V. Palm

On the role of spectral diffusion in single-molecule spectroscopy

This paper reports on measurements of fluorescence excitation spectra of single impurity molecules of terrylene in n-decane at T=1.7 K. Spectra measured within the same spectral interval but at consecutive instants of time exhibit zero-phonon (ZP) lines of single impurity molecules of several species, differing in the behavior of the line shape and frequency in time. On the one hand, one observes stable ZP lines, well approximated with a Lorentzian. On the other, one sees spectral features with a profile varying from one spectrum to another, with only individual fragments of such a profile allowing the Lorentzian approximation; such features are interpreted assuming the presence of unstable impurity molecules, the ZP lines of which display small (a few tens of MHz) spectral jumps with a time interval of about 10 s. Such molecules exhibit a substantial decrease in the spectral jump frequency within a measurement period of the order of 5000 s, which is attributed to a decrease in the contribution due to spectral diffusion resulting from sample structure relaxation.

Terrylene-doped biphenyl monocrystals for optical single-molecule spectroscopy

Biphenyl forms at normal pressure and temperatures below 40 K incommensurate crystalline structures with physical properties varying in space on a scale of nanometers. As biphenyl crystals are optically transparent, there is a possibility to study these natural nanostructures optically, by doping the host crystal with nanoscopic probes with optical properties depending on their local environment. It has already been demonstrated that terrylene impurity molecules in polycrystalline biphenyl sample can successfully play the role of such kind of sensitive nanoprobes when studied by the methods of high-resolution laser spectroscopy. We report growing of thin biphenyl monocrystals doped with terrylene molecules at very low concentrations. These sublimation-grown flakes can be studied at liquid helium temperatures using the technique of single-molecule spectroscopy. Compared to polycrystalline biphenyl samples, much higher signals and better signal-to-noise ratio can be achieved in single-molecule spectra. This allows to perform much faster spectral scans to find intensive single-molecule lines even in very dilute samples in spite of a very broad inhomogeneous absorption band of terrylene in biphenyl. Fast scanning also allows observation of single-molecule lines with much better temporal resolution, revealing processes of spectral diffusion occurring at different time scales. This can be helpful in our attempts to learn about the role of the matrix incommensurability in spectral features observed. Extremely high variability of temporal and spectral behaviour of terrylene single-molecule lines is reported, which is unusual for crystalline hosts.

Optical study of terrylene molecules in crystalline biphenyl: effects of pressure and temperature on the luminescence spectra

The photoluminescence spectra of terrylene guest molecules in polycrystalline biphenyl host were measured at various temperatures between 4.7 and 295 K under ambient pressure (1 atm) as well as under various pressures up to 4.4 kbar at 4.7 K and up to 4.9 kbar at 295 K. With increasing pressure, all four spectral peaks studied shift to the red. By elevating the temperature from 4.7 to 295 K at 1 atm, the most intense peak at 17,220 cm−1 first shifts slightly to the red, but at ∼40 K begins to shift to the blue. Comparison of the temperature and pressure shifts for this peak reveals that its temperature shift is mainly determined by the thermal expansion of the host crystal rather than by the change in the electron–phonon coupling with temperature.

Single-molecule probing of incommensurate biphenyl

Our data on the distribution of purely electronic linewidths of single molecules of terrylene in incommensurate biphenyl crystals are compared with the data obtained by other groups for different low-temperature organic solid hosts and with results of numerical simulations. The first two moments of the distributions measured within a narrow temperature interval have been used to calculate a single dimensionless parameter characterizing each of the respective hosts—the variation coefficient. It appears that different amorphous hosts have similar values of this coefficient, but the value obtained for the incommensurate crystal of biphenyl is significantly different. One can conclude that the remarkable single-molecule line broadening in biphenyl at 1.8K cannot be solely explained by the interaction with two-level systems, which is considered to cause the broadening in amorphous hosts.

