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Modelling the sharp focusing of laser light. Sergey Stafeev Samara State Aerospace University Image Processing System Institute REC -14 , Samara. Voronezh, 2010. Introduction. Decreasing the focal spot size is critical in lithography, optical memory and micromanipulation
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Modelling the sharp focusing of laser light Sergey Stafeev Samara State Aerospace University Image Processing System Institute REC-14, Samara Voronezh, 2010
Introduction • Decreasing the focal spot size is critical in lithography, optical memory and micromanipulation • Sharp focusing is a reaching a minimal focal spot size beyond the diffraction limits. • Recently plasmons with FWHM = 0.35λ [Opt.Lett. - 2009. - Vol.34, no.8. - P.1180-1182], FWHM = 0.4λ [Opt. Lett. - 2009. - vol.34, no12. - p.1867-1869] had been obtained. • In this research we used two types of axicons: refractive and diffractive which wereilluminated by radially polarized light
Radial-FDTD (1) • FDTD = finite difference time domain • This method involves the numerical solution of Maxwell's equations in cylindrical coordinate system • We used a modification for a radially polarized light (R-FDTD) • There are three equations with three components Er, Ez andHφ (2)
Refractive microaxicon Focusing of radially-polarized mode R-TEM01 (3) using refractive (conical) microaxicon Radial section of a conical glass (n=1.5) microaxicon of radius R = 7 µm and height h = 6 µm • FWHM=0.30λ • HMA=0.071λ2 The (absolute value of) radial component of the electric field strength of the mode R-TEM01
Refractive microaxicon: 3D modelling Instantaneous distributions of the amplitude Er and Ez for diffraction of the R-TEM01 laser mode by the refractive microaxicon The Intensity and FWHM of the focal spot as a function of axicons height Er Ez the intensity distribution in focal plane the intensity distribution along axicon axis
Binary microaxicon • FWHM =0.39λ • HMA= 0.119λ2 binary axicon with step height 633nm, period 1.48um, index of refraction n = 1.5 the intensity distribution along axicon axis the intensity distribution in focal plane (on the axicons surface) Focusing of radially-polarized mode R-TEM01 using binarymicroaxicon
Manufacture and experiment • Three diffractive binary axicons of periods 4 µm, 6 µm, and 8 µm and height 500nm were fabricated on silica substrate (n=1.46) using a laser writing system CLWS-200 (bought with CRDF money) and plasmo-chemical etching. • Diameter of the focal spot in the near-zone (z<40um) varies from 3.5λ to 4.5λ with a 2-um period for the axicon with period 4um. • The minimal diameter of focal spot equal to 3.6λ (FWHM=1.2λ) An oblique image of the central part of the binary axicon of period 8 µm, produced with the Solver Pro microscope (bought with CRDF money). The diameter of the light spot on the axis (in wavelengths) as a function of distance from binary axicons with period 4µm Diffraction pattern and radial section of the intensity distribution recorded with the CCD-camera from the axicons with period 4 µm at different distances: 5 µm and 2 µm (λ=532nm)
Conclusions • We have numerical shown that when illuminating a conical glass microaxicon of base radius 7 µm and height 6 µm by a radially polarized laser mode R-TEM01 of wavelength λ =1 µm, in the close proximity (20 nm apart) to the cone apex, we obtain a sharp focus of transverse diameter at half-intensity FWHM=0.30λand axial spot size at half-intensity FWHMz=0.12λ. The focal spot area at half-intensity equals HMA=0.071λ2. • Three diffractive binary axicons of periods 4 µm, 6 µm, and 8 µm and height 500nm were fabricated on silica substrate (n=1.46) using a laser writing system CLWS-200 (bought with CRDF money) and plasmo-chemical etching. • Diameter of the focal spot in the near-zone (z<40um) varies from 3.5λ to 4.5λ with a 2-um period for the axicon with period 4um. • The results of numerical simulation agree with experiment
