**Figure 5.** The cross-sectional view in vicinity of the focal point and two different types of collimations.

### **3.2. Surface roughness on inner wall**

**Laser and its irradiation condition Cell**

**Pulse energy**  **Irradi ation time** 

320 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

[20] – – 1 Through

**Table 1.** Optical fiber sensors with a microhole produced by femtosecond laser drilling.

**Fabric ation process** 

direction

drilling

direction

The following sections show results obtained in our recent study regarding the fabrication of optical fiber sensors using femtosecond laser with high-fluence regimes. As shown in **Figures 5** and **6**, the microholes are intentionally designed and machined to easily guide a liquid sample into microhole working as a sensing area without immersion liquids during fabrication process. The sensor part has microhole array penetrating through the whole fiber core by drilling from both sides of the fiber cladding in order to avoid tapering the opening apertures

The microholes were fabricated with optical train, as illustrated in **Figure 5**. A multimode optical fiber (MMF, core diameter 62.5 µm) was irradiated with femtosecond pulses from two directions (not simultaneously) to avoid tapering of the inlet/outlet of the cell. The fundamental of a 1 kHz Ti-sapphire laser (IFRIT Cyber Laser Inc., 1 mJ/pulse, λ = 800 nm, 210 fs pulse duration) was converted to the second harmonic (240 µJ/pulse, 350 fs pulse duration) in a wavelength converter [second harmonic generation (SHG) unit, manufactured by Cyber Laser Inc.]. The second harmonic pulses were introduced to an objective lens through an optical train to guide and focus the beam at the target optical fiber. The collimating optics reduce the beam diameter from 6.0 to 2.8 mm so that a longer Rayleigh distance compared with the initial beam can be obtained using a negative and positive lens array. This prevents plasma generation in the air between the lens combination, which is important because the laser beam could be defocused and the energy could be lost by plasma creation during collimation and thereby affected by diffraction before the final focusing. An optical fiber irradiated was mounted on a three-dimensional motor-controlled translation stage equipped with a rotary mechanism for

**Lens NA** 

0.68 SM: 1 MM: 3

**Number of hole**

0.25 1 Dead-end

0.40 MM: 1, 10 Through

**Shape (taper angle)** 

(7°–10°)

10 s 8 6.3 15 s 11 12

> hole (4°–18°)

Through hole (1°–4°)

hole (3.7°)

**Hole diameter (µm)** 

**Roughness Volume**

6 – 1.2

15 >3 µm 60–80

20 300 nm (no evidence)

10 <500 nm 20

**(single cell) (pL)** 

40

**Repetition (kHz)** 

[19] 120 800 1 11 µJ 5 s Single

[21] 280 1030 100 1–5 µJ 5 min Helical

[22] 350 400 1 15 µJ 1.2 s Two

**3.1. Fabrication of microholes into optical fibers**

inlet/outlet of through hole.

**References Pulse** 

**width (fs)**

**Wave length (nm)** 

> **Figure 6(a)** and **(b)** indicates the schematic images of side and cross-sectional views of the through holes embedded in optical fiber lines, respectively. Based on the microscopic image showing the fabricated through hole (c), the hole diameters were found to be approximately 18 and 10 µm at the opening aperture and waist, respectively, the volume of which was calculated to 19.8 pL by assuming that its shape could be approximated by two truncated cones with taking into account the taper angle of the microhole. Optical micrographs of the through hole of the side and top view are presented in **Figure 6(c)** and **(d)**, respectively, showing that the microholes are connected together so as to produce a through hole. Taking a look at the inside of the through hole by using scanning electron microscope (SEM) as shown in **Figure 6(e)**, a morphology modification was formed to be granulated structure, which have been reported so far in the previous works as mentioned above in the femtosecond regime [41]. The sample of **Figure 6(e)** was prepared in such a way that the inner surface of a part of hole was exposed by intentionally cleaving the fabricated fiber along an off-axis plane of the through hole to clearly see the rugged surface with SEM. The rough surface could be generated with folded debris or microrims surrounding laser-induced craters by successive laser pulses. Importantly, the size of the rugged particle is found to be approximately a few hundred nanometers. Such rough surface gives more diffusive reflection on the hole boundary, hence higher optical scattering, especially for shorter-wavelength light wave. Additionally, it should also be noted that such particle-like structure will be more water repellent to a liquid by which water inside becomes easier to flow. As shown in **Table 1**, the surface roughness should be depended on the fabrication method. The shorter wavelength laser irradiation can reduce the

increasing thermal debris and its redeposition on the inner surface, the fact of which can be seen from the comparison between Refs. [20] and [22].

