Pump Intensity Dependence Research Articles

Photovoltage measurement by modulated photoemission on the p-GaAs surface with cesium overlayers

A contactless two-beam modulated photoemission technique for measuring the photovoltage on the surface of highly doped semiconductors is proposed and verified for the p+-GaAs surface with adsorbed Cs. The surface photovoltage evolution under the Cs deposition on the highly doped p+-GaAs surface is compared with that observed on Cs/UP+-structures by the photoreflectance spectroscopy method. The pump intensity dependence of the photovoltage is measured. This allows us to determine the absolute photovoltage values.

Terahertz generation by optical rectification in chalcopyrite crystals ZnGeP2, CdGeP2 and CdSiP2.

Optical rectification of near-infrared laser pulses generates broadband terahertz radiation in chalcopyrite crystals CdGeP2, ZnGeP2 and CdSiP2. The emission is characterized using linear-polarized excitation from 0.8 eV to 1.55 eV (1550 nm - 800 nm). All three crystals are (110)-cut and polished to 0.5 mm, thinner than the coherence length across most of the excitation photon energy range, such that they all produce a bandwidth ~2.5 THz when excited with ~100 fs pulses. It is found that CdGeP2 produced the strongest emission at telecoms wavelengths, while CdSiP2 is generally the strongest source. Pump-intensity dependence provides the nonlinear coefficients for each crystal.

Terahertz emission from gold nanorods irradiated by ultrashort laser pulses of different wavelengths

Electron photoemission and ponderomotive acceleration by surface enhanced optical fields is considered as a plausible mechanism of terahertz radiation from metallic nanostructures under ultrafast laser excitation. To verify this mechanism, we studied experimentally terahertz emission from an array of gold nanorods illuminated by intense (~10–100 GW/cm2) femtosecond pulses of different central wavelengths (600, 720, 800, and 1500 nm). We found for the first time that the order of the dependence of the terahertz fluence on the laser intensity is, unexpectedly, almost the same (~4.5–4.8) for 720, 800, and 1500 nm and somewhat higher (~6.6) for 600 nm. The results are explained by tunneling currents driven by plasmonically enhanced laser field. In particular, the pump-intensity dependence of the terahertz fluence is more consistent with terahertz emission from the sub-cycle bursts of the tunneling current rather than with the ponderomotive mechanism.

Low Inhomogeneous Broadening of Excitonic Resonance in MAPbBr3 Single Crystals.

We present an optical study of MAPbBr3 single crystal grown from solution. The crystal Pm3m symmetry was confirmed by electron backscatter diffraction. Our major attention was focused on optical effects related to the excitonic states in MAPbBr3. Photoluminescence temperature dependence of narrow exciton resonance showed encouragingly low inhomogeneous broadening Γ ≈ 0.5 meV that allows one to distinguish the signals from free excitons and those arising from recombination of excitons localized on defects. Excitonic origin of the resonance was proved by its superlinear pump intensity dependence, in contrast to the linear behavior of the defect-assisted recombination bands. For the first time, the phonon replicas originating from free exciton recombination accompanied by partial energy transfer to the phonons were observed in high-resolution PL spectra and confirmed by independent low-temperature Raman scattering experiments. In turn, low-temperature, low-frequency Raman scattering studies let us resolve the structure of the low-frequency phonon spectrum.

Photoinduced giant magnetic polarons in EuTe

Photoinduced magnetic polarons in EuTe, with a magnetic moment of several hundred Bohr magnetons, were investigated as a function of pump intensity and temperature by pump-probe Faraday rotation. The quantum efficiency for optical generation of magnetic polarons is found to be 0.09. The pump-intensity dependence of the photoinduced Faraday rotation shows a sublinear increase, from which we deduce that the population of photoexcited polarons is limited by a maximum value of $4.5\ifmmode\times\else\texttimes\fi{}{10}^{15}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$. This is four orders of magnitude less than the concentration of polarons that would completely fill the crystal, which suggests that the photoexcited polarons are anchored by defects. In addition to the generation of polarons, at high pump densities the modulated pump light also causes a small alternating heating of the illuminated region. The temperature dependence of the polaron magnetic moment is well described by the Curie--Weiss law. Above 100 K, polarons are thermally quenched with an activation energy of 11 meV.

