Theoretical And Experimental Investigation Of Terahertz Parametric Sources Based On Stimulated Polariton Scattering

Terahertz parametric sources have the advantages of compactness,room temperature operation and widely tunable range among terahertz sources.Terahertz parametric sources are based on stimulated polariton scattering.As is known,polariton is the products of infrared lasers and the A1 inodes in terahertz parametric crystals.An excellent terahertz parametric crystal should have relatively high nonlinear coefficients,optical damage threshold and wide spectral transparent range,as well as strong A,modes.LiNbO₃ crystal,KTP crystal,KTA crystal and RTP crystal can all be well operated as terahertz parametric crystals.With pumping at 1064 nm,terahertz parametric sources can generate terahertz wave at 1 THz to 7 THz.Frequency gaps appear in tuning process while applying KTP crystal,KTA crystal and RTP crystal into tunable terahertz parametric sources.These frequency gaps will break the tunable ranges of terahertz wave and different peak output energies of terahertz wave would be gained in tuning process.Coupling equations of stimulated polariton scattering can be used to demonstrate the behaviors of terahertz wave,obtained from terahertz parametric sources.Given that the interaction areas are collinear and the electrical field of pump wave is constant,researchers have derived the expression of terahertz gain.However,such assumptions cannot be applied into intense terahertz parametric sources,especially with a large non-collinear phase matching angle and a strong pump depletion.According to the theory of stimulated polariton scattering,the gain coefficient of terahertz wave is consisted of two parts.One is the second-order nonlinear coefficient and the other is the equivalent second-order coefficient.In this thesis,we confirmed the dependence of the equivalent second-order coefficient on the infrared strength and the Raman strength of A1 modes.We gave reasonable explanations of THz frequency gaps and theoretical analysis of the influence of the A1 inodes on terahertz generation in stimulated polariton scattering.And with a KTP surface-emitted terahertz parametric oscillator,we obtained high output energy of terahertz wave.Based on surface-emitted injection-seeded terahertz parametric generators of MgO:LiNbO₃ crystal,we took non-collinear phase matching at rather a distinct angle and the pump depletion into consideration.We proposed a precise numerical methods to solve the coupling equations.Comparing the theoretical results and experimental results,we confirmed the eligibility of the numerical simulations.The main contents of this thesis include::1.Based on the coupling equations of stimulated polariton scattering,we investigated the gain coefficient of terahertz wave.Equivalent second-order coefficient Re(?2j"in the gain coefficient is proportional to the product of parameter ?j and ?j?WT?.According to the analysis of the three parameters ?i,?j?wT?and Re??2j"?,the parameter ?j is proportional to the product of the infrared oscillating strength and the Raman scattering strength of A1 mode,the parameter ?j?wT?is related with the frequency of terahertz wave and A1 mode.The modes near terahertz frequency wT have greater influence than those far from wT.In frequency areas very close to the A1 modes,the terahertz gain cannot win over the great absorption,the SPS process fails and no terahertz wave can be obtained.When the pumping top-hat pulse laser wavelength was 1064 nm,the external angle between the Stokes beam and the pump beam was from 1.250° to 5.625°,on a KTP surface-emitted terahertz parametric oscillator.The obtained terahertz wave could be intermittently tuned from 3.04 THz to 3.50 THz,from 4.28 THz to 4.54 THz,from 4.82 THz to 4.99 THz and from 5.47 THz to 6.04 THz.The maximum terahertz output energy was 1.56 ?J under the pump energy of 256 mJ.2.On the assumption of non-collinear phase match with a distinct angle and of great pump depletion,we proposed numerical solutions to the surface-emitted injection-seeded parametric generator of MgO:LiNbO₃.Under steady-state condition,we reasonably divided the interaction zones and brought up numerical simulations.With pumping at a wavelength of 1064.16 nm and seeding at a wavelength of 1070.76 nm,we investigated the surface-emitted injection-seeded parametric generator of MgO:LiNbO₃.When the input pump pulse energy was 63.6 mJ and seeded Stokes pulse energy was 9.8 mJ,the output terahertz energy was 0.69 ?J.We set suitable values of parameters in numerical models,the theoretical results can agree well with experimental results.