Systematic study of highly efficient white light generation in transparent materials using intense femtosecond laser pulses

We report the results of a systematic study of white light generation in different high band-gap optical media (BaF₂, acrylic, water and BK-7 glass) using ultrashort (45 fs) laser pulses. We have investigated the influence of different parameters, such as focal position of the incident laser light within the medium, the polarization state of the incident laser radiation and the pulse duration of the incident laser beam on the white light generation. Our results indicate that for intense, ultrashort pulses, the position of physical focus inside the media is crucial in the generation, with high efficiency, of white light spectra over the wavelength range 400–1100 nm. Linearly polarized incident laser light generates white light with higher intensity in the blue region than circularly polarized light. Ultrashort (45 fs) pulses generate a flatter spectrum with higher white light conversion efficiency than longer (300 fs) pulses of the same laser power. We believe that a flat response over a wide range of wavelengths in the continuum may be efficiently compressed for generation of sub-10 fs pulses. PACS 52.38.Hb; 42.65.Jx; 42.65.Tg; 33.80.Wz; 52.35.Mw     

Depolarization of white light generated by ultrashort laser pulses in optical media

We report measurements of the extinction ratio (ER) of white light generated upon irradiation of BK7 glass by ultrashort (36 fs) laser pulses with incident power approximately 10(3) times larger than the critical power for self-focusing. At low incident powers, the continuum is symmetric about the incident laser wavelength; at high powers it becomes broader and distinctly asymmetric towards the blue side. We observe that ER degrades by 100-fold after the onset of multiphoton-induced free-electron generation (at incident intensity approximately 2 TW cm-2), which also corresponds to the onset of asymmetry in white-light spectra.     

Third-order nonlinear optical response in transparent solids using ultrashort laser pulses

The third-order optical nonlinearity, χ ⁽³⁾, is measured in transparent glasses (BK7 and fused silica) and crystals (BaF₂ and quartz) using 36-fs, 800-nm laser pulses and the optical Kerr gate (OKE) technique; values are found to lie in the range 1.3–1.7×10^-14 esu, in accordance with theoretical estimates. We probe the purely electronic response to the incident ultrashort laser pulse in fused silica and BK7 glass. In BaF₂ and quartz, apart from the electronic response we also observe contribution from the nuclear response to the incident ultrashort pulses. We observe oscillatory modulations that persist for ～400 fs. The response of the media (glasses and crystals) to ultrashort pulses is also measured using two-beam self-diffraction; the diffraction efficiency in the first-order grating is measured to be in the range of 0.06–0.13 %. Third harmonic generation due to self-phase matching in the transient grating geometry is measured as a function of temporal delay between the two incident ultrashort pulses, yielding the autocorrelation signal.     

Enhanced quadratic photocurrent in commercial light emitting diodes for autocorrelation measurement of ultrashort laser pulses

The quadratic photocurrent in commercial light emitting diodes (LEDs) has been studied in reverse bias mode for autocorrelation measurement of ultrashort laser pulses. It is found that the photocurrent can be greatly enhanced by operating the LED biased just below the reverse bias breakdown threshold. The effect of aging of LEDs on laser exposure in this mode of operation is found to be similar to that for the photovoltaic mode. The large internal gain in LED junction has enabled the recording of the second order autocorrelation signal of ∼200 fs laser pulses from 100 MHz laser oscillator with two orders of magnitude smaller average and peak power product compared to the case of the photovoltaic mode. PACS 42.65.Re; 42.50.Hz; 85.60.Jb     

Visualization of focusing–refocusing cycles during filamentation in BaF<Subscript>2</Subscript>

Filamentation occurs within a 1.5 cm-long crystal of BaF₂ during the propagation of intense, ultrashort (40 fs) pulses of 800 nm light; a systematic study as a function of incident power enables us to extract quantitative information on laser intensity within the condensed medium, the electron density and the six-photon absorption cross section. At low incident power, a single filament is formed within the crystal; two or more filaments are observed along the direction transverse to laser propagation at higher incident powers. Further, due to fluorescence from six-photon absorption (6PA), we are able to map the intensity variation in the focusing–refocusing cycles along the direction of laser propagation. At still higher incident powers, we observe splitting of multiple filaments. By measuring the radius (L _min) of single filament inside BaF₂, we obtain estimates of peak intensities (I _max) and electron densities (ρ _max) to be 3.26×10¹³ W cm⁻² and 2.81×10¹⁹ cm⁻³, respectively. Use of these values enables us to deduce that the 6PA cross-section in BaF₂ is 0.33×10⁻⁷⁰ cm¹² W⁻⁶ s⁻¹.     

