A terahertz absorber with controllable and switchable bandwidth that is insensitive to polarization is of great interest. Here, we propose and demonstrate a metasurface absorber with switchable bandwidth based on a phase-change material of vanadium dioxide (VO ₂ ) and verify its performance by finite element method simulations. The metasurface absorber is composed of a hybrid cross fractal as a resonator separated from a gold ground plane by a polyimide spacer. Switching from narrowband to broadband absorber is achieved via connecting VO ₂ patches to the gold first-order cross fractal converting the resonator to a third-order cross fractal. In the insulator phase of VO ₂ , the main narrowband absorption occurs at the frequency of 6.05 THz with a 0.99 absorption and a full-width half-maximum (FWHM) of 0.35 THz. Upon insulator-to-metal transition of VO ₂ , the metasurface achieves a broadband absorption with FWHM of 6.17 THz. The simulations indicate that by controlling the partial phase transition of VO ₂ , we can tune the bandwidth and absorption level of the absorber. Moreover, the designed absorber is insensitive to polarization due to symmetry and works well for a very wide range of incident angles. In the metallic state of VO ₂ , the absorber has an absorption exceeding 0.5 in the 3.57–8.45 THz frequency range with incident angles up to 65°.

The rapid development of photonic integrated circuits demands the design of efficient and compact waveguide devices such as waveguide tapers and crossings. Some components in the silicon nitride (SiN) waveguide platform are superior to their counterparts in the silicon waveguide platform. Designing a compact SiN waveguide taper and crossing is crucial to reduce the size of SiN photonic components. In this paper, we utilize the focusing property of the Luneburg lens to design an SiN taper connecting a 10-µm-wide waveguide to a 1-µm-wide waveguide. Three-dimensional full-wave simulations indicate that the designed 13-µm-long taper has an average transmission efficiency of 92% in the wavelength range of 1500–1600 nm. We also present an in-plane SiN waveguide crossing based on the imaging property of the square Maxwell’s fisheye lens designed with quasi-conformal transformation optics. The designed waveguide crossing occupies a compact footprint of 5.65 µ m × 5.65 µ m , while its average insertion loss is 0.46 dB in the bandwidth of 1500–1600 nm. To the best our knowledge, the designed SiN waveguide taper and crossing have the smallest footprints to date.

We demonstrate a reconfigurable optical bandpass filter based on subwavelength grating (SWG) waveguide operating in the Bragg reflection regime and integrated with a cavity of the phase-change material G e 2 S b 2 T e 5 (GST). Partial crystallization of GST provides us with an efficient tool to modify the effective optical properties of the GST governed by the effective medium theory. Consequently, the resonance wavelength as well as the transmission peak can be tuned in the designed filters. Numerical simulations indicate that the presented SWG waveguide with a single GST cavity offers up to 8.8 nm redshift while the transmission amplitude can be modulated from 0.544 to 0.007. The presented Fabry–Perot structure can also be used as a nonvolatile optical switch with a high extinction ratio of about 24 dB at the wavelength of 1548.3 nm.

Subwavelength engineering and utilizing phase-change materials with large contrast in their optical properties have become powerful design tools for integrated silicon photonics. Reversible phase-transition of phase-change materials such asGe2Sb2Te5(GST) provide a new degree of freedom and open up the possibility of adding new functionalities to the designed devices. We present an optical filter based on a silicon subwavelength grating (SWG) waveguide evanescently coupled to phase-change material loading segments arranged periodically around the SWG core. The effect of the GST loading segments’ geometry and their distance from the SWG core on the filter’s central wavelength and bandwidth are studied with three-dimensional finite-difference time-domain simulations. The employment of GST in the structure adds a switching functionality with an extinction ratio of 28.8 dB. We also examine the possibility of using the proposed structure as a reconfigurable filter by controlling the partial crystallization of the GST offering a blueshift of more than 4 nm.

Mode-division multiplexing (MDM) is an emerging large-capacity data communication technology utilizing orthogonal guiding modes as independent data streams. One of the challenges of multimode waveguide routing in MDM systems is decreasing the mode leakage of waveguide crossings. In this article, a square Maxwell’s fish-eye lens as a waveguide crossing medium based on quasiconformal transformation optics is designed and implemented on a silicon-on-insulator platform. Two approaches were taken to realize the designed lens: graded photonic crystal and varying the thickness of the silicon slab waveguide. Three-dimensional numerical simulations show that the designed multimode waveguide crossing has an ultrawide bandwidth from 1260 to 1675 nm with a compact footprint of only $3.77\times 3.77{\mathrm{\mu m}}^2$. For the first three transverse electric modes (TE0, TE1, and TE2), the designed waveguide crossing exhibits an average insertion loss of 0.24, 0.55, and 0.45 dB; a crosstalk of less than $-72$, $-61$, and $-27\mathrm{dB}$; and a maximum return loss of 54, 53, and 30 dB, respectively. The designed waveguide crossing supports low-distortion pulse transmission with a high fidelity factor of 0.9857. Furthermore, the proposed method can be expanded to design waveguide crossings with an even higher number of supporting modes by increasing the size of the lens.