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Register a new userBroadband terahertz emission from two-color fs-laser-induced microplasmas
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We investigate terahertz emission from two-color fs-laser-induced microplasmas. Under strongest focusing conditions, microplasmas are shown to act as point-sources for broadband terahertz-to-far-infrared radiation, where the emission bandwidth is determined by the plasma density. Semi-analytical modeling allows us to identify scaling laws with respect to important laser parameters. In particular, we find that the optical-to-THz conversion efficiency crucially depends on the focusing conditions. We use this insight to demonstrate by means of Maxwell-consistent 3D simulations, that for only 10-$mu$J laser energy a conversion efficiency well above $10^{-4}$ can be achieved.
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We study the influence of the polarization states of femtosecond two-color pulses ionizing gases on the emitted terahertz radiation. A local-current model and plane-wave evaluations justify the previously-reported impact on the THz energy yield and an (almost) linearly-polarized THz field when using circularly-polarized laser harmonics. For such pump pulses, the THz yield is independent on the relative phase between the two colors. When the pump pulses have same helicity, the increase in the THz yield is associated to longer ionization sequences and higher electron transverse momenta acquired in the driving field. Reversely, for two color pulses with opposite helicity, the dramatic loss of THz power comes from destructive interferences driven by the highly symmetric response of the photocurrents lined up on the third harmonic of the fundamental pulse. While our experiments confirm an increased THz yield for circularly polarized pumps of same helicity, surprisingly, the emitted THz radiation is not linearly-polarized. This effect is explained by means of comprehensive 3D numerical simulations highlighting the role of the spatial alignment and non-collinear propagation of the two colors.
We report enhanced broadband Terahertz (THz) generation and detailed characterization from the interaction of femtosecond two colour laser pulses with thin transparent dielectric tape target in ambient air. The proposed source is easy to implement, exhibits excellent scalability with laser energy. Spectral characterization using Fourier transform spectrometer reveals yield enhancement of more than 150 % in the THz region of 0.1 - 10 THz with respect to conventional two-colour laser plasma source in ambient air. Further, the source spectrum extends up to 40 THz with an enhancement of flux > 30 %. Experimental results, well supported with two-dimensional particle-in-cell simulations establishes that the transient photo-current produced by the asymmetric laser pulse interaction with air plasma as well as near solid density plasma formed on the tape surface is responsible for the enhanced terahertz generation. The source will be useful for the multidisciplinary activities and ongoing applications of the laboratory-based terahertz sources.
We disclose an unanticipated link between plasmonics and nonlinear frequency down-conversion in laser-induced gas-plasmas. For two-color femtosecond pump pulses, a plasmonic resonance is shown to broaden the terahertz emission spectra significantly. We identify the resonance as a leaky mode, which contributes to the emission spectra whenever electrons are excited along a direction where the plasma size is smaller than the plasma wavelength. As a direct consequence, such resonances can be controlled by changing the polarization properties of elliptically-shaped driving laser pulses. Both, experimental results and 3D Maxwell consistent simulations confirm that a significant terahertz pulse shortening and spectral broadening can be achieved by exploiting the transverse driving laser beam shape as an additional degree of freedom.
Graphene is an ideal material for integrated nonlinear optics thanks to its strong light-matter interaction and large nonlinear optical susceptibility. Graphene has been used in optical modulators, saturable absorbers, nonlinear frequency converters, and broadband light emitters. For the latter application, a key requirement is the ability to control and engineer the emission wavelength and bandwidth, as well as the electronic temperature of graphene. Here, we demonstrate that the emission wavelength of graphene$$ s broadband hot carrier photoluminescence can be tuned by integration on photonic cavities, while thermal management can be achieved by out-of-plane heat transfer to hexagonal boron nitride. Our results pave the way to graphene-based ultrafast broadband light emitters with tunable emission.
We consider a two-color formaldehyde PLIF thermometry scheme using a wavelength-switching injection seeding Nd:YAG laser at 355 nm. The 28183.5 cm-1 and 28184.5 cm-1 peaks of formaldehyde are used to measure low temperature combustion zone. Using a burst mode amplifier and a high speed camera, high-repetition rate (20 kHz) temperature field measurement is validated on a laminar coflow diffusion flame and demonstrated on a turbulent confined jet in hot crossflow flame.
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