**Figure 6.** The schematic drawings of microholes in a fiber-optic line monitored from side view (a) and the cross section (b). The photographs of (c) and (d) give examples which show the shape of the through hole and the hole opening, respectively. (e) SEM picture of a part of hole. Adapted with permission from Ref. [22].

### **3.3. Influences of surface roughness for sensing**

To evaluate the intrusion/discharge velocity of liquids sucked into/drained out from a through hole, the real-time response of optical intensity change was measured in such a way that the sensing part with a single through hole is alternatively immersed in water and ethanol as indicated in **Figure 7**. Sample liquids were immediately sucked into microholes by capillary driving force and as was firstly confirmed by monitoring the optical intensity of transmitting light and staining the microhole with a color dye simultaneously. As soon as the sensing part was lifted from a liquid pool, the liquid held in the microhole seems to be immediately drained out in a few seconds because of the hydrophobic-repellent structure.

**Figure 8** shows transmitting spectra to investigate sensing response for RI changes in a wavelength between 400 and 1000 nm. In this measurement, two types of sensor samples which had (a) a single cell and (b) 10 cells were prepared to compare the spectroscopic measurement between them. Taking a look at **Figure 8(b)**, remarkable decreases in transmission can be confirmed in a shorter wavelength range (λ < 600 nm), because light transmitted through the fiber core was scattered at the hole surfaces by the fact that the wavelength of transmitted light becomes approximately equal to the scale of surface roughness. By reducing or controlling the surface roughness on the inner wall, sensing performances of fiber optics with an inline sample cell in terms of sensitivity, reproducibility, response time, and recovery time as well as versatility for sensing target could be improved for practical use. High-quality surface finish therefore is required for the improvement of the sensing performances even in high-fluence regimes. The roughness with a periodicity of <100 nm could be useful for reducing the scattering in a shorter wavelength range, by roughly estimating the reflectivity on the inner wall based on Mie-scattering effect.

increasing thermal debris and its redeposition on the inner surface, the fact of which can be

322 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

**Figure 6.** The schematic drawings of microholes in a fiber-optic line monitored from side view (a) and the cross section (b). The photographs of (c) and (d) give examples which show the shape of the through hole and the hole opening,

To evaluate the intrusion/discharge velocity of liquids sucked into/drained out from a through hole, the real-time response of optical intensity change was measured in such a way that the sensing part with a single through hole is alternatively immersed in water and ethanol as indicated in **Figure 7**. Sample liquids were immediately sucked into microholes by capillary driving force and as was firstly confirmed by monitoring the optical intensity of transmitting light and staining the microhole with a color dye simultaneously. As soon as the sensing part was lifted from a liquid pool, the liquid held in the microhole seems to be immediately drained

**Figure 8** shows transmitting spectra to investigate sensing response for RI changes in a wavelength between 400 and 1000 nm. In this measurement, two types of sensor samples which had (a) a single cell and (b) 10 cells were prepared to compare the spectroscopic measurement

respectively. (e) SEM picture of a part of hole. Adapted with permission from Ref. [22].

out in a few seconds because of the hydrophobic-repellent structure.

**3.3. Influences of surface roughness for sensing**

seen from the comparison between Refs. [20] and [22].

**Figure 7.** (a) Experimental setup to measure intrusion/discharge velocity of liquids. (b) The real-time response of optical intensity change measured by alternatively immersing in water and ethanol. (c) and (d) show one of a response waveform using ethanol and water, respectively.

According to the experimental data [19–22] (summarized in **Table 1**), the roughness might be reduced by immersing an optical fiber in water or immersion oils even though the fabrication period becomes unfavorably much longer. Instead of using liquid immersion processing, we have been trying to reduce the roughness using a NUV 400-nm femtosecond laser so as to achieve a high-quality inner surface of microhole in a high-throughput fashion. **Figure 9** shows SEM pictures of the inner surface of a dead-end microhole fabricated in a flat silica glass plate. The experiment was performed to make a smooth surface on the inner wall of the hole with varying laser irradiation parameters. As can be seen from **Figure 9(a)**, wave-like nanostructures with a spatial periodicity of 100–200 nm are observed on the bottom of the hole. **Figure 9(b)** and **(c)**, respectively, shows the inner surface of the hole at different depths near and far from the hole inlet. Surface roughness is still observed at the periphery of the inlet of **Figure 9(b)** because the surface roughness could be worse as the number of irradiated pulses becomes greater. On the other hand, the case of the inner surface **Figure 8(c)** seems to be much smoother than the surface (b). To figure out the formation mechanism of the granulated surface in a highenergy regime, further experiment was carried out on this point.

