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2016-Flat Helical Nanosieves(Adv Funct Mater)



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1. FULLPAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1. Introduction Benefiting from the replacement of phase accumulation in bulk…
  • 1. FULLPAPER © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1. Introduction Benefiting from the replacement of phase accumulation in bulk materials with the abrupt interfacial phase discontinuity, Flat Helical Nanosieves Shengtao Mei, Muhammad Qasim Mehmood, Sajid Hussain, Kun Huang, Xiaohui Ling, Shawn Yohanes Siew, Hong Liu, Jinghua Teng, Aaron Danner, and Cheng-Wei Qiu* Compact and miniaturized devices with flexible functionalities are always highly demanded in optical integrated systems. Plasmonic nanosieve has been successfully harnessed as an ultrathin flat platform for complex manipulation of light, including holography, vortex generation, and nonlinear processes. Compared with most of the reported single-functional devices, multifunctional nanosieves might find more complex and novel applications across nanopho- tonics, optics, and nanotechnology. Here, a promising roadmap for nanosieve- based helical devices is experimentally demonstrated, which achieves full manipulations of optical vortices, including its generation, hybridization, spatial multiplexing, focusing and nondiffraction propagation, etc., by controlling the geometric phase of spin light via over 121 thousands of spatially rotated nanos- ieves. Thanks to such spin-conversion nanosieve helical elements, it is no longer necessary to employ the conventional two-beam interferometric meas- urement to characterize optical vortices, while the interference can be realized natively without changing any parts of the current setup. The proposed strategy makes the far-field manipulations of optical orbital angular momentum within an ultrathin interface viable and bridges singular optics and integrated optics. In addition, it enables more unique extensibility and flexibility in versatile optical elements than traditional phase-accumulated helical optical devices. DOI: 10.1002/adfm.201601345 S. Mei, Prof. M. Q. Mehmood, Dr. S. Hussain, Dr. X. Ling, S. Y. Siew, Prof. A. Danner, Prof. C.-W. Qiu Department of Electrical and Computer Engineering National University of Singapore 4 Engineering Drive 3, Singapore 117583, Singapore E-mail: S. Mei, Prof. C.-W. Qiu Graduate School for Integrative Sciences and Engineering National University of Singapore, Centre for Life Sciences (CeLS) #05-01, 28 Medical Drive, Singapore 117456, Singapore Prof. M. Q. Mehmood Information Technology University (ITU) 346-B, Ferozepur Rd, Lahore 54600, Pakistan Dr. K. Huang, Dr. H. Liu, Dr. J. Teng Institute of Materials Research and Engineering Agency for Science Technology and Research (A*STAR) #08-03, 2 Fusionopolis Way, Innovis, Singapore 138634 Dr. X. Ling College of Physics and Electronic Engineering Hengyang Normal University Hengyang 421002, P.R. China metasurface is considered as a promising 2D metamaterial to design integrated optical devices by controlling electro- magnetic wave’s phase, amplitude, and polarization state in a desired manner.[1–5] Varieties of metasurface devices have been fabricated to demonstrate their powerful ability based on the general- ized laws of reflection and refraction. Examples include optical vortex plate,[6] ultrathin flat lens,[7–10] propagating wave to surface wave convertor,[11,12] broad- band quarter-wave plate,[13] directional surface plasmon polaritons excitation,[14] and meta-hologram.[15–20] Especially for the Pancharatnam–Berry (PB) metasur- face,[21] it transfers the interfacial phase distribution into orientations of base units, which largely simplifies the design process of optical antenna compared with those metasurfaces based on spa- tially varying geometric parameters (e.g., V-shaped metasurface, slot antenna array metasurface[22–24] ). Thus, metasurface provides new opportunities and strategies to develop ultrathin optical devices that are compatible with integrated and compact systems. However, most of the afore- mentioned metasurface-based optical devices are limited to single-specific-purpose devices, which restricts their application value. On the other hand, traditional integrated optical devices, which are always fabricated by direct laser writing on bulky optical material, accumulate phase change through light’s prop- agation in the refractive optical material. Therefore, smooth phase change requires continuous thickness change, which demands stringent spatial resolution. The smaller the device gets, the more challenging requirements on the spatial resolu- tion will be. It would be more difficult for traditional strategy when complex surface is required. Specifically, the topological charge-dependent thickness of the Spiral Phase Plate will induce more uncertainties on the required performance and also increase the total thickness of the whole device,[25–27] which may not be compatible with integrated optical systems. Recently, methods of microscopic generation of optical vortex beam,[28] including plasmonics,[29] integrated optics,[30,31] spiral zone plate,[32] optical coordinate transformations,[33] and fiber optics,[34] have been developed. Potential strategies of new generation method are proposed.