The photon spectrum in macro-coherent atomic deexcitation via radiative emission of neutrino pairs has been proposed as a sensitive probe of the neutrino mass spectrum, capable of competing with conventional neutrino experiments. In this paper, we revisit this intriguing possibility, presenting an alternative method for inducing large coherence in a target based on adiabatic techniques. More concretely, we propose the use of a modified version of coherent population return (CPR), namely two-photon CPR, that turns out to be extremely robust with respect to the experimental parameters and capable of inducing a coherence close to 100 % in the target.

A novel technique to achieve atomic macro-coherence as a tool to determine the nature of neutrinos

We present computations of high-order harmonic generation in atoms by using the coherent population return technique. This approach consists in two steps: firstly, the active medium is prepared in a coherent superposition of two bound states using a relatively long laser pulse which has a photon energy close to the energy gap between the states and then, the system is driven by an ultrashort infrared laser pulse. We demonstrate, by employing the numerical solution of the time-dependent Schrodinger equation in one and three dimensions, that using this scheme it is possible to enhance the harmonic conversion efficiency by several orders of magnitude. Our numerical results show that, on one hand the increase in ionization efficiency is responsible of an enhancement in the harmonic signal and on the other hand, the coherent preparation of the medium produces a much richer structure of the harmonic emission.

High-order harmonic generation enhanced by coherent population return

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Exploring Ultrafast Quantum Dynamics of Electrons by Attosecond Transient Absorption

We carry out the theoretical study for the generation of vacuum-ultraviolet pulses with a Doppler-broadened gas utilizing high atomic coherence. It is essentially a difference-frequency generation scheme induced by the two-photon near-resonant pump and probe pulses, where the key point is to generate high atomic coherence between the ground and two-photon near-resonant states through a variant of stimulated Raman adiabatic passage with a time-dependent detuning. The advantage of our scheme is that the degree of coherence is sensitive to neither the exact amount and even sign of the detuning, nor the exact timing between the pump, auxiliary, and probe pulses. Hence our scheme is practically insensitive to Doppler broadening. As a specific example, we consider the generation of picosecond Lyman-α pulses with a Kr gas, and quantitatively study the influence of Doppler broadening as well as the intensity and incident timing of the picosecond probe pulse with respect to the pump pulse. The numerical results indicate that our scheme has a certain advantage over the conventional scheme which utilizes two-photon resonant excitation.

Generation of vacuum-ultraviolet pulses with a Doppler-broadened gas utilizing high atomic coherence.

