We present a metasurface consisting of W-shaped resonators to realize broadband reflective linear and circular polarization conversions. We find that the cross polarization conversion ratio for normal incidence is over 0.95 from 9.2 to 18.7 GHz, covering 68.1% of the central frequency. We also show that, the conversion performance is almost insensitive to the angle of incident waves. Furthermore, by simply adjusting the geometrical parameters of the W-shaped metasurface, the broadband circular polarization conversion is also achieved. We emphasize that the bandwidth of axis ratio less than 3.0 dB covers from 10.1 to 17.7 GHz, equivalent to 54.7% relative bandwidth. Due to these broadband and high-efficiency polarization conversion features, our proposal may have a wide application prospect.

We report two-photon excited ultrafast carrier trapping and recombination in n-type β-Ga₂O₃ crystals by using femtosecond transient absorption spectroscopy. The broadband absorption spectra arising from the defect are polarization dependent, especially, two absorption peaks can be observed by subtracting the absorption transients under two probe polarizations. We attribute these observed defect-related absorption features to optical transitions from the valence band to different charge states of a native defect (such as gallium vacancies). A model for carrier capture by multilevel of a single defect is proposed to interpret the data, wherein holes are captured more efficiently than electrons by the defects, and the absorption cross sections for the defects are at least ten times larger than that for free carriers. Our results reveal the potential applications of β-Ga₂O₃ in ultrafast and broadband optoelectronic devices.

We present a broadband electromagnetically induced transparency-like (EIT-like) effect in a microwave waveguide system by employing a bright meta-atom side-coupled to a linear chain of dark meta-atoms with a hybridization bandgap (HBG). We find that such a configuration can generate a wideband EIT response of up to 145 MHz, covering 23.7% of the central frequency. We also show that the EIT bandwidth can even extend to 211 MHz by further broadening the HBG while keeping the device volume unchanged. We emphasize that the EIT-like meta-molecules are subwavelength structures with only about one tenth of the operating wavelength. Our findings will be beneficial for realizing compact broadband slow light components in communication systems.