[35] However, more com- plex manipulations of the generated optical vortex beam are always required in practical applications such as optical twee- zers/optical spanner,[36–38] optical communication,[39–42] and Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601345
  • 2. FULLPAPER 2 © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim even stimulated emission depletion (STED) Microscopy.[43] For example, optical spanners that are used to rotate small particles are real- ized by highly focused vortex beams.[36,37] Up to now, most of the metasurface-generated vortex beams are Laguerre-Gauss-like beams, which will diverge after its generation.[6,44] Nondiffracting vortex beams are desired, e.g., trapping of long and thin objects such as rods and E-coli, simultaneously trapping of both high and low refractive index parti- cles,[45–48] and implementing the Bessel-beam STED,[43] while additional manipulations are difficult to implement after such microscale generation. Here, we would like to achieve complete control of optical vortex beam’s gen- eration, nondiffraction, focusing, and charac- terization via multifunctional helical nanos- ieves, which may bridge the fields of singular optics and integrated optics. In this paper, a series of complex controls of optical vortex beams are reported based on flat helical devices. Without loss of generality, we employ the Babinet PB gold metasurface, or called nanosieve array, as the platform to achieve varieties of multiple orbital angular momentum (OAM) loaded light control. Nanosieves for dealing spin photons and circularly polarized (CP) lights are particu- larly demanded with higher signal-to-noise ratio (SNR) (SNR = 10 log10 (Pd/Pud) dB) by increasing desired transmission power (Pd: cross-CP transmitted power, Pud: co-CP trans- mitted power), which cannot be addressed by reflection-type ones (though with higher effi- ciency). Strategy of improving efficiency has been investigated in many reported works, our work, however, provides a comprehen- sive recipe for designing spin-dependent multifunctional helical nanosieves enabling complete control of vortex manipulations based on our available fabrication conditions verified with fairly good experimental results. Unlike the traditional nanofabricated OAM optical elements, our multifunctional flat helical nanosieves can not only manipulate optical vortex, but also show unique extensi- bility and flexibility during the process of fab- rication and applications. 2. Materials and Methods The two processes of superimposing phase profiles for processing OAM are depicted in Figure 1a. The first row presents the pro- cess of superimposing phase profiles of two concentric axicons and two concentric spiral phase plates. The second row presents the process of superimposing phase profiles Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601345 Figure 1.  Procedure of superimposing phase profiles, schematic diagram, and SEM images of the two-region flat helical nanosieves, and functioning demo of the vortex nanosieves for processing OAM. a) Processes of obtaining the phase profiles of helical axicon and vortex lens by combining spiral phase with different manipulation phases. Insets: SEM images of the same position on the two fabricated vortex nanosieves which correspond to different designs. b) Schematic diagram of the two- region helical nanosieves consisting of rotating perforated photon nanovoid—complementary nano- antennas (shown in the inset)—on the 60 nm gold film. The device contains two regions (Region I and Region II) with a gap (5 μm) between them. Region I is 0.5–45 μm and Region II is 50–100 μm. The values of the parameters in the inset are d1 = d2 = 500 nm, l = 150 nm, and w = 75 nm. ϕ is the local orientation angle of each nanovoid with respect to x axis; c) SEM image of one two-region helical device. All of the designs are fabricated in this shape. The orientation angle of every nanovoid is determined by a specific design. Two of them are shown in the insets of (a); d) Illumination of dif- ferent functional (hybridizing and multiplexing) flat helical devices (Helical Axicons and Vortex Lens) with simulated intensity profiles and the corresponding interference patterns.
