Creation of multi-frequency terahertz waves by optimized cascaded difference frequency generation

A new scheme which generates multi-frequency terahertz(THz)waves from planar waveguide by the optimized cascaded difference frequency generation(OCDFG)is proposed.A THz wave with frequencyω_(T1)is generated by the OCDFG with two infrared pump waves,and simultaneously a series of cascaded optical waves with a frequency intervalω_(T1)is generated.The THz wave with a frequency of M-timesω_(T1)is generated by mixing the m-th-order and the(m+M)-th-order cascaded optical wave.The phase mismatch distributions of cascaded difference frequency generation(CDFG)are modulated by changing the thickness of planar waveguide step by step,thereby satisfying the phase-matching condition from first-order to high-order cascaded Stokes process step by step.As a result,the intensity of THz wave can be enhanced and modulated by controlling the cascading order of OCDFG.

Broadband mid-infrared pulse via intra-pulse difference frequency generation based on supercontinuum from multiple thin plates

We report on the generation of optical pulses with a nearly one octave-spanning spectrum ranging from 1300 nm to2500 nm at 1 kHz repetition rate, which are based on intra-pulse difference frequency generation(DFG) in β-barium borate crystal(β-BBO) and passively carrier-envelope-phase(CEP) stabilized. The DFG is induced by few-cycle pulses initiated from spectral broadening in multiple thin plates driven by a Ti: sapphire chirped-pulse amplifier. Furthermore, a numerical simulation is developed to estimate the conversion efficiency and output spectrum of the DFG. Our results show that the pulses from the DFG have the potential for seeding intense mid-infrared(MIR) laser generation and amplification to study strong-field physics and attosecond science.

Theoretical research on terahertz wave generation from planar waveguide by optimized cascaded difference frequency generation

A novel scheme for high-efficiency terahertz(THz)wave generation based on optimized cascaded difference frequency generation(OCDFG)with planar waveguide is presented.The phase mismatches of each-order cascaded difference frequency generation(CDFG)are modulated by changing the thickness of the waveguide,resulting in a decrement of phase mismatches in cascaded Stokes processes and an increment of phase mismatches in cascaded anti-Stokes processes simultaneously.The modulated phase mismatches enhance the cascaded Stokes processes and suppress the cascaded anti-Stokes processes simultaneously,yielding energy conversion efficiencies over 25%from optical wave to THz wave at 100 K.

High-efficiency terahertz wave generation with multiple frequencies by optimized cascaded difference frequency generation

High-efficiency terahertz(THz) wave generation with multiple frequencies by optimized cascaded difference frequency generation(OCDFG) is investigated at 100 K using a nonlinear crystal consisting of a periodically poled lithium niobate(PPLN) part and an aperiodically poled lithium niobate(APPLN) part.Two infrared pump waves with a frequency difference ω_(T1) generate THz waves and a series of cascaded optical waves in the PPLN part by cascaded difference frequency generation(CDFG).The generated cascaded optical waves with frequency interval ω_(T1) then further interact in the APPLN part by OCDFG,yielding the following two advantages.First,OCDFG in the APPLN part is efficiently stimulated by inputting multi-order cascaded optical waves rather than the only two intense infrared pump waves,yielding unprecedented energy conversion efficiencies in excess of 37% at 1 THz at 100 K.Second,THz waves with M timesω_(T1) are generated by mixing the mth-order and the(m+M)th-order cascaded optical waves by designing poling period distributions of the APPLN part.

Energy Scaling of Terahertz Pulses Produced through Difference Frequency Generation

We study the energy scaling of terahertz (THz) emission through difference frequency generation of near-infrared pulses, and demonstrate that Gigawatt few-cycle THz transients at the central frequency of 30 THz are produced from GaSe crystal pumped by two pulses at 1.65 and 1.95 micrometers, with the high quantum yield of 28%. Our analysis indicates that the high yield of DFG originates from the largely reduced group velocity mismatch as the long-wavelength pumping pulses are employed.

