Single molecule spectroscopy in a solid

LUMINESCENCE JOURNAL OF

Journal of Luminescence 53 (1992) 165—169 North-Holland

Single molecule spectroscopy in a solid M. Orrit and J. Bernard Centre de Physique Moléculaire Optique et Hertzienne, CNRS (URA 283) and Unicersité de Bordeaux 1.351, Cours de Ia Liberation, F-33405 Talence Cedex, France

By exciting a very small volume of an inhomogeneously broadened solid solution at a very low concentration of guest molecules, it is possible to spectrally resolve its inhomogeneous fluorescence excitation spectrum into the set of sharp homogeneous peaks of the composing single molecules. Intensity, shape, width and distribution of the peaks are all consistent with their attribution to single molecules, but a definite proof of this is given by the correlation properties of the fluorescence light: due to intersystem crossing, the fluorescence of a single molecule exhibits photon bunching. The study of single molecule peaks as temperature is varied displays the variety of possible molecular microenvironments.

1. Introduction The optical detection of single molecules or chromophores in liquid or solid solutions would open many new possibilities in such diverse fields as nanophysics, molecular engineering and biology. While single molecule detection by optical means remains difficult in general, new experiments at room [1] and helium temperatures [2—4] on model systems have recently demonstrated its feasibility. Several applications to trace detection and sensing, to spectroscopy of localized neighborhoods and to optical addressing of local spots in solids may now be envisaged, In what follows, we discuss the optical spectroscopy of single pentacene molecules in a transparent host crystal at liquid helium temperatures. The absorption spectra of such solid solutions consist of comparatively broad bands resulting from the superposition of the much narrower homogeneous lines of individual molecules. When the number of absorbing molecules within the illuminated volume is reduced, a reproducible absorption “noise” appears, which is caused by Correspondence to: Dr. M. Orrit, Centre de Physique Moléculaire Optique et Hertzienne, CNRS (URA 283) and Université de Bordeaux 1,351, Cours de Ia Liberation, F-33405 Talence Cedex, France. 0022-23t3/92/$05.OO © 1992

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the statistical fluctuations of the number of resonant molecules [51.In the limit of very dilute samples, when either one or no molecule is in resonance with the exciting laser, the homogeneous resonances of single molecules are resolved from each other. Consequently, the absorption spectrum of the sample will display resonance peaks distributed at random within the inhomogeneous profile, each peak corresponding to a single molecule. The main difficulty in isolating the signal of a single molecule arises from its weakness. In the detection scheme proposed here, the molecule’s absorption peak must be identified as a function of the exciting frequency. To be able to discriminate the weak molecular signal against the frequency-independent background from matrix molecules, this signal must be accumulated over comparatively long time intervals; this means that the host—guest couple must present a high stability, in order for the molecule’s resonance to remain constant for times as long as possible. In particular, both hole burning and spectral diffusion processes change the resonance frequency and will therefore limit the useful acquisition time. This limitation must be kept in mind when trying to generalize to other systems the results obtained so far in crystalline matrices.

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For pentacene in p-terphenyl crystal. the matrix is stable enough to allow for long acquisition ttmes In their first singlc moiccule detcction experiment. Moerner and Kador [2] measured directly the absorption signal of a single pentacene molecule. The relative absorption signal was given by the ratio of the molecular absorption cross-section to the beam section. As this ratio lay in the range It) to 10 the photon noise of the necessarily weak prching beam severely limited the signal/ noise ratio, even with the very sensitive differential detection they used. By recording the fluorescence excitation of pentacene instead of its absorption, we could improve the signal/noise ratio dramatically. Using this technique, a true spectroscopy of single molecules becomes possible. Our experimental setup was fairly simple, of the type used for hole burning measurements. The excitation source was a single-mode ring dye laser (CR-699, instantaneous frequency jitter about 1 MHz. but subject to slow drifts). To limit the number of excited molecules, we had to decrease both the concentration of pentacene in the crystal, and the excited crystal volume. A simple solution was to use a thin sublimation flake (pentacene and terphenyl were co-sublimated, resulting in concentrations around 10—n mol/mol) and ,

to contact it optically to the end of a single-mode optical fiber. The number of pentacene molecules in the exciting beam was on the order of a few hundreds. The sample was placed at the focus of a parabolic mirror inside the cryostat (see fig 1) The emission, collected by the mirror, was focused on the photomultiplier tube through a redpass cutoff filter to eliminate stray light at the exciting frequency. 3. Results Figure 2 shows the excitation spectrum (A) ofa 3 with a large macroscopic (a few mmcompared concentration of aboutcrystal 10 mol/mol) to the spectrum (B) of our microscopic crystal at the fiber’s end. The broad Gaussian hands O~

