The patent's assignee for patent number 8646319 is
News editors obtained the following quote from the background information supplied by the inventors: "This invention is related to highly localized Infrared (IR) spectra on a sample surface utilizing an Atomic Force Microscope (AFM) and a variable wavelength pulsed IR source, and specifically to dynamic IR power control for maximizing dynamic range while minimizing sample damage.
"IR spectroscopy is a useful tool in many analytical fields such as polymer science and biology. Conventional IR spectroscopy and microscopy, however, have resolution on the scale of many microns, limited by optical diffraction. It would be particularly useful to perform IR spectroscopy on a highly localized scale, on the order of biological organelles or smaller, at various points on a sample surface. Such a capability would provide information about the composition of the sample, such as location of different materials or molecular structures. Conventional infrared spectroscopy is a widely used technique to measure the characteristics of material. In many cases the unique signatures of IR spectra can be used to identify unknown material. Conventional IR spectroscopy is performed on bulk samples which gives compositional information but not structural information. Infrared Microscopy allows collection of IR spectra with resolution on the scale of many microns resolution. Near-field scanning optical microscopy (NSOM) has been applied to some degree in infrared spectroscopy and imaging. Recently, a technique has been developed based on use of an AFM in a unique fashion to produce such localized spectra. This work was described in a publication entitled 'Local Infrared Microspectroscopy with Sub-wavelength Spatial Resolution with an Atomic Force Microscope Tip Used as a Photo-thermal Sensor' Optics Letters, Vo. 30, No. 18,
"Referring to FIG. 1, the PTIR technique basically uses an Atomic Force Microscope (AFM). A typical AFM cantilever probe 2 is brought in interaction with a region of a sample 1. A beam from pulsed IR radiation source 3 is directed to the sample 1. When a brief, intense radiation pulse illuminates the sample 1, it causes a rapid sample expansion due to thermal shock, stimulating a resonant oscillation 4 of the cantilever probe 2, which is measurable by the AFM's probe deflection detection system. The amplitude of the thermal shock depends on the degree of IR absorption, which will depend on the material characteristics of the sample in the area immediately under the probe tip. The degree of absorption will also depend on the wavelength of the IR radiation. Thus varying the wavelength of the source and repeating the deflection measurement across a range of wavelengths yields an absorption spectrum 5 for a highly localized region of a sample. The measurement may be repeated at a plurality of points on the sample surface, to create an absorption spectra map, enabling characterization and identification of sample material composition at a previously unattainable resolution. Related techniques detect the local temperature change in the sample via a temperature sensing AFM probes, as described by Hammiche and others in the scientific literature.
"It has become apparent during applications testing of a commercial nanoscale IR spectroscopy platform there is a challenge between optimizing the signal to noise ratio and the risk of sample damage. The issue is related to the dynamic range of the measurement technique. If the amount of IR energy absorbed is small in some areas of the sample at particular wavelengths, it creates a signal that is below the limit of detection. To increase the signal to noise ratio, the IR laser power can be turned up to increase the amount of absorbed radiation. But if the absorbed IR energy is too high, it causes substantial heating of the sample in other areas and/or wavelengths which can lead to melting, burning and/or chemical changes in the sample. Even at temperatures below a thermal damage threshold, the sample may soften to the point that the pressure of an AFM tip can cause local plastic deformation, altering the topography of the sample. It is desirable to avoid any or all of these potential types of sample damage.
"The temperature rise in the sample is a function of both the laser power at a given wavelength and the sample absorption at that wavelength, as illustrated in FIG. 2. A suitable radiation source using an optical parametric oscillator (OPO) laser has power variations of almost 20.times. over the range of 1000-2000 cm.sup.-1. In addition to this, sample absorption peaks can vary by orders of magnitude. In recently obtained spectrums, the ratio between the largest and smallest peak heights can be 20.times.. So even over the range of 1000-2000 cm.sup.-1 the sample temperature change could vary by 20.times.20 =400.times. over peaks of interest. If the laser power is turned up large enough to resolve the smallest peaks, the sample can easily be damaged at the higher peaks. (Note that the x-axes in FIG. 2 are labeled in 'wavenumbers (cm.sup.-1)' a convention used in spectroscopy. The wavelength in microns is given by 10,000/wavenumber. In this application we use the terms wavenumber and wavelength interchangeably, i.e. measuring a property as a function of wavelength provides equivalent information as measuring a property as a function of wavenumber.)"
As a supplement to the background information on this patent, VerticalNews correspondents also obtained the inventors' summary information for this patent: "The Invention is a method and an apparatus for producing an absorption spectrum in a localized region of a sample, using an Atomic Force Microscope. A beam of variable wavelength IR radiation, typically pulsed, is directed at the sample, and the AFM probe tip interacts with the sample. The response of the probe due to absorption of the IR radiation is measured and the IR source power is automatically adjusted based on the response. Typically the response of the probe is measured at a plurality of center wavelengths of the IR radiation, and the adjusting step may occur at one or more of these wavelengths. Versions of the method may include the step of automatically adjusting an angle of the beam of radiation as a function of wavelength.
"In preferred embodiments, adjusting the power of radiation according to the probe response is done to limit damage to the sample. The adjusting step may be performed if the cantilever amplitude exceeds a threshold value. Alternatively determining a damaging power density threshold may be based on a rapid decrease in contact resonant frequency due to sample softening.
"It is preferable to normalize the probe response to the adjusted power density and to use the normalized data to create an absorption spectra. The adjusting step and the normalizing step may enable measurements of normalized cantilever amplitude with a dynamic range of at least 100, and preferably 1000. In various preferred embodiments, the IR source may an OPO and/or the power adjustment may be accomplished with a variable attenuator. The variable attenuation may be, among others, accomplished with a wire grid polarizer, variable pump current control, adjustable iris, or adjusting beam focus. In various versions of the invention the probe may be a cantilever probe and the probe response is cantilever oscillation, or the probe may be a thermal sensor and the probe response is temperature change, or a combination of the two.
"Feedback may be employed to maintain a target amplitude of probe response substantially constant and/or under a threshold value. The invention may also include the step of determining power of the beam of radiation using attenuation values of the variable attenuator. The attenuation value of the variable attenuator may be determined by scaling the current attenuation value by the ratio between the measured probe response and a target probe response. Also for the case where spectra are gathered at a plurality of points covering an area of the sample, look-ahead feedback may be employed to adjust the laser power as the probe nears a wavelength previously measured, such as a corresponding point in an adjacent spectral measurement."
For additional information on this patent, see: Prater, Craig; Kjoller, Kevin. Dynamic Power Control for Nanoscale Spectroscopy. U.S. Patent Number 8646319, filed
Keywords for this news article include: Nanoscale, Nanotechnology, Emerging Technologies,
Our reports deliver fact-based news of research and discoveries from around the world. Copyright 2014, NewsRx LLC
Most Popular Stories
- Koch Brothers Step up Anti-Obamacare Campaign
- Vybz Kartel Convicted of Murder
- Stocks Close Lower Ahead of Crimea Vote
- Ulta Shares Look Good on Strong Q4
- FDIC Sues Big Banks Over Rate Manipulation
- Jittery Investors Dumping Russian Stocks
- JLo Turns the Tables in New Vid: 'I Luh Ya Papi'
- U.S. Consumer Sentiment Falls in Early March
- FDIC Accuses Big Banks of Fraud, Conspiracy
- Is Malaysian Airlines Flight 370 in Andaman Sea?