Cultural and Generational Differences in The Joy Luck Club

Amy Tan is one of the prominent Chinese American writers that have emerged since the 1980s. Her first novel The Joy Luck Club, which was published in 1989, brought her instant success. The story explores conflicts between two generations and two different cultures. Set in China and in the United States, the novel is woven by stories of four Chinese mothers and their four daughters. Though short in form, it is really worth detailed interpretation and appreciation. This paper will mainly discuss the cultural and generational differences reflected in the story with the conflict of one pair of the mother and daughter--Suyuan Woo and Jing-mei Woo as the main examples. The author of this paper believes that with the realization of the globalization, different cultures are fusing now

Mid-IR dual-wavelength difference frequency generation in uniform grating PPLN using index dispersion control

A novel widely tunable dual-wavelength mid-IR difference frequency generation （DFG） scheme with uniform grating periodically poled lithium niobate （PPLN） is presented in this paper. By using the temperature-dependent dispersion property of PPLN, the quasi-phase matching （QPM） peak for the pump may evolve into two separate ones and the wavelength spacing between them increases with the decrease of the crystal temperature. Such two pump QPM peaks may allow simultaneous dual-wavelength mid-IR laser radiations while properly setting the two fundamental pump wavelengths. With this scheme, mid-IR dual-wavelength laser radiations at around 3.228 and 3.548, 3.114 and 3.661, and 3.019 and 3.76 μm, are experimentally achieved for the crystal temperatures of 90, 65, and 30 ℃, respectively, based on the fiber laser fundamental lights.

Generation of Higher Terahertz Harmonics in Nonlinear Paraelectrics under Focusing in a Wide Temperature Range

It is theoretically investigated the generation of higher harmonics of two-dimensional and three-dimensional terahertz electromagnetic beams in nonlinear crystals. The attention is paid to crystalline paraelectrics like SrTiO3 under the temperatures 60 - 200 K, these crystals possess the cubic nonlinearity. The bias electric field is applied to provide the dominating quadratic nonlinearity. The initial focusing of the beams not only increases the efficiency of generation of higher harmonics, but alto makes possible to select maxima of different higher harmonics at some distances from the input. At lower temperatures the nonlinearity behaves at smaller input amplitudes, whereas at higher temperatures the harmonic generation can be observed at higher frequencies up to 1.5 THz. In three-dimensional beams the peak amplitudes of higher harmonics can be bigger than in two-dimensional beams, but the ratios of these peak values to the maximum values of the focused first harmonic are smaller than in two-dimensional beams.

Compact mid-infrared dual-comb spectrometer over 3-4μm via intra-pulse difference frequency generation in LiNbO_(3) waveguides

The mid-infrared optical frequency comb is a powerful tool for gas sensing.In this study,we demonstrate a simple midinfrared dual-comb spectrometer covering 3±4μm in Li Nb O_(3)waveguides.Based on a low-power fiber laser system,the mid-infrared comb is achieved via intra-pulse difference frequency generation in the Li Nb O_(3)waveguide.We construct pre-chirp management before supercontinuum generation to control spatiotemporal alignment for pump and signal pulses.The supercontinuum is directly coupled into a chirped periodically poled Li Nb O_(3)waveguide for the 3±4μm idler generation.A mid-infrared dual-comb spectrometer based on this approach provides a 100 MHz resolution over25 THz coverage.To evaluate the applicability for spectroscopy,we measure the methane spectrum using the dualcomb spectrometer.The measured results are consistent with the HITRAN database,in which the root mean square of the residual is 3.2%.This proposed method is expected to develop integrated and robust mid-infrared dual-comb spectrometers on chip for sensing.

