Pulse Front Tilt Research Articles

Effect of spatio-temporal coupling on ultrafast laser direct writing in glass

Spatio-temporal coupling characteristics of ultrafast laser pulses are quantitatively tailored. An asymmetric microstructure is induced in the focal volume when the laser scans perpendicularly to the direction of the spatial chirp in fused silica. The tilted direction reverses when adding a Dove prism into the light path. The sign of the pulse front tilt can be turned from positive to negative by changing the group delay dispersion by steps. We reveal that the tilted direction of a microstructure depends on spatial chirp, and the interplay between spatio-temporal chirp leads to the change of tilted angles.

Spatio-spectral characterization of broadband fields using multispectral imaging.

Spatio-temporal coupling in the field of ultrashort optical pulses is a technical enabler for applications but can result in detrimental effects such as increased on-target pulse duration and decreased intensity. Spectrally resolved spatial-phase measurements of a broadband field are demonstrated using a custom multispectral camera combined with two different wavefront sensors: a multilateral spatial shearing interferometer based on an amplitude checkerboard mask and an apodized imaged Hartmann sensor. The spatially and spectrally resolved phase is processed to quantify the commonly occurring pulse-front tilt and radial group delay, which are experimentally found to be in good agreement with models.

Enhanced energy gain in a dielectric laser accelerator using a tilted pulse front laser.

Using an 800 nm, 45 fs pulse-front-tilted laser we demonstrate a record 315 keV energy gain in a dual grating dielectric laser accelerator (DLA) and average accelerating gradients of 560 MV/m over 0.5 mm. These results open a new regime in DLA characterized by significant evolution of the beam distribution in the longitudinal phase space, corresponding to > 1/4 of a synchrotron oscillation. By tilting the laser wavefront we control the resonant velocity of the DLA and observe a net energy gain, indicating that a tapered optical phase could be used to achieve very high energy gain.

Nonlinear distortion and spatial dispersion of intense terahertz generation in lithium niobate via the tilted pulse front technique

We systematically investigate the influences of the input infrared spectrum, chirp, and polarization on the emitted intense terahertz spectrum and spatial dispersion in lithium niobate via optical rectification. The terahertz yield and emission spectrum depend on both the chirp and spectrum of the input pump laser pulses. We also observe slight non-uniform spatial dispersion using a knife-edge measurement, which agrees well with the original predictions. The possible mechanism is the nonlinear distortion effect caused by high-energy laser pumping. Our study is very important and useful for developing intense terahertz systems with applications in extreme terahertz sciences and nonlinear phenomena.

Stimulated Raman near-backscattering with a pulse-front-tilted pump

A new type of Raman plasma amplifier has been proposed using a stimulated Raman near-backscattering scheme with a pulse-front-tilted pump. In order for the seed to overlap spatiotemporally with the pump, the propagation angle of the pump with respect to the seed path is set at twice the angle of the pulse-front tilt. The length of the pump passing each point of a plasma is dependent on the angle of the tilt. As a result, the tilt angle can be tuned to control the plasma temperature as well as the Raman spontaneous radiation growth length, which can be optimized for the seed amplification and significantly suppress the precursors growing from the plasma noise.

Tilted Electron Pulses.

We report the all-optical generation and characterization of tilted electron pulses by means of single-cycle terahertz radiation at an electron-transmitting mirror at slanted orientation. Femtosecond electron pulses with a chosen tilt angle are produced at an almost arbitrary target location. The experiments along with theory further reveal that the pulse front tilt in electron optics is directly connected to angular dispersion. Quantum mechanical considerations suggest that this relation is general for particle beams at any degree of coherence. These results indicate that ultrashort electron pulses can be shaped in space and time as versatilely as femtosecond laser pulses, but at 10^{5} times finer wavelength and subnanometer imaging resolution.

A tilted pulse-front setup for femtosecond transient grating spectroscopy in highly non-collinear geometries

We demonstrate a tilted pulse-front transient grating (TG) technique that allows to optimally utilize time resolution as well as TG line density while probing under grazing incidence as typically done in extreme ultraviolet (EUV) or soft x-ray (SXR) experiments. Our optical setup adapts the pulse front tilt of the two pulses that create the TG to the grazing incident pulse. We demonstrate the technique using all 800 nm femtosecond laser pulses for TG generation on a vanadium dioxide film. We probe that grating via diffraction of a third 800 nm pulse. The time resolution of 90 fs is an improvement by a factor of 30 compared to our previous experiments on the same system. The scheme paves the way for EUV and SXR probing of optically induced TGs on any material.