Extreme multiphoton luminescence in GaAs

We demonstrate experimentally extreme multiphoton-absorption cascades in GaAs. We show (2 + 3)- and (2 + 4)-photon luminescence, in which a two-photon transition occurs from the valence to the lower conduction band, followed by another 3- or 4-photon transition to the upper conduction band —inducing luminescence corresponding to several inter-band transitions in GaAs. Our systematic study of the observed effects verifies the cascaded multiphoton absorption, by monitoring the pump intensity and wavelength dependence of the observed luminescence spectra. The measurements are in good agreement with our modeling of multiphoton effects including Auger recombination. Our results open new directions in bulk-material band-structure exploration.

Study on the partial self-recovery process in Fe-doped lithium niobate crystals

We study the temperature and pump-intensity dependence of the temporal evolution of photoinduced light scattering in congruent, hafnium-codoped and near-stoichiometric LiNbO3:Fe crystals. A partial self-recovery process is found in the evolution trend of photoinduced light scattering in all crystals at room temperature while it disappears at −190 °C or above 120 °C. Moreover, a “scattering acceleration effect” is found for both Hf-doped and near-stoichiometric crystals. Basing the saturation refractive-index modulation the partial self-recovery process is explained by the slow recombination of light-induced space charges of the noise gratings with high wave vectors. The “scattering acceleration effect” is connected with the activation of the protons located around Fe ions and their impact on the photoconductivity.

Phonon-assisted nonlinear optical processes in ultrashort-pulse pumped optical parametric amplifiers.

Optically active phonon modes in ferroelectrics such as potassium titanyl phosphate (KTP) and potassium titanyl arsenate (KTA) in the ~7–20 THz range play an important role in applications of these materials in Raman lasing and terahertz wave generation. Previous studies with picosecond pulse excitation demonstrated that the interaction of pump pulses with phonons can lead to efficient stimulated Raman scattering (SRS) accompanying optical parametric oscillation or amplification processes (OPO/OPA), and to efficient polariton-phonon scattering. In this work, we investigate the behavior of infrared OPAs employing KTP or KTA crystals when pumped with ~800-nm ultrashort pulses of duration comparable to the oscillation period of the optical phonons. We demonstrate that under conditions of coherent impulsive Raman excitation of the phonons, when the effective χ(2) nonlinearity cannot be considered instantaneous, the parametrically amplified waves (most notably, signal) undergo significant spectral modulations leading to an overall redshift of the OPA output. The pump intensity dependence of the redshifted OPA output, the temporal evolution of the parametric gain, as well as the pump spectral modulations suggest the presence of coupling between the nonlinear optical polarizations PNL of the impulsively excited phonons and those of parametrically amplified waves.

Observation of coherent phonon-plasma coupled modes in wide gap semiconductors by transmission pump-probe measurements.

We have investigated coherent LO phonon properties in zinc-based II-VI widegap semiconductors, focusing on phonon-plasma coupled modes. By a careful treatment of the time evolution of the signals in ZnS, ZnSe, and ZnTe, we found a frequency upshift as the pump intensity increases. Using a classical coupled oscillator model, we have explained the pump intensity dependence of both the shift and the decay rates by a mixing of highly damped two-photon generated plasma. From the linear dependence between them we can estimate the photo-excited carrier mobilities, leading to a new powerful estimation method to measure the mobility.

Terahertz emission upon the band-to-band excitation of Group-IV semiconductors at room temperature

Terahertz emission upon the band-to-band excitation of Group-IV semiconductors (Si:B and Ge:Ga) at room temperature by a semiconductor laser emitting in the visible range (660 nm) is observed and investigated. It is established that, as the crystal temperature is elevated above room temperature, the emission intensity increases considerably, while the emission spectrum shifts to higher frequencies. The terahertz-emission spectra of germanium and silicon are quite similar to each other. The pump-intensity dependence of the terahertz-emission intensity is nearly linear. The above features make it possible to attribute the observed terahertz emission to the effect of crystal heating by absorbed pump radiation.

