SignificanceAbsorption microscopy is a promising technique that can detect single nonfluorescent molecules. However, fundamental limitations of existing optical absorption methods result in noisy detection signals for single molecules, which has hindered many anticipated applications. A promising method is to optically measure the photothermal heating of single molecules. In this paper, we present a photothermal microscopy technique where we detect the photothermal heating of single molecules mechanically with a temperature-sensitive nanomechanical drum. With our method, we achieve an unprecedented optical absorption sensitivity, enabling the detection of single molecules with large signal-to-noise ratios. This enables interesting applications such as the accurate localization of naturally occurring marker molecules or the identification of single molecules by measuring their absorption spectrum.

Thermal detectors are a cornerstone of infrared and terahertz technology due to their broad spectral range. These detectors call for efficient absorbers with a broad spectral response and minimal thermal mass. A common approach is based on impedance-matching the sheet resistance of a thin metallic film to half the free-space impedance. Thereby, one can achieve a wavelength-independent absorptivity of up to 50%. However, existing absorber films typically require a thickness of the order of tens of nanometers, which can significantly deteriorate the response of a thermal transducer. Here, we present the application of ultrathin gold (2 nm) on top of a surfactant layer of oxidized copper as an effective infrared absorber. An almost wavelength-independent and long-time stable absorptivity of 47(3)%, ranging from 2 μm to 20 μm, can be obtained. The presented absorber allows for a significant improvement of infrared/terahertz technologies in general and thermal detectors in particular.

The sensitive detection of infrared (IR) radiation is a essential task in today's modern world. The sensitivity of the state-of-the-art uncooled thermal infrared detectors is still several orders of magnitude above the fundamental photon noise limit. Thermal detectors based on temperature sensitive micro-and nanomechanical resonators are a promising approach to obtain improved thermal IR detectors. Here, we present an uncooled infrared detector based on a 1 mm×1 mm large nanoelectromechanical drum resonator made of 50 nm thick low-stress silicon nitride (SiN). The detector features a thin film absorber with an absorptivity of ∼30% over the entire mid-IR range. The detector drum is driven at its resonance frequency by means of a phase-locked loop. Absorbed IR radiation results in an observable detuning of the drum's oscillation frequency. We measured an Allan deviation of σ A = 5.5 × 10 −7 at room temperature at a noise bandwidth of 25 Hz. With a responsivity of R = 343 W −1 this results in a sensitivity defined as noise equivalent power (NEP) of NEP = 320 pW/rtHz for an IR beam at a wavelength of 9.5 µm. For this measurement, the IR beam focus spot diameter was equal to the drum size. The drum's responsivity improves by a factor of ten for a focal spot size smaller than ∼ 100 µm. For smaller spots the responsivity remains constant. Based on this analysis we predict a sensitivity of ∼ 30 pW/rtHz for an IR spot size smaller than 100 µm. The detector can be improved further by e.g. optimizing the tensile pre-stress to a lower value or by improving the absorptivity.

In situ spectroscopic ellipsometry is combined with cyclic voltammetry to discover and quantify potential-dependent surface adsorbates and the electronic charge on copper single crystals in HCl solution. In comparison with electrochemical scanning tunneling microscopy, it is demonstrated that ellipsometry provides not only an extremely high finger print sensitivity to sub-monolayer surface modifications but that it is furthermore possible to determine qualified values with an appropriate optical model. As a critical bench mark we use the amount of adsorbed Cl– at the Cu(111) surface. In this context, we found clear optical evidence for a densified water layer at the Cu(111) surface. Particular attention is drawn to the potential range of the hydrogen evolution reaction and the catalytic efficiency of the relatively stable Cu(111) and the more open corrugated (110) surface. With the introduced ellipsometric method, we disclose a steplike increase of the surface electron excess and a decreasing lateral surface electron mobility at the onset of the hydrogen evolution reaction. Both are explained by protonation of the surface or the adsorbed water layer and demonstrate an unexpected inhibiting effect to the hydrogen evolution reaction.

Single-molecule microscopy has become an indispensable tool for biochemical analysis. The capability of characterizing distinct properties of individual molecules without averaging has provided us with a different perspective for the existing scientific issues and phenomena. Recently, super-resolution fluorescence microscopy techniques have overcome the optical diffraction limit by the localization of molecule positions. However, the labeling process can potentially modify the intermolecular dynamics. Based on the highly sensitive nanomechanical photothermal microscopy reported previously, we propose optimizations on this label-free microscopy technique toward localization microscopy. A localization precision of 3 Å is achieved with gold nanoparticles, and the detection of polarization-dependent absorption is demonstrated, which opens the door for further improvement with polarization modulation imaging.