Sequential Two-photon Excitation Research Articles

Multipartite entanglement encoded in the photon-number basis by sequential excitation of a three-level system.

We propose a general scheme to generate entanglement encoded in the photon-number basis, via a sequential resonant two-photon excitation of a three-level system. We apply it to the specific case of a quantum dot three-level system, which can emit a photon pair through a biexciton-exciton cascade. The state generated in our scheme constitutes a tool for secure communication, as the multipartite correlations present in the produced state may provide an enhanced rate of secret communication with respect to a perfect GHZ state.

Sequential Two-Photon Delayed Fluorescence Anisotropy for Macromolecular Size Determination

Time-resolved fluorescence anisotropy (FA) uses the fluorophore depolarization rate to report on rotational diffusion, conformation changes, and intermolecular interactions in solution. Although FA is a rapid, sensitive, and nondestructive tool for biomolecular interaction studies, the short (∼ns) fluorescence lifetime of typical dyes largely prevents the application of FA on larger macromolecular species and complexes. By using triplet shelving and recovery of optical excitation, we introduce optically activated delayed fluorescence anisotropy (OADFA) measurements using sequential two-photon excitation, effectively stretching fluorescence anisotropy measurement times from the nanosecond scale to hundreds of microseconds. We demonstrate this scheme for measuring slow depolarization processes of large macromolecular complexes, derive a quantitative rate model, and perform Monte Carlo simulations to describe the depolarization process of OADFA at the molecular level. This setup has great potential to enable future biomacromolecular and colloidal studies.

Excited-state resonance Raman spectroscopy probes the sequential two-photon excitation mechanism of a photochromic molecular switch.

Some diarylethene molecular switches have a low quantum yield for cycloreversion when excited by a single photon, but react more efficiently following sequential two-photon excitation. The increase in reaction efficiency depends on both the relative time delay and the wavelength of the second photon. This paper examines the wavelength-dependent mechanism for sequential excitation using excited-state resonance Raman spectroscopy to probe the ultrafast (sub-30fs) dynamics on the upper electronic state following secondary excitation. The approach uses femtosecond stimulated Raman scattering (FSRS) to measure the time-gated, excited-state resonance Raman spectrum in resonance with two different excited-state absorption bands. The relative intensities of the Raman bands reveal the initial dynamics in the higher-lying states, Sn, by providing information on the relative gradients of the potential energy surfaces that are accessed via secondary excitation. The excited-state resonance Raman spectra reveal specific modes that become enhanced depending on the Raman excitation wavelength, 750 or 400nm. Many of the modes that become enhanced in the 750nm FSRS spectrum are assigned as vibrational motions localized on the central cyclohexadiene ring. Many of the modes that become enhanced in the 400nm FSRS spectrum are assigned as motions along the conjugated backbone and peripheral phenyl rings. These observations are consistent with earlier measurements that showed higher efficiency following secondary excitation into the lower excited-state absorption band and illustrate a powerful new way to probe the ultrafast dynamics of higher-lying excited states immediately following sequential two-photon excitation.

Two-photon dissociation dynamics of the mercapto radical.

Two-photon dissociation dynamics of the SH/SD radicals are investigated using the high-n Rydberg atom time-of-flight (HRTOF) technique. The H/D(2S) + S(1D) and H/D(2S) + S(1S) products are observed in the dissociation of the SH/SD radicals on the 22Π and B2Σ+ repulsive states, from sequential two-photon excitation via the A2Σ+ (v' = 0, J' = 0.5-2.5) state. The angular distributions of both H/D(2S) + S(1D) and H/D(2S) + S(1S) product channels are anisotropic. The anisotropy parameter (β) of the H(2S) + S(1D) products is ∼-0.8 ± 0.1 (-0.9 ± 0.05 for SD), and that of the H(2S) + S(1S) products is ∼1.3 ± 0.3 (1.2 for SD). The anisotropic angular distributions indicate that the SH/SD radicals promptly dissociate on the repulsive 22Π and B2Σ+ potential energy curves (PECs) along with some non-adiabatic crossings, leading to the H/D(2S) + S(1D) and H/D(2S) + S(1S) products, respectively. The bond dissociation energy of the ground-state X2Π3/2 SH/SD to the ground-state H/D(2S) + S(3P2) products is measured to be D0(S-H) = 29 253 ± 20 cm-1 and D0(S-D) = 29 650 ± 20 cm-1, respectively. The dissociation energy of the A2Σ+ state SH/SD to the H/D(2S) + S(1D) products is derived to be D0[S-H(A)] = 7659 ± 20 cm-1 and D0[S-D(A)] = 7940 ± 20 cm-1.

