**3. Realization of desired plasmonic nanostructures**

Different fabrication methods of PNSs have been demonstrated, each possessing its own advantages and drawbacks. For each application, particular metallic NPs or PNSs are required and then a specific fabrication method could be adopted. To form Au NPs and NSs in a small area but with a desired configuration, electron-beam lithography [35, 36] and direct laser writing (DLW) method [37] are two best choices and both are commercially available. Generally, these two methods allow creating PNSs by an indirect way, namely, PNSs are obtained by evaporation of metals on polymeric templates and lift-off. Recently, a direct fabrication method was also demonstrated by using optically induced thermal effect via DLW technique [38]. In this section, the use of DLW will be presented in detail for realization of desired PNSs indirectly and directly.

### **3.1. Direct laser writing method**

**Figure 5(a)** illustrates the experimental setup of the DLW system. For realization of 2D PNSs, this DLW involves with a one-photon absorption (OPA) mechanism by using a continuouswave (cw) laser beam. The sample is a commercial positive photoresist, S1805, for fabrication of PNS by the indirect way. This setup is also used to realize PNSs by the direct way using a sputtered Au film sample. The laser beam, whose wavelength (532 nm) locates the absorption

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**Figure 5.** (a) Illustration of the DLW technique used to realize arbitrary 2D structures on photoresist and Au film. PZT: piezoelectric translator; DM: dichroic mirror; OL: objective lens. (b) Control of filling factor of structures fabricated on a positive photoresist by adjusting the exposure dose. A 2D square structure is obtained by scanning continuously the focal spot in x- and y-directions. Top: theoretical light pattern; bottom: experimental demonstration. The separation between lines, that is, the period of structure, is Λ = 0.8 μm, and the structures change from negative (air-holes) to positive (polymeric cylinders) forms.

spectrum of both S1805 and Au materials, is tightly focused into samples by a high numerical aperture (NA) objective lens (OL). Since the DLW operates with an OPA mechanism, the required laser power is very modest, in the range of few microwatts for S1805 photoresist and a few dozens of milliwatts for Au films. Thanks to the use of a high NA OL, the light intensity at the focusing region is, however, very high, which is enough for depolymerizing the S1805 photoresist and thermally dewetting the Au films. 3D piezoelectric translator (PZT) connected to a computer control allows the focusing spot to move through the sample following a desired trajectory. By controlling the laser power and the exposure time, the exposure doses are adjusted resulting in structures with desired sizes and forms, as illustrated in **Figure 5(b)**. A detection system consisting of a lenses ensemble, a pinhole, and an avalanche photodiode is used to determine the focusing position, which should be practically located on substrate surface. It should be noted that this DLW is time consuming, like e-beam lithography. Also, in order to keep high resolution, the total area of fabricated structures is limited, generally of about 100x100 μm2 . This surface should be enough for various applications, and in case necessary, it could be enlarged by using a PZT with a larger scanning range, together with an increase of fabrication time.

### **3.2. Realization of plasmonic structure by an indirect method**

band selection is poor. Conversely, smaller holes allow fewer transmission resonance modes, which make transmission peak sharper but also decrease the transmission coefficients. Based on this insight along with advantages and disadvantages of the direct laser writing method, an optimum NHA should have the following parameters: tAu = 50 nm, tCr = 3 nm, Λ = 1000 nm,

**Figure 4.** Calculated transmission spectra of Au NHAs as a function of Au layer thickness, tAu (a); of nano-hole diameter,

Different fabrication methods of PNSs have been demonstrated, each possessing its own advantages and drawbacks. For each application, particular metallic NPs or PNSs are required and then a specific fabrication method could be adopted. To form Au NPs and NSs in a small area but with a desired configuration, electron-beam lithography [35, 36] and direct laser writing (DLW) method [37] are two best choices and both are commercially available. Generally, these two methods allow creating PNSs by an indirect way, namely, PNSs are obtained by evaporation of metals on polymeric templates and lift-off. Recently, a direct fabrication method was also demonstrated by using optically induced thermal effect via DLW technique [38]. In this section, the use of DLW will be presented in detail for realization of

**Figure 5(a)** illustrates the experimental setup of the DLW system. For realization of 2D PNSs, this DLW involves with a one-photon absorption (OPA) mechanism by using a continuouswave (cw) laser beam. The sample is a commercial positive photoresist, S1805, for fabrication of PNS by the indirect way. This setup is also used to realize PNSs by the direct way using a sputtered Au film sample. The laser beam, whose wavelength (532 nm) locates the absorption

and dhole = 400 nm, respectively.

