Single-molecule nano-optics at low temperature

Abstract

The work presented in this thesis involves experiments on single molecules at low temperature. At low temperature, many disturbing temperature activated processes are frozen out, resulting in extremely sharp zero-phonon lines in the fluorescence ex- citation spectra of certain molecule-matrix systems. The presence of sharp absorp- tion lines is accompanied by the fact that the absorption cross-section is significantly increased, approaching a value of about 10% of the theoretical limit for an oscillating dipole. The first part of this thesis provides background information for understanding the experimental results. It introduces the energy level scheme of a single mole- cule in a solid matrix at low temperature, the methods to get down to the single- molecule level, the requirements for a molecule-matrix system for single-molecule spectroscopy, the absorption cross-section and saturation. After this theoretical part, the experimental techniques used for the experiments in this thesis, confocal microscopy and aperture scanning near-field optical microscopy, are discussed. This theoretical part is followed by an experimental part, which describes the combined home-built confocal and aperture scanning near-field optical microscope working at 1.8 K in a helium bath cryostat. It starts with a general overview, followed by detailed descriptions of the most important parts of the set up. The last part of the experimental section describes the preparation of the two different samples used in these studies: terrylene doped into crystals of p-terphenyl and terrylene in a stretched film of linear low-density polyethylene. The second part of this thesis describes the experimental results. It starts with a study of the imaging properties of single molecules at low temperature as a function of excitation frequency and excitation intensity. Molecules are imaged at several spectral positions on their resonance curve. The spot sizes of single molecules appear increased in resonance compared to the out-of-resonance values and increase with excitation intensity. With the help of Monte Carlo simulations, addressing the measured spot size as a function of detuning (i.e. decreasing signal-to-background ratio) and as a function of intensity, the observed effects could be attributed to pronounced saturation effects in single-molecule imaging. In fact, the spot size of a single molecule turns out to increase with intensity, even below saturation. After this first experiment, the main experiment of this thesis is described. It starts with the characterisation of a new sample for single-molecule spectroscopy at low temperature, terrylene in a stretched film of linear low-density polyethylene. Terrylene molecules in a stretched film of linear low-density polyethylene are all ori- ented along the stretching direction and show su±cient spectral stability. The degree of orientation turns out to include all three dimensions. The molecules all have their transition dipole moments aligned in the sample plane, which leads to a maximised absorption cross-section. The maximised absorption cross-section of the terrylene molecules makes this sample a good candidate for single-molecule detection by ab- sorption. Single molecule detection by absorption is the main experiment described in this thesis. Despite the conceptual ease of performing a bulk absorption exper- iment, single-molecule detection by absorption is often considered hardly possible: the excitation and emission wavelength are exactly the same and the detector is di- rectly exposed to high intensity laser light. Single-molecule detection by absorption exploits the fact that the coherent part of light emitted by the molecule can interfere in the far field with the excitation light. Due to a phase difference between the light scattered resonantly by the molecule and the reflected excitation light, a dispersive signal is observed on scanning the excitation frequency over the resonance of a single molecule. Single molecules are thus detected in absorption as dispersive features. Absorption and fluorescence excitation spectra are recorded simultaneously. From fluorescence excitation spectra, the exact spectral position and line width of single molecules were determined, which facilitated the analysis of the absorption spectra. Dynamical features, like blinking and spectral jumping, are observed. From the amplitudes of the absorption signal and the line width of the signals, a negative correlation between line width and amplitude was found. This confirms the physical principle that the amount of coherently scattered light decreases upon saturation or due to dephasing. Unfortunately, the signal-to-background ratio of absorption spectra is still rather low compared to fluorescence excitation spectra. A method to improve the signal-to-background ratio could be to go to near-field excitation.