Generation Mechanism And Characteristic Of Nonlinear Spectral Phase In Ultrafast Optical Parametric Processes

Ultra-short ultra-intense laser has gained rapid development since the invention of chirpedpulse amplification(CPA)and optical parametric chirped-pulse amplification(OPCPA)schemes.Compared with conventional CPA based on laser gain media,OPCPA based on optical parametric amplifier(OPA)has higher gain and broader gain bandwidth.So far,OPCPA has been able to produce laser peak power exceeding petawatt within a pulse width as short as few optical-cycle.Such an ultra-short and ultra-intense laser pulse provides a promising tool for ultrafast science and strong field physics.Broadband phase-matching(PM)is the core of OPCPA,which requires that the PM condition is highly insensitive to signal wavelength.In general,PM at central wavelength can be achieved by tuning the PM angle of birefringent crystal.The group velocity matching between signal and idler pulses(i.e.the first-order differential of PM condition)shall be realized by exploiting collinear wavelength degeneracy or noncollinear interaction.Specially,“magic” PM(i.e.the noncollinear angle is insensitive to the signal wavelength)can be obtained at some specific wavelengths,which can support an octave-spanning PM bandwidth.However,an ultrabroadband PM bandwidth does not necessarily lead to an ultra-short pulse.One reason is that broadband OPCPA is always subject to certain amounts of phasemismatch,even in the “magic” PM condition.Only the central frequency of signal satisfies perfect PM,and other frequency components experience frequency-dependent phase-mismatch.During parametric amplification,such a frequency-dependent phase-mismatch results in nonlinear spectral phase distortion of the signal,which hampers ideal compression of amplified signal pulse and consequently necessitates precise control and compensation.However,there are few dedicated investigations on the nonlinear spectral phase in OPCPA to date,and its physical mechanism has remained unclear: why does this effect that occurs in second-ordernonlinearity-based OPCPA behaves as third-order nonlinearity? In addition,the signal pulse will also accumulate linear spectral phase induced by the crystal dispersion.The final spectral phase of the output pulse is determined by the nonlinear spectral phase induced during amplification and the linear dispersion phase together.Thus,it is necessary to explore the correlation between these two kinds of spectral phase.The research results are of importance to guide the design of few-cycle intense OPCPA systems.In addition,whether this “harmful” nonlinear spectral phase can be manipulated and utilized is also a physics problem of great research value.Focusing on the nonlinear spectral phase in OPCPA,this thesis is dedicated to a fundamental research on its generation mechanism,evolution characteristic as well as potential applications.The main works and research achievements include the following three aspects: 1.The generation mechanism of nonlinear spectral phase in OPCPA is newly revealed,and the phase evolution characteristic during parametric amplification as well as its effect on pulse compression are also comprehensively studied.In a broadband OPCPA system,the nonlinear spectral phase is inevitably produced by the intrinsic phase-mismatch due to crystal dispersions.By viewing OPCPA as a series of narrowband OPAs in the time domain with individual signal frequencies,we link the nonlinear spectral phase to the cascaded nonlinear phase shifts due to intrinsic phase-mismatch and demonstrate that nonlinear spectral phase essentially originates from the cascaded quadratic nonlinearity.Based on the numerical studies,we show that the nonlinear spectral phase increases with parametric gain,which approaches to half the intrinsic phase-mismatch in the high gain limit.In this way,nonlinear speatral phase directly links with the crystal-dispersionrelated linear spectral phase.We also find that the effect of nonlinear spectral phase in the spatial domain is typically negligible,which makes the nonlinear spectral phase compensation in the spectral domain sufficient.2.The effect of idler absorption on the signal nonlinear spectral phase in quasi-parametric chirped pulse amplification(QPCPA)is thoroughly studied.It is newly found that the nonlinear spectral phase can be flat by designing a nonuniform idler absorption spectrum.QPCPA is a novel scheme wherein back conversion of OPCPA get inhibited,so it enables amplification with high efficiency and broadband bandwidth.The suppression of back conversion not only allows signal efficiency approaching to the quantum limit but also greatly increases the phase-mismatch tolerance of QPCPA.In consequence,the characteristics of cascaded nonlinearity and nonlinear spectral phase in QPCPA scheme are of dramatic difference compared to that in conventional OPCPA.The simulation results show that the nonlinear spectral phase in QPCPA is much less than that in OPCPA under the condition of same phasemismatch.Specially,by designing the idler absorption spectrum as inversed-Gaussian profile,the nonlinear spectral phase becomes almoat flat and thus the compression pulse gets significantly improved.3.A novel ultrafast group-velocity control platform based on OPCPA in chirped quasi-phase-matching(QPM)structures is proposed.The cascaded nonlinearity induced by group-velocity mismatch(GVM)possesses a broadband resonance characteristic,enabling group-velocity control in the femtosecond regime.Previous group-velocity control platform based on the resonant response of the material syatems is always limited by the absorption,response bandwidth and fidelity.In our proposed control platform,the resonant cascaded nonlinear phase underlies the group-velocity control.Specifically,the effect of GVM contributes to a resonant cascaded nonlinearity that influences group velocity by a large time delay.And the bandwidth limitation in resonant cascaded nonlinearities can be resolved by using chirped QPM,which makes group-velocity control possible in the femtosecond regime.As numerically demonstrated in the OPA based on lithium niobate crystal,the signal and idler pulse can be significantly delayed in time comparable to the signal duration while keep high fidelity for durations down to 100 fs until the crystal dispersion becomes effective.Furthermore,the group delay can be switched from positive to negative by changing the sign of GVM.Finally,we also study the group-velocity control in sum frequency generation processes and find that larger group delay can be achieved at a lower pump intensity.These results enhance our understanding of cascaded nonlinearity with respect to its resonance nature and pave the way for putting ultrafast group-velocity control into reality.