Ultra-broadband multi-frequency Raman generation is a long-established technique for producing temporal bandwidths that can be of the order of the pump frequency. However, higher-than-optimal intensity beams reduce the bandwidth generated. Many works have considered different media and effects such as coherence preparation and decoration of sidebands with more lines, but the essential phenomenon and its characteristics have proved relatively invariant. \n \nHere, we report the discovery of a profoundly new regime of multi- frequency Raman generation. The simple inclusion of strong cavity feedback has uncovered a fundamental modification to the process and its overall characteristics. \n \nFirstly, we find that the magnitude of bandwidth generated may be enhanced by up to 100% in comparison with the equivalent cavity-less context. Secondly, this novel feature is accompanied by a self-synchronization of the generated spectrum towards a global fixed- point. This self-organisation of frequency lines can be consistent with formation of pulse- trains with individual pulsewidths less than 300 attoseconds. Importantly, self- synchronization is not only stable against reduction in mirror bandwidth but it has also been identified as an intrinsic effect that is independent of cavity mistuning in strong-cavity regimes. \n \nOur investigations show that self-synchronization is indeed most stable in regimes where generated pulsewidths are shortest.
Ultra-broadband multi-frequency Raman generation is a long-established technique for producing temporal bandwidths that can be of the order of the pump frequency. However, higher-than-optimal intensity beams reduce the bandwidth generated. Many works have considered different media and effects such as coherence preparation and decoration of sidebands with more lines, but the essential phenomenon and its characteristics have proved relatively invariant. \n \nHere, we report the discovery of a profoundly new regime of multi- frequency Raman generation. The simple inclusion of strong cavity feedback has uncovered a fundamental modification to the process and its overall characteristics. \n \nFirstly, we find that the magnitude of bandwidth generated may be enhanced by up to 100% in comparison with the equivalent cavity-less context. Secondly, this novel feature is accompanied by a self-synchronization of the generated spectrum towards a global fixed- point. This self-organisation of frequency lines can be consistent with formation of pulse- trains with individual pulsewidths less than 300 attoseconds. Importantly, self- synchronization is not only stable against reduction in mirror bandwidth but it has also been identified as an intrinsic effect that is independent of cavity mistuning in strong-cavity regimes. \n \nOur investigations show that self-synchronization is indeed most stable in regimes where generated pulsewidths are shortest.
Subfemtosecond pulses can be generated in a Raman generator by using electromagnetically induced transparency if ultrabroad Raman sidebands are generated and then propagate in such a way that the generated sidebands at higher frequencies are temporally advanced with regard to those at lower frequencies. However, it was recently demonstrated by experiments [1] that above-resonance Raman excitations ensured the establishment of anti-phased Raman coherence for pulse compression but only very limited number of Raman sidebands could be generated due to Raman self-defocusing [2], while broadband sidebands could be generated under below-resonance Raman excitations, which supported no pulse compression in the same Raman media. In order to establish anti-phased Raman coherence and simultaneously overcome Raman self-defocusing to benefit efficient generation of broadband sidebands, we suggest in this report a new technique to generate sub-fs pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through nonadibatic processes. The simulation on the basis of the fundamental rotational Raman transitions in H2 shows that sub-fs pulse can be generated by above-resonance or below-resonance Raman excitations, or even Raman excitations with opposite detunings, as is shown in Fig.1. This opens a new way for experiments to observe enhanced generation of broadband Raman sidebands, together with compression of the phase-locked Raman sidebands into sub-fs pulse trains in the same Raman media
Throughout the past few decades, science has progressed towards the ability to probe many extremely fast processes and a large amount of research has been aimed at the area of few-femtosecond pulse generation. This thesis describes the generation of coherent broadband radiation through two-colour pumping of molecular hydrogen confined to a unidirectional ring cavity, and the subsequent synthesis of high peak power and few-femtosecond pulses. A set of normalised semi-classical field equations are derived in Bloch form describing the process of ultra-broadband multi-frequency Raman generation or UMRG, and a 3-wave gain suppression analysis is derived from a subset of the plane wave UMRG field equations which describes gain suppression within the ring cavity in terms of both medium and cavity parameters. The gain suppression analysis is further generalised to include finite levels of linear two-photon frequency detuning of the pump beams. Simulations of the plane wave ultra-broadband multi-frequency Raman (UMRG) equations show that a broad frequency spectrum of mutually coherent sideband can be generated. The inverse Fourier transform of spectra generated in this way yields a train of high power near Fourier limited pulses in the time domain which can range from a few-femtoseconds in duration to tens of attoseconds with repetition rates equal to the Raman transition frequency. Pulses synthesised in this way are limited only by the level of medium dispersion, the reflection bandwidth of the chosen coupling mirror and the chosen Raman medium. Simulations of the transverse UMRG equations within the ring cavity geometry have shown ring cavity enhanced UMRG to be resilient to transverse effects such as finite beam width, beam diffraction and the transverse beam separation of the applied pump beams.
