Continuous Wave Gaussian Beam Research Articles

Comparison of photothermal responses under spatial and temporal modulations of CW Gaussian beam excitation: A numerical study

Obtaining stronger photothermal response of optical materials is of great interest in the measurement of optical losses using the photothermal tools, in particular the thermal mirror. In our previous work, we experimentally demonstrated a more effective method of thermal mirror detection employing the spatial modulation (SM) of a continuous wave Gaussian beam excitation instead of the conventional temporal modulation (TM). A complete analytical model is presented to describe the laser-induced three-dimensional dynamic thermal fields in a semi-infinite sample and the time-dependent thermal mirror due to the thermoelastic response of the material as well as the evolution of the intensity distribution of a probe beam after interacting with the thermal mirror, in both the SM and TM schemes under the low absorption and exponentially-decaying absorption cases. With the model, we obtained the variations of temperature, surface displacement, and diffraction intensity giving rise to the photothermal response of the material. The results of the comparison between the two schemes reveal that the SM scheme is more sensitive than the TM scheme originating from a large variation range of the temperature in the SM scheme. We also explained the complex relation between the surface displacement and the diffraction intensity of the probe beam at the center of the detection plane on the basis of the phase shift caused by the thermal mirror. The presented model and the results of comparison are of great importance to gain a deeper insight into the photothermal characterization in both the SM and TM schemes.

On the inverse phase characteristics of Gaussian beams in negative refractive index materials

The phase characteristics of Gaussian beams in negative refractive index (NRI) slabs have been quantitatively investigated via the finite-difference time-domain (FDTD) method. The phase and magnitude distributions for both the normal and oblique incidences of continuous wave Gaussian beams are presented. For the normal incidence, all three contributions of the phase, i.e., plane-wave phase, radial phase and Guoy phase shift, have been shown to be the reverse of their counterparts in free space, which evidently reveals how phase compensation is caused by a NRI slab. Moreover, it is worth noting that our FDTD results are more accurate than the paraxial results in the case of a Gaussian beam propagating beyond the paraxial approximation.

Influence of spatiotemporal coupling on the capture-and-acceleration-scenario vacuum electron acceleration by ultrashort pulsed laser beam

This paper investigates the properties of the ultrashort pulsed beam aimed to the capture-and-acceleration-scenario (CAS) vacuum electron acceleration. The result shows that the spatiotemporal distribution of the phase velocity, the longitudinal component of the electric field and the acceleration quality factor are qualitatively similar to that of the continuous-wave Gaussian beam, and are slightly influenced by the spatiotemporal coupling of the ultrashort pulsed beam. When the pulse is compressed to an ultrashort one in which the pulse duration TFWHM<5T0, the variation of the maximum net energy gain due to the carrier-envelope phase is a crucial disadvantage in the CAS acceleration process.

3D finite element analysis of evaporative laser cutting

A three-dimensional computational model of evaporative laser-cutting process has been developed using a finite element method. Steady heat transfer equation is used to model the laser-cutting process with a moving laser. The laser is assumed continuous wave Gaussian beam. The finite element surfaces on evaporation side are nonplanar and approximated by bilinear polynomial surfaces. Semi-infinite elements are introduced to approximate the semi-infinite domain. An iterative scheme is used to handle the geometric nonlinearity due to the unknown groove shape. The convergence studies are performed for various meshes. Numerical results about groove shapes and temperature distributions are presented and also compared with those by semi-analytical methods.

Pulsed and CW Gaussian beam interactions with double negative metamaterial slabs

The interactions of pulsed and continuous wave (CW) Gaussian beams with double negative (DNG) metatmaterials are considered. Subwavelength focusing of a diverging, normally incident pulsed Gaussian beam with a planar DNG slab is demonstrated. The negative angle of refraction behavior associated with the negative index of refraction exhibited by DNG metamaterials is demonstrated. The transmitted beam resulting from both 3- cycle and CW Gaussian beams that are obliquely incident on a DNG slab are shown to have this property. Gaussian beams that undergo total internal reflection from a DNG metamaterial slab are also shown to experience a negative Goos-Hänchen (lateral) shift. Several potential applications for these effects in the microwave and optical regimes are discussed.