Few-cycle pulse retrieval using amplitude swing technique.

Ultrashort pulses have garnered significant attention across various scientific disciplines and applications. In this paper, we demonstrate that the recently introduced amplitude swing technique is a robust method for characterizing pulses in the few-cycle temporal domain by analyzing compressed and chirped pulses from a Ti:Sapphire laser oscillator. The duration of the measured pulse for the case of best compression was 5.98 fs (Fourier limit 5.50 fs) corresponding to 2.2 cycles, while the chirped pulses were up to 15 times temporally stretched. The results obtained have been validated using the d-scan technique, showing excellent agreement in all situations. Therefore, the capability of the amplitude swing technique to measure ultra-broadband pulses in the few-cycle regime is demonstrated, as well as very far from optimum compression, while only being limited by the transparency and birefringence of its elements.

Characterization of ultrashort vector pulses from a single amplitude swing measurement.

Ultrashort vector pulses exhibit time- and frequency-dependent polarization, sparking significant interest across various fields. Simple, robust, and versatile characterization techniques are crucial to meet this rising demand. Our study showcases how complete polarization dynamics are encoded within a single amplitude swing trace, demonstrated both theoretically and experimentally. We have developed a reconstruction strategy to effectively extract all this information. The amplitude swing technique's sensitivity to vector pulses offers a robust, compact in-line setup adaptable across diverse pulse bandwidths, durations, and spectral ranges. This self-referenced method offers effective measurement of ultrashort vector pulses, addressing the growing interest in these complex pulses.

Generalizing amplitude swing modulation for versatile ultrashort pulse measurement.

In this work we broaden the amplitude modulation concept applied to the temporal characterization of ultrashort laser pulses with the amplitude swing technique. We theoretically study the effect of diverse types of relative amplitude and phase modulations. This variation of the replicas can be implemented by means of rotating zero-order waveplates to manipulate the delayed pulse replicas produced in a following multi-order waveplate, which can be more practical under certain conditions. We numerically simulate and study different scenarios under different modulations and for different noise levels and pulses. The proposed schemes are validated and compared through the experimental application to compressed and chirped pulses, confirming the applicability of the work. The simplicity, robustness and versatility of this ultrashort pulse measurement benefits the applications of ultrafast optics.

Dynamical behaviour of ultrathin [CoFeB (tCoFeB)/Pd] films with perpendicular magnetic anisotropy.

CoFeB-based ultrathin films with perpendicular magnetic anisotropy are promising for different emerging technological applications such as nonvolatile memories with low power consumption and high-speed performance. In this work, the dynamical properties of [CoFeB (tCoFeB)/Pd (10 Å)]5 multilayered ultrathin films (1 Å ≤ tCoFeB ≤ 5 Å) are studied by using two complementary methods: time-resolved magneto-optical Kerr effect and broadband ferromagnetic resonance. The perpendicular magnetization is confirmed for multilayers with tCoFeB ≤ 4 Å. The effective perpendicular magnetic anisotropy reaches a clear maximum at tCoFeB = 3 Å. Further increase of CoFeB layer thickness reduces the perpendicular magnetic anisotropy and the magnetization became in-plane oriented for tCoFeB ≥ 5 Å. This behaviour is explained by considering competing contributions from surface and magnetoelastic anisotropies. It was also found that the effective damping parameter αeff decreases with CoFeB layer thickness and for tCoFeB = 4 Å reaches a value of ~ 0.019 that is suitable for microwave applications.

Robustness and capabilities of ultrashort laser pulses characterization with amplitude swing.

In this work we firstly study the influence of different parameters in the temporal characterization of ultrashort laser pulses with the recently developed amplitude swing technique. In this technique, the relative amplitude of two delayed replicas is varied while measuring their second-harmonic spectra. Here we study the retrieval of noisy traces and the implications of having different delays or phase retardations (relative phases) between the two replicas. Then, we study the capability of the technique to characterize the pulses when the second-harmonic signal is spectrally uncalibrated or incomplete, presenting the analytical calculation of the marginal, which is used to calibrate the traces and to perform the pulse retrievals. We experimentally show the retrieval of different pulses using diverse delays and phase retardations to perform the amplitude swing trace and demonstrate that, from an uncalibrated trace, both the pulse information and the response of the nonlinear process can be simultaneously retrieved. In sum, the amplitude swing technique is shown to be very robust against experimental constraints and limitations, showing a high degree of soundness.