Self-focusing in a plasma due to ponderomotive forces and relativistic effects

The propagation of intense laser pulses in a plasma is discussed in terms of a constant shape, paraxial ray approximation. Self-focusing due to ponderomotive forces and relativistic effects is investigated. It is found that the stationary self-focusing behaviour of each mechanism treated separately is similar, with several orders of magnitude difference in critical power. In stationary self-focusing due to the combined mechanisms, complete saturation of ponderomotive self-focusing prevents the occurrence of relativistic effects. Self-focusing lengths and minimum radii are given for a large range of beam powers. A characteristic focal spot radius is found which depends only on the plasma density.     

Self-focusing of laser radiation in cluster plasma

The self-focusing of laser radiation in plasma with ionized gaseous clusters is studied both analytically and numerically. An electrodynamic model is proposed for cluster plasma in a field of ultrashort laser pulse. The radiation self-action dynamics are studied using the equation for wave-field envelope with allowance for the electronic nonlinearity of the expanded plasma bunches and the group-velocity dispersion in a nanodispersive medium. It is shown that, for a laser power exceeding the self-focusing critical power, the wave-field self-compression occurs in a medium with dispersion of any type (normal, anomalous, or combined). Due to the strong dependence of the characteristic nonlinear field on the size of ionized cluster, the corresponding processes develop faster than in a homogeneous medium and give rise to the ultrashort pulses.     

Yellow nanosecond sum-frequency generating optical parametric oscillator using periodically poled LiNbO<Subscript>3</Subscript>

Nanosecond yellow light has been generated through simultaneously phase matched sum-frequency generation and optical parametric oscillation in a periodically poled LiNbO₃ crystal. 300 mW of yellow light at a wavelength of 586 nm has been generated from 1.3 W of laser power from a Q-switched Yb:YAG laser operating at 1031 nm. The conversion efficiency of the device is 23%. PACS 42.65.Ky; 42.65.Yj     

Enhanced harmonic conversion efficiency in the self-guided propagation of femtosecond ultraviolet laser pulses in argon

Harmonic generation during the self-guided propagation of femtosecond ultraviolet (UV) laser pulses (248-nm, 450-fs) in argon is investigated. The third (82.7-nm) and fifth (49.6-nm) harmonics are generated in the UV filament. The energy-conversion efficiencies for the harmonics are found to be at least two orders of magnitude higher than those reported in the literature for similar gas pressures. The enhancement is attributed to the quasi-phase matching of the harmonics due to the self-guiding of the driving pulse. PACS 42.25.Bs; 42.65.Jx; 42.65.Re; 42.65.Ky     

Room temperature femtosecond optical parametric generation in MgO-doped stoichiometric LiTaO3

We demonstrate room temperature femtosecond optical parametric generation with high average output power in periodically poled MgO-doped stoichiometric LiTaO₃. Direct pumping with 725-fs pulses from a passively mode-locked thin disk laser at 1030 nm resulted in stable 1.5 W average signal power at 1484 nm at the full laser repetition rate of 59 MHz. With this demonstration we achieved a significant simplification of our recently presented red-green-blue laser source because no temperature stabilization of any nonlinear crystal is required. PACS 42.65.Yj; 42.70.Mp; 42.79.Nv     

An efficient control of ultrashort laser filament location in air for the purpose of remote sensing

The unavoidable hot spots in a practical terawatt level laser pulse will self-focus in air at a short distance. The short distance cannot be changed significantly by only controlling the chirp or divergence. We overcome such early self-focusing by using a telescope, which enlarges the diameter of the beam, thus that of the hot spots. The telescope’s effective focal length is much shorter than the self-focusing distance of both the enlarged beam and the hot spots. Then, the resulting filaments merge into the geometrical focus whose position is controllable by the telescope. This technique also minimizes the generation of white light. PACS 52.38.Hb; 42.65.Jx; 52.35.Mw     

Tunable second harmonic generation by phase-modulated ultrashort laser pulses

We report on the generation of tunable light around 400?nm by frequency-doubling ultrashort laser pulses whose spectral phase is modulated by a sum of sinusoidal functions. The linewidth of the ultraviolet band produced is narrower than 1?nm, in contrast to the 12?nm linewidth of the non-modulated incident spectrum. The influence of pixellation of the liquid crystal spatial light modulator on the efficiency of the phase-modulated second harmonic generation is discussed.     

