Direction‐Dependent Plasmonic Chiroptical Response With High Asymmetry and Dynamic Tunability

ABSTRACT

A direction‐dependent chiroptical response breaks forward‐backward symmetry, introducing an additional degree of freedom beyond conventional circular dichroism (CD) modulation in wavelength, intensity, and handedness. However, most direction‐asymmetric chiroptical materials rely on isotropic films incorporating chiral molecules, showing a relatively weak signal. Here, a plasmonic‐based chiroptical film with a direction‐dependent CD signal is achieved via linear assembly of gold nanospheres (AuNSs) on an elastomeric polymer substrate. This film demonstrates reversal of CD handedness upon changing the light propagation direction from the forward to the backward configuration, accompanied by a significantly enhanced CD intensity from −0.8 to 10.8 deg. This behavior originates from wrinkle‐induced geometrical chirality in the AuNSs assemblies under mechanical stretching, as well as the interplay between the linear dichroism of the AuNSs chains and the strain‐induced linear birefringence of the polymer film. Furthermore, the direction‐dependent response is programmable by varying the mechanical stretching ratio, applied angle, and x‐/y‐orientation, enabling dynamical tuning of signal on/off, magnitude of ellipticity, and reversible handedness. This plasmonic chiroptical film not only provides a strong direction‐dependent CD response, exceeding the performance of most reported systems by 2–3 orders of magnitude, but also possesses multiple degrees of freedom for dynamical chiroptical modulation, presenting a promising platform for circularly‐polarized light‐based information encryption.

A plasmonic chiroptical film capable of light‐propagation‐direction‐dependent circular dichroism (CD) is achieved based on the linear assembly of gold nanospheres (AuNSs) on an elastomeric polymer film. The film exhibits not only a reversal of the CD response but also a more than 10‐fold enhanced CD intensity. Moreover, the CD response can be dynamically modulated by mechanical treatment, including stretching direction, strain, and angle, providing multiple degrees of freedom for programmable chiroptical regulation.

1. Introduction

Chiroptical materials exhibiting direction‐dependent signals, which invert the handedness of the circular dichroism (CD) signal by reversing the light propagation direction, offer promising opportunities for advanced information security, chiral photonics, and spintronics [1, 2, 3]. These materials expand degrees of freedom for manipulating optical signals, facilitated by their preferred interactions with left‐/right‐handed circularly polarized light and their direction‐asymmetric responses under forward/backward illumination [4, 5, 6]. Up to now, the most representative direction‐dependent chiroptical materials are isotropic films incorporated with organic π‐conjugated small molecules and polymers [7, 8]. The direction‐dependent chiroptical activities in such a chiral organic system originate from the synergetic combination of two contributions. One is the intrinsic circular polarized absorption generated from the molecular chirality, and the other is the direction‐dependent contribution arising from the coupling between nonparallel axes of linear dichroism (LD) and linear birefringence (LB) within the film [9]. However, CD intensity in these materials is generally within the range of 10⁻³–10⁻¹ deg, and the dissymmetry factor (g‐factor) is in the order of 10⁻², far below the theoretical limit of ±2 [10]. Moreover, such CD responses are intrinsically associated with electronic absorption bands of molecules, thus, typically confined to the UV–visible region. Extending the CD response toward the near‐infrared (NIR) region, while enabling high programmability over both the spectral position and magnitude of direction‐dependent CD signals, remains a challenge.

In contrast, chiral plasmonic materials represent a good candidate for realizing strong direction‐dependent CD signals due to their strong light–matter interaction and the polarization rotation of incident light [11, 12, 13, 14]. Chiral plasmonic structures can be constructed via “top‐down” micro/nanofabrication approaches, such as two‐photon direct laser writing or electron beam lithography [15, 16, 17], as well as “bottom‐up” assembly strategies, including DNA‐ and helical polymer fibrils‐directed self‐assembly [18, 19, 20, 21, 22]. In both cases, the obtained CD responses are closely related to the chiral configurations. However, the top‐down strategies usually result in chiral architectures in a static state, lacking reconfiguration [23, 24]. In contrast, the bottom‐up approaches enable rearrangeable chiral configurations and corresponding tunable CD spectra under external stimuli [25, 26, 27, 28, 29, 30, 31, 32]. Nevertheless, the CD signals arise from the averaged response of isotropically distributed chiral structures. Both of the approaches generate inherently chiroptical signals governed by the structural chirality. The direction‐dependent CD responses based on chiral plasmonic architectures have not yet been realized.

