Haut PDF Terahertz Wave Generation in Air by Femtosecond Optical Vortex Pulses

Terahertz Wave Generation in Air by Femtosecond Optical Vortex Pulses

Terahertz Wave Generation in Air by Femtosecond Optical Vortex Pulses

HAL Id: hal-02327613 https://hal.archives-ouvertes.fr/hal-02327613 Submitted on 22 Oct 2019 HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

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Vortex terahertz wave generation in air by femtosecond optical vortex pulses

Vortex terahertz wave generation in air by femtosecond optical vortex pulses

more than octave spectral bandwidth of vortex generation. First SH was circularly polarized by a quarter wave plate (QWP) and converted to an optical vortex (OV) beam by an S-waveplate (RPC-405-06-557, Workshop of Photonics) (SWP). Subsequent polarization filtering by a second quarter wave plate (QWP) and a polarizer (POL) ensured generation of a linearly polarized vortex in femtosecond SH beam over all spectral bandwidth of the SH pulse. A 0.5 mm thick Si wafer along with various commercial THz filters was used to remove the high frequency part and to transmit only THz radiation, which was collimated and shrank by parabolic mirrors in a telescope configuration to match the detector aperture. Imaging of the generated THz beam was performed with a thermal camera detector (VarioCAM head HiRes 640, InfraTec GmbH), sensitive in the range 0.1 40 THz (3000 7.5 µm). Spectra of THz radiation were obtained from Fourier transformed interferometric measurements of THz radiation using pyroelectric detector (TPR-A-65 THz, Spectrum Detector Inc.), sensitive in the range 0.1 - 300 THz (3000 - 1 µm) with a flat response function from ∼3 to ∼100 THz. Typically generated THz radiation spectrum spans from 10 to 40 THz (right-bottom panel of Fig. 9 ). Deeps in the spectrum are associated with absorbance in the guiding elements such as Si filters and pellicle beam splitters. The efficiency of THz generation with the regular Gaussian pulses was about 10 −4 , but dropped to about 10 −5 in the case of the SH vortex pump due to the different spatial intensity distributions of the Gaussian FH and vortex SH beams.
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Intensity modulated terahertz vortex wave generation in air plasma by two-color femtosecond laser pulses

Intensity modulated terahertz vortex wave generation in air plasma by two-color femtosecond laser pulses

5. CONCLUSIONS We have investigated the properties of THz radiation generated in air plasma by focused bichromatic femtosecond laser pulses, when one of the pump beams (second harmonic) is an optical vortex. The presence of a phase singularity in the generated THz beam was confirmed by astigmatic transformation of the singular THz beams in the focus of a cylindrical lens, as well as by fully space and time resolved numerical simulations. We report that, in contrast to other nonlinear processes (second harmonic generation, parametric generation, etc.), the THz radi- ation generated by electron currents in a plasma filament can not be characterized as a THz vortex beam in the ‘classical’ sense, such as a pure Laguerre-Gaussian beam. Instead, the intensity of the THz beam is modulated along the beam azimuthal an- gle and contains two minima between two lobes of maximum intensity. This is because the relative phase between two har- monics varies azimuthally when the SH pump pulse is a vortex. Moreover, our numerical simulations demonstrate that trans- verse instabilities in the filamentary pump propagation affect the THz vortex without destroying it. They may introduce sec- ondary phase singularities, which renders the phase topology of produced structured THz fields particularly rich. One of the benefits of THz generation from plasma currents is the large ( > 40 THz) spectral range achievable, contrary to bandwidth limited external THz shaping techniques. We envisage that dif- ferent combinations of the topological charges of the FH and SH pulses open a wide playground for the creation of structured singular THz sources.
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Geometric phase shaping of terahertz vortex beams.

