependence on position is also important as the Au nanoparticles are possible to be adsorbed randomly on the micro ring resonator, the relatively position independence is necessary. To this purpose, for thirty Au naoparticles with 80nm in size, we have randomly distributed the 30 Au nanoparticles on the micro ring resonator and compared the Drop intensity in
. It is shown that the intensity at Drop port for three random positions is at the same order and this result demonstrates the relatively independence of the Au nanoparticle position on the micro ring resonator.
The Drop port intensity with three random positions of 30 Au nanoparticles with 80nm in size.
Metallic nanoparticle size uniformity is very important for practical sensing and detection, as the nanoparticle size normally has a distribution around the target nanoparticle size which we would like to use. To study the effect of uniformity of the size of the Au nanoparticles on the performance of the integrated micro ring resonator, for 5 Au nanoparticle case with 80nm size, we have randomly chosen the nanoparticle size which has certain distribution around 80nm.
is the comparison between the uniform size nanoparticles and nanoparticles with certain distribution, the overall signal from drop port is almost the same although there is some small difference. The result demonstrates the robustness of our sensing mechanism using Au nanoparticles which can tolerate certain non uniformity of Au nanoparticles.
The Drop port intensity with Au nanoparticle size distribution, 5 Au nanoparticles with uniform 80nm in diameter (solid line) and 5 Au nanoparticles with random size distribution around 80nm in size.
We also studied the dependence on Au naoparticle size, which is illustrated in
. 10nm, 40nm and 80nm Au nanoparticle are compared at Drop port with the different number of Au nanoparticles. It is observed the intensity at Drop port for 10nm size Au nanoparticle is reduced in a much slower pace than that for the 40nm and 80nm Au nanoparticle. The reduction of the intensity is mainly caused by the strong absorption of Au nanoparticles, around 1.55µm; the penetration depth of Au is about 45nm. The dependence on nanoparticle size for the Drop port reveals the correlation between the penetration depths, nanoparticle size and resonance mode evanescent tail length, which might be utilized to measure the Au naoparticle size. For optical sensing and detection purpose, the optimized Au nanoparticle size should meet two requirements, one is relatively large shift when the nanoparticle is adsorbed to the ring resonator, the other is the slow Q degradation ratio when more Au naoparticles are adsorbed on the micro ring resonator. Based on the results in and , the optimized size of Au nanoparticle is estimated as 40nm, which is around the penetration depth.
The Drop intensity vs. nanoparticle number for different size of Au nanoparticles.