Subpicosecond Laser Excitation Research Articles

Generation of terahertz radiation by a surface ballistic photocurrent in semiconductors under subpicosecond laser excitation

An analytical model describing the onset of a surface ballistic photocurrent in cubic semiconductors under femtosecond laser excitation is proposed. It is shown that the contribution of the photocurrent component parallel to the surface to the generation of terahertz pulses may be comparable to the contribution of the perpendicular component. Consideration of the cubic symmetry of a semiconductor leads to the azimuthal anisotropy of terahertz generation.

Photoinduced charge shift as the driving force for the excited-state relaxation of analogues of the Photoactive Yellow Protein chromophore in solution

Transient absorption spectroscopy with subpicosecond laser excitation is used to probe the primary photoinduced processes in two ester analogues (linear and cyclic) of the Photoactive Yellow Protein (PYP) chromophore in solution. The PYP chromophore is the thioester derivative of the deprotonated trans-4-hydroxycinnamic acid. The results found for the ester analogues are compared to those previously obtained for the deprotonated trans-4-hydroxycinnamic acid and its amide and thioester derivatives. Special attention is paid to the role of the electron donor–acceptor character of the chromophore substituents and of the molecular flexibility on the excited-state relaxation pathway and kinetics. Solvent viscosity and polarity effects on the kinetics are also analyzed. Two hypothetical relaxation pathways involving a one-bond flip mechanism are proposed to explain the observation of a transient species in the course of the excited-state relaxation of the analogues bearing the stronger electron-acceptor substituents. In the first one, the intermediate is described as a perp ground state, whereas the second one involves a twisted excited state where the conformation of the ethylenic bond deviates from 90°. In both cases, the relaxation of the transient state may lead or not to the cis isomer.

Optical studies of high-field carrier transport of InN thick film grown on GaN

Transient Raman spectroscopy has been used to study electron transport in an InN film grown on GaN at T=300 K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2 × 1 0 8 cm / s , significantly larger than those observed for other III–V semiconductors such as GaAs and InP. These experimental results suggest that InN is potentially an excellent material for ultrafast electronic devices.

Observation of large electron drift velocities in InN by ultrafast Raman spectroscopy

Electron transport in an InN film grown on GaN has been studied by transient Raman spectroscopy at T=300K. Our experimental results demonstrate that under the subpicosecond laser excitation and probing, electron drift velocity of carriers in the Γ valley can exceed its steady-state value by as much as 40%. Electron velocities have been found to cut off at around 2×108cm∕s, significantly larger than those observed for other III-V semiconductors, such as GaAs and InP. These experimental results have been compared with ensemble Monte Carlo simulations and good agreement has been obtained.

Formation time of a small electron polaron in LiNbO 3 : measurements and interpretation

Infrared optical absorption attributed to the electron polaron on a non-defective site in LiNbO3 and KNbO3 has previously been observed using pulsed electron and laser techniques. With subpicosecond laser excitation and spectroscopy, it is possible to measure a rise time of the infrared absorption, which may be interpreted as the time for a band-state conduction electron to cool by phonon scattering, collapse its wavefunction around a site made attractive by thermal disorder, and relax vibrationally to a small polaron. This is a process which is of fundamental interest, involving dynamics of self-localization from band states and vibrational relaxation of a localized electron in an otherwise non-defective lattice. For example, Gavartin and Shluger have recently performed calculations on the role of thermal fluctuations in self-trapping of holes in MgO. We report initial measurements on the rise time of infrared absorption at 0.95 eV (Mg-perturbed polaron) in LiNbO3:Mg to be τR ≈ 230 fs at T = 20 K and τR ≈ 110 fs at T = 296 K. We discuss 2 stages that together may account for the delay and its temperature dependence: free-electron cooling and vibrational relaxation of a “defect” (small polaron) in a host. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Low Quantum Yields of Relaxed Electron Transfer Products of Moderately Coupled Ruthenium(II)−Cobalt(III) Compounds on the Subpicosecond Laser Excitation

