ABSTRACT
We report THz transmission and reflection properties of an ultrafast optically excited highly resistive silicon wafer. Amplified Ti:Sapphire femtosecond laser pulses at 800 nm were used to create fluence-dependent carrier density on the front surface of the wafer which modifies the dielectric properties at the THz frequencies. Time-resolved experiments in the optical pump-THz probe configuration were conducted in which THz pulses reflected off from the surface at 0° and 45° angles of incidence make it possible to measure the pump-fluence dependent ultrafast evolution of the reflection and transmission coefficients in 0.5-6 THz range. An analytical model, where both the Drude contributions from the photo-excited electrons and holes account for the change of the dielectric constant of the photo-excited silicon, has been used to evaluate the THz reflection and transmission coefficients at steady state. Thus obtained results match well with the experimental results and demonstrate an all-optical means to convert a silicon wafer into an ultrafast, tunable and broadband neutral density filter or reflector in the THz frequency range.
ABSTRACT
We have analyzed both theoretically and experimentally the spectrum of the optical pulses produced by the interaction of optical and THz pulses in a ZnTe crystal. Recorded as a function of the delay between the two pulses, the resulting spectrogram can be viewed as a frequency resolved cross-correlation between the optical and THz pulses making it possible to characterize the optical pulse.
ABSTRACT
Terahertz time domain spectroscopy has been used to study low frequency confined acoustic phonons of silver nanoparticles embedded in poly(vinyl alcohol) matrix in the spectral range of 0.1-2.5 THz. The real and imaginary parts of the dielectric function show two bands at 0.60 and 2.12 THz attributed to the spheroidal and toroidal modes of silver nanoparticles, thus demonstrating the usefulness of terahertz time domain spectroscopy as a complementary technique to Raman spectroscopy in characterizing the nanoparticles.
ABSTRACT
A chirped pulse, spectrally broadened in a photonic bandgap optical fiber by 120 fs Ti:Sapphire laser pulses, is parametrically amplified in a BBO crystal pumped by a frequency doubled nanosecond Nd:YAG laser pulse. Without changing the frequency of the Ti:Sapphire, a spectral tunability of the amplified pulses is demonstrated. The possibility to achieve broader spectral range amplification is confirmed for a non-collinear pump-signal interaction geometry. For optimal non-collinear interaction geometry, the pulse duration of the original and amplified pulse are similar. Finally, we demonstrate that the combination of two BBO crystals makes it possible to spectrally shape the amplified pulses.
ABSTRACT
We use the electric-field-induced second-harmonic (EFISH) technique to characterize, for the first time to our knowledge, the profile of the electric field induced within the bulk of fused silica glass samples both during and after thermal-poling treatment. During thermal poling, the initially homogeneous electric field decays exponentially from the anodic to the cathodic surface. After the poling treatment, a permanent electric field is induced within the bulk of the sample. It also decays exponentially from a positive value on the anodic surface to a negative value on the cathodic surface. The exponential decay constant depends on the thickness of the sample, the poling time, and the temperature. The temperature evolution of this constant makes it possible to retrieve the activation energy of the rapidly moving ionic species. This ensemble of observations indicates that modifications to models previously proposed are necessary.
ABSTRACT
Short femtosecond pulses at 1054 nm are stretched up to 400 ps by long broadband chirped-fiber Bragg gratings. The temporal profile of the reflected pulses is measured with a cross-correlation scheme. The advantage of this technique over the usual spectral measurements is its high sensitivity, since the temporal profile of reflected pulses is revealed to be strongly affected by the presence of defects in the grating. We show that this method is a good test of the quality of fiber Bragg gratings.
ABSTRACT
A model is proposed to account for Kerr-like nonlinearity induced by femtosecond pulses via terahertz generation and electro-optical effect. This phenomenon, so far overlooked, is evidenced in a zinc blende single crystal with a heterodyne optical Kerr effect setup. The spectral evolution of this phenomenon as well as its noninstantaneous response character are reported. Its competition with a third-order optical Kerr effect is demonstrated.
ABSTRACT
We characterize the gratings induced in commercial SF57 glass by femtosecond pulses. A simple technique enables us to measure both the real and the imaginary parts of the refractive index and shows that the induced grating is mainly a phase grating. An index variation of as much as 6x10(-3) is reported. Then we study the diffraction efficiency of the grating with respect to the intensity of the femtosecond pulse. Permanent structures are induced above the damage threshold. Below this threshold, the grating relaxes. The long-time dynamics of the decay is studied and described by two models. The formation of transient and permanent gratings in various other glasses is observed.
