Long Wavelength Absorption Tail Research Articles

Urbach Rule in MnGa2Se4 Single Crystals Upon Optical Absorption

The Urbach rule in MnGa2Se4 single crystals is studied. It is found that the long-wavelength absorption tail obeys the Urbach rule at photon energies in the range 2.1–2.2 eV. It is shown that a strong electron–phonon interaction exists in MnGa2Se4 single crystals.

Tandem-pumped, tunable thulium-doped fiber laser in 2.1 μm wavelength region.

We present a continuously tunable thulium(Tm)-doped fiber laser operating in the important 2.1 μm region, which is tandem-pumped by another Tm-doped fiber laser at 1908 nm. The advantages of pumping a Tm-doped fiber laser at the long-wavelength absorption tail (>1900 nm) of the fiber include a reduced quantum-defect, and efficient suppression of the amplified spontaneous noise (and potential parasitic lasing) at the short-wavelength region. This facilitates attainment of stable lasing operation in the long-wave emission tail of the Tm fiber at ~2.1 μm. By rotating a diffraction grating inside the Tm fiber laser cavity, we experimentally achieved a wavelength-tuning range of 2000-2172 nm. At central wavelengths of 2050 nm, 2150 nm, and 2172 nm, the slope efficiencies were 23%, 16%, and 9.9%, respectively. To the best of our knowledge, this is the first demonstration of long-wavelength operation of a Tm fiber laser system tandem-pumped at >1900 nm.

Правила Урбаха в монокристаллах MnGa-=SUB=-2-=/SUB=-Sе-=SUB=-4-=/SUB=- при оптических поглощениях

AbstractThe Urbach rule in MnGa_2Se_4 single crystals is studied. It is found that the long-wavelength absorption tail obeys the Urbach rule at photon energies in the range 2 . 1–2 . 2 eV. It is shown that a strong electron–phonon interaction exists in MnGa_2Se_4 single crystals.

Charge-transfer character of the low-energy Chl a Q(y) absorption band in aggregated light harvesting complexes II.

One of the key functions of the major light harvesting complex II (LHCII) of higher plants is to protect Photosystem II from photodamage at excessive light conditions in a process called "non-photochemical quenching" (NPQ). Using hole-burning (HB) spectroscopy, we investigated the nature of the low-energy absorption band in aggregated LHCII complexes - which are highly quenched and have been established as a good in vitro model for NPQ. Nonresonant holes reveal that the lowest energy state (located near 683.3 nm) is red-shifted by ~4 nm and significantly broader (by a factor of 4) as compared to nonaggregated trimeric LHCII. Resonant holes burned in the low-energy wing of the absorption spectrum (685-710 nm) showed a high electron-phonon (el-ph) coupling strength with a Huang-Rhys factor S of 3-4. This finding combined with the very low HB efficiency in the long-wavelength absorption tail is consistent with a dominant charge-transfer (CT) character of the lowest energy transition(s) in aggregated LHCII. The value of S decreases at shorter wavelengths (<685 nm), in agreement with previous studies (J. Pieper et al., J. Phys. Chem. B 1999, 103, 2422-2428), proving that the low-energy excitonic state is strongly mixed with the CT states. Our findings support the mechanistic model in which Chl-Chl CT states formed in aggregated LHCII are intermediates in the efficient excited state quenching process (M. G. Müller et al., Chem. Phys. Chem. 2010, 11, 1289-1296; Y. Miloslavina et al., FEBS Lett. 2008, 582, 3625-3631).