Geometric phase shaping of terahertz vortex beams.

at 1 THz. Indeed, we can directly use the broadband THz pulses (0.1 2.5 THz), since the recorded data contain both the amplitude and phase information at all frequencies. This implies that transforming the temporal information into spec tral information can be numerically done by a fast Fourier transform. By doing so, the amplitude and phase spatial distri bution data are obtained for each frequency. A demonstration is illustrated in Fig. 3 , where the intensity and phase transverse patterns at 1 THz extracted from the polychromatic data are shown in panels (a) and (b), respectively, for σ  1. Again, one thus unambiguously concludes that the generation of a spin controlled unit charge THz vortex, whose linear azimuthal dependence of the phase along the circle line in Fig. 3(a) is quantitatively shown in Fig. 3(c) , is in good agree ment with the theory, Φ  σφ. In addition, we note that a broadband THz vortex could be easily obtained by mere use of achromatic quarter wave retarders instead of monochromatic ones; however, this would be at the expense of the overall vortex generation efficiency. Indeed, the frequency dependent Jones matrix Jν is expressed as
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Energy-scalable temporal cleaning device for femtosecond laser pulses
based on cross-polarized wave generation

Energy-scalable temporal cleaning device for femtosecond laser pulses based on cross-polarized wave generation

peak (see Fig. 4 ). calculated with a home-made MATLAB code in the case of the LCF laser source (2). It demonstrates the good agreement be- tween the theoretical predictions and the observed efficiencies up to 30%. This is a direct consequence of the spatial filtering through the fiber which optimizes the spatial profile (Gaussian and smooth) on the crystal. Furthermore, it mitigates Kerr fo- cusing in the crystal by seeding it with a divergent beam (see Subsection III C ). The consequent nonlinear phase mismatch- ing is reduced and the conversion of the XPW signal is opti- mized even for long crystal length (>3 mm). Therefore, we could achieve up to 25% internal efficiency even when seed- ing the set-up with 11 mJ. Hence, we obtained a XPW beam carrying 1.6 mJ pulse energy, which is the highest achieved so far to the best of our knowledge. However, we noticed that the fiber transmission was lower in this case, around 60%, due to air fluctuations in the laboratory and damages caused to the fiber entrance following repeated realigning. Furthermore, at this energy level, the beam size on the crystal (3.5 mm diam- eter) requires better crystal surface polishing quality in order to reduce the risk of damage at high intensities.
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Terahertz pulse generation by two-color laser fields with circular polarization

Terahertz pulse generation by two-color laser fields with circular polarization

4. Experimental setup and results THz waves were generated in air by bichromatic femtosecond laser pulses following the setup shown in figure 7 . A 1 kHz repetition rate femtosecond Ti:sapphire chirped pulse amplification laser system (Legend elite duo HE+, Coherent Inc.) delivering 40–45 fs (FWHM) light pulses centered at 790 nm with maximal pulse energy of 8 mJ was used as a pump source. The output laser power could be varied by inserting thin partially reflecting dielectric mirrors (DMs) into the beam path. The laser beam was divided into two arms thanks to a thin 50:50 beam splitter (BS1). One of these beams was used for SH generation through a 0.2 mm thick nonlinear BBO crystal. A temporal delay between the FH and SH pulses was introduced by using a motorized optical delay line (DL). The pulse polarization was controlled using broadband zero-order half- and quarter-wave plates (HWP and QWP, respectively) inserted into the beam paths. The QWPs allowed to vary the polarization state of both FH and SH pulses from linear to circular, while HWP inserted into the FH beam path alone was used to switch between the mutually orthogonal and parallel linear polarizations. The FH and SH beams were concentrically superimposed at a dichroic beam splitter (BS2). After passing through the hole of an aluminum-coated off-axis parabolic mirror (PM1) they were directed to a focusing spherical mirror (focal length about 22 cm). As a result, a visible few-mm long plasma filament was produced. In order to minimize optical aberrations the focused bichromatic pump beam was reflected nearly exactly in the backward direction by the focusing mirror. Despite its hole at center, the mirror PM1 was still capable to collect and collimate most of emerging THz radiation, which formed a hollow cone with ∼ 5 ◦ apex angle [ 64 , 65 ]. The second parabolic mirror (PM2) then focused the THz beam onto a pyroelectric detector (TPR-A-65 THz, Spectrum Detector Inc.), sensitive in the range 0.1–300 THz
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Coherent control of boosted terahertz radiation from air plasma pumped by femtosecond three-color sawtooth field

Coherent control of boosted terahertz radiation from air plasma pumped by femtosecond three-color sawtooth field

