1. Introduction

Metallic nanostructures have attracted much attention recently because of their unique and remarkable features [1]. Periodic metallic nanostructures with tuneable coupling between photonic modes and surface plasmon polaritons have been used extensively in scientific studies and in practical applications [2–4]. Periodic metallic nanostructures have been constructed using various techniques such as electron beam lithography [5, 6], reactive ion-beam lithography [7], nanoimprinting [8–10], interference lithography [11], solution-processing [12], and a nanoscale tensile stress approach [13]. Most fabrication techniques involve templates, high temperatures, complex procedures, or expensive processes. Recently, a laser-induced transfer method was investigated extensively because of its simplicity and flexibility [14]. With the aid of templates, periodic metallic nanostructures can be produced using this technique [15]. In previous work [16,17] we showed that random nanostructures can be fabricated successfully by a laser-induced transfer technique. However, much simpler and more effective methods are required for the fabrication of periodic nanostructures on a large scale and with low cost.

In this paper, we propose a template-free approach to create flexible periodic metallic nanostructure pairs with nanoscale periods and centimetre-scale areas using a single ultraviolet laser pulse. A thin metallic film on a soft substrate covered by a soft slab is exposed to a two-beam interference pattern of ultraviolet laser pulses. After a nanosecond single pulse exposure the metallic film and the soft slab “record” the pattern of dark fringes and bright fringes, respectively. Two metallic nanostructures are formed that are complementary to each other. The period of the one-dimensional (1D) nanostructure can be controlled by changing the angle between the two interference beams. The duty ratio of the metallic nanostructure can be adjusted by varying the laser pulse fluence. Employing a multiexposure process, two-dimensional (2D) nanostructures could be fabricated on large scale in a flexible manner. The plasmon resonance and its Fano coupling with the waveguide mode can be achieved in the proposed nanostructures. The laser-induced nanostructuring technique enables potential applications in sensing systems or in optoelectronic elements.

2. Fabrication of complementary metallic nanostructure pairs

A scheme of the proposed method for the simple fabrication of periodic metallic nanostructure pairs is shown in Fig. 1. In the laser-induced nanostructuring process, polydimethylsiloxane (PDMS) was used as soft matter. A 15 nm gold film was evaporated onto a PDMS substrate of 20 × 20 mm² in area and a thickness of 1 mm, and this acted as a “donor”. Another PDMS slab was placed on top of the gold-coated PDMS substrate as a “receiver” as shown in the inset in Fig. 1. The preparation of the PDMS slab is very similar to our previous work [16]. In a control experiment, indium tin oxide (ITO)-coated glass substrates the same size as the PDMS substrates were used as a “donor”. The ITO coating acts as a conducting layer for scanning electron microscopy (SEM) measurements. Figure 1(b) shows the light path for laser-induced nanostructuring. A 355 nm pulsed laser with a repetition frequency of 1 Hz and a pulse width of 5 ns was used as the laser irradiation source. The diameter of the laser spot was about 5 mm. A larger laser spot size can be obtained by expanding and collimating the laser pulse spatially. The sample was exposed to an interference pattern of two ultraviolet pulses each with energy of about 30 mJ. The period (Λ) of the nanostructure can be adjusted conveniently by changing the included angle α of the two pulses through the simple relation of Λ = λ/2sin(α/2), where λ is the wavelength of the writing laser. The 2D nanostructures can be obtained by employing a double exposure process. The sample is rotated by 90° about an axis normal to the substrate in the center of the area written in the first exposure before it is exposed to the second pulse of the interference pattern. More complex nanostructures can also be fabricated in a flexible manner by a multiexposure process [11, 13]. Note that laser pulses can irradiate the gold film through the transparent PDMS. The distance between the gold film and the PDMS in Fig. 1(b) is much less than 50 μm. In our experiment, the top layer of PDMS was placed on top of the donor substrate and pressed down gently. The inset in Fig. 1(b) is the photograph of the PDMS pairs. A shorter distance yields better quality nanostructures. All nanostructures are fabricated at room temperature and under atmospheric pressure. After the two-pulse exposure, periodic metallic nanostructure pairs were achieved, as shown in Fig. 1(c). The gold film in the dark fringes of the interference pattern remains on the PDMS substrate, forming a periodic nanostructure consisting of continuous gold nanolines. The gold films in the bright fringes melt and break up to form a mass of spherical nanodroplets. Most nanodroplets are ejected from the PDMS substrate to the PDMS slab and form periodic nanoisland structures. During the multiexposure process, the laser fluence is a bit higher than the threshold intensity for melting the gold film to minimize transferring nanoparticles on the receiver back to the donor. Some small nanoparticles on the receiver may be remelted and ejected back to the donor. But most of the nanopartices remain intact.

