Multiple modulation heterodyne infrared spectroscopy
View Patent ↗A heterodyne detection technique for highly localized IR spectroscopy based on an AFM. A pulsed IR source illuminates a sample and causes contact resonance of an AFM probe, which is a function of localized absorption. The probe is operated in intermittent contact mode and is therefore oscillated at a resonance frequency. A secondary oscillation is mixed in to the probe oscillation such that the sum of the secondary oscillation and the IR source pulse frequency is near another harmonic of the probe. A mixing effect causes measurable probe response at the other harmonic allowing data to be taken away from the pulse frequency.
1. A method of obtaining nanoscale measurements of optical absorption of a sample with an oscillating cantilever probe of a probe microscope and a source of infrared radiation, the method comprising:
modulating the infrared source at a frequency f m ;
illuminating a region of a sample with a beam from the modulated, infrared source of infrared radiation; oscillating the cantilever at a first frequency f 1 ;
modulating the tip-sample separation at at least one second frequency f 2 ; and, detecting an oscillation of the cantilever at a frequency f 2 +nf m , where n in an integer.
2. The method of claim 1 further comprising the step of deriving an infrared absorption spectrum for a region of the sample under the probe tip from the detected oscillation at a frequency f 2 +nf m .
3. The method of claim 2 wherein a detected amplitude is measured with a spatial resolution of less than 100 nm.
4. The method of claim 1 further comprising the step of measuring the detected amplitude at a plurality of locations on the sample surface to create an image indicative of infrared absorption across a region of the sample.
5. The method of claim 4 further comprising the step of correcting the absorption imaged based on a measurement of surface topography of the region of the sample.
6. The method of claim 4 further comprising the step of correcting the absorption imaged based on measurements of a signal proportional to dC/dZ at a plurality of locations on the sample.
7. The method of claim 4 wherein the IR absorption image has a spatial resolution of better than 20 nm.
8. The method of claim 4 wherein the detected amplitude is measured with a spatial resolution of less than 100 nm.
9. The method of claim 1 wherein the frequency f 1 substantially corresponds to a flexural resonance of the cantilever and the probe microscope is operated in intermittent contact mode.
10. The method of claim 1 wherein the frequency f 1 substantially corresponds to a flexural resonance of the cantilever and the probe microscope is operated in non-contact mode.
11. The method of claim 1 wherein the modulated source is a pulsed IR laser.
12. The method of claim 1 wherein the beam modulation frequency, f m , is at least 1 kHz.
13. The method of claim 1 wherein the frequency f 2 +nf m is greater than 50 kHz.
14. A method of obtaining nanoscale measurements of optical absorption of a sample with an oscillating cantilever probe of a probe microscope and a source of infrared radiation, the method comprising:
oscillating the cantilever at a first frequency f 1 , substantially corresponding to a frequency f c1 of a resonant mode of the cantilever;
modulating the relative tip-sample distance at a at least one second frequency f 2 ;
illuminating a sample with a modulated beam of radiation with a modulation frequency f m ; and,
selecting f 2 and f m such that combinations including (f 2 +nf m ) are approximately equal to f c2 , where f c2 is the frequency of another resonant mode of the cantilever, and where n is an integer.
15. The method of claim 14 wherein sample interaction with the modulated beam at f m results in a probe response at f m resulting in an excitation of the cantilever at resonance f c2 .
16. The method of claim 14 further comprising the step of measuring an amplitude of cantilever vibration at or around f 2c .
17. The method of claim 16 further comprising the step of repeating the measurement of cantilever amplitude at a plurality of wavelengths of the beam of radiation.
18. The method of claim 17 wherein the cantilever amplitude as a function of wavelength or wavenumber is indicative of the absorption spectrum of material interacting with the tip of the probe microscope.
19. The method of claim 14 , wherein the resonant peak of the cantilever oscillation at f c2 has a half width of F c2hw , and f m is chosen to be greater than f c2hw , to ensure that f 2 will not excite a significant cantilever response at f c2 absent sample/tip interaction at f m .
20. The method of claim 14 wherein f c2 /f m is not equal to an integer, to avoid direct excitation of the cantilever by harmonics of f m at resonance f 2c .
21. A method of obtaining nanoscale measurements of optical absorption of a sample with an oscillating cantilever probe of a probe microscope and a source of infrared radiation, the method comprising:
modulating the infrared source at a frequency f m ;
illuminating a region of a sample with a beam from the modulated, infrared source of infrared radiation; and,
modulating the tip-sample separation at at least one frequency f 1 not centered on a cantilever resonance;
detecting an oscillation of the cantilever at a frequency f 1 +nf m ˜f c1 where n is an integer.