Deflection of electromagnetic waves by pseudogravity in distorted photonic crystals

We demonstrate electromagnetic waves following a gravitational field using a photonic crystal. We introduce spatially distorted photonic crystals (DPCs) capable of deflecting light waves owing to their pseudogravity caused by lattice distortion. We experimentally verify the phenomenon in the terahertz range using a silicon DPC. Pseudogravity caused by lattice distortion reveals alternative approaches to achieve on-chip trajectory control of light propagation in PCs.

Figure 1

(a) Schematic of lattice point positions of the DPC based on a square-lattice PC. The DPC introduces a lattice distortion in which the interval of lattice points gradually expands in the $+y$ direction. The $x$ and $y$ axes are defined along the Γ-X direction of the square-lattice PC. To obtain a homogenous and averaged refractive index, the radii increase along the $+y$ direction. (b, c) Simulation results of light propagation in the DPC with $\beta =0.005$ ($H$ mode): (b) air-hole DPC and (c) dielectric rod DPC. The light wave propagates with the same degree of bending in both results. The solid red lines show the exact solutions of the geodesic equation derived in Ref. [22]. The dashed black lines show the straight lines of input beam direction.

Figure 2

Design of (a) square-lattice PC and (b) DPC. Both samples have one input port (port A) and two output ports (ports B and C), which use a GRIN lens structure. (c, d) Micrographs of fabricated samples at the locations indicated in (a) and (b), respectively.

Figure 3

(a) Schematic diagram of experimental setup. (b) Overview image of experimental setup. (c) Output signal ratio of ports C-B. Dots represent raw data and lines represent rolling average of 15 adjacent raw data. The variation in results is due to Fabry-Perot reflections at the input and output ports [31].

Figure 4

(a) Simulation model diagram showing the 9% miniaturized version of the experimental structure, where eight perfectly matched layer (PML) absorbing boundaries are employed. The input-output monitor is located on the Si waveguide before port A's GRIN lens or after ports B's and C's GRIN lenses. (b), (c) Light propagation after input and before output for the PC and DPC, respectively. (d) Output signal ratio of ports C-B, where the solid lines represent simulation results and the dashed lines represent experimental results [as shown in Fig. 3]. The upper lines show the results in the case of DPC, and the lower lines show that in the case of PC. The solid purple line with circle accounts for the 4 dB loss due to the tapered air-hole structure.

Figure 5

(a) Schematic of the DPC approximated as a series of PCs: PC1 represents a square-lattice PC, PC2 represents a rectangular lattice PC with $a_y=1.2a^{\left(0\right)}$ (corresponding DPC at $n=20$), and PC3 represents a rectangular lattice PC with $a_y=1.4a^{\left(0\right)}$ (corresponding DPC at $n=40$). (b)–(d) Calculation results of EFC in $H$ mode for each PC3–1, respectively. $k, k^{\prime }$, and $k^{″}$ represent the wave number vector of light waves propagating in each PC, and $v_g, {v^{\prime }}_g$, and ${v^{″}}_g$ represent the group velocity vector.