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

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

We investigate the wavelength and the polarization dependence of the absorption of a subwavelength tip illuminated by an ultrashort laser pulse using laser-assisted atom-probe tomography. We show that it is possible to localize the absorption at the tip apex by choosing the appropriate laser polarization and wavelength even for excitation wavelength far from the plasmonic resonance. This effect can be explained by the diffraction theory of Sommerfeld applied on a tip geometry. Furthermore, numerical calculations are performed on a real subwavelength tip to confirm this absorption localization and to explain the role of the polarization and the wavelength.

Optical near-field absorption at a metal tip far from plasmonic resonance

We report the local study of an operating double hetero-junction GaAs/GaAlAs laser diode by apertureless Scanning Near-field Optical Microscopy (SNOM). This study allows the characterization of the spatial properties of the light emitted by the laser source. The optical probe used is a tungsten tip vibrating (at frequency f ) perpendicularly to the emitting surface. Suitable experimental parameters permitting us to accede to near-field information have been defined. This has leaded to an experimental configuration in which the probe vibration amplitude is "optimal". A far-field detection at grazing angle is used and a second harmonic (at frequency 2 f ) lock-in detection is performed. This configuration has permitted us to obtain SNOM images in stimulated emission as well as in spontaneous emission. Above the threshold current, information concerning the emitted laser mode are obtained, whereas in spontaneous emission the SNOM images are linked to the index contrast of the laser diode front facet.

Imaging a GaAlAs laser diode in operation using apertureless scanning near-field optical 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.

R. Neumann

Papers

Thermal expansion of scanning tunneling microscopy tips under laser illumination