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# Ultrasmall radial polarizer array based on patterned plasmonic nanoslits

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We developed an ultrasmall radial or azimuthal polarization converter of size ranged from 10 to 100 μm. The converter consists of a half-wave plate divided into four quadrants, each of which is made of periodic gold nanoslits at different orientations in steps of 45°. The converter and its array were fabricated and followed by an evaluation with the Sénarmont method and the polarization microscopy. Due to the giant birefringence of the gold nanoslits, each slit segment achieved 180° retardation at the wavelength of 633 nm, and the conversion from linear to radial polarization was demonstrated.
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Ultrasmall radial polarizer array based on patterned plasmonic nanoslits
Kentaro Iwami, Miho Ishii, Yuzuru Kuramochi, Kenichi Ida, and Norihiro Umeda
Citation: Appl. Phys. Lett. 101, 161119 (2012); doi: 10.1063/1.4761943
View online: http://dx.doi.org/10.1063/1.4761943
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Ultrasmall radial polarizer array based on patterned plasmonic nanoslits
Kentaro Iwami,
a)
Miho Ishii, Yuzuru Kuramochi,
b)
Kenichi Ida, and Norihiro Umeda
Department of Mechanical Systems Engineering, Tokyo University of Agriculture and Technology,
2-24-16 Nakacho, Koganei, Tokyo 184-8588, Japan
(Received 4 September 2012; accepted 8 October 2012; published online 19 October 2012)
We developed an ultrasmall radial or azimuthal polarization converter of size ranged from 10 to
100 lm. The converter consists of a half-wave plate divided into four quadrants, each of which is
made of periodic gold nanoslits at different orientations in steps of 45. The converter and its array
were fabricated and followed by an evaluation with the S
enarmont method and the polarization
microscopy. Due to the giant birefringence of the gold nanoslits, each slit segment achieved 180
retardation at the wavelength of 633 nm, and the conversion from linear to radial polarization was
demonstrated. V
C2012 American Institute of Physics.[http://dx.doi.org/10.1063/1.4761943]
Radially and azimuthally polarized lights have attracted
much interest because of their unique axially symmetric dis-
tribution of the polarization state in a beam cross section, in
contrast to the homogeneously polarized beams. These polar-
izations have been widely studied and applied to many ﬁelds,
including ﬁne focusing for high resolution optical micros-
copy,
1
laser machining,
2
optical manipulation,
3
plasmonic
focusing,
4
Raman spectroscopy,
5
and photothermal therapy.
6
In particular, the generation of a large longitudinal electric
ﬁeld component is a signiﬁcant characteristic of focused
radially polarized light, and it has opened up unique applica-
tions such as particle acceleration.
7
The generation of these polarizations relies on the local
control of both the polarization orientation and phase. Sev-
eral methods have been studied to accomplish this, including
polarization selection at a laser cavity
8,9
polarization converter (RPC). Of the wide variety of RPCs
that have been studied, examples include orientation-tailored
liquid crystal (LC),
10,11
LC-based spatial light modulators
(SLMs),
1
spatially varying retardation (SVR) based on opti-
cal crystal
5,12
or photonic crystal,
13
sub-wavelength gra-
ting,
14
and glass nanostructuring.
15
On the other hand, recent advances in microelectrome-
chanical systems (MEMS) have opened up a world of
“parallel optics,” represented by micromirror array or micro-
lens array devices, which have been applied in areas from
basic science to the telecommunications of the future.