In the present work the frequency up-conversion of (ps) laser pulses towards the EUV spectral regime was investigated. We addressed the problem of limited conversion efficiency in prevalent gaseous nonlinear media by three approaches.
On the atomic scale, the source term for the harmonic generation in form of the nonlinear polarizability was strongly enhanced by tuning the laser frequency in the vicinity of a five-photon resonance in argon. By monitoring the relative conversion efficiency for fifth harmonic generation versus the multi-photon detuning for several laser intensities of the visible driving laser, we identified pronounced AC Stark shifts of the transition frequency in excess of 50 THz, exceeding the pulse bandwidth by more than one order of magnitude already at an intensity of the order of 10 TW/cm². We concluded, that for a given peak intensity, due to the strong level shifts a transient enhancement is achieved. This enhancement occurs when the laser is tuned to a wavelength such, that the dynamically shifted level comes in resonance at intensities slightly lower than the peak intensity of the pulse. By tuning the laser accordingly, we achieved an enhancement of about one order of magnitude for the fifth harmonic and also strong enhancements of the seventh and ninth harmonic generated simultaneously. This form of resonance enhancement of multiple harmonics emphasizes the importance of intermediate resonances also in higher harmonic generation.
In a second approach, we focused on coherent control of frequency up-conversion towards the EUV regime by actively controlling the nonlinear polarization in a bi-color laser field. A four photon transition to the 5p^5 6p²[5/2] level in xenon is driven by intense laser pulses in the visible regime around 512 nm and can interfere with a two-photon transition, driven by the second field at a wavelength of 256 nm. By tuning the relative phase between the two fields, we achieve control of the excitation probability as confirmed by a pronounced modulation of the laser-induced fluorescence from the excited level. A further photon at 512 nm serves to drive fifth harmonic generation and simultaneous four-wave mixing with the UV photons, both yielding radiation at 102 nm wavelength. In systematic measurements, we examined the several preconditions to achieve maximum control in an experiment involving high nonlinear orders and ultra-short laser pulses at intensities around 1 TW/cm². As a result, we gained a visibility of 90 % in the interference of the two conversion pathways - to our knowledge the highest achieved visibility in phase control of harmonic generation so far. While coherent control can only gain a limited enhancement of less than a factor of 4, the data exhibit a convincing demonstration of the feasibility of coherent control also with ultra-short pulses at TW/cm² intensity, gaining a factor of 18 in modulation between destructive and constructive interference. Furthermore, we examined the dependence of absolute signal and control strength (visibility) concerning the detuning from the resonance and showed a change in the temporal modulation period with respect to the detuning. In a simultaneous measurement of the excited state population and frequency conversion, both processes show similar interference with equal modulation period and large modulation depth, but phase lag of 0.03 π between the two interferograms. We attributed this phase lag to the contribution of further atomic levels (in this case especially the Rydberg levels and the ionization continuum) to the nonlinear polarization, possibly introducing additional phase compared to the excitation channel.
We finally enhanced the product of number density N and interaction length L by about a factor of 1000 by confining the gas medium inside a hollow core waveguide and balancing the gas dispersion with the waveguide dispersion. We showed that the combination of resonance enhancement and phase-matched harmonic generation at high N*L is possible in argon in the vicinity of the strong 3p^6 -> 3p^5 4s'² [1/2]° transition. Because of the effect of the AC Stark shift of the transition frequency on the refractive index of argon at the fifth harmonic frequency, the Stark shift could be determined directly by analyzing the shift in phase matching pressure at a constant fundamental wavelength. The obtained (averaged) energy shift of 0.85 Φ, is close to the ponderomotive energy Φ and in good agreement with recent publications. In comparing the relative experimental conversion efficiency versus gas pressure and resonance detuning with an extensive numerical simulation, we unveiled the important contribution of a quasi-phase matching scheme resulting from the mode beating at the fundamental frequency, even with only less than 3% of power guided in modes higher than EH11. Furthermore, we reproduced the resonance enhancement, which is significantly detuned even from the shifted resonance and explained the detuning and enhancement lineshape by the phase matching precondition and the required phase matching bandwidth to convert the full spectrum of the pulses. Within these constraints, we rate the investigated coupling scheme capable of resonantly enhanced frequency up-conversion of pulses as short as 100 fs. At optimal conditions, we achieved more than a factor of 800 higher conversion efficiency compared with an atomic jet operated at 1 bar of stagnation pressure and could even enhance this efficiency by another factor of 1.5 in admixing a positively dispersive buffer gas.

Efficient frequency conversion towards ultrashort VUV pulses using coherent control and multi-photon resonances

We present an experimental demonstration of sum-frequency generation toward tunable picosecond, broadband radiation pulses in the extreme-ultraviolet regime, supported by adiabatically prepared atomic coherences. We drive a two-photon transition in a dense medium of xenon atoms with a long (nanosecond) pump laser pulse at the Fourier-transform-limited bandwidth and with a wavelength of 225 nm. The frequency of the pump pulse is slightly detuned from exact two-photon resonance. In this configuration, the medium is adiabatically driven by a process of coherent population return. The adiabatic passage process generates a maximal coherent superposition of two quantum states with large energy spacing. An additional, short (picosecond) probe laser pulse at a wavelength of 540 nm beats with the maximal atomic coherence and generates a short (picosecond) signal radiation pulse at 93 nm. As compared to conventional (diabatic) frequency conversion, the adiabatically driven maximal atomic coherence yields enhanced conversion efficiency and significantly enhanced stability.

Adiabatically driven frequency conversion towards short extreme-ultraviolet radiation pulses

We present experimental data on the coherent control of frequency conversion toward the vacuum-ultraviolet regime in xenon atoms, applying intense ultrashort picoseconds (ps) radiation pulses. We report on quantum interference between two resonantly enhanced frequency conversion processes (i.e., fifth harmonic generation and four-wave mixing). The conversion processes are driven by a fundamental (ps) laser pulse at 530 nm and the phase-locked second harmonic at 265 nm. Both fifth harmonic generation and four-wave mixing yield vacuum-ultraviolet radiation at 106 nm. The two frequency conversion pathways interfere with each other - either constructively or destructively, depending on the relative phase of the driving laser pulses. Thus, by variation of the relative phase, we substantially enhance or reduce the conversion efficiency (i.e., the yield of generated vacuum-ultraviolet radiation).

Holger Muench

Papers

Adiabatically driven frequency conversion towards short extreme-ultraviolet radiation pulses

Coherent control of frequency conversion toward short (picosecond) vacuum-ultraviolet radiation pulses