Fast IR imaging with sub-wavelength resolution using a transient near-field probe

Abstract

We present a method for the remote generation of a transient near-®eld probe using conventional IR microscopy optics. Photo-induced re¯ectivity generated by picosecond pulses of visible light incident on the surface of a semiconductor substrate is used to create transient mirrors with dimensions determined by the spot size of the visible light. The IR light scattered by such sub-wavelength-size mirror is collected after propagating through the sample. As the sample is located on the semiconductor substrate, no near-®eld distance control is required, and the image can be taken at the speed of typical laser scanning microscope. And since the near-®eld probe is generated remotely ± using light ± the sample to be imaged can be covered by, or encased in, a transparent liquid or solid. The resolution of such an IR microscope is determined by the dimensions of the transient mirror, i.e., by the spot size of the visible light and its penetration depth into the substrate. To prevent resolution degradation due to diusion of the photo-excited carriers in the substrate, the probe (IR) pulse duration should not exceed a few tens of picoseconds. Ó

Infrared near-field microscopy with the Vanderbilt free electron laser: overview and perspectives

“…Extensive tests did demonstrate IR lateral resolution well beyond the diffraction limit. The most attractive possibility is to perform IR SNOM experiments in a ''spectroscopic'' mode, by selecting the wavelength to excite specific vibration modes and to identify the spatial distribution of the corresponding chemical species [8,[14][15][16][17][18]. Our present tests demonstrate, by coupling SNOM with the infrared radiation coming from the Vanderbilt free electron laser, that this is possible not only for materials-science systems but also for bio-systems.…”

“…The best sources are FELs [15][16][17] and optical parametric amplifiers [24]. The Vanderbilt FEL is continuously tunable over a broad wavelength range (2.1-9.8 lm), very intense and very reliable--and therefore ideal for IR spectroscopic SNOM.…”

“…IR spectroscopic SNOM is one of the most powerful versions of near-field microscopy [8,[14][15][16][17][18]. Images are taken using transmitted or reflected IR light with a wavelength close to one of the vibrational modes of the specimen.…”

“…Recent instrumentation advances--and in particular the use of a tunable free electron laser (FEL) as the IR source--removed most of these difficulties [10,[14][15][16][17][18][19][20][21]. We present here successful IR spectroscopic SNOM experiments on different systems in materials science and the first tests in the life sciences.…”

Development of terahertz wave microscopes

“…This method employs a sub-wavelength aperture to suppress the reaction area. This method was developed by Palanker and coworkers in Stanford University [14][15][16] and introduced in the mid-IR frequency regime. A focused near-IR beam in a semiconductor wafer produces a conducting region, which is much smaller than a mid-IR light spot, to have its transmitting property of the mid-IR light modulated by the near-IR beam.…”