Kinetic Hole Burning Research Articles

Microscopic modeling of transverse mode instabilities in mode-locked vertical external-cavity surface-emitting lasers

The generation and control of higher order transverse modes within a mode-locked vertical external-cavity surface-emitting laser with a semiconductor saturable absorber mirror are studied using a numerical solver for the two dimensional Maxwell Semiconductor Bloch Equations. In this work, the complex spatiotemporal evolution of the pulse toward a mode-locked state depends sensitively on the pumping level and pump to pulse spot size ratio. Microscopic physics sourced effects such as kinetic hole burning and filling, occurring on femtosecond timescales, play a central role. In particular, unsaturated charge carriers, both spectrally and transversally, initiate the development of asymmetric pulse profiles that transform over various characteristic time scales in a carrier chasing behavior.

Multipulse instabilities of a femtosecond SESAM-modelocked VECSEL.

Optically pumped passively modelocked vertical external-cavity surface-emitting lasers (VECSELs) can generate pulses as short as 100 fs with an intracavity semiconductor saturable absorber mirror (SESAM). Very stable soliton modelocking can be obtained, however, the high-Q-cavity, the short gain lifetime, and the kinetic-hole burning can also support rather complex multipulse instabilities which we analyze in more details here. This onset of multipulse operation limits the maximum average output power with fundamental modelocking and occurs at the roll-over of the cavity round trip reflectivity. Unfortunately, such multipulse operation sometimes can mimic stable modelocking when only limited diagnostics are available.

Ultrafast nonequilibrium carrier dynamics in semiconductor laser mode locking

Semiconductor disk lasers have been shown to be ideal as wavelength-agile, high-brightness sources for producing high average power under various pulsed mode-locking scenarios. Systematic microscopic modeling reveals that ultrafast nonequilibrium kinetic hole burning in electron/hole carrier distributions dictates the outcome of femtosecond duration mode-locked pulse formation. The existence of a large reservoir of unsaturated carriers within the inverted distributions leads to the emergence of multiple pulse waveforms (not necessarily harmonically mode-locked pulse trains) that inefficiently draw on these carrier reservoirs. The concept of gain is no longer meaningful in this limit, and the dynamical inversion of electrons and holes primarily in the active medium establishes the final dynamical state of the system. The simulation results explain much of the generic behavior observed in key recent experiments and point to the difficulty of pushing semiconductor mode-locked lasers to pulse durations below 100 fs.

Effects of the Distributions of Energy or Charge Transfer Rates on Spectral Hole Burning in Pigment–Protein Complexes at Low Temperatures

Effects of the distributions of excitation energy transfer (EET) rates (homogeneous line widths) on the nonphotochemical (resonant) spectral hole burning (SHB) processes in photosynthetic chlorophyll-protein complexes (reaction center [RC] and CP43 antenna of Photosystem II from spinach) are considered. It is demonstrated that inclusion of such a distribution results in somewhat more dispersive hole burning kinetics. More importantly, however, inclusion of the EET rate distributions strongly affects the dependence of the hole width on the fractional hole depth. Different types of line width distributions have been explored, including those resulting from Förster type EET between weakly interacting pigments as well as Gaussian ones, which may be a reasonable approximation for those resulting, for instance, from so-called extended Förster models. For Gaussian line width distributions, it is possible to determine the parameters of both line width and tunneling parameter distributions from SHB data without a priori knowledge of any of them. Concerning more realistic asymmetric distributions, we demonstrate, using the simple example of CP43 antenna, that one can use SHB modeling to estimate electrostatic couplings between pigments and support or exclude assignment of certain pigment(s) to a particular state.

Microscopic simulation of nonequilibrium features in quantum-well pumped semiconductor disk lasers

A microscopically motivated nonequilibrium theory is applied to study the power characteristics of an in-well pumped vertical external cavity surface emitting Laser for varying pump energies. Dynamic simulations yield steady state nonequilibrium carrier distributions resulting in gain reduction due to kinetic hole burning. Pauli blocking effects become prominent for more resonant pumping and increased pump powers. The reduced pump absorption results in a sublinear input-output power characteristics even for the optimized case where heating of the active mirror plays no role.

Application of the Broad Hole Burning Technique for Studying Low-Temperature Photoreactions in Impurity Amorphous Bodies

A new experimental method is developed for measuring kinetics of low-temperature photoreactions—the broad hole burning technique, which eliminates the nonexponentiality of the hole burning kinetics caused by the inhomogeneous broadening. The method is successfully used to study the low-temperature photoreactions in various impurity amorphous systems. Examples of single-and two-exponential kinetics have been found, as well as kinetics with a moderate or large dispersion of the hole burning quantum efficiency. The results obtained show a good outlook for the broad hole burning technique for studying low-temperature photoreactions and illustrate the diversity of photoreactions occurring in specific chromophore-matrix pairs.

