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Abstract
Chiral structures have a wide range of applications, such as biometric identification, chemical analysis, and chiral sensing. The simple fabrication process of chiral nanostructures that can produce a significant circular dichroism (CD) effect remains a challenge. Here, a three-dimensional (3D) cantilever-shaped nanostructure, which inherits the chiral advantages of 3D nanostructures and simplicity of 2D nanostructures, is proposed. The nanostructure can be prepared by the combination of one-time electron beam lithography and oblique-angle deposition and consists of a thin metal film with periodic holes such that two hanging arms were attached to the edges of holes. The length of the cantilever and the height difference between the two arms can be adjusted by controlling the tilt angle of beam current during the deposition processes. Numerical calculations showed that the enhancement of CD signal was achieved by plasmon distortion on the metal film by the lower hanging part of the cantilever structure. Furthermore, signals can be actively adjusted using a temperature-sensitive polydimethylsiloxane (PDMS) material. The angle between the lower cantilever and the top metal film was regulated by the change in PDMS volume with temperature. The results provide a new way to fabricating 3D nanostructures and a new mechanism to enhance the CD signal. The proposed nanostructure may have potential applications, such as in ultra-sensitive detection and remote temperature readout, and is expected to be an ultra-compact detection tool for nanoscale structural and functional information.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Chirality shows the geometric properties of an object which cannot be superimposed to its mirror image. The chirality of molecules is crucial in various fields of science and technology, including enantioselective synthesis and catalysis [1,2], molecular spintronics [3,4], and drug design [5]. However, natural chiral materials often exhibit an extremely weak chiroptical response due to scale mismatch between molecular size and the wavelength of circularly polarized light (CPL) [6,7]. In recent years, researchers have enhanced the interaction between light and matter by two methods; one using standing waves generated by far-field interference to enhance the optical field and the other using rationally designed plasmonic nanostructures to modulate the optical near-field at length scales comparable to the molecular size. Given the manipulation and concentration of electromagnetic fields in the near-field region of optical frequencies, artificial plasmonic nanostructures are considered pioneer in the development of light–matter interactions, spinning toward numerous important nanophotonic technologies such as super configuration lenses, polarization control [8], chiroptical spectroscopy [9], and chiral sensing [10,11].
These artificial chiral structures often result in different interactions with left-handed circularly polarized (LCP) and right-handed circularly polarized (RCP) light, causing different absorbances or transmissions called circular dichroism (CD) [12–14]. In general, two types of chiral nanostructures, including elements of true three-dimensional (3D) chiral nanostructures and 2D nanostructures, have been enveloped. Compared with 3D micro- and nanostructures, 2D nanostructures greatly simplify the preparation process while retaining the special capability to manipulate the propagation properties of light waves [15]. U-shaped [16], S-shaped [17,18], heptamer nanostructure [19], and gammadion nanostructure [20,21] exhibit a CD signal due to the coupling between different parts of the nanostructure. The adjustment of CD signal can be achieved by adjusting the shape, size, and period of the planar structure [22–24]. However, CD is prominent in 3D nanostructures and weak in 2D nanostructures due to the requirement of intrinsic optical chirality for a structure without any planar mirror symmetry [25].
Metal nanohole arrays, as typical 2D nanostructures, have attracted increasing attention due to the need for a strongly confined electromagnetic field inside subwavelength nanoholes [26,27]. Metal nanoholes excite two surface plasmon modes, propagating surface plasmon polaritons (SPPs) through along the metal surface, whereas localized surface plasmon excitations (LSPs) are located at the edge of the nanohole. A unique advantage of nanoholes over other plasmonic biomolecular sensors is that the sample liquid can flow across the surface, which increases the binding rate and improves the detection limit under certain conditions. Furthermore, the coupled SPP and LSP modes can be specified by controlling the geometry of the structure, including the shape, diameter, and lateral period or quasi-periodic spacing of the nanohole [28–31].
