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Abstract
An optical vortex with orbital angular momentum (OAM) can be used to induce microscale chiral structures in various materials. Such chiral structures enable the generation of a nearfield vortex, i.e. nearfield OAM light on a sub-wavelength scale, thereby leading to further nanoscale mass-transport. We report on the formation of a nanoscale chiral surface relief in azo-polymers due to nearfield OAM light. The resulting nanoscale chiral relief exhibits a diameter of ca. 400 nm, which corresponds to less than 1/5–1/6th of the original chiral structure (ca. 2.1 µm). Such a nanoscale chiral surface relief is established by the simple irradiation of uniform visible plane-wave light with an intensity of <500 mW/cm2.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
The orbital angular momentum (OAM) of light [1–6], that is associated with a helical wavefront, has been intensively investigated for use in a variety of applications, including optical tweezers and traps/guides [7–9]. In fact, several researchers have successfully demonstrated that OAM forces microscale dielectric or metal particles into particular orbits in solution or in a vacuum [10–12]. In recent years, we and our collaborators have discovered that a pulsed or continuous-wave (CW) OAM beam, i.e. an optical vortex, twists materials to form various chiral structures (so-called OAM-induced chiral structures) at the microscale through OAM transfer effects [13–21]. These structures with the chirality reflecting the wavefront of the irradiated OAM beam act like an extremely high numerical aperture hyper-lens formed of hyperbolic metamaterial [22–24], generating the nearfield with OAM, i.e. nearfield OAM light, at the nanoscale.
Twisting materials at the nanoscale, i.e. selective mass transport of polymers and aggregations of molecules in the clockwise or counter-clockwise directions at the sub-wavelength scale, enables the development of chiral metamaterials and metasurfaces, chiral sensitive sensors [25,26], chiral chemical reactors [27,28], and bio-microelectromechanical systems (MEMS) with high efficiency [29]. The aforementioned nearfield OAM light can potentially twist materials in the clockwise or counter-clockwise direction at the nanoscale, which is expected to open up new avenues towards the development of advanced chiral devices [30–34]. Furthermore, nearfield OAM light is expected to yield new physical phenomena, such as coupling effects between spin angular momenta (SAM)—arising from the helical electric field of circularly polarized light—and OAM; for instance, OAM-induced SAM or SAM-induced OAM, through interactions with matter at the nanoscale [35–38].
However, there have been no reports concerning nearfield OAM light generation from OAM-induced chiral structures. Furthermore, the nanoscale chiral mass transport of molecules (or polymers) and their aggregations by nearfield OAM light has not yet been established.
Here, we report on the presence of nearfield OAM light at the nanoscale, in which plasmon-enhanced nearfield OAM light is generated from an OAM-induced chiral structure. We also address the fact that such nearfield OAM light induces nanoscale chiral mass-transport of a large amount of azo-polymers, which results in the formation of a sub-diffraction-limit sized chiral surface relief.
2. Numerical model
The amplitude and phase distributions of nearfield OAM light generated from an OAM-induced chiral surface relief by irradiation of CW circularly polarized plane-wave light were numerically analyzed using the finite-difference time-domain (FDTD) method. The software ‘Poynting’ (Fujitsu) was used for the simulation. The OAM-induced chiral surface relief on an azo-polymer thin film was shaped to be a semi-sphere, which exhibited a diameter of 2–4 µm and a height of ca. 1 µm, as previously reported [16,20]. To reduce the computational cost and time, a 3-D simulation model was adopted, in which the OAM-induced clockwise-chiral (right-handed) surface relief exhibits a flat top surface without any curvature, and its shape can be fit to a logarithmic spiral function expressed as
where r is the distance from the origin, ϕ is the azimuthal angle around the origin, r0 and r1 are the radii of the outer and inner rings, respectively, and b is the spiral constant, as shown in Fig. 1(a). The parameters r0, r1, and b were fixed at 360 nm, 1 µm, and 0.14 rad−1, respectively, to fit the outline of the experimentally observed chiral surface relief. The surface relief, which had a thickness of 500 nm and a refractive index of n = 1.5, was then covered with a 200 nm thick Au thin film (n = 0.54 + 2.23i) with an etched slot (depth = 150 nm, width = 80 nm). A 500 nm thick film (n = 1.5), corresponding to an azo-polymer layer laminated on the device, was furthermore superimposed onto the Au thin film as a top layer. All boundaries of the device were assumed to be a perfect matching layer under the absorption boundary conditions. This device, in which the Au film acted as a generator of plasmon-enhanced nearfield light, enabled the creation of plasmon-enhanced nearfield light around the chiral structure by the illumination of circularly polarized light. The observation plane of the nearfield light was placed 50nm above the Au film surface. Right-handed (clockwise) CW circularly polarized visible plane-wave light was irradiated onto the device from behind (the substrate side), and the wavelength was varied within the range of 500-580 nm. As shown in Fig. 1(b), the nearfield light generated along the z-direction (i.e. the laser propagation direction) was significantly enhanced by irradiation with 532-580 nm plane-wave light. When the replica expands (or shrinks), i.e. its diameter increases (or decreases), its plasmon resonance wavelength will shift towards red (or blue).
