Huiyun Zhang

Independently tunable dual-band perfect absorber based on graphene at mid-infrared frequencies.

We design a dual-band absorber formed by combining two cross-shaped metallic resonators of different sizes within a super-unit-cell arranged in mirror symmetry. Simulations indicate that absorption efficiencies greater than 99% can be achieved at two different frequencies under normal incidence. We employ a design scheme with graphene integration, which allows independent tuning of individual absorption frequencies by electrostatically changing the Fermi energy of the graphene layer. High absorbance is maintained over a wide incident angle range up to 50 degrees for both TE and TM polarizations. It thus enables a promising way to design electrically tunable absorbers, which may contribute toward the realization of frequency selective detectors for sensing applications.

Realization of tunable plasmon-induced transparency by bright-bright mode coupling in Dirac semimetals

A numerical and theoretical study is presented on the realization of the tunable plasmon-induced transparency (PIT) effect in Dirac semimetal films (DSFs) that are known as “three-dimensional graphene”. The weak hybridization between the two parallel bright modes leads to the novel PIT optical response. The properties of the PIT system can be controlled by adjusting the geometric parameters of the DSF strips. Meanwhile, the resonant frequency of the PIT can be dynamically tuned by varying the Fermi energy of the DSFs instead of refabricating the structures. Correspondingly, by adjusting the Fermi energy of the DSFs, a large group delay of more than 1.86 ps is obtained in the vicinity of the transparency peaks. Such proposed DSFs-based PIT system may open up avenues for tunable terahertz switching, slow-light devices, sensing technology and some other THz devices.

Femtosecond pulse generation with an a-cut Nd:CaYAlO 4 disordered crystal

We experimentally demonstrated a diode-pumped 587 fs ultrafast laser by using an a-cut Nd:CaYAlO4 crystal. Pumped by an 808 nm fiber-coupled laser diode, a stable continuous-wave mode-locked ultrafast laser was achieved with a semiconductor saturable absorber. The ultrafast pulses had a repetition rate of 75 MHz at the center wavelength of 1080.8 nm. A maximum average output power of the mode-locked laser reached 375 mW delivering a slope efficiency of 9%.

Electromagnetically Induced Transparency Based on Cascaded π-Shaped Graphene Nanostructure

We demonstrate a cascaded π-shaped graphene nanostructure and investigate its transmission properties which could realize electromagnetically induced transparency (EIT). We also explore the influence of the distance between two π structures and the position of the top dipole strip on the EIT phenomenon, while the results show that when the distance reduces, there exhibits a small change of the transparency window as well as the intensity of this phenomenon. However, the window emerges and becomes stronger as the top dipole antenna moves from left to right. Moreover, the first transmission dip becomes stronger with the shift of the top dipole antenna while the second dip gets weaker. In addition, we design a more compact nanostructure and this structure could also achieve EIT. What is more, a tunable transmission window could be obtained through changing the Fermi energy of graphene, as well as a large group delay. This work has applications in biosensing, photonics, and optical filters.

Tunable omnidirectional photonic band gap of one-dimensional photonic crystals containing Dirac semimetals

We have theoretically investigated the tunability of the omnidirectional bandgap (OBG) of a one-dimensional photonic crystal consisting of alternating Dirac semimetals (DSs) and SiO2 dielectrics by adjusting the structural Fermi level. This photonic bandgap (PBG) is strongly dependent on the Fermi level and thickness ratio of the DSs and SiO2 layers. The effects of different parameters such as Fermi level, incident angle, and lattice constant on PBG are analyzed in detail. It is found that the first gap does not change with the change in lattice constant, but it is sensitive to the Fermi level; the width of the omnidirectional PBG increases with the structural Fermi level. The second gap is also sensitive to the Fermi level, the upper and lower frequency limits of this PBG shift to higher frequency, and the width becomes narrower as the Fermi level is increasing, where only one OBG exists in the range of 3.6–4.3 THz for transverse electric polarization. However, as the angle of incidence increases, the ph...

Dynamically tunable broadband linear-to-circular polarization converter based on Dirac semimetals

We propose a dynamically tunable broadband linear-to-circular cross polarization converter based on Dirac semimetals. The proposed converter unit cell consists of a center-cut cross-shaped metallic patterned structure with a sandwiched Dirac semimetal ribbon on a polyimide substrate bottomed with gold. The proposed system converts linear waves to right-hand circular polarized waves in the frequency ranges of 1.5–2.8 THz or to left-hand circular polarized waves in two narrow frequency ranges of 1.20–1.25 and 3.04–3.07 THz. The polarization conversion is dynamically tunable by varying the Fermi energy without re-optimizing the microstructures, which may further tunable polarizers and polarization switchers developments.