Adiabatic frequency conversion, a recent advance in frequency conversion allowed the achievement of efficient scalable and ron\bust broadband frequency conversion. I recent years, it was successfully applied to the conversion of ultrashort pulses, demonstrating near-100% efficiency for ultrabroadband spectrum. Also, it offers spectral pulse shaping for coherent control experiments. Here we review the development and recent advances in the field.
Adiabatic frequency conversion, a recent advance in frequency conversion allowed the achievement of efficient scalable and ron\bust broadband frequency conversion. I recent years, it was successfully applied to the conversion of ultrashort pulses, demonstrating near-100% efficiency for ultrabroadband spectrum. Also, it offers spectral pulse shaping for coherent control experiments. Here we review the development and recent advances in the field.
Adiabatic evolution, an important dynamical process in a variety of classical and quantum systems providing a robust way of steering a system into a desired state 1-5, was introduced only recently to frequency conversion 6-9. Adiabatic frequency conversion allowed the achievement of efficient scalable broadband frequency conversion 8, 9 and was applied successfully to the conversion of ultrashort pulses, demonstrating near-100% efficiency for ultrabroadband spectrum 10-16. The underlying analogy between undepleted pump nonlinear processes and coherently excited quantum systems was extended in the past few years to multi-level quantum systems, demonstrating new concepts in frequency conversion, such as complete frequency conversion through an absorption band 17-20. Additionally, the undepleted pump restriction was removed, enabling the exploration of adiabatic processes in the fully nonlinear dynamics regime of nonlinear optics 21-26. In this article, the basic concept of adiabatic frequency conversion is introduced, and recent advances in ultrashort physics, multi-process systems, and the fully nonlinear dynamics regime are reviewed.
Adiabatic frequency conversion is a method recently developed in nonlinear optics [H. Suchowski, D. Oron, A. Arie, and Y. Silberberg, Phys. Rev. A 78, 063821 (2008)], using ideas from the technique of rapid adiabatic passage (RAP) via a level crossing in quantum physics. In this method, the coupling coefficients are constant and the phase mismatch is chirped adiabatically. In this work, we propose another method for adiabatic frequency conversion, in which the phase mismatch is constant and the coupling is a pulse-shaped function with a sign flip (i.e., a phase step of $\ensuremath{\pi}$) at its maximum. Compared to the RAP method, our technique has comparable efficiency but it is simpler to implement for it only needs two bulk crystals with opposite ${\ensuremath{\chi}}^{(2)}$ nonlinearity. Moreover, because our technique requires constant nonzero frequency mismatch and has zero conversion efficiency on exact frequency matching, it can be used as a frequency filter.
We present experimental data on adiabatically driven frequency conversion with ultrashort (picosecond) laser pulses towards the vacuum-ultraviolet regime, enhanced by coherent population return and preparation of maximal atomic coherences. We generate the sum frequency of an intense pump pulse and a probe pulse via a two-photon resonance in xenon. When we slightly detune the pump laser from two-photon resonance, the atomic populations are adiabatically driven forth and back from the atomic ground state to an excited state. This coherent population return (CPR) prepares the medium in a transient state of maximal atomic coherence, which enhances frequency mixing with a probe laser. We thoroughly study the variation of CPR-enhanced frequency conversion with experimental parameters and compare CPR with conventional, resonantly enhanced frequency conversion. In particular, we investigate the temporal evolution of the atomic coherence and the effect of inevitable ac Stark shifts. We find that ac Stark shifts induce an asymmetry in the spectral characteristics of CPR, which nevertheless permits enhanced frequency conversion. For the case of resonantly enhanced frequency conversion, we show that the atomic coherences are maintained for several tens of picoseconds after the pump pulse, which permits time-delayed frequency conversion. Moreover, we analyze the pressure dependence of the atomic coherence' lifetime and observe a free induction decay at the two-photon coherence with the generation of a second harmonic field. These findings shall push applications of adiabatic light-matter interactions also to the regime of nonlinear optics, driven by intense laser pulses.
In this work, we thoroughly investigated frequency conversion towards ultrashort picosecond (ps) laser pulses in the vacuum ultraviolet (VUV) spectral regime via nonlinear optics in coherently-driven quantum systems at high intensities. We applied approaches of adiabatic quantum dynamics relying on coherent light-matter interactions, which modify the "bare" eigenstates of a quantum system to "dressed" states and exploit slow (i.e., adiabatic) population evolutions via such dressed states. This enables enhancement of typically low-order, low-intensity nonlinear optics (at MW/cm², GW/cm²), where the light field perturbs the atomic level structure only weakly. On the other hand, aiming at nonlinear optics at higher orders, e.g., high-harmonic generation, strong light fields are applied (100 TW/cm², PW/cm²), which modify the Coulomb potential of the atom strongly. In this case, resonances play no role anymore and adiabatic quantum dynamics via atomic resonances will not work. With this work, we demonstrated the implementation of adiabatic quantum dynamics and coherently prepared media for nonlinear optics in an intermediate intensity regime (TW/cm²), i.e., at Keldysh parameters approaching γ≈1. The fundamental question was, whether the high intensities still permit us to drive and exploit coherent-adiabatic dynamics via dressed states for enhancement of frequency conversion processes. We investigated two approaches to enhance the frequency conversion yield applying nonlinear optics in coherently-driven quantum systems (rare gases). In the first approach, we resonantly enhanced harmonic generation via dressed states with large Autler-Townes splittings. With this approach, we demonstrated how to make atomic resonances available for an initially far detuned fixed-frequency pump laser and showed a compensation for inevitable, perturbing Stark shifts. In the second approach, we investigated frequency conversion, enhanced by coherent population return (CPR) and preparation of maximal atomic coherences at high intensities. With this approach, we showed how to maximally benefit from an atomic resonance via adiabatically driven frequency conversion, i.e., by detuning the strong pump laser slightly from the resonance.
Abstract The method of adiabatic frequency conversion, in analogy with a two‐level atomic system, has been put forward recently and verified experimentally to achieve robust frequency mixing processes such as sum and difference frequency generation. Here a comparative study of efficient frequency mixing using various techniques of shortcuts to adiabaticity such as counter‐diabatic driving and invariant‐based inverse engineering is presented. It is shown here that it is possible to perform sum frequency generation by properly designing the poling structure of a periodically poled crystal and the coupling between the input lights and the crystal. The required crystal length for frequency conversion significantly decreases beyond the adiabatic limit. This approach significantly improves the robustness of the process against the variation in temperature as well as the signal frequency. By introducing a single parameter control technique with constant coupling and combining with the inverse engineering, perturbation theory, and optimal control, it is shown that the phase mismatch can be further optimized with respect to the fluctuations of input wavelength and crystal temperature that results into a novel experimentally realizable mixing scheme.