Two-photon Excitation Spectroscopy Research Articles

Two-photon fluorescence excitation spectroscopy by pulse shaping ultrabroad-bandwidth femtosecond laser pulses

A fast and automated approach to measuring two-photon fluorescence excitation (TPE) spectra of fluorophores with high resolution (~2 nm) by pulse shaping ultrabroad-bandwidth femtosecond laser pulses is demonstrated. Selective excitation in the range of 675-990 nm was achieved by imposing a series of specially designed phase and amplitude masks on the excitation pulses using a pulse shaper. The method eliminates the need for laser tuning and is, thus, suitable for non-laser-expert use. The TPE spectrum of Fluorescein was compared with independent measurements and the spectra of the pH-sensitive dye 8-hydroxypyrene-1,3,6-trisulfonic acid (HPTS) in acidic and basic environments were measured for the first time using this approach.

Ligand-Induced Anisotropy of the Two-Photon Luminescence of Spherical Gold Particles in Solution Unraveled at the Single Particle Level

Here we report on the visible luminescence properties of individual spherical gold particles in solution, obtained by two-photon excited fluorescence correlation spectroscopy and by an original dual Rayleigh-fluorescence method, correlating the Rayleigh scattering and the luminescence fluctuations of the same particle. The results demonstrate that the power needed to observe the two-photon excited visible luminescence depends on the illuminated particle and that the corresponding emission is anisotropic at low power. These observations combined with the evolution of the dynamics of the luminescence with respect to excitation power are interpreted by the presence of unique emissive surface states that are randomly switched off and on by the heat-induced movement of the molecular coating. These characteristics, which remain hidden in macroscopic experiments, have important implications with respect to the potential use of the particles as labels in two-photon imaging in aqueous samples.

Extended Förster theory: a quantitative approach to the determination of inter-chromophore distances in biomacromolecules

This review highlights recent theoretical and experimental advances in the study of biomacromolecular structure by using electronic transfer. The considered electronic transport in the extended Förster theory occurs within donor-acceptor pairs, donor-donor pairs, as well as within regular arrangements of many donors which may undergo reorienting and translational dynamics. The classical and the extended Förster theory are compared. Applications concern the determination of structural properties of proteins and non-covalent protein polymers. Studies of energy migration by means of two-photon excited fluorescence spectroscopy, as well as the relevant extension of the Förster theory are presented.

Two-Photon Excitation Spectroscopy of Carotenoid-Containing and Carotenoid-Depleted LH2 Complexes from Purple Bacteria

We applied two-photon fluorescence excitation spectroscopy to LH2 complex from purple bacteria Allochromatium minutissimum and Rhodobacter sphaeroides . Bacteriochlorophyll fluorescence was measured under two-photon excitation of the samples within the 1200-1500 nm region. Spectra were obtained for both carotenoid-containing and -depleted complexes of each bacterium to allow their direct comparison. The depletion of carotenoids did not alter the two-photon excitation spectra of either bacteria. The spectra featured a wide excitation band around 1350 nm (2x675 nm, 14,800 cm(-1)) which strongly resembled two-photon fluorescence excitation spectra of similar complexes published by other authors. We consider obtained experimental data to be evidence of direct two-photon excitation of bacteriochlorophyll excitonic states in this spectral region.

Two-photon fluorescence correlation spectroscopy through a dual-clad optical fiber

We report on the utilization of a dual-clad optical fiber for two-photon excited fluorescence correlation spectroscopy (FCS). High excitation efficiency is obtained by sending prechirped ultrafast pulses through the single-mode core of the fiber, while the fluorescence collection efficiency is enhanced because of the larger numerical aperture of the inner cladding. We show that the utilization of a dual-clad fiber is critical for ultrasensitive two-photon fluorescence detection. Our system has the ability to detect fluorescent nanospheres as small as 12 nm in radius. Quantum dots of radius 7 nm are also measured and show excellent signal to noise ratio. The particle sizes obtained from the fiber FCS system were confirmed by measurements using a commercial dynamic light scattering (DLS) system.

