Graphene has attracted much attention due to its unique optical properties as a new kind of plasmonic metamaterial in the terahertz regime. Here, we theoretically investigated a wavelength tunable plasmon induced transparency (PIT) device based on graphene metamaterials which is composed of periodically patterned graphene nanostructures. The interactions and coupling between plasmonic modes are investigated in detail by analyzing the field distributions and spectral responses. The coupled Lorentz oscillator models are used to explain the physical mechanism of the PIT. The finite-difference-time-domain (FDTD) method is used to investigate the tunable properties of the structure. It is shown that the coupling strength between the bright mode and dark mode is tuned by the coupling distance between the elements of the proposed structure. By varying the Fermi level of graphene, the PIT resonant frequency can be dynamically tuned. Furthermore, we demonstrate numerically that tunable slow light can be realized in our patterned graphene metamaterials.
Studies on the application of a parabolic reflector in spontaneous Raman scattering for low background Raman analysis of gas samples are reported. As an effective signal enhancing sample cell, photonic bandgap fiber (HC-PBF) or metallined capillary normally result in a strong continuous background in spectra caused by the strong Raman/fluorescence signal from the silica wall and the polymer protective film. In order to obtain enhanced signal with low background, a specially designed sample cell with double-pass and large collecting solid angle constructed by a parabolic reflector and a planar reflector was applied, of which the optical surfaces had been processed by diamond turning and coated by silver film and protective film of high-purity alumina. The influences of optical structure, polarization characteristic, collecting solid-angle and collecting efficiency of the sample cell on light propagation and signal enhancement were studied. A Raman spectrum of ambient air with signal to background ratio of 94 was acquired with an exposure time of 1 sec by an imaging spectrograph. Besides, the 3σ limits of detection (LOD) of 7 ppm for H<sub>2</sub>, 8 ppm for CO<sub>2</sub> and 12 ppm for CO were also obtained. The sample cell mainly based on parabolic reflector will be helpful for compact and high-sensitive Raman system.
Diode laser pump source with narrow emitting spectrum for optically pumped metastable rare gas laser (OPRGL) of argon was achieved by employing a complex external cavity coupled with volume Bragg grating (VBG). A commercially available c-mount laser diode with rated power of 6 W was used and studied in both the free running mode and VBG external cavity. The maximum output power of 3.9 W with FWHM less than 25 pm and peak wavelength locked around 811.53 nm was obtained from the VBG external cavity laser diode. Precise control of VBG temperature enabled fine tuning of the emission wavelength over a range of 450 pm. Future researches on OPRGL of argon will benefit from it.
Spectroscopy applications of free-running laser diodes (LD) are greatly restricted as its broad band spectral emission.
And the power of a single blue-violet LD is around several hundred milliwatts by far, it is of great importance to obtain
stable and narrow line-width laser diodes with high efficiency. In this paper, a high efficiency external cavity diode laser
(ECDL) with high output power and narrow band emission at 405 nm is presented. The ECDL is based on a
commercially available LD with nominal output power of 110 mW at an injection current of 100 mA. The spectral width
of the free-running LD is about 1 nm (FWHM). A reflective holographic grating which is installed on a home-made
compact adjustable stage is utilized for optical feedback in Littrow configuration. In this configuration, narrow line-width
operation is realized and the effects of grating groove density as well as the groove direction related to the beam
polarization on the performances of the ECDL are experimentally investigated. In the case of grating with groove density
of 3600 g/mm, the threshold is reduced from 21 mA to 18.3 mA or 15.6 mA and the tuning range is 3.95 nm or 6.01 nm
respectively when the grating is orientated in TE or TM polarization. In addition, an output beam with a line-width of 30
pm and output power of 92.7 mW is achieved in TE polarization. With these narrow line-width and high efficiency, the
ECDL is capable to serve as a light source for spectroscopy application such as Raman scattering and laser induced