Nonlinear Spectroscopy on a Single Quantum System: Two-Photon Absorption of a Single Molecule

Abstract

was approximately 1%; this yield U~S P. Wild?included the solid angle of collection, optical losses, and the quantum efficiency of the photomultiplier (15). This A,,, was several Two-photon fluorescence excitation spectra of single diphenyloctatetraene molecules times higher than under one-photon excitrapped in an n-tetradecane matrix were measured at cryogenic temperatures. The purely tation, because the laser wavelength is electronic zero-phonon line (transition at 444 nanometers) of these single molecules with twice the transition wavelength and the a width of about 60 megahertz was excited by a continuous-wave, single-mode laser at whole luminescence spectrum of DPOT in-888 nanometers. Even though the two-photon absorption cross section is extremely cluding the strongest 0-0 line can be colsmall, a high photon count rate and low background allowed nonlinear spectroscopy to lected without interference from stray light. be extended to the single-molecule level. This experiment also suggests the possibilityThe fluorescence excitation spectra were of two-photon single-molecule scanning microscopy.recorded as a function of the laser wavelength. All measurements were performed in supetfluid He at 1.7 K.A two-photon excitation spectrum of Optical experiments with single quantum minescence spectra of DPOT in TD (Fig. the inhomogeneous band (Fig. 3) had a systems (SQSs) have been steadily progress-I), where a small peak at 444 nm corre-maximum intensity at about 888 nm and aing. Single atoms in a beam, single ions in a sponding to the one-photon transition to S1 linewidth of about 120 GHz. The average Paul trap (I), single molecules on a surface is observed. In a solid matrix, the observed intensity of the band was proportional to (2), and single atoms in a magnetooptical lifetime T, of the S, state is about 6 ns, the square of the laser intensity and was trap (3) are promising systems that can be whereas the radiative lifetime Tmd is 66 ns roughly proportional to the concentration used to test quantum physics. Since 1989, (13). The blue luminescence ( Fig. 1) was of DPOT at room temperature (16). The optical spectroscopy of single molecules in used in this work for SM detection. peak position is in agreement with the solids (4, 5) has been achieved by several We prepared the sample by dissolving a value observed in the low-resolution specgroups (6).small amount of solid 1,8-diphenyl-1,3,5,7-tra (Fig. I), and the band shape is domi-In all of these previous experiments, the octatetraene (Aldrich) in TD (Fluka) at a nated by fluctuations called statistical fine SQS absorbed only one photon to undergo concentration of 2.5 x M at room structure (SFS) (17). This SFS is caused a transition from its ground state to the temperature. The solution was placed be-by fluctuations in the number of molecules excited state, that is, the interaction of the tween two microscope cover-glass plates and per frequency interval in the excited vol-SQS and the electromagnetic wave was lin-was rapidly cooled to liquid He temperature ume and i...

Light Induced Single Molecule Frequency Shift

“…For DPOT in TD, C FC 0.04 [5]. For two neighboring molecular states having identical symmetry, one can assume ͗ejr 2 je͘ ഠ ͗gjr 2 jg͘ ഠ ͗ejr 2 jg͘ and estimate the line shift as jDnj , ͓R det G͑͞A tot hC FC ͔͒ 1͞2 ഠ 50 MHz, where G ഠ 100 MHz, R det ഠ 1 kHz is the photomultiplier count rate (at 300 mW laser power), A tot ഠ 0.01 the total collection efficiency, h ഠ 0.1 the luminescence quantum yield of DPOT [1]. The resulting maximal coefficient of 165 MHz͞W is 3 times smaller than in the case when the electric field vector is parallel to the z axis indicating a tilt of the axis with respect to the polarization plane of the laser beam.…”

“…In contrast to the line frequencies, the linewidths were almost insensitive to the laser power though they were 2-3 times larger than under one-photon excitation [5]. A small broadening was observed at powers above 250 mW, but this can be explained by saturation due to the triplet bottleneck [1]. To exclude saturation broadening only linewidths measured at powers below 250 mW have been taken into account.…”

“…Two-photon single molecule spectroscopy (SMS) in solids [1], a further development of the SMS technique providing observations of very fine effects without ensemble averaging [2], opened new possibilities for high resolution spectroscopy on a single quantum system (SQS). Precise control of interactions between a SQS and external fields is required for many potential applications of SQSs, e.g., quantum computation [3].…”

“…The experimental setup has already been described [1]. The sample was immersed in a superfluid He bath.…”

“…The purely electronic 1 1 A g ! 2 1 A g zero-phonon line of DPOT at 444 nm was excited by two photons with a wavelength at 888 nm when laser light with a power of a few hundred mW was focused onto a spot of 2 mm diameter. The luminescence emitted by DPOT was recorded as a function of the laser frequency detuning.…”