Femtosecond Spectroscopy of Native and Carotenoidless Purple-Bacterial LH2 Clarifies Functions of Carotenoids

EET between the two circular bacteriochlorophyll compartments B800 and B850 in native (containing the carotenoid rhodopin) and carotenoidless LH2 isolated from the photosynthetic purple sulfur bacterium Allochromatium minutissimum was investigated by femtosecond time-resolved transient absorption spectroscopy. Both samples were excited with 120-fs laser pulses at 800nm, and spectral evolution was followed in the 720–955nm range at different delay times. No dependence of transient absorption in the B800 band on the presence of the carotenoid rhodopin was found. Together with the likewise virtually unchanged absorption spectra in the bacteriochlorophyll Qy region, these observations suggest that absence of rhodopin does not significantly alter the structure of the pigment-protein complex including interactions between bacteriochlorophylls. Apparently, rhodopin does also not accelerate B800 to B850 EET in LH2, contrary to what has been suggested previously. Moreover, “carotenoid-catalyzed internal conversion” can also be excluded for the bacteriochlorophylls in LH2 of A. minutissimum. Together with previous results obtained with two-photon fluorescence excitation spectroscopy, it can also be concluded that there is neither EET from rhodopin to B800 nor (back-)EET from B800 to rhodopin.

Uncovering Forbidden Optical Transitions in PbSe Nanocrystals

The 1S(h,e)-1P(e,h) exciton transition energy of PbSe nanocrystals was determined via two-photon photoluminescence excitation spectroscopy and was found to be in good agreement with predictions from a tight-binding calculation. The two-photon excitation peak occurs at energies very close to a strong feature in the one-photon absorption spectrum and suggests that it should be assigned as a formally forbidden S-P transition. Leading explanations for the unusual strength of the forbidden transition are discussed.

Two-photon excitation spectroscopy of coupled asymmetric GaN/AlGaN quantumdiscs

By using two-photon excitation spectroscopy we take advantage of the increased spatialresolution to perform time-integrated and time-resolved photoluminescence measurementson several discrete stacks of GaN quantum discs. The stack structure consisted of coupledasymmetric GaN quantum discs with embedded AlGaN barriers, which were grown at thetip of a GaN nanocolumn. We observed that with increasing optical excitation power thecarrier lifetime decreased due to free-carrier screening, with an enhancement of thescreening effect in the larger quantum disc due to carrier tunnelling from the smallerquantum disc.

Oxygen-Related Band Gap State in Single Crystal Rubrene

A molecular exciton signature is established and investigated under different ambient conditions in rubrene single crystals. An oxygen-related band gap state is found to form in the ambient atmosphere. This state acts as an acceptor center and assists in the fast dissociation of excitons, resulting in a higher dark and photoconductivity of oxidized rubrene. The band gap state produces a well-defined photoluminescence band at an energy 0.25 eV below the energy of the 0-0 molecular exciton transition. Two-photon excitation spectroscopy shows that the states are concentrated near the surface of naturally oxidized rubrene.

Energy levels and hypersensitivity of samarium(III) in the elpasoliteCs2NaSmCl6

Electronic absorption spectra ofneat Cs2NaSmCl6 between 300 and 10 K are reported for the spectral region between 900 and40 000 cm−1. Interpretation of the intricate vibronic structure enables 91 conclusiveand 45 tentative Kramers doublet and quartet energy levels of the4f5 ionSm3+ in this system to be assigned. Configuration interaction assisted crystalfield calculations employing the diagonalization of the combined4f5 and 4f46p1 matrices fit the energy levels better than the conventional4f5 crystal fieldcalculation. The 4f5 calculation is more accurate, however, than for thePr3+ and Nd3+ elpasolite systems, showing the greater importance of intra- rather thaninter-configuration interaction for the more complex electronic structure ofSm3+. The experimental dataset generally shows agreement with that from two-photon excitationspectroscopy and the differences are highlighted. The intensity of structure at and near thezero phonon line of the hypersensitive infrared absorption transition increases relative to the intensity of the vibronic sideband whenz isincreased in Cs2NaSm(Cl1−zBrz)6. This is attributed to the introduction of electric dipole transition intensityinto the pure electronic transition by lowering the site-symmetry ofSmCl63−, or by thepresence of SmCl5Br3−.