  • 3. FULLPAPER© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim of two concentric lenses and two concentric spiral phase plates. The two concentric regions in Figure 1a correspond to the Regions I and II in Figure 1b, respectively. The insets in Figure 1a show the scanning electron microscopy (SEM) images of the same position on the fabricated samples of the two flat helical devices in which the nanovoid’s different ori- entation angles are resulted from the different phase value of helical axicon and vortex lens. The flat helical nanosieve is fabricated by patterning on gold film with perforated nano- voids: Babinet-inverted (complementary) photon nanosieves on quartz substrate. It is composed of nanosieves with spatially varying orientation in two different regions marked by I and II [cf. Figure 1b]. The radii of Region I and II are kept as 0.5–45 μm and 50–100 μm, respectively. In our design, the nanosieve has its length l = 150 nm, width w = 75 nm, and depth 60 nm. All of the nanosieves are equally spaced (d1 = 500 nm) on the concentric circles, and all the circles are separated with the same period (d2 = 500 nm) [cf. Figure 1b inset]. An SEM image shown in Figure 1c and its insets in Figure 1a clearly show the rotating rectangular nanosieves. There are 121 486 nanosieves in total with different orientation angles. The abrupt phase delay of the flat helical device is realized by controlling the ori- entation angle ϕ of the nanosieves relative to the x axis. As a result, the designed phase distribution x y x y( , ) ±2 ( , )ϕΦ = (“+”: right circularly polarized (RCP) incident light; “-”: left circularly polarized (LCP) incident light) can be controlled precisely and smoothly on nanosieves. The basic idea is that the two regions can process the loaded optical vortex indepen- dently by integrating different manipulating phases. Figure 1d depicts the illumination of the two kinds of proposed flat hel- ical devices: helical axicon and vortex lens, which show the basic functions (hybridizing and multiplexing) of these helical nanosieves. The experimental setup for characterization is shown in Figure S6 (Supporting Informa- tion). The fabrication process can be found in the Experimental Section. 3. Results and Discussion 3.1. Cascaded Bessel Beam Generator We first show two-region helical axicon to generate a piecewise Bessel beam—cascaded nondiffraction optical vortex beams, which is obtained by superimposing the phase pro- files of axicon and spiral phase plate. The axicon projects a point source on to a line segment along the optical axis, whose length is regarded as the depth of focus (DOF).[49] Inspired by this, helical axicon, based on conventional microfabrication, was designed to achieve the high order Bessel-Gauss-like beams.[50] For an axicon with opening angle R tan DOF 1 β =      − (R is the radius of the axicon), its phase delay increases monotoni- cally with the distance from the center. In other words, it is a conical radial phase dis- tribution expressed as r 2 sinAφ π λ β= , where λ is the working wavelength and r represents the distance between the nanosieve and the center of the device. While for a spiral phase plate with topological charge l, the phase pro- file is l y xtanS 1 φ =      − . Hence, the overall phase profile of a helical axicon is A Sφ φΦ = + , so the orientation angle distribu- tion of the nanosieves is ± 2 ± 2 A S A S ϕ ϕ ϕ φ φ = + = Φ = + . Meas- ured results of single-region helical axicons’ field distributions along the propagation direction can be found in the Supporting Information. Meanwhile, their corresponding radially averaged intensity profiles of different cross-sections are well fitting with different lth order Bessel functions [cf. Figure S1, Supporting Infor- mation]. In the two-region designs, the orientation angle distribu- tion of the nanosieves can be rewritten in the following details r r l y x r r l y x r ± sin ± 2 tan 0.5 45 m ± sin ± 2 tan 50 100 m 1 1 1 2 2 1 ϕ π λ β µ π λ β µ ( ) =       ≤ ≤       ≤ ≤        − − (1) The above distribution can be considered as two concentric hel- ical axicons with opening angle ,1 2β β and topological chargel l,1 2. In a step-by-step process, we first investigate the case of the two regions with the same opening angle 61 2β β( )= = ° and different topological charge (l l1 2≠ ). Two concentric sub- helical axicons will generate two cascaded nondiffracting fields (one hybridizing Bessel beam) but with a different radius because of the different helical phase gradient encoded onto them [cf. Figure 2]. The inner subhelical axicon generates a Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601345 Figure 2.  Examples of two-region helical axicons to generate cascaded Bessel beams (hybrid- izing nondiffracting vortices). a) Mechanism of the two-region helical axicons. b,c) Meas- ured far-field distribution along the optical path for helical axicon containing two regions encoded with the same opening angle 61 2β β( )= = ° and different topological charge (l l1 2≠ ). l l l lb) 1, 3. c) 1, 41 2 1 2= = = = . The right-side images in (b) and (c) show the intensity profiles and interference patterns of the beam’s x–y cross-sections within a different nondiffracting field segment for each design at z = 300 μm and z = 700 μm.
