Narrow Results

We reviewed the recent progress on slow and fast lights in solids at room temperature based on moving and stationary refractive index gratings. A dispersive photorefractive phase coupling associated with moving gratings results in slow and fast lights. In principle, such phase-coupling-induced slow and fast lights can be observed in any nonlinear wave mixing process with a dispersive phase coupling effect. The slow and fast lights in the stationary gratings are also discussed. One advantage of the stationary gratings is the possibility to engineer the dispersion slope of the grating through designing the grating structure and parameters. As an example, we show that the dispersion slope of the gratings is enhanced significantly by stratifying a series of identical volume index gratings with homogeneous optical buffer layers sandwiched between every two neighboring grating layers. The slow and fast lights, therefore, can be controlled more effectively in such specifically designed grating structures than in the homogeneous gratings. Another advantage is the high transparency of the slow and fast lights with appropriate grating structure and parameters. Issues such as the pulse broadening effect and the pulse distortion are addressed. The slow and fast light techniques have many important potential applications such as optical delay lines and optical buffers.

Scattering of light on periodic subwavelength arrays is studied in the framework of the resonant scattering theory. With various examples of periodic structures it is demonstrated that: (i) an enhanced reflectance or transmittance is associated with the existence of trapped modes (quasi-stationary modes of light confined in the vicinity of the scattering structure); (ii) scattering structures may have trapped modes due to peculiarities their geometry (geometrical modes) and the dispersive properties of their material (material modes); a practical criterion based on the scaling symmetry of Maxwell's equations is proposed to distinguish them; (iii) the trapped mode field can be significantly amplified, as compared to the incident wave amplitude, in some regions of the structure; (iv) the amplification increases with increasing the lifetime of the trapped mode; (v) this effect can be used to enhance nonlinear optical effects (a resonant higher harmonic generation is studied in detail as an example). The theory of coupled resonances is developed and used to prove that there exist bound states of light in the radiation continuum (resonances with the vanishing width) in periodic arrays. The bound states are neither modes in metal cavities nor modes in photonic crystal defects. Structures supporting the bound states of light can be used to enhance and control nonlinear optical effects in subwavelength periodic arrays.

We report an ultra-narrowband absorber with a dielectric grating and metal substrate. The simulation results show that we can achieve ultra-narrowband absorption with the absorption bandwidth less than 0.6 nm and the absorption rate more than 0.99 for TE-polarization (electric field is parallel to grating grooves). The simulation results also show the guide-mode resonance in the grating region and low power loss at the absorption peak. In addition, the ultra-narrowband absorption peak can be tuned by shrinking or enlarging the structure parameters. The figure of merit (FOM) is larger than 760 if this absorber is applied as a refractive index sensor.

We report a graphene-based tunable ultra-narrowband mid-infrared filter which can be tuned from 4.45122 $\mu$m to 4.44675 $\mu$m by tuning the Fermi level from 0.2 eV to 0.6 eV. Furthermore, the reflection bandwidth is less than 0.2 nm and the reflection rate is more than 0.55. The ultra-narrowband filter is designed based on the guided-mode resonance (GMR) effect. The shift of reflection peak is mainly caused by the change of the real part of the graphene’s permittivity. This tunable ultra-narrowband mid-infrared filter can be applied in the mid-infrared microscopy.

The optical transmission properties in a three-slit metal grating with different mediums are studied. The results show that the tunable phase resonances and multiple phase resonances can be obtained by filling different mediums into the three slits of the straight channel grating. When the medium in the three slits is extremely asymmetric, the phenomenon of multiphase splitting becomes very obvious. In addition, based on the field distributions, multiphase resonant mechanisms have been proposed for the physical origins of the observations. In addition, a multi-channel selector is described by flexibly controlling the light through any slit of the grating. Compared with the conventional compound grating, this grating with different mediums has obvious advantages, such as simple structure, and ease of implementation.

We report a tunable bandpass mid-infrared filter with microstructure and graphene, and the transmission peaks can be tuned from $4.5638\mu \mathrm{\text{m}}$ to $4.5461\mu \mathrm{\text{m}}$ when the graphene’s Fermi level increases from 0.2 eV to 1.0 eV. This bandpass mid-infrared filter is originated from the guided-mode resonance (GMR) effect, and the tunable mechanism is mainly attributed to the change of the refractive index of the graphene. This tunable mid-infrared filter can be applied in non-dispersive infrared analyzer.

A novel scheme for integrated optical filters with narrow bandwidth is proposed to dramatically improve its compactness on the basis of slow-light waveguide, which provides a large optical path in a short geometry length. By introducing the one-dimensional grating waveguide (ODGW), a kind of slow-light waveguide, into a micro-ring resonator (MRR), the area of MRR is reduced by three orders of magnitude compared to the MRR without ODGW. As a proof of concept, an ultra-compact on-chip filter with narrow bandwidth of 2.1 pm (263 MHz) is conceived by cascading six MRRs with ODGW. Additionally, the designed optical filter shows excellent performances of roll-down speed, out-of-band signal suppression, and in-band ripple. Our proposal offers a promising solution to miniaturize the integrated optical filters with narrow bandwidth, and hence improve the integration density of on-chip photonic networks.

Enhanced photosensitivity has been observed in hydrogen-loaded tin-phosphosilicate fibres by using a 248 nm excimer laser. Isothermal measurements up to 860 K demonstrated significant advantages over fibre gratings written in conventional H-loaded fibres. Gratings written in this fibre require a considerably shorter post-fabrication thermal annealing in order to satisfy the stability requirements of telecom components.

The theory of the Casimir effect for two parallel gratings with a coinciding period is developed.

The prospect of creation of the X-ray source with tunable wavelength on the base of a middle energy accelerator and mosaic crystals are analyzed. It is proved, that due to the contribution of diffracted bremsstrahlung, the mosaic crystals provide the essentially greater yield of hard radiation (ω ≥ 20 keV), than perfect crystals. It is shown, that using a traditional scheme with one crystal for X-ray generation isn't acceptable for medicine applications. Double-crystal scheme is offered and analyzed, in which one crystal is located on the electrons beam, and another is used for monochromatization and parallel moving of the X-ray beam. It provides suppression of background bremsstrahlung, and there is no need to move the object when change the photons energy.