GaSe晶体的双光子吸收对太赫兹输出的影响

The broadband THz generation from 0. 2 to 2. 5 THz was measured as a function of pump intensity in a single pure gallium selenide crystal based on the optical rectification of ultrafast laser pulses. Two-photon absorption at 800 nm can generate free charge carriers which can absorb the generated THz radiation,and finally attenuate THz output. The result indicates that the pump intensity dependence of ouput THz radiation changes from square to sub-linear relationship and the saturation of output THz radiation appears at high pump intensity. To study the impact of two photon absorption on THz generation,the two photon absorption coefficient of Ga Se at 800 nm is measured to be 0. 165 cm / GW,which is determined by the measured nonlinear transmission. The THz output fitting result taking account of the free charge carrier absorption cross section of 1 × 10-15cm2 is consistent with the experiment data very well. This estimation result can be used to optimize the conversion efficient of THz generation in Ga Se crystal under intense laser pump.

Terahertz emission from ZnGeP_2: phase-matching, intensity, and length scalability

Collinear phase-matched optical rectification is studied in ZnGeP$_{2}$ pumped with near-infrared light. The pump-intensity dependence is presented for three crystal lengths (0.3, 1.0 and 3.0 mm) to determine the effects of linear optical absorption, nonlinear optical absorption and terahertz free-carrier absorption on the generation. Critical parameters such as the coherence length (for velocity matching), dispersion length (for linear pulse broadening) and nonlinear length (for self-phase modulation) are determined for this material. These parameters provide insight into the upper limit of pulse intensity and crystal length required to generate intense terahertz pulse without detriment to the pulse shape. It is found that for 1-mm thick ZnGeP$_{2}$(012), pumped at 1.28 micron with intensity of ~15 GW/cm2 will produce intense undistorted pulses, whereas longer crystals or larger intensities modify the pulse shape to varying degrees. Moreover, phase-matching dispersion maps are presented for the terahertz generation over a large tuning range (1.1-2.4 micron) in longer (3 mm) crystal, demonstrating the phase-matching bandwidth and phase mismatch that leads to fringing associated with multi-pulse interference. All observed results are simulated numerically showing good qualitative agreement.

Influences of Mg2+ ion on dopant occupancy and upconversion luminescence of Ho3+ ion in LiNbO3 crystal

The effects of a Mg2+ ion on the dopant occupancy and upconversion luminescence of a Ho3+ ion in LiNbO3 crystal are reported. The birefringence gradient of the crystal is measured to investigate the optical homogeneity. The X-ray powder diffraction spectrum and the upconversion luminescence are used to investigate defect structure and spectroscopic properties of Mg,Ho:LiNbO3. Under 808-nm excitation, blue, red, and very intense yellow-green bands are observed. Based on the energy levels of Ho3+ in LiNbO3, and the pump intensity dependence of the observed emission, an excitation scheme is presented. The upconversion emission spectra reveal an enhancement of upconversion intensity when the Mg2+ ions are introduced into Ho:LiNbO3. The main upconversion mechanism is discussed in this work.

Electron Correlation Effects on the Femtosecond Dephasing Dynamics ofE22Excitons in (6,5) Carbon Nanotubes

Highly nonlinear pump fluence dependence was observed in the ultrafast one-color pump-probe responses excited by 38 fs pulses resonant with the E(22) transition in a room-temperature solution of (6,5) carbon nanotubes. The differential probe transmission (ΔT/T) at the peak of the pump-probe response (τ = 20 fs) was measured for pump fluences from ∼10(13) to 10(17) photons/pulse cm(2). The onset of saturation is observed at ∼2 × 10(15) photons/pulse cm(2) (∼8 × 10(5) excitons/cm). At pump fluences >4 × 10(16) photons/pulse cm(2) (∼1.6 × 10(6) excitons/cm), ΔT/T decreases as the pump fluence increases. Analogous signal saturation behavior was observed for all measured probe delays. Despite the high exciton density at saturation, no change in the E(22) population decay rate was observed at short times (<300 fs). The pump probe signal was modeled by a third-order perturbation theory treatment that includes the effects of inhomogeneous broadening. The observed ΔT/T signal is well-fit by a pump-fluence-dependent dephasing rate linearly dependent on the number of excitons created by the pump pulse. Therefore, the observed nonlinear pump intensity dependence is attributed to the effects of quasi-elastic exciton-exciton interactions on the dephasing rates of single carbon nanotubes. The low fluence total dephasing time is 36 fs, corresponding to a homogeneous width of 36 meV (290 cm(-1)), and the derived E(22) inhomogeneous width is 68 meV (545 cm(-1)). These results are contrasted with photon-echo-derived parameters for the E(11) transition.