 

Fig. 1 Schematic of the fabrication procedure for the complementary metallic nanostructure pairs. (a) PDMS substrate coated with a thin gold film. (b) The gold-coated glass substrate is covered by a soft elastic slab of 1 mm in thickness. The sample was exposed to a two-beam interference pattern with an included angle α, which forms periodic metallic nanostructures. The inset is the photograph of the PDMS pairs in Fig. 1(b). ① and ② identify the receiver and donor, respectively. (c) The periodic metallic nanostructure forms within the bright fringes of the interference pattern and is transferred to the soft slab. The complimentary periodic nanostructure within the dark fringes remains on the PDMS substrate.

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In the experiment, the topology of the nanostructures was investigated by SEM (Hitachi S-4800) and atom force microscopy (AFM, Witec Alpha 300-S). In the optical spectroscopic characterizations, a nonpolarized white light from a tungsten halogen lamp (HL-2000) is employed as a light source. The spectroscopic responses of the sample are measured using a spectrometer (Maya 2000 Pro, Ocean Optics, USA).

The physical laser-induced nanostructuring process is intuitive. The laser-generated heat is mainly confined in the gold film and it partly diffuses to the substrate [18]. The thermal conductivity of the substrate has a great influence on the amount of heat dissipating through the interface. Therefore, the interfacial gold-substrate interaction plays an important role in distributing heat and in the formation of the nanostructures. Different substrate materials yield different nanostructure morphologies, which will be discussed in detail later in this paper.

Figure 2 shows gold gratings with different duty ratios and these were fabricated by laser-induced nanostructuring. A 15 nm gold film was evaporated onto an ITO-coated glass substrate. In the fabrication process, the gold film in the bright fringes of the interference pattern will melt into nanodroplets, and the gold film will remain within the dark fringes as indicated by the arrows in Fig. 1. Larger laser fluences lead to more gold film removal within the dark fringes, producing more nanodroplets. Thus, it is feasible to control the amount of nanodroplets by changing the laser fluences.

 

Fig. 2 SEM images of gold gratings with different duty ratios: (a) 0.5 and (b) 0.66 fabricated at laser fluences of (a) 478 mJ/cm² and (b) 318 mJ/cm². The bright regions/dots denoted by blue/red arrows are gold films/nanodroplets. The period of the gratings is 900 nm. The substrate is ITO-coated glass.

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Figure 3 shows complementary nanostructure pairs on soft substrates. Both donors and receivers are made from PDMS materials. PDMS was prepared by mixing silicone gel with a cross-linker at a mass ratio of 10:1. The mixture was placed in an air-exhaust apparatus for 1.5 h to remove the bubbles. A few drops of PDMS were then placed on the top of a piece of glass slide. Another piece of glass slide was placed on top of the PDMS followed by gentle pressing. Silicone rubber spacers with adjustable thicknesses were placed between two pieces of glass. The sample was heated to 100°C for 40 min in an oven to achieve the cross-linking polymerization and drying process. After cooling to room temperature, two pieces of glass were peeled off and a smooth PDMS slab was obtained. A thin gold film was evaporated onto the PDMS substrate, which was then covered with another PDMS slab. It should be noted that an excellent contact between the donor and the receiver is important to retain a good quality of the periodic nanostructures. If the gap between the receiver and the donor is too large, the ejected nanoparticles will form a random nanostructure instead of a periodic nanostructure. In the experiment, the top layer of PDMS (receiver) was placed on top of the PDMS substrate (donor) and pressed down gently. Therefore, a three-layer structure was obtained consisting of PDMS/Au/PDMS. After exposing the sample to the interference pattern of two ultraviolet pulses, 1D and 2D gold nanostructures were fabricated in a flexible manner on both the donor and receiver substrates. Different diffraction colors are observed when the soft PDMS substrate is bent under stress, as shown in Figs. 3(c) and 3(f), where several patterns are fabricated on one soft substrate to demonstrate the reproducibility of the proposed technique. The repeatability on optical properties of the flexible devices are also examined. Similar optical properties can be obtained, which will be discussed later. Some gold nanolines flake away because of the elastic deformation of soft PDMS, as shown in Fig. 3(d). Several 1D patterns were fabricated on the same PDMS slab, as shown in Fig. 3(c). It is worth mentioning that the SEM image quality of the nanostructures on the PDMS slabs is strongly limited by poor conductivity even after spraying.

