Surface-enhanced Raman scattering (SERS) plays an important role in trace detection and other fields of research. The use of periodic micro-nano structures has been a popular method for realizing high-performance SERS substrates. Traditional preparation technologies can prepare micro-nano structures with high precision and excellent quality. However, most of them have the limitations of strict environmental requirements, low efficiency, and high material dependence; therefore, it is necessary to develop alternative micro-nano periodic structure preparation techniques. Laser-induced forward transfer (LIFT) has been used to transfer almost all types of materials; however, it is limited in preparing periodic micro-nano structures. Laser interference lithography (LIL) has the advantage of high efficiency in preparing periodic micro–nano structures with large areas. Therefore, combining LIFT with LIL technology, a novel technique of laser interference-induced forward transfer (LIIFT) is proposed to transfer the periodic Ag micro-dots under three-beam laser interference in this study, which can overcome the shortcomings of traditional technologies and realize large-area metal micro-nano array structure manufacturing in a rapid, low-cost manner. Finally, the SERS properties of the Ag micro-dots transferred via LIIFT are tested and analyzed.

The donor film to be transferred consists of a quartz substrate, a polyimide(PI)sacrificial layer film mixed with carbon nanoparticles (CNPs@PI), and an Ag film. The preparation process of the CNPs@PI thin film is described in Ref.[16], and the Ag film is evaporated onto the CNPs@PI film (Fig.1). The principle of transferring a micro- dot via LIIFT is shown in Fig.2. The azimuth angles (0°, 120°, and 240°) and transverse-magnetic (TM) mode of the polarization direction are chosen. Moreover, the effect of the interference period on the transferred microstructure is discussed. Finally, the Raman spectra of RhB on the transferred Ag micro-dots with different periods (9, 11, and 15 μm) are tested to analyze the SERS property of the transferred Ag micro-dots.

The transferred Ag micro-dot structure is observed on the receiving substrate, as shown in Fig. 3(a1)?(a3). While the interference periods of the three beams are 9 μm, the outline of the micro-dot structure can be observed, but there are many overlapping areas between adjacent micro-dots, as shown in Fig.3(a1). While the interference period increases to 11 μm, a micro-dot with clear edge can be observed, as shown in Fig.3(a2). Upon increasing the period to 15 μm, the clarity of the micro-dot is further improved [Fig.3(a3)]. These results, combined with Fig. 3(b1)?(b3), indicate that an increasing number of nanoparticles are distributed in the middle regions of a single micro-dot, leading to an improved resolution of each micro-dot with an increase in the period. The average diameter of the nanoparticles in the consistent area in the middle regions of the micro-dots with different periods is 129?141 nm, as shown in Fig.3(c1)?(c3). These results, combined with the statistical results of the numbers of Ag nanoparticles in the consistent area (Fig.4), indicate that, with an increase in the interference period, the average size of the Ag nanoparticles changes slightly, whereas the number of nanoparticles increases significantly. This is because the maximum intensity and contrast of the three-beam interference light field increase with the laser interference period (Fig.5), which affects the transferable mass of the Ag film and the distribution of Ag nanoparticles on the receiving substrate.

The SERS characteristics of Ag micro-dot substrates with periods of 9, 11, and 15 μm are tested by selecting Rhodamine B(RhB) as the analyte with a concentration of 10^-3 mol?L^-1 and compared with the RhB Raman spectrum on the bare Si substrate. The results show that, while using an Si substrate covered with the transferred Ag nanoparticle micro-dots, the Raman intensity of RhB is significantly enhanced compared with that of the bare Si substrate. Simultaneously, the Raman intensity of RhB on the Ag nanoparticle micro-dot substrate increases with the micro-dot period (Fig.7). This can be attributed to the local surface plasmon resonance effect. With the increase in interference period, the density of the transferred Ag nanoparticles increases evidently, which can create more and stronger “hot spots,” making it easier to excite strong Raman signals.

Periodic Ag nanoparticle micro-dots are realized efficiently with three-beam laser interference LIIFT. The distribution of Ag nanoparticles in each micro-dot is controlled by adjusting the laser interference period. With the increase in the period of the laser interference from 9 μm to 15 μm, the density of Ag nanoparticles in the micro-dot increases. Finally, the Ag nanoparticle micro-dot substrates prepared by the three-beam LIIFT technology show significant SERS characteristics. Moreover, the Raman intensity of RhB on the transferred Ag micro-dots increases with the period of the micro-dots. This study demonstrates the potential application of LIIFT technology in the field of SERS chips.