Fig. 1 (a) Top-, side- and 3D-views of the two-dimensional model of the OAM-induced right-handed chiral surface relief with the laminated azo-polymer layer (b) Amplitude and phase distribution along the z-direction of the nearfield light generated around the two-dimensional model by irradiation with right-handed circularly polarized uniform CW visible light at wavelengths of 500-580 nm.
The generated nearfield light was right-handed nearfield OAM light with a field diameter of ca. 300 nm and a total angular momentum (TAM) of 2ħ, as evidenced by the annular amplitude distribution and the 4π azimuthal phase shift [Figs. 2(a)-2(b)]. Such nearfield OAM light allow materials, such as polymers or their aggregations, to be transported in the clockwise direction at the nanoscale. Horizontal-linearly polarized plane-wave light also generated asymmetric nearfield light with a π phase shift along the horizontal axis [Figs. 2(c)-2(d)].
Fig. 2 Amplitude and phase distribution along the z-direction of the nearfield light generated around the two-dimensional model by irradiation with (a)-(b) right-handed-, (c)-(d) horizontal-linearly-, and (e)-(f) left-handed-polarized uniform CW 532 nm light.
In contrast, left-handed (counter-clockwise) circularly polarized plane-wave light produced only non-vortex nearfield light with a concentric circular spatial profile and without any TAM, thereby preventing chiral mass transport of the nanomaterials [Figs. 2(e)-2(f)].
These indicate that an intrinsic chirality of the structure couples constructively or deconstructively with the SAM of the irradiated green plane-wave light, i.e. SAM-OAM coupling, which is expressed through the following formula:
where J is the TAM of the generated nearfield OAM light, ℓ is the chirality (which approximately corresponds to OAM) of the chiral structure, and s is the SAM of the circularly polarized plane-wave light.3. Experiments & Discussions
The experiment is composed of three steps. The first step is the fabrication of a chiral surface relief by the illumination of an optical vortex. The fabricated chiral surface relief might be easily deformed during exposure of visible uniform light through cis-trans photo-isomerization or plasmon-heating processes. The second step is fixing of the chiral relief as the nearfield OAM light generator, in which a replica of the chiral surface relief in azo-polymer film is imprinted on a glass plate as a positive replica with the same height and diameter as the original relief by employing a nanoimprinting technique based on ultraviolet photo-polymerization in combination with a negative silicon rubber replica. The third step, the final step, is the nanoscale mass-transport of azo-polymers by the nearfield OAM light.
Firstly, the OAM-induced chiral surface in the azo-polymer was fabricated by the illumination of an optical vortex. The azo-polymer used was poly-orange-Tom 1, which has a molecular weight of ca. 190,000 g/mol, a molecular volume of ca. 0.27 cm3/mol, and an absorption band in the wavelength region of 300-550 nm. It was spin-coated on a glass substrate to form a film with a thickness of ca. 1 µm. A right-handed chiral surface relief was generated by focusing a right-handed circularly polarized CW 532 nm optical vortex with a TAM of 2ħ to a 2.1 µm diameter annular spot on the azo-polymer film. The focused vortex intensity was measured to be ca. 1 kW/cm2, and the exposure time was fixed to be ca. 10 s. Wavefront-sensitive mass-transport in an azo-polymer film took place through cis-trans photo-isomerization simply by the irradiation of an optical vortex with a wavelength of 400-550 nm, resulting in the formation of a chiral surface relief, as reported in our previous publications. The fabricated OAM-induced chiral surface relief had a diameter of 2.1 µm and height of 500 nm [Fig. 3(a)].
Fig. 3 AFM images of (a) a right-handed chiral surface relief on an azo-polymer film created by irradiation with a focused right-handed circularly polarized CW 532 nm optical vortex, (b) the positive replica with the same height and diameter as the original relief on a glass plate formed by nanoimprinting, (c) the Au-coated replica, and (d) the superimposed azo-polymer on the Au-coated replica. (e) Schematic 3D and cross-sectional views of the device.