Near-infrared free-carrier absorption in silicon nanocrystals

We report quantification of the free-carrier absorption (FCA) cross section in silicon nanocrystals embedded in a thin SiO(2) film at 1540 nm using a collinear pump-probe method. To this end, we measured the pump-intensity dependence of both the light transmission through the film and the photoexcited carrier density in the nanocrystals. From these measurements, we extracted a FCA cross section of sigma(FCA)=(3.6+/-1.4)x10(-17) cm(2), consistent with previous results in the visible range and the known lambda(2) scaling behavior of this quantity. Given the rapidly rising prevalence of silicon-based active photonic devices, our finding assumes particular significance for Si-nanocrystal-sensitized rare-earth-atom lasers and all optical switches at important telecom wavelengths.

Ultrafast time-resolved spectroscopy of one-dimensional metal-dielectric photonic crystals

We study the all-optical switching behavior of one-dimensional metal-dielectric photonic crystals due to the nonlinearity of the free metal electrons. A polychromatic pump-probe setup is used to determine the wavelength and pump intensity dependence of the ultrafast transmission suppression as well as the dynamics of the process on a subpicosecond timescale. We find ultrafast (sub-picosecond) as well as a slow (millisecond) behavior. We present a model of the ultrafast dynamics and nonlinear response which can fit the measured data well and allows us to separate the thermal and the electronic response of the system.

Polarization dependence of optical Kerr effect in metallophthalocyanine-doped inorganic–organic hybrid materials

Ultrafast optical Kerr effect of metallophthalocyanine-doped inorganic–organic materials was investigated using a femtosecond optical Kerr shutter at wavelength of 800 nm. Experimental results showed that the dependence of the Kerr signals on the polarization angle between the pump beam and the probe beam could be controlled by changing the pump-probe intensity ratio. The pump-intensity dependence of the polarization characters of the Kerr signals probably arose from the contribution of light induced transient grating (LITG) to the Kerr signals.

Spectral and dynamical study of nonlinear luminescence from silicon nanocrystals excited by ultrashort pulses

We report on the spectral and dynamical properties of photoluminescence (PL) under ultrashort (femtosecond and picosecond) laser pulse excitation in Si nanocrystals prepared by Si +-ion implantation into a silica matrix. We concentrate on the luminescence properties that make it possible to distinguish several luminescence bands within a broad luminescence spectrum. In time-integrated luminescence, saturation-related changes suggest a presence of two different recombination channels connected to the nanocrystal volume states and the nanocrystal–SiO 2 interface. The saturation of the luminescence can be explained in terms of Auger recombination. We investigated in detail fast sub-nanosecond luminescence, where significant differences were observed under UV excitation (linear pump-intensity dependence, PL maximum at 450 nm) and for the excitation wavelengths in the visible part of the spectrum (superlinear pump-intensity dependence, broad luminescence band including wavelengths as short as 300 nm). We propose a rate-equations model based on the Auger and radiative/nonradiative recombination to interpret the experimental data.

High-Order Harmonic Transient Grating Spectroscopy in a Molecular Jet

We study high-order harmonic generation in excited media using a four-wave-mixing-like configuration. We analyze the spatial profile of high harmonics emitted by a grating of rotationally excited molecules as a function of the pump-probe delay. We demonstrate a dramatic improvement in the contrast of the diffracted signal relative to the total high harmonic signal. This allows us to observe subtle effects in the rotational wave packet excitation such as the pump-intensity dependence of the wave packet dynamics. High harmonic transient grating spectroscopy can be extended to all forms of molecular excitation and to weak resonant excitation.

Strongly sublinear growth of the photorefractive effect for increasing pump intensities in doped lithium-niobate crystals

The pump intensity dependence of the birefringence change due to the photorefractive effect in Hf-doped and Mg-doped congruent lithium niobate (LN) crystals is reported. The birefringence change is found to be very weakly dependent on the light intensity. This is in strong contrast with the behavior of undoped congruent LN, and much more similar to what happens in stoichiometric LN. The data also indicate that the minimum HfO2 concentration necessary to strongly reduce photorefractivity is lower than the value of 4 mol % indicated by previous experiments.