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I. Goykhman et al.
The device consists of a silicon waveguide covered by a thin silicon dioxide
layer. A metal electrode is positioned on top of the oxide. Two modes (photonic and
plasmonic) are supported by the structure. By properly designing the dimensions
of the structure, the photonic mode can be brought to a near cut-off condition for
the wavelength of operation around 1550 nm. If no voltage is applied, both modes
propagate in the waveguide and can interfere destructively if the length of the device
is chosen such that both modes are in anti-phase at the output. Due to the destructive
interference, a small output signal is measured. Upon the application of voltage to the
gate, the photonic mode is brought to cut-off such that it can no longer interfere with
the plasmonic mode. As a result, higher output signal is obtained. Using this approach,
a substantial modulation was obtained with very low power consumption, in the
range of femtojoules per bit. Although high speed operation was not demonstrated,
the device has the potential for gigahertz modulation frequencies owing to its small
dimensions allowing the obtaining of small RC time constants.
While the PlasMOStor concept has the potential for becoming a viable candidate
as an SPM, there is still a long way to go before it can replace current technologies. First, on chip coupling schemes need to be employed in order to facilitate the
demonstration of a low loss device. Additionally, the coupling of signals to both
modes needs to be precisely controlled such that the extinction ratio of the device
will be high under varying operation conditions. Finally, in spite of the low power
consumption, heat issues related to generating power within a very small volume
need to be addressed.
Another approach for demonstrating SPM devices is by using the hybrid structure
of metal - oxide-silicon (MOS) and exploring accumulation or inversion operation
of this MOS capacitor for the realization of an electro-absorption SPM. This concept was demonstrated by several authors [34–37]. For example, Kwong et al. [34]
demonstrated about 6 dB attenuation under the application of 7 V for a device length
of about 3 μm. The demonstration by Leuthold et al. [36] is based on integrating
an active section consisting of a stack of silver (Ag), indium tin oxide (ITO), and
SiO 2 layers into a silicon waveguide platform. This structure supports a strongly
confined asymmetric SPP in the 1.55 μm telecommunication wavelength window.
The absorption coefficient of the SPP is modulated by applying a voltage between
the two silver electrodes, generating free carriers in the ITO layer. This configuration
is expected to operate at rates faster than 100 Gbit/s owing to the ultra small RC time
constant. Finally, SPM which is based on all optical modulation was demonstrated as
well [37]. In this work, a gold-silicon grating coupler is used to couple pump pulses
(800 fs pulse width, central wavelength around 775 nm). The pump signal generates
free carriers in the silicon which in turn modifies the effective refractive index of
the silicon. As a result, the coupling of a probe signal (in the spectral regime of
1300–1700 nm) into SPP mode on the gold-silicon interface can be controlled. The
authors demonstrate resonance shift larger than the resonance linewidth by using a
pump energy density of 2.2 mJ/cm 2 , with time response of about 100 ps, practically
limited by the recombination time of the free carriers in silicon. Time response can be
further enhanced by generating free carriers in the metal, as previously demonstrated
by Zheludev et al. [38].
I. Goykhman et al.
The device consists of a silicon waveguide covered by a thin silicon dioxide
layer. A metal electrode is positioned on top of the oxide. Two modes (photonic and
plasmonic) are supported by the structure. By properly designing the dimensions
of the structure, the photonic mode can be brought to a near cut-off condition for
the wavelength of operation around 1550 nm. If no voltage is applied, both modes
propagate in the waveguide and can interfere destructively if the length of the device
is chosen such that both modes are in anti-phase at the output. Due to the destructive
interference, a small output signal is measured. Upon the application of voltage to the
gate, the photonic mode is brought to cut-off such that it can no longer interfere with
the plasmonic mode. As a result, higher output signal is obtained. Using this approach,
a substantial modulation was obtained with very low power consumption, in the
range of femtojoules per bit. Although high speed operation was not demonstrated,
the device has the potential for gigahertz modulation frequencies owing to its small
dimensions allowing the obtaining of small RC time constants.
While the PlasMOStor concept has the potential for becoming a viable candidate
as an SPM, there is still a long way to go before it can replace current technologies. First, on chip coupling schemes need to be employed in order to facilitate the
demonstration of a low loss device. Additionally, the coupling of signals to both
modes needs to be precisely controlled such that the extinction ratio of the device
will be high under varying operation conditions. Finally, in spite of the low power
consumption, heat issues related to generating power within a very small volume
need to be addressed.
Another approach for demonstrating SPM devices is by using the hybrid structure
of metal - oxide-silicon (MOS) and exploring accumulation or inversion operation
of this MOS capacitor for the realization of an electro-absorption SPM. This concept was demonstrated by several authors [34–37]. For example, Kwong et al. [34]
demonstrated about 6 dB attenuation under the application of 7 V for a device length
of about 3 μm. The demonstration by Leuthold et al. [36] is based on integrating
an active section consisting of a stack of silver (Ag), indium tin oxide (ITO), and
SiO 2 layers into a silicon waveguide platform. This structure supports a strongly
confined asymmetric SPP in the 1.55 μm telecommunication wavelength window.
The absorption coefficient of the SPP is modulated by applying a voltage between
the two silver electrodes, generating free carriers in the ITO layer. This configuration
is expected to operate at rates faster than 100 Gbit/s owing to the ultra small RC time
constant. Finally, SPM which is based on all optical modulation was demonstrated as
well [37]. In this work, a gold-silicon grating coupler is used to couple pump pulses
(800 fs pulse width, central wavelength around 775 nm). The pump signal generates
free carriers in the silicon which in turn modifies the effective refractive index of
the silicon. As a result, the coupling of a probe signal (in the spectral regime of
1300–1700 nm) into SPP mode on the gold-silicon interface can be controlled. The
authors demonstrate resonance shift larger than the resonance linewidth by using a
pump energy density of 2.2 mJ/cm 2 , with time response of about 100 ps, practically
limited by the recombination time of the free carriers in silicon. Time response can be
further enhanced by generating free carriers in the metal, as previously demonstrated
by Zheludev et al. [38].