Advancements in Silicon Photonics - Optical Signal Processing by Silicon Photonics - page 47

}

Conduction band

Free-
Carrier
Scattering

photon

phonon

}

photons

Two-photon Absorption

Valence band

B. Jalali. Nature Photonics 1. 193

Crystal Momentum

Fig. 4.11 Two photon and free carrier absorption [ 67 ]

In present days, one of the limitations in electronic circuits is the interconnection
of technologies. The most up-to-date IC chips technology has practically touched
109 on-chip transistors by means of using 4 nm thick gate oxides and 100 nm mini-
mum feature size running at 4 GHz clock speeds [ 66 ]. It is believable that in mate-
rials and fabrication technology the incremental advances in chip architecture will
compliment improvements. However, signal propagation delay (between transistors)
is still larger than device gate delay (delay within device) (Fig. 4.11 ).

4.11 Two-Photon Absorption

Two-photon absorption or TPA is defined as the absorption done simultaneously
between two photons that can be of the same frequency or unlike frequencies, for
exciting a molecule from one state (usually the ground state) to another state, i.e.,
electronic state of higher energy. The difference between the energy involving
lower states and upper states of the molecule is equivalent to the summation of the
individual energies of the two photons.
Two-photon absorption is numerous times of magnitude weaker than the lin-
ear absorption. It also differs from linear absorption in a way that the strength
of absorption which depends on the square of the light intensity, hence it is also
termed as nonlinear optical process.

References

1. Lipson M (2005) Guiding, modulating, and emitting light on silicon—challenges and oppor-
tunities. Lightwave Technol 23:4222-4238
2. Joannopoulos JD, Meade RD, Winn JN, Johnson SG (2008) Photonic crystals: molding the
flow of light: one dimensional photonic crystals, 2nd edn. pp 44-104