1 Nanoplasmonics: From Present into Future
61
1.5.3.1 Nanospaser with Semiconductor Gain Media
It is of both fundamental and applied importance to develop nanoscale-size spasers
(nanospasers) with semiconductor gain media. The photochemical and electrochemical stability of the semiconductor gain media is the main attraction of such a
design. Belonging to this class, spasers have recently been fabricated and their operation observed, comprised of a InGaN-core/InN-shell semiconductor-nanorod gain
medium and silver film as a plasmonic component [282, 283]. They generate on
ocalized SP modes. One of these [283] is a nanospaser with a deeply sub-wavelength
mode size based on an epitaxial silver nanofilm [283]. Such a design bears a promise
of practical applications due to its stability and small modal volume leading to high
operational speed—see below Sect. 1.5.6.
In Fig. 1.27, we display geometry of this InGaN-core/InN-shell nanorod spaser
and properties of its spasing mode. The active region of the spaser (Fig. 1.27a, left
panel) is a core-shell nanocylinder with a 30-nm diameter core of InGaN surrounded
by think shell if GaN. The latter is a wide band-gap semiconductor that plays a
role of insulator. The active nanorod is separated by the metal by a 5-nm layer of
silica. The plasmonic component of this spaser is a flat layer of epitaxial silver.
The high monocrystalline quality of the silver film is instrumental in reducing the
threshold of the spaser and increasing its output. The calculated intensity for the
spasing eigenmode is shown in the right panel of Fig. 1.27a. Similar to the gap
modes introduced in Ref. [284], this eigenmode is concentrated in the thin layer of
a low-permittivity dielectric (silica) between the two high-permittivity media: GaN
and silver. The modal fields do penetrate sufficiently into the gain medium providing
the feedback necessary for the spaser functioning.
Under 8.3 kW/cm 2 optical pumping with frequency above the band gap of InGaN,
a series of the emission spectra of a single spaser is displayed in Fig. 1.27b, At a
room temperature, T = 300 K, the emission is a spontaneous fluorescence in a wide
yellow–green spectral band near the band gap of InGaN. The first evidence of the
spasing appears at T = 120 K as a small notch at the green side of the spectrum.
As the temperature decreases to T = 8 K, the narrow line at λ ≈ 500 nm becomes
dominant and narrow. This change of the spectrum over the threshold is in a qualitative
agreement with theory—see below Sect. 1.5.5 and, in particular, Figs. 1.29d–f.
The light–line (L–L) line is the dependence of the light intensity out (the intensity
of the radiation emitted by the spaser within the linewidth spectral range) versus the
intensity of the pumping radiation. The theoretical prediction for the spaser is that
after reaching the spasing threshold, the L–L line becomes linear with universally
unit slope—see Fig. 1.29a and its discussion in Sect. 1.5.5.
The experimentally obtained L–L line of the nanorod spaser shown in Fig. 1.27c
is in an excellent agreement with this prediction. Note that this figure is presented
in the double-logarithmic scale. There are two curves in this figure taken at different
temperatures, which are similar though at a lower temperature the intensity out is
higher and the threshold is lower. The parts of the curves at lower pumping intensities
are also unit-slope straight lines corresponding to spontaneous fluorescence. With the
increased intensity, the curves enter a transitional regime of amplified spontaneous
61
1.5.3.1 Nanospaser with Semiconductor Gain Media
It is of both fundamental and applied importance to develop nanoscale-size spasers
(nanospasers) with semiconductor gain media. The photochemical and electrochemical stability of the semiconductor gain media is the main attraction of such a
design. Belonging to this class, spasers have recently been fabricated and their operation observed, comprised of a InGaN-core/InN-shell semiconductor-nanorod gain
medium and silver film as a plasmonic component [282, 283]. They generate on
ocalized SP modes. One of these [283] is a nanospaser with a deeply sub-wavelength
mode size based on an epitaxial silver nanofilm [283]. Such a design bears a promise
of practical applications due to its stability and small modal volume leading to high
operational speed—see below Sect. 1.5.6.
In Fig. 1.27, we display geometry of this InGaN-core/InN-shell nanorod spaser
and properties of its spasing mode. The active region of the spaser (Fig. 1.27a, left
panel) is a core-shell nanocylinder with a 30-nm diameter core of InGaN surrounded
by think shell if GaN. The latter is a wide band-gap semiconductor that plays a
role of insulator. The active nanorod is separated by the metal by a 5-nm layer of
silica. The plasmonic component of this spaser is a flat layer of epitaxial silver.
The high monocrystalline quality of the silver film is instrumental in reducing the
threshold of the spaser and increasing its output. The calculated intensity for the
spasing eigenmode is shown in the right panel of Fig. 1.27a. Similar to the gap
modes introduced in Ref. [284], this eigenmode is concentrated in the thin layer of
a low-permittivity dielectric (silica) between the two high-permittivity media: GaN
and silver. The modal fields do penetrate sufficiently into the gain medium providing
the feedback necessary for the spaser functioning.
Under 8.3 kW/cm 2 optical pumping with frequency above the band gap of InGaN,
a series of the emission spectra of a single spaser is displayed in Fig. 1.27b, At a
room temperature, T = 300 K, the emission is a spontaneous fluorescence in a wide
yellow–green spectral band near the band gap of InGaN. The first evidence of the
spasing appears at T = 120 K as a small notch at the green side of the spectrum.
As the temperature decreases to T = 8 K, the narrow line at λ ≈ 500 nm becomes
dominant and narrow. This change of the spectrum over the threshold is in a qualitative
agreement with theory—see below Sect. 1.5.5 and, in particular, Figs. 1.29d–f.
The light–line (L–L) line is the dependence of the light intensity out (the intensity
of the radiation emitted by the spaser within the linewidth spectral range) versus the
intensity of the pumping radiation. The theoretical prediction for the spaser is that
after reaching the spasing threshold, the L–L line becomes linear with universally
unit slope—see Fig. 1.29a and its discussion in Sect. 1.5.5.
The experimentally obtained L–L line of the nanorod spaser shown in Fig. 1.27c
is in an excellent agreement with this prediction. Note that this figure is presented
in the double-logarithmic scale. There are two curves in this figure taken at different
temperatures, which are similar though at a lower temperature the intensity out is
higher and the threshold is lower. The parts of the curves at lower pumping intensities
are also unit-slope straight lines corresponding to spontaneous fluorescence. With the
increased intensity, the curves enter a transitional regime of amplified spontaneous