Herein, a chiral plasmonic film exhibiting direction‐dependent chiroptical properties is constructed through linearly assembling gold nanospheres (AuNSs) onto an elastic polymeric film (Figure 1). Upon mechanically induced wrinkling, unidirectional wrinkles with a period of hundreds of nanometers are generated on the surface of the film, which induces the 2D pre‐assembled, highly ordered AuNSs chains to turn into 3D twisted “S‐like” chiral morphology. This behavior contributes to an obvious CD response, which originates from the geometry‐induced chirality. Meanwhile, by reversing the light propagation direction, an inverted CD signal is achieved, demonstrating switched handedness and significantly enhanced intensity in comparison with that obtained from structural chirality. This reversed CD response is attributed to the interplay between the LD of the AuNSs chains and the linear birefringence (LB) of the stretching‐state polymeric film. Furthermore, the direction‐dependent CD responses obtained under forward/backward light propagation can be facilely and dynamically tuned in magnitude, wavelength, and handedness by changing the wrinkling direction and amplitude, as well as the mechanical stretching ratio. Such a chiral plasmonic film with direction‐asymmetric chiroptical activity, which exhibited distinct CD intensity from 10⁻¹ to 10¹ deg exceeding the mostly reported performance by 2–3 orders of magnitude, together with broadband wavelength coverage from visible to NIR wavelengths, dynamic programmability, and multiple degrees of freedom for optical modulation, has rarely been reported. By integrating outstanding CD performance with the mechanically tunable and direction‐dependent CD responses in a flexible, mechanically programmable plasmonic/polymer platform, this system shows great potential for optical encryption and logic‐gate‐like operations.

FIGURE 1.

Schematic illustration of the plasmonic‐based polymeric film exhibiting direction‐dependent chiroptical activity under mechanical stretching.

2. Results and Discussion

2.1. Direction‐Dependent Chiroptical Response Induced by Mechanical Wrinkling

Chiral plasmonic films capable of generating direction‐dependent chiroptical signals were obtained by assembling AuNSs (average diameter: 63 ± 2 nm, Figure S1) on a plasma pre‐treated polydimethylsiloxane (PDMS) substrate by a capillary‐assisted particle assembly process. In which another PDMS slab with unidirectional wrinkles was utilized as a template, a parallel‐ordered array of AuNSs dimer/singlet chains was obtained over macroscopic areas on the flat PDMS substrate following the wrinkle templates (Figures 2a, right, see Methods for details). The AuNSs chain assemblies exhibited uniform grating color over a centimeter‐scale area (Figure S2). And the SEM images, which were collected from different locations within the assembled region under varied magnifications, revealed a highly oriented and uniform arrangement of AuNSs chains across the whole region (Figure S3). The particles within the chains were closely packed with interparticle gaps of 1–2 nm (Figure 2a right, and Figure S3). Such close‐packing contributed to a strong plasmonic coupling, giving rise to a longitudinal surface plasmonic resonance (LSPR) mode (Figure S4). Consequently, the linear assembly of AuNSs exhibited a strong plasmonic absorption peak extending to the NIR region, with a dominant peak centered at 1000 ± 50 nm. In contrast, a relatively weak plasmonic absorption located at 532 nm corresponded to the transversal plasmonic resonance mode (TSPR).

FIGURE 2.

Direction‐dependent chiroptical response of the chiral plasmonic‐based polymeric film. Photographs (with iridescent color), and AFM image of the linearly assembled AuNSs chains on polymeric film before (a) and under (b) mechanical stretching (strain: 20%). (c) CD spectra of the film under 0% and 20% stretching collected in forward light propagation configuration (θ = 45°, film thickness: 2.5 mm). (d) Schematic illustration of the light propagation path in the forward configuration, where light met AuNSs chains first, then the polymeric film. (e) CD spectra of the film under 0% and 20% stretching were collected in a backward light propagation configuration (θ = 45°, film thickness: 2.5 mm). (f) Schematic illustration of the light propagation path in the backward configuration, where light met the polymeric film first, then the AuNSs chains. (g) Linearly polarized light‐dependent extinction spectra of the film (P: parallel, S: perpendicular to the long axis of AuNSs chains). (h) The dichroic ratio (DR) of linear AuNSs chains on the non‐stretched film. (i) The phase retardance of the polymer film under stretching state (without AuNSs, strain: 0% and 20%).

