Picosecond Photon Echo Research Articles

Infrared photon echo experiments on small molecules isolated in condensed phase

The dephasing times of CO and DCl isolated in van der Waals solids have been obtained by means of infrared picosecond photon echo experiments using the Orsay Free Electron Laser (CLIO). For CO in argon matrices, even at low temperature, an homogeneous behaviour has been observed. When trapping DCl in nitrogen matrices, isolated molecules as well as dimers are studied, allowing to probe different interaction strengths (van der Waals for the monomer and weak hydrogen bond for the dimer). We observed three different time-resolved structures corresponding to the excitation of the monomer and of the two stretching modes of the dimer.

Line broadening mechanisms in spectra of organic amorphous solids: photon echo study of terrylene in polyisobutylene at subkelvin temperatures

We have developed an experimental setup with a 3He bath cryostat for picosecond photon echo (PE) measurements in a broad range of temperatures. In this Letter, a two-pulse PE investigation of the amorphous polymer polyisobutylene (PIB) doped with tetra-tert-butylterrylene (TBT) in the temperature region 0.4–4 K is presented. The results are compared with single-molecule spectroscopy (SMS) data for the same system. The PE yields a linewidth which is close to the average of the SMS linewidth distribution. We conclude that a dispersion of the dephasing time T 2 contributes significantly to the linewidth distribution as measured by SMS. Furthermore, the temperature dependence of the linewidth is analyzed.

Nanosecond spectral diffusion and optical dephasing in organic glasses over a wide temperature range: incoherent photon echo study of resorufin in D- and D 6-ethanol

Incoherent photon echoes (IPE) decay curve of two differently deuterated ethanol glasses: resorufin in d-ethanol (r/d-e) and resorufin in d 6-ethanol (r/d 6-e) have been measured in wide temperature interval (1.7–35 K). At low temperatures the difference in decay times between results of IPE and two-pulse picosecond photon echoes (2PE) were observed and attributed to nanosecond spectral diffusion (SD). The T-dependence of the SD contribution in IPE data was defined and analyzed using Bai-Fayer theory. IPE high temperature results have shown that high-temperature dephasing in r/d-e is caused mainly by interaction with single low frequency mode (LFM). LFM dephasing was analyzed using Osad'ko nonperturbative dynamical theory. The difference in temperature dependencies of dephasing time between r/d-e and r/d 6-e was observed: the deuteration influenced both on LFM and two-level systems (TLS) contribution to dephasing.

Ultrafast dephasing of Resorufin in d-ethanol glass from 17 to 40 K studied by incoherent photon echo

The temperature dependence of the electronic dephasing of Resorufin in d-ethanol glass was measured from 1.7 to 40 K by incoherent photon echo (IPE) techniques. At T = 8–11 K good agreement between IPE and two-pulse picosecond photon echo (PPE) results was observed. For T 11 K confirm the hypothesis that high-temperature dephasing in this system is due mainly to interaction with a pseudolocal mode.

Dynamics in a low-temperature glass: Fast generation and detection of optical holes

High-resolution, fast optical hole-burning results are reported for the amorphous system cresyl violet in ethanol glass at 1.3 K. Holes are burned and detected using a novel technique which allows precise detection of narrow (∼0.03 cm−1 ), shallow (∼1%) holes 10 μs to 50 ms after their generation. The technique is described in detail along with careful tests demonstrating the validity of its results. The hole width is observed to increase linearly with time when plotted against log time. Using the four time correlation function description of optical hole burning, the time-dependent increase in hole width (spectral diffusion) is shown to arise from a broad distribution of fluctuation rates in the glass with the probability of having a fluctuation at rate R proportional to 1/R. The 10 μs to 50 ms data is combined with hole-width data spanning the range 100 ms to 10 000 s and with two-pulse picosecond photon echo data. The two-pulse photon echo linewidth is calculated by extrapolating the fluctuation rate distribution obtained from the hole-width data to short times. The results are in excellent agreement with experimental echo results. The combined data from the two sets of hole-burning experiments provides a detailed description of the glass dynamics over nine decades of time, 10 000 s to 10 μs. Together with the two-pulse photon echo results, the data provide information on the glass dynamical behavior over seven decades faster in time as well. The net result is a description of the dynamics in low-temperature ethanol glass on time scales spanning 16 decades.