Different relaxations in myoglobin after photolysis.

To clarify the interplay of kinetic hole-burning (KHB), structural relaxation, and ligand migration in myoglobin (Mb), we measured time-resolved absorption spectra in the Soret region after photolysis of carbon monoxide Mb (MbCO) in the temperature interval 120-260 K and in the time window 350 ns to 200 ms. The spectral contributions of both photolyzed (Mb*) and liganded Mb (MbCO) have been analyzed by taking into account homogeneous bandwidth, coupling to vibrational modes, and static conformational heterogeneity. We succeeded in separating the "time-dependent" spectral changes, and this work provides possibilities to identify the events in the process of ligand rebinding. KHB is dominant at T <190 K in both the Mb* and the MbCO components. For MbCO, conformational substates interconversion at higher temperatures tends to average out the KHB effect. At 230-260 K, whereas almost no shift is observed in the MbCO spectrum, a shift of the order of approximately 80 cm(-1) is observed in Mb*. We attribute this shift to protein relaxation coupled to ligand migration. The time dependence of the Mb* spectral shift is interpreted with a model that enables us to calculate the highly nonexponential relaxation kinetics. Fits of stretched exponentials to this kinetics yield Kohlrausch parameter values of 0.25, confirming the analogy between proteins and glasses.

The influence of orientational effects on the dispersion of the kinetics of hole burning in impurity centers

The dynamics of laser hole burning in a three-dimensional system of fixed isotropically oriented oscillators is simulated. The kinetic curves obtained are analyzed using methods of solving the inverse problem. The contribution of orientational effects to the total dispersion is studied and a quantitative characteristic of this contribution in the form of the standard deviation is obtained. The influence of the broadening of impurity centers on the dispersion of the burning kinetics of persistent and dynamical holes is studied. It is shown how some experimental instrumental factors can distort the results of the analysis of kinetic curves. On the basis of spectral and fluorescence experimental kinetic data, a method for determining the rate constants of the hole burning is proposed.

Quaternary structure dependence of kinetic hole burning and conformational substates interconversion in hemoglobin.

Using a sol-gel encapsulation technique, we have prepared samples of CO saturated human adult hemoglobin locked in the R or T quaternary conformation. We report time-resolved spectra of these samples in the Soret region following flash photolysis, in the time interval ranging from 250 ns to 200 ms and in the temperature interval of 100-170 K. A suitable analysis of the measured difference spectra enables us to obtain the spectral contribution of deoxyHb and HbCO molecules as a function of time and/or of the fraction N(t) of deoxyHb molecules. In our experimental time window geminate CO rebinding to hemoglobin in the T quaternary conformation is about 2 orders of magnitude slower than to hemoglobin in the R conformation: this suggests that the barrier distribution for the CO rebinding, g(H), depends strongly on the protein quaternary structure. In our temperature interval, spectral shifts due to kinetic hole burning (KHB) are present: for HbCO the KHB effect is large in the R conformation and small in the T conformation. For deoxyHb the opposite is true. We attribute the observed behavior to the effect of interconversion between the relevant substates. This effect is stronger for HbCO molecules in the T conformation and for deoxyHb molecules in the R conformation; it confirms the quaternary structure dependence of the hemoglobin energy landscape and suggests enhanced dynamics of ligation intermediate species such as T-state HbCO or R-state deoxyHb.

Broadband hole burning: a new technique for the measurement of hole burning kinetics, removing the influence of inhomogeneous broadening

Abstract One of the problems associated with measurement of hole burning (HB) kinetics in disordered media is their non-exponential time dependence. There are two sources of non-exponentiality inherent to all glassy systems: random orientation of the chromophores and strong inhomogeneous broadening of their optical spectra. The non-exponentiality caused by the above factors conceals other, more interesting sources of dispersive burning kinetics such as dispersion of burning quantum efficiency and distribution of homogeneous line widths. A modification of the HB method, the broadband hole burning (BHB), is proposed, which eliminates the strongest source of non-exponentiality—the influence of inhomogeneous broadening. The new technique was applied to measurements of BHB kinetics on a simple, well-characterized system: H 2 -tetraphenylporphine (TPP) in polystyrene (PS). Only photochemical HB (i.e., proton tautomerization) with negligible dispersion of burning efficiency was expected in this sample. The BHB technique revealed the existence of a second, previously unknown photoreaction, with quantum efficiency about one order of magnitude lower than proton tautomerization. The ordinary HB technique does not detect this photoreaction because its manifestation in burning kinetics is smeared out by the above-mentioned sources of non-exponentiality.