Compact in-line temporal measurement of laser pulses with amplitude swing.

A method of ultrashort laser pulse reconstruction is presented, consisting on the analysis of the nonlinear signal obtained from the interference of the pulse with a replica of itself at a given time delay while varying the relative amplitude between the pulses. The resulting spectral traces are analyzed both analytically and numerically, showing the encoding of the input pulse spectral phase. A reconstruction algorithm is discussed and applied to extract the spectral phase and, jointly to the measured spectral amplitude, reconstructing the pulse. In order to validate the technique, an experimental in-line implementation of the characterization concept is compared to the results from a stablished technique, obtaining a good agreement at different input pulse cases. In sum, a new technique is presented, showing the capability to reconstruct a broad range of temporal pulse durations while its implementation is robust and straightforward, able to be easily adapted to diverse pulse duration and central wavelength ranges.

All-optical measurement of the complete waveform of octave-spanning ultrashort light pulses.

We demonstrate the complete temporal characterization of the optical waveform of visible near-infrared octave-spanning ultrashort laser pulses, using an all-optical, all-solid-state, and fully inline dispersion-scan device based only on second-harmonic generation.

Theoretical analysis of single-cycle self-compression of near infrared pulses using high-spatial modes in capillary fibers: erratum.

The first equation in [Opt. Express26, 6345 (2018)] contains an error and is corrected in this erratum.

Tailoring the spatio-temporal distribution of diffractive focused ultrashort pulses through pulse shaping.

Focusing control of ultrashort pulsed beams is an important research topic, due to its impact to subsequent interaction with matter. In this work, we study the propagation near the focus of ultrashort laser pulses of ~25 fs duration under diffractive focusing. We perform the spatio-spectral and spatio-temporal measurements of their amplitude and phase, complemented by the corresponding simulations. With them, we demonstrate that pulse shaping allows modifying in a controlled way not only the spatio-temporal distribution of the light irradiance in the focal region, but also the way it propagates as well as the frequency distribution within the pulse (temporal chirp). To gain a further intuitive insight, the role of diverse added spectral phase components is analyzed, showing the symmetries that arise for each case. In particular, we compare the effects, similarities and differences of the second and third order dispersion cases.

Theoretical analysis of single-cycle self-compression of near infrared pulses using high-spatial modes in capillary fibers.

Soliton self-compression is demonstrated during the propagation of high spatial modes in hollow core fibers in the near-infrared spectral region, taking advantage of their negative dispersion response. We have found that there is always an optimum spatial mode to observe this phenomenon, compressing the pulses down to the single-cycle regime without needing any external compression device and with a consequent increase in the output peak power. Our result is relevant for any ultrashort laser application in which few- or single-cycle pulses are crucial.

Self-calibrating d-scan: measuring ultrashort laser pulses on-target using an arbitrary pulse compressor.

In most applications of ultrashort pulse lasers, temporal compressors are used to achieve a desired pulse duration in a target or sample, and precise temporal characterization is important. The dispersion-scan (d-scan) pulse characterization technique usually involves using glass wedges to impart variable, well-defined amounts of dispersion to the pulses, while measuring the spectrum of a nonlinear signal produced by those pulses. This works very well for broadband few-cycle pulses, but longer, narrower bandwidth pulses are much more difficult to measure this way. Here we demonstrate the concept of self-calibrating d-scan, which extends the applicability of the d-scan technique to pulses of arbitrary duration, enabling their complete measurement without prior knowledge of the introduced dispersion. In particular, we show that the pulse compressors already employed in chirped pulse amplification (CPA) systems can be used to simultaneously compress and measure the temporal profile of the output pulses on-target in a simple way, without the need of additional diagnostics or calibrations, while at the same time calibrating the often-unknown differential dispersion of the compressor itself. We demonstrate the technique through simulations and experiments under known conditions. Finally, we apply it to the measurement and compression of 27.5 fs pulses from a CPA laser.

Strategies for achieving intense single-cycle pulses with in-line post-compression setups.

Intense few- and single-cycle pulses are powerful tools in different fields of science Today, third- and higher-order terms in the remnant spectral phase of the pulses remain a major obstacle for obtaining high-quality few- and single-cycle pulses from in-line post-compression setups. In this Letter, we show how input pulse shaping can successfully be applied to standard post-compression setups to minimize the occurrence of high-order phase components during nonlinear propagation and to directly obtain pulses with durations down to 3 fs. Furthermore, by combining this pulse shaping of the input pulse with new-generation broadband chirped mirrors and material addition for remnant third-order phase correction, pulses down to 2.2 fs duration have been measured.