Single-shot real-time detection technique for pulse-front tilt and curvature of femtosecond pulsed beams with multiple-slit spatiotemporal interferometry.

Spatiotemporal coupling (STC) of femtosecond pulsed beams could significantly reduce the focal-spot intensity of ultra-intense lasers. We theoretically present a very simple method for single-shot real-time detecting pulse-front tilt, curvature, or tilt and curvature (PFT, PFC or PFT&PFC) by using multiple-slit spatiotemporal interferometry (MSTI). An unknown input pulsed beam is spatially cut by a high-density multiple-slit and changed into a series of spatially separated sub-pulses. By only measuring the spatial distribution of the interference pattern in the far-field, PFT, PFC, or PFT&PFC can be detected. Comparing with recent methods, no reference pulses or beams, no temporal or spatial scanning, and no temporal or spectral measurement is required. The single-shot and spatial-only measurement will greatly simplify the real-time detection of PFT and PFC.

Adaptive spatiotemporal optical pulse front tilt using a digital micromirror device and its terahertz application.

We report a new method to temporally and spatially manipulate the pulse front tilt (PFT) intensity profile of an ultrashort optical pulse using a commercial microelectromechanical system, also known as a digital micromirror device (DMD). For our demonstration, we show terahertz generation in a lithium niobate crystal using the PFT pumping scheme derived from a DMD chip. The adaptive functionality of the DMD could be a convenient alternative to the more conventional grating required to generate a laser beam with a PFT intensity profile that is typically used for efficient optical rectification in noncollinear phase-matching conditions. In contrast to a grating, PFT using DMD does not suffer from wavelength dispersion, and exhibits overlap properties between grating and a stair-step echelon mirror.

Efficient semiconductor multicycle terahertz pulse source

Multicycle THz pulse generation by optical rectification in GaP semiconductor nonlinear material is investigated by numerical simulations. It is shown that GaP can be an efficient and versatile source with up to about 8% conversion efficiency and a tuning range from 0.1 THz to about 7 THz. Contact-grating technology for pulse-front tilt can ensure an excellent focusability and scaling the THz pulse energy beyond 1 mJ. Shapeable infrared pump pulses with a constant intensity-modulation period can be delivered for example by a flexible and efficient dual-chirped optical parametric amplifier. Potential applications include linear and nonlinear THz spectroscopy and THz-driven acceleration of electrons.

Terahertz pulse generation by the tilted pulse front technique using an M-shaped optical system

To achieve the phase matching condition in terahertz (THz) pulse generation by the tilted pulse front technique, it is necessary to rebuild the entire optical setup if the optical conditions, such as excitation wavelength, temperature of nonlinear crystal, and output THz frequency, are changed. We propose THz pulse generation by the tilted pulse front technique using an M-shaped configuration. This system allows us to change the optical conditions only by tuning a few optics and without rebuilding the entire setup. We change the excitation wavelength at a fixed radiation frequency and assess the performance of the proposed system.

Highly efficient generation of 0.2 mJ terahertz pulses in lithium niobate at room temperature with sub-50 fs chirped Ti:sapphire laser pulses.

We demonstrate generation of 0.2 mJ terahertz (THz) pulses in lithium niobate driven by Ti:sapphire laser pulses at room temperature. Employing tilted pulse front technique, the 800 nm-to-THz energy conversion efficiency has been optimized to 0.3% through chirping the sub-50 fs pump laser pulses to overcome multi-photon absorption and to extend effective interaction length for phase matching. Our approach paves the way for mJ-level THz generation via optical rectification using existing Ti:sapphire laser systems which can deliver Joule-level pulse energy with sub-50 fs pulse duration.

Spectrally resolved single-shot wavefront sensing of broadband high-harmonic sources.

Wavefront sensors are an important tool to characterize coherent beams of extreme ultraviolet radiation. However, conventional Hartmann-type sensors do not allow for independent wavefront characterization of different spectral components that may be present in a beam, which limits their applicability for intrinsically broadband high-harmonic generation (HHG) sources. Here we introduce a wavefront sensor that measures the wavefronts of all the harmonics in a HHG beam in a single camera exposure. By replacing the mask apertures with transmission gratings at different orientations, we simultaneously detect harmonic wavefronts and spectra, and obtain sensitivity to spatiotemporal structure such as pulse front tilt as well. We demonstrate the capabilities of the sensor through a parallel measurement of the wavefronts of 9 harmonics in a wavelength range between 25 and 49 nm, with up to λ/32 precision.

Optical design for increased interaction length in a high gradient dielectric laser accelerator

We present a methodology for designing and measuring pulse front tilt in an ultrafast laser for use in dielectric laser acceleration. Previous research into dielectric laser accelerating modules has focused on measuring high accelerating gradients in novel structures, but has done so only for short electron–laser coupling lengths. Here we demonstrate an optical design to extend the laser–electroninteraction to 1 mm.