16,17
The use of radially or azimuthally polarized light in the ﬁeld
of parallel optics may offer many opportunities, for example,
high-throughput nanoscale surface imaging, sensing, and
modiﬁcation by combining them with MEMS probe array
system. However, such an application has not yet been real-
ized because axially symmetric beams cannot be arranged in
parallel. Therefore, a RPC array is critically needed. Con-
ventional RPCs, however, have disadvantages for this pur-
pose. The main obstacle is the difﬁculty in fabricating
microscale RPCs. The size and resolution of SLMs are not
MEMS compatible, and the low damage threshold of LC-
based devices restricts their application. SVR plates, which
consist of half-wave plate crystal pieces divided into four or
eight segments, are difﬁcult to miniaturize. The sub-
wavelength gratings formed on dielectric substrates have
advantages for micromachining, but the application is lim-
ited to the mid-infrared range because the micromachining
of optically transparent material with the sub-wavelength re-
solution is difﬁcult. Although photonic crystals would satisfy
the above requirements, its fabrication is difﬁcult because
multilayer deposition is essential. Minimum featuring size of
glass-nanostructured RPC is still unclear.
Periodic nanoslit arrays on noble metal thin ﬁlms were
utilized to induce giant birefringence, and more than a 180
phase shift was achieved with nanoscale-thickness.
18
This
structure is most compatible with miniaturization because it
can be fabricated through conventional electron beam (EB)
or nano-imprint lithography combined with a lift-off process,
as is expected for future mass-produced wave plates.
18,19
Furthermore, by arranging nanoslit arrays into a pattern, the
spatial distribution of polarization can be theoretically tai-
lored. In this paper, we design, develop, and demonstrate an
ultrasmall RPC based on a half-wave plate divided into four
quadrants that are composed of patterned plasmonic nanoslit
arrays.
Figure 1shows a schematic drawing of a plasmonic
RPC and its cross section. As shown in Fig. 1(a), the RPC
consists of four segments of a nanoslit array of a gold thin
ﬁlm deposited on a transparent substrate. Each array segment
has identical slit width and period. Figure 1(b) shows the ori-
entation of polarization. Since the phase shifts are opposite
between the transverse electric (TE: the electric ﬁeld is par-
allel to the nanoslits) and the transverse magnetic (TM: the
electric ﬁeld is perpendicular to the nanoslits) polarizations,
a giant birefringence can be achieved.
18
As shown in Fig.
1(a), the four segments, or quadrants, are arranged by chang-
ing each slit orientation in steps of 45. In this structure, the
TE axis (fast axis) is treated as the principal axis of the slit
array because the phase shift for the TE wave is positive, in
contrast to the negative phase shift for the TM wave. There-
fore, the principal axes of the four quadrants from 1 to 4 can
be described as 45,90
, 135, and 0, respectively. If each
quadrant achieves a retardation of 180, the arrays work as a
half-wave plate divided into fourths. Consequently, 45and
a)
Author to whom correspondence should be addressed. Electronic mail:
k_iwami@cc.tuat.ac.jp. URL: http://nmems.lab.tuat.ac.jp/en/.
b)
Present address: Department of Precision Engineering, the University of
Tokyo, Tokyo, 113-8656, Japan.
0003-6951/2012/101(16)/161119/4/$30.00 V C2012 American Institute of Physics101, 161119-1 APPLIED PHYSICS LETTERS 101, 161119 (2012) 135linear-polarized incident light can be converted into radially and azimuthally polarized light, respectively. When developing a plasmonic half-wave plate of metal nanoslits, design parameters should be optimized; however, it is difﬁcult to estimate the phase shift accurately on the ba- sis of analytical waveguide theory. For example, propagation constants for the TE and TM waves are given by, 18 kTE ¼ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ k2 0ðp=wef f Þ2 q;(1) tanh w 2ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ k2 TM k2 0 q  ¼ ﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ k2 TM mk2 0 p mﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ k2 TM k2 0 p;(2) where kTE and kTM are propagation constants for the TE and TM modes, respectively; and wand wef f are the raw and effective widths of the slit, respectively; and mis a dielectric function of the metal. As shown in Eq. (1),kTE has a sharp cutoff at k¼wef f =2. However, for gold thin ﬁlms, cutoff is not sharp but gradual around this wavelength. 