Spectral splitting in phase mismatched harmonics.

Spectral splitting of high harmonic radiation is observed when a gas target is irradiated with a high-energy laser pulse, having an extreme amount of frequency chirp. The phenomenon, which may be observed only by using a multi-TW laser system, originates from the temporal evolution of the phase-matching conditions. We illustrate how these conditions are mapped to the spectral domain, and present experimental evidence which is validated by our model.

Undepleted direct laser acceleration.

Intense lasers enable generating high-energy particle beams in university-scale laboratories. With the direct laser acceleration (DLA) method, the leading part of the laser pulse ionizes the target material and forms a positively charged ion plasma channel into which electrons are injected and accelerated. The high energy conversion efficiency of DLA makes it ideal for generating large numbers of photonuclear reactions. In this work, we reveal that, for efficient DLA to prevail, a target material of sufficiently high atomic number is required to maintain the injection of ionization electrons at the peak intensity of the pulse when the DLA channel is already formed. We demonstrate experimentally and numerically that, when the atomic number is too low, the target is depleted of its ionization electrons prematurely. Applying this understanding to multi-petawatt laser experiments is expected to result in increased neutron yields, a perquisite for a wide range of research and applications.

Intense laser interaction with micro-bars.

Intense laser fields interact very differently with micrometric rough surfaces than with flat objects. The interaction features high laser energy absorption and increased emission of MeV electrons, ions, and of hard x-rays. In this work, we irradiated isolated, translationally-symmetric objects in the form of micrometric Au bars. The interaction resulted in the emission of two forward-directed electron jets having a small opening angle, a narrow energy spread in the MeV range, and a positive angle to energy correlation. Our numerical simulations show that following ionization, those electrons that are pulled into vacuum near the object's edge, remain in-phase with the laser pulse for long enough so that the Lorentz force they experience drive them around the object's edge. After these electrons pass the object, they form attosecond duration bunches and interact with the laser field over large distances in vacuum in confined volumes that trap and accelerate them within a narrow range of momentum. The selectivity in energy of the interaction, its directionality, and the preservation of the attosecond duration of the electron bunches over large distances, offer new means for designing future laser-based light sources.

Annular coherent wake emission.

Coherent wake emission may be generated only by sufficiently high-contrast driving laser pulses. When the laser contrast is too low, the formed long-scale-length plasma cannot support its generation. In this Letter, we show how, by gently spoiling a pristine laser contrast in an engineered way, coherent wake emission becomes inhibited in the center of the irradiated substrate only, thus forming an annular-shaped source of coherent extreme ultraviolet (XUV) pulses. We present an analytical model that describes the phenomenon and validation of its predictions in the experiment and the simulation. We also show how the ion-acoustic velocity dependency on the laser intensity may be obtained from the emission patterns and offer examples for future applications.

High-resolution multi-scan compact Fourier transform-infrared spectrometer.

The Fourier transform-infrared (FT-IR) spectrometer is a widely used high-resolution spectral characterization method in materials, chemicals, and more. However, the inverse relation between the spectral resolution and the interferometer's arm length yields a tradeoff between spectral resolution and spectrometer footprint. Here, we introduce a novel method to overcome this traditional FT-IR resolution limit. The enhanced high-resolution multi-scan compact FT-IR spectrometer we present achieves an effectively long interferogram by combining multiple short FT-IR scans. Simulation and experimental results demonstrate a significant increase in the spectral resolution of a FT-IR spectrometer by employing our interferogram stitching algorithm.

Pulse shaping of broadband adiabatic SHG from a Ti-sapphire oscillator.

We experimentally demonstrate an efficient broadband second-harmonic generation (SHG) process with a tunable mode-locked Ti:sapphire oscillator. We have achieved a robust broadband and efficient flat conversion of more than 35 nm wavelength by designing an adiabatic aperiodically poled potassium titanyl phosphate crystal. Moreover, we have shown that with such efficient flat conversion, we can shape and control broadband second-harmonic pulses. More specifically, we assign a spectral phase of absolute value and π-step, which allows wavelength tunable intense pump-probe and amplitude modulation of the broadband second-harmonic output. Such spectral phases serve as a proof of concept for other pulse-shaping applications for nonlinear spectroscopy and imaging.

Ultrafast adiabatic second harmonic generation.

We introduce a generalization of the adiabatic frequency conversion method for an efficient conversion of ultrashort pulses in the full nonlinear regime. Our analysis takes into account dispersion as well as two-photon processes and Kerr effect, allowing complete analysis of any three waves with arbitrary phase mismatched design and any nonlinear optical process. We use this analysis to design an efficient and robust second harmonic generation, the most widely used nonlinear process for both fundamental and applied research. We experimentally show that such design not only allows for very efficient conversion of various of ultrashort pulses, but is also very robust to variations in the parameters of both the nonlinear crystal and the incoming light. These include variation of more than 100 °C in the crystal temperature, a wide bandwidth of up to 75 nm and a chirp variation of 300 fs to 3.5 ps of the incoming pulse. Also, we show the dependency of the adiabatic second harmonic generation design on the pump intensity and the crystal length. Our study shows that two photon absorption plays a critical role in such high influence nonlinear dynamics, and that it must be considered in order to achieve agreement with experimental results.

Capability of long distance 100 GHz FMCW using a single GDD lamp sensor.

Millimeter wave (MMW)-based imaging systems are required for applications in medicine, homeland security, concealed weapon detection, and space technology. The lack of inexpensive room temperature imaging sensors makes it difficult to provide a suitable MMW system for many of the above applications. A 3D MMW imaging system based on chirp radar was studied previously using a scanning imaging system of a single detector. The radar system requires that the millimeter wave detector will be able to operate as a heterodyne detector. Since the source of radiation is a frequency modulated continuous wave (FMCW), the detected signal as a result of heterodyne detection gives the object's depth information according to value of difference frequency, in addition to the reflectance of the 2D image. New experiments show the capability of long distance FMCW detection by using a large scale Cassegrain projection system, described first (to our knowledge) in this paper. The system presents the capability to employ a long distance of at least 20 m with a low-cost plasma-based glow discharge detector (GDD) focal plane array (FPA). Each point on the object corresponds to a point in the image and includes the distance information. This will enable relatively inexpensive 3D MMW imaging.

Performance quantification of a millimeter-wavelength imaging system based on inexpensive glow-discharge-detector focal-plane array.

Inexpensive millimeter-wavelength (MMW) optical digital imaging raises a challenge of evaluating the imaging performance and image quality because of the large electromagnetic wavelengths and pixel sensor sizes, which are 2 to 3 orders of magnitude larger than those of ordinary thermal or visual imaging systems, and also because of the noisiness of the inexpensive glow discharge detectors that compose the focal-plane array. This study quantifies the performances of this MMW imaging system. Its point-spread function and modulation transfer function were investigated. The experimental results and the analysis indicate that the image quality of this MMW imaging system is limited mostly by the noise, and the blur is dominated by the pixel sensor size. Therefore, the MMW image might be improved by oversampling, given that noise reduction is achieved. Demonstration of MMW image improvement through oversampling is presented.