Computational Analysis of the Effect of Nano Particle Material Motion on Mixed Convection Flow in the Presence of Heat Generation and Absorption

The present study is concerned with the physical behavior of the combined effect of nano particle material motion and heat generation/absorption due to the effect of different parameters involved in prescribed flow model.The formulation of the flow model is based on basic universal equations of conservation of momentum,energy and mass.The prescribed flow model is converted to non-dimensional form by using suitable scaling.The obtained transformed equations are solved numerically by using finite difference scheme.For the analysis of above said behavior the computed numerical data for fluid velocity,temperature profile,and mass concentration for several constraints that is mixed convection parameterλt,modified mixed convection parameterλc,Prandtl number Pr,heat generation/absorption parameterδ,Schmidt number Sc,thermophoresis parameter Nt,and thermophoretic coefficient k are sketched in graphical form.Numerical results for skin friction,heat transfer rate and the mass transfer rate are tabulated for various emerging physical parameters.It is reported that in enhancement in heat,generation boosts up the fluid temperature at some positions of the surface of the sphere.As heat absorption parameter is decreased temperature field increases at position X=π/4 on the other hand,no alteration at other considered circumferential positions is noticed.

THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves

We propose a novel scheme for THz wave generation by repeated and continuous frequency conversions from pump wave to high-order Stokes waves(HSWs).The repeated frequency conversions are accomplished by oscillations of Stoke waves in resonant cavity(RC)where low-order Stokes waves(LSWs)are converted to high-order Stokes waves again and again.The continuous frequency conversions are accomplished by optimized cascaded difference frequency generation(OCDFG)where the poling periods of the optical crystal are aperiodic leading to the frequency conversions from low-order Stokes waves to high-order Stokes waves uninterruptedly and unidirectionally.Combined with the repeated and continuous frequency conversions,the optical-to-THz energy conversion efficiency(OTECE)exceeds 26%at 300 K and 43%at 100 K with pump intensities of 300 MW/cm^(2).

Transient analysis of Casson fluid thermo-convection from a vertical cylinder embedded in a porous medium: Entropy generation and thermal energy transfer visualization

Thermal transport in porous media has stimulated substantial interest in engineering sciences due to increasing applications in filtration systems,porous bearings,porous layer insulation,biomechanics,geomechanics etc.Motivated by such applications,in this article,a numerical study of entropy generation impacts on the heat and momentum transfer in time-dependent laminar incompressible boundary layer flow of a Casson viscoplastic fluid over a uniformly heated vertical cylinder embedded in a porous medium is presented.Darcy’s law is used to simulate bulk drag effects at low Reynolds number for an isotropic,homogenous porous medium.Heat line visualization is also included.The mathematical model is derived and normalized using appropriate transformation variables.The resulting non-linear time-dependent coupled governing equations with associated boundary conditions are solved via an implicit finite difference method which is efficient and unconditionally stable.The outcomes show that entropy generation and Bejan number are both elevated with increasing values of Darcy number,Casson fluid parameter,group parameter and Grashof number.To analyze the heat transfer process in a two-dimensional domain,plotting heat lines provides an excellent approach in addition to streamlines and isotherms.It is remarked that as the Darcy number increases,the deviations of heat lines from the hot wall are reduced.

Analysis of the Per Capita Generation of Municipal Solid Waste by Socioeconomic Strata in Veracruz, Mexico, Using a Statistical Experimental Design

The management of municipal solid waste （MSW） is currently one of the major environmental concerns in Mexico. Therefore, municipalities must develop local programs for prevention and integrated management of MSW based on the data of MSW generation; however, these are scarce and diffuse. It is known that the MSW generation is affected by many factors, including life level. Given the importance of having reliable data, the objective of this study was to determine the per capita MSW generation of a small urban locality in the state of Veracruz and test whether there were significant statistic differences of per capita M SW generation among the life levels. The households were randomly sampled according to the Mexican standard of reference and were classified as popular, middle and residential. The per capita MSW generation determined was 0.355 kg·inhab^-1.day^-1. This value was lower than the value estimated by the national reference, suggesting that it might be overestimated and consequently lead to errors in estimating installations costs for the MSW treatment. The result of the experimental design revealed that there were significant statistic differences in the per capita MSW generation among the three socioeconomic strata. This study will allow the municipality to establish better strategies within the municipal program and differential rate for the provision of public clean service and it also provides a solid basis for future research under similar conditions.