20 Therefore, numerical analysis is essential to the design of a plasmonic nanoslit half-wave plate. To determine the optimal slit design, a numerical simu- lation based on commercially available ﬁnite-difference time-domain (FDTD) software (RSoft, FULLWAVE 8.1) was performed. As a half-wave plate, identical transmittance between the TE and the TM waves is important for the plas- monic nanoslit array, together with a 180phase shift. Figure 2(a) shows contour plots of the phase shifts and the ratio of amplitude transmittance (ATE=ATM) for a 500-nm-period slit array under a 633 nm incident wavelength. In Fig. 2(a), the solid and dashed contours correspond to the phase shifts and transmittance ratios, respectively. Since the contours of a 180phase shift and an identical transmittance ratio intersect at a gold ﬁlm thickness of 320 nm and a slit width of 240 nm, a plasmonic nanoslit half-wave plate can be developed at this point. Fig. 2(b) shows the regions at which phase shifts can be achieved between 175and 185together with trans- mittance ratios between 0.95 and 1.05 for each incident wavelength on a 500-nm-period gold slit array. As shown in Fig. 2(b), a plasmonic nanoslit half-wave plate can be achieved for a broad range in the visible spectrum. Fig. 2(c) shows amplitude transmittance of above-mentioned regions. The RPC pattern was deﬁned on a quartz glass substrate of thickness 525 lm through EB lithography. A 5-nm-thick chromium layer was deposited by the EB deposition for ad- hesion, followed by the deposition of a gold layer. Next, the EB resists were removed by an organic solvent, and a metal nanoslit pattern was developed (lift-off process). Sizes of single RPC ranged from 10 to 100 lm. Two types of slit arrays were fabricated, one with a slit width of 300 nm and the other, 350 nm with a slit pitch of 525 nm due to lithogra- phy limitation. In order to achieve a 180phase shift at the 300-nm-wide slit, the gold thin ﬁlm was deposited with a thickness of 445 nm. Fig. 3(a) shows a scanning electron mi- croscopy (SEM) image of the fabricated 3 3 array of the 10 lm RPC array with the slit width of 300 nm. FIG. 1. (a) Schematic drawing of the RPC based on a plasmonic gold nano- slit pattern. It consists of four quadrants, each having 180retardation. The RPC is arranged by changing the slit orientation in steps of 45. As the nano- slit array has its principal axis (fast axis) along a slit, a 45linear-polarized light can be converted to a radially polarized light. (b) Schematic of a cross section of each slit array allocated on each quadrant. FIG. 2. (a) Contour plots of phase shifts (solid curves) and the ratio of am- plitude transmittance (ATE=ATM, dashed curves) for the 633 nm incident wavelength. The pitch of the slit array was ﬁxed to 500 nm. (b) Regions to achieve both 175185phase shift and 0:951:05 amplitude ratio for each incident wavelength. (c) Amplitude transmittance spectrum of above- mentioned regions. FIG. 3. (a) An SEM image of a fabricated 3 3 RPC array. The size of an RPC unit is 10 lm. (b) Experimental setup for the retardation measurement based on the S enarmont method. (c) Retardation spectra on 525-nm-period and 300-nm-wide gold nanoslit by S enarmont method measurement (circle), FDTD calculation (square), and theoretical estimation (solid line), respec- tively. Error bars indicate standard deviations over four quadrants (n¼50 for each quadrant). 