Some Fundamental Criteria of the Scientific Method and the Internal Structure of Science

The author’s intention is to represent a brief review of the mainly normative methodological criteria, which enable one to distinguish the scientific approach from the non-scientific or arbitrary one. These normative criteria are applicable, if the completed text pretending to be a scientific one is available. They are indifferent in respect of the way such text has been created. And they are not directly usable for the evaluation of the degree of originality of the data and conceptions represented in such text. Therefore a plain use of these rules can not substitute an evaluation by the competent expert, so far as the positive aspect of this activity is considered. But, in many cases they enable the detection of the non-or antiscientific approach without need for going deep into details. And they may foster the critical attitude of the investigators to their own work.

Excitation of surface plasmons in Al-coated SNOM tips

The mesoscopic effect of spectral modulation occurring due to the interference of two photonic fiber modes filtered out by a metal-coated SNOM tip is used to observe the surface plasmon polariton (SPP) excitation in SNOM tips. In a spectrum of the broadband light transmitted by a SNOM tip a region of highly regular spectral modulation can be found, indicating the spectral interval in which only two photonic modes (apparently HE11 and TM01) are transmitted with significant and comparable amplitudes. The modulation period yields the value of optical path difference (OPD) for this pair of modes. Due to the multimode fiber’s inherent modal dispersion, this OPD value depends linearly on the fiber tail length l. An additional contribution to OPD can be generated in a metal-coated SNOM tip due to a mode-dependent photon-plasmon coupling strength resulting in generation of SPPs with different propagation velocities. For an Al-coated 200 nm SNOM tip spectra of transmitted light have been registered for ten different l values. An extrapolation of the linear OPD (l) dependence to l=0 yields a significant residual OPD value, indicating according to our theoretical considerations a mode-selective SPP excitation in the metal-coated tip. The modal dispersion is shown to switch its sign in the SNOM tip. First results of analogous experiments with an Al-coated 150 nm SNOM tip confirm our conclusions.

High pressure narrowing of zero-phonon lines in polymer glasses at different temperatures

Abstract Pressure-induced narrowing of zero-phonon lines, stronger at higher temperatures, established for chlorin molecules in a glassy polystyrene by using the spectral hole burning technique, is explained theoretically by pressure dependence of pseudolocal vibrations and two-level systems.

Observation of Surface Plasmons in Metal-Coated Tapered Fiber Terminated by a Subwavelength Aperture

The effect of a tapered Al-coated optical fiber terminated by a subwavelength aperture (SWA) on the spectrum of the transmitted light is investigated experimentally. Under certain conditions a remarkable spectral modulation of the transmitted light can be observed [1]. This effect is of a mesoscopic origin, occurring only for a certain interval of SWA diameters. A noticeable modulation appears when the number of the transmitted fiber modes is small but exceeds unity, thus indicating the presence of a phase shift between different modes.

Mesoscopic Spectral Modulation of Light Transmitted by a Subwavelength Aperture

We are currently studying the transmission of light through a tapered metal-coated optical fiber with a subwavelength aperture (SWA). The problem under investigation is the effect of SWA on the spectrum of the transmitted light. According to our experimental findings, one can observe, under certain conditions, a remarkable modulation of the spectrum of the transmitted light. The effect has a mesoscopic origin: the modulation takes place if the number of transmitted light modes is small but exceeds unity, which indicates the phase shifts between different modes. One possible source of such phase shift could be the different propagation speed for different modes in the fiber, but this effect should be small. In our opinion, the origin of the phase shifts is in the (different for different modes) slowdown of the light near the tip with SWA due to the interaction of propagating modes with surface plasmons of the metal coating of the fiber. One can expect that the interaction strength depend on the actual shape of the light field in the mode, which results in different modes getting different delays before passing through the tip. In case of sufficiently small SWA diameter only few modes can pass through the tapered fiber region [1], and their delay differences can cause an observable modulation of the transmitted light spectrum. In case of larger diameters many light modes can pass out, and no significant spectral modulation can be observed due to the effect of averaging. An observable modulation also disappears for SWA diameters as small as 100 nm, because in this case only one (the fundamental) light mode passes out [1].