1 Nanoplasmonics: From Present into Future
87
of angles. This was a threshold phenomenon with the threshold increasing with the
Kretschmann angle. At the maximum of the pumping intensity, the widest range of
the outcoupling angles was observed, and the frequency spectrum at every angle
narrowed to a peak near a single frequency ω ≈ 2.1 eV.
These observations of Ref. [262] can be explained by the spasing where the
feedback is provided by roughness of the metal. At the high pumping, the localized
SPs (hots spots), which possess the highest threshold, start to spase in a narrow
frequency range around the maximum of the spasing criterion—the left-hand side of
Eq. (1.103). Because of the sub-wavelength size of these hot spots, the Kretschmann
phase-matching condition is relaxed, and the radiation is outcoupled into a wide
range of angles.
The SPPs of Ref. [262] excited by the Kretschmann coupling are short-range SPPs,
very close to the antisymmetric SPPs. They are localized at subwavelength distances
from the surface, and their wave length in the plane is much shorter the ω/c. Thus
they can be well described by the quasistatic approximation and the present theory
is applicable to them. Substituting the above-given parameters of the dye and the
extinction cross section σ e = 4×10 −16 cm 2 into Eq. (1.104), we obtain a point shown
by the black diamond in Fig. 1.31, which is clearly above the threshold, supporting our
assertion of the spasing. Likewise, the amplified spontaneous emission and, possibly
spasing, appear to have prevented the full loss compensation in a SPP system of
Ref. [274]. Note that recently, random spasing for rough surfaces surrounded by dye
gain media was shown experimentally in two independent observations [281, 303].
Note that the long-range SPPs of Ref. [277] are localized significantly weaker (at
distances ∼λ) than those excited in Kretschmann geometry. Thus the long-range
SPPs experience a much weaker feedback, and the amplification instead of the
spasing can be achieved. Generally, the long-range SPPs are fully electromagnetic
(non-quasistatic) and are not describable in the present theory. Similarly, relatively
weakly confined, full electromagnetic are symmetric SPP modes on thin gold strips
in Ref. [288] where the amplification has been demonstrated.
As we have already discussed in conjunction with Fig. 1.28, the spasing is readily
achievable with the gain medium containing common DBGSs or dyes. There have
been numerous experimental observations of the spaser. Among them is a report of a
SP spaser with a 7-nm gold nanosphere as its core and a laser dye in the gain medium
[252], observations of the SPP spasers (also known as nanolasers) with silver as a
plasmonic-core metal and DBGS as the gain medium with a 1d confinement [253,
256], a tight 2d confinement [254], and a 3d confinement [255]. There also has been a
report on observation of a SPP microcylinder spaser [304]. A high efficiency roomtemperature semiconductor spaser with a DBGS InGaAS gain medium operating
near 1.5 µm (i.e., in the communication near-ir range) has been reported [256].
The research and development in the area of spasers as quantum nano-generators
is very active and will undoubtedly lead to further rapid advances. The next in line
is the spaser as an ultrafast nanoamplifier, which is one of the most important tasks
in nanotechnology.
In contrast to this success and rapid development in the field of spasing and
spasers, there has so far been a comparatively limited progress in the field of loss
87
of angles. This was a threshold phenomenon with the threshold increasing with the
Kretschmann angle. At the maximum of the pumping intensity, the widest range of
the outcoupling angles was observed, and the frequency spectrum at every angle
narrowed to a peak near a single frequency ω ≈ 2.1 eV.
These observations of Ref. [262] can be explained by the spasing where the
feedback is provided by roughness of the metal. At the high pumping, the localized
SPs (hots spots), which possess the highest threshold, start to spase in a narrow
frequency range around the maximum of the spasing criterion—the left-hand side of
Eq. (1.103). Because of the sub-wavelength size of these hot spots, the Kretschmann
phase-matching condition is relaxed, and the radiation is outcoupled into a wide
range of angles.
The SPPs of Ref. [262] excited by the Kretschmann coupling are short-range SPPs,
very close to the antisymmetric SPPs. They are localized at subwavelength distances
from the surface, and their wave length in the plane is much shorter the ω/c. Thus
they can be well described by the quasistatic approximation and the present theory
is applicable to them. Substituting the above-given parameters of the dye and the
extinction cross section σ e = 4×10 −16 cm 2 into Eq. (1.104), we obtain a point shown
by the black diamond in Fig. 1.31, which is clearly above the threshold, supporting our
assertion of the spasing. Likewise, the amplified spontaneous emission and, possibly
spasing, appear to have prevented the full loss compensation in a SPP system of
Ref. [274]. Note that recently, random spasing for rough surfaces surrounded by dye
gain media was shown experimentally in two independent observations [281, 303].
Note that the long-range SPPs of Ref. [277] are localized significantly weaker (at
distances ∼λ) than those excited in Kretschmann geometry. Thus the long-range
SPPs experience a much weaker feedback, and the amplification instead of the
spasing can be achieved. Generally, the long-range SPPs are fully electromagnetic
(non-quasistatic) and are not describable in the present theory. Similarly, relatively
weakly confined, full electromagnetic are symmetric SPP modes on thin gold strips
in Ref. [288] where the amplification has been demonstrated.
As we have already discussed in conjunction with Fig. 1.28, the spasing is readily
achievable with the gain medium containing common DBGSs or dyes. There have
been numerous experimental observations of the spaser. Among them is a report of a
SP spaser with a 7-nm gold nanosphere as its core and a laser dye in the gain medium
[252], observations of the SPP spasers (also known as nanolasers) with silver as a
plasmonic-core metal and DBGS as the gain medium with a 1d confinement [253,
256], a tight 2d confinement [254], and a 3d confinement [255]. There also has been a
report on observation of a SPP microcylinder spaser [304]. A high efficiency roomtemperature semiconductor spaser with a DBGS InGaAS gain medium operating
near 1.5 µm (i.e., in the communication near-ir range) has been reported [256].
The research and development in the area of spasers as quantum nano-generators
is very active and will undoubtedly lead to further rapid advances. The next in line
is the spaser as an ultrafast nanoamplifier, which is one of the most important tasks
in nanotechnology.
In contrast to this success and rapid development in the field of spasing and
spasers, there has so far been a comparatively limited progress in the field of loss