Picosecond Photon Echo and Coherent Raman Measurements on Molecular Solids in a High Pressure Diamond Anvil Cell

Abstract The centrosymmetric molecule, dianthracene, has four low-lying excited singlet states arising due to interactions between the four equivalent benzene chromophores in the molecule. An empirical model identifies the lowest excited state as 1B3g with the other three states at increasingly higher energies being 1B1u, 1B2u, and 1Ag. The energy gap between the low temperature emission and fluorescence excitation spectra of crystalline dianthracene as well as the shape of the origin absorption spectrum and the lack of site-selected spectra in this region for dianthracene in a low temperature polymer host provide strong support for this predicted state ordering. The temperature dependence of the quantum yield for photodissociation of the dianthracene molecule and the emission spectrum of the anthracene pairs formed by this reaction are also presented.

Picosecond Photon Echo and Coherent Raman Measurements on Molecular Solids in a High Pressure Diamond Anvil Cell

Abstract We report the first picosecond photon echo measurements on a molecular solid in a high pressure diamond anvil cell at low temperature. We also present time-resolved coherent Raman measurements of vibrational relaxation in pure molecular crystals as a function of pressure in a diamond anvil cell at low temperature. Since photon echo spectroscopy eliminates inhomogeneous dephasing, we utilize this technique to yield the homogeneous dephasing rate for high pressure samples where sample inhomogeneities and inhomogeneous pressure gradients may exist. Since the Fourier transform of the photon echo decay yields the absorption lineshape, these experiments provide a means of obtaining high resolution spectra under extreme high pressure conditions. In addition, pressure induced changes in vibrational relaxation rates observed by picosecond coherent Raman measurements are used as a sensitive probe of the intermolecular interactions that give rise to relaxation phenomena.

Picosecond photon echo experiments using a superconducting accelerator-pumped free-electron laser

A superconducting accelerator-pumped free-electron laser (SCA/FEL) is used to perform photon echo experiments on a near-IR dye molecule in a polymeric glass. The SCA/FEL produces Fourier-transform-limited picosecond pulses at a repetition rate of 11.8 MHz. Its wavelength is continuously tunable. The experiments are conducted at 0.776 μm by doubling the output of the SCA/FEL tuned to 1.55 μm. At this wavelength, the SCA/FEL is operated in a burst mode. The duration of the bursts (macropulses) is 2 ms with a repetition rate of 10 Hz. Each individual pulse (micropulse) has a length of 3.2 ps and is approximately 1 μJ in energy. In the experiment a number of single micropulses are selected from each macropulse. The doubling, stabilization, synchronization, single-pulse selection, and detection electronics are described. These are the first photon echo experiments and, to our knowledge, the first optical coherence experiments performed using a free-electron laser as a source.

Optical dephasing of a near infrared dye in PMMA: photon echoes using the superconducting accelerator pumped free electron laser

A superconducting accelerator pumped free electron laser is used to perform picosecond photon echo experiments on the near infrared dye 1,1′,3,3,3′,3′-hexamethylindotricarbocyanine iodide (HITCI) in poly(methyl methacrylate) (PMMA). The temperature dependence of the optical dephasing is measured from 1.5 to 10 K. At the lowest temperatures a temperature dependence, T−1.4, characteristic of two-level system glass dynamics is observed. Above 5 K, the optical dephasing is exponentially activated with an activation energy of 15 cm−1. This is the same activation energy reported for rhodamine B in PMMA, demonstrating that 15 cm−1 corresponds to an intrinsic glass mode. These are the first photon echo experiments, and to our knowledge, the first nonlinear optical coherence experiments, performed using a FEL as a source.

High temperature optical dephasing mechanism for dye molecules in PMMA glass

Picosecond photon echo experiments on rhodamine B and octadecyl rhodamine B in a PMMA matrix between 1.0 and 12.5 K are compared. The two dyes have identical first electronic excited state transitions but different masses and moments of inertia. Echo decay times are identical and individual decays at the same temperature from the two systems are superimposable. The high-temperature echo data can be fit to an activated process with an energy of 15 cm −1. We conclude that in this glassy matrix, high-temperature dephasing is caused by a mode of the host and not a mode of the guest as has been suggested for glasses and is common in crystals.

Solvation shell effects and spectral diffusion: Photon echo and optical hole burning experiments on ionic dyes in ethanol glass

Results of picosecond photon echo and optical hole burning experiments are reported for four ionic dyes in ethanol glass. At low temperatures, the dephasing times deduced from the hole widths are as much as nine times shorter than those measured by the two-pulse echo because of the effect of spectral diffusion. The temperature dependences found are of the form aTα+b exp (−ΔE/kT) due to glass two level system dynamics (T<4 K) and a process that activates exponentially at higher temperatures, possibly from a pseudolocal mode or glass optical phonon. Comparing the ratios of echo to hole burning measured dephasing times for the four dyes suggests that the dephasing is influenced by the existence of distinct local ethanol solvation shells in addition to the dynamics of the bulk solvent. A theoretical description of solvent shell effects is achieved through the use of a two spatial domain model of the glass dynamics. Calculations of dynamic perturbations from distinct solvation shell and bulk solvent regions show that the observed differences between the dyes’ dephasing ratios can be explained if the ionic chromophores alter glass dynamics locally.

