Polarization Gating Technique Research Articles

Control of the electron dynamics in solid-state high harmonic generation on ultrafast time scales by a polarization-skewed laser pulse.

Since high-order harmonic generation (HHG) from atoms depends sensitively on the polarization of the driving laser field, the polarization gating (PG) technique was developed and applied successfully to generate isolated attosecond pulses from atomic gases. The situation is, however, different in solid-state systems as it has been demonstrated that due to collisions with neighboring atomic cores of the crystal lattice strong HHG can be generated even by elliptically- and circularly-polarized laser fields. Here we apply PG to solid-state systems and find that the conventional PG technique is inefficient for the generation of isolated ultrashort harmonic pulse bursts. In contrast, we demonstrate that a polarization-skewed laser pulse is able to confine the harmonic emission to a time window of less than one-tenth of the laser cycle. This method provides a novel way to control HHG and to generate isolated attosecond pulses in solids.

Generation of the isolated highly elliptically polarized attosecond pulse using the polarization gating technique: TDDFT approach.

This paper theoretically investigates the generation of isolated elliptically polarized attosecond pulses with a tunable ellipticity from the interaction of Cl2 molecule and a polarization-gating laser pulse. A three-dimensional calculation based on the time-dependent density functional theory is done. Two different methods are proposed for generating elliptically polarized single attosecond pulses. The first method is based on applying a single-color polarization gating laser and controlling the orientation angle of the Cl2 molecule with respect to the polarization direction of the laser at the gate window. An attosecond pulse with an ellipticity of 0.66 and a pulse duration of 275 as is achieved by tuning the molecule orientation angle to 40° in this method and superposing harmonics around the harmonic cutoff. The second method is based on irradiating an aligned Cl2 molecule with a two-color polarization gating laser. The ellipticity of the attosecond pulses obtained by this method can be controlled by adjusting the intensity ratio of the two colors. Employing an optimized intensity ratio and superposing harmonics around the harmonic cutoff would lead to the generation of an isolated, highly elliptically polarized attosecond pulse with an ellipticity of 0.92 and a pulse duration of 648 as.

Narrow Bandwidth Gamma Comb from Nonlinear Compton Scattering Using the Polarization Gating Technique.

Nonlinear Compton scattering is a promising source of bright gamma rays. Using readily available intense laser pulses to scatter off the energetic electrons, on the one hand, allows us to significantly increase the total photon yield, but on the other hand, leads to a dramatic spectral broadening of the fundamental emission line as well as its harmonics due to the laser pulse shape induced ponderomotive effects. In this Letter we propose to avoid ponderomotive broadening in harmonics by using the polarization gating technique-a well-known method to construct a laser pulse with temporally varying polarization. We show that by restricting harmonic emission only to the region near the peak of the pulse, where the polarization is linear, it is possible to generate a bright narrow bandwidth comb in the gamma region.

High-order harmonic generation by static coherent states method in single-electron atomic and molecular systems.

We solve the time-dependent Schrodinger equation using the coherent states as basis sets for computing high harmonic generation (HHG) in a full-dimensional single-electron "realistic" system. We apply the static coherent states (SCS) method to investigate HHG in the hydrogen molecular ion induced by a linearly polarized laser field. We show that SCS gives reasonable agreement compared to the three dimensional unitary split-operator approach. Next, we study isolated attosecond pulse generation in . To do so, we employ the well-known polarization gating technique, which combines two delayed counter-rotating circular laser pulses, and opens up a gate at the central portion of the superposed pulse. Our results suggest that the SCS method can be used for full-dimensional quantum simulation of higher dimensional systems such as the hydrogen molecule in the presence of an external laser field.

Polarization gating technique extracts depth resolved fluorescence redox ratio in oral cancer diagnostics.

Mortality of oral cancer is often due to late diagnosis. Effective non-invasive diagnostic techniques may increase the survival rate based on an earlier diagnosis.. We report on the application of the polarization gating technique for isolating weakly scattered and highly scattered components of fluorescence emission from the superficial and deeper layers of tissue due to intrinsic fluorophores NADH and FAD. The fluorescence polarization spectra were collected from 21 normal and 67 oral squamous cell carcinoma biopsy tissues. The tissues were excited at 350 nm and the fluorescence emission had two peaks corresponding to NADH, and FAD respectively. The spectra were analyzed using the polarization gating technique along with the spectral deconvolution method to derive the optical redox ratio from different layers of tissue. The fractional change in redox ratio between superficial and deeper layers of tissue exhibits excellent statistical significance (p<10-3) which may be due to a shift in the metabolic pathway from oxidative phosphorylation to glycolysis in the cancer cell. Further, variation in collagen intensity in deeper layers of tissue is observed which may be attributed to the breakdown of collagen fibers in the stroma. Linear discriminant analysis showed that oral cancer tissue is discriminated with a better accuracy using polarization gating technique than that of conventional fluorescence spectroscopy.

