Train of high-power femtosecond pulses: Probe wave in a gas of prepared atoms

Abstract

We present a new method for generating a regular train of ultrashort optical pulses in a prepared two-level medium. The train develops from incident monochromatic probe radiation travelling in a medium of atoms, which are in a quantum mechanical superposition of dressed internal states. In the frame of linear theory for the probe radiation, the energy of individual pulses is an exponentially growing function of atom density and of interaction cross section. Pulse repetition rate is determined by the pump field's generalized Rabi frequency and can be around 1 THz and greater.We also show that the terms, extra to the dipole approximation, endow the gas by a new property: non-saturating dependence of refractive index on dressing monochromatic field intensity. Contribution of these nonsaturating terms can be compatible with the main dipole approximation term contribution in the wavelength region of about ten micrometers (the range of CO 2 laser) or larger.

Coherence as ultrashort pulse train generator

“…This rate in fact determines the space and time repetition distance between the pulses: ∆t repetition = 2π/Ω and ∆z repetition = c· ∆t repetition . The regime of propagation, amplification or weakening, is determined by the detuning ω− ω pump (See Fig.1 in [14]). This dependence is especially sharp near the scattering resonances.…”

“…The additional terms C 1 ( − → r , t) and C 2 ( − → r , t) arise due to interaction with probe radiation and are proportional to the probe field intensity in frame of linear theory. Note that in distinction from [14], where the pump field was assumed to be strictly monochromatic,…”

“…In particular in [13] Harris and co-workers have suggested and used a Raman-type three level interaction scheme in D2-molecular gas to get series of femtosecond pulses. In our recent paper [14], discussing propagation of radiation probe wave in a medium of dressed two-level atoms initially prepared in a quantum superpositional state of ground and excited energy levels, we showed that it splits into a sequence of ultrashort pulses with easy and precise tunning of possible control parameters. Later we will refer to this scheme as QS (quantum superposition) generator.…”

Coherence as ultrashort pulse train generator

“…Note that in distinction from [14], where the pump field was assumed to be strictly monochromatic, here the problem of atomic states in the field of pump radiation is time dependent.…”

“…In particular in [13] Harris and coworkers have suggested and used a Raman-type three level interaction scheme in D2-molecular gas to get series of femtosecond pulses. In our recent paper [14], we suggest and analyze a theoretical model for producing a train of ultrashort optical pulses We show that an incident quasimonochromatic radiation "probe" wave attains a periodic modulation of intensity accompanied, in general, by amplification, if the medium atoms have been prepared in a quantum superposition state of two internal states, dressed by the pump field. For appropriate gas parameters, concentration, and resonance detuning, the probe wave intensity modulates in a form of regular train of sharp ultrashort pulses.…”