Given the advantages of metal nanoholes themselves, different nanostructures are combined onto the metal nanohole arrays to enhance the chiral optical response. Spatially complementary chiral nanostructures can be prepared by one-time electron beam lithography (EBL) combined with electron beam deposition [32]. The generation of CD signal in nanoholes and their complementary structures is achieved through the modulation the near-field interactions between the localized and propagating surface plasmon modes. Given that spatially complementary nanostructures strongly depend on the shape of the nanohole, the shape of the bottom layer is also determined in the preparation process when the upper layer is decided, which hinders the adjustment of the CD signal [33,34]. The stacked nanoholes can be prepared using polystyrene sphere templates in combination with glancing angle deposition (GLAD) coatings. The CD signal is achieved by adjusting the size of the holes, the number of stacked layers, and the direction of stacking rotation. However, the accuracy of the structure is low due to its dependence on the masking of the template and multiple depositions [35,36]. Curled meta-surfaces can be bent by a focused ion beam-induced folding technique on the parts attached to the nanohole. The CD signal occurs due to the curled part of the meta-surface, which breaks the mirror symmetry of the whole nanostructure. Curled meta-surfaces are spontaneously bent or folded using differential strain between adjacent objects and require precise metrological control and careful pre-design and rigorous sequencing [37,38]. Thus, the production of 3D nanostructures with strong CD responses using a simple fabrication process remains a huge challenge.
Here, we proposed and verified a simple 3D cantilever-shaped nanostructure (CSN) that inherits the chiral advantages of 3D geometry and the simplicity of 2D structure fabrication. This can be fabricated by one-time electron beam exposure and oblique-angle deposition (OAD). The length of the cantilever and the height difference between the two arms can be adjusted/modified by controlling the tilt angles. Numerical calculations showed that the enhancement of the CD signal was achieved by plasmon distortion on the metal film by the hanging arms of the cantilever structure. In addition, the CD signal can be actively adjusted with temperature by polydimethylsiloxane (PDMS). A layer of PDMS was spin-coated on the cantilever-shaped nanostructure, and the coupling between the two hanging arms was changed through the temperature, causing the PDMS to expand and squeeze the hanging arms of the cantilever-shaped nanostructure. These results provide new ways to fabricate 3D structures, novel mechanisms for enhancing chiral optical response, and potential micro/nanophotonic devices.
2. Preparation of CSN arrays and computational method
The detailed fabrication process of CSN (Fig. 1) is as follows: (a) The ITO substrate was ultrasonically cleaned for 15 min using acetone, alcohol, and ultrapure water. At a homogenizer speed of 4000 rpm, PMMA (AR-P 679.04) was spin-coated on ITO for 60 s, which resulted in PMMA film with thickness T = 270 nm. Using a heating plate, the PMMA film on substrate was dried at 150°C for 3 min. (b) PMMA was then exposed to EBL to engrave a square nanohole pattern. Adjusting the calibration and image dispersion, EBL was performed with a spot size of 3, high voltage equal to 10 kV, and exposure dose of 100 µC/cm2. (c) Ag was deposited at α1 = 61° and ϕ1 = 90° and covered the PMMA film and one edge of each nanohole. (d) Ag was again deposited at α2 = 71° and ϕ2 = 180° and covered another edge of each nanohole. Here, α is the polar angle defined as the angle between the normal to the substrate plane and vapor deposition direction, whereas ϕ is the azimuthal angle [39,40]. The difference in the angle of deposition (α1 and α2) leads to structures with different heights of the lower cantilever arms (h1 and h2). The greater the difference between α1 and α2, the greater the difference between h1 and h2. In comparison with the conventional EBL fabrication process, the method reported here does not require a lift-off step, thereby avoiding sample detachment from the substrate which usually occurs during lift-off. Figure 1(e) shows the schematic unit cell of the CSN array, which consists of a metal film with periodic nanohole, and two hanging cantilever arms at adjacent edges of the nanohole. The cantilever-shaped nanostructure was suspended above the substrate and supported by PMMA.