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(16882.7 cm t) and OU (16886.8 cm I) of the large sample resolve for the small sample into the unresolved dots in (B) of fig. 2. When these dots are recorded on narrower frequency scans, they

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M. Orrit, J. Bernard

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display the nearly Lorentzian shape expected for a homogeneous resonance, as shown in fig. 3. All the features of these peaks (shape, width, intensity, distribution) are consistent with their attribution to single molecules. Thus, the fluorescence from a single molecule is seen to dominate both Raman scattering from the surrounding matrix and broad-band fluorescence of residual impurities. Several years ago, experiments on single atoms in atomic beams or single ions in electromagnetic traps have demonstrated the non-classical properties of the light emitted by a single quantum system. According to the level scheme of the system studied, several phenomena were ohserved such as photon bunching or antibunching, quantum jumps, etc. [6—81.In the case of a single pentacene molecule, photon bunching can be expected due to intersystem crossing to the triplet state. For a sufficiently high excitation flux, the

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TIME/1o~~ Fig. 4. Time-domain autocorrelation function of the fluorescence intensity of a single molecule. The exciting intensity is large enough to saturate the singlet—triplet three-level system. The exponential decay shows photon bunching at short times, due to the dark intervals during which the molecule is “shelved” in the triplet state. The observed contrast and decay time are compatible with a single molecule’s emission only.

molecule will perform a number of excitation— emission cycles between the ground and excited singlet states, before crossing over to the triplet. There, it will remain “shelved” for the triplet lifetime TT, during which no fluorescence is emitted. Eventually, after intersystem crossing from the thethe ground theover emissions will triplet resumetoand cycle singlet, begins all again: The triplet parameters of the pentacene/ terphenyl system are known from the literature

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________________________________ EXCITATION FREQUENCY Fig. 3. Section of the spectrum of fig. 2(B) with frequency scales blown up, showing the Lorentzian shape of a single molecule resonance. The peak’s measured width is about 12 MHz, while the homogeneous width expected from photon echoes would be 8 MHz. The difference could arise from spontaneous motion (spectral diffusion) in the molecule’s surroundings,

separated by dark intervals lasting some 45 p.s on average. Due to the rather low quantum yield of our detection (about i0~), it was impossible to visualize directly the photon bunches. However, the auto-correlation function of the emitted intensity presents a characteristic exponential decay, which demonstrates bunching indirectly (see fig. 4). The decay of the intensity correlation is specific of very small numbers of independent quantum systems: the correlation effects decrease like the inverse number of independent systems participating in the signal. Indeed, the high con-

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trast of’ the measured exponential decay (fig. 4). together with the known triplet lifetime, are consistent only with emission by a single quantum system md definitivcly confirm our itO ihutton of the peaks to single pentacene molecules. Imniediately after detection of a fluorescence photon, we know that the molecule is in the ground singlet state. Then, a time interval of the order of the Rabi period must elapse before the molecule gets excited again to emit a second photon. Therefore. we expect photon antihunching oii much shorter timescales (a few tens of ns). This would he a specific quantum effect, never observed so far with large molcculcs The widths of the single molecule peaks we measured in our first experiments ranged from 10 to thout 20 MHz which is significantly broader than the homogeneous line width expected from the decay time of photon echoes, about 8 MHz [9]. Recently. Ambrose and Moerner [10,11] found at least in one case the much narrower width of 7.8 MHz, in agreement with photon echces measurements, The discrepancy between our measurement and theirs cannot he attributed to saturation broadening, because the width we mea— sured remained the same for very weak exciting fluxes, similar to the width extrapolated to zero flux. We think that the broadening we observe is due to small changes of the molecule’s frequency by spectral diffusion processes such as those reported and investigated by Ambrose and Moerner in ref. [101. Then, the difference with “class I” molecules of ref. [10] would arise from the much larger concentration of defects in our sampie contacted to the fiber tip, as compared to the unsupported crystals of ref. [10]. Nonetheless, we plan to improve the frequency stabilization of our laser to completely eliminate slow laser drifts as a source of broadening. Single molecule peaks can be studied as ternperature changes. Although our data do not sufflee yet for a systematic study, fig. 5 presents an instance of the possible temperature dcpendences of the peaks. The excitation spectra of fig. S were recorded during a thermal cycle lasting some 20 mm. The temperature was first raised slowly (from bottom to top) up to 6.5 K, then the sample was cooled down again to liquid helium

