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• Filament-induced aerosol fluorescence emission appears a strong angular dependence. • The backward angular distribution of the aerosol fluorescence can be attributed to Mie scattering. • The divergence angle of aerosol fluorescence radiation is affected by the particle size distribution. Backward angular distributions of the sodium fluorescence in NaCl aerosols induced by femtosecond laser filamentation in air with a distance of 10 m are measured. The recorded signals appear as strong angular dependence. Further study indicates that it could be associated with the Mie scattering of the filamentation-induced "aerosol lasing" in the forward direction. These results will benefit the remote-sensing of air pollutants by femtosecond laser filamentation-based LIDAR (light detection and ranging). [ABSTRACT FROM AUTHOR]

• Acoustic method was adopted to quantitatively measure the long-distance filamentation. • A simple physical model was presented to analyze the spatial distribution of filament. • The onset and length of filament can be conveniently obtained from the model. • The increase of the filament length can be obtained for longer pulse with more energy. The promising application of femtosecond laser filamentation in atmospheric remote sensing brings imperative demand for diagnosing and controlling the spatiotemporal dynamics of long-distance filamentation. Here, the acoustic method was adopted to quantitatively diagnose the long-distance filamentation of femtosecond pulses manipulated by energy and temporal domain. The onset and length of filament can be conveniently obtained from a simple analytic formula based on reasonable approximation and further demonstrated by the experimental results. The influence of pulse energy and initial pulse chirp under three different focal lengths (∼10 m, 20 m, and 30 m) on the filamentation in the air were studied experimentally. These findings provide a guiding significance for the optimal control of the long-distance propagation of filament, thereby laying a firm foundation for femtosecond laser-based atmospheric remote sensing. [ABSTRACT FROM AUTHOR]

• A graphene foam-based MHz ultrasonic source is excited by femtosecond laser. • Its bandwidth is 100 times larger than that by the piezoelectric transducer. • The ultrasonic source has a dipole-like pressure distribution. The performance of a photo-thermo-acoustic (PTA) ultrasonic source can be significantly improved by utilizing graphene-based materials. Graphene is an excellent PTA material owing to its wide electromagnetic absorption spectrum, low heat capacity per unit area, and high thermal conductivity. In this study, a broadband graphene-foam-based PTA ultrasonic source covering the frequency range of 50 kHz to 1.8 MHz was excited by a near-infrared femtosecond laser beam. The lower and upper frequency limits of the acoustic wave that can be detected in the experiments were determined by the responsivity of the microphone and the attenuation of the acoustic wave in air, respectively. The experimental results show that the sound pressure of this ultrasonic source was independent of the laser polarization and incident angle. The peak-to-peak magnitude of the ultrasonic wave was proportional to laser energy when the single-pulse energy of the femtosecond laser varied from 0.4 to 1.0 mJ. Therefore, the sound intensity of the ultrasonic source could be easily controlled by modulating the laser energy. The experimental results also show that the ultrasonic wave emitted from the graphene foam had a dipole-like acoustic pressure distribution, and its principal emission direction was normal to the sample surface, regardless of the laser incidence angle. This characteristic may benefit future applications in directed message transfer/acquisition and nondestructive testing/imaging. [ABSTRACT FROM AUTHOR]

• Large birefringence of THz wave inside a femtosecond laser filament was observed. • The birefringence is attributed to the strong confinement of THz wave inside the filament. • THz wave with arbitrary polarization orientation, ellipticity and chirality can be produced. Large birefringence of THz wave inside a femtosecond laser filament in air has been found in this work. By varying the laser pulse energy, the filament length is changed, inducing variation in the phase delay between two perpendicularly polarized THz pulses. The birefringence could be explained by the delayed rotational molecular wave packet of air molecules inside the femtosecond laser-induced filament. The newly discovered THz birefringence is essentially attributed to the strong confinement of the THz pulse inside the filament. This phenomenon has opened a new way to freely control the polarization of THz pulses emitted from an optical filament and significantly benefits polarization-sensitive THz spectroscopy and control of matter. [ABSTRACT FROM AUTHOR]