The scanning electron microscopy (SEM) image of the CSN array is shown in Fig. 2(a), where pink and green dashed lines indicate the positions of the two lower cantilever arms. Figure 2(b) shows the SEM image of the cantilever-shaped structure after the transfer of the L-shaped underhanging part of the feature upward. The contrast of the two arms of the L-shaped structure differed, which indirectly indicated that the CSN had a height difference. Structural parameters of unit cell were obtained by averaging the SEM measurements. The obtained parameters of the rectangular hole were about w1 = 400 nm, w2 = 380 nm, and the thickness of the metal film was t = 80 nm, which is the sum of the thicknesses of the two OADs. The heights of L-shaped bottom cantilever arms were h1 = 120 nm and h2 = 220 nm, which were determined by α. The relationship between α and h is α = arctan(h/w), where h is the height of the lower cantilever. The widths of the bottom cantilever were about 60 and 80 nm, which were caused by differences in the tilt angle during the OAD.
Fig. 2. (a) SEM image of fabricated CSN arrays. (b) SEM image of the reverse side of the fabricated CSN array. The pink and green dashed lines indicate the positions of the arm of L-shaped cantilevers.
The 3D simulations with finite element method (COMSOL Multiphysics) were implemented to analyze the mechanism of the cantilever-shaped nanostructure. In the simulation, the periodic boundary conditions were imposed in the x-and y-directions to characterize the periodic structure, and two perfectly matched layers were located at the bottom and top along the z direction. The refractive index values of Ag were derived from the experimental data of Johnson and Christy [41], whereas that of silicon oxide was obtained from the work of Malitson [42].
3. Results and discussion
3.1 CD spectra and modes
A beam emitted from a halogen lamp was transmitted through the CSN and was analyzed with a spectrometer in the wavelength range 500–1000 nm. As shown in Fig. 3(a), the nanostructure had a distinct transmission peak (Mode I) and a transmission valley (Mode II) at 780 and 910 nm, respectively, under LCP and RCP light illumination. The transmission difference led to two CD modes, namely, Modes I (peak) and II (valley), which slightly shifted compared with the transmission modes [Fig. 3(c)]. To avoid errors in the measurements, the spectrum of the enantiomeric CD signal is given in Fig. 3(c). The CSN array and enantiomer (L-CSN) have opposite spectral signals and similar spectrum shapes. To theoretically analyze the origin of transmission difference, we performed simulations in COMSOL Multiphysics (finite element method) using the dimensional parameters obtained from SEM images. Figures 3(b) and 3(d) show the simulated results. Figure 3(b) shows the transmission spectra obtained by simulating the designed CSN arrays. The CD spectrum of the enantiomer is also included in Fig. 3(d). The pink and green bands in Fig. 3(b) correspond to the modes in Fig. 3(a). From the resonance wavelength of the spectral lines, the number of resonance peaks, shape of the spectrum, and line shapes of experimental measurements are in good agreement with the numerically computed results.
Fig. 3. Transmittance and CD spectra of L- and R-CSN under RCP and LCP light illuminations. (a), (c) Measured in experiment; (b), (d) calculated during numerical simulations.
To understand the mechanism of chiral optical response of CSN arrays, we analyzed the charge distributions given in Fig. 4, which depicts the charge distributions on four consecutive unit cells and on a single unit cell with the hole and hanging cantilever shown separately. The positive and negative signs indicate positive and negative charges, respectively. In Mode I, under LCP excitation, positive and negative charges were distributed transversely on the metal film around the hole. This distribution on the film was characterized as SPP. The charge distribution on the cantilever was the same as that on the metal film around the hole, the distribution on the lower part of larger arm was opposite to that on the metal film (Fig. 4(a)). The same is the case with the distribution under RCP excitation with a difference that the charge distribution on the metal film around the hole is longitudinal (Fig. 4(b)). The charge distribution on cantilever was thus a continuation of that on the metal film, but it was due to the angle between the cantilever and the metal film. Certain distortion occurred in the resonance path of the charge. Therefore, Mode I was regarded as a distorted SPP resonance mode. The distorted SPPs excited by RCP light were stronger than those of the LCP light, which led to a strong positive CD value in Mode I (Fig. 3(b)). In Mode II, under LCP excitation, the charge was concentrated at the edges of the nanohole, and charge distribution on the cantilever was an extension of that on the metal film around the hole (Fig. 4(c)). Under RCP excitation, positive charges appeared on horizontal sides of the metal film, whereas negative charges were observed on vertical sides of the metal film around the hole, with the charge distribution on cantilever being an extension of that of the metal film (Fig. 4(d)). Mode II was regarded as a distorted LSP resonant mode. The bottom cantilever changed the resonance path of the charges on the membrane, which distorted the path of resonance and created transmission differences under various circularly polarized illumination.