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EXCITATION FREQUENCY / GHz Fig. 5. Temperature dependence of the excitation peaks of a lcw molecules. The sample underwent a thermal cycle lasting .

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some 21) mmmi (t rom hi)! tom to top). while spectra were recorded at dittereni temperatures. The general shift of all strumcturcs is a pressure effect. Between .75 and 3.3 K. line h is seen to troaden, svhcrcas the lines of the other molecules remain narrow, Moreover, the relative positions of lines a and h have ehan~edafter the cycle. which is evidence of activ’ateit spec-

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temperature. At about 4 K. the activation of lihrations [12] begins to broaden the peaks considerahly. Even below this temperature, different behaviors can he observed for different molecules, as fig. 5 shows. Both lines labelled “a” and “h” are narrow at 1.75 K. At 3.3 K, line a remains fairly narrow, while line h broadens very clearly. From such spectra, it is possible to gain information about the distribution of the microscopic parameters that determine the optical line width of individual molecules. Although it is impossible from the spectra of fig..~to decide whether the broadening of molecule h at 3.3 K is due to homogeneous broadening (on the T2 timescale) or to spectral diffusion, we can conclude from the comparison of molecules a and h that the surroundings of molecule h most probably include defects with lower activation energies than the surroundings of molecule a.

M. Orrit, .1. Bernard

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Single molecule spectroscopy in a solid

It may be seen in fig. 5 that the relative positions of resonance frequencies a and b have changed after the thermal cycle (here, only the relative position of the peaks is significant as the temperature changes, because of a pressure-induced shift of the whole spectrum). Also, the set of three peaks on the blue side of b gives rise after cooling to two peaks at new frequencies. All of this provides evidence for thermally activated changes in the neighborhoods of the different molecules, in other words for activated spectral diffusion.

References [1] E. Brooks Shera, N.K. Seitzinger, L.M. Davis, R.A. Keller and S.A. Soper, Chem. Phys. Lett. 174 (1990) 553.

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[2] WE. Moerner and L. Kador, Phys. Rev. Lett. 62 (1989) 2535. [31L. Kador, D.E. Home and WE. Moerner, J. Phys. Cheni. 94 (1990) 1237. [4] M. Orrit and J. Bernard, Phys. Rev. Lett. 65 (1990) 2716. [5] T.P. Carter, M. Manavi and WE. Moerner, J. Chem. Phys. 89 (1988) 1768. [6] Hi. Kimble, M. Dagenais and L. Mandel, Phys. Rev. Lett. 39 (1977) 691. [7] J.C. Bergquist, RU. Hulet, W.M. Itano and D.J Wineland, Phys. Rev. Lett. 57 (1986) 1699. [8] Th. Sauter, R. Blatt, W. Neuhauser and P.E. Toschek, Opt. Commun. 60 (1986) 287. [9] H. de Vries and D.A. Wiersma, J. Chem. Phys. 70 (1979) 5807. [101 W.P. Ambrose and WE. Moerner, Nature 349 (1991) 225. [11] WE. Moerner and W.P. Ambrose, Phys. Rev. Lett. 66 (1991) 1376. [12] T.E. Orlowski and A.H. Zewail, J. Chem. Phys. 70(1979) 1390 (see p. 1413).