High-brightness switchable multi-wavelength remote laser in air

Abstract: Remote laser in air based on amplified spontaneous emission (ASE) has produced rather well-collimated coherent beams in both backward and forward propagation directions, opening up possibilities for new remote sensing approaches. The remote ASE-based lasers were shown to enable operation either at ~391 and 337 nm using molecular nitrogen or at ~845 nm using molecular oxygen as gain medium, depending on the employed pump lasers. To date, a multi-wavelength laser in air that allows for dynamically switching the operating wavelength has not yet been achieved, although this type of laser is certainly of high importance for detecting multiple hazard gases. In this Letter, we demonstrate, for the first time to our knowledge, a harmonic-seeded switchable multi-wavelength laser in air driven by intense mid-infrared femtosecond laser pulses. Furthermore, population inversion in the multi-wavelength remote laser occurs at an ultrafast time-scale (i.e., less than ~200 fs) owing to direct formation of excited molecular nitrogen ions by strong-field ionization of inner-valence electrons, which is fundamentally different from the previously reported pumping mechanisms based either on electron recombination of ionized molecular nitrogen or on resonant two-photon excitation of atomic oxygen fragments resulting from resonant two-photon dissociation of molecular oxygen. The bright multi-wavelength laser in air opens the perspective for remote detection of multiple pollutants based on nonlinear spectroscopy.

• The paper demonstrates a novel harmonic-seeded mechanism for dynamic multi-wavelength remote lasing in air.
• It utilizes mid-infrared femtosecond pulses and filamentation to induce ultrafast (<200 fs) population inversion, yielding a 391 nm output 2–3 orders of magnitude stronger than typical fluorescence.
• These findings pave the way for advanced environmental sensing and atmospheric spectroscopy, with applications in air quality monitoring and biological threat detection.

High-Brightness Switchable Multi-Wavelength Remote Laser in Air: A Technical Overview

The paper presents a notable advancement in the domain of remote sensing and spectroscopy, introducing a novel harmonic-seeded switchable multi-wavelength laser capable of operating remotely in ambient air. Historically, remote laser action based on amplified spontaneous emission (ASE) has been established using molecular nitrogen and oxygen as gain media, with typical wavelengths of 391, 337, and 845 nm, contingent on the employed pump laser. This study proposes a groundbreaking method of achieving a dynamically switchable multi-wavelength laser through the seeding of nitrogen laser in air with self-generated harmonics from a tunable mid-infrared femtosecond laser.

Methodology and Experimental Setup

The experimental setup leverages intense mid-infrared femtosecond laser pulses operationalized through an optical parametric amplifier (OPA) system. The employed Ti:Sapphire laser delivers ultrafast pulses with the capability of producing harmonics up to the fifth order. When focused in air, these mid-infrared laser pulses undergo filamentation due to the dynamic balance between Kerr self-focusing and plasma defocusing, which is conducive to nonlinear optical processes, such as harmonic generation.

The authors report that the seeding effect of self-generated harmonics in the gain zone enables the remote laser emission at distinct wavelengths, including 330, 357, 391, 428, and 471 nm, varying with the pump wavelengths within the range of 1682 nm to 2050 nm. A crucial aspect of the study is that the seeding enables high-brightness emission, particularly observed at 391 nm, with a linear polarization mimicking that of the pump laser.

Results and Analysis

The analysis reveals that population inversion is achieved on an ultrafast timescale, less than 200 femtoseconds, owing to the ionization of inner-valence electrons of nitrogen molecules. This ionization distinctly contrasts with previously reported mechanisms such as the recombination of ionized molecular nitrogen, indicating a direct inversion method.

One of the paper’s significant empirical observations is that the intensity of the 391 nm lasing line is 2 to 3 orders of magnitude higher than the fluorescence lines at 357 nm and 337 nm, with the polarization analysis reinforcing the harmonic-seeded lasing action.

Implications and Future Directions

The research implications are two-fold, extending both practical and theoretical realms of laser technology. Practically, the high-brightness switchable multi-wavelength remote laser provides enhanced tools for environmental monitoring, capable of analyzing multiple atmospheric trace gases simultaneously over extensive distances. This development carries potential in monitoring global warming, air quality, and detecting biological threats. Theoretically, the method introduces novel insights into the interaction of strong-field ionization processes with air molecules, unveiling new dynamics in the field of nonlinear optics and femtosecond laser filamentation.

The paper suggests further advancements could be realized with improvements in mid-infrared ultrafast laser technology. Increasing the pump laser pulse energy beyond current limits could potentially augment the system’s peak power efficacy and extend its applicability in various atmospheric conditions and remote locations.

In conclusion, this study marks a substantial scientific contribution in remote laser technologies, offering innovative methodologies for environmental spectroscopy. Continued research and development could refine the technique for broader atmospheric and practical applications, underlining the importance of interdisciplinary collaboration in advancing complex laser systems.