ABSTRACT
Quasi phase matching was induced within a few minutes by optical means in a thermally poled glass sample. The spatial modulation of chi((2)) that was induced by two-photon absorption of the sample subjected to an intense sinusoidal pump intensity pattern resulted in a chi((2)) grating. The second-harmonic generation produced by a probe beam was diffracted in specific directions by this chi((2)) grating, thus providing a way to monitor its inscription inside the glass. This chi((2)) grating is extremely stable at room temperature and in ambient laboratory conditions. During the process of inscription, a linear index grating was also induced. It was not stable, and it relaxed rapidly.
ABSTRACT
The second harmonic generation in a thin ss-barium borate crystal is used to measure chi ((2)) cascading phenomena in the spectral domain. The harmonic generation is induced by two pulses produced by spectrally filtering a femtosecond pulse and centered at the wavelength lambda -Delta lambda and lambda +Delta lambda. New spectral components appear in spectral density of both the fundamental and harmonic pulses. High order cascading phenomena are evidenced. In good agreement with theoretical predictions, for large phase mismatch the evolution of the spectra demonstrates the competition between cascaded chi(2) and chi(3) phenomena.
ABSTRACT
We propose a simple technique based on electric field induced second harmonic (EFISH) generation of femtosecond pulses to measure the third order susceptibility x ;(3) (2 omega,0,omega,omega) in glasses. First we present the principle of the method, then we validate our experimental set-up and develop unexpected aspects of EFISH technique. Finally, we give a numerical value of the third order susceptibility in various glasses and discuss these results.
ABSTRACT
Noncollinear frequency conversion of two femtosecond pulses is used to characterize the nonlinear profile induced in thermally poled fused silica. This technique permits one to deduce the nonlinear profile and the charge distribution induced by the poling treatment. These data and the current that passes through the sample during the poling are consistent with the occurrence of a built-in electric field associated with the diffusion of two ionic species.
ABSTRACT
Because of the large number of nucleated domains and the polydispersity of the droplet distribution, the early-stage kinetics of a first-order phase transition remains difficult to explore. We investigate this issue by using laser-induced concentration variations to drive a transition locally in a liquid mixture with two intersecting pump beams whose interference pattern traps the nucleated droplets on the optical fringes. The time-resolved reflectivity of a third probe wave on the induced droplet grating allows us to characterize the droplet growth at its early stage, which illustrates the efficiency of transient grating methods in quantifying the kinetics of out-of-equilibrium processes.
ABSTRACT
We have studied the influence of cascaded second-order effects on the spectral density of a second-harmonic pulse generated in thin KDP and beta-barium borate crystals by an intense femtosecond pulse. A noticeable evolution of spectral density is recorded for any value of phase mismatch. This evolution is in good agreement with the solution of the nonlinear propagation equation and allows a simple direct measurement of the effective susceptibility of the studied crystals.
ABSTRACT
We propose a scheme for optimizing the time delay between the pump and seed pulses of an optical parametric amplifier (OPA) over a large spectral range. The efficiency of this method is demonstrated for a femtosecond BBO parametric amplifier seeded with a white-light continuum pulse. The error signal used for intensity stabilization results from a modulation of the temporal delay between the pump and the continuum pulses and phase-sensitive detection of the amplified signal. It allows us to lock the delay to the position that maximizes the OPA gain.
ABSTRACT
Temporal and spectral characteristics of pulses resulting from second-harmonic generation of 120-fs amplified Ti:sapphire laser pulses up to 0.1 mJ at a wavelength of 815 nm in type I KDP crystal were experimentally and theoretically analyzed. Widely different behaviors were observed, according to the sign of the phase mismatch. Comparison between the theoretical simulation and experimental data demonstrates that the competition between third- and second-order nonlinear phenomena strongly modifies the structure of the pulses generated.
ABSTRACT
Second-harmonic generation (SHG) experiments at the surfaces and in the volumes of thermally poled commercial fused-silica samples have been performed. The experimental results show, for the first time to our knowledge and in agreement with our model, that second-order nonlinearity is induced on both cathodic and anodic interfaces. No contribution from the bulk of the sample could be detected. Moreover, our data also reveal that reorientable moieties are at the origin of the induced nonlinearity. The second-order susceptibility, evaluated by a surface SHG experiment, is chi((2))(333)~1 pm/V . The estimated susceptibility associated with the dc Kerr effect is negligible.
ABSTRACT
Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water-surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water molecules. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.