3. Experimental results We first compared the THz emissions obtained by two-color and three-color sawtooth wave excitation. For the two-color case, the insertion of the fused silica wedge was optimized for the most intense THz generation [ 3 ]. The energy of the incident 800 nm pump pulse was 1.6 mJ and the second harmonic was 0.2 mJ. The measured THz waveform is presented in Fig. 2(a) (red line). Since both 800 nm and 400 nm optical fields are linearly polarized in the horizontal plane ( Fig. 3(a) ), the resulting THz electric field is observed to be horizontally polarized with a weak vertical component, in agreement with previous results [ 13 ]. For the three-color field excitation, it is observed that both the rotation and azimuth angle of the SFG crystal determine the intensity of the 266 nm field, as well as the polarization state of the exiting three optical fields. Moreover, the distance of the SFG BBO crystal with respect to the focus determines the phase Δ . In general
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THz Emissions from Air-Plasmas Created by Mid-and Far-Infrared Two-Color Femtosecond Pulses

THz Emissions from Air-Plasmas Created by Mid-and Far-Infrared Two-Color Femtosecond Pulses

for CO 2 and water absorption [8]. Peak intensities of ∼ 200 TW/cm 2 produce electron densities above 10 17 cm −3 . When increasing the pump wavelength and pump energy (P cr ∝ λ 0 2 ), longer filaments are promoted and they start to self-focus at shorter propagation distances. Figure 1 (a) illustrates the resulting THz energy yields below 10 THz that reach the 0.1 mJ level for 3.9-µm pumps and several mJ for 10.6-µm lasers. Figures 1 (b,c,d) detail the THz spectra computed at the distance of maximum THz field production, i.e., where the on-axis THz fields attain their maxima. Because an important question has been the role of Kerr-induced four-wave mixing compared to that of photocurrents [7], Figs. 1 (b,c,d) also compare the spectra computed with or without the plasma terms before the linear focus. The presence of plasma clearly increases the THz spectral intensity by at least three orders of magnitude and shifts the frequency centroid down to 0.2 THz for the three pump wavelengths. Hence, plasma generation – even when it involves low electron densities ∼ 10 15−17 cm −3 – is here the key player in THz pulse generation.
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Boosting terahertz generation in laser-field ionized gases using a sawtooth wave shape

Boosting terahertz generation in laser-field ionized gases using a sawtooth wave shape

Broadband ultrashort terahertz (THz) pulses can be produced using plasma generation in a noble gas ionized by femtosecond two-color pulses. Here we demonstrate that, by using multiple-frequency laser pulses, one can obtain a waveform which optimizes the free electron trajectories in such a way that they reach the highest velocity at the electric field extrema. This allows to increase the THz conversion efficiency to the percent level, an unprecedented performance for THz generation in gases. Besides the analytical study of THz generation using a local current model, we perform comprehensive 3D simulations accounting for propagation effects which confirm this prediction. Our results show that THz conversion via tunnel ionization can be greatly improved with well-designed multicolor pulses.
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Terahertz spectroscopy from air plasmas created by two-color femtosecond laser pulses: The ALTESSE project

Terahertz spectroscopy from air plasmas created by two-color femtosecond laser pulses: The ALTESSE project

L. Berg´e et al. Fig. 3: (a) Mechanisms generating THz waves by intense two- color laser pulses, distributed according to the optical intensity. The first region involves the Kerr effect (four-wave mixing) and photoionization. The second region accentuates the contribu- tion of photoionization in the tunnel regime (photocurrents) and involves plasma waves created by ponderomotive forces. (b) Photocurrent process: The two-color electric field gener- ates free electrons in stepwise increase via tunneling ionization occurring near the field extrema at t = t n . This builds a slow
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Terahertz Radiation Source in Air Based on Bifilamentation of Femtosecond Laser Pulses

Terahertz Radiation Source in Air Based on Bifilamentation of Femtosecond Laser Pulses

2) THz generation in a stratified plasma It has been predicted that a short laser pulse propagating in a periodically varying (stratified) plasma can generate electromagnetic radiation in the THz domain [3], if the modulation period is of the order of a few hundred microns. However, such a modulation cannot be created spontaneously in the time between two pulses. The plasma oscillations created by the first pulse are decaying in a time scale less than 1 ps, and they cannot induce any specific large-scale plasma motion. Moreover, the plasma column is expected to be fairly homogeneous along the filament because of the strong clamping effect upon the pulse intensity [4]. Therefore, this undulator effect is not likely at the origin of the THz emission in our experiments.
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Self-compression of optical laser pulses by filamentation