By applying a mechanical stretching to the polymeric substrate (PDMS) at a tilted angle (θ) relative to the long axis of the AuNSs chains, unidirectional wrinkles were formed on the PDMS film surface with a submicron periodicity. This wrinkling induced a distortion of the AuNSs assemblies, transforming the 2D line chains into 3D asymmetric chiral geometries, forming twisted “S‐like” morphologies, as revealed by in situ AFM imaging (Figure 2b). Notably, the iridescent grating color of the plasmonic film in the wrinkling state was different from that of the flat PDMS film bearing 2D straight AuNSs chains, even when observing under identical viewing angles. This optical variation arose from the coupled diffraction effects of the periodic AuNSs chains and the formed wrinkles. It was worth mentioning that the mechanical wrinkling was facilitated by the gradient of elastic modulus between the top glassy layer and the beneath elastic bulk substrate of the PDMS film due to the plasma pre‐treatment [33, 34].

The chiroptical response of the film was analyzed utilizing spectroscopic ellipsometry in a transmission mode. As the incident light propagated through AuNSs assemblies first, and then passed through the stretched PDMS substrate, named as forward measurement, an obvious CD spectrum was observed, exhibiting three characteristic modes (CDI–CDIII) spanning over the visible to NIR range. The positions of CD modes were associated with the plasmonic resonance wavelength (Figure 2c,d). In which the positive CDI appeared at 528 nm originated from the transverse surface plasmon resonance, while the negative CDIII with an intensity of −0.8 deg at 1035 nm arose from the LSPR mode [30, 35]. In stark contrast, the AuNSs assemblies in a 2D‐straight chain state showed no observable CD signals (Figure 2c). Here, the mechanically induced wrinkling led to the formation of a chiral geometry of AuNSs assemblies with a 3D “S‐like” morphology. These chiral plasmonic structures selectively absorbed the left‐ and right‐circularly polarized light, giving rise to an intrinsic CD response (Figure 2d). In this case, the CD measurement was collected in the light path of the AuNSs assemblies first, then the PDMS substrate. Although the birefringence of PDMS could change the polarization state of the transmitted light after its interaction with the AuNSs chains, it did not alter the total transmitted intensity. Thus, in this optical path, the phase retardance of PDMS had a negligible influence on the measured CD signal. Besides, a control measurement on the stretched PDMS substrate without AuNSs assemblies was performed to evaluate its possible contribution to LD. The stretched PDMS showed nearly identical transmission spectra under linearly polarized light with the polarization direction parallel (P‐) and perpendicular (S‐) to the stretching direction, indicating no LD response (Figure S5). Therefore, in the forward configuration, the CD signal originated from the structural chirality of the mechanically induced AuNSs assemblies.

In contrast, by reversing the light propagation direction, passing first through the stretched PDMS substrate and subsequently through the AuNSs chains (named as backward measurement, Figure 2e,f), an inverted CD spectrum was achieved, also consisting of three characteristic modes (CD1–CD3). In this case, a negative peak appeared at 535 nm (CD1), while a positive one was located around 954 nm (CD3) (Figure 2e). In addition to the handedness inversion, the chiroptical intensity was significantly enhanced in the backward configuration to 10.8 deg, more than one order of magnitude higher than that measured in the forward mode. And a g‐factor, which could be calculated according to g = CD/(32.982*Abs) (details in Supporting Information), reached up to 1.01. It was worth noting that only the plasmonic film with stretching‐induced wrinkles demonstrated CD signals. These results indicated that the mechanically wrinkled plasmonic film not only possessed structural chirality but also broke propagation symmetry, contributing to a pronounced direction‐dependent chiroptical response with significant intensity enhancement.