Photoinduced electron-transfer reactions of [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarterpyridyne), [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ (tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene), and [(bpy)2RuII(tpphz)CoIII(bpy)2]5+ (bpy = 2,2‘-bipyridine, and tpphz = tetrapyrido[3,2-a:2‘,3‘-c:3‘ ‘,2‘ ‘-h::2‘ ‘‘,3‘ ‘‘-j]phenazine) were studied in the range 140−298 K by means of subpicosecond transient absorption spectroscopy. 3MLCT(Ru) of [(tpy)RuII(L−L)CoIII(tpy)]5+ (L−L: tpy-tpy and tpy-ph-tpy) underwent intramolecular electron-transfer (EET) reaction to form [2RuIII(tpy)(L−L)2CoII(tpy)]5+ in a shorter time than 10 ps. Low quantum yields of the EET products (0.53 for tpy-tpy and 0.41 for tpy-ph-tpy in butyronitrile at 298 K) measured before the fast return electron-transfer were found as in the intermolecular electron-transfer quenching of 3MLCT(Ru) by [Co(tpy)2]3+. The quantum yield of [(tpy)RuIII(tpy-tpy)CoII(tpy)]5+ (Φ) decreases to 0.38 in a slowly reorganizing solvent of propylene carbonate and that of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ to 0.21 at 180 K. The reductions in the EET yields, 1 − Φ, can be ascribed to a fast transition of the nonrelaxed EET product to the lowest triplet d−d* state of [CoIII(tpy)2] moiety during the solvent reorganization. A tunneling transition of the nonrelaxed EET products to the lowest lying d−d excited-state of [CoIII(tpy)2] moiety takes place as a hole transfer, HT, from the dπ-orbital of Ru(III) to that of Co(II) with a configuration of dπ6dσ*. An electronic coupling of dπ(Ru)−dπ(Co) estimated from the intensity of inter-valence transition of [(tpy)RuIII(L−L)RuII(tpy)]5+ (Collin, J.-P.; Laine, P.; Launay, J.-P.; Sauvage, J.-P.; Sour, A. J. Chem. Soc., Chem. Commun. 1993, 434) is large enough to carry out the HT. The weak coupling of dπ(Ru)−dπ(Co) in the case of [(bpy)2Ru(tpphz)Co(bpy)2]5+ attenuates the HT rate to bring about a high yield of EET (0.8). The possibility that energy-transfer of 3MLCT(Ru) to the Co(III) moiety via an intermediate super-exchange coupling between dπ(Ru) and dπ(Co) occurs in competition with the EET of [(tpy)RuIII(tpy-ph-tpy)CoII(tpy)]5+ is also pursued.

Doublet−Quartet Intersystem Crossing and Electron Transfer of Cobalt(II) Moieties in [LRuIII(L−L)CoIIL]5+ (L = 2,2‘:6‘,2‘ ‘-Terpyridine and L−L = A Bridging Ligand)

Return electron transfer (RET) and intersystem crossing (ISC) of [(tpy)RuIII(tpy-ph-tpy)2,4CoII(tpy)]5+ (tpy = 2,2‘:6‘,2‘ ‘-terpyridine and tpy-ph-tpy = 1,4-bis[2,2‘:6‘,2‘ ‘-terpyridine-4‘-yl]benzene) and [(tpy)RuII(tpy-tpy)CoIII(tpy)]5+ (tpy-tpy = 6‘,6‘ ‘-bis(2-pyridyl)-2,2‘:4‘,4‘ ‘:2‘ ‘,2‘ ‘‘-quarter-pyridyne) produced on the subpicosecond laser excitation have been investigated in a wide temperature range. A biexponential recovery process of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ and [CoIII(t)] = [2CoII(0)](1 − [fα exp(−kαt) + fβ exp(−kβt)]) occurs immediately after the production of [2RuIII(tpy)(tpy-pph-tpy)2CoII(tpy)]5+ in the excited-state electron-transfer quenching of [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. Four parameters of biexponential recovery, kα, fα, kβ, and fβ in a range of 180−300 K, are interpreted in terms of RET and ISC reactions among three states: [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+, [(tpy)2RuIII(tpy-tpy)4CoII(tpy)]5+, and [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+. RET of [(tpy)2RuIII(tpy-ph-tpy)2CoII(tpy)]5+ regenerating [(tpy)RuII(tpy-ph-tpy)CoIII(tpy)]5+ with kRET(D) of ∼4 × 1010 s-1 competes with doublet−quartet ISC with kQD of 0.20 × 1010 s-1 at of 0.044 eV and 2.8 × 1010 s-1 at of −0.01 eV. Meanwhile, the rate constant of the quartet−doublet ISC, kDQ (∼1.6 × 1010 s-1at of −0.01 eV), is much larger than RET of [(tpy)2RuIII(tpy-ph-tpy)4CoII(tpy)]5+. Dependences of kQD on give rise to a rough estimation of reorganizational free energy (0.03 eV). As for [(tpy)RuIItpy-tpy)CoIII(tpy)]5+, kRET(D), kDQ, and kQD are evaluated to be 2.3 × 1011 s-1, 1.5 × 1011 s-1 and 1.5 × 1011 s-1 at of −0.01 eV.

