White-light Continuum Z-scan Technique Research Articles

Picosecond dynamic of aqueous sodium-copper chlorophyllin solution: An excited state absorption study

This work reports on the singlet excited-state absorption spectrum of sodium-copper chlorophyllin using three distinct methods of Z-Scan measurement: white-light continuum Z-Scan technique ranging from 470 to 780 nm, tunable Z-Scan technique with femtosecond pulses (490–700 nm) and 532 nm Z-Scan technique with 100 ps pulse width. A transition from reverse saturation absorption (RSA) to saturable absorption (SA) was observed at 570 nm, and at 532 nm RSA was tested showing potential application as optical limiting device. Pump-probe technique was used to measure lifetime of the first singlet excited state (22 ps) and no evidence of triplet conversion was observed.

Excited-state absorption of meso-tetrasulfonatophenyl porphyrin: Effects of pH and micelles

The influence of the environment on the excited state transitions of meso-tetrakis(p-sulfonatophenyl) porphyrin (TPPS) is reported. TPPS was investigated in protonated and non-protonated forms, and in the presence of the cationic cetyltrimethylammonium bromide (CTAB) micelles. The singlet excited-state absorption spectra were measured by using the white-light continuum Z-scan technique and the triplet–triplet absorption spectra were acquired employing an association of laser flash photolysis and Z-scan techniques. Our results show that the perseveration of the molecular symmetry, upon excitation, depends on the state of multiplicity of the molecules, as well as on the environment and structural characteristics of the porphyrin. Additionally, it was observed that for excited molecules, the ring distortion caused by the protonation of porphyrin ring has great influence on the changes observed for the symmetry and vibronic structure. The results clearly show that the porphyrin investigated is a promising candidate for optical limiting applications for all investigated environments.

Two-photon absorption of perylene derivatives: Interpreting the spectral structure

This work investigates the two-photon absorption spectrum of perylene tetracarboxylic derivatives using the white-light continuum Z-scan technique. Perylene derivatives present relatively high two-photon absorption cross-section, which makes them attractive for applications in photonics. Because of the spectral resolution of the white-light continuum Z-scan, we were able to observe a well defined structure in the two-photon absorption spectrum, composed by two distinct peaks. These peaks, as well as the resonant enhancement of the nonlinearity, were modeled using the sum-over-states approach considering a four-level energy diagram with two final two-photon states. The existence of such states was confirmed using the response function formalism within the DFT framework.

Excited state absorption in conjugated polymers: Photoinduced transparency

The understanding of excited states of conjugated polymers is of great interest for photonics, especially for fabricating electroluminescent and optical devices. This work concerns the excited state absorption of three conjugated polymers: poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene) (MEH-PPV), poly[2-[ethyl-[4-(4-nitro-phenylazo)-phenyl]-amino]-ethane-(3-thienyl)-ethanoate] (PAzT), and poly(3-octyl-thiophene) (POT). The nonlinear spectra from 450nm up to 700nm for the three polymers were obtained by means of the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion for the three polymers. In addition to this effect, we also observed a small excited state absorption for POT. The absorption from the first to a higher excited state is negligible for MEH-PPV and PAzT. In contrast, POT presented absorption for a higher excited state, which allowed the determination of its excited state absorption cross-section.

Excited state absorption spectrum of chlorophyll a obtained with white-light continuum

The study of excited state properties of chlorophyll a is a subject of foremost interest, given that it plays important roles in biological process and has also been proposed for applications in photonics. This work reports on the excited state absorption spectrum of chlorophyll a solution from 460 to 700 nm, obtained through the white-light continuum Z-scan technique. Saturation of absorption was observed due to the ground state depletion, induced by the white-light continuum region that is resonant with the Q band of chlorophyll a. The authors also observed reverse saturation of absorption related to the excitation from the first excited state to a higher energy level for wavelengths below 640 nm. An energy-level diagram, based on the electronic states of chlorophyll a, was employed to interpret their results, revealing that more states than the ones related to the Q and B bands participate in the excited state absorption of this molecule.

Two-photon absorption cross-section spectrum of a π-conjugated polymer obtained using the white-light continuum Z-scan technique

We have used the white-light continuum (WLC) Z-scan technique to determine the degenerate two-photon absorption (2PA) cross-section spectrum of poly(2-methoxy-5-(2′-ethylhexyloxy)-1,4-phenylenevinylene). The results obtained show good agreement with those employing a single-wavelength source, although much faster because of the wavelength multiplexing introduced by the use of a broadband source. The improved spectral resolution of the WLC Z-scan allowed the observation of a 2PA peak around 675nm, a subtle feature in the nonlinear spectrum that could be fitted using a sum-over-states model. The WLC Z-scan technique has proved to be an important tool to determine the 2PA spectrum of polymers.

Nonlinear absorption spectrum of ytterbium bis-phthalocyanine solution measured by white-light continuum Z-scan technique

In this work we have measured the nonlinear absorption spectrum of ytterbium bis-phthalocyanine (YbPc 2) in chloroform solution employing the Z-scan technique with white-light continuum (WLC) pulses. Two distinct excited state absorption behaviors were observed: a saturable absorption (SA) at the Q-band region and a reverse saturable absorption (RSA) around 530 nm. A three-energy-level diagram was used to explain the experimental results, leading to the excited state absorption cross-section determination from 500 up to 675 nm.

White-light continuum Z-scan technique for nonlinear materials characterization

We present a technique for rapid characterization of degenerate nonlinear absorption and refraction spectra using a femtosecond white-light continuum (WLC) pulse to perform Z-scans. The spectral components of the WLC source are temporally and spatially dispersed to minimize nondegenerate two-photon absorption (2PA) processes. We demonstrate the validity of the method by measuring the 2PA spectrum of a well-characterized semiconductor, ZnSe.