Strong excitation with high electromagnetic fields can generate lasing in gaseous media by resonant excitation. Previous efforts have shown that the lasing signal can be emitted perpendicular to the direction of the pump laser if the shape of the gain volume can be controlled. This paper reports on the characterization of sideways emitted lasing obtained through two-photon pumping in oxygen atoms present in a flame. Intense 35-fs laser pulses of 226-nm wavelength focused into a sheet by a cylindrical mirror are used to form a gain volume geometry that yields an 845-nm lasing beam that is emitted perpendicular to the pump-beam direction. The sideways lasing has been investigated in terms of its pump laser energy dependence, excitation and emission spectra, and spatial characteristics. Single-shot images of the emission exhibit interference fringes, revealing that the emission contains multiple coherent spatial modes. The results indicate that superfluorescence constitutes a significant contribution to the sideways lasing. We believe that this is the first important step for understanding and using sideways lasing as a phenomenon and tool for future applications and inventions.
Strong excitation with high electromagnetic fields can generate lasing in gaseous media by resonant excitation. Previous efforts have shown that the lasing signal can be emitted perpendicular to the direction of the pump laser if the shape of the gain volume can be controlled. This paper reports on the characterization of sideways emitted lasing obtained through two-photon pumping in oxygen atoms present in a flame. Intense 35-fs laser pulses of 226-nm wavelength focused into a sheet by a cylindrical mirror are used to form a gain volume geometry that yields an 845-nm lasing beam that is emitted perpendicular to the pump-beam direction. The sideways lasing has been investigated in terms of its pump laser energy dependence, excitation and emission spectra, and spatial characteristics. Single-shot images of the emission exhibit interference fringes, revealing that the emission contains multiple coherent spatial modes. The results indicate that superfluorescence constitutes a significant contribution to the sideways lasing. We believe that this is the first important step for understanding and using sideways lasing as a phenomenon and tool for future applications and inventions.
Lasing refers to the generation of highly directional laser-like emission remotely pumped by laser pulses in a medium such as ambient air or a flame.It has been intensively investigated because of its potentials to revolutionize the field of remote sensing.Earlier studies were carried out in flames, where lasing effect has been observed in various radicals, for example H, O, N etc., through two-photon resonant excitation.This scheme was later applied to generate lasing of O and N in ambient air, which can be dramatically improved by predissociation of the air constituents.However, the underlying physical mechanism of two-photon-excited lasing effect is still unsettled although it has been studied for more than one decade.Currently there are three different proposals: (1) Amplified Spontaneous Emission (ASE), in which case population inversion is responsible for optical gain;(2) Superfluorescence (SF), where a macroscopic dipole occurs in the population inverted medium; (3) Both hyper-Raman scattering and four-wave mixing are involved in the generation of lasing emission.Here, we experimentally studied 656 nm lasing emission of atomic hydrogen via two-photon resonant excitation by focusing 125-fs laser pulses of 205-nm wavelength in a methane/oxygen welding flame (see Fig. 1), where hydrogen atoms are naturally present.Lasing occurs in both forward and backward directions, and the forward signal strength is almost one order of magnitude stronger than the backward one.It has been found that the durations of lasing pulses are around 20 ps, and decreases with increasing pump laser energies.Furthermore, the delay of the lasing pulse with respect to the pump pulse decreases with increasing pump energies.These results show high similarities with the behaviors of SF, suggesting that the femtosecond two-photonexcited lasing emission of atomic hydrogen might be SF.
There exists a lack of laser-based remote sensing methods that give signals with high intensity and directionality. According to previous research, mirrorless lasing can meet these requirements; however, neither the dependence on the gain medium geometry nor the light-matter interaction responsible for the emission has been investigated in detail. This thesis project examined the sideways lasing emission from a gain medium with a Fresnel number much smaller than one, generated by the overlap of two counterpropagating femtosecond laser pulses in xenon gas at atmospheric pressure. Both spatial and temporal measurements of the sideways lasing emission were made, with the latter being obtained using a streak camera. The emission was highly localised in space, suggesting it originated from the overlap. It also yielded a power dependence with a clear threshold and an interference pattern indicating coherence, both of which showed strong agreement with the predicted behaviour for superfluorescence (SF). The recorded temporal profile displayed a pulse with oscillations on the incline and the decline, appearing with a delay of about 70 ps relative to the excitation pulse and thus corresponding well to the theoretical predictions for SF emitted from a gain medium with a Fresnel number much smaller than one. It can therefore be concluded that the observed sideways lasing was generated by SF. Additionally, the dependence of the emission on the gain medium geometry was in good accordance with theory. These measurements, to the best of the author's knowledge, provide the first thorough investigation of mirrorless lasing at atmospheric pressure for this gain medium configuration, thus opening the way for both further fundamental research and the implementation of these techniques in applied laser diagnostics.