Surface-Enhanced Raman Scattering in Local Optical Fields of Silver and Gold NanoaggregatesFrom Single-Molecule Raman Spectroscopy to Ultrasensitive Probing in Live Cells

This Account discusses surface-enhanced Raman scattering at extremely high enhancement levels that can occur for molecules attached to silver and gold nanoclusters. Strongly enhanced and highly confined local optical fields enable surface-enhanced Stokes and anti-Stokes Raman spectroscopy of single molecules even under nonresonant excitation conditions as well as extremely large effective cross sections in two-photon excited Raman spectroscopy. The ability for very sensitive and spatially confined molecular structural probing makes gold and silver nanoclusters very promising tools for studies of small structures in biological materials, such as cellular compartments.

Doppler-Free Two-Photon Excitation Spectroscopy and the Zeeman Effect in the “Channel Three” Region of C6H6

Abstract Doppler-free two-photon excitation spectra and the Zeeman effects for the 1021401 and 1011401 bands, whose vibrational excess energies are 3412 and 2492 cm−1, respectively, of the S11B2u ← S01A1g transition in gaseous benzene-h6 have been measured. Rotationally resolved lines up to a rotational excess energy of 945 cm−1 in the 1011401 band, whose excess energy is isoenergetic with the one of the 1021401 band at low J, have been observed. The density of perturbation is observed to increase around the K = 0 and K = J levels in the 1011401 band as the rotational energy increases, but line broadening is not observed. It was reported that only the K = 0 lines at low J and the K = J lines at high J were observed as a sharp line and the rest was washed out by broadening in the 1021401 band. The assignments are confirmed by the Zeeman spectra. The character and magnitude of Zeeman splittings in both bands could be understood as originating from the electronic orbital angular momentum arising from a mixing of the S11B2u and S21B1u states via J–L coupling. The levels of the S11B2u state are found not to be mixed with a triplet state by the Zeeman effect.

Exciton binding energies in carbon nanotubes from two-photon photoluminescence

One- and two-photon luminescence excitation spectroscopy showed a series of distinct excitonic states in single-walled carbon nanotubes. The energy splitting between one- and two-photon-active exciton states of different wavefunction symmetry is the fingerprint of excitonic interactions in carbon nanotubes. We determine exciton binding energies of 0.3-0.4 eV for different nanotubes with diameters between 0.7 and 0.9 nm. Our results, which are supported by ab-initio calculations of the linear and non-linear optical spectra, prove that the elementary optical excitations of carbon nanotubes are strongly Coulomb-correlated, quasi-one dimensionally confined electron-hole pairs, stable even at room temperature. This alters our microscopic understanding of both the electronic structure and the Coulomb interactions in carbon nanotubes, and has direct impact on the optical and transport properties of novel nanotube devices.

Structural Dependence of Excitonic Optical Transitions and Band-Gap Energies in Carbon Nanotubes

The optical transitions of semiconducting carbon nanotubes have been ascribed to excitons. Here we use two-photon excitation spectroscopy to measure exciton binding energies, as well as band-gap energies, in a range of individual species of semiconducting SWNTs. Exciton binding energies are large and vary inversely with nanotube diameter, as predicted by theory. Band-gap energies are significantly blue-shifted from values predicted by tight-binding calculations.

The Optical Resonances in Carbon Nanotubes Arise from Excitons

Optical transitions in carbon nanotubes are of central importance for nanotube characterization. They also provide insight into the nature of excited states in these one-dimensional systems. Recent work suggests that light absorption produces strongly correlated electron-hole states in the form of excitons. However, it has been difficult to rule out a simpler model in which resonances arise from the van Hove singularities associated with the one-dimensional band [corrected] structure of the nanotubes. Here, two-photon excitation spectroscopy bolsters the exciton picture. We found binding energies of approximately 400 millielectron volts for semiconducting single-walled nanotubes with 0.8-nanometer diameters. The results demonstrate the dominant role of many-body interactions in the excited-state properties of one-dimensional systems.

