Broadband, robust, and tunable beam splitter based on topological unidirectional surface magnetoplasmons

TL;DR

This work tackles the need for broadband, tunable, and robust beam splitters in integrated photonic and microwave systems by leveraging topological unidirectional surface magnetoplasmons (USMP) in gyromagnetic waveguides. The authors design a GDG/GDM junction that supports topologically protected USMPs, yielding a bandgap with unidirectional propagation, and derive SMP dispersion relations consistent with bulk-edge predictions. A 3D realistic splitter achieves near-100% transmission across the USMP band with a frequency-independent 50:50 split in the symmetric case, and allows continuous tuning of the splitting ratio by adjusting the output air-layer thicknesses; corner fillets further broaden the usable bandwidth. The splitter demonstrates robustness against obstacles due to topological protection and can be dynamically tuned via the external magnetic field, offering a practical path to reconfigurable, backscattering-free beam splitters for advanced photonic and microwave systems. $[ ext{USMP band}] = [\omega_{sp}, \omega_b]$, where $\omega_{sp}=0.5\omega_m+\omega_0$, $\omega_b=\omega_m+\omega_0$, and $\omega_0=2\pi\gamma H$, enabling magnetic-field control of operating frequencies.$

Abstract

Beam splitters are pivotal components in integrated microwave and photonic systems. However, conventional designs based on directional coupling or multi-mode interference often suffer from back scattering, frequency-dependent splitting ratios, and limited bandwidth. To overcome these limitations, here, we propose a new physical mechanism to achieve a broadband, robust, and tunable beam splitter by manipulating the mode coupling of the topological unidirectional surface magnetoplasmons (USMP) at the input and output waveguides. We show that the beam splitter not only exhibits strong robustness against obstacles but also achieves a broad bandwidth across nearly the entire USMP band with arbitrarily tunable and frequency-independent splitting ratios. Moreover, the operating band of the beam splitter can be actively tuned by adjusting the external magnetic field, while its robust and broadband characteristics are retained. Our results extend the research frontier of beam splitters and may have potential applications in integrated photonic devices and modern communication systems.

Figures (7)

• Figure 1: Schematic of (a) the GDM waveguide: gyromagnetic-dielectric-metal under one EMF, and (b) the GDG waveguide: gyromagnetic–dielectric–gyromagnetic under two opposing EMFs. (c), (d) Dispersion diagrams of the GDM and GDG waveguides, respectively. The circles and solid lines represent the SMP modes in 3D and 2D waveguides, while the gray and yellow areas represent the bulk modes and the USMP band, respectively. (e), (f) Simulated $Ez$-field distribution in the GDM and GDG waveguides for an excitation source (pentagram) at $\omega = 1.15\,\omega_m$. The parameters are $h=6$ mm and $\omega_0 = 0.4\omega_m$, $H=715$ Oe.
• Figure 2: Schematic of (a) 3D with width $d$ and (b) 2D splitters. The input port is a GDG waveguide, and the two output ports are GDM waveguides, with the dielectric layer widths $h_1, h_2, h_3$, respectively. (c) Simulated $E$-field distribution of the 3D splitter at $\omega = 1.15\,\omega_m$. (d) Transmission spectra ($\eta_{2}, \eta_{tot}$) of the 3D (circles) and 2D (lines) symmetric splitter. The yellow-shaded region denotes the USMP band. The parameters are $h_1=h_2=h_3= 6$ mm, $d= 6$ mm , and $H=715$ Oe.
• Figure 3: Broadband tunable asymmetric splitter. Schematic of (a) unfilleted ($r = 0\ \mathrm{mm}$) and (d) filleted ($r = 3\ \mathrm{mm}$) asymmetric splitter for $h_2 \neq h_3$. Transmission spectra ($\eta_{1}, \eta_{2}, \eta_{tot}$) versus $h_3$ for (b) unfilleted and (e) filleted splitters at $\omega = 1.15\omega_m$. Transmission spectra for $\eta_{1}:\eta_{2}=$ 67:33 (dashed lines) and 75:25 (solid lines) in the USMP band. (c) unfilleted splitter for $h_3 =$ 9.2 mm and $h_3 =$ 11.3 mm, and (f) filleted splitter for $h_3 =$ 9.5 mm and $h_3 =$ 11.8 mm. The other parameters are the same as in Fig. \ref{['fig2']}.
• Figure 4: Dispersion diagrams of TIR (Green curves) and SMP modes (red curves) for $h>h_c$. (a) GDG and (b) GDM waveguides for $h=25\,\text{mm}$. The yellow and blue areas represent USMP and bidirectional TIR, where $\omega_{f}$ indicates the cutoff frequency of TIR modes. (c) USMP bandwidth $\Delta\omega$ versus $h$. (d) Transmission spectrum of the splitter with $h_1=25\,\text{mm}$, $h_2=12\,\text{mm}$, and $h_3=15\,\text{mm}$, where the yellow and blue regions correspond to frequency ranges marked in (a), respectively. (e,f) Comparison of the $E$-field distributions in the unidirectional splitter (e) with conventional bidirectional splitter (f).
• Figure 5: Tunable-Band beam splitter. (a) USMP band versus magnetic field $H$ for the splitter. The red dashed lines represent $H = 715\ \mathrm{Oe}$ ($\omega_0 = 0.4\omega_m$) and $H = 536\ \mathrm{Oe}$ ($\omega_0 = 0.3\omega_m$). (b)-(d) Transmission spectra for $H = 536\ \mathrm{Oe}$. The $\eta_{1}:\eta_{2}=$ 50:50 (b), 67:33 (c), and 75:25 (d), corresponding to $h_3 = 6$ mm, 9.5 mm and 11.8 mm, respectively. Simulated $E$-field distributions of the splitters with splitting ratios of 67:33 (e) and 75:25 (f), where the excitation sources (pentagram) are located at $\omega = 1.15\,\omega_m$. The other parameters identical to those in Fig. \ref{['fig3']}(d).