We investigate the excited-state dynamics of hydrogen (H) atoms in flames by using a femtosecond (fs) pump-probe scheme and measuring directional emission signals. An approximately 100-fs pump pulse at 205.1 nm excites H atoms through a two-photon process (n = 3 ←← n = 1), which is followed by detection of the forward emission signals induced by a broadband fs probe pulse near 656 nm. Above a certain threshold, we observe a quadratic dependence of the emission signal on the pump laser energy. Moreover, the linewidth of the forward emission signal varies with the probe delay and the probe laser energy. This behavior can be explained in terms of superradiance. We perform a theoretical analysis and compare the experimental results with the theory, and conclude that, within the conditions of our experiment, the behavior of the atomic system involves atomic coherence, which is produced non-adiabatically and corresponds to a superradiant process. Variations in the duration of the gain time window and lifetime of the excited-state H atoms in flames are explored at different flame conditions (i.e., equivalence ratio and heights above the burner). The fs pump-probe technique demonstrated here can also be extended to characterize the time-resolved population dynamics and the corresponding collisional energy transfer rates for energy levels involved in laser-induced fluorescence detection of H atoms in flames relevant to practical combustion applications.
Among different approaches to generate mirrorless lasing, resonant multiphoton pumping of gas constituents by deep-UV laser pulses exhibits so far the highest efficiency and produces measurable lasing energies, but the underlying mechanism was not yet fully settled. Here, we report lasing generation from atomic oxygen in a methane-air flame via femtosecond two-photon excitation. Temporal profiles of the lasing pulses were measured for varying concentrations of atomic oxygen, which shows that the peak intensity and time delay of the lasing pulse approximately scales as N and 1/N, respectively, where N represents the concentration. These scaling laws match well with the prediction of oscillatory superfluorescence (SF), indicating that the lasing we observed is essentially SF rather than amplified spontaneous emission. In addition, the quantum-beating effect was also observed in the time-resolved lasing pulse. A theoretical simulation based on nonadiabatic Maxwell-Bloch equations well reproduces the experimental observations of the temporal dynamics of the lasing pulses. These results on fundamentals should be beneficial for the better design and applications of lasing-based techniques.
Argon gas excited by resonant femtosecond ultraviolet pulses gives rise to cavity-free lasing emission in the near-infrared (NIR) range. Here we reported on a pump-probe study of the optical gain of this lasing phenomenon. With the injection of an external seeding pulse, the forward signal was significantly enhanced, confirming the existence of optical gain. The temporal dynamics of the optical gain were characterized by a time-resolved measurement. It was found that the optical gain decays on a time scale of ∼ 10 ps and it does not present a significant dependence on the gas pressures. Moreover, the intensity of the forward NIR emission signal shows a linear dependence on the gas pressure. These features suggest that the nature of this forward NIR radiation is amplified spontaneous emission, not superradiance when multiple-photon resonant excitation is involved.
Optical phase-conjugation properties of the backward-stimulated radiation from a two-photon-pumped frequency-upconversion lasing medium are studied. The gain medium is a new dye solution pumped with ∼11-ns and ∼1064-nm laser pulses; a highly directional and phase-conjugate backward-stimulated emission at ∼616-nm wavelength could be obtained with a net conversion efficiency of ∼10%. The spectral, temporal, and output–input characteristics of this kind of stimulated emission are presented in detail. The phase-conjugation properties of the backward-stimulated emission have been demonstrated by both the far-field (beam-divergence) measurement and the near-field (image-reconstruction) measurement. The wave-front reconstruction behavior is not perfect, especially under a larger aberration influence.