This chapter describes ultrafast dynamics of elementary excitations induced by femtosecond near infrared pulses. The concept of elementary excitations is often used to describe the behavior of electronic excited states in solids. Investigating ultrafast dynamics of elementary excitations is important to understand the ultrafast dynamics of materials. We studied the ultrafast dynamics of phonons and plasmons in a semiconductor crystal of gallium arsenide (GaAs) using quantum model calculations and femtosecond time-resolved spectroscopy.