In order to gain a better understanding of the origin of the inverted CD signal in backward mode, the LD of the aligned AuNSs chains and the LB of the stretched PDMS were explored, respectively. The linear assembly of AuNSs demonstrated polarization‐dependent absorption, showing distinct transmission spectra under linear‐polarized light parallel (P‐) and perpendicular (S‐) to the long axis of AuNSs chains (Figure 2g). A dominant extinction at 990 nm was observed only in the P‐polarization, and a relatively weak absorption at 525 nm appeared under S‐polarization. This optical response resulted in LD of the AuNSs chains, with a high dichroic ratio (DR) up to 0.55 at 990 nm and −0.07 at 525 nm, which is related to the polarization‐dependent extinction of the longitudinal and transversal plasmonic modes, respectively (Figure 2h). On the other hand, the stretching‐state polymer film exhibited a measurable phase retardance due to the strain‐induced anisotropy. A retardance of 34 deg at the wavelength of 1000 nm (38 deg at 900 nm) was obtained under a strain ratio of 20% (Figure 2i). Its optical fast axis was aligned with the stretching direction, which was also in the same orientation as unidirectional wrinkles. Thus, in the backward configuration, the incident circularly‐polarized light met the birefringent polymer film prior to interacting with the linear dichroic AuNSs chains (Figure 2f). Owing to the phase retardance of the polymer film, the circularly polarized light was converted into orthogonal linear polarization components before reaching the plasmonic nanoparticle chains. These linearly polarized components were subsequently differentially transmitted due to the LD of the aligned AuNSs chains, leading to distinct transmitted intensities of the left‐ and right‐circularly polarized (LCP and RCP) light [36, 37]. The interplay between the LB of the stretched film and the LD of the aligned AuNSs chains gave rise to the observed CD signal in the backward mode. To be noted, in the backward configuration, the measured CD signal contained contributions from both the LD/LB coupling‐induced apparent CD and the intrinsic CD of the “S‐like” chiral AuNSs chains. However, the intrinsic CD of the AuNSs chains measured in the forward configuration was only about −0.8 deg at the wavelength around 1000 nm, whereas the backward CD signal reached about 10.8 deg with an opposite sign. The sign of the LD‐/LB‐induced CD was determined by the relative orientation between the LD and LB axes. In the current film, the LB axis at the wavelength of ∼ 1000 nm was oriented at +45° clockwise in relation to the LD axis, giving rise to a positive CD signal. Moreover, since the LD‐/LB‐generated CD intensity was more than one order of magnitude higher than that of intrinsic CD, the LD‐/LB‐generated CD response dominated in the backward configuration. Consequently, the backward chiroptical response exhibited a switched CD signal compared with that measured in the forward configuration.

Therefore, the observed direction‐dependent chiroptical response stemmed from both the structural chirality of AuNSs chains and the interplay between LD and LB within the plasmonic polymer film. Notably, the LD/LB interplay generated an obviously enhanced CD signal with switched handedness compared to that originating from the intrinsic structural chirality, leading to the inverted response upon sample flipping. In contrast, under the forward configuration, the birefringence of the stretched film did not contribute to the differential absorption of left‐ and right‐handed circularly polarized light. The CD signal resulted only from the chirality of AuNSs assemblies. In comparison with previously reported systems exhibiting direction‐dependent chiroptical signals, the plasmonic film demonstrated significantly improved performance in both propagation modes, with the g‐factor enhanced by 1–2 orders of magnitude (Table S1) [1, 2, 6, 7]. Moreover, CD spectra measured at different positions on the sample showed consistent CD spectral profiles, with only slight variation in CD intensity, particularly for the intrinsic chiroptical response measured in the forward configuration (Figure S6). The variation likely arose from minor differences in the local chiral geometry, as the intrinsic CD was highly sensitive to the chiral geometry.

