Hole-burning Study Research Articles

Spectral Hole-Burning Studies of a Mononuclear Eu(III) Complex Reveal Narrow Optical Linewidths of the 5D0→7F0 Transition and Seconds Long Nuclear Spin Lifetimes.

Coordination complexes of rare-earth ions (REI) show optical transitions with narrow linewidths enabling the creation of coherent light-matter interfaces for quantum information processing (QIP) applications. Among the REI-based complexes, Eu(III) complexes showing the 5D0→7F0 transition are of interest for QIP applications due to the narrow linewidths associated with the transition. Herein, we report on the synthesis, structure, and optical properties of a novel Eu(III) complex and its Gd(III) analogue composed of 2,9-bis(pyrazol-1-yl)-1,10-phenanthroline (dpphen) and three nitrate (NO3) ligands. The Eu(III) complex-[Eu(dpphen)(NO3)3]-showed sensitized metal-centred emission (5D0→7FJ; J=0,1,2,3, 4, 5, or 6) in the visible region, upon irradiation of the ligand-centred band at 369 nm, with the 5D0→7F0 transition centred at 580.9 nm. Spectral hole-burning (SHB) studies of the complex with stoichiometric Eu(III) concentration revealed a narrow homogeneous linewidth (Γh) of 1.55 MHz corresponding to a 0.205 μs long optical coherence lifetime (T2opt). Remarkably, long nuclear spin lifetimes (T1spin) of up to 41 s have been observed for the complex. The narrow optical linewidths and long T1spin lifetimes obtained for the Eu(III) complex showcase the utility of Eu(III) complexes as tuneable, following molecular engineering principles, coherent light-matter interfaces for QIP applications.

High-Resolution Frequency-Domain Spectroscopic and Modeling Studies of Photosystem I (PSI), PSI Mutants and PSI Supercomplexes.

Photosystem I (PSI) is one of the two main pigment-protein complexes where the primary steps of oxygenic photosynthesis take place. This review describes low-temperature frequency-domain experiments (absorption, emission, circular dichroism, resonant and non-resonant hole-burned spectra) and modeling efforts reported for PSI in recent years. In particular, we focus on the spectral hole-burning studies, which are not as common in photosynthesis research as the time-domain spectroscopies. Experimental and modeling data obtained for trimeric cyanobacterial Photosystem I (PSI3), PSI3 mutants, and PSI3-IsiA18 supercomplexes are analyzed to provide a more comprehensive understanding of their excitonic structure and excitation energy transfer (EET) processes. Detailed information on the excitonic structure of photosynthetic complexes is essential to determine the structure-function relationship. We will focus on the so-called "red antenna states" of cyanobacterial PSI, as these states play an important role in photochemical processes and EET pathways. The high-resolution data and modeling studies presented here provide additional information on the energetics of the lowest energy states and their chlorophyll (Chl) compositions, as well as the EET pathways and how they are altered by mutations. We present evidence that the low-energy traps observed in PSI are excitonically coupled states with significant charge-transfer (CT) character. The analysis presented for various optical spectra of PSI3 and PSI3-IsiA18 supercomplexes allowed us to make inferences about EET from the IsiA18 ring to the PSI3 core and demonstrate that the number of entry points varies between sample preparations studied by different groups. In our most recent samples, there most likely are three entry points for EET from the IsiA18 ring per the PSI core monomer, with two of these entry points likely being located next to each other. Therefore, there are nine entry points from the IsiA18 ring to the PSI3 trimer. We anticipate that the data discussed below will stimulate further research in this area, providing even more insight into the structure-based models of these important cyanobacterial photosystems.

