102
M. Zhong and P. Fan
Furthermore, attributing to the isotropic and symmetric distribution of the Cu
SMNS, this broadband infrared antireflection property shall be independent from
the incident angles or polarizations of the incoming light. A simulation study on the
hemispherical reflectance of the Cu micro cone structures with oxide nanowires at
oblique incident angles has been conducted. Also, the specular reflectance of the same
Cu SMNS has been experimentally measured at variable incidence angles in the MIR
region. The results validate that neither the simulated hemispherical reflectance nor
the measured specular reflectance have apparent change at different incident angles
up to 60°. It is demonstrated that the multiscale hierarchical structures fabricated
via the top-down and bottom-up combined approach can provide us unique infrared
antireflection properties.
2.6.2 Mechanism of Enhanced IR Antireflection
of Multiscale Structures
Within the macro-micronano-nanowire multiscale structures, the relatively larger
features, e.g., micro cones, function as the skeleton of the entire surface architectures. Since these micro cones have spacings between them that are larger than
the wavelengths of incident light investigated in this chapter, they can perform the
multiple internal reflection and geometrical light trapping effect, which is the main
reason accounting for the improved antireflection properties of ultrafast laser fabricated SMNS compared to the polished surfaces. Specifically, the number that the
internal reflection occurs for an incident light before escaping the SMNS is determined by their geometrical dimensions. And the overall reflectance of the SMNS
should be the product of reflectance for each internal reflection of the incident light
within the SMNS.
Here, for the sake of brevity, we assume that the reflectance for each internal
reflection is the same, termed as R0. Then, the difference between R0 of the micro
cone arrays without nanowires and that with nanowires is mainly triggered by the
oxide nanowire features. It is known that the oxide nanowires can induce phonon
dissipation and eliminate the energy of incident photons, resulting in a lower R0.
Then, the reduced R0 gets multiplied through the multiple internal reflection among
the micro structures produced by ultrafast laser, reaching a much lower overall surface
reflectance, as schematically indicated by the thickness difference of the arrows in
Fig. 2.29c, d.
The macro-micronano-nanowire multiscale structures combine the phonon dissipation effect of the oxide nanowires with the light trapping effect of the larger scale
metal structural features. Indeed, it is reasonable to regard the whole multiscale structure as an effective medium between the metal surface and the free space. The oxide
nanowires are metallurgically connected to the bulk metal through the micro-nano
structures produced by ultrafast laser. A gradual and seamless transition in structural features from the nanowires to the micro-nanostructures and to the original
M. Zhong and P. Fan
Furthermore, attributing to the isotropic and symmetric distribution of the Cu
SMNS, this broadband infrared antireflection property shall be independent from
the incident angles or polarizations of the incoming light. A simulation study on the
hemispherical reflectance of the Cu micro cone structures with oxide nanowires at
oblique incident angles has been conducted. Also, the specular reflectance of the same
Cu SMNS has been experimentally measured at variable incidence angles in the MIR
region. The results validate that neither the simulated hemispherical reflectance nor
the measured specular reflectance have apparent change at different incident angles
up to 60°. It is demonstrated that the multiscale hierarchical structures fabricated
via the top-down and bottom-up combined approach can provide us unique infrared
antireflection properties.
2.6.2 Mechanism of Enhanced IR Antireflection
of Multiscale Structures
Within the macro-micronano-nanowire multiscale structures, the relatively larger
features, e.g., micro cones, function as the skeleton of the entire surface architectures. Since these micro cones have spacings between them that are larger than
the wavelengths of incident light investigated in this chapter, they can perform the
multiple internal reflection and geometrical light trapping effect, which is the main
reason accounting for the improved antireflection properties of ultrafast laser fabricated SMNS compared to the polished surfaces. Specifically, the number that the
internal reflection occurs for an incident light before escaping the SMNS is determined by their geometrical dimensions. And the overall reflectance of the SMNS
should be the product of reflectance for each internal reflection of the incident light
within the SMNS.
Here, for the sake of brevity, we assume that the reflectance for each internal
reflection is the same, termed as R0. Then, the difference between R0 of the micro
cone arrays without nanowires and that with nanowires is mainly triggered by the
oxide nanowire features. It is known that the oxide nanowires can induce phonon
dissipation and eliminate the energy of incident photons, resulting in a lower R0.
Then, the reduced R0 gets multiplied through the multiple internal reflection among
the micro structures produced by ultrafast laser, reaching a much lower overall surface
reflectance, as schematically indicated by the thickness difference of the arrows in
Fig. 2.29c, d.
The macro-micronano-nanowire multiscale structures combine the phonon dissipation effect of the oxide nanowires with the light trapping effect of the larger scale
metal structural features. Indeed, it is reasonable to regard the whole multiscale structure as an effective medium between the metal surface and the free space. The oxide
nanowires are metallurgically connected to the bulk metal through the micro-nano
structures produced by ultrafast laser. A gradual and seamless transition in structural features from the nanowires to the micro-nanostructures and to the original