2 Ultrafast Laser Enabling Versatile Fabrication of Surface …
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of brightness on the four Cu surfaces in Fig. 2.32. Compared to the uniform micro
cone arrays, the disordered particle aggregates can absorb light more effectively.
Specifically, Structure 2 reaches higher light absorption than Structure 1 while
Structure 4 reaches higher light absorption than Structure 3. In particular, a steady
broadband spectrum with below 10% reflectance from UV to the infrared regions
is obtained in Structure 4. An average hemispherical reflectance of ~2% has been
realized in the spectrum of 200–800 nm, suggesting that ~98% of the irradiated solar
energy in this wavelength range can be collected by the structure. In contrast, the
light trapping capability of Structure 4 is even more significant than the commercial
antireflection blue coating over broader spectrum ranges.
Further, the specular reflectance under varied incident angles is measured. At
higher incident angles, the reflection spectra of the blue coating rise to higher levels
successively (see Fig. 2.33b). Compared to that, the reflection spectra of Structure 4,
namely the ultrafast laser fabricated cauliflower-shaped structure, stay at much lower
levels at all incident angles tested, with no increment being observed till the incident
angle of 50°. Even at the incident angle of 60°, its specular reflectance spectrum is
only around 0.1%. Such results illustrate that the SMNS fabricated by ultrafast laser,
particularly the cauliflower-shaped hierarchical structure, can benefit applications
of light absorption with their remarkable capabilities of broadband antireflection
without sensitivity to incident angles.
The enhanced light absorption properties of the ultrafast laser fabricated SMNS
are more intuitively validated by their thermograms under the irradiation of a solar
simulator (analogue to the AM 1.5 Global condition). As shown in Fig. 2.34, all the
copper sample surfaces look similar at the starting point under the infrared view.
Only slight contrast can be observed between the sample surfaces and the circumjacent environment. After being irradiated for 10 min, obvious differences on their
appearances are shown. The temperature rise on the polished Cu surface is very
insignificant. The blue coating gets a little more temperature rise but still far from
significant. For the SMNS fabricated by ultrafast laser, the grouping characteristics,
in accordance with discussed in their structural forms in Fig. 2.32, are also illustrated in their infrared thermograms. The uniform micro cone arrays obtain limited
temperature increments, while the disordered hierarchical particle aggregates manifest distinct temperature rises. The latter show bright white appearances instantly
when irradiated by the sunlight. Such performances persist at incident angles of 0–
60°. Although the heating capabilities of the ultrafast laser fabricated SMNS seem
to weaken at higher incident angles, the cauliflower-shaped structure presents the
most prominent heating effect all the time. Therefore, it is verified that the unique
cauliflower-shaped Cu SMNS is an efficient medium for absorbing solar light and
converting it to heat.
For further demonstrating the solar harvesting and the photothermal conversion
capabilities of the SMNS produced by ultrafast laser, a water evaporation experiment
has been conducted. As Fig. 2.35a shows, the copper samples are employed to absorb
the incident light and convert it to thermal energy. Driven by the heated Cu surfaces,
accelerated evaporation occurs in the surrounding water. Through such a water evaporation test, the light-to-heat conversion properties of the samples studied can be more
107
of brightness on the four Cu surfaces in Fig. 2.32. Compared to the uniform micro
cone arrays, the disordered particle aggregates can absorb light more effectively.
Specifically, Structure 2 reaches higher light absorption than Structure 1 while
Structure 4 reaches higher light absorption than Structure 3. In particular, a steady
broadband spectrum with below 10% reflectance from UV to the infrared regions
is obtained in Structure 4. An average hemispherical reflectance of ~2% has been
realized in the spectrum of 200–800 nm, suggesting that ~98% of the irradiated solar
energy in this wavelength range can be collected by the structure. In contrast, the
light trapping capability of Structure 4 is even more significant than the commercial
antireflection blue coating over broader spectrum ranges.
Further, the specular reflectance under varied incident angles is measured. At
higher incident angles, the reflection spectra of the blue coating rise to higher levels
successively (see Fig. 2.33b). Compared to that, the reflection spectra of Structure 4,
namely the ultrafast laser fabricated cauliflower-shaped structure, stay at much lower
levels at all incident angles tested, with no increment being observed till the incident
angle of 50°. Even at the incident angle of 60°, its specular reflectance spectrum is
only around 0.1%. Such results illustrate that the SMNS fabricated by ultrafast laser,
particularly the cauliflower-shaped hierarchical structure, can benefit applications
of light absorption with their remarkable capabilities of broadband antireflection
without sensitivity to incident angles.
The enhanced light absorption properties of the ultrafast laser fabricated SMNS
are more intuitively validated by their thermograms under the irradiation of a solar
simulator (analogue to the AM 1.5 Global condition). As shown in Fig. 2.34, all the
copper sample surfaces look similar at the starting point under the infrared view.
Only slight contrast can be observed between the sample surfaces and the circumjacent environment. After being irradiated for 10 min, obvious differences on their
appearances are shown. The temperature rise on the polished Cu surface is very
insignificant. The blue coating gets a little more temperature rise but still far from
significant. For the SMNS fabricated by ultrafast laser, the grouping characteristics,
in accordance with discussed in their structural forms in Fig. 2.32, are also illustrated in their infrared thermograms. The uniform micro cone arrays obtain limited
temperature increments, while the disordered hierarchical particle aggregates manifest distinct temperature rises. The latter show bright white appearances instantly
when irradiated by the sunlight. Such performances persist at incident angles of 0–
60°. Although the heating capabilities of the ultrafast laser fabricated SMNS seem
to weaken at higher incident angles, the cauliflower-shaped structure presents the
most prominent heating effect all the time. Therefore, it is verified that the unique
cauliflower-shaped Cu SMNS is an efficient medium for absorbing solar light and
converting it to heat.
For further demonstrating the solar harvesting and the photothermal conversion
capabilities of the SMNS produced by ultrafast laser, a water evaporation experiment
has been conducted. As Fig. 2.35a shows, the copper samples are employed to absorb
the incident light and convert it to thermal energy. Driven by the heated Cu surfaces,
accelerated evaporation occurs in the surrounding water. Through such a water evaporation test, the light-to-heat conversion properties of the samples studied can be more