161119-2 Iwami et al. Appl. Phys. Lett. 101, 161119 (2012) The phase shifts of each quadrant were measured by the S enarmont method. Figure 3(b) shows the experimental setup. A He-Ne laser (k¼633 nm) beam passes through a linear polarizer whose transmission axis is 0(LP0), and is focused onto each quadrant. The RPC with the size of 100 lm was used. The RPC was rotated with a degree of / to keep the angle between the fast axis of each quadrant at 45. Therefore, /for the quadrants 1, 2, 3, and 4 were 0, 45,90, and 135, respectively. The transmitted beam was collimated by an objective lens and passed through a 90Babinet-Soleil compensator as a quarter-wave plate (QWP90) and a 90linear polarizer as an analyzer. Finally, the power of the transmitted beam was measured by rotating the analyzer (LP90þh). Table Ishows the summary of retardation for RPCs with slit widths of 350 and 300 nm for 633 nm light. For both slits, the slit period was 525 nm. As expected, the 300 nm-slit RPC shows retardation of about 180for every quadrant. This result conﬁrms that each quadrant of the 300-nm-wide nano- slit array acts as a half-wave plate. In 350-nm-wide slit, the large retardation difference between orthogonal arrangement (quadrants 2 and 4) and diagonal arrangement (quadrants 1 and 3) is observed. It is possibly due to shape error since variable shaped e-beam was used. Figure 3(c) shows the retardation spectra of a 300- nm-wide slit under laser irradiation at four wavelengths (532, 576, 614, and 633 nm) measured by the S enarmont method (circle), FDTD calculation (square), and theoretical estimation (solid line), respectively. As shown in Fig. 3(c), theoretical estimation based on Eqs. (1) and (2) does not agree with the other methods in the short wavelength region. In contrast, the FDTD calculation agrees well with the experimental result. To determine the principal axes of each quadrant, the dependence of the rotation angle on transmitted intensity was measured for the 350-nm-slit RPC, and the results were evaluated by ﬁtting to the theoretical curve of the S0compo- nent of Stokes vector. In this setup, we assumed each quad- rant as an optical retarder with the principal axis of /þ45 and a retardation of D. For example, the S0component of transmitted light through the quadrant 1 (/¼0)is expressed as follows: S0¼1 41þcosðD2ð90þhÞÞ fg :(3) Fig. 4shows the normalized intensity and the ﬁtted S0 curves for each RPC quadrant. As shown here, all the experi- mental results agree well with the retarder model, and the principal axes of each quadrant are conﬁrmed as the TE direction (parallel to the slit). Therefore, it is conﬁrmed that the fabricated nanoslit array pattern can act as four-quadrant half-wave plates, that is, an RPC. To demonstrate the fabricated RPC visually, a polariza- tion microscope was used to observe it. Fig. 5shows optical microscope images under polarized illumination with and without an optical bandpass ﬁlter at 633 nm. As illustrated in Fig. 5,a45 input linear polarization should be converted into radial polarization by the RPC. The transmitted light is imaged by a charge-coupled device (CCD) camera through an analyzer with the transmission axis at 45(upper images) and 135(lower images), with and without the 633-nm opti- cal bandpass ﬁlter. As shown in the images with the 633 nm ﬁlter, the RPC pattern exhibits the expected contrast; the quadrants 1 and 3 are bright with the 45analyzer, and the quadrants 2 and 4 are bright with the 135analyzer. In con- trast, the white light images do not show clear RPC-like response because of the wavelength-dependent retardations from the nanoslit. The differences in brightness between kitty-cornered quadrants obtained with 633 nm ﬁlters can be attributed to the differences of transmittance between the TE and the TM polarizations resulting from the fabrication ﬂuc- tuation in slit dimensions. Intra-quadrant inhomogeneity of transmittance is also obtained, the reason can be due to par- tial peeling or sticking occurred during the development pro- cedure of EB resist. TABLE I. Retardation (deg) for 633-nm irradiation of each nanoslit array quadrant with slit widths of 350 and 300 nm. The period and thickness of both slit arrays are 525 and 445 nm, respectively. Slit width Quadrant 350 nm 300 nm 1 116.3 175.7 2 71.65 184.9 3 147.9 184.7 4 70.24 181.1 FIG. 4. Normalized intensity of transmitted light through each quadrant of RPC and ﬁtting with S0component of Stokes vector. FIG. 5. Polarization microscope images of the 300-nm-wide slit RPC illumi- nated by white light with and without a 633 nm bandpath ﬁlter. Dashed lines at RPC symbol indicate fast axes of quadrants (slit direction). Symbols Q1-Q4 indicate the number of each quadrant. 