Ido Barth

Quantum and Classical Chirps in an Anharmonic Oscillator

We measure the state dynamics of a tunable anharmonic quantum system, the Josephson phase circuit, under the excitation of a frequency-chirped drive. At small anharmonicity, the state evolves like a wave packet-a characteristic response in classical oscillators; in this regime, we report exponentially enhanced lifetimes of highly excited states, held by the drive. At large anharmonicity, we observe sharp steps, corresponding to the excitation of discrete energy levels. The continuous transition between the two regimes is mapped by measuring the threshold of these two effects.

Distinguishing Raman from strongly coupled Brillouin amplification for short pulses

Plasma-based amplification by strongly coupled Brillouin scattering has recently been suggested for the compression of a short seed laser to ultrahigh intensities in sub-quarter-critical-density plasmas. However, by employing detailed spectral analysis of particle-in-cell simulations in the same parameter regime, we demonstrate that, in fact, Raman backscattering amplification is responsible for the growth and compression of the high-intensity, leading spike, where most of the energy compression occurs, while the ion mode only affects the low-intensity tail of the amplified pulse. The critical role of the initial seed shape is identified. A number of subtleties in the numerical simulations are also pointed out.

Beyond nonlinear saturation of backward Raman amplifiers

Backward Raman amplification is limited by relativistic nonlinear dephasing resulting in saturation of the leading spike of the amplified pulse. Pump detuning is employed to mitigate the relativistic phase mismatch and to overcome the associated saturation. The amplified pulse can then be reshaped into a monospike pulse with little precursory power ahead of it, with the maximum intensity increasing by a factor of two. This detuning can be employed advantageously both in regimes where the group velocity dispersion is unimportant and where the dispersion is important but small.

Plasma Wave Seed for Raman Amplifiers

It is proposed to replace the traditional counterpropagating laser seed in backward Raman amplifiers with a plasma wave seed. In the linear regime, namely, for a constant pump amplitude, a plasma wave seed may be found by construction that strictly produces the same output pulse as does a counterpropagating laser seed. In the nonlinear regime, or pump-depletion regime, the plasma-wave-initiated output pulse can be shown numerically to approach the same self-similar attractor solution for the corresponding laser seed. In addition, chirping the plasma wave wavelength can produce the same beneficial effects as chirping the seed wave frequency. This methodology is attractive because it avoids issues in preparing and synchronizing a frequency-shifted laser seed.

Ladder Climbing and Autoresonant Acceleration of Plasma Waves

When the background density in a bounded plasma is modulated in time, discrete modes become coupled. Interestingly, for appropriately chosen modulations, the average plasmon energy might be made to grow in a ladderlike manner, achieving upconversion or downconversion of the plasmon energy. This reversible process is identified as a classical analog of the effect known as quantum ladder climbing, so that the efficiency and the rate of this process can be written immediately by analogy to a quantum particle in a box. In the limit of a densely spaced spectrum, ladder climbing transforms into continuous autoresonance; plasmons may then be manipulated by chirped background modulations much like electrons are autoresonantly manipulated by chirped fields. By formulating the wave dynamics within a universal Lagrangian framework, similar ladder climbing and autoresonance effects are predicted to be achievable with general linear waves in both plasma and other media.

Quantum phenomena in a chirped parametric anharmonic oscillator.

Parametric ladder climbing and the quantum saturation of the threshold for the classical parametric autoresonance due to the zero point fluctuations at low temperatures are discussed. The probability for capture into the chirped parametric resonance is found by solving the Schrödinger equation in the energy basis and the associated resonant phase-space dynamics is illustrated via the Wigner distribution. The numerical threshold for capture into the resonance is compared with the classical and quantum theories in different parameter regimes.

Two-photon ladder climbing and transition to autoresonance in a chirped oscillator

The two-photon ladder climbing (successive two-photon Landau-Zener-type transitions) in a chirped quantum nonlinear oscillator and its classical limit (subharmonic autoresonance) are discussed. An isomorphism between the chirped quantum-mechanical one and two-photon resonances in the system is used in calculating the threshold for the phase-locking transition in both the classical and quantum limits. The theory is tested by solving the Schrodinger equation in the energy basis and illustrated via the Wigner function in phase space.

Multi-frequency Raman amplifiers

In its usual implementation, the Raman amplifier features only one pump carrier frequency. However, pulses with well-separated frequencies can also be Raman amplified while compressed in time. Amplification with frequency-separated pumps is shown to hold even in the highly nonlinear, pump-depletion regime, as derived through a fluid model, and demonstrated via particle-in-cell simulations. The resulting efficiency is similar to single-frequency amplifiers, but, due to the beat-wave waveform of both the pump lasers and the amplified seed pulses, these amplifiers feature higher seed intensities with a shorter spike duration. Advantageously, these amplifiers also suffer less noise backscattering, because the total fluence is split between the different spectral components.