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.
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
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.
This paper established a model of three-energy-level system from the laser rate equation theory, and then analyzed the molecular dynamic processes and laser mechanism of the optically pumped CH3OH. According to the experimental parameters, the CO2 laser is used to pump CH3OH to emit pulsed THz laser lines. When the 9P(36) line of CO2 laser pumps CH3OH, the 118.8 μm line radiation is obtained, and then we analyze the three-energy-level system’s the number of particles and the characteristics of the output waveform, and the influence of the pump light, buffer gas pressure and output coupler reflectivity on laser output power. The results show that this model reveals the generate terahertz optical pump laser process correctly.
Laser produced plasma (LPP) light sources for extreme ultraviolet (EUV) lithography currently has been extensively studied. Most of the studies are based on CO2 laser induced plasma from mass limited tin targets. In this work, a droplet dispenser that produces uniform droplets size of about 150μm was established. A pulsed TEA-CO2 laser and a Nd: YAG laser irradiated the droplets producing plasma respectively to get EUV emission. An X-ray Spectrometer and EUV photodiodes were used to collect the spectra and EUV radiation. The different EUV spectral composition and angular distribution of EUV emission from plasmas induced by the CO2 and Nd: YAG laser were studied.
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.
A distributed feedback (DFB) fiber laser is compact, and is very suitable for using as a hydrophone to sense acoustic pressure. A DFB fiber laser hydrophone was researched. In the fiber laser hydrophone signal demodulating system, an unbalanced Michelson fiber interferometer and a Phase Generated Carrier (PGC) method were used. The PGC method can be used to demodulating the acoustic signal from the interference signal. Comparing with the Naval Research Laboratory (NRL) method and Naval Postgraduate School (NPS) method, the digitized PGC method requires a greater amount of computation because of the high signal sampling, but it demands only one interference signal which makes the less fiber connections of the fiber laser hydrophone array. So the fiber laser hydrophone array based on the PGC method has lower complexity and higher reliability than that based on the NRL method or NPS method. The experimental results approve that the PGC method can demodulate acoustic signal between 20~2000 Hz frequency range with good signal-to-noise ratio (SNR) when the PZT driving frequency is 20 kHz.
Recent studies on signal enhancement of spontaneous Raman scattering for developing of sensitive Raman gas detectors
are reported. Raman scattering is a gas detecting method with high feasibility, but usually its signal is very low. To
improve the level and the quality of the Raman signal, the effects of pumping laser source, sample cell, and optical
arrangement are studied in detail. It is found that not only the wall of sample cell will give a wide Raman or fluorescence
background which will decrease the sensitivity, but the dichroic beam splitter will also contribute considerable
background if it is not aligned properly. The sample cell of hollow fiber is characterized by its high responsibility as well
as its high background and low signal contrast. When the hollow fiber is replaced by a free-space sample cell consisted
of metal-coated parabolic reflector, the wide background is largely suppressed. If there is no common optical elements
between the pumping and collecting optical systems, the wide background will be cut down obviously, which is proved
by the intracavity-enhanced Raman scattering in a He-Ne laser. These experimental results will be helpful for the
research and developing of highly sensitive Raman gas detectors.
An coherent terahertz (THz) source was reported by quasi-phase matched different frequency generation in a stacked GaAs wafers pumped by one CO2 laser with dual-wavelength output. The THz generation was increased with the increase of the number of GaAs wafers. The maximum single pulse energy of 12 nJ was generated at a frequency of 0.94 THz (319 μm) by using ten GaAs wafers, corresponding to a peak output power 200 mW
Experiments of pulse CO2 laser produced tin plasma had been carried out. Plasma parameters of electron temperature and density measurements both in axial and radial direction had been performed from a two-dimensional time and space resolved image spectra analysis. Debris speed of laser produced plasma in various buffer gas was quantitatively estimated by means of a fast gated intensified charge coupled device imaging system. The stopping power of the hydrogen buffer gas was assessed under ambient pressure ranging from 30 to 104 Pa.
We succeeded in obtaining gas discharge in 250, 150, and 50μm bore-diameter hollow-core fibers by using longitudinal
DC excitation. Stable glow discharges of at least several minutes were observed for these hollow-core fibers, and a flash
glow was also observed for a hollow-core capillary with a diameter of ~20μm. Breakdown of helium and argon gases in
a 26.2cm-length 250μm-inner-diameter hollow-core fiber was achieved with a voltage of less than 30kV. Breakdown
voltages of helium and argon gases were measured for various bore-sizes, fiber-lengths, and a pressure range of from
below 1 to 50 Torr. Experimental results deviated from previous theoretical models and further theoretical and
experimental investigations are needed to understand the unique characteristics of gas-discharge in miniature capillaries
with bore-diameter below 250μm.
A theoretical model is established to describe the α-RF discharge in slab Oxygen Iodine lasers, according to the
continuity equation of electron density, the electron energy equilibrium equation and the continuity equation of current
density. Assuming a Maxwellian energy distribution, the spatial distributions of electron density and electric field in RF
plasma are obtained by numerical method. The effects of parameters on discharge characteristics have been analyzed.
