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Single molecule spectroscopy using tunable lasers

MAX PLANCK INSTITUTE FOR THE SCIENCE OF LIGHT, ERLANGEN, GERMANY

The fluorescence excitation spectra of single organic molecules in a solid state crystal are measured at cryogenic temperatures using a single frequency tunable laser light source based on optical parametric oscillator technology. This laser exhibits promising features as a light source for spectroscopy applications, including a broad tuning range from 450 to 650 nm, narrow linewidth < 1 MHz and mode-hop-free tuning over > 25 GHz. This applications note presents the experimental setup, measured spectra and discusses the applicability of this kind of laser for high-resolution spectroscopy.

Introduction

Investigations of molecular processes and their characteristics play an import­ant role in many scientific disciplines, like medicine, biology, chemistry, and physics. Fluorescence spectroscopy of single molecules gives fundamental insights into their properties and the in­fluence of their surroundings [1,2]. How­ever, studying single molecules in a host material is a challenging task with high demands on the detectors and on the laser light sources [3]. This report exam­ines the applicability of a new tunable continuous-wave (cw) laser light source for various spectroscopy methods.

Experiment

The experimental setup is illustrated in Fig. 1. A frequency-tunable laser beam is focused onto a sample containing dibenzanthanthrene (DBATT) molecules hosted in a naphthalene crystal. The laser beam is tuned to resonances of the DBATT molecules, leading to their excitation and subsequent fluorescence. The fluorescence light is filtered and measured with a single photon detector, SPCM-AQR from Perkin Elmer. A High­Finesse WS/6-200 wavemeter monitors the frequency of the excitation laser. A more detailed description of the experi­mental methods is given, e.g. in [3].

Suitable excitation lasers

Single molecules in a solid state crystal can be regarded as nearly ideal two-lev­el systems with natural linewidths in the range of 10 - 50 MHz. Due to imperfec­tions in the crystal the transitions of indi­vidual molecules are inhomogeneously distributed over more than 1 THz. With a narrow-band laser tuned to an individ­ual transition single molecules can be isolated from the ensemble. Therefore, the laser is a crucial component for the excitation of single molecules: It requires a linewidth below the natural linewidth of the molecules and a mode-hop-free tunability over a broad spectral range to address many molecules. Furthermore the flexibility in choosing the center wavelength for different kinds of mole­cule-host combinations, an output pow­er well above 200 mW, and low intensity noise are other relevant parameters. Commonly, dye lasers, Ti:Sa lasers, or tunable diode lasers are used for such experiments. Ti:Sa lasers are restricted to the wavelength ranges 700 – 1000 nm and 350 – 500 nm when frequen­cy-doubled. Dye lasers require a change of the dye or even of the pump laser used, when switching between wave­length ranges. Diode lasers are typically restricted to small tuning ranges and low output powers in the visible wavelength range. Optical parametric oscillators (OPO) offer an attractive alternative: The wavelength coverage of OPOs can be designed according to the experimental requirements, OPOs are solid-state sys­tems which do not rely on consumables such as dyes, and they deliver relatively high output powers.

In this applications note, the commercially available cw tunable OPO, C-WAVE, was used. This OPO covers the wavelength ranges 450 – 650 nm (frequency doubled SHG option) and 900 – 1300 nm. The output power is in the 500 mW range and the line­width is below 1 MHz. C-WAVE can be swept mode-hop-free over more than 25 GHz. Changes of the center frequency of several GHz as well as large variations of the wave­length are fully computer controlled. The good beam profile allows high coupling efficiencies into optical fibers and therefore a simple integration into existing setups.

Fluorescence spectra of DBATT mole­cules have been measured at a center wavelength of 618 nm and over a range of 1 THz by stitching several mode-hop-free frequency sweeps. Figure 2 shows a zoom in one of the mode-hop-free scans. The molecules’ redshifted fluorescence signal is measured with the APD while the laser frequency is scanned. The measurement shows several narrow spectral features corre­sponding to individual DBATT molecules [4]. With C-WAVE it is also possible to lock the laser frequency with waveme­ter-precision to one single molecule resonance in order to study its pho­to-physics and photon dynamics.

Summary

The presented setup enables the measurement of fluorescence exci­tation spectra of single molecules in transparent host materials. Both laser light sources, Ti:Sa lasers and C-WAVE, are well suited for measuring narrow spectral features over wide frequency ranges. A combination of these laser sources allows a nearly complete wave­length coverage from 450 – 1300 nm with fully automatic wavelength control and no need to change laser media or perform realignments. This makes the presented setup a user friendly, flexible and sensitive spectrometer for charac­terizing single molecules, color centers and semiconductor quantum dots.

For additional information,

please contact:

Dr. Tobias Utikal, Max Planck Institute for the Science of Light, Erlangen, Ger­many, tobias.utikal@mpl.mpg.de,

or

Dr. Niklas Waasem, Hübner GmbH & Co. KG, Kassel, Germany, niklas.waas­em@hubner-germany.com