A study of Photonic Crystals Bending Waveguide Laser Tip for Dental Treatments
Abstract — We propose a design and two dimensional (2D) simulation of a Photonic Crystals (PhC) slab waveguide laser tip with triangular lattices for dental treatments. The band diagram is computed in the transverse electric (TE) mode for triangular lattices of air holes in silicon su bstrate. The power transmission efficiencies of 90 and 120 bending structure with different sizes of defects are measured and compared with the straight waveguide. The result shows that for the bending structure, the efficiency depends relativ ely on both bending degree and defect size and for this particular design, the highest efficiency is achieved using the 120 structure with W2 defect.
Keywords-component; Photonic Crystals slab waveguide; bending structure; triangular lattices;dental laser tips;
I. I NTRODUCTION Infrared lasers are widely used in applications of dentistry for coral cavity treatment. In a small area operation, the tiny probe is employed to guide the laser beam to the problematic area. The laser tip is preferably designed to be sharp with small spot size and with some bendi ng angle to enable a treatment in the difficult to reach area [1-4]. For the insert-type tip, loss increases with the bending angle and the fixed-bending tip with a radius of 2-3 mm can cause the heat around the tip surface up to 100 C . For the chisel-shaped fiber tip  and a fiber- based desktop diode with ultra-short-pulsed (1552 nm ) laser , the ablation slows down when the distance treatment increases. The important issue in a small area application is the ability to control the light to a specific location with high accuracy and efficiency. Photonic Crystals (PhC) is an artificial periodic structure of dielectrics possessing great potential for controlling and manipulating the light propagation in nano structures with high efficiency . The possibilities of absolute band gaps in their band structures have found several applications such as waveguides, filters, and splitters [5-6]. This paper focuses on the application of a low loss p hotonic crystals slab waveguide to be used as the laser bending tip for dental treatment. The advantages of the proposed tip compared with the conventional laser tip includes small spot size [1-4], high efficiency even with bending geometry, and simple design and fabrication. The simulation is performed to evaluate the efficiency of bending PhC waveguides at 90 and 120 that will be compared to that of the straight (180 ) waveguide. The size of the guiding path or defect is varied by removing the arrays of holes. Section II presents the theoretical design and simulation setup. Section III presents the results and discussion. Conclusions are provided in Section IV.
The 2D power flow plots of the proposed PhC tip are shown in Fig. 5. These images show how lights are propagated inside the structure. It is noticed that scattering occurs distinctively near bending spots of 90 and 120 structures. Relatively large scattering is observed for W1 and W2 defects of the 90 structure and for the W3 defect of the 12 structure, respectively. This result suggests that the power efficiency depends certainly on the bending angle and the defect size and the correlation between these two input variables can be expected. Fig. 6 illustrates the plot of the transmission efficiency of the photonic crystals slab waveguide laser tip. It is first found that the overall highest efficiency of around 90% is achieved using the straight waveguide, as expected. The overall efficiency decreases as the bending angle decreases and in particular for the bending structures, W2 gives higher transmission efficiency than W1 and W3. It appears that this W2 defect is designed more properly for the wave propagation at the studied wavelength.
A laser tip used in dentistry application is designed using the Photonic Crystals slab waveguide with the TE mode propagation. The simulation results show that the transmission efficiency depends certainly on the bending angle and the defect size. The overall efficiency decreases as the bending angle decreases and for our design, the highest efficiency of bending structures is achieved with 120 bending and W2 defect. Based on these results presented in this paper, the future of the laser tip PhC waveguide for dental devices is highly promising. Future work includes 3D simulation of the PhC tip, prototype fabrication and experimental validation.