Effects of squeezed vacuum on transient optical response in a ∧ system

The transient nonlinear optical properties of direct gap semiconductors are discussed in relation to picosecond time resolved experiments of the excite-and-probe type. In particular, degenerate four-wave mixing and transient laser induced grating spectroscopies, in two-beam and three-beam configurations, are being used to investigate the nonlinear optical resonances associated with exciton interactions. The coherence and dephasing of these resonances are also related to the exciton interactions and relaxations. Specific results from the binary compound CdSe and from mixed crystals of CdSexS1-x are presented, and some possible applications of the transient nonlinear optical response are discussed.

Two dimethylamino-carbaldehyde derivatives with different π-bridge lengths were prepared, and their transient optical properties and photophysical mechanisms were investigated by transient absorption spectroscopy and Z-scan measurements. Owing to the difference in molecular structures, the two compounds exhibit different populations of locally excited states and, therefore, they also produce different transient absorption spectra. After photoexcitation, both molecular materials exhibit a wide excited state absorption band from 450 nm to 1000 nm. Meanwhile, the excited state lifetimes are dramatically different, 2 ns and 100 ps, for the two molecules. A figure of merit greater than 2 at the wavelength of 1000 nm is obtained. The results show that modulating the population of the locally excited states in this type of molecule can be a promising approach for obtaining optical switching and solar cell materials.

Ultrashort pulse laser heating of a germanium nanofilm was simulated using a combined continuum–atomistic method that couples the molecular dynamics and a self-consistent energy model for ultrafast laser–semiconductor interaction. Both a single pulse and a pulse burst were considered. To accurately describe laser energy deposition, the transient optical properties were computed based on the Drude formula. It was found that for a single pulse at low fluence (e.g., 0.02 J/cm2), the pulse duration had little impact on the lattice temperature response. In contrast, a higher lattice temperature could be obtained for a longer pulse (e.g., 5 ps) at higher fluences (e.g., 0.06 J/cm2) due to lower surface reflectivity. A strong thermal stress wave could be induced by the laser heating, with its maximum compression and tension occurring in the front and rear film regions, respectively. The investigations of laser burst heating revealed that a laser burst not only can retain the advantages of ultrashort pulse lasers but can enhance the photon efficiency for material melting as well.

Enhanced two temperature modeling of ultrashort laser ablation for the investigation of thermionic emission characteristics

The effect of surface modification on femtosecond optical Kerr effect of PBS nanoparticles

By tracking the spatiotemporal distribution of the free electron density/temperature and laser intensity, the ablation threshold, depth and crater shape of fused silica for femtosecond laser processing are investigated theoretically and experimentally. The electron dynamics as well as the transient optical and thermophysical properties of laser irradiated fused silica is quantitatively determined. The numerical model is validated by comparing the calculated threshold fluence, depth and crater shape of ablation with the experimental data at the wavelength of 800 nm. The electron relaxation time at different laser fluences and pulse durations throughout the photoionization and impact ionization processes are probed. In the present paper, it is found that (1) the electron relaxation time significantly affects the material optical properties and femtosecond laser energy absorption. The optical properties change dramatically, and the fused silica becomes opaque during laser irradiation. Moreover, the transition from electron-phonon collision to electron-ion collision accompanies by the laser ablation of fused silica in the femtosecond laser irradiation process. (2) The experimentally observed saturation of the ablation depth at high laser fluence is elucidated by the proposed model, which is due to the significantly changed optical reflectivity and absorption coefficient. Both theoretical simulations and experimental observations found that laser fluence has a strong influence on the shape of the ablation crater. The ablation volume increases sharply with increasing laser fluence for femtosecond laser irradiation compared to that for picosecond laser irradiation. (3) With the increment of laser fluence, a saturation of the ablation depth removal efficiency and ablation efficiency occurs, followed by slight decrements. The ablation depth removal efficiency peaks at laser fluence close to 1.4 times of the ablation threshold. Whereas, the accuracy is slightly low due to the higher sensitivity of the ablation characteristics (ablation crater depth and ablation volume) to the shorter pulse laser. For the laser fluence higher than 3.5 times of the ablation threshold, good repeatability over a very wide fluence range enables accurate processing results, because a more consistent flat-bottom ablation profile tends to appear. However, the heat-affected zone leads to a decrement of the processing quality compared to that of laser close to the ablation threshold.