Excitonic structure and dynamics in various photosynthetic antenna protein complexes: Holeburning and modeling studies

Excitonic structure and dynamics in various photosynthetic antenna protein complexes: Holeburning and modeling studies

The quest for energy traps in the CP43 antenna of photosystem II

To identify energy traps in CP43, a subcomplex of the photosystem II antenna system, site energies and excitonic couplings of the QY transitions of chlorophyll (Chl) a pigments bound to CP43 are computed using electrostatic models of pigment–protein and pigment–pigment interactions. The computations are based on recent crystal structures of the photosystem II core complex with resolutions of 1.9 and 2.1Å and compared to earlier results obtained at 2.9Å resolution. Linear optical spectra (i.e., absorption, linear dichroism, circular dichroism, and fluorescence) are simulated using the computed excitonic couplings, a refinement fit for the site energies, and a dynamical theory of optical lineshapes. A comparison of the obtained root mean square deviation of about 100cm−1 between directly calculated and refined site energies with the maximum range of about 350cm−1 of directly calculated site energies shows that the combined quantum chemical/electrostatic approach provides a semi-quantitative agreement with experiment. Possible reasons for the deviations are discussed, including limits of the electrostatic models and the lineshape theory as well as structural alterations of CP43 upon detachment from the core complex. Based on the simulations, an assignment of the two low-energy exciton states A and B of CP43, that where observed earlier in hole burning studies, is suggested. State A is assigned to a localized exciton state on Chl 37 in the lumenal layer of pigments. State B is assigned to an exciton state that is delocalized over several pigments in the cytoplasmic layer. The delocalization explains the smaller inhomogeneous width of state B compared to state A observed in hole burning spectra, which is proposed to be due to exchange narrowing. The assignment of states A and B largely confirms our earlier suggestion that was based on a fit of linear optical spectra and electrostatic calculations using the 2.9Å resolution structure. Interestingly, for the latter structure, the site energy of Chl 37 is obtained closer to the refined value than for 1.9 and 2.1Å resolution. This is explained by a variation of the site energy due to the influence of lipids that might be different in the core complex and isolated CP43. To remove remaining uncertainties in the assignment of states A and B, target sites for mutagenesis experiments are proposed based on the electrostatic computations. In particular, it is suggested to mutate Trp C63 close to Chl 37 to probe the identity of state A and to mutate Arg C41 close to Chl 47 to probe state B.

Optical properties and coherent vibrational oscillations of gold nanostars

Hot electron relaxation and coherent vibrational oscillations of gold nanostars (AuNS) were studied using femtosecond laser spectroscopy. Transient profiles feature a fast decay attributed to electron–phonon interaction and a slower decay due to phonon relaxation. Superimposed on the slow decay are oscillations attributed to coherent vibrations of the AuNS with a period dependent on probe wavelength, due to size variation of the AuNS. In addition, persistent spectral hole-burning studies indicate depletion of AuNS at the excitation wavelength and formation of redder-absorbing AuNS aggregates. In conjunction with experiments, theoretical calculations were carried out to gain insight into the coherent vibrational oscillations.

Solvatochromic shift mechanisms of centrosymmetric chromophores in polar liquids

Origins of absorption shifts in polar liquids are considered for centrosymmetric chromophores devoid of dipole moments. The refractive index (n) dependent bathochromism shows that the excited state is always more polarizable than ground state. With increasing solvent dielectric constant (ɛ), mainly red shifts occur, but there are exceptions. Hypsochromism of n–π* transition in s-tetrazine is ascribed to a decrease of quadrupole moment in the excited state. In this case the upper level is pushed up in the reaction field of the ground-state quadrupole. Red shifts of intense bands in polyenes (11Bu or S2) and polyarenes (1Bb or β), reaching ∼−250 cm−1 are assigned to local polarization in the reaction fields of chromophore bond dipoles. The hyperbolic-like ɛ-dependence of solvent shift, approximated to an empirical function (ɛ − n2)/(ɛ − n2 + c), is much steeper (with “stretching” factor c = 3 ± 0.5) in this case, as compared to that for the linear effect (for s-tetrazine, c = 9). Polarization mechanism also causes less band broadening in polar environments. Contrary to a common view, the spectral shift is probing local reaction fields of chromophores, rather than random fields in the cavities accommodating the dye. Negligible polarization in solvent “cavity” fields is in accordance with earlier hole-burning study of matrix induced dipole moments in tetrapyrrolic pigments that depend surprisingly little on the polarity of polymeric hosts (R.B. Altmann, I. Renge, L. Kador, D. Haarer, J. Chem. Phys. 97 (1992) 5316 [30]).

