Absorption Of Femtosecond Light Pulses Research Articles

Multiphoton intraband absorption of femtosecond light pulses in crystals: I. General relations

We have developed a theory of the transient absorption of femtosecond light pulses by free electrons in the course of intraband transitions involving the participation of phonons. General expressions have been obtained for the probability of transition of an electron from the bottom of the conduction band to a highly excited state due to the absorption of several photons and either the absorption or emission of a phonon.

Multiphoton intraband absorption of femtosecond light pulses in crystals: II. Processes with the participation of acoustic and optical phonons

For arbitrary integers n, we have calculated probabilities P n of n-photon intraband transitions with the participation of longitudinal acoustic and longitudinal optical phonons. Dependences of P n on the duration of femtosecond pulses have been obtained. These dependences differ from those that are observed in the case of quasisteady electromagnetic radiation.

A transient two-photon–one-photon double resonance on interband transitions in crystals: I. General relations

A theory of the transient absorption of femtosecond light pulses at a two-photon–one-photon double resonance on adjacent interband transitions in the electronic system has been developed. Approximate expressions for the reactive component of the nonlinear polarization of the three-level system, Ps, which determines the absorbed power in the medium, have been obtained, and the dependences of Ps on the light intensity and detunings of resonances have been calculated.

Non-Linear Optical Phenomena in Detecting Materials as a Possibility for Fast Timing in Detectors of Ionizing Radiation

The time resolution of the detectors currently in use is limited by 50–70 ps due to the spontaneous processes involved in the development of the response signal, which forms after the relaxation of carriers generated during the interaction. In this study, we investigate the feasibility of exploiting sub-picosecond phenomena occurring after the interaction of scintillator material with ionizing radiation by probing the material with ultra-short laser pulses. One of the phenomena is the elastic polarization due to the local lattice distortion caused by the displacement of electrons and holes generated by ionization. The key feature of the elastic polarization is its short response time, which makes it prospective for using as an optically detectable time mark. The nonlinear optical absorption of femtosecond light pulses of appropriate wavelength is demonstrated to be a prospective tool to form the mark. This study was aimed at searching for inorganic crystalline media combining scintillation properties and non-linear absorption of ultra-short laser pulses. The nonlinear pump-and-probe optical absorption technique with 200 fs laser pulses was used to study the effects in lead tungstate, garnet-type, and diamond scintillator crystals.

Absorption of femtosecond light pulses due to indirect interband transitions in crystals

The theory of absorption of femtosecond light pulses due to indirect interband transitions in crystals is developed. The absorption occurs due to two-quantum processes with participation of a photon and a phonon. Expressions governing J, the density of energy absorbed during the laser pulse action, and its dependence on laser pulse duration, temperature, and frequency of light are obtained.

Nonlinear absorption of light pulses under conditions of two-photon resonance in bulk crystals and nanostructures in the femtosecond pump-probe spectroscopy mode

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of different dimensions in the pump-probe spectroscopy mode has been developed. Expressions for the energy absorbed from the probe pulse under conditions of two-photon resonances are derived for transitions between the discrete or band electronic states and between the size-quantization subband states in quantum wells and quantum wires. The dependences of the energy absorbed from the probe pulse on the two-photon resonance detuning and on the time delay between the pulses are analyzed. The dependence of the power absorbed from the probe pulse on the time is also obtained.

Interferometric detection of extensional modes of GaN nanorods array

Femtosecond pump probe spectroscopy experiments were carried out to observe extensional modes of GaN nanorods. Different orders of extensional modes were generated and observed following the absorption of femtosecond light pulses. This observation confirms that with a diameter on the order of 100 nm, no mechanical change is expected compared to bulk GaN. We propose and demonstrate that the detection of these modes is achieved through the modulation of the Fabry-Pérot cavity formed by the nanorod array. The extensional modes change the nanorods length and thus modify the reflectivity of the rod-array cavity.

Nonlinear absorption of femtosecond light pulses under conditions of multiphoton resonances in solids

A theory of nonlinear absorption of femtosecond light pulses by bulk crystals and nanostructures of differing dimensionality is developed. The pulse width is assumed to be small compared to the relaxation times of the electron and hole momenta. Expressions for the absorbed energy under conditions of multiphoton resonance are derived for transitions between discrete or band-related electron states and between sub-band states of size quantization in quantum wells and quantum wires. The dependence of the absorbed energy on the multiphoton resonance detuning and pulse width is analyzed.

Nonlinear absorption of femtosecond light pulses accompanying two-photon resonance in bulk crystals and nanostructures

A theory has been developed for the nonsteady-state absorption of supershort light pulses in bulk materials and heterostructures with quantum wells when two-photon resonance occurs at interband transitions as well as at transitions between discrete states or between size-quantization sublevels (subbands). Analytical expressions have been obtained for the nonlinear polarizabilities that determine how much energy is absorbed from a femtosecond light pulse. The dependences of the absorbed energy on the detunings of the two-photon resonances and on the pulse widths have been obtained.

Optical manipulators of microparticles using femtosecond laser radiation

Two setups for manipulating (translocation, stretching, rotation) microscopic objects (with the dimensions from several nanometers to tens of micrometers) are developed based on optical trapping by femtosecond laser radiation. The possibility of single-cell translocation is shown. The possibility of destructing malignant cells as well as of cutting off a fragment from a malignant cell cluster due to the rapture of bonds is demonstrated in multiphoton absorption of femtosecond light pulses. The possibility of the holographic control to move several particles simultaneously is shown.