As mentioned above, the fabricated chiral surface relief might be easily deformed during the exposure of visible uniform light through cis-trans photo-isomerization or plasmon-heating processes. To fix the chiral relief as the generator of plasmon-enhanced nearfield light, a replica of the chiral surface relief in azo-polymer film was imprinted on a glass plate as a positive replica with the same height and diameter as the original relief by employing a nanoimprinting technique based on ultraviolet photo-polymerization in combination with a negative silicon rubber replica [Fig. 3(b)] [39] as the second step. The ultraviolet light source and cure resin used were a UVGL-58 (UVP Inc.) lamp with a center wavelength of 254 nm and Norland Optical Adhesive 61 (NOA 61) [40], respectively. The replica was coated with a 100 nm thick Au film, so as to act as the generator of plasmon-enhanced nearfield light [Fig. 3(c)]; it exhibited <1% transmission for 532 nm. Furthermore, an azo-polymer film with a thickness of 500 nm was superimposed on the Au-coated replica [Fig. 3(d)] so that the amplitude and phase of the nearfield OAM light could be visualized by deformation of the film, i.e. mass transport of the azo-polymers. Figure 3(e) shows schematic 3D view as well as cross-sectional views of the Au-coated replica with the laminated azo-polymer layer.
As the third step, i.e. the generation of the nearfield OAM light for the nanoscale mass-transport of azo-polymers, right-handed or left-handed circularly polarized 532 nm plane-wave light with an intensity of ca. 500 mW/cm2 was uniformly irradiated on this device, i.e. the Au-coated replica covered with a 500 nm thick azo-polymer film, from the backside (the glass plate substrate side). The corresponding intensity irradiated on the azo-polymer was estimated to be <5 mW/cm2, which was less than ca. 10−5 times the laser intensity used in our previous experiments concerning chiral surface relief formation in azo-polymer, and it was significantly lower than the threshold of surface relief formation.
The azo-polymer film started to deform within 5 minutes after the start of laser irradiation. After a total of 15 minutes, the formation of a right-handed chiral surface relief was completed [Fig. 4] and it had a diameter of ca. 400 nm, which was 1/5-1/6th that of the original relief, as shown in Fig. 5(a). Such chiral surface relief Horizontal-linearly polarized plane-wave light also induced a chiral deformation in the azo-polymer film along the polarization direction, resulting in the formation of an elliptical right-handed surface relief [Fig. 5(b)]. In contrast, left-handed circularly polarized plane-wave light prevented such chiral mass transport of the laminated azo-polymer, thereby forming only a shallow non-chiral relief in the azo-polymer film, even for an exposure time of 10 min, as shown in Fig. 5(c). Also, note that any chiral relief was never established by the device without any Au film, even by the right-handed circularly polarized light [Fig. 5(d)].
Fig. 4 Temporal evolution of the nearfield OAM light-induced right-handed chiral surface relief in the superimposed azo-polymer. AFM images of surface relief in the superimposed azo-polymer film formed by irradiation with uniform right-handed circularly polarized 532 nm plane-wave light.
Fig. 5 (a) Right-handed-, (b) linearly- and (c) left-handed- polarized CW 532 nm light for an exposure time of 10 min. (d) Right-handed circularly polarized light was irradiated on the device without Au thin film by for the exposure time of 10 min. (e) Original chiral surface relief in azo-polymer film formed using a focused right-handed circularly polarized 532 nm optical vortex.
These amplitude and phase distributions indicate that the light-induced chiral (right-handed) surface relief provides right-handed nearfield OAM light due to SAM-OAM coupling between a circularly polarized beam and an OAM-induced chiral structure; it forces the chiral (right-handed) mass-transport of azo-polymers to establish a chiral (right-handed) surface relief at the nanoscale. The right-handed circularly polarized plane-wave light significantly reinforced the mass transport of azo-polymers, which resulted in a chiral surface relief height that reached ca. 150 nm on the azo-polymer film. In contrast, irradiation with left-handed circularly polarized plane-wave light limited the height of the surface relief to within 10–20 nm [Fig. 6].
Fig. 6 Height of the nearfield-induced surface relief in a superimposed azo-polymer at various exposure times. RCP, LCP, and LP indicate right-handed, horizontal-linear, and left-handed circular polarizations, respectively.