Temperature dependence of the R1 linewidth in Al 2O 3:Mn 4+: A spectral hole-burning and FLN study

The temperature dependence of the R 1 linewidth in Al 2O 3:Mn 4+ is reported. Transient holes broaden rapidly from 2.5 to 40 K due to the one-phonon process between the E ¯ and 2 A ¯ levels of the 2 E excited state. The linewidth varies from 3 MHz at 2.5 K to 2640 GHz at 294 K and the line shifts by −1440 GHz. Above 50 K the two-phonon Raman process becomes dominant. The quadratic electron–phonon coupling constant is larger for Al 2O 3:Mn 4+ compared to ruby, resulting in a stronger phonon sideband in Al 2O 3:Mn 4+. The best description of the linewidth data are yielded by a model using pseudo-local modes.

Optical Properties and Persistent Spectral Hole Burning of Near Infrared-Absorbing Hollow Gold Nanospheres

We report on the characteristics and mechanism of persistent spectral hole burning of hollow gold nanospheres (HGNs) with near IR (NIR) surface plasmon resonance (SPR) absorption induced by femtosecond (fs) laser pulses. The HGNs were exposed to pulsed laser light at 810 nm, on resonance with their absorption band, at various laser powers while the total photon flux incident on the samples was kept constant. Depletion of HGNs responsible for 830 nm absorption was observed while simultaneous growth of bluer-absorbing HGNs, solid gold nanoparticles (AuNPs), or a mixture thereof was observed. This occurred only with fs pulsed laser irradiation and did not occur with continuous wave (CW) laser irradiation. Based on combined UV−vis spectroscopy and transmission electron microscopy data, the mechanism behind the persistent hole burning with fs laser irradiation is proposed to involve two possible processes: (i) conversion of NIR-absorbing HGNs to bluer-absorbing HGNs, and (ii) breakdown of NIR-absorbing HGNs into solid AuNPs directly. The branching ratio between these two processes depends on the peak power of the fs laser pulses with higher peak power favoring the second process. This hole-burning study is useful for understanding and exploring the potential use of HGNs in drug delivery with a laser as a trigger for release. In conjunction, theoretical calculations have been carried out to gain further insight into the optical absorption of HGNs, especially in terms of how the extinction coefficient depends on the structure of the HGNs in comparison to solid AuNPs. One interesting finding is that HGNs with thicker shells not only absorb at bluer wavelengths, but also have lower absorption as well as extinction coefficient. An HGN with the same outer diameter as a solid AuNP has a substantially larger extinction coefficient. These theoretical findings are supported by experimental results. The enhanced extinction coefficient is advantageous for applications such as photothermal ablation therapy (PTA) for cancer treatment. The dependence of the absorption coefficients on the HGN structure is also important for interpreting the hole-burning results that rely on changes in spectral position as well as intensity.

Insight into the Electronic Structure of the CP47 Antenna Protein Complex of Photosystem II: Hole Burning and Fluorescence Study

We report low temperature (T) optical spectra of the isolated CP47 antenna complex from Photosystem II (PSII) with a low-T fluorescence emission maximum near 695 nm and not, as previously reported, at 690-693 nm. The latter emission is suggested to result from three distinct bands: a lowest-state emission band near 695 nm (labeled F1) originating from the lowest-energy excitonic state A1 of intact complexes (located near 693 nm and characterized by very weak oscillator strength) as well as emission peaks near 691 nm (FT1) and 685 nm (FT2) originating from subpopulations of partly destabilized complexes. The observation of the F1 emission is in excellent agreement with the 695 nm emission observed in intact PSII cores and thylakoid membranes. We argue that the band near 684 nm previously observed in singlet-minus-triplet spectra originates from a subpopulation of partially destabilized complexes with lowest-energy traps located near 684 nm in absorption (referred to as AT2) giving rise to FT2 emission. It is demonstrated that varying contributions from the F1, FT1, and FT2 emission bands led to different maxima of fluorescence spectra reported in the literature. The fluorescence spectra are consistent with the zero-phonon hole action spectra obtained in absorption mode, the profiles of the nonresonantly burned holes as a function of fluence, as well as the fluorescence line-narrowed spectra obtained for the Q(y) band. The lowest Q(y) state in absorption band (A1) is characterized by an electron-phonon coupling with the Huang-Rhys factor S of approximately 1 and an inhomogeneous width of approximately 180 cm(-1). The mean phonon frequency of the A1 band is 20 cm(-1). In contrast to previous observations, intact isolated CP47 reveals negligible contribution from the triplet-bottleneck hole, i.e., the AT2 trap. It has been shown that Chls in intact CP47 are connected via efficient excitation energy transfer to the A1 trap near 693 nm and that the position of the fluorescence maximum depends on the burn fluence. That is, the 695 nm fluorescence maximum shifts blue with increasing fluence, in agreement with nonresonant hole burned spectra. The above findings provide important constraints and parameters for future excitonic calculations, which in turn should offer new insight into the excitonic structure and composition of low-energy absorption traps.

