**4. Potential applications of fabricated plasmonic nanostructures**

Nowadays, plasmonics appear in many different domains with numerous interesting applications and continue to attract more and more attention. Of course, these applications depend strongly on the fabrication technologies that may or may not allow to produce PNSs as desired. In this section, a few interesting applications related to PNSs fabricated by the DLW method will be presented, particularly for plasmonics-based sensor, optical filter, data storage, and color nanoprinter.

### **4.1. Applications of plasmonic nano-hole arrays**

**Figure 11(a)** shows an experimental setup used to measure the transmission of NHAs fabricated previously by the indirect method. A supercontinuum laser (λ = 1200–1700 nm) is used as the illumination source. The beam was expanded and focused through NHA structure by an OL (NA = 0.4). The surrounding media of NHA can be easily changed from air to water

**Figure 11.** (a) Experimental setup of Au NHA transmission measurement. (b) Experimental and theoretical results of transmission spectra in air of fabricated Au NHA structures. (c) Experimental and (d) calculating transmission spectra of Au NHA structures in different media. For all simulations: Λ = 1000 nm, dhole = 400 nm, tAu = 50 nm, and tCr = 15 nm.

and to oil by simply casting a drop of desired medium. The transmission spectra was collected by another OL and transmitted to a spectrometer. The experimental results were summarized and compared to predicted simulation results.

A remarkable similarity between experimental results and theoretical calculations is emphasized in **Figure 11(b)** for a sample immersed in air. We can observe a transmission dip around 1535 nm and a transmission peak located at about 1750 nm. The experimental transmission spectrum follows the evolution trend of calculation. However, due to the limitation of the detection range of the spectrometer, it is not possible to fully characterize this transmission band of the fabricated NHA, in particular, for the transmission peak. Therefore, the transmission dip corresponding to SPR band on the interface of gold-glass substrate is exploited further. From simulation (**Figure 11(c)**), a transmission dip is predicted approximately close to the wavelength of Λ × n with n as the refractive index of surrounding media, that is, transmission dips around 1330 and 1500 nm for water and oil, respectively. Those dips are effectively observed experimentally and shown in **Figure 11(d)**. The transmission dip at λG ≈ 1535 nm (corresponding to SPR at glass-Au interface, called glass-mode) was unchanged in all three cases of water, oil, and air. Another dip appears at λW ≈ 1335 nm when the NHA was embedded in water, called water-mode. This dip red-shifted to λO ≈ 1510 nm when the NHA was immersed in oil, called oil-mode. This suggests a great application as plasmonics-based sensor if the surrounding medium of the NHA changes.

### **4.2. Plamonic-based data storage and color nanoprinters**

**Figure 11.** (a) Experimental setup of Au NHA transmission measurement. (b) Experimental and theoretical results of transmission spectra in air of fabricated Au NHA structures. (c) Experimental and (d) calculating transmission spectra of Au NHA structures in different media. For all simulations: Λ = 1000 nm, dhole = 400 nm, tAu = 50 nm, and tCr = 15 nm.

examples of the PNSs consisting of Au NIs: a "NANO" letter consisted of Au NPs as demonstrated by the SEM image and a "Mario" image consisted of Au NPs having different sizes, as

Nowadays, plasmonics appear in many different domains with numerous interesting applications and continue to attract more and more attention. Of course, these applications depend strongly on the fabrication technologies that may or may not allow to produce PNSs as desired. In this section, a few interesting applications related to PNSs fabricated by the DLW method will be presented, particularly for plasmonics-based sensor, optical filter, data stor-

**Figure 11(a)** shows an experimental setup used to measure the transmission of NHAs fabricated previously by the indirect method. A supercontinuum laser (λ = 1200–1700 nm) is used as the illumination source. The beam was expanded and focused through NHA structure by an OL (NA = 0.4). The surrounding media of NHA can be easily changed from air to water

**4. Potential applications of fabricated plasmonic nanostructures**

seen by different colors.

78 Plasmonics

age, and color nanoprinter.

**4.1. Applications of plasmonic nano-hole arrays**

The newly developed optically induced thermal effect by DLW technique allows imaging many applications at nanoscale. For a demonstration, different PNS were realized and several examples are shown in **Figure 12**. By this way, stereoscopic images can be encoded in the NSs and can be potentially used as elements for data storage and color nanoprinter applications. The stored data can be coded (binary code, alphabet letter, etc.) and programmed into the trajectory of laser scanning to directly write data on metallic materials.

