Shorter Pump Wavelength Research Articles

Excitation Energy-Dependent Decay Dynamics of the S1 State of N-Methyl-2-pyridone.

The UV-induced decay dynamics of N-methyl-2-pyridone is investigated using a femtosecond time-resolved photoelectron spectroscopy method. Irradiation in the wavelength range of 339.3-258.9 nm prepares N-methyl-2-pyridone molecules with very different vibrational levels of the S1(11ππ*) state. For v' = 0 (origin) and a few low-energy vibrational levels slightly above the S1 state origin, the radiative decay channel is in operation for some specific vibrations. This is revealed by the excited-state lifetime of ≫1 ns. In addition, some other nearby S1 vibronic states have a much shorter lifetime in the range of several picoseconds to a few tens of picoseconds, indicating that the radiation-less decay to the ground state (S0) via internal conversion is the dominant channel for them. As the pump wavelength slightly decreases, the radiative decay is suddenly not important at all, and the deactivation rate of the S1 state becomes faster. At shorter pump wavelengths, the lifetime of highly excited vibrational states of the S1 state further decreases with the increase in the vibrational excess energy. This study provides quantitative information about the excitation energy-dependent decay dynamics of the S1 state of N-methyl-2-pyridone. Methyl substitution effects on the excited-state dynamics of 2-pyridone are also discussed.

Ultrafast decay dynamics of electronically excited 2-ethylpyrrole.

The excited-state decay dynamics of 2-ethylpyrrole following UV excitation in the wavelength range of 254.8-218.0 nm is investigated in detail using the femtosecond time-resolved photoelectron imaging method. The time-resolved photoelectron spectra and photoelectron angular distributions at all pump wavelengths are carefully analysed and the following picture is derived: at the longest pump wavelengths (254.8, 248.3 and 246.1 nm), 2-ethylpyrrole is excited to the S1(1πσ*) state having a lifetime of about 50 fs. At 248.3, 246.1 and 237.4 nm, another excited state of Rydberg character is excited. The lifetime of this state is ∼570 fs at 237.4 nm and becomes slightly longer at other two pump wavelengths. At the shortest pump wavelengths (230.8 and 218.0 nm), 2-ethylpyrrole is excited to a state which is tentatively assigned to the 11ππ* state, having a lifetime of 75 ± 15 and 48 ± 10 fs for the longer and shorter pump wavelengths, respectively. Internal conversion to the S1(1πσ*) state might be one of the decay mechanisms of the 11ππ* state.

Photodissociation dynamics of CF3I investigated by two-color femtosecond laser pulses

We report an experimental study of the multiphoton dissociation dynamics of CF3I performed on a home-built femtosecond laser pump-probe system; with time-of-flight mass spectrometer. The first repulsive A band and the 5p pi(3)7s sigma v(2) = 1 Rydberg state of CF3I were accessed by one- and two-photon transitions at 267 nm, respectively; with the latter two-photon absorption followed by a further two-photon probe transition at 401 nm to the (B) over tilde state of the parent ion. The observed signals from the CF3+ and I+ fragments show similar multi-component exponential decay patterns but the former is 4 times stronger than the latter. However; the parent CF3I+ signal was observed to evolve in a very different manner; decreasing sharply when probed in the first 289 fs following excitation and subsequently rising again after 860 fs to a constant level below that measured at negative pump-probe delay times when the pump and probe pulses exchange roles. This dip observed in the parent ion profile; is very different from that previously reported at shorter pump wavelengths of 264 nm or 265 ran, and is interpreted as the competition between two different ionization channels. One from the vibrationally excited v(2) = 1 of the irradiated Rydberg state and the other from the dissociative vibrational origin of the same electronic state which is populated by internal vibrational relaxation.

Gain Properties of multi-wavelength time division multiplexed Raman amplifier

We report gain properties of the multi-wavelength pulsed Raman pump. Based on fiber dispersion property, we obtain the group velocity difference between pump pulses which imposes the upper limit of pulse repetition rate beyond which pump-to-pump interaction occurs. The pump-to-pump interaction can be severe when pump instantaneous power is high due to small duty cycle. However, bigger duty cycle imposes much lower limit to the pulse repetition rate. Therefore, an optimized level of duty cycle is required. In addition, shorter pump wavelength separation helps to increase the upper limit of pulse repetition rate. Ultimately this will improve the noise performance of TDM Raman amplifier.

Generation and characterization of ultrafast white-light continuum in condensed media.

The generation of white-light continuum by femtosecond laser pulses in transparent condensed media is investigated comprehensively with 262-, 393-, and 785-nm pump wavelengths. We find that the ratio of the medium's bandgap energy to the photon energy of the incident wavelength determines the amount of anti-Stokes broadening, independently of the pump wavelength and the medium's bandgap. It is also shown that, although the amount of anti-Stokes broadening is greater for the longer pump wavelength, the shorter pump wavelength is more advantageous for generating shorter-wavelength continua in the UV region. In addition, a self-induced change in polarization of the white-light continuum that is generated is observed in an isotropic material with a cubic crystal structure, such as CaF2 and LiF. After the investigation of polarization, the frequency chirp of the continuum is characterized by the Kerr-gate method with 70-fs temporal and 10-nm wavelength resolution.

Absorption spectra of small semiconductor quantum dots

A density matrix approach has been employed to study analytically the absorption spectra of small semiconductor quantum dots under the strong confinement regime. The results are obtained for a single quantum dot (SQD) as well as for inhomogeneous distribution of quantum dots (IQDs) with Gaussian distribution of quantum dot sizes. A numerical analysis has been made for a SQD and IQDs in a CdS crystal with data taken from recent experimental work. A negative change in the absorption coefficient occurs in the shorter pump wavelength side of the spectrum due to the biexcitonic contribution. The wavelength at which crossover from positive to negative values of the change in absorption coefficient occurs is found to depend upon both the QD size as well as the excitation intensity. The results agree satisfactorily with the experimental observations in small CdS quantum dots.

Dependence of four-photon mixing spectrum around zero-dispersion wavelength on fiber length

It is found experimentally that the dependence of the spectral profile of four-photon mixing on the fiber length changes with the pump wavelength around zero dispersion band. The rising edge of the Stokes spectrum shifts toward the pump frequency side and the spectrum broadens as the fiber becomes longer under shorter pump wavelength, while no changes except for the power level appear in the spectrum under longer pump wavelength. A physical interpretation of this phenomenon is given from a view point of the effect of the characteristic length and the fiber length on the gain spectrum.

Numerical calculation of the small signal gain in Er:Ti:LiNbO/sub 3/ channel waveguides

The overlap integral describing the small signal gain coefficient in Er:Ti:LiNbO/sub 3/ channel waveguides is numerically evaluated, and the results are used to discuss the polarization dependence and the influence of the erbium ion distribution. It is shown that bleaching is obtained at a substantially lower pump power for an extraordinarily polarized mode. A study of the influence of the overlap and the mode confinement at different pump bands indicates that the poorer overlap between the pump and signal mode at shorter pump wavelengths can be compensated for by an improved overlap with the erbium ions.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>