Transformation of the short-range order upon melting of CuHal nanocrystals in glasses

The reduced density of states at the edge of interband transitions in CuCl and CuBr nanocrystals is described by a step at T → 0. As the temperature increases, the step is smeared toward the band gap and the long-wavelength absorption tail agrees with the Urbach rule. A change in the phase state of CuCl or CuBr nanoparticles (the crystal-melt transition) is accompanied by an insignificant change in the band gap, and the long-wavelength absorption tail for the interband transitions considerably extends deeper into the band gap as compared to that for crystals at the same temperature. The change observed in the band gap upon melting is explained by the transformation of short-range order in CuCl and CuBr melts. An increase in thermal smearing of the long-wavelength absorption tail for nanomelts is associated with the severalfold increase in the amplitude of thermal fluctuations of the band gap, possibly, due to the disappearance of the long-range order in the melts. A lower melting temperature of large (>30 nm) CuCl and CuBr particles as compared to the melting temperature of bulk samples indicates the presence of the second component NaCl or NaBr. Consequently, the CuCl-NaCl and CuBr-NaBr eutectic nanosystems undergo melting and crystallization. A small shift in the band gap observed in this case upon melting confirms the conclusion drawn by Zalkin that eutectic melts have a microheterogeneous structure.

Excited Singlet (S1) State Interactions of 6,11-Dihydroxy-5,12-naphthacenequinone with Aromatic Hydrocarbons

Interaction in the excited singlet state (S1) of 6,11-dihydroxy-5,12-naphthacenequinone (DHNQ) with aromatic hydrocarbons (AH) has been investigated using steady-state (SS) and time-resolved (TR) fluorescence quenching measurements. In both nonpolar (cyclohexane; CH) and polar (acetonitrile; ACN) solvents, the fluorescence quenching of DHNQ by AHs is accompanied with the appearance of exciplex emissions. The emission maxima of the exciplexes correlate linearly with both ionization potentials (IP) and oxidation potentials {E(AH/AH.+)} of the quenchers (AH), indicating the charge transfer (CT) type of interaction between the S1 state of DHNQ (acceptor) and the ground state of the AHs (donor). The kinetic details of the exciplex formation have been evaluated by analyzing the SS and TR fluorescence quenching results at different temperatures following a suitable mechanistic scheme. Picosecond laser flash photolysis (LFP) studies on the DHNQ−AH systems show a major transient absorption band in the 530 to 630 nm region along with a weak long-wavelength absorption tail. The transient lifetimes for the 530−630 nm absorption band are very similar to the exciplex lifetimes estimated from the fluorescence quenching results. At the long wavelength absorption tail, the transient lifetime could not be estimated due to very weak absorption. It is inferred that the 530−630 nm transient absorption band is mostly due to the S1→ Sn transition. The long wavelength absorption tail has been attributed to the anion radical of DHNQ, drawing an analogy with the anion radical absorption spectrum of 1,4-dihydroxy-9,10-anthraquinone (quinizarin; QZ), a lower analogue of DHNQ. The picosecond LFP results largely correlate with the results obtained from the fluorescence quenching studies.

Luminescence of Zinc Oxide Crystals Controlled by Electrode Potential and Electrochemical Reactions

The voltage dependence of the green luminescence of ZnO crystals used as electrodes in an electrochemical cell was investigated and compared with the voltage dependence of the photocurrent. When the exciting light is of shorter wavelength than the band edge of ZnO, this luminescence can be varied from its maximal value to complete extinction by applying small positive voltages. The luminescence resulting from excitation within the long wavelength tail of absorption, however, cannot be influenced. The potential dependence of the luminescence has been investigated with differently doped crystals under various conditions. The addition of formic acid to the electrolyte causes an increase of luminescence; this is found to be a consequence of photoelectrochemically induced radical reaction by which electrons are injected into the conduction band of the electrode. The potential dependence of the green luminescence is explained by a mechanism in which the trapping of the holes in recombination centers competes with their extraction, which leads to the generation of the photocurrent. The average concentration of photogenerated holes in the crystal is therefore rate limiting for the intensity of luminescence, provided that an excess of electrons is available for recombination. At sufficiently large positive potentials, however, when electrons are being depleted in the space charge layer, their concentration also becomes limiting, and electron injection can stimulate luminescence. The experimentally observed potential dependence of luminescence and photocurrent, as well as their interrelation, is shown to be consistent with results which were derived from a theoretical treatment of these effects. It is suggested that this green luminescence may be used as a probe for the study of the space charge layer and of electrochemical reactions.