72 Plasmonics

dhole (b); and of Cr layer thickness, tCr, (c), respectively.

desired PNSs indirectly and directly.

**3.1. Direct laser writing method**

**3. Realization of desired plasmonic nanostructures**

The indirect fabrication of PNSs consists of two steps: (i) fabrication of photoresist templates by DLW method and (ii) transferring templates to metallic structures by evaporation of method and template lift-off. **Figure 6(a)** illustrates the fabrication process of Au NSs by this indirect method. This process is very similar to the fabrication of PNSs by e-beam lithography [35–37]. To fabricate desired structures, the positive photoresist was first coated on cleaned glass substrates and exposed by the DLW system. The samples were then developed, removing all exposed parts and leaving unexposed parts as desired structures. It is demonstrated that the final structures depend strongly on the exposure dose, that is, the laser intensity and the exposure time. Therefore, the filling factor is controlled precisely and both air-holes and dielectric-cylinder structures can be easily and reliably realized (**Figure 5(b)**).

**3.3. Direct laser writing of gold nanostructures**

In Section 2.3, it is shown that the thermal annealing dewetting technique is a simple and cheap way to realize Au NSs [19, 26, 27]. Conventionally, the dewetting effect is performed on a hot plate or in an oven with an annealing temperature of about 500°C, which results in a large Au NIs areas. In the above section, it is demonstrated that that DLW is a very efficient method that allows the realization of any NS on demand. Recently, this technique is also demonstrated as an excellent method to realize PNSs consisting of Au NPs [38]. For that, the DLW technique employed a cw laser to generate a strong and local heating effect in Au material. This is called optically induced thermal effect by the DLW. This opens up numerous

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A theoretical model is proposed to investigate the optically induced thermal effect at the focusing spot of the optical system. Physically, due to the strong absorption of the Au material at the excitation wavelength and thanks to the optical intensity distribution, a temperature distribution is produced at the focusing region. **Figure 7** illustrates a tightly focused light beam inside an absorbing medium and the light intensity distribution as well as the heat profile at the focusing region. The light intensity distribution (**Figure 7(b)**) is rigorously calculated by using the vector Debye method [39]. By using finite element method with MATLAB PDE solver, the thermal effect model is numerically solved providing the heat profile as shown in **Figure 7(c)**. By using a DLW method with a cw laser, it is theoretically demonstrated that the induced temperature at the focusing spot rises up quickly as a function of exposure time

**Figure 7.** (a) Illustration of a tightly focused light beam inside an absorbing medium. (b) Theoretical calculation of light intensity distribution in xz-plane at the focal region of a high NA OL. (c) Corresponding temperature distribution at the focal region. (d) Temperature pattern (NANO letter) obtained by scanning the focusing spot in xy-plane. (e) Illustration

of the formation of arbitrary Au NSs by optically induced thermal effect.

applications of PNSs, which could be experimentally achievable at low cost.

The thin Au film (typical thickness is about 50 nm) is then deposited on the top surface of the polymeric structures by a thermal evaporation technique. As indicated previously, a thin layer of Cr (3 nm) was sputtered in between polymeric templates and the Au film in order to enhance the adhesion of the Au film. The samples were then immersed in acetone to remove the polymeric template, leaving the Au PNSs.