2.2. Mechanically Tunable Direction‐Dependent Chiroptical Responses

The direction‐dependent chiroptical properties were closely associated with both the structural chirality of the plasmonic assemblies and the interplay of LD and LB within the film. In turn, both of them could be dynamically tuned via successive mechanical stretching (Figure 3). The degree of asymmetrical twisting of the chiral AuNSs assemblies increased with the enhanced strain ratio (Figures 3a and S7). The amplitude of twisted AuNSs chains increased more than twice, changing from 69 ± 17 to 177 ± 10 nm as the strain increased from 10% to 40%, while the period showed a slight increment from 1180 ± 104 to 1320 ± 59 nm (Figure 3b). Correspondingly, the twisted distance (d) of 3D “S‐like” AuNSs chains between the peak and valley in the x–y plane increased from 144 to 164 nm, yielding a highly twisted chiral arrangement of AuNSs (Figure 3d). As a result, the CDIII mode collected in the forward manner, which is related to the chirality of AuNSs chains, increased by one time from −0.61 to −1.10 deg as the stretching changed from 10% to 40% (Figure 3c). Similar increasing trends were also observed for the other CD modes. Notably, micrometer‐scale cracks were generated in the film during the stretching process. These cracks, which were generally formed perpendicularly to the stretching direction, might introduce local optical inhomogeneity, anisotropic scattering, causing additional optical loss, and thus they could slightly influence the absolute magnitude of the measured chiroptical signals. However, because the cracks did not possess intrinsic handedness or any well‐defined plasmonic chirality, they were not expected to have an obvious effect on the CD response.

FIGURE 3.

Mechanically induced modulation of the direction‐dependent chiroptical response via stretching ratio. (a) In situ AFM height images of the twisted “S‐like” AuNSs assemblies on the polymer film under 10%, 30%, and 40% strains. (b) Geometric parameters (period and amplitude) of “S‐like” AuNSs assemblies as a function of strain. (c) CD spectra of the plasmonic film under varied strains collected in the forward configuration. (d) The twisting distance (d, as marked in panel a) of the “S‐like” AuNSs geometry and the corresponding CD intensity as a function of strain. (e) Reversibility of the CDI (522 nm) and CDIII (1190 nm) intensities during alternating stretching/releasing treatment in the forward configuration. (f) CD spectra of the plasmonic film at different strain levels collected in the backward configuration. (g) The phase retardance of the polymer film and the corresponding CD intensity as a function of strain were measured in a backward manner. (h) Reversibility of the CD1 and CD3 intensities during cyclic stretching/releasing in backward configuration.

Meanwhile, the center‐to‐center distance between AuNSs became nonuniform after stretching‐induced wrinkling. Based on AFM analysis, the center distance of AuNSs located on the wrinkle crest increased from 83 ± 10 to 101 ± 18 nm as the strain ratio increased from 0% to 30%, whereas that for AuNSs located in the wrinkle valley decreased to 75 ± 2 nm at the strain of 30% (Table S2). Such local expansion and compression of the AuNSs spacing modulated the longitudinal plasmonic coupling. The enlarged spacing weakened the coupling and might induce a blueshift, while the reduced spacing strengthened the coupling and might induce a redshift. This might account for the strain‐dependent wavelength shift of the NIR CD features in the forward configuration (Figure 2c).

In the backward configuration, the intensity of the CD3 mode also enhanced significantly from 6.26 to 16.50 deg as the applied strain increased from 10% to 40% (Figure 3f). Correspondingly, the g‐factor reached 1.14 at the 40% strain ratio. This distinctive enhancement of CD magnitude mainly originated from the increased LB of the polymer film under higher strain (Figure S8). For the CD signal arising from the LB/LD coupling, the maximum intensity is theoretically achieved when the phase retardance approaches 90°. With increasing strain, the phase retardance of the film exhibited approximately linear growth. Consistently, the CD intensity collected in the backward configuration indicated a similar strain‐dependent enhancement as the strain increased from 10% to 40% (Figure 3g). It was worth noting that the phase retardance also increased with increasing film thickness at a fixed strain ratio (Figure S9), which was a common trend for linearly birefringent films. Accordingly, the CD intensity in the backward configuration varied depending on the deviation of the phase retardance from 90° (Figure S9b). Notably, when the thickness reached 5 mm, strains exceeding 40% induced CD signal inversion at wavelengths below 900 nm, corresponding to phase retardance values surpassing 180°.