Carrier-envelope phase fluctuations of amplified femtosecond pulses: characterization with a simple spatial interference setup

We demonstrate a direct and versatile scheme to determine the carrier-envelope phase fluctuations of tunable ultrashort optical pulses. The spatial interferogram between the high frequency components and the parametrically amplified and frequency doubled low frequency components of an octave broad white light continuum is measured for every single pulse. It directly reveals the carrier-envelope phase fluctuations of the pump pulses from the regenerative amplifier, as well as of the white light and the tunable pulses generated from it. PACS 42.25.Kb; 42.65.Yj; 42.65.Re     

Plasma density gratings induced by intersecting laser pulses in underdense plasmas

Electron and ion density gratings induced by two intersecting ultrashort laser pulses at intensities of 10¹⁶ W/cm² or lower are investigated. The ponderomotive force generated by the inhomogeneous intensity distribution in the intersecting region of the interfering pulses produces deep electron and ion density modulations at a wavelength less than a laser wavelength in vacuum. Dependence of the density modulation on the plasma densities, temperatures, and the ion mass, as well as the laser pulse parameters are studied analytically and by particle-in-cell simulations. It is found that the density peaks of such gratings can be a few times that of the initial plasma density and last as long as a few picoseconds. It is also demonstrated that the scattering of signal pulses by such a bulk density grating results in high-harmonic generation. The density gratings may be incorporated into ion-ripple lasers [K.R. Chen and J.M. Dawson, Phys. Rev. Lett. 68, 29 (1992)] to generate ultrashort X-ray pulses of a few angstroms by using electron beams at only a few tens of MeV only. PACS 52.35.Mw; 42.65.Ky; 52.25.Os     

The influence of divergence on the filament length during the propagation of intense ultra-short laser pulses

We have questioned the conventional criterion, which is the critical power for self-focusing, to predict the stopping position of a filament. Our experimental result suggests that during the filamentation process the plasma induces an additional diffraction of the laser beam. This is another crucial factor that may lead to the termination of the filament even if the power is still much higher than the critical power. This should be taken into account in order to estimate the end of the filament as well as its length. PACS 52.38.Hb; 42.65.Jx; 52.35.Mw     

Multiple filamentation of femtosecond laser pulses

The formation of fluorescent channels with color centers in LiF crystals under the action of the multiple filamentation of femtosecond laser pulses is studied experimentally and theoretically for pulse powers around four orders of magnitude higher than the critical self-focusing value.     

Improvement of temporal contrast for ultrashort laser pulses by cross-polarized wave generation

Temporal contrast is a key factor influencing the performance of high-field physics experiments for ultraintense and ultrashort laser systems. The purpose of this paper is to improve the temporal contrast for ultraintense and ultrashort pulses in a laser system by cross-polarized wave (XPW) generation, a new method developed recently to enhance the temporal contrast effectively. Experiments were carried out to study the XPW generation efficiency and the enhancement of temporal contrast at the SILEX-I laser facility with a wavelength of 800 nm. The results show that the relationships between the efficiency of cross-polarized wave generation and crystal orientation/input intensity are in good agreement with theoretical predictions. Both the prepulse and the amplified spontaneous emission (ASE) pedestal are suppressed significantly and the temporal contrast is improved by two orders of magnitude by the method. With the amplification of the temporally cleaner pulses in the future, great opportunities will be created for high-field physics experiments at the laser facility.     

THz generation via optical rectification with ultrashort laser pulse focused to a line

We report on efficient THz pulse generation via optical rectification with femtosecond laser pulses focused to a line by a cylindrical lens. This configuration provides phase-matched conditions in the superluminal regime. 35 pJ THz pulses have been generated with this technique in a stoichiometric LiNbO₃ crystal pumped by 2 μJ femtosecond laser pulses at room temperature. An unusual superquadratic rise of the THz pulse energy with the laser pulse energy has been observed at high laser energies. This extraordinary energy dependence of the THz generation efficiency is explained by self-focusing of the laser beam in the crystal. Z-scan measurements and comparison of the THz pulse spectra created with laser pulses having different energies confirm this interpretation.     

Generation of a variable linear array of phase-coherent supercontinuum sources

We investigate the coherence properties of a linear array of white-light sources produced in bulk media by ultrashort laser pulses. The array is generated out of the spatial interference pattern between two laser pump pulses, so that the number of supercontinuum sources and their separations can be easily manipulated by varying the geometry of the laser beam interaction. We find that all the secondary white-light sources which arise from the generation of filaments in the optical medium are well phase-locked and are thus able to generate stable and high-visibility multiple-beam interference patterns in the far-field. Observations are compared to the results of a simple model which takes into account a clamping of the peak laser intensity inside the filaments and includes intensity-dependent phase shifts among the different sources. PACS 42.65.Jx; 42.65.Ky; 42.65.Re     

Power enhancement of a ruby laser by an intracavity plasma

An under dense plasma introduced into the cavity of a free running ruby laser is found to produce a giant pulse with a peak power two orders of magnitude larger than in the normal mode pulses.A possible plasma amplification in the cavity is suggested.