Bessel beam

About: Bessel beam is a research topic. Over the lifetime, 1946 publications have been published within this topic receiving 42264 citations.

Application of vector diffraction theory in geometric phase based metasurfaces

Abstract: In geometric phase based metasurfaces, phase modulation of the output light can be achieved by designing the shape and layout of the subwavelength structure. For traditional electromagnetic simulation software, the operating principle is often based on finite-difference time-domain, finite element method, or finite integration technique (FIT). Their disadvantages include, for example, the time-consuming simulation and the complicated modeling process. In addition, the computational accuracy may be affected by the size of divided grids. In this paper, an improved vector diffraction theory is investigated to calculate the distribution of the transmitted light field when the incident circular polarization light (CPL) passes through the geometric phase based metasurfaces. The structure acts as a diffraction screen when using the method to calculate the diffraction field. When CPL is incident, the geometric phase is imparted to the output light, and the transmission light field is calculated by normal vector diffraction theory. Three kinds of structures, which can be seen as nano-object arrays, have been designed and characterized to validate our method. The first and second devices realized the separation of cross-polarization and co-polarization components, and the third one realized the function of the focusing lens with a subwavelength catenary array. The transmission fields calculated by the refined vector diffraction theory are in good agreement with the FIT method. We believe this simple yet generalized method can be employed for efficient design of geometric phase based metasurfaces, such as Bessel beam generators, focusing lenses, holographic coding, etc.

Generation of an asymmetric hollow-beam

Abstract: We studied the optical properties of the combination between a tilted collimated light beam and the wave emergent from a conical lens The resulting optical beam is an asymmetric beam, whose shape gives them quasi-nondiffracting and self-healing properties Our experimental results are in good agreement with the simulation results

Bessel beam induced deep-penetrating bioimaging and self-monitored heating using Nd/Yb heavily doped nanocrystals

Abstract: Biological probes facilitate optical imaging and disease diagnosis and treatment. However, the large absorption and scattering loss in the tissue highly limit the depth during the application. In the present research, an NIR-I bioprobing system, which utilizes the Bessel beam to excite heavily doping nanocrystals, has been developed for deep tissue applications. On the one hand, the capillary mode selection method generates the Bessel excitation beam, lowering the excitation energy loss. On the other hand, a strong energy harvest of NaYbF4:90%Nd nanocrystals enables effective fluorescence and heat generation upon 800 nm excitation. By considering the advantages of Bessel excitation and heavily doping nanocrystals, up to ∼3 cm penetration depth for ex vivo bioimaging and the potential self-monitored photothermal treatment are demonstrated. The resultant bioprobing system allows deep tissue imaging and photothermal therapy, showcasing broad prospects in medical research and clinical applications.

Extended Depth of Field Optical Projection Tomography

Abstract: A limiting factor with regard to resolution in OPT is the limited depth of field (DoF) due to light detection with a Gaussian beam profile. The further a source in the sample is removed from the centre of rotation in the focal plane, the more distorted the image is in tangential direction due to the limited DoF. The goal of this research is to extend the depth of field in Optical Projection Tomography. The DoF limitations due to diffraction are mainly caused by the Gaussian beam shape and its inherent limitations such as a small high intensity spot and a small region of focus. An alternative to Gaussian beams is found sporadically in the literature in the form of non-diffracting beams. Non-diffracting beams are beams that propagate without diffraction and show regenerative properties after obstruction. The Bessel beam is a non-diffracting beam that is rotationally symmetric and displays a transversal high-intensity core. It can be generated without energy loss with a lens shaped like a rotationally symmetric prism, called an axicon. A propagation simulation of the axicon generated Bessel beam, using the Hankel transform as a rotationally symmetric alternative to the 2D Fourier transform, is performed and used to verify the analytic description of the axicon generated Bessel beam. A numerical OPT simulation shows that OPT reconstructions of point sources show virtually no blurring, but do show concentric rings due to the intensity distribution of the Bessel beam. These rings can be removed by deconvolution of the projection or deconvolution of the reconstruction of simulated OPT results with the imaging point spread function (PSF) of the axicon-generated Bessel beam. The PSF describes the response of the imaging system to a point source. Practical work is presented with the imaging set-up of an axicon with an objective lens as described earlier. The resolution of the PSF is analysed over paraxial distance from the objective lens for both coherent and incoherent illumination. The same is done for a resolution target in transmission. Comparison of the Bessel system with Gaussian models show that the DoF increase shown by the Bessel system is significant in all cases. It is found that for sources with spatially narrow intensity distributions (near-point source) the PSF resolution matches theoretical predictions. However, as the spatial light source distribution increases slightly, Bessel distributions overlap spatially. This creates artefacts and deteriorates the resolution It is concluded that extended depth of field in OPT can be achieved with non-diffracting axicon generated Bessel beams. However, for objects larger than point sources the resolution deteriorates. Furthermore, the means of illumination are a major influence on the resulting images when using an axicon. Further research on the optimization of illumination is recommended. Additionally, recommendations for further research on the significance of self-regeneration are made. Recommended applications for use of Bessel beams in optical imaging are those where a large DoF is desired and high resolution is less of a priority, or imaging and OPT of very sparse but large samples.