Fig. 4. (a)–(d) Normalized charge distribution near-field profiles for the CSN arrays with their resonance wavelength. The schematic illustration in the middle shows the simplified resonance model for the two resonance modes.
The schematic illustration in the middle of Fig. 4 shows the simplified resonance model for the two resonance modes. The red and green arrows graphically represent the distorted plasmons. The introduction of the lower cantilever part not only transformed the structure from 2D to 3D but also changed the resonance path of the charges on the metal film, thus enhancing the CD signal.
3.2 Influence of structural parameters on CD spectra
Based on the above analysis, the effects of structural parameters on CD properties of cantilever-shaped nanostructures were investigated by systematically varying the parameters of the metal thin film with periodic holes at the top and two hanging arms attached to edges of the holes. Figure 5(a) shows the effect of period P variation on CD signal while keeping other parameters constant. For Mode I, the increase in the metal film length increased the SPP propagation path in the horizontal direction, which led to a significant redshift of Mode I. For Mode II, the increase in the metal film length led to the elongation of the LSP oscillation path on the film, which eventually led to a slight redshift of Mode II. Figure 5(b) shows the spectra for CD signal with thickness t variations from 40 nm to 120 nm. With the increase of t, Mode I red-shifted and Mode II blue-shifted. In Mode I, the charges distribute on the surface around the nanohole. The total length of the charge oscillation was the sum of the period and film thickness. As t increased, the total length of charge oscillations increased, leading to a red-shift of Mode I. In mode II, the charge accumulated on the longitudinal sides of the nanohole, the cross-section of the charge of oscillation increased, resulting in a decreased effective aspect ratio of the charge oscillation and a blue shift of mode II. Finally, the effect of varying the height difference of the bottom cantilever on the CD signal was investigated. Figure 5(c) shows influence on CD spectra impacted by variation in h1 from 180 nm to 260 nm while keeping h2 fixed at 120 nm. Mode I slightly redshifted due to the increase in the resonance path length of SPP caused by the increase in h1. Given that mode II was caused by LSP along the nanohole edge, it showed no red nor blue shift because any change in h1 caused no effect on the electron resonance length along the gap. Figure 5(d) shows CD spectra for varying h2 from 80 nm to 60 nm while keeping h1 fixed at 220 nm. With the increase in h2, Mode I redshifted, and Mode II hardly shifted, and when h2 increased to 120 nm, the two modes almost merged into one mode resulting in a maximum CD of about 13.5%. The redshift of Mode I was due to the increase in h2, which lengthened the resonance path of distorted SPP on the metal film. Similar to the above reasons, the increase in h2 had less effect on changes in the resonance path of the distorted LSP. Thus, Mode II hardly shifted. The CD signal and the height of the lower cantilever were closely related. Thus, the CD signal can be adjusted by modifying the tilt angle of the coating, thus regulating the height difference of the lower cantilever in the tilt-angle evaporation coating process.
Fig. 5. Simulated CD spectra of the CSN arrays at varying (a) periods P of the structure from 660 nm to 740 nm, (b) thicknesses t of metal film from 40 nm to 120 nm, (c) height h1 of cantilever from 180 nm to 260 nm, and (d) height h2 of cantilever from 80 nm to 160 nm.
3.3 Temperature modulation CD signal
As discussed above, the cantilever-shaped structure achieved the enhancement of CD signal by distorted plasmon. However, for recognition of chiral molecules and detection of structures of other chiral compounds, dynamic modulations of wavelength and intensity of the CD signal are essential. PDMS is a highly temperature-sensitive transparent material with a highly negative thermo-optic coefficient and is widely used in various sensor devices due to its excellent properties.