**Figure 6(b)**–**(d)** shows some examples of 2D PNSs. Depending on the polymeric templates, it is possible to obtain both Au films containing air-holes, that is, NHAs and Au microdisks. The minimum size of the Au microdisks or air-holes is about 300 nm, which is larger than the size limit (about 100 nm) of the polymeric structure fabricated by the DLW. That is because when the polymeric holes or cylinders are small (100 nm), their walls are not vertical and the evaporated Au film is connected between the top and bottom parts of the polymeric structures, influencing the lift-off process. Besides, due to the diffraction limit of the DLW system and also the quality of the positive photoresist during development process, the size of fabricated metallic structures is still large, as compared to those realized by the e-beam lithography technique. A strong effort is currently being made to realize polymeric structures with smaller size and vertical wall, by using, for example, an OL having higher NA, a super-resolution technique to reduce the focusing spot, or a laser with shorter wavelength. Finally, as the most major advantage of the DLW, arbitrary Au NSs can be also realized as shown in **Figure 6(d)**.

**Figure 6.** (a) Fabrication of plasmonic structures by using polymeric templates. The S1805 structures with controllable period and filling factor are fabricated by the DLW technique. Plasmonic structures are then obtained by a combination of Au evaporation and lift-off techniques. (b–d) SEM images of experimental plasmonic structures: (b) 2D periodic airholes Au array; (c) periodic array of Au submicrometer disks; (d) arbitrary Au structure.

### **3.3. Direct laser writing of gold nanostructures**

that the final structures depend strongly on the exposure dose, that is, the laser intensity and the exposure time. Therefore, the filling factor is controlled precisely and both air-holes and

The thin Au film (typical thickness is about 50 nm) is then deposited on the top surface of the polymeric structures by a thermal evaporation technique. As indicated previously, a thin layer of Cr (3 nm) was sputtered in between polymeric templates and the Au film in order to enhance the adhesion of the Au film. The samples were then immersed in acetone to remove

**Figure 6(b)**–**(d)** shows some examples of 2D PNSs. Depending on the polymeric templates, it is possible to obtain both Au films containing air-holes, that is, NHAs and Au microdisks. The minimum size of the Au microdisks or air-holes is about 300 nm, which is larger than the size limit (about 100 nm) of the polymeric structure fabricated by the DLW. That is because when the polymeric holes or cylinders are small (100 nm), their walls are not vertical and the evaporated Au film is connected between the top and bottom parts of the polymeric structures, influencing the lift-off process. Besides, due to the diffraction limit of the DLW system and also the quality of the positive photoresist during development process, the size of fabricated metallic structures is still large, as compared to those realized by the e-beam lithography technique. A strong effort is currently being made to realize polymeric structures with smaller size and vertical wall, by using, for example, an OL having higher NA, a super-resolution technique to reduce the focusing spot, or a laser with shorter wavelength. Finally, as the most major advantage of the DLW, arbitrary Au NSs can be also realized as shown in **Figure 6(d)**.

**Figure 6.** (a) Fabrication of plasmonic structures by using polymeric templates. The S1805 structures with controllable period and filling factor are fabricated by the DLW technique. Plasmonic structures are then obtained by a combination of Au evaporation and lift-off techniques. (b–d) SEM images of experimental plasmonic structures: (b) 2D periodic air-

holes Au array; (c) periodic array of Au submicrometer disks; (d) arbitrary Au structure.

dielectric-cylinder structures can be easily and reliably realized (**Figure 5(b)**).

the polymeric template, leaving the Au PNSs.

74 Plasmonics

In Section 2.3, it is shown that the thermal annealing dewetting technique is a simple and cheap way to realize Au NSs [19, 26, 27]. Conventionally, the dewetting effect is performed on a hot plate or in an oven with an annealing temperature of about 500°C, which results in a large Au NIs areas. In the above section, it is demonstrated that that DLW is a very efficient method that allows the realization of any NS on demand. Recently, this technique is also demonstrated as an excellent method to realize PNSs consisting of Au NPs [38]. For that, the DLW technique employed a cw laser to generate a strong and local heating effect in Au material. This is called optically induced thermal effect by the DLW. This opens up numerous applications of PNSs, which could be experimentally achievable at low cost.