<p dir="ltr">It has been shown that liquid crystals can be used as the basis for new types of biosensors that detect bacteria, viruses, and gases. The application of liquid crystal biosensors is relatively new in the biotechnology, medicine, and food industries. Over the past few decades, this advanced material has shown its potential in a variety of applications, including electro-optical devices. Liquid crystals are viscoelastic and anisotropic materials that respond to analytes and change their optical properties accordingly. As a result, changes in optical properties can be visualized and used to detect analytes. When the liquid crystal is in contact with an aqueous interface, the chemical or biological agent is adsorbed by the liquid crystal and results in a change in the liquid crystal orientation in film or within droplets. The result of changing the bulk orientation of liquid crystal creates a texture that can be visualized by optical methods, such as a polarizing optical microscope. Understanding liquid crystal biosensors requires an understanding of bulk orientation and interface interaction in various geometries. In addition to investigating the created texture or optical pattern when polarized light is transmitted through the liquid crystal system. This research will utilize a mathematical model and computational approach to gain an understanding of the liquid crystal biosensor's performance. In this study, biosensors with the aqueous interface will be examined. The unsteady two-dimensional mathematical model is composed of the Leslie Ericksen theory incorporating the Frank free energy during the usage of liquid crystal biosensors. The model is solved numerically using the Galerkin finite element method. We examined the transient optical pattern and response time with various initial bulk and boundary conditions. In nematic liquid crystal biosensors, surface viscosity and homeotropic anchoring strength are investigated and their impact is characterized. This series of studies has been presented for the first time for the use of liquid crystal biosensors with the aqueous interface.</p>

Femtosecond laser Z-Scan measurements are used to characterize the nonlinear optical properties of AgOx super resolution near-field structures (super-RENS). Open aperture Z-Scan measurements reveal a complex transient nonlinear optical absorption response of AgOx super-RENS under intensive laser irradiation. This is attributed to the thermal decomposition of AgOx into Ag nanoparticles in the near-field optical mask layer. A bi-compositional model is proposed to explain this complex nonlinear optical response. The threshold laser peak intensity for the phase transition and the nonlinear optical absorption coefficient of an Ag nanoparticle are determined to be Ith∼0.3 GW/cm2 and β=(-5.6±0.5)×103 cm/GW at λ=800 nm, respectively.

The search for synthetic materials that mimic natural photosynthesis by converting solar energy into other more useful forms of energy is an ever-growing research endeavor. Graphene-based materials, with their exceptional electronic and optical properties, are exemplary candidates for high-efficiency solar energy harvesting devices. High photoactivity can be conveniently achieved by functionalizing graphene with small molecule organic semiconductors whose band-gaps can be tuned by structural modification, leading to interactions between the π-conjugated electronic systems in both the semiconductor and graphene. Here we investigate the ultrafast transient optical properties of a cross-linked graphene–dye (diphenyl-dithiophenediketopyrrolopyrrole) nanohybrid material, in which oligomers of the organic semiconductor dye are covalently bound to a random network of few-layer graphene flakes, and compare the results to those obtained for the reference dye monomer. Using a combination of ultrafast transient absorption and two-dimensional electronic spectroscopy, we provide substantial evidence for photoinduced charge transfer that occurs within 18 ps in the nanohybrid system. Notably, subpicosecond photoinduced torsional relaxation observed in the constituent dye monomer is absent in the cross-linked nanohybrid system. Through density functional theory calculations, we compare the competing effects of covalent bonding, increasing conjugation length, and the presence of multiple graphene flakes. We find evidence that the observed ultrafast charge transfer process occurs through a superexchange mechanism in which the oligomeric dye bridge provides virtual states enabling charge transfer between graphene–dye covalent bond sites.