**Figure 8.** Transmission spectra for the case of sensor samples (a) single cell and (b) 10 cells when immersion in three types of liquids. Adapted with permission from Ref. [22].

The surface morphology of shallow craters was generated by single and multiple pulses with an emphasis on rims surrounding the craters. In this experiment, shallow craters were formed on a fused silica plate by overlapping focused laser pulses on the same spot as shown in **Figure 10**. The shape of the crater created in this experiment is depended on incident laser beam, the pattern of which is deformed through a second harmonic generator (SHG unit manufactured by Cyber Laser Inc.). A crater surrounded by a single rim was observed by SEM analysis of **Figure 11(b)**. The average thickness of the first rim is approximately 500 nm. **Figure 11(c)** shows an overlapped crater created by two pulses. Molten materials are driven away from the crater to almost 10 µm distance. The second image (**Figure 11(d)**) shows that a new rim is formed inside the first one by overlapping a second pulse. The distance between the two rims is approximately equal to the wavelength of the irradiated laser beam. We also found a belt-shape structure on a part of the rim. The composition and formation mechanism still need to be explored. For the case of **Figure 11(f)**: overlapping three pulses, it was interestingly found that the height of third rim is comparatively low simultaneously accompanied with nanofibers [79–81] with a diameter ranging from a few to few tens of nanometers. It should be noted that, instead of the growth of the rim, nanofibers were grown by only a few pulses. In order to obtain optimum surface conditions for the fiber-optic inline spectrometer, further investigations are in progress to improve the surface finish.

varying laser irradiation parameters. As can be seen from **Figure 9(a)**, wave-like nanostructures with a spatial periodicity of 100–200 nm are observed on the bottom of the hole. **Figure 9(b)** and **(c)**, respectively, shows the inner surface of the hole at different depths near and far from the hole inlet. Surface roughness is still observed at the periphery of the inlet of **Figure 9(b)** because the surface roughness could be worse as the number of irradiated pulses becomes greater. On the other hand, the case of the inner surface **Figure 8(c)** seems to be much smoother than the surface (b). To figure out the formation mechanism of the granulated surface in a high-

324 Applications of Laser Ablation - Thin Film Deposition, Nanomaterial Synthesis and Surface Modification

**Figure 8.** Transmission spectra for the case of sensor samples (a) single cell and (b) 10 cells when immersion in three

The surface morphology of shallow craters was generated by single and multiple pulses with an emphasis on rims surrounding the craters. In this experiment, shallow craters were formed on a fused silica plate by overlapping focused laser pulses on the same spot as shown in **Figure 10**. The shape of the crater created in this experiment is depended on incident laser beam, the pattern of which is deformed through a second harmonic generator (SHG unit manufactured by Cyber Laser Inc.). A crater surrounded by a single rim was observed by SEM analysis of **Figure 11(b)**. The average thickness of the first rim is approximately 500 nm. **Figure 11(c)** shows an overlapped crater created by two pulses. Molten materials are driven away from the crater to almost 10 µm distance. The second image (**Figure 11(d)**) shows that a new rim is formed inside the first one by overlapping a second pulse. The distance between the two rims is approximately equal to the wavelength of the irradiated laser beam. We also found a belt-shape structure on a part of the rim. The composition and formation mechanism still need to be explored. For the case of **Figure 11(f)**: overlapping three pulses, it was interestingly found that the height of third rim is comparatively low simultaneously accompanied with nanofibers [79–81] with a diameter ranging from a few to few tens of nanometers. It should be noted that, instead of the growth of the rim, nanofibers were grown by only a few pulses. In order to obtain optimum surface conditions for the fiber-optic inline spectrometer, further

energy regime, further experiment was carried out on this point.

types of liquids. Adapted with permission from Ref. [22].

investigations are in progress to improve the surface finish.

**Figure 9.** SEM images of a microhole generated on a flat silica glass plate, by successive laser pulses of the energy of 20 µJ and using focusing optics (inserted table (b) in **Figure 5**). (a) The whole fabricated microhole at 5000× magnification and higher resolution SEM pictures at hole bottom (b), the side wall near the hole inlet (c), and the side wall near the bottom (d).

**Figure 10.** The schematic image of crater rims produced by overlapping focused laser pulses on the same spot. (a) One laser pulse. (b) Two overlapping pulses.

**Figure 11.** SEM images of shallow craters with thin rim generated on a flat silica glass plate with varying the number of pulses. The whole crater at 5000× magnification (a, c, e) and higher resolution (b, d, f).