 

Fig. 3 SEM images of complementary nanostructure pairs on soft PDMS. 1D nanostructures on the (a) receiver and (d) donor substrates. 2D nanostructures on the (b) receiver and (e) donor substrates. Optical images of the (c) receiver and (f) donor substrates upon bending. Different diffraction colours of the nanostructures indicate the elastic deformation of soft PDMS under stress. Several patterns are fabricated on one soft substrate in (c) and (f). The bright regions in (a), (b), (d), (e) are gold. The laser fluence was 450 mJ/cm².

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3. Influence of substrates on complementary metallic nanostructure pairs

Note that the nanostructures on the donor substrates in Figs. 3(d) and 3(e) are quite different from those in Fig. 2. To clarify this phenomenon, a control experiment was performed. The fabrication of complementary metallic nanostructure pairs was performed on PDMS/Au/ITO glass samples. A PDMS slab was used to cover the top of a piece of Au-coated ITO glass. The sample was then exposed from the PDMS side to an interference pattern. The resultant complementary nanostructure pairs are shown in Fig. 4. The duty ratios of the nanostructure pairs can be controlled in a flexible manner by tuning the laser fluences, as shown in Figs. 4(a), 4(b), 4(d) and 4(e). 2D nanostructures can be constructed by double exposures. Clearly, the nanostructures on the donor substrates consist of gold films and nanodroplets while those on the receiver substrates comprise a mass of nanodroplets.

 

Fig. 4 SEM images of complementary nanostructure pairs on ITO glass fabricated by laser-induced transfer. (a), (b), (d) and (e) 1D nanostructures with different duty ratios. (c) and (f) 2D nanostructures. The upper panels show the receivers. The lower panels denote the corresponding donors. The bright regions are gold. The laser fluence was 200 mJ/cm² for (a) and 450 mJ/cm² for (b).

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The nanostructures on the receiver substrates are obviously similar whether the receiver is ITO glass or PDMS. The difference between the nanostructures on the donors in Fig. 3 and Fig. 4 can be attributed to an interfacial gold-substrate interaction. One factor is the thermal conductivity of the substrates. The thermal conductivity of the PDMS materials (≈0.23 W/mK) [19] is much lower than that of the ITO layer (≈5 W/mK) [20]. Thus, a relatively small amount of laser-generated heat can dissipate through the gold-PDMS interface within nanoseconds. The heat is mostly confined within the gold film and dissipates laterally leading to a significant increase in temperature of the gold film. As a result, fewer residual gold films are present on the PDMS substrates for the same laser fluence, as shown in Fig. 3 and Fig. 4. Another factor is the wettability of gold towards different substrates [21]. The wettability of gold towards PDMS and the ITO layer can influence the morphology of the nanoparticles.

4. Uniformity and adhesion of nanostructures on PDMS substrates

The nanoparticles generated by laser-induced nanostructuring are characterized systematically. The adhesions of nanodroplets to different material surfaces are quite different due to their different surface energy properties. In a control experiment, the nanodroplets were transferred to receivers made of indium-tin-oxide (ITO) glass and PDMS, respectively. Under the same experimental conditions, the nanoparticle densities on different receiver substrates are quite different as shown in Figs. 5(a) and 5(b). The cohesion between Au and ITO is smaller than the adhesion between Au and the PDMS substrate due to the non-wetting property of Au/ITO interfaces. For PDMS substrates, the mean diameter of gold nanoparticles is in the range of 50-100 nm. For example, the mean diameter of nanoparticles in Fig. 4(b) is 88 nm as shown in Fig. 5(c). The histograms (blue boxes) and the Gaussian fittings (red curves) indicate the nanoparticle diameter distributions of the SEM image. It can be seen that the variance of the size is approximately within 50 nm. The height of gold nanoparticles is around 60 nm as shown in the inset in Fig. 5(c). The uniformity of nanoparticle size can be altered by changing the laser fluence per pulse and the number of pulses. As reported in our previous work [16], increasing the laser fluence per pulse and the pulse number can improve the uniformity of the nanoparticles. The optimized parameters have been used in the present work.