Substantial Non-linear Optical Response of New Polyads Based on Ru and Os Complexes of Modified Terpyridines

The third-order non-linear optical behavior of two new series of multicomponent systems based on coordination compounds of transition metals is reported. These systems, soluble in acetonitrile, are made of a metal (Ru or Os) bis-terpyridyl (tpy) unit, as photosensitizer P, connected to one or two electron acceptor groups, or to both electron donor (D) and electron acceptor (A) groups to form polyads of D−P−A type. The non-linear absorption and non-linear refraction response was substantial and was examined using the z-scan technique. The results are presented for two different excitation conditions. For excitation under laser pulses of nanosecond duration the compounds with the Ru center exhibit reverse saturable absorption (RSA). The corresponding Os samples show saturable absorption (SA) for low incident intensities that turns to RSA above an intensity threshold that depends on the concentration of the solutions. In contrast, under sub-picosecond laser excitation the Ru and the Os samples each exhibit s...

Ultrafast electron transfer in acceptor substituted bianthryl derivatives

Transient absorption spectra with subpicosecond laser excitation are reported for 9,9 ′-bianthryl (BA) and 9-anthryl-carbazole (C9A), their 10-cyano derivatives CBA and CC9A and for the 2-6- tert-butyl derivative of CC9A (TBCC9A). For BA and C9A, the rise time of the 690-nm absorption band assigned to the charge-transfer (CT) state, in about 20 ps in ethanol and 70 ps in butanol, is consistent with the average solvation time in these solvents. An initial fast loss of structure (⩽1 ps) in the gain spectra of BA is interpreted as due to the inertial component of the conformational relaxation within the locally excited (LE) state. A CT reaction involving both an intramolecular torsion coordinate and a solvation coordinate is proposed for BA and C9A. Quite different to the unsubstituted compounds, the CT process is found to occur during the 500-fs excitation pulse in CBA, CC9A and TBCC9A in both ethanol and butanol. The key parameter differentiating the systems is proposed to be the relative energy of the CT state with respect to the LE state, the substitution leading to a strong overlap of the two states in the Franck–Condon transition. On the other hand, diffusive solvation effects and torsion within the instantaneously populated CT state are discussed as possible causes for the picosecond-time-scale evolution of the transient spectra observed for the cyano derivatives.

Investigation of the laserlike behavior of polymeric scattering gain media under subpicosecond laser excitation

The narrowing effects of scatterers on the lifetime and the spectral width of the laser-induced fluorescence of organic dyes hosted in poly(methyl methacrylate) polymer sheets were studied. The excitation source was a distributed-feedback dye laser emitting 0.5-ps pulses at 496 nm. Spectral and temporal features were recorded simultaneously on a spectrograph-streak-camera detection system. The results were then compared with those obtained from dye solutions in methanol that were recorded in previous studies. The effects of the different host environments on the fluorescence characteristics of the dye were thus investigated. These effects are currently studied when the dye is inserted into human tissue in an attempt to boost tumor detection and photodynamic-therapy efficiency. Some initial results are presented.

Excited‐State Dynamics of Fully Reduced Flavins and Flavoenzymes Studied at Subpicosecond Time Resolution

AbstractThe photophysics of the fully reduced states of a number of flavins (flavin mononucleotide, flavin adenine dinucleotide and 3‐N‐methyllumiflavin) and flavoenzymes (glucose oxidase from Aspergillus niger and the flavodehydrogenase component isolated from flavocytochrome b2) was studied using subpicosecond laser excitation at Λ. = 312 nm. The prompt transient absorption spectra (measured from 400 to 850 nm) were all closely similar in the case of the free flavins in aqueous solution. The decay of the transient absorbance obeyed biexponential kinetics with a fast component of lifetime ranging from 4 to 130 ps and a slower phase with a lifetime above 1 ns. The spectral structure changed appreciably during the rapid decay phase. In contrast, in the case of the enzymes only a very slight decay was apparent over the probed time interval (1 ns) and the shape of the spectrum remained unchanged. It is proposed that the two transient spectra appearing in the free flavins correspond to two conformations differing by their degree of nonplanarity, whereas in the flavoenzymes only one conformation is possible.

