Nonlinear Absorption Spectroscopy Research Articles

Photonic-crystal fiber sources for nonlinear spectroscopy

Photonic-crystal fibers allow the creation of novel compact and convenient light sources for nonlinear spectroscopy. In experiments discussed in this paper, photonic-crystal fibers provide efficient nonlinear-optical transformations of femtosecond Cr:forsterite laser pulses, delivering linearly chirped frequency-shifted broadband light pulses with central wavelengths ranging from 400 to 900 nm. These pulses were cross-correlated with the femtosecond second-harmonic output of the Cr:forsterite laser in toluene solution, used as a test object, in boxcars geometry to measure the spectra of coherent anti-Stokes Raman scattering (CARS) of toluene molecules (XFROG CARS). The blue-shifted chirped-pulse output of a photonic-crystal fiber with a spectrum stretching from 530 to 680 nm is shown to be ideally suited for time-resolved nonlinear absorption spectroscopy of exciton bands in molecular aggregates excited by femtosecond laser pulses.

Pump-probe nonlinear absorption spectroscopy of molecular aggregates using chirped frequency-shifted light pulses from a photonic-crystal fiber

Photonic-crystal fibers provide efficient nonlinear-optical transformations of femtosecond Cr: forsterite laser pulses, delivering linearly chirped frequency-shifted broadband light pulses optimized for pump-probe nonlinear absorption spectroscopy of molecular aggregates. The blue-shifted output of a photonic-crystal fiber with a spectrum stretching from 530 to 680 nm is used to probe one-and two-exciton bands of thiacarbocyanine J aggregates in a polymer film excited by femtosecond second-harmonic pulses of the Cr: forsterite laser.

Nonlinear ultrafast optical absorption and pump-probe spectroscopy in biased semiconductor superlattices

We present theoretical simulations of two important nonlinear optical phenomena in biased semiconductor superlattices: nonlinear ultrafast absorption and pump-probe spectroscopy. We find that for both these techniques, the excitonic Wannier-Stark ladder absorption peaks shift as the optical intensity is increased. We show that both of these measurements can be used to probe the ultrafast internal intraband electric field within the superlattice. We employ a nonperturbative formalism in an excitonic basis to model the dynamic response. This allows us to self-consistently include the self-generated internal intraband field that generates the spectral peak shifts.

Exciton delocalization length of merocyanine J-aggregates in Langmuir–Blodgett films studied from linear and nonlinear absorption measurements

A delocalization length and dimensionality of Frenkel excitons in 6-methyl merocyanine J-aggregates in Langmuir–Blodgett films have been investigated by means of linear and nonlinear absorption spectroscopy. The absorption spectrum of dye molecules in an LB film shows the energy shift due to the formation of J-aggregates with respect to the transition energy of monomers. From the differential absorption spectra measured by the femtosecond pump and probe experiment, we have obtained exciton energies of the lowest and second lowest states in J-aggregates. Using both spectroscopic data and a dipole model of molecular arrangement, we have determined the exciton delocalization size of two-dimensional J-aggregates (26(±1)×40(±9) molecules).

Low-temperature near-field nonlinear absorption spectroscopy of InGaAs single quantum dots

Nonlinear absorption spectroscopy of InGaAs single quantum dots (QDs) was realized by means of a low-temperature near-field optical microscope. The spatial distribution of the nonlinear absorption change in single QDs was determined. The dependence of the nonlinear absorption change on the pump power density demonstrates that the nonlinearity originates from the state filling of the ground state. The nonlinear absorption spectrum showed a homogeneous broadening of the ground state of about 5 meV. Furthermore, the change in the absorption cross section of the single QD when the ground state is saturated with carriers is estimated to be 2.8±0.6 nm2, which agrees with the result predicted on the basis of theoretical consideration.

Investigation of energy structure of lead silicate glasses by non-linear absorption spectroscopy technique

Regularities of color center formation in the lead silicate glasses under photoexcitation have been studied. It is shown for the first time that the long-wave boundary of carrier mobility of vitreous media can be placed considerably higher then the fundamental absorption edge of the material matrix. For this reason the detection of linear formation of color centers in the lead silicate glasses is not possible. Their coloration arises only from the multi-photon matrix ionization and the excitation occurs through virtual states that are located in the fundamental absorption region. The numerical estimates of the three-photon absorption coefficient and the forming color centers cross-section at 527 nm are obtained for the glasses with different content of lead oxide.

