Tight focusing of laser light using a chromium Fresnel zone plate.

E S Kozlova,M V Kotlyar,L O’Faolain,A G Nalimov,V V Kotlyar,S S Stafeev

doi:10.1364/oe.25.019662

Abstract

Using near-field scanning microscopy, we demonstrate that a 15-µm zone plate fabricated in a 70-nm chromium film sputtered on a glass substrate and having a focal length and outermost zone's width equal to the incident wavelength λ = 532 nm, focuses a circularly polarized Gaussian beam into a circular subwavelength focal spot whose diameter at the full-width of half-maximum intensity is FWHM = 0.47λ. This value is in near-accurate agreement with the FDTD-aided numerical estimate of FWHM = 0.46λ. When focusing a Gaussian beam linearly polarized along the y-axis, an elliptic subwavelength focal spot is experimentally found to measure FWHMx = 0.42λ (estimated value FWHMx = 0.40λ) and FWHMy = 0.64λ. The subwavelength focal spots presented here are the tightest among all attained so far for homogeneously polarized beams by use of non-immersion amplitude zone plates.

Highlights

Zone plates (ZP) have been known in optics for quite long, they still arouse an incessant interest on the part of researchers
We study a Fresnel zone plates (ZP) on a quartz glass both theoretically and experimentally, showing it to focus an incident linearly polarized beam into a subwavelength focal spot smaller than FWHM = 0.45λ
We experimentally show that a simple non-immersion zone plate with NA=1 fabricated in a chromium film is able to focus a circularly polarized Gaussian beam, producing at a 600-nm distance a circular subwavelength focal spot of diameter FWHM= (0.47±0.02)λ

Summary

Introduction

Zone plates (ZP) have been known in optics for quite long, they still arouse an incessant interest on the part of researchers. An SPP-lens for radially polarized light discussed in [15] was reported to produce a focal spot of size FWHM = 0.46λ (λ = 632.8 nm). We study a Fresnel ZP on a quartz glass both theoretically and experimentally, showing it to focus an incident linearly polarized beam into a subwavelength focal spot smaller than FWHM = 0.45λ. We experimentally show that a simple non-immersion zone plate with NA=1 fabricated in a chromium film is able to focus a circularly polarized Gaussian beam, producing at a 600-nm distance a circular subwavelength focal spot of diameter FWHM= (0.47±0.02)λ. Measurements on the near-field scanning optical microscope show that the fabricated ZP focuses a linearly polarized Gaussian incident beam into a subwavelength elliptic spot measuring FWHMx=0.42λ and FWHMy=0.64λ. A near-circular focal spot measuring FWHMx=0.47λ and FWHMy=0.48λ is experimentally observed

Numerical simulation results

Fabrication and modeling with regard for fabrication errors

Findings

Conclusion