Shermineh Rostami

Air filaments and vacuum

The use of a new diagnostic tool combined with an aerodynamic window forming a boundary between vacuum and air provides differentiation between the “preparation phase” and the “confinement phase” of filaments.

The effect of propagation in air on the filament spectrum

Filamentation studies traditionally start from letting a beam focus in air. We present filament studies with control over the preparation propagation, in air or vacuum, using an aerodynamic window. The spectral content of the filament strongly depends on its preparation medium.

Dramatic enhancement of supercontinuum generation in elliptically-polarized laser filaments

Broadband laser sources based on supercontinuum generation in femtosecond laser filamentation have enabled applications from stand-off sensing and spectroscopy to the generation and self-compression of high-energy few-cycle pulses. Filamentation relies on the dynamic balance between self-focusing and plasma defocusing – mediated by the Kerr nonlinearity and multiphoton or tunnel ionization, respectively. The filament properties, including the supercontinuum generation, are therefore highly sensitive to the properties of both the laser source and the propagation medium. Here, we report the anomalous spectral broadening of the supercontinuum for filamentation in molecular gases, which is observed for specific elliptical polarization states of the input laser pulse. The resulting spectrum is accompanied by a modification of the supercontinuum polarization state and a lengthening of the filament plasma column. Our experimental results and accompanying simulations suggest that rotational dynamics of diatomic molecules play an essential role in filamentation-induced supercontinuum generation, which can be controlled with polarization ellipticity.

Polarization evolution of ultrashort pulses in air.

Measurements of polarization of filamenting light pulses at 800 nm are presented. Electronic nonlinearity, molecular alignment and nonlinear losses all contribute to modify the polarization of a femtosecond filamenting pulse. The polarization is modified in each stage of preparation, filamentation and divergence after the filament.

Videos of light filamentation in air

Light filaments are of interest for applications in remote sensing, communications, and the control of electronic discharges. Different plasma dynamics and emitted radiation have been observed according to the initial pulse characteristics and beam collimation. To help observing and understanding these observations, a new technique of creating a movie of the moving light pulse and the plasma emission in its wake is presented. Over 1,000 synchronized frames of a streak camera are combined to produce the 4D (2 D space, time in ps and wavelength) movie.

Enhanced ablation with a femtosecond-nanosecond dual-pulse

Single-shot ablation of GaAs samples by a collinear femtosecond-nanosecond (fs-ns) dual-pulse is investigated. Significantly enhanced material removal is achieved by optimally combining a single 8 ns pulse at 1064 nm and a single 50 fs pulse at 800 nm in time. The resulting ablation craters are examined for inter-pulse delays ranging from -50 ns (ns first) to +1 μs (fs first) as well as very long delays of ±30 s. Crater profilometry is conducted with white light interferometry and optical microscopy to determine the volume of ablated material and identify surface features that reveal information about the physical mechanism of material removal during fs-ns dual-pulse ablation.

Dynamic properties of large light filament arrays for complex photonic meta-structures in air (Conference Presentation)

High-power femtosecond filaments—laser-light beams capable of kilometer-long propagation—attract interest of nonlinear-optics community due to their numerous applications in remote sensing, lightning protection, virtual antennas, and waveguiding. Specific arrangements of filaments, into waveguides or hyperbolic metamaterials, allow for efficient control and guiding of electromagnetic radiation, radar-beam manipulation, and resolution enhancement. These applications require spatially uniform distribution of densely packed filaments. In order to address this challenge, we investigate the dynamic properties of large rectangular filament arrays propagating in air depending on four parameters: the phase difference between the neighboring beams, the size of the array, separation between the beams, and excitation power. We demonstrate that, as a result of the mutual interaction between the filaments, the arrays where the nearest neighbor beams are out-of-phase are more robust than the arrays with all the beams in phase. Our analysis of the array stability reveals that there exist certain trade-offs between the stability of a single filament and the stability of the entire array. We show that in the design of the experiment, the input parameters have to be chosen in such a way that they ensure a sufficiently high filling fraction, but caution has to be used in order not to compromise the overall array stability. In addition, we show the possibility of filament formation by combining multiple beams with energies below the filamentation threshold. This approach offers additional control over filament formation and allows one to avoid the surface damage of external optics used for filamentation.

Enhanced supercontinuum generation by polarization control of filamentation in molecular gases

Supercontinuum generation by filamentation in molecular gases is optimized by studying the ellipticity of the pulse polarization during the interaction with the species of the gas medium via strong field ionization and molecular alignment effects.

Movies of filaments and plasma

Using a streak camera, and combining over 1,000 synchronized frames, 4D (2 D space, time in ps and wavelength) movies of the light and plasma emission in the wake of filaments are produced.

Evolution of ellipticity of ultrashort infrared pulses propagating in air

We measure the polarization evolution of ultrashort pulses (filaments) propagating in air for various pulse widths and energies. We observe the change of light ellipticity close to circular polarization.