Highly efficient continuous-wave mid-infrared generation based on intracavity difference frequency mixing

We report on a new scheme for efficient continuous-wave(CW)mid-infrared generation using difference frequency generation(DFG)inside a periodically poled lithium niobate(PPLN)-based optical parametric oscillator(OPO).The pump sources were two CW fiber lasers fixed at 1018 nm and 1080 nm.One worked as the assisted laser to build parametric oscillation and generate an oscillating signal beam while the other worked at low power(3 W)to induce DFG between it and the signal beam.The PPLN temperature was appropriately adjusted to enable OPO and DFG to synchronously meet phase-matching conditions.Finally,both low-power 1018 nm and 1080 nm pump beams were successfully converted to 3.1μm and 3.7μm idler beams,respectively.The conversion efficiencies of the 1018 nm and 1080 nm pumped DFG reached 20% and 15%,respectively,while their slope efficiencies reached 19.6% and 15%.All these data were comparable to the OPOs pumped by themselves and never realized before in traditional CW DFG schemes.The results reveal that high-efficiency frequency down-conversion can be achieved with a low-power nearinfrared pump source.

Recent progress on terahertz generation based on difference frequency generation:from power scaling to compact and portable sources

The progress achieved on power scaling and compact and portable THz sources is reviewed.By reversely stacking the GaP plates,the photon conversion efficiency is improved from 25% to 40% which corresponds to the maximum value.When the number of the plates is increased from four to five,the output power decreases because of back conversion.The THz generation is also investigated by mixing the two frequencies generated by a single Nd：YLF solid-state laser.The average output power reaches 1 μW.The introduction of two Nd：YLF crystals significantly improves the output power to 4.5 μW.This configuration facilitates the generation of different output frequencies.

Tunable terahertz wave difference frequency generation in a graphene/AlGaAs surface plasmon waveguide

Graphene-based surface plasmon waveguides(SPWs) show high confinement well beyond the diffraction limit at terahertz frequencies. By combining a graphene SPW and nonlinear material, we propose a novel graphene/AlGaAs SPW structure for terahertz wave difference frequency generation(DFG) under near-infrared pumps.The composite waveguide, which supports single-mode operation at terahertz frequencies and guides two pumps by a high-index-contrast AlGaAs∕Al Oxstructure, can confine terahertz waves tightly and realize good mode field overlap of three waves. The phase-matching condition is satisfied via artificial birefringence in an AlGaAs∕Al Ox waveguide together with the tunability of graphene, and the phase-matching terahertz wave frequency varies from 4 to 7 THz when the Fermi energy level of graphene changes from 0.848 to 2.456 eV. Based on the coupled-mode theory, we investigate the power-normalized conversion efficiency for the tunable terahertz wave DFG process by using the finite difference method under continuous wave pumps, where the tunable bandwidth can reach 2 THz with considerable conversion efficiency. To exploit the high peak powers of pulses, we also discuss optical pulse evolutions for pulse-pumped terahertz wave DFG processes.

Generation of intensity difference squeezed state at a wavelength of 1.34 μm

The intensity difference squeezed state, which means that the fluctuation of the intensity difference between signal and idler beams is less than that of the corresponding shot noise level（SNL）, plays an important role in high sensitivity measurement, quantum imaging, and quantum random numbers generation. When an optical parametric oscillator consisting of a type-Ⅱ phase-matching periodically poled KTiOPO4 crystal operates above the threshold, an intensity difference squeezed state at a telecommunication wavelength can be obtained. The squeezing of 7.7 ± 0.5 dB below the SNL is achieved in an analysis frequency region of 2.4–5.0 MHz.

