Bessel beams (BBs) appear to be immune to diffraction over finite propagation distances due to the conical nature of light propagation along the optical axis. This offers promising advantages in laser fabrication. However, BBs exhibit a significant intensity variation along the direction of propagation. We present a simple technique to engineer the axial intensity of the BBs over centimeter-long propagation distances without expansion of the incoming laser beam. This method uses two diffractive optical elements (DOEs), one converts the input Gaussian intensity profile to an intermediate intensity distribution, which illuminates the second DOE, a binary axicon. BBs of a desired axial intensity distribution over a few centimeters length can be generated. © 2018 IOP Publishing Ltd.

Shanti Bhattacharya

Department of Electrical Engineering

Raghu Dharmavarapu

Bessel functions

DIFFRACTIVE OPTICS

Laser beams

BEAM-SHAPING

Bessel beam

FINITE PROPAGATION

GAUSSIAN INTENSITY PROFILE

INTENSITY DISTRIBUTION

Intensity variations

Laser fabrication

PROPAGATION DISTANCES

Gaussian beams

IIT Madras is a public technical and research university located in Chennai, Tamil Nadu. Founded in 1959, it is recognised as an Institute of National Importance.

IIT Madras has been ranked as the top engineering institute in India for four years in a row by the National Institutional Ranking Framework of the MHRD

It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Journal of Optics (United Kingdom)

We develop an open source software “GDoeSII” for the simulation of Fresnel (near field) and Fraunhofer (far field) diffraction integrals of diffractive optical elements (DOE). It can compute the intensity distributions at a desired plane from the DOE. This software can also convert the phase profiles of the DOEs from standard image formats such as JPG and PNG to lithography graphic format GDSII for fabrication purposes. The conversion algorithm used in this program groups adjacent same-valued pixels and creates a single cell rather than creating a cell for each and every pixel in the image file. This results in a faster and smaller output GDSII file size. Conversion to the multi-layer GDSII format is also possible for grayscale lithography. We show, as an example, the simulation, GDSII conversion, fabrication and experimental results for the case of an Airy beam generator. GDoeSII offers a complete platform for researchers working in diffractive optics from simulations to lithography file preparation. © 2019 The Authors

Fulltext

SoftwareX

GDOESII: Software for design of diffractive optical elements and phase mask conversion to GDSII lithography files

Recently, metasurfaces have gained popularity due to their ability to offer a spatially varying phase response, low intrinsic losses and high transmittance. Here, we demonstrate numerically and experimentally a silicon meta-surface at THz frequencies that converts a Gaussian beam into a Vortex beam independent of the polarization of the incident beam. The metasurface consists of an array of sub-wavelength silicon cross resonators made of a high refractive index material on substrates such as sapphire and CaF2 that are transparent at IR-THz spectral range. With these substrates, it is possible to create phase elements for a specific spectral range including at the molecular finger printing around 10 μm as well as at longer THz wavelengths where secondary molecular structures can be revealed. This device offers high transmittance and a phase coverage of 0 to 2π. The transmittance phase is tuned by varying the dimensions of the meta-atoms. To demonstrate wavefront engineering, we used a discretized spiraling phase profile to convert the incident Gaussian beam to vortex beam. To realize this, we divided the metasurface surface into eight angular sectors and chose eight different dimensions for the crosses providing successive phase shifts spaced by π/4 radians for each of these sectors. Photolithography and reactive ion etching (RIE) were used to fabricate these silicon crosses as the dimensions of these cylinders range up to few hundreds of micrometers. Large 1-cm-diameter optical elements were successfully fabricated and characterised by optical profilometry. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

Proceedings of SPIE - The International Society for Optical Engineering

All-dielectric metasurface for wavefront control at terahertz frequencies

This paper describes a simplified mesh generation technique that is based on the finite element method of calculation of beam-shaping diffractive optical elements (DOEs). The mesh generation technique uses the inherent symmetry of the incident beam to generate a mesh. Using the meshes so generated, DOEs that convert a Gaussian intensity beam to one of a specified shape, are calculated. Simulations of the results of such beam-shaping elements will be presented. Such elements have uses in industrial and medical applications where both the shape as well as the intensity distribution on the material that is to be processed is very important. For example, in industrial applications the beam may be used to uniformly heat up a specific area in which case the intensity has to be uniform across the beam. The Gaussian intensity variation of a laser has to be converted to a flat-top beam in order to achieve this. To reiterate, beam shaping refers to changing both the intensity distribution and the shape of the beam. Experimental results of the fabricated gratings will also be presented. These results will include experimental data on the method of additive lithography which can be used to improve the efficiency of DOEs. © 2007 Elsevier GmbH. All rights reserved.

Optik

Simplified mesh techniques for design of beam-shaping diffractive optical elements

Optics Express

Flattening axial intensity oscillations of a diffracted Bessel beam through a cardioid-like hole

Science

Metalenses: Versatile multifunctional photonic components

Light: Science & Applications

Generation of wavelength-independent subwavelength Bessel beams using metasurfaces

Diffractive optics for axial intensity shaping of Bessel beams

Journal	Journal of Optics (United Kingdom)
Publisher	Institute of Physics Publishing
ISSN	20408978
Open Access	No