Effect of Al on the spectral and spectral-hole-burning properties of sodiumborate glasses doped with Sm 2+

Photoluminescence and absorption measurements of Sm 2+-doped aluminoborate glasses are reported. The effect of Al on the valence state of samarium and on its photoluminescence properties has been studied. Special attention was paid to the inhomogeneous widths of the 7F 0– 5D 0 transitions. Persistent spectral hole burning was performed at different temperatures to study the variation of homogeneous linewidths and hole-burning kinetics.

Spectral hole burning in naphthalocyanines derivatives in the region 800 nm for holographic storage applications

Persistent spectral hole burning is studied for several free-based and metallo-naphthalocyanine derivatives in polymer hosts. These materials exhibit a strong 0–0 absorption band in the region 800 nm matching the wavelength range of most semiconductor diode lasers and Ti : Sapphire lasers. Metallo-naphthalocyanines demonstrate a nonphotochemical hole-burning mechanism that is likely related to rotations of small molecular groups attached to a relatively rigid molecular ring. Free-base molecules exhibit the regular proton phototautomerisation mechanism of hole burning. Spectral- and hole-burning parameters were determined for eight materials; in particular, the hole-burning kinetics was analyzed and the quantum efficiencies were determined to be between 0.1% and 1%. Holograms (data pages) in the transmission geometry were successfully recorded in the materials studied using single-frequency laser diodes.

Determination of quantum efficiency of persistent spectral hole burning using dispersive kinetics

A practical approach for the determination of the quantum efficiency of persistent spectral hole burning (PSHB) is suggested. The quantum efficiency is found from the experimental kinetics of hole formation. The method takes into account all primary mechanisms of dispersion for the hole burning kinetics and can be applied to the characterization and the comparison of materials with different mechanisms of hole burning. The quantum efficiency of PSHB has been found for free-base and zinc naphthalocyanines, which have photochemical and photophysical mechanisms of PSHB, respectively. These materials are useful for PSHB applications in the spectral range around 800 nm.

Nonphotochemical hole burning in Zn-TBP/PMMA. Unusual burning kinetics and very narrow holewidth limit

Nonphotochemical hole burning (NPHB) kinetics was investigated on PMMA samples doped with Zn-TBP molecules. The combination of a very low triplet state saturation threshold of Zn-TBP and a large dispersion of NPHB quantum efficiencies produces very specific hole burning kinetics. The most interesting effects are: (a) very high sensitivity of hole width and burning quantum efficiency to burning power; (b) narrow persistent hole width limit as compared to transient hole burning and 3-pulse photon echo data. The experimental results are interpreted in terms of a kinetic hole burning model which includes a broad distribution of the NPHB quantum efficiencies, the triplet state saturation effect, and the influence of spectral diffusion.

Structural factors controlling ligand binding to myoglobin: a kinetic hole-burning study.

Using temperature-derivative spectroscopy in the temperature range below 100 K, we have studied the dependence of the Soret band on the recombination barrier in sperm whale carbonmonoxy myoglobin (MbCO) after photodissociation at 12 K. The spectra were separated into contributions from the photodissociated species, Mb*CO, and CO-bound myoglobin. The line shapes of the Soret bands of both photolyzed and liganded myoglobin were analyzed with a model that takes into account the homogeneous bandwidth, coupling of the electronic transition to vibrational modes, and static conformational heterogeneity. The analysis yields correlations between the activation enthalpy for rebinding and the model parameters that characterize the homogeneous subensembles within the conformationally heterogeneous ensemble. Such couplings between spectral and functional parameters arise when they both originate from a common structural coordinate. This effect is frequently denoted as "kinetic hole burning." The study of these correlations gives direct insights into the structure-function relationship in proteins. On the basis of earlier work that assigned spectral parameters to geometric properties of the heme, the connections with the heme geometry are discussed. We show that two separate structural coordinates influence the Soret line shape, but only one of the two is coupled to the enthalpy barrier for rebinding. We give evidence that this coordinate, contrary to widespread belief, is not the iron displacement from the mean heme plane.

Dynamics of Gain and Stimulated Emission in II-VI Laser Diodes

The dynamics of gain and stimulated emission of II–VI laser diodes is investigated after picosecond optical pumping. For this purpose we use time-resolved variable stripe-length measurements to study the gain dynamics after pulse excitation in waveguide geometry. Features of carrier heating and pronounced kinetic hole burning are observed in the transients of stimulated emission and gain. At high excitation densities spectral hole burning effects are additionally observed. The spectral hole recovers on a time scale of some picoseconds. We give an explanation in terms of a microscopic laser theory which is based on the non-equilibrium Green's function technique.