Mono‐ and bi‐excitonic states in j‐aggregates of pseudoisocyanine studied by nonlinear polarization and nonlinear absorption spectroscopy

The aggregation of pseudoisocyanine (PIC) is of interest because of the unusual properties of the aggretages which make them useful in photographic science as sensitizers as well as in photosynthesis research as model systems. Here, the room‐temperature substructure of the J‐absorption band of PIC–Cl aggregates is discussed on the basis of the results from nonlinear polarization spectroscopy in the frequency domain. The results are compared with the known substructure at 4.2 K.

Linear and nonlinear spectroscopy of highly oriented thin films of α-sexithienyl: a model for polythiophene

α-sexithienyl (T 6) is a large oligomer of polythiophene with twelve conjugated double bonds forming highly ordered thin films. The chemical definition and the ordered solid state aggregation allows to investigate the energetics of the molecule as a model for polythiophene. Linear spectroscopy including single crystal and ordered thin film absorptiion and one-photon excitation allows to locate and to assign the lowest Frenkel 1 1B u exciton at 17450 cm −1. Nonlinear two-photon absorption spectroscopy on the film allows to locate the lowest 2 1AA g Frenkel exciton at about 900 cm −1 above the 1 1B u e-e correlation is therefore crucial in determining the energetics of T 6. The level crossing of the 1 1B u and 2 1A g states provides an efficient radiationlless relaxation pathway. We suggest that the energitics of T 6 provides a good model for the understanding of polythiophene.

Excited-state properties of 20-chloro-chlorophyll a

The excited-state and lasing properties of 20-chloro-chlorophyll a in ether solution were compared to those of chlorophyll a. Desactivation parameters and cross-sections were obtained from non-linear absorption spectroscopy in combination with a physico-mathematical methods package. The Cl substituent at C-20 (1) increases both intersystem crossing and internal conversion, (2) produces a blue-shift of the S 1 absorption spectrum, and (3) leads to pronounced photochemistry.

Nonlinear-absorption spectroscopy and materials technology of crystals

Nonlinear-absorption spectroscopy and materials technology of crystals

High-power pulsed dye laser with Fourier-limited bandwidth

A high-peak-power, narrow-linewidth light source with a homogeneous beam profile has been constructed by modifications to a commercially available pulsed-dye-laser system. Output pulses of up to 10 mJ were generated with linewidths of about 50 MHz for 12-nsec pulses. The pulse-to-pulse frequency stability was better than the linewidth, and the center frequency could be scanned over a frequency range of 142.5 GHz at a wavelength of 600 nm. The performance of the system was demonstrated by observing the 6s2 1S0–6s7s1S0 transition in atomic mercury at 2λ = 312.8 nm and the 6s2S1/2–8s2S1/2 transition in atomic gold at 2λ = 308.9 nm using up to 1 mJ of frequency-doubled output for two-photon nonlinear absorption spectroscopy. The lifetimes of both excited states, the magnetic hyperfine splitting of the 8s2S1/2 state in gold, and the isotope shift of the 6s7s1S0 state of mercury were determined.

Investigation of gases by the method of nonlinear absorption spectroscopy

An investigation is made of the possibility of using nonlinear absorption spectroscopy in the analysis of gas mixtures. This method enables accurate concentration measurements to be made in those spectral regions where normal spectroscopic methods are inaccurate because of the overlapping of the neighboring absorption lines. It was used to study hydrocarbons in the Zeeman tuning range of a He–Ne laser with λ = 3.39 μ. The promise shown by this method in studies of isotopic mixtures is remarked upon.

Nonlinear absorption spectroscopy of the ethane molecule

By means of nonlinear absorption spectroscopy data necessary for the interpretation of molecular spectra can be obtained in such spectral regions, where conventional spectroscopic methods yield only a quasi-continuous absorption curve. Using a Zeeman-tuned He−Ne laser at the wavelength of 3.39μm we have measured significant differences in the intensity and temperature behaviour for several ethane absorption lines, the analysis of which made possible a preliminary qualitative assignment of these lines.