Furthermore, this mechanical wrinkling induced twisted “S‐like” morphology of the AuNSs assemblies, which was capable of being fully reversible under cyclic stretching/releasing treatments. The 3D “S‐like” morphology turned back to 2D linear assemblies upon releasing the mechanical stretching. Correspondingly, the chiroptical signals detected in both forward and backward configurations disappeared (Figure 3e,h). Upon reapplying the stretching, the twisted “S‐like” chiral morphology was restored, together with the recovery of the structural chiroptical response. This reversible behavior could be maintained for more than 50 stretching/releasing cycles without noticeable hysteresis (Figures S10 and S11), although more cracks gradually developed on the film substrate (Figure S12). Such robust reversibility was attributed to the good elasticity of the PDMS substrate.

Moreover, the direction‐dependent chiroptical response could be tuned by varying the mechanical stretching angle (θ) relative to AuNSs chains (Figure 4). The highest ellipticity was observed at θ = ±45° in both forward and backward configurations (Figure 4a,b). For the forward measurement configuration, when θ drifted from ±45° toward either higher or lower angles, the CD intensity gradually decreased (Figure 4c), accompanied by a redshift of the response wavelength (Figure 4f). As revealed by in situ AFM analysis, the unidirectional nano‐wrinkles aligned with the stretching direction. Consequently, the intersection angle between the AuNSs chains and the wrinkle axis varied with the applied stretching angles, leading to different twisting degrees of the “S‐like” chiral morphologies (Figures 4e and S13).

FIGURE 4.

Programming of the direction‐dependent chiroptical properties by varying the mechanical stretching direction. (a,b) CD spectra as a function of stretching angle (θ) in relative to the long axis of AuNSs chains at 10% strain, measured in forward (a) and backward (b) configurations. (c,d) The intensity of CDIII (c) and CD3 (d) under varied stretching angles was measured in forward and backward configurations, respectively. (e) In situ AFM height image of the twisted “S‐like” AuNSs assemblies on polymer film, under 20% strain at different stretching angles (θ = 30°, 45°, 90°, and −45°, film thickness: 2.5 mm). (f) The wavelength corresponding to the maximum CDIII/CD3 signal at varied stretching angles in both forward and backward configurations. (g) Correlation between “‐cos(2θ‐90°)” and CD3 intensity as a function of stretching angles measured in backward configuration.

Indeed, the AuNSs chains under 45° and −45° stretching angles exhibited the highest geometric asymmetry, generating the most pronounced chiroptical signal in the forward configuration. In contrast, no significant CD response was observed for the film stretched at θ = 0° or 90°, where the AuNSs chains aligned either parallel or perpendicular to the wrinkle axis, respectively, resulting in the absence of chiral geometry. Besides, the mirror‐symmetrical CD spectra were obtained upon reversing the stretching angle, consistent with the mirrored “S‐like” geometries of the AuNSs chains observed by in situ AFM images (Figure 4e, 45° and −45°).

On the other hand, in the backward measurement configuration, a similar angular dependence of the CD response was observed upon varying the stretching angle. Because the stretching‐induced LB of the polymer film possessed a fast axis aligned with the stretching direction, changing θ altered the crossing angle between the LD and LB axes. The CD response originated from the LD/LB configuration, which was determined by the magnitudes of LD and LB, as well as their relative orientation, following the relation [38]. Thus, the maximum CD was generated at θ = ±45° according to the expression (Figure 4d,g). As θ deviated from ±45°, the CD intensity decreased. In comparison with the CD signal from the forward configuration, the wavelength of the maximum CD3 showed a slight shift over varied θ (Figure 4g), meaning that the wavelength of maximum LD for the AuNSs chains nearly changed.