Two-photon dissociation dynamics of hydroxyl radical

Two-photon dissociation dynamics of the OH radical is studied using the high-n Rydberg atom time-of-flight (HRTOF) technique. The H(2S)+O(1D) and H(2S)+O(1S) product channels are observed in the dissociation of the OH radical on the 22Π and B2Σ+ repulsive states, respectively, from sequential two-photon excitation via the A2Σ+ (v′=2, J′=0.5−2.5) state. Both H+O product channels have anisotropic angular distributions, with β=−0.97 for H(2S)+O(1D) and 1.97 for H(2S)+O(1S). The anisotropic angular distributions are consistent with a mechanism of OH direct dissociation on the repulsive potential energy curves (PECs) leading to the H+O products. The OH bond dissociation energy D0(O−H) is determined to be 35580±15 cm−1.

Hot Electrons Generated from Doped Quantum Dots via Upconversion of Excitons to Hot Charge Carriers for Enhanced Photocatalysis.

We show that hot electrons exhibiting the enhanced photocatalytic activity in H2 production reaction can be efficiently generated in Mn-doped quantum dots via the "upconversion" of the energy of two excitons into the hot charge carriers. The sequential two-photon-induced process with the long-lived Mn excited state serving as the intermediate state is considered as the pathway generating hot electrons. H2 production rate from doped quantum dots is significantly higher than that from undoped quantum dots and also exhibited the quadratic increase with the light intensity, demonstrating the effectiveness of the hot electrons produced in doped quantum dots in photocatalytic reaction. Due to the very long lifetime of Mn excited state (∼6 ms) in the doped quantum dots, the sequential two-photon excitation requires relatively low excitation rates readily achievable with a moderately concentrated solar radiation, demonstrating their potential as an efficient source of hot electrons operating at low excitation intensities.

Deployment of a sequential two-photon laser-induced fluorescence sensor for the detection of gaseous elemental mercury at ambient levels: fast, specific, ultrasensitive detection with parts-per-quadrillion sensitivity

Abstract. The operation of a laser-based sensor for gas-phase elemental mercury, Hg(0), is described. It utilizes sequential two-photon laser excitation with detection of blue-shifted laser-induced fluorescence (LIF) to provide a highly specific detection scheme that precludes detection of anything other than atomic mercury. It has high sensitivity, fast temporal resolution, and can be deployed for in situ measurements in the open atmosphere with essentially no perturbation of the environment. An ambient sample can also be pulled through a fluorescence cell, allowing for standard addition calibrations of the concentration. No type of preconcentration is required and there appears to be no significant interferences from other atmospheric constituents, including gas-phase oxidized mercury species. As a consequence, it is not necessary to remove oxidized mercury, commonly referred to as reactive gaseous mercury (RGM), from the air sample. The instrument has been deployed as part of an instrument intercomparison and compares well with conventional instrumentation that utilizes preconcentration on gold followed by analysis using cold-vapor atomic fluorescence spectroscopy (CVAFS). Currently, the achievable detection sensitivity is ~ 15 pg m−3 (~ 5 × 104 atoms cm−3, ~ 2 ppq) at a sampling rate of 0.1 Hz, i.e., averaging 100 shots with a 10 Hz laser system. Preliminary results are described for a 50 Hz instrument that utilizes a modified excitation sequence and has monitored ambient elemental mercury with an effective sampling rate of 10 Hz. Additional work is required to produce the precision necessary to perform eddy correlation measurements. Addition of a pyrolysis channel should allow for the measurement of total gaseous mercury (TGM) and hence RGM (by difference) with good sensitivity and time resolution.

