# Special reflecting features of a three-dimensional photonic crystal depending on cut surfaces oriented in the direction [001] within photonic bandgap

## TL;DR

This study investigates how the reflecting spectrum and phase of a 3D terahertz photonic crystal depend on cut-surface orientation within the same direction, revealing patterns linked to unit cell types and surface shifts.

## Contribution

It introduces a detailed analysis of the influence of cut-surface orientation on reflection features in a 3D photonic crystal within the bandgap, using FEM simulations.

## Key findings

- Reflecting features depend on cut-surface orientation and shift.
- Two main reflection patterns are identified, linked to unit cell types.
- Reflection phase can invert or remain the same depending on surface shift.

## Abstract

The reflecting features (reflected spectrum and phase) on the surface of pure-material plate depend on refractive index, extinction coefficient, incident angle, polarization direction and so on. The features, however, do not depend on a cut surface that is another uniformly processed surface in the same direction, which is obtained by being cut, etched and polished for example. In this work, the special reflecting features of a three-dimensional (3D) terahertz photonic crystal (PC) depending on the cut-surface oriented in the same direction [001], have been studied especially within photonic band gap by using a FEM (finite element method). The 3D-PC is a silicon inverse diamond structure whose lattice point shape is vacant regular octahedrons. The characteristic features were studied on four types of cut-surface that were shifted per a/4 (a: lattice constant) in the same direction [001], especially with two types of orientation, [(1,1) and (1,-1) in an XY-plane] of electric-field of the incident wave in normal incidence. It was found that eight types of reflected spectrum and those of phase in all four cut-surface types are able to be classified in two patterns, they are correlated with two types of unit cell suggested, and the reflecting features invert or do not invert according to the cut-surface.