| Terahertz waves have important research value and clinical applications in the life sciences because of their fingerprint spectrum,non-ionization and sensitivity to water molecules.The transmission and scattering characteristics of terahertz waves in fresh or dried tissues can affect the accuracy of terahertz spectroscopy analysis and the contrast of terahertz imaging.In recent years,the study of terahertz waves has been gradually extended to shaped beams,and the use of 3D printing technology to produce devices such as phase plates and axial cones can realize the modulation of the phase of terahertz waves to produce terahertz vortex beams and Bessel beams,expanding the application of terahertz technology.In the optical band,the study of tissue optics has been a research hotspot in the biomedical field,and the use of optics to achieve rapid detection,diagnosis and treatment of diseases has attracted increasing attention.The non-uniformity of biological soft tissues at the micron scale causes strong scattering of light beams in the near-infrared band in the "optical window",which limits the depth of detection of tissues and related applications by optical techniques.In recent years,researchers have found that modulating the phase,spatial coherence,and polarization distribution of the optical field can improve the ability of the beam to overcome refractive index fluctuations.For these reasons,this dissertation is centered on the study of plane and vortex beams in the terahertz band and structured beams in the optical band in biological tissue transmission and applications,and the main work is outlined as follows.1.The transmission and scattering characteristics of terahertz waves in biological tissues were analyzed.Based on the dielectric properties of biological soft tissues in the terahertz band,the absorption coefficients of different tissues were calculated,and based on the BeerLambert law,the intensity attenuation of terahertz waves in homogeneous tissues was studied.Based on Rayleigh scattering and Mie scattering theories,the scattering characteristics of molecules and cells in biological tissues to terahertz waves were analyzed.Models of red blood cells,white blood cells and blood clots of different shapes and sizes in blood were established,and the scattering intensity of blood components to terahertz waves was studied by using the time-domain finite difference method.By establishing a layered skin model,the transmittance and reflectance of terahertz TE and TM waves when transmitted in uniformly layered skin are investigated based on a feature matrix approach.For the difference between normal skin and cancerous skin,i.e.,the increased absorption coefficient of terahertz waves by the cancerous tissue changes its dielectric properties,resulting in a slight difference in the Brewster angle between cancerous and normal skin for the reflection of electromagnetic waves at specific frequencies,the expression of the reflected electric field of terahertz vortex beam on the tissue surface is derived based on the theory of angular spectrum expansion,and the effects of the incident angle of terahertz vortex beam,skin cancer and topological charge on the reflected field intensity are discussed.2.Based on the refractive index fluctuation power spectrum of biological soft tissues and the generalized Huygens-Fresnel diffraction theory,the analytical expressions for the light intensity of the Hermite-Gaussian correlated Schell-model beam when it is transmitted in refractive index perturbed tissues are derived,and the evolution characteristics and transmission quality of this partially coherent beam with special spatial coherence structure in tissues are investigated.The effects of the beam order,transverse coherence length,and the fractal dimension,inhomogeneous structure length and small scale factor of tissues on the average light intensity,spectral coherence and beam transmission factor are analyzed.3.By deriving the cross-spectral density function of the partially coherent vortex(PCV)beam in refractive index disturbed tissues,the evolution properties of the light intensity distribution and the spectral coherence of this beam in tissues were investigated,and the effects of the correlation parameters of the PCV beam and the tissue refractive index on the normalized root mean square beam width,angular width and normalized beam transmission factor were systematically analyzed.The transmission quality of the partially coherent vortex beam with column vector polarization in the tissue is explored based on the fractal power spectrum of the tissue refractive index.The transmission of the Bessel-Gaussian beam within the turbulent tissue is numerically simulated using the fractal propagation method.4.Based on the Collins diffraction formula,the analytical expression of the Bessel-Gaussian correlated Schell-model(BGCSM)beam after passing through the focusing system is derived,and the focusing characteristics of this beam are studied.Based on the Rayleigh scattering theory,the radiation force of the BGCSM beam near the focal plane on two Rayleigh particles(with relative refractive index greater than 1 or less than 1)is analyzed,and the effects of the coherence parameters of the beam and particles parameters on the radiation force are specifically studied.The capture stability is analyzed in terms of Brownian force and axial radiation force.5.The photonic nanojet produced by irradiation of terahertz plane waves,terahertz vortex beams,and column-vector Gaussian and Bessel-Gaussian beams in the visible wavelength band on a microlens is studied.The structure,size,refractive index,and other properties of the microlens affect the focal length,peak intensity,jet length,and full width at half height of the photonic nanojet effect.When the terahertz vortex beam topological charge is 0 and1,the resulting photonic nanojet spot has a Gaussian distribution with the light intensity maximum on the z-axis,which is the same as the photonic nanojet effect of plane waves;when the topological charge is 2,the resulting photonic nanojet is ring-shaped,and its full width at half height is better than the results when the topological charge is 0 and 1.In the optical band,the radially polarized Gaussian beam can achieve a narrower full width at half height,and the angular polarization beam can obtain a circular converging beam.In addition,the effects of topological charge,half-cone angle of Bessel-Gaussian vortex beams and particle radius on the convergence effect are analyzed. |
PDF Full Text Request