CHAPTER 2: STIMULARED RAMAN SCATTERING IN SILICON
3.7 Concluding remarks and future work
In conclusion cavity engineering of anti Stokes – Stokes scattering intensity ratio has been shown. By changing the cavity size, variations by over a factor of ten have been observed (0.035-0.45) in the room temperature anti Stokes to Stokes scattering ratio. Nanowire diameter, excitation wavelength and cavity structure dependent measurements have confirmed that this ratio depends on the relative confinement of the anti Stokes and Stokes electric field intensity inside the cavity. A higher confinement of anti Stokes electric field intensity inside the nanowire (relative to Stokes electric field intensity) leads to higher relative (to Stokes scattering) emission of the anti Stokes scattering. Thus it is possible to enhance phonon-photon interactions by simply increasing the density of states (in the cavity) at the wavelength at which the new photon is expected to be emitted. While the absolute Stokes and anti Stokes scattering intensity also depend on the confinement of pump intensity, the ratio is completely independent of the confinement of the pump electric field inside the cavity.
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been reported before. While this is very interesting purely as a scientific phenomenon, it could also be extremely critical in mitigating system heat and hence in the design of Raman lasers. During operation of Raman lasers, phonons are constantly being generated which lead to heating of the gain medium. This heating has detrimental effects on the quality of emission14. Since anti Stokes scattering leads to destruction of a phonon and hence cooling, it provides a natural mechanism for cooling of the gain medium. A Raman laser cavity which also has a strong confinement at the anti Stokes wavelength would also have enhanced anti Stokes emissions and would be heated to a lesser degree in the process.
High temperature measurements reveal that the tuning of anti Stokes-Stokes scattering intensity ratio on cavity mode is independent of the pump power and the system temperature. In other words the ratio continues to stay tuned even at elevated temperatures. This tuning ratio is important to understand and estimate from the application perspective of temperature determination from anti Stokes to Stokes ratio. The cavities can even be tuned to reach a state in which emission at anti Stokes wavelength is greater than the emission at Stokes wavelength leading to a net cooling effect in the cavity. While at above bandgap temperatures, due to high absorption related losses, net cooling was not observed, the cooling capacity of such systems is worth investigating with below bandgap excitation. Moreover when anti Stokes emissions is greater than Stokes emissions, the system is in a state of “population” inversion which was described as an important consideration for building an anti Stokes Raman laser15.
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lower temperatures and experiments on such cavities could reveal interesting results. The applications of optical cooling in silicon are immense given the demand of more energy efficient silicon based devices and understanding photon-phonon interactions is the key to achieving it
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