161119-3 Iwami et al. Appl. Phys. Lett. 101, 161119 (2012) In conclusion, a radial polarization converter that made use of a gold nanoslit pattern was designed and fabricated on a glass substrate. The size of the RPC ranged from 10 to 100 lm in length and was 445 nm in thickness. With proper slit width and gold thickness design, the RPC will be useful in a large portion of the visible spectrum ranged 442 to 780 nm. Improvement of the low transmittance of nanoslit is the fur- ther challenge to a practical device, which will be achieved by optimizing slit pitch, modifying pattern from slit to for exam- ple rectangle, 21 or possibly adopting a technique of electro- magnetically induced transparency. 22 We are expecting to broaden the usefulness to the ultraviolet region by changing the plasmonic material from gold to silver or aluminum. The authors would like to thank the Grant-in-Aid for Young Researchers (A) No. 23686016 from the Japanese So- ciety for the Promotion of Science (JSPS) for ﬁnancially sup- porting this research. A part of this research was supported by the “Nanotechnology Network” of the Ministry of Educa- tion, Culture, Sports, Science and Technology (MEXT), Ja- pan for electron beam lithography at Toyota Technological Institute and VLSI Design and Education Center (VDEC), the University of Tokyo. 1 T. Wilson, F. Massoumian, and R. Juskaitis, Opt. Eng. 42, 3088 (2003). 2 V. G. Niziev and A. V. Nesterov, J. Phys. D: Appl. Phys. 32, 1455 (1999). 3 Q. Zhan, Opt. Express 12, 3377 (2004). 4 W. Chen, D. C. Abeysinghe, R. L. Nelson, and Q. Zhan, Nano Lett. 9, 4320 (2009). 5 Y. Saito, N. Hayazawa, H. Kataura, T. Murakami, K. Tsukagoshi, Y. Inouye, and S. Kawata, Chem. Phys. Lett. 410, 136 (2005). 6 H. Kang, B. Jia, J. Li, D. Morrish, and M. Gu, Appl. Phys. Lett. 96, 063702 (2010). 7 Y. Liu, D. Cline, and P. 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Thin binary transmission masks have weak birefringence [26] and low transmission, hence, the efficiency of vortex generation are expected to be low. The retardance is defined by a phase shift between TE and TM modes through the thin film (50 nm in our case) [26]. The pair (∆ , ∆ " ) is defining the purity (Eqn. ... ... Fig. 5(a)). Fit was made with a function proportional to the Stokes parameter S 0 = 1 4 [1 + cos(∆ • − 2(90 • + θ))] [26]. The best fit was achieved for the retardance (in degrees) ∆ • = (11 ± 2) • or 3.1% of the wavelength, or ∆nd = 19.3 ... Preprint Full-text available Fast fabrication of micro-optical elements for generation of optical vortex beams based on the q-plate design is demonstrated by femtosecond (fs) laser ablation of gold film on glass. Q-plates with diameter of ~0.5 mm were made in ~1 min using galvanometric scanners with writing speed of 5 mm/s. Period of gratings of 0.8 micrometers and groove width of 250 nm were achieved using fs-laser ablation at 343 nm wavelength. Phase and intensity analysis of optical vortex generators was carried out at 633 nm wavelength and confirmed the designed performance. Efficiency of spin-orbital conversion of the q-plates made by ablation of 50-nm-thick film of gold was ~3%. Such gratings can withstand thermal annealing up to 800C. They can be used as optical vortex generators using post-selection of polarisation. ... Thin binary transmission masks have weak birefringence [26] and low transmission, hence, the efficiency of vortex generation are expected to be low. The retardance is defined by a phase shift between TE and TM modes through the thin film (50 nm in our case) [26]. ... ... Thin binary transmission masks have weak birefringence [26] and low transmission, hence, the efficiency of vortex generation are expected to be low. The retardance is defined by a phase shift between TE and TM modes through the thin film (50 nm in our case) [26]. The pair ( , ) is defining the purity (Eq. ... Article Fast fabrication of micro-optical elements for generation of optical vortex beams based on the q-plate design is demonstrated by femtosecond (fs) laser ablation of gold film on glass. Q-plates with diameter of ∼0.5 mm were made in ∼1 min using galvanometric scanners with writing speed of 5 mm/s. Period of gratings of 0.8μm and groove width of 250 nm were achieved using fs-laser ablation at λ=343 nm wavelength. Phase and intensity analysis of optical vortex generators was carried out at 633 nm wavelength and confirmed the designed performance. Efficiency of spin–orbital conversion of the q-plates made by ablation of 50-nm-thick film of