Fluorescence hole-burning and site-selective studies of LHCII

We report the observation of two types of changes in fluorescence spectra of LHCII at 4.2 K following intense illumination of the sample with a spectrally narrow laser beam at wavelengths between 678 and 686 nm. Nonspecific changes (burning-wavelength independent) are characterized by two relatively broad bands: a positive one at ∼ 678.7 nm and a negative one at ∼ 680.8 nm. These changes reveal a ∼1.3-nm blue shift of the distribution of final emitters in LHCII, from 680.3 nm to ∼ 679.0 nm independent of the excitation wavelength. Specific fluorescence changes (burning-wavelength dependent) are characterized by a sharp hole exactly at the burning wavelength, and positive changes directly to the shorter-and longer-wavelength side of the narrow hole. The negative changes are interpreted as zero-phonon holes, while the positive features are assigned to non-photochemical products. In the low-burning intensity experiment, in addition to the zero-phonon holes, we observed also the holes to the longer wavelength of the zero-phonon hole, which were assigned to a sum of phonon and pseudo-phonon side bands. The shapes of these extra holes are identical to the shapes of the holes revealed in the fluorescence line narrowing experiment. On the basis of the low-burning intensity experiment we estimated the upper limit of the electron-phonon coupling strength for LHCII, characterized by a Huang-Rhys factor of 1.5.

Transient spectral hole-burning studies of the R 2 line in ruby

We report transient spectral hole-burning studies of the R 2(±1/2) line in 130, 20 ppm and 0.6 ppm ruby between 2.4 and 70 K. The homogeneous line width Γ h is found to be 142 ± 6 MHz at 2.4 K for both 20 and 130 ppm crystals; this value is in agreement with the T 1 value of 1.1 ns ( Γ h = 1/2 πT 1 = 140 MHz) for the 2 A ¯ state obtained by Rives and Meltzer who used pulsed, non-selective laser excitation. The line width is dominated by a direct one-phonon relaxation process 2 A ¯ → E ¯ up to 50 K whereas at higher temperatures two-phonon Raman scattering becomes dominant. Cross-relaxation and spin-lattice relaxation times significantly affect pseudo-CW experiments.

Infrared-Optical Double-Resonance Measurements on O−H···H−Ge Dihydrogen-Bonded Phenol−Triethylgermanium Hydride Complex in the Gas Phase

Spectroscopic investigation of a dihydrogen-bonded complex between phenol and triethylgermanium hydride is reported here. Laser-induced fluorescence excitation, fluorescence-detected infrared, and IR-UV hole-burning spectroscopic studies were carried out in supersonic jet to investigate the complex formation between phenol and triethylgermanium hydride. The lowering of the O-H stretching frequency of the phenol moiety in the complex with triethylgermanium hydride clearly establishes the role of phenol as hydrogen bond donor. The experimental results together with the ab-initio calculations unambiguously confirm formation of an O-H...H-Ge dihydrogen-bonded complex between phenol and triethylgermanium hydride.

