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US 2015/0308947 A1Published application
Method to obtain absorption spectra from near-field infrared scattering using homo-dyne detection
OwnerXiaoji Xu, Gilbert C Walkerinventors
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Abstract
The present disclosure provides a procedure and an apparatus to obtain the absorption profiles of molecular resonance with ANSOM. The method includes setting a reference field phase to φ=0.5π relative to the near-field field, and reference amplitude A≧5|αeff|. The requirement on phase precision is found to be <0.3π. The apparatus includes a phase stabilization mechanism in addition to the ANSOM to maintain high phase precision homodyne phase condition of less than 0.1 rad. This method enables ANSOM performing vibrational spectroscopy at nanoscale spatial resolution.
Claims26
An apparatus for measuring an absorption spectrum of nanoscale objects, comprising:
- a) a scanning atomic force microscope, a lock-in amplifier connected to said scanning atomic force microscope, a computer controller connected to said lock-in amplifier, a detector for detecting electromagnetic radiation connected to said lock-in amplifier, a coherent light source which emits a coherent beam of light;
- b) an interferometer integrated with said atomic force microscope, said coherent light source and said detector, said interferometer having a reference arm and a sample arm, the interferometer being positioned for directing light along the sample arm from said coherent light source to a sample mounted on a scanning stage of said scanning atomic force microscope and for directing a light beam scattered from the sample and a scanning probe tip of said atomic force microscope into said detector, said interferometer also being positioned for directing a light beam along the reference arm from said coherent light source to a position adjustable reflector and directing a light beam reflected from said adjustable reflector to recombine with said scattered light beam into said detector;
- c) a positioning mechanism for adjusting and maintaining a position of the adjustable reflector to give a relative optical phase difference between said light beams in said reference arm and said sample arm to be one of in a range from about 0.4π to about 0.6π, or −0.4π to about −0.6π;
- d) wherein said lock-in amplifier is configured to receive a reference frequency from said atomic force microscope and an output voltage signal from said detector and, based on said reference frequency and said output voltage signal, demodulate said output voltage signal into harmonic components and relaying second and higher harmonic components to said computer controller which is programmed with instructions to form an apertureless near-field scanning optical microscope image of said nanoscale objects.
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A method of measuring an absorption spectrum of nanoscale objects, comprising:
- a) rastering a nanoscale object under a scanning probe tip of a scanning atomic force microscope;
- b) illuminating said nanoscale object and said scanning probe tip with a frequency tunable coherent light source along a sample arm of an interferometer;
- c) combining a scattered coherent light beam from said nanoscale object and said scanning probe tip along said sample arm with a beam of light from said frequency tunable light source reflected from an adjustable reflector in a reference arm of the interferometer and directing the combined coherent light beams into a detector;
- d) demodulating an output signal from said detector to give second or higher harmonic demodulation signals and, using the second or higher harmonic demodulation signals, adjusting a position of the adjustable reflector to give an optical phase difference between said light beams in said reference arm and said sample arm to be in a range from about 0.4π to about 0.6π, or from about −0.4π to about −0.62π, and maintaining said position using feedback to maintain the optical phase difference;
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- e) obtaining an image of signal intensity being proportional to the imaginary part of infrared absorption coefficient of the sample through the combination of scanning probe microscopy and light scattering from a sharp scanning probe tip with said procedure of setting specific homodyne phase in the interferometer.
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Dependencies are inferred from the claim text.