Cycloreversion dynamics of a photochromic molecular switch via one-photon and sequential two-photon excitation.

Ultrafast pump-probe (PP) and pump-repump-probe (PReP) measurements examine the ring-opening reaction of a photochromic molecular switch following excitation to the first and higher excited states. Sequential two-photon excitation is a sensitive probe of the excited-state dynamics, because the secondary excitation maps the progress along the S1 reaction coordinate onto the higher excited states of the molecule. In this contribution, secondary excitation at 800 nm accesses more reactive regions of the excited-state potential energy surfaces than are accessible with direct vertical excitation in the visible or UV. The quantum yield for cycloreversion increases by a factor of 3.5 ± 0.9 compared with one-photon excitation when the delay between the 500 nm pump and 800 nm repump laser pulses increases beyond ~100 fs, in contrast with a slower ~3 ps increase that was previously observed for one-color sequential excitation at 500 nm. The evolution of an excited-state absorption band reveals the dynamics of the higher-lying excited state for both one-photon excitation in the UV and sequential two-photon excitation. The spectroscopy and dynamics of the higher-lying excited state are similar for both excitation pathways, including a lifetime of ~100 fs. The complementary PP and PReP measurements provide a detailed picture of the ultrafast excited-state dynamics, including new insight on the role of excited states above S1 in controlling the photochemical cycloreversion reaction.

Controlling the Excited-State Reaction Dynamics of a Photochromic Molecular Switch with Sequential Two-Photon Excitation.

Sequential two-photon excitation increases the cycloreversion yield of a diarylethene-type photochromic molecular switch compared with one-photon excitation. This letter shows for the first time that an optimal delay of ∼5 ps between primary and secondary excitation events gives the largest enhancement of the ring-closing reaction. Pump-probe (PP) and pump-repump-probe (PReP) measurements also provide detailed new information about the excited-state dynamics. The initially excited molecule must first cross a barrier on the excited-state potential energy surface before secondary excitation enhances the reaction. The PReP experiments demonstrate that the reaction path of a photochromic molecular switch can be selectively controlled through judicious use of time-delayed femtosecond laser pulses.

Spatially Modulated Two-Photon Luminescence from Si−Au Core−Shell Nanowires

Si−Au core−shell nanowires were synthesized with a two-step strategy combining chemical vapor deposition and wet chemistry methods. Si core diameter, Au shell thickness, and wire length were controlled during synthesis. Under a far-field two-photon excitation scanning microscope, spatially modulated two-photon luminescence (TPL) was observed from the core−shell nanowires prepared with several micrometers in length. The mode spacing scales linearly with the length of nanowires for samples with 15 and 40 nm Au shells. The spatial modulation pattern was found to be independent of the excitation wavelength. The TPL intensity displayed a cos2 dependence on the excitation field polarization, indicating a sequential two-photon excitation process enhanced by the longitudinal plasmon field. The spatially modulated TPL is interpreted by a standing wave model of surface plasmon polaritons along the core−shell nanowire which acts as a Fabry−Perot resonator. These results suggest TPL imaging of Si−Au core−shell nanowi...

