The effect of the length and pressure of an argon gas jet on low-order harmonics (5th, 7th, 9th, and 11th) generation by 4.5-μm femtosecond laser radiation of a Fe:ZnSe laser system has been studied experimentally. It has been shown that an increase in the length of the generation medium up to the waist length allows one to increase the generation efficiency by a factor of 12. It has also been demonstrated that a change in the length of the gas medium changes the pressure dependence of the energy of the generated radiation because of change in the phase matching conditions, whose correct simulation requires the inclusion of nonlinear propagation effects for pump pulse and generated harmonics.

The generation of low-order harmonics in the gas medium by femtosecond laser radiation of unique mid-IR Fe: ZnSe laser system operating at $4.5\ \mu\mathrm{m}$ wavelength is investigated. The dependence of the harmonics generation efficiency on the experimental parameters is studied, that allows optimization of the harmonics yield. The present work is a new step towards high-order harmonics generation driven by low-frequency laser fields.

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense (I ~ 1014 W/cm2) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is 4.55 μm, pulse duration by the FWHM level of intensity is 160 fs, and the pulse energy is up to 3.5 mJ) with an argon jet (pressure is up to 10 bar) in the tunneling ionization regime (Keldysh parameter is $$\gamma = 0.2$$ ). The maximum energy efficiencies of the 5th, 7th, and 9th harmonic generation are 2 × 10–7, 6 × 10‒9, and 3 × 10–10, respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation.

В данной работе экспериментально исследовано влияние длины и давления газовой струи аргона на процесс генерации гармоник низкого порядка (5, 7, 9, 11) фемтосекундным излучением лазерной системы на кристалле Fe:ZnSe c длиной волны 4.5 мкм. Экспериментально установлено, что увеличение длины среды генерации вплоть до длины перетяжки позволяет более чем на порядок (в 12 раз) увеличить эффективность генерации. Кроме того, показано, что изменение длины газовой среды также приводит к изменению зависимости энергии генерируемого излучения от давления, что является следствием изменения условий фазового согласования, для корректного моделирования которых необходимо учитывать нелинейно-оптические эффекты распространения генерирующего излучения.

Low-order (fifth, seventh, and ninth) harmonics have been generated under the interaction of intense ( I 10 14 W/cm 2 ) femtosecond mid-infrared radiation of a laser system based on a Fe:ZnSe crystal (wavelength is 4.55 μm, pulse duration by the FWHM level of intensity is 160 fs, and the pulse energy is up to 3.5 mJ) with an argon jet (pressure is up to 10 bar) in the tunneling ionization regime (Keldysh parameter is γ = 0.2 ). The maximum energy efficiencies of the 5 th , 7 th , and 9 th harmonic generation are 2 × 10 –7 , 6 × 10 ‒9 , and 3 × 10 –10 , respectively. It has been established that nonlinear effects of propagation of generating radiation under an increase in the pressure of the gas jet begin to significantly affect the process of generation.