1 Nanoplasmonics: From Present into Future
29
1.3.6 Experimental Examples of Nanoplasmonic Hot Spots
There has been a significant number of experimental studies of near-field distributions
of optical fields in plasmonic nanostructures. In all cases, a pronounced picture
of the hots spots [157, 158] has been exhibited, see, e.g., Refs. [123, 155, 168].
The inhomogeneous localization of the SP eigenmodes (see Sect. 1.3.2), which is
inherently related to hot spots, has recently been confirmed experimentally [161].
The photoemission electron microscope (PEEM) is a powerful tool of analyzing
the distribution of the local field intensity without perturbing it in any way. In the
PEEM approach, the plasmonic nanosystem to be analyzed serves as a cathode and
an object of an electron microscope. The electron emission is caused by the local field
E(r, ω) of the plasmonic system. The photoelectrons are analyzed by the electron
optics of the PEEM that creates a magnified image of the system in “light” of the
photo-emitted electrons.
For silver, the work function W f (i.e., the minimum energy needed to excite an
electron from the Fermi surface to the zero energy that is the energy in vacuum outside of the metal) is approximately 4.2 eV. The highest energy of an optical quantum
(at the vacuum wavelength of 390 nm) is 3.2 eV, i.e., it is significantly less than W f .
Thus, a single optical photon cannot emit an electron from a silver surface. Such an
emission can, however, occur through two-photon absorption, leaving for the emitted electron the kinetic energy at infinity of E ∞ ≤ 2ω − W f . Such a two-photon
electron photoemission is in the foundation of the so-called two-photon photoemission PEEM (or, 2PP-PEEM). On the other hand, for ultraviolet radiation (say, from
a Hg lamp), the energy of a photon is sufficient for the one-photon photoemission
PEEM (1PP-PEEM). The 2PP-PEEM electron intensity mirrors the distribution of
I 2 = |E(r, ω)|
4 .
A model system to illustrate the hot spots used in a 2PP-PEEM experiment of
Ref. [123] is shown in Fig. 1.10a. This is a diffraction grating covered with a silver layer with roughness of a < 10 nm RMS grain size, as the scanning electron
micrograph (SEM) shows in the insert. The Hg lamp illumination (the energy of the
quantum ω = 4.89 eV exceeds W f = 4.2 eV, thus allowing one-photon photoemission, 1PP-PEEM) shows a smooth image of the underlying diffraction grating
with the resolution of the PEEM (100 nm).
A dramatically different picture is observed in Fig. 1.10b. In this case, the irradiation is with femtosecond laser pulses of λ = 400 nm vacuum wavelength. The
corresponding energy of the quantum is below the work function, ω = 3 eV <
W f = 4.2 eV. Thus the electron photoemission is two-photon. The corresponding
2PP-PEEM image in Fig. 1.10b exhibits a pronounced picture of the hot spots due
to the fact that in this case the optical frequency is in the plasmonic range. These
hot spots are localized SPs that are excited by the p-polarized radiation with a significantly greater efficiency than by an s-polarized one. This suggests that SPPs
excitation may play a role as an intermediate process for the localized SP excitation. In a full qualitative agreement with theory (see Sect. 1.3.2), these hot spots are
29
1.3.6 Experimental Examples of Nanoplasmonic Hot Spots
There has been a significant number of experimental studies of near-field distributions
of optical fields in plasmonic nanostructures. In all cases, a pronounced picture
of the hots spots [157, 158] has been exhibited, see, e.g., Refs. [123, 155, 168].
The inhomogeneous localization of the SP eigenmodes (see Sect. 1.3.2), which is
inherently related to hot spots, has recently been confirmed experimentally [161].
The photoemission electron microscope (PEEM) is a powerful tool of analyzing
the distribution of the local field intensity without perturbing it in any way. In the
PEEM approach, the plasmonic nanosystem to be analyzed serves as a cathode and
an object of an electron microscope. The electron emission is caused by the local field
E(r, ω) of the plasmonic system. The photoelectrons are analyzed by the electron
optics of the PEEM that creates a magnified image of the system in “light” of the
photo-emitted electrons.
For silver, the work function W f (i.e., the minimum energy needed to excite an
electron from the Fermi surface to the zero energy that is the energy in vacuum outside of the metal) is approximately 4.2 eV. The highest energy of an optical quantum
(at the vacuum wavelength of 390 nm) is 3.2 eV, i.e., it is significantly less than W f .
Thus, a single optical photon cannot emit an electron from a silver surface. Such an
emission can, however, occur through two-photon absorption, leaving for the emitted electron the kinetic energy at infinity of E ∞ ≤ 2ω − W f . Such a two-photon
electron photoemission is in the foundation of the so-called two-photon photoemission PEEM (or, 2PP-PEEM). On the other hand, for ultraviolet radiation (say, from
a Hg lamp), the energy of a photon is sufficient for the one-photon photoemission
PEEM (1PP-PEEM). The 2PP-PEEM electron intensity mirrors the distribution of
I 2 = |E(r, ω)|
4 .
A model system to illustrate the hot spots used in a 2PP-PEEM experiment of
Ref. [123] is shown in Fig. 1.10a. This is a diffraction grating covered with a silver layer with roughness of a < 10 nm RMS grain size, as the scanning electron
micrograph (SEM) shows in the insert. The Hg lamp illumination (the energy of the
quantum ω = 4.89 eV exceeds W f = 4.2 eV, thus allowing one-photon photoemission, 1PP-PEEM) shows a smooth image of the underlying diffraction grating
with the resolution of the PEEM (100 nm).
A dramatically different picture is observed in Fig. 1.10b. In this case, the irradiation is with femtosecond laser pulses of λ = 400 nm vacuum wavelength. The
corresponding energy of the quantum is below the work function, ω = 3 eV <
W f = 4.2 eV. Thus the electron photoemission is two-photon. The corresponding
2PP-PEEM image in Fig. 1.10b exhibits a pronounced picture of the hot spots due
to the fact that in this case the optical frequency is in the plasmonic range. These
hot spots are localized SPs that are excited by the p-polarized radiation with a significantly greater efficiency than by an s-polarized one. This suggests that SPPs
excitation may play a role as an intermediate process for the localized SP excitation. In a full qualitative agreement with theory (see Sect. 1.3.2), these hot spots are