2.3. Multiple Degrees of Freedom for Modulating Chiroptical Signals

The direction‐dependent chiroptical responses were closely associated with the chiral morphology of AuNSs structures and the LD/LB interplay within the film, in which the structural chirality of AuNSs assemblies and the LB of the polymeric film could be dynamically reversed via sequential biaxial mechanical stretching. This behavior added an additional degree of freedom for modulation of chiroptical signals aside from the direction‐dependent optical signals (Figure 5a). The plasmonic polymeric film was mounted on a customized bidirectional stretching device, with the stretching directions set at θ = 45° (x) and θ = −45° (y) relative to the long axis of the AuNSs chains. First, a 10% strain was applied along the x‐axis, producing surface wrinkles and inducing birefringence with the fast axis along the x‐axis. The wrinkling behavior induced the chiral “S‐like” morphology of AuNSs chains, giving rise to negative CDIII and positive CDI signals in the forward measurement mode (Figure 5b). In contrast, in the backward measurement mode, the coupling between the stretching‐induced LB of the polymer film along the x‐direction and the LD of the aligned AuNSs chains generated strong positive CD3 and negative CD1 signals.

FIGURE 5.

Dynamic tunability of the direction‐dependent CD response via sequential bi‐directional stretching treatment. (a) Schematic illustration of sequential bi‐directional stretching of the polymer film bearing linear AuNSs assemblies. (b–f) CD spectra collected under sequentially applied different strains along the x and y directions, collected in the forward (top panels) and backward (bottom panels) configurations. (g) Scheme of multiple degrees of freedom for modulating the chiroptical response.

Since mechanical mismatch between the rigid oxidized silica layer and the soft bulk PDMS substrate, linear wrinkles were formed under either unidirectional or biaxial stretching treatments. The orientation of the wrinkles was governed by the stretching direction with a higher strain ratio. Therefore, upon subsequent mechanical stretching along the y‐direction, the twisted “S‐like” morphology of AuNSs assemblies gradually turned into straight linear chains. Correspondingly, the magnitudes for both the forward/backward collected CD spectra progressively decreased (Figure 5c,d). When the applied strain along the y‐direction reached the same ratio as the pre‐applied one in the x‐direction (10% in this case), the wrinkles disappeared and the phase retardance of the polymer film was reduced to nearly zero (Figure S14). As a result, the CD signals in both configurations vanished (Figure 5d).

Further increasing the strain ratio in the y‐direction beyond the initial x‐direction strain, unidirectional wrinkles oriented along the y‐direction were formed, orthogonal to the original wrinkle direction. This reorientation induced a reversal of the “S‐like” morphology of the AuNSs chains, forming a mirrored configuration, named the reversed “S‐like” shape. Accordingly, the CD spectrum originated from structural chirality, which inverted the handedness as measured in forward mode, in which the CDIII and CDI modes switched to positive and negative signs, respectively, opposite to those generated in the initial state (Figure 5b,e, top panels). Meanwhile, the optical fast axis of the stretching‐state polymer film also rotated toward the y‐direction, altering the crossing angle between the LB and LD axes. This rotation effectively switched the LB/LD coupling configuration. Consequently, the CD spectra measured in the backward configuration also exhibited mirror‐symmetric inversion, with the CD3 and CD1 reversing their signs (Figure 5b,f, bottom panel).

This sequential biaxial stretching strategy enabled additional programmable control over the chiral geometries of “S‐like” AuNSs assemblies and the LD/LB interplay, providing multiple degrees of freedom for modulating chiroptical response. Specifically, the optical signals could be tuned in a two‐dimensional strain parameter, corresponding to independently adjustable strain levels along two orthogonal directions. When combined with bi‐directional direction‐dependent chiroptical signals, this approach yielded a total of 2 × m × n addressable modulation states for the chiral plasmonic polymer film (Figure 5g). Notably, the CD spectra obtained from different states did not interfere with each other. Such a multiplicity of independently addressable optical states offered unique advantages for advanced optical encryption and photonic information processing applications.

2.4. Application for Advanced Encryption With Multiple Channels and Logic Gate

Based on the dynamically programmable and light‐propagating direction‐dependent chiroptical activities, this chiral plasmonic film was utilized for advanced information encryption. A flexible information pad composed of a 1 × 2‐pixel array was fabricated by assembling AuNSs on a 5 mm‐thick polymer film. The assembly orientations of AuNSs chains on pixels 1 and 2 were set to be orthogonal, specifically, the AuNSs chains in pixel 1 were oriented at −45° anticlockwise, while those in pixel 2 were oriented at +45° clockwise (Figure 6a, top panel). By applying vertical stretching, both of the two pixels formed twisted “S‐like” AuNSs assemblies with mirror‐symmetrical geometries, leading to CD spectra with opposite signs. For information encryption, the CD3/CDIII modes located in the NIR region (around a wavelength of 900 nm) were selected, which cannot be read under sunlight. A negative CD signal was assigned as “0,” while a positive or zero signals ware considered as “1.” The chiroptical response of the information pad was detected in both forward and backward configurations under different stretching ratios, which further expanded the information encryption channels (Figures 6a and S15).

