Mapping electron-beam-injected trapped charge with scattering scanning near-field optical microscopy

Abstract

Scattering scanning near-field optical microscopy (s-SNOM) has been demonstrated as a valuable tool for mapping the optical and optoelectronic properties of materials with nanoscale resolution. Here we report experimental evidence that trapped electric charges injected by electron beam at the surface of dielectric samples affect the sample-dipole interaction, which has direct impact towards the s-SNOM image content. Nanoscale mapping of surface trapped charge holds significant potential for the precise tailoring of the electrostatic properties of dielectric and semi-conductive samples, such as hydroxyapatite, which has particular importance with respect to biomedical applications. The methodology developed here is highly relevant to semiconductor device fabrication as well.OCIS codes : (180.4243) Near-field microscopy; (160.1435) Biomaterials; (090.1995) Digital holography; (120.6650) Surface measurements, figure. http://dx.doi.org/10.1364/OL.99.099999Scattering Scanning Near-field Optical Microscopy (s-SNOM) has been demonstrated in the past years as an extremely effective tool for imaging and mapping the surface properties of materials in the nanoscale. The complex contrast mechanisms of s-SNOM have enabled a wide range of applications such as electric permittivity mapping [1,2], infrared spectroscopy measurements [3][4][5], and plasmonic response investigations [6,7] with resolution beyond the diffraction limit [8,9]. The functionality of this microscopy tool is based on local interactions between the tip of a nano-probe that is scanned across the sample surface and the molecules on the sample surface in the presence of an incident laser beam focused on the tip. The electric field component of the incident laser beam creates in the tip an oscillating electric dipole which emits light with the same wavelength as the incident beam [10,11]. When the tip is in close proximity to the surface of a sample (on distances much shorter than the wavelength), the light emitted by the electric dipole is influenced (in terms of amplitude and phase of the electric field component) by the local dielectric function of the sample. Detecting the light emitted by the oscillating electric dipole requires higher-harmonic demodulation and interferometric detection techniques to suppress the background light (mainly composed by direct or multiple reflections from the probe and sample) and to facilitate the reconstruction of the near-field optical signal amplitude and phase.Our experiment brings evidence that trapped charges (e.g. injected by an electron beam or an ion beam, both of which are common in semiconductor device processing and biomaterials sterilization) influence the oscillating dipole from the tip and thus directly impact the image contrast in s-SNOM. We exploit this phenomenon to use s-SNOM imaging for mapping trapped surface charge with nano-scale resolution.The interaction between trapped electrical charges and an oscillating dipole in s-SNOM can be understood by recalling that the latter models the oscillations of...

Characterization of Nanomaterials by Locally Determining Their Complex Permittivity with Scattering-Type Scanning Near-Field Optical Microscopy

“…Optical near-field signals were collected by using the same objective used for excitation and were directed onto a photodiode connected to the SR844 RF lock-in amplifier, locked on two successive harmonics of the signal, which facilitates the extraction of optical signals of interest from the intense background. This homemade system was previously demonstrated in various experiments focused on the characterization of materials and biological samples. ,,, …”

“…16−22 With respect to imaging biological species, a limited number of experiments have been performed so far with s-SNOM, but these demonstrate nonetheless its potential in this regard. 23−26 s-SNOM has also been fruitfully employed in various correlative imaging approaches such as the correlation of the infrared optical contrast with the structural state of a phase changing material, 27 establishing the relation between conductivity and crystal structure in ZnO nanowire cross sections, 28 mapping of surface charge domains, 29 or correlative nanoimaging of areas with strongly enhanced electromagnetic fields and Raman scattering. 30 These previous experiments demonstrate s-SNOM's potential to complement other nano-or microscale techniques, which is of great benefit with respect to a better understanding of advanced materials.…”

“…The broad range of light sources that can be used in association with s-SNOM, performing in continuous, high-repetition-rate or low-repetition-rate modes and emitting wavelengths ranging from visible light to terahertz, − propose s-SNOM to become a standard for high-spatial-resolution surface analysis. Its complex but reliable contrast mechanism enabled so far a wide range of discoveries in the condensed phase materials and two-dimensional materials. − With respect to imaging biological species, a limited number of experiments have been performed so far with s-SNOM, but these demonstrate nonetheless its potential in this regard. − s-SNOM has also been fruitfully employed in various correlative imaging approaches such as the correlation of the infrared optical contrast with the structural state of a phase changing material, establishing the relation between conductivity and crystal structure in ZnO nanowire cross sections, mapping of surface charge domains, or correlative nanoimaging of areas with strongly enhanced electromagnetic fields and Raman scattering . These previous experiments demonstrate s-SNOM’s potential to complement other nano- or microscale techniques, which is of great benefit with respect to a better understanding of advanced materials.…”

Correlative imaging of biological tissues with apertureless scanning near-field optical microscopy and confocal laser scanning microscopy

“…The optical interaction between this enhanced near-field and the sample volume underneath modifies both the amplitude and the phase of the scattered excitation light, depending on the local dielectric properties of the sample [12]. Interferometric detection of the backscattered light yields nanoscale-resolved amplitude and phase images, revealing besides local structural properties [13,14], also additional aspects such as material composition [15,16], free-carrier concentration [17,18] or surface charge domains [19]. The resolution achievable with s-SNOM is connected to the tip's radius of curvature, falling well under 40 nm [12,20], and is independent of the illumination wavelength [21].…”