Self-compression of optical laser pulses by filamentation

The peak intensity exhibits a 35 cm long plateau at 7 × 10 13 W cm −2 corresponding to the filament. The density of the associated plasma channel slightly exceeds 10 17 cm −3 . Figure 1 (b) shows the evolution of the duration (FWHM) of the radially integrated intensity profile over a radius of 100 µm. This corresponds to the maximum value reached during propagation by the radius of the light filament, defined as the half width at half maximum of the fluence (time integrated intensity) distribution. The minimum duration of 5 fs is obtained at the end of the filament. This final duration depends on the input conditions but this self-compression behaviour is generally similar for all filaments. Under slightly different conditions, the minimum pulse duration at the end of the filament was shown to be as short as a single cycle [ 17 ]. The moving focus model [ 37 ] brings a simple explanation to this effect: self-focusing of the most powerful central part of the pulse should occur faster, hence for smaller propagation distances, than in the trailing part. In addition, self-focusing of the trailing part is delayed by plasma defocusing and therefore occurs at the end of the filament while the leading part undergoes nonlinear losses, diffraction and dispersion.
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Optimal control of vortex core polarity by resonant microwave pulses

Optimal control of vortex core polarity by resonant microwave pulses

reverse the polarity from p = −1 to p = +1 and Π + pulses from p = +1 to p = −1 (see Fig.2b). The experimental data is acquired as follows: for Π − (Π + ) pulses, the p = −1 (p = +1) state is first reset using an initialization pulse whose result is known to be fully deterministic 15 . Then, a single microwave pulse of given duration w, frequency f and power P is applied, and the final polarity state is read using MRFM. An opaque coloured pixel marks pulse settings for which reversals are recorded with a 100% success rate, while a blank pixel means that no reversal is recorded. The contour plots presented in Fig.2a with different shades of red (blue) show the superposition of the results for Π − (Π + ) pulses with three different durations w: 100, 50 and
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Terahertz control of air lasing

Terahertz control of air lasing

FIG. 3. Spectrograms of the nonlinear interaction. Panels (a) and (b) both show normalized data of the radiation emitted in the UV spectral region after the THz and NIR pump pulse interaction (the color scale represents the intensity in log scale). Data in (a) are recorded at a high NIR pump energy U1, and in (b) at a low-energy U2. The horizontal axis is the delay τ between the NIR pump and the THz single-cycle pulse. The thick horizontal segments in (a) identify the spectral regions of integration (0.8 nm) used to evaluate the traces reported in panel (c): the absolute value squared of the THz electric field (black dashed) is shown as a function of the temporal coordinate. It is overlapped to the integrated portion of the high-energy spectrogram signal around 391.5 nm (red), 400 nm (orange), and 428 nm (blue). All curves are normalized to their maximum value. (d) Measured THz field via the air-biased coherent detection method [22].
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Robust Quantum Dot Exciton Generation via Adiabatic Passage with Frequency-Swept Optical Pulses

Robust Quantum Dot Exciton Generation via Adiabatic Passage with Frequency-Swept Optical Pulses

DOI: 10.1103/PhysRevLett.106.166801 PACS numbers: 73.21.La, 78.55.Cr, 78.67.Hc Photon correlation measurements along with resonant laser scattering have established the atomlike character of the interband transitions in quantum dots [ 1 – 3 ]. Excitation of a two level system by a short, intense laser pulse can induce an oscillation of the system between the upper and lower state during the pulse, at the Rabi frequency ðtÞ ¼ AðtÞ=@ where  is the dipole moment of the transition and AðtÞ the electric field envelope of the laser pulse. Any quantum state (qubit) manipulation scheme benefits from the long coherence times in quantum dots [ 4 , 5 ] and necessitates fast and robust initial state prepara- tion [ 6 – 8 ]. In principle a two level system can be initialized in the upper state with a maximum fidelity of 100% if the laser power is optimized in order to induce exactly half a Rabi oscillation during the pulse duration, a so-called  pulse [ 9 – 11 ]. Although Rabi oscillations observed in a single dot or for individual atoms [ 12 ] are a beautiful example of strong coupling between laser light and a single dipole, this commonly used technique presents two major drawbacks: (i) the upper state population is highly sensitive to fluctuations in the system, such as laser power, and (ii) in measurements on dot ensembles, an inhomoge- neous distribution of dipole moments and transition ener- gies among dots requires different laser intensities and frequencies for inducing a full population inversion in the dot ensemble.
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Backward lasing of singly ionized nitrogen ions pumped by femtosecond laser pulses