gold was ∼3%. Such gratings can withstand thermal annealing up to 800 °C. They can be used as optical vortex generators using post-selection of polarisation. ... M ETASURFACES have been successfully applied in several advanced micro-electro-mechanical systems (MEMS) sensors in the terahertz [1]- [11] and gigahertz [12]- [16] bands through both plasmonic and dielectric regimes. Owing to recent improvements in microfabrication technologies, metasurfaces have also been used in optical wavelengths [17]- [19] for various applications, including metasurface lenses (metalenses) [20]- [23], holography [24]- [29], waveplates [30]- [32], gratings [33]- [35], and functional optical devices [37]- [39]. ... Article Full-text available Conventionally, silicon is less often selected as the material of dielectric metasurfaces in the visible band than other lossless materials, including titanium dioxide, silicon nitride, and gallium nitride. The reason is its relatively high extinction coefficient and resulting low transmittance. This study demonstratedthat accurately designednanopillars made of single-crystal silicon could be used satisfactorily on a metasurface, even in the visible band. Four line-focusing metasurface lenses were designed to verify the lens performance effectiveness of silicon nanopillars in the visible band. In addition, a combination of the character projection and the variable-shaped beam modes in the electron beam lithography was operated to evaluate the compatibility between mass productivity and accuracy. We successfully obtained a highly efficient line-focusing metasurface lens composed of single crystalline silicon nanopillars. The parameters of the metasurface lens at a wavelength of 532 nm were as follows: lens thickness, 300 nm; focal length, 3.91 mm; square aperture, 2 mm; numerical aperture, 0.25; measured transmittance, 38.4% to 46.8%; and measured beam spot width, 3.68 μm (full width at half maximum, FWHM) at the focal point. The results obtained in this study show a promising use of silicon metasurface for optical sensor applications in the visible band. ... In the recent years, metasurfaces, which are a planar arrangement of subwavelength-scale phase retarders, have been applied to CGH holograms to achieve a wide viewing angle [14][15][16][17][18][19][20][21][22][23]. Metasurfaces can tailor to the properties of light not only amplitude, phase, and polarization, but also wavefront; hence, they have been applied to various optical elements, including waveplates [24][25][26][27][28], vector beam converters [29][30][31], and lenses [32][33][34][35][36]. Metasurface holograms can have a period smaller than the working wavelength; thus, their viewing angle can widen. A unit cell or a pixel of metasurface holograms, also known as meta-atoms, can be made both of metals [14][15][16] and dielectrics [17][18][19][20][21]. Dielectric metasurfaces are promising from the viewpoint of efficiency because the transmittance of the former is low due to the Ohmic loss of the metallic material [37]. ... ... [1][2][3] They are able to show unique optical properties that cannot be achieved in natural materials and have a high affinity for micro/nanofabrication methods including lithography, deposition, and etching. Therefore, metasurfaces have opened up many research fields and applications including lens [4][5][6][7], retarders and waveplates [8][9][10][11], vector beam converters [12][13][14], color filters [15][16][17], holography [18][19][20][21][22], zero refractive index materials [23], and others. It should be emphasized that mechanical deformation or change of mutual geometric position of metasurfaces offers novel functionality and tunability for their optical properties. ... Article Full-text available This paper reports an experimental demonstration of moiré metalens which shows wide focal length tunability from negative to positive by mutual angle rotation at the wavelength of 900 nm. The moiré metalens was developed using high index contrast transmitarray meta-atoms made of amorphous silicon octagonal pillars, which is designed to have polarization insensitivity and full 2π phase coverage. The fabricated moiré metalens showed focal length tunability at the ranges between ±1.73-±5 mm, which corresponds to the optical power ranges between ±578-±200 m<sup>−1</sup> at the mutual rotation between ±90 degrees. ... Moreover, it can be used to modify and tailor optical wavefronts into designed distribution [1], for example, the parabolic phase for focusing as a lens or complex diffraction pattern for metasurface holography. To date, various applications have been studied, including very thin metalenses [2][3][4], waveplates [5][6][7], and polarization converters [8]. ... Article Full-text available Animation for a metasurface hologram was achieved using a cinematographic approach. Time-lapsed images were reconstructed using sequentially arranged metasurface hologram frames. An Au rectangular nanoaperture was adopted as a meta-atom pixel and arrayed to reproduce the phase distribution based on the help of a Pancharatnam-Berry phase. We arrayed 48 hologram frames on a 2-cm2 substrate and measured and assessed the retardation of fabricated meta-atoms to reconstruct the holographic image, successfully demonstrating the movie with a frame rate of 30 frames per second. ... 1-3 They can demonstrate unique optical properties that cannot be achieved with natural materials and have high affinity for micro/nanofabrication methods, including lithography, deposition, and etching. Therefore, metasurfaces have opened many research fields and have several applications, including lenses, 4-6 retarders and waveplates, 7-10 vector beam converters, [11][12][13] color filters, [14][15][16] and holography. [17][18][19][20] It should be emphasized that mechanical deformation or change of mutual geometric position of metasurfaces offer novel functionalities and tunability for their optical properties. ... Preprint Full-text available This paper reports experimental demonstration of moir\'e metalens with a wide focal length tunability from negative to positive through mutual angle rotation at a wavelength of 900 nm. The moir\'e metalens was developed using high-index contrast transmit array meta-atoms composed of amorphous silicon octagonal pillars designed to have polarization insensitivity and full 2$\pi$phase coverage. The designed moir\'e metalens was fabricated on a glass substrate using simple a-Si sputter deposition, electron beam lithography through character projection, metal mask lift-off, and reactive-ion silicon etching. The moir\'e metalens has focal length tunability ranges from$-\infty$to$-1.73$mm and from 1.73 mm to$\infty$corresponding to an optical power in the range -578$-$578 m$^{-1}$at the mutual rotation between$\pm 90^\circ$. Our results reveal a proof of concept for the focal length tuning with mutual rotation of lens components based on moir\'e lens configuration at optical frequencies. Article Metasurface lenses (metalenses) offer an ultrathin and simple optical system with dynamic functions that include focal length tuning. In this study, a rotational varifocal (i.e., moiré) metalens based on octagonal single-crystal silicon pillars was designed and fabricated to realize a high transmittance, whole 2π phase coverage, and polarization insensitivity for visible wavelengths. The moiré metalens consists of a pair of cascaded metasurface-based phase lattices and the focal length can be adjusted from negative to positive by mutual rotation. The fabricated moiré metalens demonstrated a focal length that can be tuned from −36 mm to −2 mm and from 2 to 12 mm by mutual rotation from −90° to 90°, and the experimental measurements agreed well with theoretical values at the design wavelength of 633 nm. Imaging was demonstrated at three distinct wavelengths of 633, 532, and 440 nm. Article This paper presents an approach to design the all-dielectric metasurface with multi-function in the near-infrared range of 1.5-1.6 µm. Based on the geometric phase principle, the all-dielectric metasurface is composed of the Si nanopillar and the SiO2 substrate as an emitter unit distributed in a 21×21 array. Under the incidence of the circularly polarized light at 1550 nm, the metasurface works as a vortex-beam generator with high performance which generates the vortex beam with topological charges of ±1, and the mode purity of the vortex beam is 90.66%. Under the incidence of the linearly polarized light at 1550 nm, the metasurface also works as the azimuthally/radially polarized beam generator with high performance, and