Generation of single isolated attosecond pulse from a monolayer hexagonal boron nitride using a polarization gating technique

An ab initio approach is presented to generate an isolated attosecond pulse (IAP) by gating high-order harmonic emission from a monolayer hexagonal boron nitride (h-BN), where in this case the ellipticity of the fields are varying rapidly with time. The high harmonic generation from a monolayer h-BN lattice is studied when it is exposed to the single circularly- and bicircularly-polarized laser fields. The effect of positive and negative chirped laser pulses on the high harmonics are considered. The role of ellipticity in the HHG to create single IAP from 2D h-BN under counter-rotating bicircularly polarized laser pulses is investigated in mid-infrared wavelength region. In order to generate single IAP, the polarization gating (PG) technique is implemented. By adjusting properly positive chirped parameters and implementing the conventional polarization gating to an input intensity of 20 TW cm−2, an isolated 80 as pulse from the superposition of several harmonics is generated.

Vibrational population transfer between electronic states of N2+ in polarization-modulated intense laser fields

Recently, the significant enhancement of ${{\mathrm{N}}_{2}}^{+}$ lasing realized with the help of a polarization-gated intense 800-nm pump provides crucial clues to optimize both ionization and population coupling processes occurring during ${{\mathrm{N}}_{2}}^{+}$ lasing generation [Li et al., Phys. Rev. Lett. 122, 013202 (2019)]. In this work, we carried out an experimental and theoretical study on the generation of ${{\mathrm{N}}_{2}}^{+}$ lasing at 391 nm by the use of two different polarization-modulation schemes for pump pulses. It was found that the probe spectra in the vicinity of 391 nm are readily transformed from absorption to lasing due to population depletion of the ground state of ${{\mathrm{N}}_{2}}^{+}$ just via changing the pump polarization state. More importantly, ${{\mathrm{N}}_{2}}^{+}$ lasing intensity can be further increased when the pump polarization is modulated with a half-wave plate and a multiple-order quarter-wave plate compared with that obtained by the polarization-gating technique. All the experimental observations are well explained by the different population inversions between the states $B\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{u}^{+}$ and $X\phantom{\rule{0.16em}{0ex}}^{2}\mathrm{\ensuremath{\Sigma}}_{g}^{+}$ of ${{\mathrm{N}}_{2}}^{+}$ in polarization-modulated pump laser fields.

Extreme nonlinear terahertz electro-optics in diamond for ultrafast pulse switching

Polarization switching of picosecond laser pulses is a fundamental concept in signal processing [C. Chen and G. Liu, Annu. Rev. Mater. Sci. 16, 203 (1986); V. R. Almeida et al., Nature 431, 1081 (2004); and A. A. P. Pohl et al., Photonics Sens. 3, 1 (2013)]. Conventional switching devices rely on the electro-optical Pockels effect and work at radio frequencies. The ensuing gating time of several nanoseconds is a bottleneck for faster switches which is set by the performance of state-of-the-art high-voltage electronics. Here we show that by substituting the electric field of several kV/cm provided by modern electronics by the MV/cm field of a single-cycle THz laser pulse, the electro-optical gating process can be driven orders of magnitude faster, at THz frequencies. In this context, we introduce diamond as an exceptional electro-optical material and demonstrate a pulse gating time as fast as 100 fs using sub-cycle THz-induced Kerr nonlinearity. We show that THz-induced switching in the insulator diamond is fully governed by the THz pulse shape. The presented THz-based electro-optical approach overcomes the bandwidth and switching speed limits of conventional MHz/GHz electronics and establishes the ultrafast electro-optical gating technology for the first time in the THz frequency range. We finally show that the presented THz polarization gating technique is applicable for advanced beam diagnostics. As a first example, we demonstrate tomographic reconstruction of a THz pulse in three dimensions.