Excited-state dynamics of fully reduced flavins and flavoenzymes studied at subpicosecond time resolution.

The photophysics of the fully reduced states of a number of flavins (flavin mononucleotide, flavin adenine dinucleotide and 3-N-methyllumiflavin) and flavoenzymes (glucose oxidase from Aspergillus niger and the flavodehydrogenase component isolated from flavocytochrome b2) was studied using subpicosecond laser excitation at lambda = 312 nm. The prompt transient absorption spectra (measured from 400 to 850 nm) were all closely similar in the case of the free flavins in aqueous solution. The decay of the transient absorbance obeyed biexponential kinetics with a fast component of lifetime ranging from 4 to 130 ps and a slower phase with a lifetime above 1 ns. The spectral structure changed appreciably during the rapid decay phase. In contrast, in the case of the enzymes only a very slight decay was apparent over the probed time interval (1 ns) and the shape of the spectrum remained unchanged. It is proposed that the two transient spectra appearing in the free flavins correspond to two conformations differing by their degree of nonplanarity, whereas in the flavoenzymes only one conformation is possible.

Quantitative assessment of the effects of carrier screening on the average electric field in a GaAs-based p–i–n nanostructure under subpicosecond laser excitation

We have used electric-field-induced Raman scattering to quantitatively assess the effects of carrier screening on the average electric fields in a GaAs-based p–i–n nanostructure semiconductor under subpicosecond laser photoexcitation. Our experimental results demonstrated that the effects of carrier screening on the average electric field were negligible for a photoexcited electron–hole pair density of n⩽1015 cm−3. As the density of photoexcited carriers increased, we observed a significant decrease of the average electric field. In particular, for n=1018 cm−3, a decrease of electric field of about 50% was found. All of these experimental results were explained by ensemble Monte Carlo simulations and very good agreement has been obtained.

Ultrafast intramolecular charge transfer in the merocyanine dye DCM

Time-resolved transient absorption and gain spectra with subpicosecond laser excitation are reported for weakly polar and highly polar solutions of the merocyanine dye DCM (4-dicyanomethylene-2-methyl-6-(p-dimethylamino-styryl)-4-pyran) in the 380–800 nm spectral range. A temporary isosbestic point is observed in the transient gain spectra during 3 to 18 ps after excitation depending on the solvent. The results are interpreted in terms of the rapid formation of an emissive intramolecular charge-transfer state from the emissive initially excited singlet state. A further evolution of the transient spectra includes a blue-shift of the gain band during a few tens of picoseconds in solvents having ultrashort solvation time whereas a red-shift is found for solvent of slow response. A relaxation mechanism including vibrational cooling and solvation of the charge-transfer state is discussed to explain the observed resolved differential spectra. Other processes such as isomerization or single bond twisting are also discussed.

Sub-picosecond laser ionization of free C60 and C70 at 248 nm

Delayed ionization is found to be absent for sub-picosecond laser excitation of free C60 and C70 at 248 nm. The autocorrelation trace obtained for C60+ in a laser time-of-flight (TOF) mass spectrometer using two time-delayed and collinear 248 nm ultrashort laser pulses has a width of 1.1 ps (715 fs for sech2 pulses), in agreement with the laser pulse duration measurement in NO gas. Both above observations can be explained by direct ionization of C60 via coherent two-photon absorption by the high intensity sub-picosecond 248 nm laser excitation avoiding the channel leading to delayed ionization.

Characterization of an optoelectronic terahertz beam system

The performance of an optoelectronic terahertz (THz) beam system is described. The transmitter operation is based on the repetitive, subpicosecond laser excitation of a Hertzian dipole antenna embedded in a charged coplanar line. With this transmitter electromagnetic beams of 1/2 cycle THz pulses at a repetition rate of 100 MHz are produced. The associated optoelectronic receiver is gated in synchronism with the excitation of the transmitter by subpicosecond pulses from the same laser source. With this receiver, the 10-nW beams of THz pulses were observed with a signal-to-noise ratio greater than 10000:1. Several sources contributing to the noise of the receiver are discussed, together with ways to reduce them. With an integration time of 125 ms, a signal-to-noise ratio of 1 is obtained for a THz beam with an average power of 10/sup -16/ W. The receiver operates in the sampling mode and has a time resolution of 0.5 ps. >