A theoretical model is proposed to investigate the optically induced thermal effect at the focusing spot of the optical system. Physically, due to the strong absorption of the Au material at the excitation wavelength and thanks to the optical intensity distribution, a temperature distribution is produced at the focusing region. **Figure 7** illustrates a tightly focused light beam inside an absorbing medium and the light intensity distribution as well as the heat profile at the focusing region. The light intensity distribution (**Figure 7(b)**) is rigorously calculated by using the vector Debye method [39]. By using finite element method with MATLAB PDE solver, the thermal effect model is numerically solved providing the heat profile as shown in **Figure 7(c)**. By using a DLW method with a cw laser, it is theoretically demonstrated that the induced temperature at the focusing spot rises up quickly as a function of exposure time

**Figure 7.** (a) Illustration of a tightly focused light beam inside an absorbing medium. (b) Theoretical calculation of light intensity distribution in xz-plane at the focal region of a high NA OL. (c) Corresponding temperature distribution at the focal region. (d) Temperature pattern (NANO letter) obtained by scanning the focusing spot in xy-plane. (e) Illustration of the formation of arbitrary Au NSs by optically induced thermal effect.

and reaches a stable temperature, approximately 500°C, with an excitation laser power of 40 mW. By moving the focusing spot following a desired trajectory, it is possible to create an arbitrary pattern, such as the "NANO" letter as shown in **Figure 7(d)**. This high temperature can significantly change the morphologies and optical responses of the Au thin film [38]. This thus allows one to realize in a direct way the desired PNSs.

The DLW setup used to directly realize PNSs is same as that illustrated in **Figure 5(a)**, except that the photoresist sample is replaced by a sputtered Au thin sample. These samples were prepared on pretreated glass substrates by a magnetron sputtering technique, with an Au thickness of about 12 nm. The laser focusing spot is moved in 2D space (xy-plane) following a desired trajectory. The typical laser power is a range of 40 mW, which results in a local light intensity of about 1011 W∕m<sup>2</sup> . This high intensity induces a local temperature of about 500°C, which could then locally form Au NIs via the dewetting effect. Using an appropriate laser power, the reasonable exposure times are about 1–100 ms for a single spot or the scanning speeds are about 1–50 μm∕s for patterns.

First of all, the optically induced thermal effect is demonstrated by the creation of Au NIs as a function of the scanning speed and of the laser power. In order to obtain a well-defined area of Au NPs, the raster scan is used by the DLW technique. **Figure 8(a)** shows the optical microscope image of the fabricated samples. As the laser exposure dose changes, the color of Au NIs samples changes, suggesting a variation of the NIs sizes. **Figure 8(b)** shows the SEM image confirming the creation of Au NPs, whose size was estimated by a histogram to be about 60 nm. **Figure 8(c)** shows the measured absorption spectra of the corresponding samples, obtained before and after light exposure. The plasmonic resonance peak is clearly observed at a wavelength of about 550 nm.

It is worth to mention that there is some limit for the laser intensities, otherwise the induced temperature becomes too high, in particular after the formation of NPs, resulting in an explosion effect. **Figure 9** shows the effect of exposure dose, that is, laser intensity, on the formation of Au NPs. First, when the exposure dose is increased, the induced temperature increased resulting in the formation of well-separated Au NPs (**Figure 9(b)**). However, if the exposure dose increased strongly, the Au NPs exploded resulting in very small Au NPs (**Figure 9(c)**). When the laser intensity became too high, the Au material totally evaporated. Thus, by controlling the fabrication parameters, the morphology of Au NIs could be varied, which is quite important for further development of this method in various applications such as data stor-

**Figure 9.** Dependence of the exposure dose on the formation of Au NPs. (a) SEM image of an Au discontinuous film, sputtered on a glass substrate. (b), (c) SEM images of Au NPs obtained by scanning a focused laser beam with a power of 40 mW and with scanning velocities of 20 μm/s (b) and 0.5 μm/s (c), respectively. Bottom line: Illustrations of Au film

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Indeed, thanks to the flexibility and versatility of the laser induced local thermal dewetting method, it is possible to write PNSs at high levels of complexities. **Figure 10** shows two

**Figure 10.** Optical microscope images of plasmonic patterns consist of Au NPs, realized by the optically induced thermal

effect via DLW technique: "NANO" letter and "Mario" image. In the left: a SEM image of the Au NPs.

age, plasmonic band-pass filter, and color printing.

morphological transformation and mechanism as a function of the exposure dose.

