Nian-Hua Liu

Enhanced absorption of graphene with one-dimensional photonic crystal

The optical absorption of graphene layers prepared on top of a one-dimensional photonic crystal (1DPC) is investigated theoretically. The absorption of graphene with 1DPC is enhanced greatly over a broad spectral range due to photon localization. The absorption of graphene can also be tuned by varying either the incident angle or the distance between the graphene and the 1DPC.

Time delay of light propagation through defect modes of one-dimensional photonic band-gap structures

Light propagation through one-dimensional photonic band-gap structures with defect layers embedded at the center was investigated. The localization of the defect mode results in a great delay of propagation time. The average time spent in each layer is given. From the temporal behavior we can deduce the energy distribution in the interior of the photonic band-gap structure and calculate the average velocity of light propagation. The average velocity depends sensitively on the number of layers and the contrast of the refractive indexes.

Enhanced absorption of monolayer MoS2 with resonant back reflector

The optical absorption of monolayer MoS2 on top of one-dimensional photonic crystal (1DPC) or metal films with spacer layers is theoretically investigated by extracting the permittivity of monolayer MoS2 from existing experimental results [K. F. Mak et al., Phys. Rev. Lett. 105, 136805 (2010)]. The absorption of graphene with 1DPC across a broad spectral range is substantially enhanced because of the photonic localization at the optical micro-cavity on top of the 1DPC or metal films. The absorption of monolayer MoS2 can be tuned by varying either the distance between the monolayer MoS2 and the back reflector or the thickness of the cover layers.

Omnidirectional bandgaps in Fibonacci quasicrystals containing single-negative materials.

The band structure and bandgaps of one-dimensional Fibonacci quasicrystals composed of epsilon-negative materials and mu-negative materials are studied. We show that an omnidirectional bandgap (OBG) exists in the Fibonacci structure. In contrast to the Bragg gaps, such an OBG is insensitive to the incident angle and the polarization of light, and the width and location of the OBG cease to change with increasing Fibonacci order, but vary with the thickness ratio of both components, and the OBG closes when the thickness ratio is equal to the golden ratio. Moreover, the general formulations of the higher and lower band edges of the OBG are obtained by the effective medium theory. These results could lead to further applications of Fibonacci structures.

Negative and positive lateral shift of a light beam reflected from a grounded slab

We consider the lateral shift (LS) of a light beam reflecting from a dielectric slab backed by a metal. It is found that the LS of the reflected beam can be negative while the intensity of reflected beam is almost equal to the incident one under a certain condition. The explanation for the negativity of the LS is given in terms of the interference of the reflected waves from the two interfaces. It is also shown that the LS can be enhanced or suppressed under some other conditions. The numerical calculation on the LS for a realistic Gaussian-shaped beam confirms our theoretical prediction.

Tunable THz absorption in graphene-based heterostructures

We investigate THz absorption properties of graphene-based heterostructures by using characteristics matrix method based on conductivity. We demonstrate that the proposed structure can lead to perfect THz absorption because of strong photon localization in the defect layer of the heterostructure. The THz absorption may be tuned continuously from 0 to 100% by controlling the chemical potential through a gate voltage. By adjusting the incident angle or the period number of the two PCs with respect to the graphene layer, one can tailor the maximum THz absorption value. The position of the THz absorption peaks can be tuned by changing either the center wavelength or the thicknesses ratio of the layers constituting the heterostructure. Our proposal may have potentially important applications in optoelectronic devices.

Gate-tunable nearly total absorption in graphene with resonant metal back reflector

The gate-tunable absorption of graphene layers with a resonant metal back reflector (RMBF) is theoretically investigated. We demonstrate that the absorption of graphene with RMBF can vary from nearly negligible to nearly total by tuning the external gate voltage within the terahertz (THz) spectra range. Total THz absorption is less affected by the incident angle of THz beams. This peculiar nearly total THz absorption can be attributed to the Fabry-Perot cavity effect, which enhances the absorption and reduces the reflection of graphene. The absorption spectra of the graphene-RMBF structure can also be tailored in bandwidth and center frequency by changing the thickness and dielectric constant of the spacer layer. These findings can lead to the development of tunable THz photonic devices and have potential applications in studies on the ultrafast dynamics of Dirac fermions in graphene.

Design of a Novel Polarized Beam Splitter Based on a Two-Dimensional Photonic Crystal Resonator Cavity

We propose and analyze a novel ultra-compact polarization beam splitter based on a resonator cavity in a two-dimensional photonic crystal. The two polarizations can be separated efficiently by the strong coupling between the microcavities and the waveguides occurring around the resonant frequency of the cavities. The transmittance of two polarized light around 1.55 μm can be more than 98.6%, and the size of the device is less than 15 μm×13 μm, so these features will play an important role in future integrated optical circuits.

Enlargement of polarization-independent omnidirectional band gaps in the photonic heterostructures containing single-negative materials

We show that the frequency range of the zero effective phase gap in a photonic heterostructure containing single-negative materials can be enlarged owing to the property that its lower and upper frequency edges depend on the thickness ratio of the epsilon-negative and mu-negative materials. Compared to the zero effective phase gap of a single photonic crystal, the frequency range of the zero effective phase gap in a photonic heterostructure can be notably enlarged. Moreover, it is shown that the band edges of the zero effective phase gap is insensitive to incident angle and polarization of light.

Magnetoresistance and shot noise in graphene-based nanostructure with effective exchange field

Based on the transfer-matrix method, we investigated the spin transport through a graphene-based nanostructure with effective exchange field. It is found that the effective exchange field induces a spin-dependent Klein tunneling. The magnetoresistance becomes a number of times larger than that in the case of the Rashba spin-orbit interaction. With increasing the effective exchange field strength, the magnetoresistance and the Fano factor exhibit periodic oscillation features. In graphene superlattice, when the effective exchange field satisfies a certain condition, the Fano factor can be tuned from nearly zero to 1/3 by applying an appropriate periodic gate voltage.