Enhancing optical signal is a subject of long term interest with many applications, such as trace chemical detection in the lab and standoff detection in the atmosphere. It is well known that optical properties of multi-level atomic and molecular system can be controlled and manipulated efficiently using quantum coherence and interference resulting an enhancement in optical signals. In this dissertation we investigate methods both with/without utilizing coherence of atomic and molecular systems to enhance optical signals. We use resonant Raman scattering and surface-enhanced Raman scattering which do not rely on molecular coherence to boost Raman signal from molecules, several orders of magnitude enhancement has been achieved. When coherence is introduced into atomic systems, cooperative emission is produced as a result of coherence. The emission is named Superfluorescence (SF) or superradiance (SR) depending on the initial coherence of the prepared systems. We study the properties of SF, yoked SF (YSF) and SR, the transition from YSF to SR and quantum beat exhibits in YSF/SR signal. More than thirty folds of pulse energy is obtained from SR compare to that of YSF. Possible applications of these results are also discussed.