the purities of the azimuthally and the radially polarized beams are 92.52% and 91.02%, respectively. Moreover, the metasurface generates different output spots under the different incident lights which can be applied to optical encryption, and the metasurface with the phase gradient also can be used as the dual-channel encoder/decoder in optical communication. The simulated results are in good agreement with the theoretical derivation. The designed metasurface may become a potential candidate as a multi-function photon device in the integrated optical system in the future. Article Full-text available Cited By (since 1996): 18, Export Date: 3 January 2013, Source: Scopus, Art. No.: 063702, CODEN: APPLA, doi: 10.1063/1.3302461, Language of Original Document: English, Correspondence Address: Gu, M.; Centre for Micro-Photonics, Faculty of Engineering and Industrial Sciences, Swinburne University of Technology, Hawthorn, VIC 3122, Australia; email: mgu@swin.edu.au, References: Hirsch, L.R., Stafford, R.J., Bankson, J.A., Sershen, S.R., Rivera, B., Price, R.E., Hazle, J.D., West, J.L., Nanoshell-mediated near-infrared thermal therapy of tumors under magnetic resonance guidance (2003) Proceedings of the National Academy of Sciences of the United States of America, 100 (23), pp. 13549-13554. , DOI 10.1073/pnas.2232479100; Article Full-text available A form-birefringent plasmonic metamaterial of the subwavelength thickness is used to convert the light's polarization state in a way to cover the whole Poincar\'e sphere's surface by adjusting the experimental configuration. This optical anisotropy is induced by grating surface plasmon polaritons of a nanoslit array made in a thin golden film with the narrow spectral Fano resonance. Phase delay between linearly polarized states introduced by the sample reaches the value of$0.85$\pi${}$in the visible corresponding to the effective ordinary-extraordinary refractive index difference of$$\Delta${}n$\simeq${}10.4\$.
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Radially polarized radiation of 1.8 kW was first obtained in an industrial high-power CO2 laser. Special reflective elements with an axial polarization selectivity of 22% were used as a rear mirror in the laser. The output radiation consisted mainly of an unpolarized mode TEM00 and a radially polarized mode R-TEM01*.
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The three-dimensional (3D) theory of laser cutting is presented. The cutting efficiency determined by its ultimate parameters at different types of polarization is estimated. The physical reasons for limitations of ultimate cutting parameters at a plane P-polarized beam are displayed. In the case of cutting metals with a large ratio of sheet thickness to width of the cut, the laser cutting efficiency for a radially polarized beam is 1.5 - 2 times larger than for plane P-polarized and circularly polarized beams. The possibility of generating the radially polarized beam is discussed.
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Plasmonic waveplates with enhanced transmission can be achieved in the proposed structure with a periodic array of nanorectangles. Based on the model of nanoslits, both the transverse electric (TE) and transverse magnetic (TM) waves can be transmitted through the nanorectangles, enhancing the transmission of the incident polarized light. Simulation results show that the transmission ratio and phase shift of TE and TM waves can be manipulated by the nanorectangles structures, enabling plasmonic waveplates with enhanced transmission.
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Large phase differences between transverse electric (TE) and transverse magnetic (TM) waves were investigated in plasmonic nanoslit arrays. The phase of the TE wave shifts ahead because of its low propagation constant. On the other hand, the phase of the TM wave is retarded due to the propagation of surface plasmons. The opposite phase shift forms a giant birefringence. Its magnitude was dependent on the width of nanoslits. The birefringence magnitude was ∼ 1 for 300-nm-wide nanoslits and up to ∼ 2.7 for 100 nm ones. The spectroscopic measurements indicate that waveplates made of gold nanoslits have large bandwidths.
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