The photophysics of metalloporphyrins excited in their Soret and higher energy UV absorption bands

Photophysical processes involving the higher electronic excited states of diamagnetic porphyrins and metalloporphyrins are critically reviewed. Intramolecular electronic relaxation of one-photon Soret-excited molecules in solution is now known to involve processes other than S 2 - S 1 internal conversion; dark electronic states are implicated. Sequential two-photon excitation to produce gerade excited singlet states ( S n , n > 2) results in relaxation dynamics that are quantitatively different from those resulting from one-photon excitation to ungerade states of about the same energy. Intermolecular electron and electronic energy transfer involving Soret-excited metalloporphyrins and intramolecular electron and electronic energy transfer in Soret-excited dyads and larger arrays containing porphyrins are reviewed. Metalloporphyrins containing main group metals or transition metals with filled d orbitals exhibit relaxation dynamics that differ from metalloporphyrins containing transition metals with unfilled d orbitals. Non-linear phenomena associated with multi-photon excitation of diamagnetic metalloporphyrins are also reviewed.

Photophysical processes in electronic states of zinc tetraphenyl porphyrin accessed on one- and two-photon excitation in the soret region

Photophysical processes in the model metalloporphyrin, 5,10,15,20-tetraphenyl-21 H,23 H-porphine zinc (ZnTPP), have been investigated by means of conventional electronic spectroscopy, and by picosecond transient emission and femtosecond transient absorption methods. The radiative and radiationless decay parameters of ZnTPP have been determined under conditions of low solute concentration where dimer formation is unimportant, and the effects of dimer formation at higher concentrations have been assessed. Careful measurements of the relative S 1–S 0 fluorescence quantum yields produced on excitation to higher states compared with direct excitation to S 1 itself reveal that a second radiationless decay process that bypasses S 1 operates when ZnTPP is excited in the Soret region. A dark state, assigned most probably to a triplet ( T n , n > 2) or an upper singlet of gerade parity, is involved. The relative importance of this second process is a function of the nature of the solvent, the excitation wavelength (vibrational energy content of the excited state) and the parity of higher electronic states accessed when two-photon excitation is employed. Sequential two-photon excitation occurs when exciting with fs pulses in the Soret region and becomes significant at even modest excitation pulse energies with increasing sample absorbance. The implications of these measurements in reinterpreting the dynamics of electronically excited ZnTPP are discussed.

Blue luminescence in Tm 3+-doped KGd(WO 4) 2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO 4) 2 single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO 4) 2 single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the 3F 2+ 3F 3 energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the 3H 6 ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.

Effective schemes of samarium atom three-photon ionization

A number of frequencies have been found when studying the Sm atom three-photon ionization, being characterized by extremely high efficiency of Sm + ion production. All of them correspond to the sequential two-photon excitation of high-lying even states via the lower odd states. Such transitions could be used for effective ionization of samarium atoms when dealing with different applied problems.

Alkyl-substituted magnesium phthalocyanine: phototoxicity after excitation of higher electronic states in cellsin vitro

Photodynamic tumour therapy (PDT) on the basis of a sequential two-photon excitation of suitable sensitizers is expected 1) to prevent skin phototoxicity caused by day-light and 2) may occur via an oxygen-independent mechanism of photosensitization. Here we investigated cellular uptake, localization and phototoxicity of ( t-butyl )4- PcMg , a promising dye for a sequential two-step activation, in Jurkat cells (human T cell line) and murine hybridoma cells (B cells) in vitro. Pheophorbide a (pheo) was used as a classical Type II reference sensitizer. Excitation of higher singlet states was performed both by direct UV-excitation and by sequential two-step laser excitation in the Q-band with maximum photon flux densities of about 1026phot.cm−2.s−1. Irradiation of ( t-butyl )4- PcMg in cells in vitro is accompanied by a rapid destruction of the dye, as was proved by confocal fluorescence microscopy. Irradiated cells exhibit a less pronounced decrease of viability and a transient cell cycle arrest of about 24 h. Pheo incubated cells are characterized by a complete stop of proliferation. In contrast, no long-lasting phototoxicity was observed with ( t-butyl )4- PcMg . Low fluorescence levels and rapid photobleaching prevented the identification of intracellular dye-localization. Our data indicate the presence of a radical mechanism as origin of an irreversible dye destruction that accounts for the low phototoxicity of ( t-butyl )4- PcMg .