Excitonic optical nonlinearities and dynamics in II–VI heterostructures and laser diodes

The extremely strong Coulomb correlation in wide-gap II–VI semiconductors results in considerable excitonic contributions to optical nonlinearities still at excitation densities sufficient for stimulated emission and lasing. In pure ZnSe and ZnCdSe QWs, even more complex exciton systems like biexcitons may dominate the emission spectra at moderately high densities. In ternary wells of compositions typical for laser diodes, the exciton-biexciton kinetics is still described without the assumption of strong localization. In highest-quality ZnSe wells with excitonic line widths as small as 0.75 meV and large biexciton binding energies, the nontrivial polarization and intensity dependence of exciton and biexciton contributions to coherent four-wave mixing is for the first time systematically investigated and explained by a microscopic density-matrix model taking into accounthigher-order correlations. Whereas biexcitons have also been proved to give rise to stimulated emission at temperatures ⩽ about 150 K in undoped QW structures, they play no role for stimulated emission in favor of a strongly Coulomb-correlated electron-hole plasma in operational, injection laser diodes. Its dynamical behavior after ps-pulse excitation reveals novel features of kinetic and spectral hole burning and a coupling of the longitudinal ground and higher lateral modes in resonator structures. Eventually, an overall understanding of the lasing properties of II–VI diode structures is nowadays possible.

Spectro-temporal gain dynamics of optically pumped II–VI multiple-quantum-well structures

The real-time resolved stimulated emission and gain dynamics of optically pumped conventional edge-emitting II–VI quantum-well-laser structures are investigated. In the emission as well as in the gain, features of carrier heating and pronounced kinetic hole-burning are observed. It will be shown that these effects develop already just above the lasing threshold intensity. At high pump fluxes, spectra of the emission clearly show that the formation of the kinetic hole is accompanied by the occurrence of a spectral hole in resonatorless samples. This complex spectro-temporal dynamics has never been observed before and points to a more rapid gain depletion at the peak energy compared to the spectral wings of the stimulated emission.

Kinetic hole burning, hole filling, and conformational relaxation in heme proteins: direct evidence for the functional significance of a hierarchy of dynamical processes.

Band III is a disorder and conformation-sensitive near-infrared (approximately 760 nm) charge transfer absorption band characteristic of equilibrium and nonequilibrium five coordinate ferrous high-spin hemes. The time evolution of this absorption band subsequent to photodissociation of six coordinate ferrous hemoglobin or myoglobin can provide detailed information regarding conformational relaxation, including the thermally driven fluctuations that result in the transition from inhomogeneous to homogeneous ligand rebinding kinetic. Such time-resolved measurements over a range of temperatures are difficult due to long sample recovery times at cryogenic temperatures. A new restoring technique that allows for the rapid movement of a large optically accessible cryostat is used in combination with nanosecond time-resolved near-infrared absorption spectroscopy to generate band III as a function of time for the photoproducts of the carbon monoxide derivative of adult human hemoglobin (COHbA) and, to a more limited extent, horse myoglobin (COMb). The measurements are made over a wide range of temperatures extending from well below the solvent (75% glycerol:water) glass transition at approximately 180 K to ambient temperatures. Three temperature- and/or viscosity-dependent phenomena are observed. At the highest temperatures, only conformational relaxation is observed for the 75% glycerol sample. At very high viscosity (> or = 400 cp), conformational relaxation slows dramatically, and both kinetic hole burning followed by the filling in of the "hole" (dynamic hole filling) are observed. As the temperature is lowered, conformational relaxation slows and finally ceases. Kinetic hole burning and dynamic hole filling as well as additional broadening of band III are observed down to 140 K. The observation of kinetic hole burning (KHB) is indicative of the sample being inhomogeneous on the time scale of the ligand rebinding giving rise to KHB. The onset of hole filling is a direct manifestation of the thermal homogenization of the initial inhomogeneous distribution of conformational substates responsible for KHB. The observed dynamics are used to explain the inverse temperature effect associated with the non-Arrhenius slow down of geminate rebinding above approximately 180 K. The inverse temperature effect appears to arise not only from the onset of conformational relaxation but also from the increase in the rate on thermal averaging of the initial inhomogeneous distribution of conformational substates.

Nonphotochemical hole burning in hyperquenched glassy films of water: A pronounced deuteration effect

The effects of deuteration on hole burning of aluminum phthalocyanine tetrasulfonate (APT) in glassy films of water is reported. Deuteration has no effect on the zero phonon hole width of the APT electronic transition, but a large effect on the hole burning kinetics. These effects are discussed in terms of the two level systems of glassy water.