Method and apparatus for direct measurement of the amplitude and/or phase of a molecular vibration

Patent metadata

Field	Value
Publication number	US-9658162-B2
Application number	US-201514642655-A
Country	US
Kind code	B2
Filing date	Mar 9, 2015
Priority date	Mar 7, 2014
Publication date	May 23, 2017
Grant date	May 23, 2017

Abstract

An apparatus and method for measuring amplitude and/or phase of a molecular vibration uses a polarization modulated pump beam and a stimulating Stokes beam on a probe of a scanning probe microscope to detect a Raman scattered Stokes beam from the sample. The detected Raman scattered Stokes beam is used to derive at least one of the amplitude and the phase of the molecular vibration.

First claim

What is claimed is:

1. A method for measuring amplitude and/or phase of a molecular vibration, the method comprising:
displacing a probe of a scanning probe microscope to approach a sample;
illuminating the probe with a polarization modulated pump beam;
introducing a stimulating Stokes beam onto the probe; and
detecting a Raman scattered Stokes beam from the sample to derive at least one of the amplitude and the phase of the molecular vibration.

2. The method according to claim 1 , wherein the polarization modulated pump beam is modulated from P polarization to S polarization at frequency fm, which excites one or molecules from ground state to a higher virtual or real state.

3. The method according to claim 1 , wherein the stimulating Stokes beam is frequency tuned at frequency fs to amplify a selected Raman mode of the molecules.

4. The method according to claim 1 , wherein the Raman scattered Stokes beam is detected using an optical interferometer.

5. The method according to claim 4 , wherein the optical interferometer is a heterodyne interferometer or a serrodyne interferometer.

6. The method according to claim 1 , wherein a portion of the stimulating Stokes beam is frequency shifted by frequency fB to create a reference arm of a heterodyne interferometer and wherein another portion of the stimulating Stokes beam that is not frequency shifted is focused on the probe.

7. The method according to claim 1 , wherein a reference Stokes beam is obtained by back scattered light from the probe.

8. The method according to claim 1 , wherein the stimulated Stokes beam from the tip end of the probe and a backscattered Stokes beam from the tip shank of the probe both interfere with a frequency shifted reference beam at frequency fB to create fB+fm and fB−fm sidebands at an optical detector, where fm is the modulation frequency for the polarization modulated pump beam.

9. The method according to claim 8 , further comprising filtering and mixing an optically detected signal at the frequency fB and either one of the fB+fm and fB−fm sidebands in a balanced mixer to recover a molecular vibration signal at the frequency fm.

10. The method according to claim 9 , wherein the molecular vibration signal at the frequency fm is detected in a lock-in amplifier to recover the amplitude and/or the phase of the molecular vibration.

11. The method according to claim 1 , wherein the introducing of the stimulating Stokes beam comprises focusing only a single stimulating Stokes beam on the probe, thereby generating a backscattered Stokes beam from the tip shank of the probe and an amplified Stokes signal from the sample.

12. The method according to claim 1 , further comprising detecting optical signal modulation at a polarization modulation frequency fm using an optical detector and displaying the detected optical signal modulation as the amplitude of the molecular vibration.

13. The method according to claim 1 , wherein the stimulating Stokes beam has a P polarization or a S polarization.

14. The method according to claim 1 , wherein the polarization modulated pump beam is produced by the interference of two frequency shifted P polarized pump beams focused on the tip end of the probe.

15. The method according to claim 1 , where the polarization modulated pump beam is a pulse with a width in a picosecond range and the stimulating Stokes beam is in the 50 femtosecond range.

16. The method according to claim 14 , wherein the polarization modulated pump beam consists of two pulsed pump beams, where one of the pulsed pump beams is not frequency shifted and the other is frequency shifted by frequency f 2 and delayed using a delay path.

17. The method according to claim 15 , wherein the stimulating Stokes beam consists of two pulsed beams, wherein one of the two pulsed Stokes beams is frequency shifted by frequency f 1 and delayed using a delay path ganged with the same delay path as the delayed pulsed pump beam so that the delayed pulsed Stokes beam overlaps in time and space with the delayed pulsed pump beam.

18. The method of claim 16 , further comprising detecting a signal at frequency f 1 −f 2 generated by interference of the two pulsed Stokes beams that are backscattered from the tip end of the probe and the tip shank at an optical detector, the signal at frequency f 1 −f 2 providing the amplitude and phase of the molecular vibration.

19. The method according to claim 17 , wherein the amplitude and phase of the signal at frequency f 1 −f 2 is detected using a lock-in amplifier.

20. The method according to claim 18 , wherein the output of the lock-in amplifier is recorded as a function of the delay between the delayed pulsed pump beam and the other pulsed pump beam.

21. The method of claim 19 , wherein the recorded output of the lock-in amplifier is Fourier transformed to yield the Raman spectrum of the sample beneath the probe.

22. An apparatus for measuring amplitude and/or phase of a molecular vibration, the apparatus comprising:
a mechanism configured to displace a probe of a scanning probe microscope to approach a sample;
a first beam generator configured to generate a polarization modulated pump beam to illuminate the probe;
a second beam generator configured to generate a stimulating Stokes beam to introduce a stimulating Stokes beam onto the probe; and
an optical detection system configured to detect a Raman scattered Stokes beam from the sample to derive at least one of the amplitude and the phase of the molecular vibration.

Assignees

• Univ California

Inventors

• Wickramasinghe H Kumar

Classifications

• G01J3/44

  Raman spectrometry; Scattering spectrometry {; Fluorescence spectrometry} · CPC title

• G01J2009/0261

  polarised · CPC title

• G01N2021/655

  Stimulated Raman · CPC title

• G01J3/45

  Interferometric spectrometry · CPC title

• G01J3/0224

  using polarising or depolarising elements · CPC title