Attosecond pulse generation isolated with an asymmetric polarization gating

High harmonics generated using the polarization gating technique are simulated under the strong-field approximation for the cases of equal and unequal ratios between the amplitudes of the two counter-rotating pulses. The effect of the field asymmetry is observed on the high harmonic yield and cut-off in the frequency domain and on the attosecond pulse duration and satellite peak contrast in the time domain. The case where the strong pulse comes later is more favourable in terms of generating a higher cut-off and a shorter pulse duration. It is predicted that thin metal filters can reduce the impact of satellite pulses while simultaneously reducing the pulse duration.

Polarization gating of high harmonic generation in the water window

We implement the polarization gating (PG) technique with a two-cycle, 1.7 μm driving field to generate an attosecond supercontinuum extending to the water window spectral region. The ellipticity dependence of the high harmonic yield over a photon energy range much broader than previous work is measured and compared with a semi-classical model. When PG is applied, the carrier-envelope phase (CEP) is swept to study its influence on the continuum generation. PG with one-cycle (5.7 fs) and two-cycle (11.3 fs) delay are tested, and both give continuous spectra spanning from 50 to 450 eV under certain CEP values, strongly indicating the generation of isolated attosecond pulses in the water window region.

Ultrafast single-shot measurement of optical Kerr effect based on supercontinuum pulse.

We present an ultrafast single-shot measurement method for the optical Kerr effect based on a polarization gating technique. The advantages of this single-shot technique are demonstrated via a 26-ps chirped supercontinuum pulse used to measure the optical Kerr effect for three transparent organic liquids. The single-shot measurement results agree well with those of the time-resolved optical Kerr gate method, as regards both time and intensity. This method facilitates real-time observation of ultrafast optical Kerr responses of samples and simultaneous high-time-resolution data acquisition at ∼260 fs. We demonstrate that the single-shot measurement method is potentially a powerful tool for investigating the optical Kerr effects of unstable samples, and for application to high-power laser systems.

Versatile attosecond beamline in a two-foci configuration for simultaneous time-resolved measurements

We present our attoline which is a versatile attosecond beamline at the Ultrafast Laser Physics Group at ETH Zurich for attosecond spectroscopy in a variety of targets. High-harmonic generation (HHG) in noble gases with an infrared (IR) driving field is employed to generate pulses in the extreme ultraviolet (XUV) spectral regime for XUV-IR cross-correlation measurements. The IR pulse driving the HHG and the pulse involved in the measurements are used in a non-collinear set-up that gives independent access to the different beams. Single attosecond pulses are generated with the polarization gating technique and temporally characterized with attosecond streaking. This attoline contains two target chambers that can be operated simultaneously. A toroidal mirror relay-images the focus from the first chamber into the second one. In the first interaction region a dedicated double-target allows for a simple change between photoelectron/photoion measurements with a time-of-flight spectrometer and transient absorption experiments. Any end station can occupy the second interaction chamber. A surface analysis chamber containing a hemispherical electron analyzer was employed to demonstrate successful operation. Simultaneous RABBITT measurements in two argon jets were recorded for this purpose.

Attosecond x-ray source generation from two-color polarized gating plasmonic field enhancement

The plasmonic field enhancement from the vicinity of metallic nanostructures as well as the polarization gating technique has been utilized to the generation of the high order harmonic and the single attosecond x-ray source. Through numerical solution of the time-dependent Schrödinger equation, for moderate the inhomogeneity and the polarized angle of the two fields, we find that not only the harmonic plateau has been extended and enhanced but also the single short quantum path has been selected to contribute to the harmonic. As a result, a series of 50 as pulses around the extreme ultraviolet and the x-ray regions have been obtained. Furthermore, by investigating the other parameters effects on the harmonic emission, we find that this two-color polarized gating plasmonic field enhancement scheme can also be achieved by the multi-cycle pulses, which is much better for experimental realization.

Isolated attosecond pulse generation combining propagation effects and a modulated chirped polarization gating technique

We propose a theoretical scheme to generate a broadband supercontinuum using a modulated chirped polarization gating technique. By this technique, a broadband continuum of 155 eV can be obtained at microscopic level. The modulations on the supercontinuum can be eliminated after propagation. Then a smooth broadband supercontinuum with the bandwidth of 155 eV can be generated. As a result, isolated sub-100-attosecond pulses with tunable central wavelengths could be obtained straightforwardly. Our simulations also reveal that an isolated attosecond pulse can still be generated by using multicycle driving laser pulses, and the central wavelength of isolated attosecond pulse can be controlled by adjusting the delay τd (τd is the delay of the two counter-rotating circularly polarized pulses.).