Photon-echo detection by six-wave mixing

Picosecond photon echoes have been generated and detected by using a resonant six-wave mixing effect. In this process, the induced echo polarization is used to drive a four-wave mixing process. The merits of this method compared with other techniques of photon-echo detection are discussed. Further examples of spectroscopically useful six-wave mixing effects are presented.

Picosecond photon echo and optical hole burning studies of chromophores in organic glasses

The range of relaxation rates present in amorphous systems is shown to affect the results of optical dephasing measurements. The interpretation of these experiments in terms of four-time correlation functions is introduced. Photon echo and hole burning results are shown for resorufin in three glass hosts: ethanol, d-ethanol, and glycerol. The hole burning data are shown to be affected by slow relaxation processes whereas the echo results are not. The implications of a slowest relaxation rate in amorphous systems are also considered.

Homogeneous optical dephasing and line broadening processes in an organic glass: comparison of the temperature dependences of picosecond photon echo and hole burning experiments

The homogeneous optical dephasing times ( T 2) and the non-photochemical hole burning (NPHB) linewidths of resorufin in glycerol glass are reported from 1.1 to 25.5 K. At low temperature, the linewidths obtained from NPHB are substantially wider than the linewidths (1/π T 2) obtained from photon echo measurements, but the two become equal at high temperatures. The extra broadening of the NPHB is found to increase as T 0.8. A model of homogeneous dephasing including both two-level system and pseudolocal mode contributions is successful over the entire temperature range.

Probing intermolecular interactions with picosecond photon echo experiments

Probing intermolecular interactions with picosecond photon echo experiments

Ultrafast optical dephasing in a low-temperature organic glass

The optical dephasing of pentacene in an o-terphenyl glass at 1.5 K is examined with picosecond photon echo and fluorescence line narrowing experiments. The homogeneous T 2 is found to be 1–3 ps, in contrast to all previous measurements on glass—chromophore systems in which the homogeneous T 2are two to three orders of magnitude longer.

A picosecond photon echo study of a chromophore in an organic glass: Temperature dependence and comparison to nonphotochemical hole burning

The first two-pulse photon echo experiments on a chromophore in an organic glass are reported. The homogeneous electronic dephasing of resorufin in ethanol glass is measured from 1.5–11.4 K. The temperature dependence of the dephasing time does not fit the power law frequently predicted by theory for the dephasing characteristic of glasses. However, the temperature dependence can be accounted for by including dephasing from librations or acoustic phonons, mechanisms known to be important in crystals. The dephasing decay is found to be a single exponential for over six factors of e. The dephasing is also shown to be uncorrelated with the extent of nonphotochemical hole burning (NPHB). However, the homogeneous linewidth deduced from the photon echo is four times narrower than the linewidth obtained from NPHB, demonstrating that the hole is broadened by additional processes.

Delocalization of electronic excitations in disordered molecular systems

Picosecond photon echo experiments are used to examine the nature of the electronic states of interacting molecules in the mixed molecular crystal system, pentacene in p-terphenyl at 1.5 K. The onset of delocalization of the pentacene first singlet electronic excited state with increasing pentacene concentration is manifest in very fast photon echo decays. The concentration dependence of the echo decay rates accords with theoretical estimates.

Optical dephasing of chromophores in an organic glass: picosecond photon echo and hole burning experiments

The homogeneous optical dephasing time T2 for the organic glass system resorufin in ethanol is measured by picosecond two-pulse photon echoes from 1.5 to 11.4 K. A non-power law temperature dependence of T2 is measured. Non-photochemical hole burning (NPHB) linewidths are four times broader than the homogeneous linewidth, demonstrating that additional factors contribute to NPHB.

Optical dephasing in organic amorphous systems. A photon echo and hole-burning study of pentacene in polymethylmethacrylate

Picosecond photon echo experiments on pentacene in polymethylmethacrylate (PMMA) show that the homogeneous width exhibits a T1.3 temperature dependence. This behavior is ascribed to coupling of the pentacene transition to randomly distributed low-frequency excitations in the amorphous host. A calculation of the resulting homogeneous width is presented which uses optical Redfield theory as a starting point. From the results of this calculation we conclude that the coupling between the pentacene molecule and the host low frequency excitations is of static dipolar nature and that the density of states of these excitations varies as ω0.3. Results of nonphotochemical hole-burning experiments on the same system are also reported. Comparison of these results with the ones obtained by the photon echo method indicates, that in the hole-burning experiments, the hole width and its temperature-induced broadening are dominated by spectral diffusion.