  • 4. FULLPAPER 4 © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim nondiffracting field from 0 to 430 μm, while the outer one generates a nondiffracting field from 470 to 950 μm. The agreement concerning the propagating behavior of the light field emerging from the two flat helical axicons between the measured and the sim- ulated data is fairly good (refer Figure S2, Supporting Information for the simulation results). Figure 2a shows the mechanism of the two-region helical axicon, which clearly shows that the two regions can manipulate OAM independently. Figure 2b,c shows the experimental results of the two designed helical axicons. The measured intensity pro- files and interference patterns are shown at the right side of Figure 2b,c. Both the devices are designed for RCP illumination and LCP detection. Based on the spin-dependent phase response of the device, the output beams carry opposite chirality to that of the incident beam, because of which there is no restriction on the incident light beam’s waist. However, when considering conven- tional microoptical helical axicon[50] (combi- nation of spiral phase plate and axicon), the incident Gaussian beam’s waist is limited by the size of the device, which may impose restrictions on real applications because an extra focusing lens is needed to confine the incident beam. In characterization, circular polarizer is used at the transmission side to filter out the background light (light with the same chirality as that of the incident beam). Only light with opposite chirality is captured by charge coupled device. When the back- ground light is partially allowed to transmit by tuning the cir- cular polarizer, a single-beam interferometric scheme allows for the unambiguous characterization of the phase structure of any encoded OAM state. The measured interference pattern results have shown good agreement with the simulation ones, which validates such an embedded approach as a simple and stable characterization technique. This expandable multiregion flat helical axicons can be really complex to fabricate by con- ventional techniques by direct laser writing using femtosecond laser nanopolymerization. 3.2. Helicon Wave Generator Two-region helical axicons with two different opening angles (helicon wave generators) are further investigated to show flexi- bility of this flat fabrication strategy [cf. Figure 3]. As validated in the single-region cases [cf. Figure S1, Supporting Information], nondiffracting field generated by inner subhelical axicon with 31β = ° covers the nondiffracting field generated by the outer subhelical axicon with 62β = ° . The range of the covered field is from 470 to 950 μm. Two designs are presented in Figure 3, in which the two nondiffracting beams merge together to form a helicon wave[51] showing the rotating interference patterns during their propagation [cf. Figure S3, Supporting Informa- tion for the simulation results]. To be more specific, Figure 3a exhibits the experimentally detected helicon wave resulted from the superposition of the first order and the third order Bessel beams. Their two-lobe intensity images rotate along the propa- gating direction as depicted in the transverse profiles [cf. right side of Figure 3a]. Similarly, the rotational three-lobe intensity images [cf. Figure 3b] correspond to the superposed patterns of the first order and the fourth order Bessel beams.[52] The total field can be decomposed into the contributions from the two subhelical axicons, given respectively as , , e1 i z 1 1 1 1 U r z J k rl l r k lz ϕ( ) ( )∝ ϕ( )+ (2a) , , e2 i z 2 2 2 2 U r z J k rl l r k lz ϕ( ) ( )∝ ϕ( )+ (2b) where k k k k m and km msin , cos 1 or 2 2 .mr 0 mz 0 0β β π λ = = = =       If only considering the azimuthal (ϕ) and longitudinal (z) components, the superposed field is given by the following proportionality 1total ,1 2 1 1 2 1 2 1 U U U e ez l l z i k z k z i k k z l lz z z z ( )( )( ) = + ∝ +ϕ ϕ ϕ( )( ) ( ) ( )+ − + − (3) Adv. Funct. Mater. 2016, DOI: 10.1002/adfm.201601345 Figure 3. Examples of helicon wave generators (spatially multiplexed nondiffracting vor- tices). Measured far-field longitudinal intensity distributions for two-region helical axicons encoded with different opening angles ( 1 2β β≠ ) and different topological charges (l l1 2≠ ). l l l la) 3 , 1,and 6 , 3. b) 3 , 1,and 6 , 41 1 2 2 1 1 2 2β β β β= = = = = = = =° ° ° ° . The right-side images in (a) and (b) show the intensity profiles of the beam’s x–y cross-sections within the nondif- fracting field for each design at z = 520 μm and z = 700 μm. c) The rectangular images are two 3D schematics showing two helicon waves ( 2 and 32 1l l l∆ = − = ) within one period (from 600 to 760 μm). The circular images represent the measured cross-section intensities at inter- vals of 40 μm along the propagating direction (z) from 600 to 760 μm. The first row corresponds to the design of l l3 , 1,and 6 , 31 1 2 2β β= = = =° ° . The second row corresponds to the design of l l3 , 1,and
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