Double-grating monochromatic beamline with ultrafast response for FLASH2 at DESY.

The preliminary design of a monochromatic beamline for FLASH2 at DESY is presented. The monochromator is tunable in the 50-1000 eV energy range with resolving power higher than 1000 and temporal response below 50 fs over the whole energy range. A time-delay-compensated configuration using the variable-line-spacing monochromator design with two gratings is adopted: the first grating disperses the radiation on its output plane, where the intermediate slit performs the spectral selection; the second grating compensates for the pulse-front tilt and for the spectral dispersion due to diffraction from the first grating.

Temporal Response of Ultrafast Grating Monochromators

The temporal response of double-grating monochromators is analyzed considering two effects on the ultrafast pulse given by the configuration. The first effect is the compensation of the pulse-front tilt, i.e., all the rays emitted by the source in different directions at the same wavelength have to travel the same optical path. The second effect is the group delay introduced by the two gratings, i.e., different wavelengths within the bandwidth transmitted by the slit travel different paths. The methodology to calculate the group delay introduced by the double-grating configuration is presented. Some practical design examples are provided to show the design rules and the achieved performances.

Chromatic effect in a novel THz generation scheme

Deriving single or few cycle terahertz (THz) pulse by an intense femtosecond laser through cascaded optical rectification is a crucial technique in cutting-edge time-resolved spectroscopy to characterize micro-scale structures and ultrafast dynamics. Due to the broadband nature of the ultrafast driving laser, the chromatic effect limits the THz conversion efficiency in optical rectification crystals, especially for those implementing the pulse-front tilt scheme, e.g. lithium niobate (LN) crystal, has been prevalently used in the past decade. In this research we developed a brand new type of LN crystal utilizing Brewster coupling, and conducted systematically experimental and simulative investigation for the chromatic effect and multi-dimensionally entangled parameters in THz generation, predicting that an extreme conversion efficiency of ∼10% would be potentially achievable at the THz absorption coefficient of ∼0.5 cm−1. Moreover, we first discovered that the chirp of the driving laser plays a decisive role in the pulse-front tilt scheme, and the THz generation efficiency could be enhanced tremendously by applying an appropriate chirp.

Coherent combination of ultrashort pulse beams using two diffractive optics

A novel coherent beam combiner, capable of combining large numbers of femtosecond pulse beams using two diffractive optics, is presented. The diffractive optic pair cancels pulse front tilt, while uncorrected dispersions are minimized. An example using four beams is modeled numerically and tested experimentally, demonstrating 120fs pulses combined without degradation of pulse width. Scaling the concept, we show analytically that combining loss due to uncorrected dispersions is only a few percent for ∼200 beams with 130fs pulses.

Direct Writing with Tilted-Front Femtosecond Pulses

Shaping light fields in both space and time provides new degrees of freedom to manipulate light-matter interaction on the ultrafast timescale. Through this exploitation of the light field, a greater appreciation of spatio-temporal couplings in focusing has been gained, shedding light on previously unexplored parameters of the femtosecond light pulse, including pulse front tilt and wavefront rotation. Here, we directly investigate the effect of major spatio-temporal couplings on light-matter interaction and reveal unambiguously that in transparent media, pulse front tilt gives rise to the directional asymmetry of the ultrafast laser writing. We demonstrate that the laser pulse with a tilted intensity front deposits energy more efficiently when writing along the tilt than when writing against, producing either an isotropic damage-like or a birefringent nanograting structure. The directional asymmetry in the ultrafast laser writing is qualitatively described in terms of the interaction of a void trapped within the focal volume by the gradient force from the tilted intensity front and the thermocapillary force caused by the gradient of temperature. The observed instantaneous transition from the damage-like to nanograting modification after a finite writing length in a transparent dielectric is phenomenologically described in terms of the first-order phase transition.

Study of femtosecond laser-induced circular optical properties in silica by Mueller matrix spectropolarimetry.

Transmission Mueller-matrix spectroscopic ellipsometry is applied to femtosecond laser-induced modifications in silica glass in the spectral range of 450-1000nm. Within a type II regime, the modifications exhibit not only circular dichroism, but also circular birefringence. We suggest that the laser polarization orientation with respect to pulse front tilt determines the amplitude and the sign of the circular properties. By using differential decomposition of the Mueller matrix, optical rotation is revealed for the first time, to the best of our knowledge. A maximum value of the effective optical activity of 143°/mm at 550nm is found.