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Tunable orbital angular momentum in high-harmonic generation

View Article: PubMed Central - PubMed

ABSTRACT

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

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Yield of the modes as a function of the intensity and gas pressure for the 16th harmonic.(a) Evolution of the generated signal as a function of the intensity of the generating beams at focus. The intensity is normalized to an estimated intensity ratio I2ω/Iω of about 18%, with Iω∼1.7 × 1014 W cm−2. The intensity is calculated from the measured iris aperture and transmitted energy. The signal is normalized to 1.7 × 107 photons per shot on the CCD (top left corner of each image). (b,c) Evolution of the generated signal as a function of the argon pressure in the cell. (c) The curves display the integrated signal for each individual mode.
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f4: Yield of the modes as a function of the intensity and gas pressure for the 16th harmonic.(a) Evolution of the generated signal as a function of the intensity of the generating beams at focus. The intensity is normalized to an estimated intensity ratio I2ω/Iω of about 18%, with Iω∼1.7 × 1014 W cm−2. The intensity is calculated from the measured iris aperture and transmitted energy. The signal is normalized to 1.7 × 107 photons per shot on the CCD (top left corner of each image). (b,c) Evolution of the generated signal as a function of the argon pressure in the cell. (c) The curves display the integrated signal for each individual mode.

Mentions: For a given high-harmonic order, the signal is not equally distributed between the modes. This is a known effect in highly non-linear wave mixing2530, due to both the microscopic response of the medium and the macroscopic effects (propagation and phase matching) in HHG. This property can be exploited to optimize and favour the emission of a specific vortex by modifying the generation conditions. Figure 4 shows the evolution of the generated signal for three modes of the 16th harmonic order, when varying the iris aperture (that is, the transmitted energy) in each arm of the interferometer and the pressure in the gas cell. In Fig. 4a, the iris apertures modify the intensity as well as the size of both beams at focus. These parameters impact the individual atomic response in the gas jet (amplitude and phase of the dipoles), the phase matching conditions, and the propagation and reshaping of the fundamental beams in the medium3132. The intensity ratio of the second harmonic to the fundamental beam (2ω/ω) was varied from a few percent up to 50%, spanning both perturbative and non-perturbative regimes. The relative intensity of the two colours, which affects the dipole amplitude, is the relevant parameter to explain the evolution of the signal within each mode25. This can be understood in terms of the probability that a certain pair (n1, n2) contributes to the emission of a given harmonic. A low 2ω/ω intensity ratio favours generation from pairs requiring a low n2 of photons absorbed from the 2ω field. On the other hand, a large intensity ratio 2ω/ω favours pairs with a high n2, particularly when the absolute intensity of the ω field is low. Figure 4b,c shows how the gas pressure impacts the yield of different modes. As expected, the yield increases for all modes when the pressure is increased. Surprisingly, at about 20 mbar the ℓ=3 mode overcomes the ℓ=1 mode. This feature cannot be explained by single atom effects. Including phase matching effects in the analysis of the experiment allows us to find the origin of this unexpected behaviour.


Tunable orbital angular momentum in high-harmonic generation
Yield of the modes as a function of the intensity and gas pressure for the 16th harmonic.(a) Evolution of the generated signal as a function of the intensity of the generating beams at focus. The intensity is normalized to an estimated intensity ratio I2ω/Iω of about 18%, with Iω∼1.7 × 1014 W cm−2. The intensity is calculated from the measured iris aperture and transmitted energy. The signal is normalized to 1.7 × 107 photons per shot on the CCD (top left corner of each image). (b,c) Evolution of the generated signal as a function of the argon pressure in the cell. (c) The curves display the integrated signal for each individual mode.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382323&req=5

f4: Yield of the modes as a function of the intensity and gas pressure for the 16th harmonic.(a) Evolution of the generated signal as a function of the intensity of the generating beams at focus. The intensity is normalized to an estimated intensity ratio I2ω/Iω of about 18%, with Iω∼1.7 × 1014 W cm−2. The intensity is calculated from the measured iris aperture and transmitted energy. The signal is normalized to 1.7 × 107 photons per shot on the CCD (top left corner of each image). (b,c) Evolution of the generated signal as a function of the argon pressure in the cell. (c) The curves display the integrated signal for each individual mode.
Mentions: For a given high-harmonic order, the signal is not equally distributed between the modes. This is a known effect in highly non-linear wave mixing2530, due to both the microscopic response of the medium and the macroscopic effects (propagation and phase matching) in HHG. This property can be exploited to optimize and favour the emission of a specific vortex by modifying the generation conditions. Figure 4 shows the evolution of the generated signal for three modes of the 16th harmonic order, when varying the iris aperture (that is, the transmitted energy) in each arm of the interferometer and the pressure in the gas cell. In Fig. 4a, the iris apertures modify the intensity as well as the size of both beams at focus. These parameters impact the individual atomic response in the gas jet (amplitude and phase of the dipoles), the phase matching conditions, and the propagation and reshaping of the fundamental beams in the medium3132. The intensity ratio of the second harmonic to the fundamental beam (2ω/ω) was varied from a few percent up to 50%, spanning both perturbative and non-perturbative regimes. The relative intensity of the two colours, which affects the dipole amplitude, is the relevant parameter to explain the evolution of the signal within each mode25. This can be understood in terms of the probability that a certain pair (n1, n2) contributes to the emission of a given harmonic. A low 2ω/ω intensity ratio favours generation from pairs requiring a low n2 of photons absorbed from the 2ω field. On the other hand, a large intensity ratio 2ω/ω favours pairs with a high n2, particularly when the absolute intensity of the ω field is low. Figure 4b,c shows how the gas pressure impacts the yield of different modes. As expected, the yield increases for all modes when the pressure is increased. Surprisingly, at about 20 mbar the ℓ=3 mode overcomes the ℓ=1 mode. This feature cannot be explained by single atom effects. Including phase matching effects in the analysis of the experiment allows us to find the origin of this unexpected behaviour.

View Article: PubMed Central - PubMed

ABSTRACT

Optical vortices are currently one of the most intensively studied topics in optics. These light beams, which carry orbital angular momentum (OAM), have been successfully utilized in the visible and infrared in a wide variety of applications. Moving to shorter wavelengths may open up completely new research directions in the areas of optical physics and material characterization. Here, we report on the generation of extreme-ultraviolet optical vortices with femtosecond duration carrying a controllable amount of OAM. From a basic physics viewpoint, our results help to resolve key questions such as the conservation of angular momentum in highly nonlinear light–matter interactions, and the disentanglement and independent control of the intrinsic and extrinsic components of the photon's angular momentum at short-wavelengths. The methods developed here will allow testing some of the recently proposed concepts such as OAM-induced dichroism, magnetic switching in organic molecules and violation of dipolar selection rules in atoms.

No MeSH data available.


Related in: MedlinePlus