Danfeng Zhu

α-Ga₂O₃ has attracted significant attention for high-power electronics and deep ultraviolet optoelectronics. Herein, density functional theory calculations were employed to systematically investigate the effects of metal doping, including Cu, Ag, Zn, and Au, on the structural stability, electronic structure, and optical properties of α-Ga₂O₃. The formation energy analysis reveals that all metal dopants preferentially substitute at the octahedral Ga sites, with lower formation energies under O-rich conditions compared to Ga-rich conditions, among which Zn doping exhibits the lowest formation energy under O-rich conditions, followed closely by Cu, indicating that both Zn and Cu can be readily incorporated into the α-Ga₂O₃ lattice. Metal doping introduces impurity levels within the band gap through the strong hybridization between dopant d orbitals and O 2p orbitals near the Fermi level, significantly narrowing the effective band gap and transforming the material from a pristine n-type to a p-type semiconductor. The charge density difference analysis further confirms the electron transfer from dopant metal atoms to neighboring O atoms, generating holes as the dominant charge carriers. Among the four dopants, Zn introduces the shallowest acceptor level near the valence band maximum, making it the most promising candidate for achieving effective p-type conductivity, while Cu, Ag, and Au create relatively deeper acceptor states. Furthermore, the optical properties demonstrate that metal doping causes a pronounced red shift in the absorption edge from the ultraviolet to the visible light region, as the impurity levels provide additional electronic transition pathways at lower photon energies. The enhanced visible light absorption, combined with the increased static dielectric constant and modified reflectivity spectra, indicates improved light-harvesting capability of the doped systems. These findings provide important theoretical guidance for the design of α-Ga₂O₃-based optoelectronic devices.

Nonvolatile switching is emerging and shows potential in integrated optics. A compact nonvolatile reconfigurable mode converter implemented on a 4H-silicon-carbide-on-insulator (4H-SiCOI) platform with a footprint of 0.5 × 1 × 1.8 μm³ is proposed in this study. The functional region features an Sb₂S₃ film embedded in a 4H-SiC strip waveguide. The functionality is achieved through manipulating the phase state of the Sb₂S₃. The high refractive index contrast between the crystalline Sb₂S₃ and 4H-SiC enables high-efficiency mode conversion within a compact footprint. The incident TM0 mode is converted to the TM1 mode with a high transmittance (T) beyond 0.91 and a mode purity (MP) over 91.72% across the 1500–1600 nm waveband. Additionally, when the Sb₂S₃ transitions to its amorphous state, the diminished refractive index contrast efficiently mitigates the mode conversion effect. In this state, the TM0 mode propagates through the functional region with minimal perturbation, exhibiting T ≥ 0.99 and MP_TM0 ≥ 97.65% within a 1500–1600 nm waveband. Furthermore, the device performances were investigated under partially crystallized states of Sb₂S₃. The proposed structure offers a broad range of transmittance differences (−16.42 dB ≤ ΔT ≤ 17.1 dB) and mode purity differences (−90.91% ≤ ΔMP ≤ 96.11%) between the TM0 mode and TM1 mode. The proposed device exhibits a high robustness within ±20 nm Δl and ±10 nm Δw. We believe that the proposed multi-level manipulation can facilitate a large communication capacity and that it can be deployed in neuromorphic optical computing.

Three ultracompact integrated photonic devices are proposed, assisted by etched structures. A mode-order converter (MOC) is achieved with a footprint of 0.85 × 1.4 μm², which exhibits high performances with insertion loss (IL) below 0.34 dB and mode purity (MP) above 90% within 100 nm waveband. Moreover, a mode blocking filter (MBF) is proposed based on a subwavelength grating with a footprint of 0.8 × 4.12 μm², which allows the propagation of TM0 mode but blocks the TM1 mode with a remarkably high extinction ratio of 31.6 dB at the wavelength of 1550 nm. Finally, a compact reciprocal optical diode (ROD) is presented by cascading the abovementioned MOC and MBF with a footprint of 0.85 × 5.9 μm². This ROD enables one-way transmissions for both TM0 mode and TM1 mode with contrast ratios of 19.74 dB and 32.04 dB, respectively. The design methodology paves a new way for mode manipulation in integrated multimode photonic circuits.

A high-contrast and compact wavelength diplexer is presented for conventional 1550 nm and emerging 2000 nm based on a subwavelength-grating (SWG) coupler. The SWG silicon waveguide thoroughly blocks the light propagation around 1550 nm but fully supports 2000 nm (extinction ratio: 43.11 dB). This grating type anisotropic metamaterial not only efficiently reduces the coupling length but also expands the operational bandwidth. Simulated by 3D finite-difference time-domain method, the proposed diplexer possesses a remarkably low insertion loss and a high contrast of 25.24 (31.7) dB at 1550 (2000) nm. The operational bandwidth of 200 (108) nm is achieved with contrast over 15 dB and insertion loss below 0.22 dB. The footprint of diplexer is only 5.27 μm × 11.85 μm. Moreover, such design has the scalability by simply tuning the geometrical parameters of SWG.