Terrylene in hexadecane revisited: A hole burning study

Holes burnt into the absorption spectrum of terrylene in hexadecane have quite unusual features: spectral diffusion behavior under thermal cycles shows a narrowing regime at very low temperatures (2-5 K) followed by a plateau region (up to about 13 K) and a broadening regime (T>13 K). Thermal line broadening (quasihomogeneous linewidth) shows a nonmonotonous behavior as a function of temperature: at around 4 K there is a maximum followed by a flat minimum and the onset of strong broadening at higher temperatures. Finally, the central hole shows one-sided narrowly spaced side features. This behavior is interpreted within the frame of a two-site model. One of the two sites can be well described by a standard two level system; the other, however, shows characteristic features of a multilevel system. The two sites are characterized by strongly different optical linewidths, phototransformation yields, and thermal stabilities.

Assignment of the Lowest QY-state and Spectral Dynamics of the CP29 Chlorophyll a/b Antenna Complex of Green Plants: A Hole-burning Study ‡

Low-temperature absorption, fluorescence and persistent nonphotochemical hole-burned spectra are reported for the CP29 chlorophyll (Chl) a/b antenna complex of photosystem II of green plants. The absorption-origin band of the lowest Qy-state lies at 678.2 nm and carries a width of ∼130 cm−1 that is dominated by inhomogeneous broadening at low temperatures. Its absorption intensity is equivalent to that of one of the six Chl a molecules of CP29. The absence of a significant satellite hole structure produced by hole burning, within the absorption band of the lowest state, indicates that the associated Chl a molecule is weakly coupled to the other Chl and, therefore, that the lowest-energy state is highly localized on a single Chl a molecule. The electron–phonon coupling of the 678.2 nm state is weak with a Huang–Rhys factor S of 0.5 and a peak phonon frequency (ωm) of ∼20 cm−1. These values give a Stokes shift (2Sωm) in good agreement with the measured positions of the absorption band at 678.2 nm and a fluorescence-origin band at 679.1 nm. Zero-phonon holes associated with the lowest state have a width of ∼0.05 cm−1 at 4.2 K, corresponding to a total effective dephasing time of ∼400 ps. The temperature dependence of the zero-phonon holewidth indicates that this time constant is dominated at temperatures below 8 K by pure dephasing/spectral diffusion due to coupling of the optical transition to the glass-like two-level systems of the protein. Zero-phonon holewidths obtained for the Chl b bands at 638.5 and 650.0 nm, at 4.2 K, lead to lower limits of 900 ± 150 fs and 4.2 ± 0.3 ps, respectively, for the Chl b→ Chl a energy-transfer times. Downward energy transfer from the Chl a state(s) at 665.0 nm occurs in 5.3 ± 0.6 ps at 4.2 K.

An analysis of erbium excited state absorption in silicon-rich silica

Erbium-doped silicon-rich silica is a material that has generated great interest, as it holds considerable promise for practical silicon-based optical sources. There are numerous reports of sensitization of erbium 1.5 μm luminescence by silicon nanoclusters (Si nc), and it is now accepted that this process increases the effective absorption cross-section of Er ions by three orders of magnitude. There have been reports of optical gain in a planar waveguide pumped at wavelengths away from Er absorption bands. However, no lasing action has yet been achieved. In this paper, we discuss a key interaction—excited state absorption (ESA)—which has been neglected in this material. Typically, Si nc responsible for sensitization have a broad photoluminescence peak centred around 800 nm. Moreover, spectral hole-burning studies suggest that resonant energy transfer occurs from the Si to the Er in this wavelength region. However, this energy transfer route may be problematic, due to an ESA transition known from early work on erbium-doped fibre amplifiers (EDFAs). Excitation around 800 nm causes the Er metastable state to be excited to higher-level states. Such transitions serve no useful purpose, but constitute an energy drain: EDFAs pumped at 800 nm require an order of magnitude more pump power to achieve the same gain as those pumped at 980 nm. We have conducted an analysis of the erbium-doped silicon-rich silica system, incorporating ESA in the model. We compare the results of our analysis with reported experimental data and extract the Si nc–Er excited state energy transfer coefficient, yielding a value of 1×10 −15 cm 3/s. This is comparable to the Si nc–Er ground state transfer coefficient, confirming that ESA is potentially significant.

