Numerical and Experimental Study on Multi-Focal Metallic Fresnel Zone Plates Designed by the Phase Selection Rule via Virtual Point Sources

Abstract

Abstract:We propose a novel design method for multi-focal metallic Fresnel zone plates (MFZPs), which exploits the phase selection rule by putting virtual point sources (VPSs) at the desired focal points distant to the MFZP plane. The phase distribution at the MFZP plane reciprocally formed by the VPSs was quantized in a binary manner based on the phase selection rule, thereby leading to a corresponding on-off amplitude pattern for the targeted MFZP. The resultant phase distribution was dependent on the complex amplitudes of the VPSs, so that they could be determined from the perspective of both multi-focal functionality and fabrication feasibility. As a typical example, we utilized the particle swarm optimization algorithm to determine them. Based on the proposed method, we designed and numerically analyzed two types of novel MFZPs-one for a monochromatic multi-focal application and the other for a multi-chromatic mono-focal application-verifying the effectiveness and validity of the proposed method. We also fabricated them onto Au-deposited glass substrates, using electron beam evaporation and a focused ion beam milling process. We experimentally characterized them and also verified that they successfully demonstrated their feasibilities. The former produced distinct hot spots at three different focal distances of 10, 15, and 20 µm for monochromatic incidence at 650 nm, and the latter produced a single hot spot at a focal distance of 15 µm for multi-chromatic incidence at 660, 532, and 473 nm. The experimental results were also in good agreement with their corresponding numerical results. We expect that both MFZPs will have various applications, such as laser micromachining, optical trapping, biomedical sensing, confocal collimation, achromatic optics, etc.

Sub-Wavelength Slit-Assisted Binary Metallic Lens Design for Effective Multifocusing via Phase Superposition Method

“…The calculation of the field pattern was performed by assuming that each lens surface was a point charge source having the same intensity and phase with the incident light, and then superpositioning the set of charges. The same approach was performed on our previous work, and we showed a considerably good match compared to numerical simulations and experiments [23].…”

“…It is possible to divide the lens section that acts as a different lens [21] or to consider the phase information of the targeted foci [15], [22]. We have also proposed a design principle, virtual point method (VPM), that assumes the focused light to be a virtual source and calculates the field of the lens plane, then positions multiple virtual sources to design the multifocal lens [23]. However, the information obtained using the VPM includes not only the phase but also the intensity and polarization.…”

Plasmonics on Optical Fiber Platforms

“…These schemes will be useful for various applications that require accurate, flexible, and centrosymmetric optical focusing with a broad focal-length tuning range, such as in micro/nanomachining and optical trapping. In addition, these schemes can also be exploited for mono-chromatic-multi-focal or multi-chromatic-mono-focal lenses [43,44].…”

Bifocal Ultrasound Focusing Using Bi-Fresnel Zone Plate Lenses

“…These devices have been widely used in various fields of physics, such as X-rays [13,14], optics [15][16][17], microwaves [18,19], and acoustics [20][21][22]. In [23], J. Kim et al presented a method to achieve multifocal focusing profiles based on the phase conjugation principle and the phase-opposition blocking criterion used in conventional FZPs. Recently, novel lenses based on traditional FZPs have been introduced in optics [ [24][25][26][27] and later demonstrated in acoustics [28][29][30], which increases the versatility of these devices.…”