**Figure 12(a)** shows an example of the "NANO LETTERS" words made by Au NIs. The height of the word can be as small as 1 μm. Generally, any nano text can be written by this DLW

**Figure 12.** (a) Optical microscope image of the plasmonic text ("NANO LETTERS" with different sizes) realized by the DLW technique. (b) Optical image of a quick response (QR) code, which links to the website of the author's laboratory (LPQM).

method and read by plasmonic effect. **Figure 12(b)** illustrates a quick response (QR) code, which links to the website of the author's laboratory (LPQM). Bar code (1D) or QR code (2D) is a fast, easy, and accurate data storage method enabling products to be tracked efficiently and accurately. In particular, QR code attracts more attention in e-commerce because it also improves mobile user experience by convenient and easy operation.

changes of refractive index; therefore, such a sensor is not suitable for gas, chemical, and biological sensing applications. Recently, it has been discovered that when the MO active media is coupled with PNSs, the strong local EM fields produced through LSPR or SPP interact strongly with ambient magnetic materials. Consequently, the MO property of the magnetic media could be significantly enhanced at these LSPR/SPP resonance wavelengths [41, 42]. Since magnetic materials possess weak plasmonic property, most current PMO materials for sensors are layered films made by noble metal (Au, Al, and Ag) films and magnetic (Ni, Co, Fe, or magnetic insulators) films [43]. The DLW method is an excellent tool for fabrication on demand of arbitrary NHAs, whose plasmonic property and hence PMO performance may be significantly enhanced. It is expected that the PMO sensors have advantages over the cor-

Arbitrary Form Plasmonic Structures: Optical Realization, Numerical Analysis and Demonstration…

http://dx.doi.org/10.5772/intechopen.79236

81

In summary, this chapter reports systematically most aspects related to arbitrary plasmonic nanostructures, in particular those realized by the direct laser writing technique. In the first section, the optical properties of very basic nanostructures are completely investigated by using a well-known FDTD simulation method. Real fabricated metallic structures are also imported to a simulation model and calculated accurately. These investigations offer a short but understandable image of plasmonic properties of various nanostructures and guide for applications of plasmonic nanostructures in different domains. In the second section, the direct laser writing technique is demonstrated as an excellent method for realization of desired plasmonic nanostructures on demand. The fabrication of plasmonic nanostructures is demonstrated in two ways: indirectly via the use a polymeric template and directly by exploiting the optically induced local thermal effect. Any plasmonic microstructures with desired size, shape, and color were obtained by controlling the writing pattern and the exposure doses (laser power and writing speed). Finally, several important applications of plasmonic nanostructures, in particular those realized by direct laser writing method, have been demonstrated. Namely, the nano-holes arrays are demonstrated as excellent optical bandpass filters and also sensitive plasmonics-based sensor. The plasmonic nano-islands realized by optically induced thermal effect offered an excellent way for data storage and color nanoprinter. It is clear that those fabricated structures could be useful for a wide range of applications in numerous fields

This work is supported by a public grant overseen by the French National Research Agency (ANR) (project: GRATEOM) and by a public grant of Ministry of Science and Technology of

responding plasmonic sensors and will be fully explored.

**5. Conclusion**

(physics, chemistry, biology, etc.).

Vietnam (project: DTDLCN.01/2017).

**Acknowledgements**

Furthermore, as mentioned previously, the DLW technique allows production of any structures at nanoscale. **Figure 13** shows the result of a real image: a real "experiment book" of French laboratories. The real photo was imported to a MATLAB image, and each pixel was transferred to an exact dose of the light exposure, resulting in a plasmonic image of this "experiment book" at microscale. The image color is quite faithful to the original one, but theoretically and experimentally limited in the green and yellow color domain. That could be explained by the result shown in the simulation section, which predicted that the plasmonic resonance shifts only about 48 nm when the particles size changes from 20 to 100 nm. It is theoretically expected that a variety of colors could be obtained by organizing these Au NIs in order, like 1D and 2D PNSs [10, 11]. The combination of LSPR and PNS will offer a large possibility to tune the color.

### **4.3. About resonantly enhanced plasmonic magneto-optics**

The magneto-optical (MO) sensors are a powerful sensing platform based on the Faraday or Kerr effects, that is, the rotation of linearly polarized light when it passes through or reflects from a magnetic thin film under influence of an external magnetic field [40]. However, conventional MO sensor cannot be used as a refractometer since it is not sensitive to minute

**Figure 13.** The color printed image fabricated by the DLW method on Au film. Left: a real "experiment book". Right: a plasmonic image of corresponding book at microscale.

changes of refractive index; therefore, such a sensor is not suitable for gas, chemical, and biological sensing applications. Recently, it has been discovered that when the MO active media is coupled with PNSs, the strong local EM fields produced through LSPR or SPP interact strongly with ambient magnetic materials. Consequently, the MO property of the magnetic media could be significantly enhanced at these LSPR/SPP resonance wavelengths [41, 42]. Since magnetic materials possess weak plasmonic property, most current PMO materials for sensors are layered films made by noble metal (Au, Al, and Ag) films and magnetic (Ni, Co, Fe, or magnetic insulators) films [43]. The DLW method is an excellent tool for fabrication on demand of arbitrary NHAs, whose plasmonic property and hence PMO performance may be significantly enhanced. It is expected that the PMO sensors have advantages over the corresponding plasmonic sensors and will be fully explored.