Backward lasing of singly ionized nitrogen ions pumped by femtosecond laser pulses

Fig. 4. Pressure dependence of the backward (a) and forward (b) lasing emission from nitrogen. The pump pulse energy was 2.2 mJ. In order to get further insight into the gas pressure dependence, we compared the intensity of the backward and forward 391.4 nm emissions to the corresponding sideway fluorescence signal in Fig. 5 . There are three particular features that deserve our attention. First, the three signals present a rapid increase in the pressure range below 10 mbar. In this low gas pressure regime, the laser pulse propagates almost linearly and the laser intensity is independent of the gas pressure. Considering a Gaussian beam with width of 12 mm (1/e 2 ) focused by an f = 300 mm lens, the size of the focus is found to be 2w 0 =
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Postcompression of high-energy terawatt-level femtosecond pulses and application to high-order harmonic generation

Postcompression of high-energy terawatt-level femtosecond pulses and application to high-order harmonic generation

O. Hort current address: Photonics Institute, Vienna University of Technology, Gusshausstrasse 27-29, A-1040 Vienna, Austria We perform a post-compression of high energy pulses by using optical-field ionization of low pres- sure helium gas in a guided geometry. We apply this approach to a TW chirped-pulse-amplification based Ti:Sapphire laser chain and show that spectral broadening can be controlled both with the input pulse energy and gas pressure. Under optimized conditions, we generate 10 fs pulses at TW level directly under vacuum and demonstrate a high stability of the post compressed pulse duration. These high energy post-compressed pulses are thereafter used to perform high harmonic generation in a loose focusing geometry. The XUV beam is characterized both spatially and spectrally on a single shot basis and structured continuous XUV spectra are observed.
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Confined longitudinal acoustic phonon modes in free-standing Si membranes coherently excited by femtosecond laser pulses

Confined longitudinal acoustic phonon modes in free-standing Si membranes coherently excited by femtosecond laser pulses

immediately apparent with the fundamental frequency of the thinner sample being higher than that of the thicker sample. 17 In Figs. 2共a兲 and 2共c兲 the numerical Fourier transforma- tions of the transients are depicted on a log scale. In both cases a series of equidistant peaks with decreasing ampli- tudes can be observed. A detailed evaluation leads to the conclusion that in both cases the peaks are located at odd harmonics of the fundamental mode at 19.15 GHz 共221 nm

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Nanoparticle generation inside Ag-doped LBG glass by femtosecond laser irradiation

Nanoparticle generation inside Ag-doped LBG glass by femtosecond laser irradiation

rather low spatial densities of NPs. In the case of laser-modified samples, the samples were first cut and polished perpendicularly to the written lines. Thus, the resulting longitudinal cross-sections of the structured areas gave a direct visual access to their thicknesses, as well as to the nature of the photo-induced modification, either in terms of index profile change or of the precise localization of the NPs. Indeed, laser-modified volumes (Figs. 1(c) and 3(c)) showed darkened profiles with restricted thickness (typically 10 μm measured for both spectra δ and ε of Fig. 2(b)) but containing very high NP concentrations, which led to extremely high absorption coefficients, typically up to 1200 cm − 1 . Moreover, by considering
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Laser structuration of dielectric materials by a train of femtosecond pulses through cumulative effects

Laser structuration of dielectric materials by a train of femtosecond pulses through cumulative effects

2 + , which exhibits luminescence properties rendering it possible to observe their spatial distribution. Fig. 1(a) shows the emergence of a micrometric ring structure after an irradiation by 10 7 laser pulses [1] with parameters: 1030 nm, 470 fs, 1.2 μm of waist, 100 nJ, 10MHz. Due to the migration of charged species, a static electric field originates leading to the formation of local nonlinear optical properties. A model based on laser heating, heat diffusion, and thermally-activated diffusion and kinetic reactions of the various silver species, allows us to account for the observed structure as shown in Fig. 1(b). The details of the modelling, mechanisms for cumulative effects, and influence of laser parameters on the ring characteristics will be presented.
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