Doppler-free two-photon excitation spectroscopy and the Zeeman effects of the S1B1u1(v21=1)←SAg1(v=) band of naphthalene-d8

Doppler-free two-photon excitation spectrum and the Zeeman effect of the S1 1B1u(v21=1) <-- S0 1Ag(v=0) transition of naphthalene-d8 have been measured. 908 lines of Q(Ka)Q(J)KaKc transition of J=0-41, Ka=0-20 were assigned, and the molecular constants of the S1 1B1u(v21=1) state were determined. Perturbations were observed, and those were identified as originating from Coriolis interaction. No perturbation originating from an interaction with triplet state was observed. The Zeeman splittings from lines of a given J were observed to increase with Kc, and those of the Kc=J levels increased linearly with J. The Zeeman effects are shown to be originating from the magnetic moment of the S1 1B1u state, which is along the c axis and is induced by mixing of the S2 1B3u state to the S1 1B1u state by J-L coupling. Rotationally resolved levels were found not to be mixed with a triplet state from the Zeeman spectra. Accordingly, it is concluded that nonradiative decay of an isolated naphthalene excited to low rovibronic levels in the S1 1B1u state does not occur through the intersystem mixing. This is at variance with generally accepted understanding of the pathways of the nonradiative decay.

Experimental study of the K2392Πg3 state by perturbation facilitated infrared-infrared double resonance and two-photon excitation spectroscopy

The 39K2 2 3Pi(g) state has been observed by perturbation facilitated infrared-infrared double resonance and two-photon excitations. The vibrational numbering of the 2 3Pi(g) levels was determined by resolved fluorescence into the bound levels as well as to the continuum of the a 3Sigma(u)+ state. The rotational assignment of the 2 3Pi(g) levels excited by two-photon transitions was determined from excitation frequencies and resolved fluorescence into the bound levels of the a 3Sigma(u) + and b 3Pi(u) states. Molecular constants obtained from these observed levels agree with theoretical constants.

Two-photon excitation of confined biexcitons in CuCl quantum dots

We have studied direct creation processes of confined biexcitons in CuCl quantum dots by polarization-dependent resonant two-photon excitation spectroscopy. The two-photon absorption band for the lowest state of the biexciton (total angular momentum J = 0 ) which appears on the lower energy side of confined exciton band was identified from the analysis of the polarization dependence of the photoluminescence excitation spectrum of the biexciton. Furthermore, the two-photon excitation process for the excited state of the biexciton ( J = 2 ) was also found with polarization dependence different from the J = 0 biexciton state.

Optical properties of epitaxially grown submonolayerCdSe∕ZnSenanostructures

The optical properties of epitaxially grown $\mathrm{Cd}\mathrm{Se}∕\mathrm{Zn}\mathrm{Se}$ nanostructures with nominal CdSe coverage between 0.15 monolayer (ML) and 0.58 ML are investigated by photoluminescence (PL) and PL excitation spectroscopy at low and high excitation density, time-resolved PL, temperature dependent linewidth analysis, and two-photon excitation spectroscopy. We discuss exciton-phonon complexes in a potential and analyze their dominant dimensionality. The exciton-polaron picture, which is mostly applied to the extreme case of very strong exciton-phonon coupling, is introduced here as a well-suited description of the optical properties of ultra-small semiconductor nanostructures with relaxed quantum confinement.

Doppler-free two-photon excitation spectroscopy and the Zeeman effect of the 141 band of the S1 1B2u←S 1A1g transition of benzene-d6

The Doppler-free two-photon excitation spectrum and the Zeeman effect of the 1401 band of the S1←S0 transition of C6D6 were measured from 39 842 to 39 856 cm−1. The Zeeman splittings of the Q(J)Q(K) lines of a given J were observed to increase proportionally with K2 and reach a maximum at K=J. The Zeeman splittings of the Q(J)Q(K=J) lines were observed to increase proportionally with J. The observed Zeeman splittings are identified as originating from the electronic orbital angular momentum arising from a mixing of the S1 1B2u and S2 1B1u states via J-L coupling. No perturbation originating from an interaction with a triplet state was observed. It became clear from the Zeeman effect that rotationally resolved levels of the S1 state are not mixed with a triplet state. Therefore, it is concluded that nonradiative decay of an isolated benzene molecule excited to the S1 state occurs through the intramolecular vibrational-rotational mixing within the S1 state even in the low vibrational levels.