 

Fig. 5 SEM images of metallic nanoparticles on (a) an ITO-coated glass and (b) a PDMS slab. (c) A typical size distribution of the nanoparticles generated by laser-induced nanostructuring. Histograms (blue boxes) of the mean diameter distribution of the SEM image in Fig. 4(b) are Gaussian fitted (red curves). The inset shows the atomic force microscopic image of the nanostructure. Scale bar, 600 nm. (d) Extinction spectra of gold nanoparticles on PDMS substrates before and after 5-min ultrasonic cleaning experiment. The inset denotes optical image of the sample after 5-min ultrasonic cleaning.

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We have checked the adhesion of nanoparticles on PDMS substrates by ultrasonic cleaning experiments. In Fig. 5(b), the peak centered at about 546 nm is primarily ascribed to the particle plasma resonance of gold nanoparticles. No significant change can be observed in both the image and the spectroscopic response after 5-min ultrasonic cleaning (250 W, 40 kHz) as shown in Fig. 5(b). Thus, the adhesion between nanoparticles and PDMS surface is strong enough for practical applications.

5. Fano resonance in the spectroscopic response of the fabricated nanostructures

The Fano coupling between plasmon resonance of gold nanostructures and waveguide resonance is the most intriguing mechanism for plasmonic biosensor and filter applications due to its remarkable sensitivity enhancement effect [22–24]. It is necessary to introduce a waveguide layer in the fabricated nanostructures to excite the waveguide mode. In the experiment, a 200-nm layer of polymethyl methacrylate (PMMA) is spin-coated onto the fabricated nanostructures. The sample is mounted on a rotating stage enabling the angle-resolved tuning property measurement. The transmission spectra through the sample is measured using a spectrometer mentioned above. It is defined that incident white light is TM polarized (TE polarized) when its electric field is perpendicular (parallel) to the incident plane. The extinction spectrum can be calculated by –lg(I_t/I₀), where I_t and I₀ denote the transmission spectrum through the patterned sample and the PMMA-coated substrate, respectively. The extinction spectra were measured by increasing the incident angle of the white light beam from 0 to 20 degrees with a step of 2 degrees. The corresponding spectroscopic response is shown in Fig. 6, where the spectra were measured for TM and TE polarizations, respectively. The Fano resonance can be recognized clearly by the dips in the extinction spectra, which are indicated by the red arrows in Fig. 6. The angle-resolve tuning rate of the Fano dips is about 5 nm per degree for both TM and TE polarizations. The Fano dips are observed at 618 nm and 628 nm at normal incidence for TM and TE polarizations, respectively, which split into two branches with increasing incident angle. A long resonance wavelength at normal incidence results from the large effective refractive index of the nanostructures [25].

 

Fig. 6 Angle-resolved tuning properties of the Fano dips for (a) TE and (b) TM polarizations. The period of the gratings is 400 nm.

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The Fano resonance is caused by the coupling between the plasmon resonance and the waveguide mode. It should be noted that the plasmon resonance of our proposed nanostructures can be excited by both TM and TE polarizations as shown in Fig. 6 due to a large number of gold nanoparticles on the sample surface. Thus, Fano resonance can be achieved in one sample for TM and TE polarizations, which is more complicated when compared with that of its counterpart without nanoparticles [12]. The unique plasmonic property can be utilized in plasmonic applications.

6. Conclusions

We fabricated 1D and 2D complementary nanostructure pairs on soft substrates using a laser-induced transfer technique. Gold nanostructures consisting of isolated nanodots or nanolines can be achieved using two soft substrates and single nanosecond pulse exposure. Fano coupling through plasmonic-photonic interactions can be observed for both TM and TE polarizations. This technique can be used to fabricate metallic nanostructures on large scale and with low cost, which may facilitate the design of plasmonic devices.