We have demonstrated that coherent molecular modulation can result in the collinear generation of mutually-coherent spectral sidebands that extend in frequency from the infrared to the far ultraviolet. Our technique is based on adiabatic preparation of a highly coherent molecular superposition-state, which is achieved by using narrow-linewidth lasers slightly detuned from a Raman resonance. The phases of the resultant Stokes and anti-Stokes sidebands are adjusted in order to synthesize desired single-cycle pulse trains at the target. In this article we review recent improvements and developments in this area, including: techniques for increasing the number of generated sidebands; synchronization of the pulse trains with the molecular motion in the given molecular system; laser self-focusing and spatial soliton formation due to the coherent interaction of light with oscillating molecules. In the future, this Raman source may produce sub-cycle optical pulses, and allow synthesis of waveforms where the electric field is a predetermined function of time, not limited to a quasi-sinusoidal oscillation.
High order Raman generation has received considerable attention as a possible method for generating ultrashort pulses. A large number of Raman orders can be generated when the Raman-active medium is pumped by two laser pulses that have a frequency separation equal to the Raman transition frequency. High order Raman generation has been studied in the different temporal regimes, namely: adiabatic, where the pump pulses are much longer than the coherence time of the transition; transient, where the pulse duration is comparable to the coherence time; and impulsive, where the bandwidth of the ultrashort pulse is wider than the transition frequency. To date, almost all of the work has been concerned with generating as broad a spectrum as possible, but we are interested in studying the spectra of the individual orders when pumped in the transient regime. We concentrate on looking at extra peaks that are generated when the Raman medium is pumped with linearly chirped pulses. The extra peaks are generated on the low frequency side of the Raman orders. We discuss how linear Raman scattering from two-photon dressed states can lead to the generation of these extra peaks.
We suggest a technique to generate subfemtosecond pulse train in a Raman generator driven by three sufficiently intense single-mode laser beams whose carrier frequencies are tuned off the Raman resonance with two different two-photon detunings. The Raman interaction establishes a molecular modulation through a nonadiabatic process. The simulation based on the first fundamental vibrational Raman transition of solid hydrogen indicates that subfemtosecond pulse train can be generated when the signs of the two-photon detunings are opposite. This opens an experimentally feasible way to observe enhanced generation of broadband Raman sidebands as well as compression of the phase-locked Raman sidebands into subfemtosecond pulse trains in the same Raman media with above- and below-resonance Raman excitations.
Ultrabroad-band multifrequency Raman generation is a (laser-driven) resonant-symmetric two-colour pumping technique for exciting polychromatic light beams that are characterized by potentially very wide “top-hat” temporal bandwidths [1]. The spectra associated with such beams may have a width of the same order as the pump frequency, and contain scores of discrete Raman lines of comparable energy [2,3]. Here, we will report findings from the first systematic analysis addressing the interplay between finite material dispersion and detuning (where the frequency difference between the two pump waves no longer exactly matches the Raman transition). Of principal concern is how the dispersion-detuning feedback loop influences bandwidth generation. For simplicity, a continuous-wave model [4] is considered so that the input pulses driving the medium excitation are long compared to the characteristic dephasing time of the polarization wave (and transient dynamics are thus neglected). A host of new results will be unveiled (see Fig. 1), a particular highlight of which is the recent identification of a unique parameter regime wherein bandwidth growth may be strongly enhanced. Fully-numerical calculations are also compared against a generalized three-wave gain-suppression analysis [2] that can accommodate linear detuning.
References
[1] G. S. McDonald, G. H. C. New, L. L. Losev, A. P. Lutsenko, and M. J. Shaw, Opt. Lett. 19, 1400 (1994).
[2] G. S. McDonald, G. H. C. New, L. L. Losev, and A. P. Lutsenko, J. Phys. B: At. Mol. Opt. Phys. 30, L719 (1997).
[3] G. S. McDonald, G. H. C. New, Y. M. Chan, L. L. Losev, and A. P. Lutsenko, J. Mod. Opt. 45, 1099 (1998).
[4] K. S. Syed, G. S. McDonald, and G. H. C. New, J. Opt. Soc. Am. B 17, 1366 (2000).