Infrared-pumped third-harmonic and sum-frequency generation in diatomic molecules

Abstract

Using molecular hydrogen as a model system, we present the first calculation which demonstrates the feasibility of efficient third-harmonic and sum-frequency generation of ir laser radiation by diatomic molecular gases. A conversion efficiency of 10% without phase matching is achievable with a modest laser intensity of about 20 MW/cm2. In addition to a two-photon resonance, the efficient conversion depends on a mechanism which makes use of virtual vibronic transitions exclusively. Possible methods for phase matching are also suggested.

Vibrational excitation of hydrogen molecules by two-photon absorption and third-harmonic generation

“…Next, we introduce the formula for THG in diatomic molecules that is based on that in She and Billman [1,7]. The definitions of the variables are slightly different from those in [1].…”

“…The lowest-order process is third-harmonic generation (THG), which is similar to coherent anti-Stokes Raman scattering. Of the many theoretical and experimental studies on general THG, She and Billman [1] investigated the theory of using the vibrational states of diatomic molecules for THG and found that a high conversion efficiency of up to 30% could be expected in hydrogen gas using an optimized column density and an input laser intensity of 34 MW cm −2 . Palastro et al [2] discussed vibrational two-photon absorption and THG by nitrogen and oxygen and their effects on the propagation of ultrafast long-wave infrared pulses.…”

“…In this study, we improved the mid-infrared system to reduce the linewidth and achieve higher coherence using two-photon absorption and thus succeeded in observing the resultant THG in parahydrogen gas. We also modified the THG formula of She and Billman [1] to consider realistic experimental conditions, especially the linewidth of the laser and decoherence time. She and Billman [1] did not consider the linewidth of the laser and the decoherence time was estimated from the Doppler width and treated as a constant.…”

“…We also modified the THG formula of She and Billman [1] to consider realistic experimental conditions, especially the linewidth of the laser and decoherence time. She and Billman [1] did not consider the linewidth of the laser and the decoherence time was estimated from the Doppler width and treated as a constant. The dependence of the intensity of the third harmonic on the mid-infrared pulse energy, target pressure, and cell length are observed and compared with the results obtained with the modified formula.…”

“…The paper is organized as follows: in section 2, we introduce the simple representation of THG in the diatomic molecules, which is based on the paper of She and Billman [1], and then modify it for our study to account for realistic conditions. Section 3 presents the experimental setup used, especially the upgraded mid-infrared system.…”