The combination of scanning tunneling microscopy (STM) with optical excitation adds new information to STM. A review is presented covering the work done on light-induced effects in STM during the past 15 years. Effects discussed include thermal effects, nonlinear effects, field enhancement at the STM tip, various effects on semiconductor surfaces, excitation of surface plasmons, detection of photoelectrons, spin-polarized tunneling, as well as light-induced nanomodifications, local optical spectroscopy, the use of ultrashort laser pulses for time-resolved STM, and the combination of STM and scanning near-field optical microscopy.

Photoassisted scanning tunneling microscopy

The present status and development of strong-field nano-optics, an emerging field of nonlinear optics, is discussed. A nonperturbative regime of light-matter interactions is reached when the amplitude of the external electromagnetic fields that are driving a material approach or exceed the field strengths that bind the electrons inside the medium. In this strong-field regime, light-matter interactions depend on the amplitude and phase of the field, rather than its intensity, as in more conventional perturbative nonlinear optics. Traditionally such strong-field interactions have been intensely investigated in atomic and molecular systems, and this has resulted in the generation of high-harmonic radiation and laid the foundations for contemporary attosecond science. Over the past decade, however, a new field of research has emerged, the study of strong-field interactions in solid-state nanostructures. By using nanostructures, specifically those made out of metals, external electromagnetic fields can be localized on length scales of just a few nanometers, resulting in signficantly enhanced field amplitudes that can exceed those of the external field by orders of magnitude in the vicinity of the nanostructures. This leads not only to dramatic enhancements of perturbative nonlinear optical effects but also to significantly increased photoelectron yields. It resulted in a wealth of new phenomena in laser-solid interactions that have been discovered in recent years. These include the observation of above-threshold photoemission from single nanostructures, effects of the carrier-envelope phase on the photoelectron emission yield from metallic nanostructures, and strong-field acceleration of electrons in optical near fields on subcycle timescales. The current state of the art of this field is reviewed, and several scientific applications that have already emerged from the fundamental discoveries are discussed. These include, among others, the coherent control of localized electromagnetic fields at the surface of solid-state nanostructures and of free-electron wave packets by such optical near fields, resulting in the creation of attosecond electron bunches, the coherent control of photocurrents on nanometer length and femtosecond timescales by the electric field of a laser pulse, and the development of new types of ultrafast electron microscopes with unprecedented spatial, temporal, and energy resolution. The review concludes by highlighting possible future developments, discussing emerging topics in photoemission and potential strong-field nanophotonic devices, and giving perspectives for coherent ultrafast microscopy techniques. More generally, it is shown that the synergy between ultrafast science, plasmonics, and strong-field physics holds promise for pioneering scientific discoveries in the upcoming years. (Less)

Strong-field nano-optics

A theoretical analysis of the lateral force as measured with a scanning force microscope is performed, showing that the lateral force signal contains a topography-induced component which is proportional to the slope of the surface, in addition to the signal arising from the tangential force. Measurements carried out on corrugated samples (gold, indium-tin-oxide, diamond) are analysed within the framework of this theory on the basis of a simple physical model for the forces, taking into account a repulsive normal force, a tangential friction force, and an adhesion force. The correlation between the lateral force and the topography is confirmed quantitatively. Furthermore, friction coefficients are determined and the influence of the adhesion force on the data is discussed.

https://iopscience.iop.org/article/10.1088/0957-4484/4/3/003/pdf

The role of topography and friction for the image contrast in lateral force microscopy

The excitation of the tunneling junction of a scanning tunneling microscope using ultrashort laser pulses combined with detection of a tunneling current component which depends nonlinearly on the laser intensity allows, in principle, to simultaneously obtain ultimate spatial and temporal resolution To achieve this goal, a laser system that produces ultrashort laser pulses is combined with an ultrahigh vacuum scanning tunneling microscope The basic technical considerations are discussed and it is shown that atomic resolution can be achieved under pulsed laser excitation of the tunneling junction The pulsed illumination gives rise to several contributions to the measured total current Experimental evidence for signal contributions due to thermal expansion, transient surface potentials and multiphoton photoemission are presented

Femtosecond laser assisted scanning tunneling microscopy

This work presents results of scanning force microscopy (SFM) of tracks induced by energetic heavy ions on the surface of LiF and CaF 2 single crystals. The samples have been irradiated at normal incidence with several ion species with kinetic energies in the range from several hundred MeV to some GeV. The analysis of the SFM micrographs provides the mean diameters and heights of tiny hillocks created by ion impact. In the case of LiF, a previously published data set of the diameters and heights as a function of energy loss has been extended. Furthermore, the topographic height distribution of an image containing numerous tracks is analyzed by plotting a grey-value histogram of the complete image. Fitting a Gaussian function to each of the two peaks of this distribution allows us to determine the relative surface coverage with ion tracks. Displaying the relative area covered with tracks versus increasing fluence results in an exponential saturation curve containing an average track diameter. This procedure examines the consistency of the different mean diameters extracted from the series of single SFM images underlying the data set presented here.

Ion tracks on LiF and CaF2 single crystals characterized by scanning force microscopy

The periodic thermal expansion of scanning tunneling microscopy (STM) tips arising under irradiation with power-modulated laser light has been investigated. The expansion was determined by comparison with a calibrated piezomotion measured in an STM, which was operated in the constant-current mode, and instrumental effects were corrected for. The experimental data concerning the frequency response of the thermal expansion for various geometries of the tip and for different positions of the laser focus are compared with theoretical results which were derived from a numerical solution of the equation of heat conduction. A very good agreement is found. The results are also interpreted in terms of simplified analytical expressions. Furthermore, the theoretical data are used to derive the response of the tip to fast transients of the light power as in the case of pulsed irradiation.

Thermal expansion of scanning tunneling microscopy tips under laser illumination

A scanning tunnelling microscope (STM) is described which uses a double-lever reduction system with a single micrometer-drive for the coarse sample-to-tip approach and a thermally compensated single-tube piezo scanner for the tip movement. The microscope is equipped with versatile analogue electronics, and the data acquisition is computer-automated. The principal considerations are two main problems in the design of the STM electronics: the current control circuit and the current-sensing preamplifier. The performance of the STM is illustrated with images of gold and graphite samples.

https://iopscience.iop.org/article/10.1088/0957-0233/1/2/007/pdf

Design and detailed analysis of a scanning tunnelling microscope

A compact system based on a Macintosh II workstation for scanning tunneling microscopy (STM) control, data acquisition, and image display and processing is presented. To access the versatile analog electronics and the piezo scanner of our STM, a multifunction board (MacAdios II) is employed as an interface. The on‐board timer/counter permits well defined time intervals between each tip movement and subsequent data acquisition. For on‐line control and image display of scanning processes an efficient code was written in Pascal and C. To achieve simple and user‐friendly operation, the program takes advantage of so‐called mouse–window–desktop techniques supported by the Macintosh operating system. The entire code has a modular structure and therefore facilitates easy replacement of existing modules or implementation of additional ones.

J. Kowalski

Papers

Thermal expansion of scanning tunneling microscopy tips under laser illumination

Design and detailed analysis of a scanning tunnelling microscope

A compact scanning tunneling microscopy control and data acquisition system based on a Macintosh II workstation