Midinfrared optical frequency comb based on difference frequency generation using high repetition rate Er-doped fiber laser with single wall carbon nanotube film

We demonstrated stable midinfrared(MIR) optical frequency comb at the 3.0 μm region with difference frequency generation pumped by a high power, Er-doped, ultrashort pulse fiber laser system. A soliton mode-locked161 MHz high repetition rate fiber laser using a single wall carbon nanotube was fabricated. The output pulse was amplified in an Er-doped single mode fiber amplifier, and a 1.1–2.2 μm wideband supercontinuum(SC) with an average power of 205 m W was generated in highly nonlinear fiber. The spectrogram of the generated SC was examined both experimentally and numerically. The generated SC was focused into a nonlinear crystal, and stable generation of MIR comb around the 3 μm wavelength region was realized.

In vitro and in vivo cell tracking of chondrocytes of different origin by fluorescent PKH 26 and CMFDA

Tissue engineering techniques for cartilage re-pair to heal defects in joint surfaces is a clinical practice. Harvested autologous chondrocytes are expanded in culture and delivered in a suitable carrier medium back into the patient>s joint de-fect. The defect is then subsequently filled by new cartilage. Whether the cells in the repair tissue originate from the engineered tissue of the host or are derived from the surrounding original cartilage remains a relevant question for the ap-plied therapy. To answer this several methods exist to track cells, such as transfection of cells with LacZ carrying viruses, radio labeling with 111 IN or 51 Cr or fluorescent labeling with FDA. However, these techniques have drawbacks such as they may influence cellular properties, are radioactive and or quickly lose their tracking ability. New fluorescent probes are easier to handle and do not to interfere with cells. PKH 26劌 is a relatively new cell-labeling agent, but few data exist on the application of this dye in chondrocytes in vitro and in vivo. 5-chloromethylfluorescein diacetate - CMFDA (¨cell tracker green〔) is an established fluores-cent probe for imaging the dynamic processes of cell proliferation in vitro and in vivo. Likewise, several studies exist on different cell types. However, little data are available for chondro-cytes. The first aim of the study was to evaluate qualitative differences in fluorescence pattern after labeling of articular, auricular and costal chondrocytes. Secondly, we evaluated the influ-ence of labeling with CMFDA on cellular adhe-sion properties. The third aim was to compare the duration of cell labeling of chondrocytes of different origin with established CMFDA as stan-dard and PKH 26潴 for 3 cell generations in vitro and 12 weeks in vivo. We show that chondro-cytes from different origin can be labeled effec-tively with both PKH 26潴 and CMFDA. The PKH 26潴 labeled articular chondrocytes maintained fluorescence longer than CMFDA in vitro and in vivo. A higher percentage of articular chondro-cytes remained stained at 63 days than auricular or costal chondrocytes.

Amplification assisted difference frequency generation for efficient mid-infrared conversion based on monolithic tandem lithium niobate superlattice

We report the investigation on the performance of an amplification assisted difference frequency generation(AA-DFG) system driven by pulsed pump and continuous-wave primary signal lasers. A monolithic tandem lithium niobate superlattice was employed as the nonlinear crystal with a uniform grating section for the DFG process, followed by a chirp section for the optical parametric amplification process. The impacts of pump pulse shape, primary signal power, input beam diameter, and crystal structure on the pump-to-idler conversion efficiency of the AA-DFG system were comprehensively studied by numerically solving the coupled wave equations. It is concluded that square pump pulse and high primary signal power are beneficial to high pump-to-idler conversion efficiency. In addition, tighter input beam focus and smaller DFG length proportion could redeem the reduction in conversion efficiency resulting from wider acceptance bandwidths for the input lasers. We believe that such systems combining the merits of high stability inherited from cavity-free configuration and high efficiency attributed from the cascaded nonlinear conversion should be of great interest to a wide community,especially when the pulse shaping technique is incorporated.