Furthermore, we directly measured the polarization and the intensity of the scattered light (nearfield) from the chiral structure using a scanning nearfield microscope equipped with a photoelastic modulator and a linear polarizer [41]. The polarization of the scattered radiation upon the linearly polarized nearfield excitation (532 nm) is characterized by the ellipticity angle η of the scattered light [Fig. 7(a)]; this indicates that the nearfield includes a right- (or left-) handed circularly polarized component, shown by blue (or yellow). The nearfield light emerging from the structure was shaped to be four lobes with an excessive intensity distribution to the right-handed circularly polarized component in both center and whole regions. Further, the chiral relief structures also showed the nearfield intensity distribution [Fig. 7(b)] with a slightly distorted Taichi-over-mark shaped spatial profile.
Fig. 7 Experimentally observed polarization of scattered light upon linearly polarized nearfield (532 nm) excitation. Its (a) ellipticity angle and (b) extinction through the probe aperture from its top. The white arrow shows the direction of the incident polarization.
These observations mean that the nearfield light emerging from the structure shows the chirality, which enables the chiral mass-transport of azo-polymers towards the clockwise or counter-clockwise direction.
The chiral structure with a semi-spherical configuration might also act as a micro-lens so as to focus near-filed light [42]. The refractive index difference between the chiral structure (n = 1.56) [40] and the azo-polymer layer (n = 1.5-1.6) [43,44] is rather small. The resulting micro-lens effects, occurring at an interface between the structure and azo-polymer thin film, should be negligible. Furthermore, the simple micro-lens effect does not well support the strong circular polarization dichroism of the structures, as seen in Fig. 6. Thus, we consider that the micro-lens effects can be neglected in this present work. In fact, as shown in Fig. 5(d), any chiral relief was never established by the device without any Au film, even by right-handed circularly polarized light.
The chiral structure used in this experiment, a positive replica of the OAM-induced chiral surface relief on the azo-polymer thin film, exhibits no significant absorption for a green laser and a rather high glass temperature (~125°C). Also, it shows a very small thermal expansion coefficient (2x10−4) and dn/dT (~2x10−4/°C) [40]. The thermal deformation of the structure should also be negligible, even by its plamon-heating.
Note that the OAM-induced left-handed surface relief also induces the left-handed mass-transport of azo-polymers to complete the left-handed relief [Fig. 8]. Even the formation of a chiral microscale surface relief in an azo-polymer film without any chiral structure and Au thin film also requires an irradiated vortex beam intensity of at least 10 W/cm2 and an exposure time of 10 min [Fig. 9]. This intensity threshold is determined by a temporal decay of the photo-isomerization of azo-polymers, viscosity of azo-polymer film etc.. Thus, the corresponding plasmon-enhancement factor of the nearfield OAM light due to the SAM-OAM coupling effect was estimated to be at least 1,000.
Fig. 8 AFM images of (a) a left-handed chiral surface relief on the azo-polymer film formed by irradiation with a focused left-handed circularly polarized CW 532 nm optical vortex and (b) a left-handed chiral surface relief on the superimposed azo-polymer formed by irradiation with uniform left-handed circularly polarized CW 532 nm light for an exposure time of 10 minutes.
Fig. 9 Height of the surface relief formed on an azo-polymer film by irradiation with a focused CW 532 nm optical vortex at various beam intensities for an exposure time of 10 minutes.
4. Conclusion
We have successfully demonstrated, for the first time, nanoscale (i.e. sub-wavelength scale) chiral surface relief formation in azo-polymers induced by plasmon-enhanced nearfield OAM light. Such nearfield OAM light was generated by SAM-OAM coupling effects between circularly polarized plane-wave light and a OAM-induced chiral structure. Also, note that the OAM-induced chiral structure can be fabricated merely by the optical vortex irradiation of an azo-polymer film, which could lead to a nanoimprinting technology without advanced apparatus requirements, such as a vacuum chamber.
Such nanoscale chiral surface relief formation, i.e. twisting materials at the nanoscale, in which an azo-polymer film around a OAM-induced chiral structure is deformed to establish a right-handed surface relief at the sub-wavelength scale, will pave a way towards the development of novel photonics devices, such as chiral metamaterials and metasurfaces for selective identification of chiral chemical composites and chemical reactions that are typically costly and time-consuming.
Funding
The authors acknowledge support in the form of a Grants-in-Aid for Scientific Research (Nos. JP 15H03571, JP15H02161, JP17H07330, JP18H03884) from the Japan Society for the Promotion of Science (JSPS). This work was also financially supported by a Grants-in-Aid for Scientific Research on Innovative Areas “Nano-Material Optical-Manipulation” (Nos. JP JP16H06505, 16H06507) from JSPS and by JSPS Core-to-Core Program (A. Advanced Research Networks).
Acknowledgments
The authors thank Mr. Nao Harada (IMS) and Dr. Tetsuya Narushima (IMS) for their assistance in the near-field optical microscope measurements.
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