Zero-Phonon Linewidth in CdSe/ZnS Core/Shell Nanorods

High-resolution spectral hole-burning studies of CdSe/ZnS core/shell nanorods reveal a sharp zero-phonon line, with a line width dependent on the measurement time scale. The zero-phonon line width is attributed to contributions from radiative decay, spectral diffusion induced by surface electric field fluctuations, and phonon-assisted migration of excitons localized in the nanorods. A decoherence rate as small as 4.5 GHz has been observed, when the effects of spectral diffusion are suppressed in the spectral hole-burning measurement. Comparison between zero-phonon line widths in nanorods and spherical nanocrystals also elucidates important differences in the decoherence process between the one- and zero-dimensional nanostructures.

Nonphotochemical Hole-Burning Study of Selectively Stained Normal and Cancerous Human Ovarian Tissues

Results are presented of nonphotochemical hole-burning (HB) experiments on cancerous ovarian and analogous normal peritoneal in vitro tissues stained with the mitochondrial-selective dye rhodamine 800. A comparison of fluorescence excitation spectra, hole-growth kinetics data, and external electric field (Stark) effects on the shape of spectral holes burned in cancerous and normal tissues stained with rhodamine 800 revealed significant differences only in the dipole moment change (fDeltamu) measured by a combination of HB and Stark spectroscopies. It is shown that the permanent dipole moment change for the S0--> S1 transition of the rhodamine 800 molecules in cancerous tissue is higher than that of normal tissue by a factor of about 1.4. The finding is similar to the HB results obtained earlier for human ovarian surface epithelial cell lines, i.e., OV167 carcinoma and OSE(tsT)-14 normal cells stained with the same mitochondria-specific dye (Walsh et al. Biophys. J. 2003, 84, 1299). We propose that the observed difference in the permanent dipole moment change in cancerous ovarian tissue is related to a modification in mitochondrial membrane potential.

Optical hole burning studies in europium doped oxide glasses

Europium doped sodium borate, silicate, borosilicate, germanate and tellurite glasses have been prepared by melt quenching technique. Optical spectroscopy techniques were used to characterize the divalent and trivalent ions. Optical hole burning technique was used to characterize rare-earth ion doped glasses for optical data storage. The hole burning mechanisms, which depend on the glass composition and the preparation methods used, are discussed.

Hole burning study of Tm 3+:YAG hyperfine structure for quantum storage applications

Quantum storage, which aims at transferring photon quantum states into matter, can be obtained by using an ensemble of atoms whose levels form a three level Λ system. In these systems, two optical transitions couple two levels to a third one. This quantum storage scheme could be obtained with rare-earth ions in single crystals, since their optical and ground-state hyperfine transitions can exhibit long coherence lifetimes and their hyperfine structures could be used to build a three level Λ system. Tm 3+ ions in Y 3Al 5O 12 are especially interesting since the 3H 6– 3H 4 transition can be driven by ultra-stable laser diodes. However, the selection rules on the nuclear spin projection can forbid the simultaneous coupling of two levels to a third one. In this paper, the hyperfine structure of Tm 3+ is investigated by hole burning spectroscopy under a magnetic field and compared to theoretical calculations based on crystal field calculations. The experimental results are found to be in good agreement with theory and show that some magnetic field orientations are able to relax the selection rules on the nuclear spin projection.

Intra- vs. intermolecular hydrogen bonding: dimers of alpha-hydroxyesters with methanol

Intermolecular hydrogen bonding competes with an intramolecular hydrogen bond when methanol binds to an alpha-hydroxyester. Disruption of the intramolecular OH...O=C contact in favour of a cooperative OH...OH...O=C sequence is evidenced by FTIR spectroscopy for the addition of methanol to the esters methyl glycolate, methyl lactate and methyl alpha-hydroxyisobutyrate in seeded supersonic jet expansions. Comparison of the OH stretching modes with quantum-chemical harmonic frequency calculations and 18O labelling of methanol unambiguously prove the insertion of methanol into the intramolecular hydrogen bond. This is in marked contrast to UV/IR hole burning studies of the homologous system methyl lactate: (+/-)-2-naphthyl-1-ethanol, where only addition complexes were found and the intramolecular hydrogen bond was conserved. This switch in hydrogen bond pattern from aliphatic to aromatic heterodimers is thought to reflect not only a kinetic propensity but also a thermodynamic preference for addition complexes when dispersion forces become more important in aromatic systems.