Hematite (Fe2O3) is a promising earth-abundant, visible light absorber, and easily processable photocatalytic material. Understanding the dynamics of photogenerated electrons and holes in hematite and their optical signatures is crucial in designing hematite thin film devices such as photoanodes for water oxidation. We report carrier dynamics in hematite films as measured by ultrafast transient absorption spectroscopy (TA) with a pump pulse centered at 400 nm (3.1 eV) and a probe pulse spanning the visible range. We observe a small negative response for wavelengths shorter than 530 nm (2.34 eV) and a large positive response for longer wavelengths. We interpret the spectrally resolved TA data based on recent electronic band structure calculations, while accounting for excited state absorption, ground state bleach, and stimulated emission within the relevant bands. We propose that the origin of the positive TA response is absorption of the probe by photoexcited electrons within the conduction bands. This in...

The sudden absorption of light by a metal nanoparticle launches a series of relaxation processes (internal thermalization, acoustic vibrations, and cooling) which induce a transient modification of its optical response. In this work, the transient optical response associated with the internal thermalization of a single gold nanodisk (occurring on a few picoseconds time scale) was quantitatively investigated by time-resolved spectroscopy experiments, and the measured signals were compared with a model accounting for the effects of both electron and ionic lattice heating. We show that experimental time-resolved signals at delays posterior to nanodisk excitation and electron gas thermalization can be simply interpreted as a combination of electron and lattice temperature evolutions, with probe wavelength-dependent weights. This demonstrates the possibility to selectively probe the electronic or lattice dynamics, through the choice of specific probe wavelengths. Additionally, the time-dependent spectral shape of transient extinction cross-section changes is shown to be successively dominated by the effects of electron and lattice heating, which present distinct spectral signatures.

We report an experimental demonstration of electromagnetically induced transparency in the transient optical response in a cascade-type three-level system of GaAs/AlGaAs multiple quantum wells, and analyze the cascade-type three-level schemes with density matrix and Maxwell equations to then obtain the phase shift, group velocity, and group-velocity dispersion. The calculated group velocity is ∼6.87×104 m/s and the corresponding pulse delay is ∼7 ps. Finally, we provide a convenient basis for investigating many-body effects in semiconductors.

Nanoparticles with non-spherical shapes are now being widely used for various photonic applications. We observe experimentally that the magnitude as well as the time dependence of the transient absorption of a colloid of silver nanoplatelets depends on the relative polarization of the pump and probe pulses. There have been a few reports about the dependence of the transient signal magnitude on polarization, but little information is available on its temporal dependence. Using a theoretical model, we show that this observed behavior arises from the fact that the energy absorption by a non-spherical nanoparticle depends on, among other factors, the nanoparticle orientation with respect to the pump and probe polarization directions. It is essential to consider this when estimating nanoparticle characteristics such as carrier thermalization time, carrier–phonon scattering time, and complex polarizability from transient absorption measurements.

Electromagnetically induced transparency and slow light in V-type three-level system of GaAs/AlGaAs multiple quantum wells

We report systematic studies of the femtosecond transient reflectivity at 2 eV in partially oxygen-depleted Y-Ba-Cu-O thin films exhibiting the resistive superconducting transition at 60 K. Our measurements, performed at room temperature, reveal that in 60 K films, the Fermi level in the Cu-O2 plane lies approximately 2 eV above the filled copper d9/d10 band. By mapping the transient optical response across a 1 cm2 film, we show that the position of the Fermi level is very sensitive to the apparent local variations of the film oxygen content, which gives rise to dramatically different transient optical responses. We also observe a strong transient reflectivity dependence on intensity, which can be attributed to the shift of the Fermi level caused by nonequilibrium hole heating.