The results show that the current density has a big effect on the electron density in discharge area. The influences of
excitation frequency on the maximum value of electric field and the thickness of boundary layer are also discussed. And
the spatial distributions of electron energy and excitation efficiency of singlet delta oxygen have been calculated. The
influences of gas mixture on the excitation efficiency of singlet delta oxygen are discussed. It provides references of
parameters for slab discharge in singlet delta oxygen generating.
Cutting brittle materials such as ceramics and glass by lasers or traditional saw method, costly fractures and associated
damage such as chips and cracks can result. In most cases, these problems were caused as a result of the stress was not
controlled properly and exceeded the critical value of the fracture. In this study, a dual-laser-beam method was proposed
to avoid fractures in glass laser cutting, where an off-focused CO2-laser beam was scanning on the top surface of glass
periodically and repeatedly and a preheated-band which has a proper temperature was built. This preheated-band will
reduce the temperature gradients when the glass is cut by the focused CO2-laser beam. Under these conditions, glass can
be cut with melting method without any fractures. The process of cutting glass by dual CO2-laser beams was simulated
numerically with FEA method and the distribution of temperature and thermal stress was investigated. The relationships
between the cutting parameters, such as laser beam diameter, laser power, cutting speed, and the profile of the cutting
groove were also discussed. The result showed that thermal stress decreased with the increasing width of
preheated-banding, and the smaller the diameter of laser beam, the better the quality of the cutting groove was in the
same laser power situation.
A dual laser beams method was proposed to reduce the thermal stresses when machining glass with CO2-laser. A numerical method based on Ansys software was used to simulate temperature and thermal stresses in glass. Thermal stresses in glass with and without CO2-laser preheating have been studied. The results showed that machining glass with dual laser beams could reduce thermal stresses.
We report the progress toward a novel waveguide gas laser based on hollow-core light guiding capillaries and hollowcore
photonic bandgap (PBG) fibers. Since Smith reported the first waveguide He-Ne laser at 0.633μm with a 20cm
length of 430μm-diameter-bore glass-capillary-tubing in 1971, no smaller size waveguide gas laser has ever been
constructed. Conventional hollow waveguide theory reveals that small bore size tubes suffer greater waveguide loss
which hinders the construction of smaller size waveguide gas lasers. Hollow-core PBG fibers guide light through PBG
effect that is different from grazing incidence mechanism of traditional simple hollow waveguides, and PBG fibers with
a loss of below 0.5dB/m have been demonstrated at various wavelengths including 633nm. This indicates that we may
construct waveguide gas lasers with such hollow-core PBG fibers. We carried out a series of experiments and succeeded
in discharging gases contained within 250μm, 150μm and 50μm bore diameter hollow-core light guiding capillaries or
fibers. Stable glow discharge of at least several minutes was observed for these waveguides. A flash glow was also
observed from a hollow-core capillary with a diameter of ~20μm. Initial measurements of current-voltage (I-V)
characteristics have been carried out for various tube sizes filled with various gases at different pressures. Theoretical IV
characteristics are also presented and compared with experimental results. Discharging miniature waveguide bore
tubes was found to exhibit unique characteristics that are different from the traditional larger diameter tubes.
Boltzmann equation was solved for the oxygen gas with the mixture of He, Ar, Xe etc. The electron energy distribution, electron average energy, ionization and attachment coefficient were obtained. The results have shown that the electron energy distribution over frequency is almost identical to that for the DC excitation at the same value of E/N, only in the microwave frequency range the frequency began to influence the electron energy distribution. The condition to obtain self-sustained discharge and the the influence of gas mixture on the production of singlet delta oxygen was discussed in this paper.
A numerical method based on ANSYS software was presented to simulate the temperature and stress distributions on the glass plate under the CO2 laser irradiation. The influences of laser modes and output power on temperature and stress distributions were discussed in this paper, results shown that the TEM00 mode laser is not suitable for preheating glass.
The principle and method of high power CO2 laser excited by switching power have been proposed. In the traditional transverse flow pin-plate CO2 laser, a high frequency switching power was overlapped on the DC discharge to obtain uniform discharge. Experimental results show that this new excited technique can improve the discharge stability, and modulate the laser output. The new method is also profitable for minimization and industrial applications of high power transverse flow CO2 laser.
We reported a discharge configuration for the slab CO2 laser excited by switching power, and the frequency of the power sources is about 300 kHz. The discharge was confined between two water-cooled narrow gap anodized aluminum plates. The uniform stable discharge was obtained in the volume of 4 X 30 X 400 mm3, the input power density is about 25W/cm3. The measurements of gain in the discharge show that this configuration is suitable for the high power CO2 laser.
Theoretical investigation has been made to describe the discharge in CO2 lasers excited by high frequency magnetic-confined discharge. The distribution of the electron energy was considered to be a Maxwellian and the most important collision cross sections were considered, the energy balance equation for electron can be put up under the influence of the magnetic field. The evaluation of the present model was performed for gas mixtures of CO2, N2, and He, the influence of the electron energy on the intensity of the magnetic field and the spatial distributions of the electron density and electric field were presented in this paper.
An all-metal CO2 laser is presented in this paper. A new electrode configuration is used and a high frequency discharge between two dielectric electrodes is used to obtain uniform stable glow discharge in a volume of 6 X 6 X 446 mm3. Some experimental results are presented in this paper and the laser output power is 12 W.