Nano-electronvolt Fourier-limited transition of a single surface-adsorbed molecule

Abstract

High-resolution spectroscopy allows one to probe weak interactions and to detect subtle phenomena. While such measurements are routinely performed on atoms and molecules in the gas phase, spectroscopy of adsorbed species on surfaces is faced with challenges. As a result, previous studies of surface-adsorbed molecules have fallen short of the ultimate resolution, where the transition linewidth is determined by the lifetime of the excited state. In this work, we conceive a new approach to surface deposition and report on Fourier-limited electronic transitions in single dibenzoterrylenes adsorbed onto the surface of an anthracene crystal. By performing spectroscopy and super-resolution microscopy at liquid helium temperature, we shed light on various properties of the adsorbed molecules. Our experimental results pave the way for a new class of experiments in surface science, where high spatial and spectral resolution can be combined.

Figures (5)

• Figure 1: Sample preparation and experimental setup.a, Microscope image of a large AC crystal surrounded by many small crystallites, which were sublimated onto a cover glass. Inset shows the structure of AC. Carbon and hydrogen atoms are shown in dark and light shades, respectively. b, Sketch of the crystal self-cleaning process. (i) After original sublimation and exposure to air, the crystal may contain surface contaminants, indicated by the black dots. (ii) Partial sublimation in vacuum results in a thinner crystal with a fresh surface. c, Molecular structure of DBT. d, Jablonski energy diagram of DBT. $\vert g \rangle$ and $\vert e \rangle$ stand for the electronic ground and excited states, respectively, while v denotes the order of the vibrational levels in each state. e, Microscope image of the custom-designed oven chip containing a heater (central part) and Pt thermometers (sides). f, Experimental setup for depositing DBT and achieving optical access for spectroscopy and microscopy. Upper panel: schematic layout; lower panel: side view to scale. The micro-oven evaporates DBT onto a fresh AC crystal surface located on a glass substrate (S). Two pinholes define the molecular beam. The oven chip is thermally isolated by a support made of polyetheretherketone (light brown). The other components are made of gold-coated oxygen-free copper. The path connecting the oven chip and the crystal surface is enclosed within a tube to avoid contamination of the cryostat. An aspheric lens is used for laser illumination and collection of fluorescence from the sample. Piezo-driven stages (gray) allow for the translation of the sample, the aspheric lens, and one of the pinholes.
• Figure 2: Spectral and temporal properties of individual DBT molecules.a, Fluorescence excitation spectrum of a DBT molecule on an AC surface. Solid line shows a Lorentzian fit function with FWHM of 18 MHz. b, Second-order autocorrelation function of the emitted photons from another single molecule. Inset: a low-power fluorescence excitation spectrum of the same molecule. c, (i) Repeated laser frequency scans at low excitation power of $8$ nW performed once every 10 minutes. 00ZPLs remain stable over several hours. (ii) Repeated laser frequency scans performed once every two seconds at higher excitation power of $150$ nW result in spectral instability. d, Distribution of 00ZPLs from about 50,000 molecules show distinct spectral sites with central frequencies and FWHM linewidths ($\nu_i, \Delta\nu_i$): $\nu_1=415.8$ THz, $\Delta\nu_1=68$ GHz; $\nu_2=416.3$ THz, $\Delta\nu_2=56$ GHz; $\nu_3=416.6$ THz, $\Delta\nu_3=65$ GHz; $\nu_4=418.1$ THz, $\Delta\nu_4=63$ GHz; $\nu_5=418.5$ THz, $\Delta\nu_5=59$ GHz. The arrow marks a possible sixth site. The sites with long-term stability are marked by an asterisk. e, Distribution of the 00ZPL linewidths from about 10,000 single molecules for the four most prominent sites with the same color code as in (d). The dashed line denotes the median linewidth at 19 MHz.
• Figure 3: Spatial and orientational distribution of DBT molecules on the AC surface.a, (Left) Top-view geometry of an AC crystal and the orientations of its a and b axes. (Right) Theoretically predicted configuration of the carbon atoms (red) of a DBT molecule with respect to the carbon atoms (black) of the AC crystal. Two orientations are found for the adsorption of DBT. (Upper row) View from above, i.e., in the ab plane. (Lower row) View from the side. b, Super-resolution maps of individual DBT molecules for the three persistent spectroscopic sites. c, (Top) Schematics for polarization analysis of the fluorescence signal after excitation with linearly-polarized light at different angles within $180^\circ$. (Bottom) Pairs of symbols mirrored through the origin of the polar plot represent the orientation of a single molecule from one of the spectral sites as marked in the legend. The distance of a point from the center corresponds to the degree of polarization, i.e., visibility $V$ of the fluorescence modulation. Dashed circles mark $V=0.5$ and $V=1$.
• Figure 4: Emission spectra and temperature dependence of 00ZPL.a, Fluorescence spectra recorded from ensembles of molecules excited to higher-lying vibrational levels of the electronic excited state in the range of $420-423$ THz. The 00ZPL is set at the origin of the frequency axis in each case. Colors indicate data from different persistent spectral sites. The signal is represented in x5 magnification for the region beyond the dashed line. b, 00ZPL linewidth as a function of temperature obtained from about 100 molecules for the three different spectral sites. Dashed curves show Arrhenius fits. For comparison, the dotted curve shows the behavior of DBT in bulk AC nicolet2007b. c, Evolution of the 00ZPL spectrum of a single molecule as a function of temperature. The orange dashed curve marks the 00ZPL center in each laser frequency scan.
• Figure 5: Spectroscopy of DBT in a vapor cell.a, Schematics of the vapor cell measurement setup. DBT is heated in a cell that is prepared in a glove box under nitrogen at atmospheric pressure. A sapphire window on the cell and a long working distance microscope objective provide optical access for laser spectroscopy (Methods). b, Absorption (solid) and emission (dashed) spectra of DBT recorded at different oven temperatures noted in the legend. Not all temperatures have associated emission data. The spectra in the two categories are normalized by the maxima of their corresponding data recorded at $340$$^\mathrm{\circ}$C. Control measurements confirmed that the rise of the signal at 600 nm is also present in a cell without DBT (i.e., empty), while the absorption peak at around 680 nm arises exclusively from DBT. The high number of vibrational and rotational degrees of freedom in the gas phase results in broad spectra. The estimated Doppler broadening of 0.35 GHz and collisional broadening below 5 GHz are both negligible on the scale of the presented spectra.