Communication: Simplified two-beam rotational CARS signal generation demonstrated in 1D

We explore a novel phase matching scheme for gas-phase rotational coherent anti-Stokes Raman spectroscopy (CARS). The scheme significantly simplifies the employment of the technique in general. Two laser beams, one broadband and one narrowband, are crossed at arbitrary angle and the generated rotational CARS signal, copropagating with the probe beam, is isolated using a polarization gating technique. The effect of phase-vector mismatch for various experimental implementations was measured experimentally and compared to calculations. The spatial resolution of the current technique is improved by more than an order of magnitude over standard gas-phase CARS experimental arrangements, providing an interaction length of less than 50 μm when desired. Both the pump and Stokes photons originate from the broadband pulse, and are therefore automatically overlapped temporally and spatially. Significantly improved signal levels are achieved because of both the ease of alignment and the higher pulse energy available to the pump and Stokes fields. We demonstrate the technique for single-laser-shot 1D rotational CARS signal generation over approximately a 1 cm field in a flame.

Polarization gating and circularly-polarized high harmonic generation using plasmonic enhancement in metal nanostructures

We investigate possibilities to utilize field enhancement by specifically designed metal nanostructures for the generation of single attosecond pulses using the polarization gating technique. We predict the generation of isolated 59-attosecond-long pulses using 15-fs pump pulses with only a 0.6 TW/cm2 intensity. Our simulations also indicate the possibility to generate previously inaccessible high-harmonics with circular polarization by using an ensemble of vertically and horizontally orientated bow-tie structures. In the numerical simulation we used an extended Lewenstein model, which includes the spatial inhomogeneity in the hot spots and collisions of electrons with the metal surface.

Generation of isolated sub-100-as pulses with 30-fs lasers

We propose a scheme to generate isolated sub-100-as pulses with 30-fs lasers. Our model is based on high-order harmonic generation from a helium atom irradiated by a two-color femtosecond laser field, in which the driving and the control fields are linearly polarized and the polarization angle between them is $\ensuremath{\pi}/4$. By mixing the polarization gating technique and optimization of two-color lasers, we destroy the symmetry of the driving field from two directions and realize quantum path control with multicycle lasers.

Chirped polarization-gating technique for isolated attosecond pulse generation

We propose a scheme to generate broadband high harmonic supercontinuum and single attosecond pulse emission in time domain by employing a chirped polarization-gating laser field. It is shown that this scheme can significantly extend the high harmonic cutoff to higher energy region, and the produced isolated attosecond pulse is shorter than 100 attoseconds after proper dispersion compensation, when a 7 fs driving laser pulse is used.

Nonsequential double ionization with polarization-gated pulses

We investigate laser-induced nonsequential double ionization by a polarization-gated laser pulse, constructed by employing two counter-rotating circularly polarized few-cycle pulses with a time delay Td. We address the problem within a classical framework and mimic the behaviour of the quantum-mechanical electronic wave packet by means of an ensemble of classical electron trajectories. These trajectories are initially weighted with the quasi-static tunnelling rate and with suitably chosen distributions for the momentum components parallel and perpendicular to the laser-field polarization in the temporal region for which it is nearly linearly polarized. We show that, if the time delay Td is of the order of the pulse length, the electron-momentum distributions, as functions of the parallel momentum components, are highly asymmetric and dependent on the carrier-envelope (CE) phase. As this delay is decreased, this asymmetry gradually vanishes. We explain this behaviour in terms of the available phase space, the quasi-static tunnelling rate and the recollision rate for the first electron for different sets of trajectories. Our results show that the polarization-gating technique may provide an efficient way to study the NSDI dynamics in the single-cycle limit without employing few-cycle pulses.

Coherent continuum extreme ultraviolet radiation in the sub-100-nJ range generated by a high-power many-cycle laser field

High-energy coherent continuum radiation is generated by the interaction of rare gases with a high-power many-cycle laser field, utilizing the interferometric polarization gating technique. A narrow temporal gate is formed in the laser pulse within which the extreme ultraviolet (XUV) emission is restricted. An analytical expression for the gate function is derived. A super-XUV continuum down to 15 nm, broad enough to support synthesis of single pulses of 260 as duration and a few tens of nanojoule energy, has been measured. These results directly challenge the perspectives of single-attosecond pulse XUV-pump-XUV-probe applications.