Up-conversion mechanisms in Pr 3+-doped Bi 4Ge 3O 12 crystal

We investigated the up-conversion mechanisms leading to RGB emission in a Pr 3+ : Bi 4Ge 3O 12(BGO) crystal excited to the 1 D 2 level. Sequential two-photon excitation process (STEP) was found to be the dominant mechanism to populate the fluorescing 3 P 0 and 3 P 1 levels, also with a minor contribution of energy transfer up-conversion (ETU). Furthermore, the initial state of the excited state absorption (ESA) in the STEP was proven to be the 3 H 6 level with a dual-wavelengths excitation experiment. This excitation scheme provides significant enhancement of fluorescence intensity compared with the single-wavelength excitation.

Resonant Raman, Hot, and Cold Luminescence of Iodine in Rare Gas Matrixes

Spontaneous light emission (SLE) spectra induced by continuous wave excitation of iodine in rare gas matrixes is analyzed from the perspective of semiclassical molecular dynamics. The three-time correlation functions that describe the process are interpreted in terms of time circuits, where the reversibility of mechanics along specified paths establishes the distinction between resonant Raman (RR) and fluorescence. The observed hot luminescence (fluorescence from the vibrationally over-damped period of dynamics) is simulated, and the underlying dynamics analyzed to describe vibrational dephasing, formation of stationary states, and predissociation of I2(B) in matrixes. The extracted predissociation time of 3−5 ps in matrixes agrees with time-domain measurements. The analysis allows the interpretation of liquid phase spectra which contain both structured and unstructured SLE as strictly RR, suggesting that the unstructured component is due to linear complexes. The spectra also contain vibrationally relaxed emissions from the cage-bound, doubly spin−orbit excited I*I* states, which are prepared via sequential two-photon excitation with an intermediate state lifetime of 2 ms, assigned to A(3P1u). The two main emission bands, which shift by 300 cm-1 in proceeding from Ar to Kr to Xe, are assigned to I*I*(1u) → I2(a(1g)), and I*I*(0g+) → I2(B‘ ‘(1u)) transitions, and analyzed.

Upconversion fluorescence of Ho 3+ ions in a BaF 2 crystal

Efficient upconversion luminescence from the Ho 3+ C 3v centre in a BaF 2 crystal is reported. The red-to-green and red-to-blue upconversion is observed upon pumping with a single cw red dye laser. The upconversion mechanism is assigned as a sequential two-photon excitation process in which the excitation wavelength is resonant with either ground state absorption (GSA) or excited state absorption transitions. The efficient green upconversion with excitation into GSA results from a double-resonant excitation between the 5I 8→ 5F 5 and 5I 7→ 5F 3 transitions.

Room-temperature up-conversion fluorescence of rare-earth centers in CaF 2

We report temperature-dependent up-conversion fluorescence from Er 3+ and Tm 3+ C 4v centers in CaF 2, upon pumping with a single red-dye laser. The up-conversion mechanism is a sequential two-photon excitation process in which the laser is resonant with either a ground-state or an excited-state absorption transition.

Sequential two-photon excitation via repulsive intermediate states

Large bond extensions ≈ 1 Å are reported in a two-photon bound-to-bound optical excitation via a repulsive intermediate electronic state. In a one-colour experiment on I 2, the ion-pair E state is accessed in the sequence E(0 + g( v′≈40–70) ← B(0 + u) ← X with the pump/probe wavelength scanned near the prominent red extremum of the E←B transition. At this wavelength the continuum of the B state is reached in the first step, rather than the bound levels conventionally used in OODR spectroscopy. The classical point of transition that conserves atomic momentum for the second step then occurs near R e of the ion-pair state.