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A Parametric Study of Mirrorless Lasing Dynamics

Klara Lozani Gerdhem

Published Jan 1, 2026• 0 citations
OpenAlex

Abstract

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.

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Spatial characterization of sideways lasing

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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.

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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.

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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.

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The transition from the amplification of spontaneous emission to superfluorescence in a three-level and swept-gain medium excited by an x-ray free-electron-laser pulse is theoretically investigated. Given the specific timescale of an x-ray free-electron-laser pulse, we investigate the swept pumping process in detail, and our results show that the temporal structure of an x-ray free-electron-laser pulse plays a more critical role than its peak intensity does for producing population inversion. The typical watershed of two radiant regions depends on the optical depth of the gain medium for a given coherence time, namely, the particle number density and medium length are equally important. However, we find that medium length plays a more important role than particle density for making the forward-backward asymmetry. The transient gain length and the total medium length are identified as two important factors to observe length-induced backward transition. The present results suggest an application of parametric controls over a single-pass-amplified light source.

arXiv:1911.00450•Physical Review A•Feb 24, 2020•7 citations
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.
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.
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.
Air lasing induced by laser filamentation opens a new route for research on atmospheric molecular physics and remote sensing. The generation of air lasing is composed of two processes, i.e., building up optical gain of air molecules in femtosecond time scale and emitting coherent radiation in picosecond time scale. Here, we focus on the emission mechanisms of N 2 + air lasing and reveal, by examining the intensities and temporal profiles of N 2 + lasing at 391 nm generated respectively in a time-varying polarization-modulated and a linearly polarized pump laser field under different nitrogen gas pressures, that the N 2 + lasing can emit through either triggered super-radiance or seed amplification. We find that the two pressure-sensitive factors, i.e., the dipole dephasing time T 2 and the population inversion density n , determine which of these two mechanisms dominates the N 2 + lasing emission process, enabling manipulation of the transition from triggered super-radiance to seed amplification or vice versa. Our findings clarify the emission mechanism of N 2 + lasing under different pressures and provide a deeper understanding of N 2 + air lasing not only in the establishment of optical gain but also in the lasing emission process.
This dissertation investigates three different optical instabilities: cw conical emission, bifurcations of optical transverse solitary waves, and asymmetric gain modification in a vertical cavity surface emitting laser (VCSEL). The first two instabilities occur in sodium vapor while the third occurs in a new type of semiconductor laser. For the conical emission instability, the first comparison and agreement between theory and experiment is given. The conical emission process is modeled as a series of nonlinear effects including self-trapping of the pump beam, Doppler-broadened Raman-gain amplification of Doppler-broadened resonance fluorescence, propagational four-wave mixing, and diffraction and pump-induced refraction of the new frequencies. Each step of the emission process is thoroughly examined and explained. Detailed comparisons are made between experimental observables and numerical calculations including near-field and far-field spatial profiles, frequency spectra, and cone angles. All comparisons show good agreement. In a different but related experiment, bifurcations of optical transverse solitary waves are studied for one-way propagation through a sodium vapor cell. Two types of phase encoding seed transverse bifurcations which result in transverse cell-exit profiles with the beauty rivaling that of a kaleidoscope. The cell-exit profiles are stationary in time, reproduce completely when the power or frequency is scanned, and agree well with one-way computations. Evidence is also presented for the first observation of a double-peaked Raman gain. In the third experiment, a temporal instability is examined in a GaAs VCSEL. Strong evidence is presented for an asymmetric gain modification induced when a strong optical signal is injected into the Fabry-Perot transmission peak of the VCSEL while it is lasing. This gain modification results in increased absorption on the high frequency side of the injected signal and increased gain on the low frequency side as manifested by the formation of an optical sideband. The frequency of the sideband depends on the intensity of the injected signal with frequencies (> 50 GHz) well beyond the predicted relaxation oscillation frequency being observed. Very efficient nondegenerate four-wave mixing is also observed.
The transition from the amplification of spontaneous emission to superfluorescence in a three-level and swept-gain medium excited by an x-ray free-electron-laser pulse is theoretically investigated. Given the specific timescale of an x-ray free-electron-laser pulse, we investigate the swept pumping process in detail, and our results show that the temporal structure of an x-ray free-electron-laser pulse plays a more critical role than its peak intensity does for producing population inversion. The typical watershed of two radiant regions depends on the optical depth of the gain medium for a given coherence time, namely, the particle number density and medium length are equally important. However, we find that medium length plays a more important role than particle density for making the forward-backward asymmetry. The transient gain length and the total medium length are identified as two important factors to observe length-induced backward transition. The present results suggest an application of parametric controls over a single-pass-amplified light source.