FIGURE 6.

Applications for the multi‐channel encryption and logic gate operation based on the direction‐dependent chiroptical response. (a) Schematic illustration of the information pad composed of two pixels and the output definition. The corresponding CD spectra in the NIR range of the two pixels under different applied strains, collected in forward/backward light propagation configurations (film thickness: 5 mm). (b) Scheme of the digital logic exclusive OR gate based on the information pad in panel a, along with the dynamically tuned logical gate outputs under different strains.

In the initial state without applying any mechanical stretching, the code “1, 1” was expressed for pixels 1 and 2 in both forward and backward configurations. Upon stretching to a strain below 40%, the code “1, 0” was decrypted in the forward mode, while a “0, 1” code was expressed under the backward mode. In contrast, as the applied strain exceeded 40%, the phase retardance of the polymer film surpassed 180° (one π), resulting in sign inversion of the CD signal in the backward mode. As a result, both modes displayed the code “0, 1” (Figure 6a). This direction‐dependent chiroptical film enabled multi‐channel data encryption, providing a promising system for versatile letter and image encoding, which could boost practical applications for information security‐ and optics‐related applications.

Furthermore, based on the pixel‐arrayed information pad capable of the stretching‐induced variable optical signals, a dynamically tunable digital exclusive OR (XOR) logic gate was designed (Figure 6b). For instance, the above‐defined codes derived from the forward and backward CD signals of pixel 1 were used as the two inputs for the XOR gate. For a two‐input system with inputs A and B, a true output (1) was obtained when A ≠ B, whereas a false output (0) occurred when A = B, in accordance with the Boolean expression (see details in the Supporting Information). The codes based on the forward and backward configurations were “1, 1” at the non‐stretching state; thus, the output of the XOR gate was “0” (Figure 6b). Upon increasing the mechanical strain to 20%, the inputs became “0, 1,” leading to a dynamic transition of the XOR gate output to “1.” When the strain was further enhanced beyond 40% (50% in this case), the backward CD signal underwent sign inversion, and the inputs became “0, 0,” thereby switching the output of the XOR gate back to “0.” Thus, based on the direction‐dependent chiroptical signal and its mechanical programming, the chiral plasmonic film could function as a dynamically tunable XOR logic gate.

3. Conclusion

In conclusion, a flexible and mechanically programmable plasmonic chiroptical film with direction‐dependent chiroptical responses were developed through linear assembly of AuNSs on an elastomeric polymer substrate. The film exhibited a high‐magnitude of CD signal, dynamic, and multiple degrees of freedom for chiroptical modulation. This direction‐dependent response originated from two effects: the mechanically induced structural chirality of the twisted “S‐like” AuNSs assemblies, together with the interplay between the linear dichroism of AuNS chains and the linear birefringence of the stretched polymer film. The film not only displayed light‐propagation‐direction‐dependent sign inversion of the CD signals, but also achieved a significantly enhanced intensity by more than 10‐fold, increasing from 0.8 to 10.8 deg. This CD magnitude was orders of magnitude higher than that of conventional chiral‐ molecular‐based direction‐asymmetric systems. Furthermore, the direction‐dependent optical response could be dynamically programmed through varying both the mechanical stretching direction and strain ratio, introducing additional degrees of freedom for chiroptical control, also showing full reversibility. Taking advantage of the multi‐dimensional modulation and the direction‐asymmetric chiroptical responses, this plasmonic chiroptical film presented a promising platform for advanced multi‐channel information encryption and dynamically tunable exclusive OR logic gate operations.

Conflicts of Interest

The authors declare no conflicts of interest.

Supporting information

Supporting File: adma74156‐sup‐0001‐SuppMat.docx.

Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.