**Figure 8.** (a) Optical microscope image of Au NPs samples realized by using different exposure doses (laser powers and scanning speeds) of the DLW. (b) SEM image of Au NPs sample. (c) Experimental measurement of the plasmon resonance spectrum of the Au NSs obtained before and after laser exposure.

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**Figure 9.** Dependence of the exposure dose on the formation of Au NPs. (a) SEM image of an Au discontinuous film, sputtered on a glass substrate. (b), (c) SEM images of Au NPs obtained by scanning a focused laser beam with a power of 40 mW and with scanning velocities of 20 μm/s (b) and 0.5 μm/s (c), respectively. Bottom line: Illustrations of Au film morphological transformation and mechanism as a function of the exposure dose.

It is worth to mention that there is some limit for the laser intensities, otherwise the induced temperature becomes too high, in particular after the formation of NPs, resulting in an explosion effect. **Figure 9** shows the effect of exposure dose, that is, laser intensity, on the formation of Au NPs. First, when the exposure dose is increased, the induced temperature increased resulting in the formation of well-separated Au NPs (**Figure 9(b)**). However, if the exposure dose increased strongly, the Au NPs exploded resulting in very small Au NPs (**Figure 9(c)**). When the laser intensity became too high, the Au material totally evaporated. Thus, by controlling the fabrication parameters, the morphology of Au NIs could be varied, which is quite important for further development of this method in various applications such as data storage, plasmonic band-pass filter, and color printing.

Indeed, thanks to the flexibility and versatility of the laser induced local thermal dewetting method, it is possible to write PNSs at high levels of complexities. **Figure 10** shows two

**Figure 10.** Optical microscope images of plasmonic patterns consist of Au NPs, realized by the optically induced thermal effect via DLW technique: "NANO" letter and "Mario" image. In the left: a SEM image of the Au NPs.

**Figure 8.** (a) Optical microscope image of Au NPs samples realized by using different exposure doses (laser powers and scanning speeds) of the DLW. (b) SEM image of Au NPs sample. (c) Experimental measurement of the plasmon

and reaches a stable temperature, approximately 500°C, with an excitation laser power of 40 mW. By moving the focusing spot following a desired trajectory, it is possible to create an arbitrary pattern, such as the "NANO" letter as shown in **Figure 7(d)**. This high temperature can significantly change the morphologies and optical responses of the Au thin film [38]. This

The DLW setup used to directly realize PNSs is same as that illustrated in **Figure 5(a)**, except that the photoresist sample is replaced by a sputtered Au thin sample. These samples were prepared on pretreated glass substrates by a magnetron sputtering technique, with an Au thickness of about 12 nm. The laser focusing spot is moved in 2D space (xy-plane) following a desired trajectory. The typical laser power is a range of 40 mW, which results in a local light

which could then locally form Au NIs via the dewetting effect. Using an appropriate laser power, the reasonable exposure times are about 1–100 ms for a single spot or the scanning

First of all, the optically induced thermal effect is demonstrated by the creation of Au NIs as a function of the scanning speed and of the laser power. In order to obtain a well-defined area of Au NPs, the raster scan is used by the DLW technique. **Figure 8(a)** shows the optical microscope image of the fabricated samples. As the laser exposure dose changes, the color of Au NIs samples changes, suggesting a variation of the NIs sizes. **Figure 8(b)** shows the SEM image confirming the creation of Au NPs, whose size was estimated by a histogram to be about 60 nm. **Figure 8(c)** shows the measured absorption spectra of the corresponding samples, obtained before and after light exposure. The plasmonic resonance peak is clearly

. This high intensity induces a local temperature of about 500°C,

thus allows one to realize in a direct way the desired PNSs.

intensity of about 1011 W∕m<sup>2</sup>

76 Plasmonics

speeds are about 1–50 μm∕s for patterns.

observed at a wavelength of about 550 nm.

resonance spectrum of the Au NSs obtained before and after laser exposure.

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 seen by different colors.

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

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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 sen-

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

**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

and compared to predicted simulation results.

sor if the surrounding medium of the NHA changes.

(LPQM).

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

trajectory of laser scanning to directly write data on metallic materials.
