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 H2, 8 ppm for CO2 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.
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
fluorescence.
Spontaneous Raman scattering is an effective technique in gas analysis, but the detection of minor constituents is difficult because of the low signal level and the usually existed background. Imaging spectrometer can provide highly spatial resolved spectra, so it should be much easier to pick up Raman signal of minor constituents from the Raman/fluorescence background of the sample cell and transporting optics compared with the widely used fiber-coupled spectrometers. For this reason, an imaging spectrometer was constructed from transmitting volume phase holographic grating, camera lenses and CCD detector. When it was used to analyze the gas sample in metal-lined capillary, which is a sample cell believed with great enhancement of Raman signal, the background was compressed obviously. When it was used to analyze the gas in a sample cell including a parabolic reflector, only weak background signal was observed, as the wide separation between the collecting zone (the focus point of the parabolic surface) and the wall of sample cell benefitted to the analysis by imaging spectrometer. By using the last sample cell, the signal from CO2 in ambient air was able to be found by an exposure time about 20 sec, and limits of detection for H2, CO2 and CO were estimated as 60 ppm, 100 ppm and 300 ppm respectively by the results of a longer exposure time. These results show that an imaging spectrometer paired with a well-arranged sample cell will lower the detecting limit effectively.
The single-filament schlieren method was based on the beam deflection in non-uniform medium. In this paper, a fourelement photodiode was used to acquire the deflection of the probing beam. The effects of electromagnetic interference (EMI) and the vibration of the blower on the output of the photodiode were investigated in detail and they have little impact on the measurements of the flowing characteristic after discharge. Then the perturbation in the discharge region was investigated. The heated gas in the discharge region can be easily detected and the gas velocity can be calculated by tracing the drift of the heated gas. This method also showed a high sensitivity and convenience to observe the acoustic waves originated from fast energy deposition. The results showed that the reflective acoustic wave existed for about 4 ms after discharge and it had a major effect on the non-uniformity of gas medium before the subsequent pulsed discharge.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.