Holographic optical element to generate achromatic vortices.

A compound holographic optical element to generate achromatic vortices with high efficiency, based on the combination of two volume phase holograms, is designed and constructed. This element is compact and easy to align. It has high damage threshold, so it can be used with ultraintense laser pulses.

Spatiotemporal characterization of few-cycle laser pulses.

In this paper we apply a broadband fiber optic coupler interferometer to the measurement of few-cycle laser pulses. Sub-8-fs pulses delivered by an ultrafast oscillator were characterized spatiotemporally using STARFISH, which is based on spatially resolved spectral interferometry. The reference pulse was measured with the d-scan technique. The pulses were focused by an off-axis parabolic mirror and were characterized at different transverse planes along the focusing region. The evolution of the retrieved pulses is analyzed, exhibiting small variations in the temporal (and spectral) amplitude and phase during propagation. Finally, the peak irradiance evolution is estimated from the integration of the spatiotemporal intensity.

Synthesis of fractal light pulses by quasi-direct space-to-time pulse shaping.

We demonstrated a simple diffractive method to map the self-similar structure shown in squared radial coordinate of any set of circularly symmetric fractal plates into self-similar light pulses in the corresponding temporal domain. The space-to-time mapping of the plates was carried out by means of a kinoform diffractive lens under femtosecond illumination. The spatio-temporal characteristics of the fractal pulses obtained in this way were measured by means of a spectral interferometry technique assisted by a fiber optics coupler (STARFISH). Our proposal allows synthesizing suited sequences of focused fractal femtosecond pulses potentially useful for several current applications, such as femtosecond material processing, atomic, and molecular control of chemical processes or generation of nonlinear effects.

Multibeam second-harmonic generation by spatiotemporal shaping of femtosecond pulses.

We present a technique for efficient generation of the second-harmonic signal at several points of a nonlinear crystal simultaneously. Multispot operation is performed by using a diffractive optical element that splits the near-infrared light of a mode-locked Ti:sapphire laser into an arbitrary array of beams that are transformed into an array of foci at the nonlinear crystal. We show that, for pulse temporal durations under 100 fs, spatiotemporal shaping of the pulse is mandatory to overcome chromatic dispersion effects that spread both in space and time the foci showing a reduced peak intensity that prevents nonlinear phenomena. We experimentally demonstrate arbitrary irradiance patterns for the second-harmonic signal consisting of more than 100 spots with a multipass amplifier delivering 28 fs, 0.8 mJ pulses at 1 kHz repetition rate.

Enhancement of filamentation postcompression by astigmatic focusing.

The energy scaling up of pulse postcompression is still an open issue. In this work we analyze the use of astigmatic focusing to improve the output pulses in a filamentation based postcompression setup. Unlike spherical conditions, astigmatic focusing enhances the output energy and the spectral broadening of the filament. This is due to the increase of critical power, allowing a considerable improvement of the postcompression energy and stability in a simple way. We demonstrated compression from FWHM 100 fs, 10 nm, 3 mJ input pulses to 13 fs, 142 nm, near 1 mJ pulses.

Spatio-temporal characterization of ultrashort pulses diffracted by circularly symmetric hard-edge apertures: theory and experiment.

We carry out a complete spatio-temporal characterization of the electric field of an ultrashort laser pulse after passing through a diffractive optical element composed of several binary amplitude concentric rings. Analytical expressions for the total diffraction field in the time and spectral domain are provided, using the Rayleigh-Sommerfeld formulation of the diffraction. These expressions are experimentally validated. The spatio-temporal amplitude and phase structure of the pulse are measured at different planes beyond the diffractive optical element using spatially-resolved spectral interferometry assisted by an optical fiber coupler (STARFISH). Our results allow corroborating theoretical predictions on the presence of multiple pulses or complex spectral distributions due to the diffraction-induced effects by the hard-edge ring apertures.

Above-millijoule super-continuum generation using polarisation dependent filamentation in atoms and molecules.

We demonstrate for the first time that input polarisation control inducing one single filamentation is a very robust technique to accurately control the filamentation dynamics enhancing throughput energy of the supercontinuum generation up to 1.2 millijoule. Reaching the above-millijoule regime opens the way to post-compression of multi-terawatt laser pulses.