6
A. Hu et al.
excited electrons can originate from the thermal excitation of impurity and/or from
tunneling and multiphoton ionization. The avalanche ionization causes the significant
increase of free electron density. Meanwhile, the laser heat enhances the plasma
frequency ω pe approaching the laser frequency. This subsequently yields a stronger
resonant absorption and improves the free electron density to a critical value n e =
π mc
2
/(e
2
λ
2
)(~10
22 cm
3 ).
For a practical photonic excitation with a high laser power, these ionizations may
synergically exist. For example, with a 100 fs pulse excitation, multiphoton ionization
creates a substantial amount of free electrons. When the electron density approaches
10
17 cm
−3 , the collisional ionization rate begins to exceed the multiphoton ionization
rate [31]. When this electron density approaches 10
22 cm
−3 , a critical electron density,
the reflectivity dramatically increases [32]. The maximum density was thus limited
to 10
22 cm
−3 , which corresponds to approximately 10% of the total valence-band
population [32].
A lattice softening has been theoretically expected when 10% of the valence electrons are excited into the conduction band [33, 34]. This lattice softening eventually
leads to lattice melting, as predicted by ab initio molecular-dynamic calculation [35,
36] and observed experimentally by time-resolved reflectivity [37]. Such melting
is an ultrafast phenomenon and is not a consequence of conventional thermal heat
transfer.
1.3 Laser-Nanomaterial Interaction
1.3.1 Scaling Law
Nanomanufacturing is further based on the laser-nanomaterials interaction. When
the size decreases from a macroscale to a nanoscale, some negligible properties at
a macroscopic world become dominant. Although some macroscopic principles are
still valid at a microsize, nanomaterials behave very unique. Specially, the quantum
effect is critical at an interatomic range or very close to the surface, i.e., within a
few nanometers distance. At this size range, a tiny nanoparticle is conventionally
named as a quantum dot. The scaling law is a useful method to observe nanoworld
based on our knowledge of the macroworld. The scaling law displaying the relation
between physical quantities, allows to investigate physical principles and variations
in the macro-, micro- and nanoworlds [38].
Size-Effect For a solid material, it is well known that surface atoms have fewer
bonds than internal atoms. Less energy is thus needed for them to leave the surface.
Considering n balls with an even diameter of R closely compacted in a 2D plane, it is
easily to calculated that the total volume is 4n/3R
3 , while the surface area is n R
2 . The
surface/volume ratio is 3/(4R), which will significantly increases when R decreases.
For a rough estimation, there are about 0.3% atoms presented at the surface for a
1 Â μm particle. However, 87.5% atoms will occupy the surface state in a 2 nm
A. Hu et al.
excited electrons can originate from the thermal excitation of impurity and/or from
tunneling and multiphoton ionization. The avalanche ionization causes the significant
increase of free electron density. Meanwhile, the laser heat enhances the plasma
frequency ω pe approaching the laser frequency. This subsequently yields a stronger
resonant absorption and improves the free electron density to a critical value n e =
π mc
2
/(e
2
λ
2
)(~10
22 cm
3 ).
For a practical photonic excitation with a high laser power, these ionizations may
synergically exist. For example, with a 100 fs pulse excitation, multiphoton ionization
creates a substantial amount of free electrons. When the electron density approaches
10
17 cm
−3 , the collisional ionization rate begins to exceed the multiphoton ionization
rate [31]. When this electron density approaches 10
22 cm
−3 , a critical electron density,
the reflectivity dramatically increases [32]. The maximum density was thus limited
to 10
22 cm
−3 , which corresponds to approximately 10% of the total valence-band
population [32].
A lattice softening has been theoretically expected when 10% of the valence electrons are excited into the conduction band [33, 34]. This lattice softening eventually
leads to lattice melting, as predicted by ab initio molecular-dynamic calculation [35,
36] and observed experimentally by time-resolved reflectivity [37]. Such melting
is an ultrafast phenomenon and is not a consequence of conventional thermal heat
transfer.
1.3 Laser-Nanomaterial Interaction
1.3.1 Scaling Law
Nanomanufacturing is further based on the laser-nanomaterials interaction. When
the size decreases from a macroscale to a nanoscale, some negligible properties at
a macroscopic world become dominant. Although some macroscopic principles are
still valid at a microsize, nanomaterials behave very unique. Specially, the quantum
effect is critical at an interatomic range or very close to the surface, i.e., within a
few nanometers distance. At this size range, a tiny nanoparticle is conventionally
named as a quantum dot. The scaling law is a useful method to observe nanoworld
based on our knowledge of the macroworld. The scaling law displaying the relation
between physical quantities, allows to investigate physical principles and variations
in the macro-, micro- and nanoworlds [38].
Size-Effect For a solid material, it is well known that surface atoms have fewer
bonds than internal atoms. Less energy is thus needed for them to leave the surface.
Considering n balls with an even diameter of R closely compacted in a 2D plane, it is
easily to calculated that the total volume is 4n/3R
3 , while the surface area is n R
2 . The
surface/volume ratio is 3/(4R), which will significantly increases when R decreases.
For a rough estimation, there are about 0.3% atoms presented at the surface for a
1 Â μm particle. However, 87.5% atoms will occupy the surface state in a 2 nm