A temperature-sensitive device that uses temperature to modulate the wavelength and intensity of CD spectra was proposed, and the steps for its preparation are as follows. In the first step, PDMS Sylgard-184 10:1 solution was prepared together with its crosslinker and stirred with a magnetic bar for 30 min. In the second step, the two components were mixed thoroughly, and self-exhausting bubbles were performed by static. In the third step, the static PDMS was dropped onto the CSN array. During this step the glue machine was set at 4000 rpm for 60 s. In the last step, the substrate was placed on a heating plate set at 60°C for 20 min. The spectrum of the temperature-sensitive device was measured by a self-assembled optical path, and a dry blank glass sheet was measured as the background noise. To obtain a wide experimental temperature range, we varied the test temperature between 80°C and 120°C. In addition, to ensure the plausibility of the experimental data, we plotted each spectral line in Fig. 6(a) by averaging three separate measurements of different regions of the sample at various temperatures. As shown in Fig. 6(a), the CD spectra at each temperature showed a slight blue-shift with the increase in temperature. In addition, the CD intensity at the resonance valley decreased with the increase in temperature. This result predicts that our proposed temperature-sensitive device can achieve the modulation of CD intensity and wavelength. Promising applications in chiral molecule detection, such as the modulation of the wavelength of biomolecules to match the resonance wavelength through the CD signal of temperature modulation, can also achieve the survival temperature requirements of different chiral molecules.
Fig. 6. Experimentally measured (a) and numerically calculated (b) spectra of the CD signal at different temperatures. The green arrow indicates the movement of the resonance valley.
Simulations were performed to understand the physical mechanism behind this phenomenon. The refractive indices of PDMS at different temperatures were obtained from [43], where is a negative photothermal coefficient. At a temperature of 22°C, the refractive index was 1.4204, which decreased with the increase in temperature. The simulation results [Fig. 6(b)] were consistent with the experimental findings [Fig. 6(a)] with respect to the shape and variation pattern of the spectra. The blue shift of the spectral lines was due to the refractive index of PDMS, which decreased as the temperature increased, resulting in a change in the refractive index of the metal and the dielectric surface. In addition to the variation in the CD spectra with temperature, the intensity of CD and the number of CD valleys were modulated because a portion of PDMS entered the nanohole and as a result of temperature variation, caused changes in the PDMS volume; this result changed the coupling between the lower cantilever and upper metal film. The results of the variation in the intensity of the CD signal with temperature in experiments [Fig. 6(a)] and simulations [Fig. 6(b)] showed that the variation in the CD signal intensity was linear in the range of 80°C to 120°C, which is more favorable for the real-life application of the device. Under experimental and theoretical verifications, cantilever-shaped structures offer new ways to fabricate 3D structures, explore new mechanisms for enhancing CD effects, and find potential applications in micro/nanophotonic devices.
4. Conclusion
In conclusion, a simple 3D cantilever-shaped nanostructure was proposed, and it inherits the chiral advantage of 3D geometry and the simplicity of 2D structure fabrication, as shown by experimental and theoretical demonstration. Here, plasmonic distortion-enhanced CD was achieved through the introduction a cantilevered 3D hypersurface on a planar structure. The structure can be prepared by combining one-time EBL and oblique-angle deposition stage, where the length of the cantilever and height difference between the two arms can be adjusted by controlling the tilt angles during oblique-angle deposition. Numerical calculations showed that the enhancement of CD was achieved through plasmon distortion on the perforated metal film by the hanging arms of the cantilever structure. Furthermore, the CD signal can be actively adjusted using the temperature-sensitive PDMS. The cantilever-type structure has potential applications, such as ultra-sensitive detection and remote temperature readout, and has practical prospects for ultra-small detection tools for nanoscale structural and functional information.
Funding
National Natural Science Foundation of China (61575117, 62005213); the Fundamental Research Funds for the Central Universities of Ministry of Education of China (201601008, 2021TS092).
Disclosures
The authors declare no conflicts of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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