System and Method for terahertz frequency measurements

• US 6,479,822 B1
• Filed: 07/07/2000
• Issued: 11/12/2002
• Est. Priority Date: 07/07/2000
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology
• Original Assignee: Massachusetts Institute of Technology
• Inventors: Crimmins, Timothy, Nelson, Keith A.
• Primary Examiner(s): Hannaher, Constantine
• Assistant Examiner(s): Moran, Timothy J
• Application Number:
  US09/611,834
• Time in Patent Office: 858 Days
• Field of Search: 250/330, 250/358.1, 250/338.1, 250/341.1, 359/239, 356/432, 356/445, 356/451, 356/484, 356/521
• US Class Current: 250/341.1
• CPC Class Codes: G01N 21/3563 for analysing solids; Preparation of samples therefor
• CPC Class Codes: G01N 21/3581 using far infrared light; using Terahertz radiation

Abstract

A spectroscopic method for characterizing a sample including: positioning the sample adjacent to a non-centrosymmetric material; directing at least one temporal pulse of coherent EM radiation into the non-centrosymmetric material to generate a polariton therein and cause EM radiation from the polariton to propagate into the sample, wherein the polariton has a frequency less than or equal to the bandwidth of the pulse; and measuring a response of the sample to the EM radiation from the polariton.

Claims

1. A spectroscopic method for characterizing a sample comprising:

positioning the sample adjacent to a non-centrosymmetric material;

directing at least one temporal pulse of coherent EM radiation into the non-centrosymmtric material to generate a polariton therein and cause EM radiation from the polariton to propagate into the sample, wherein the polariton has a frequency less than or equal to the bandwidth of the pulse; and

measuring a response of the sample to the EM radiation from the polariton.

2. The method of claim 1, wherein the measuring comprises:

positioning a second non-centrosymmetric material to receive EM radiation from the sample in response to the EM radiation from the polariton in the first-mentioned non-centrosymmetric material, wherein the EM radiation from the sample propagates into the second non-centrosymmetric material to form another polariton;

directing additional EM radiation to interact with the polariton in the second non-centrosymmetric material; and

measuring a response of the second non-centrosymmetric material to the interaction of the additional EM radiation and the polariton in the second non-centrosymmetric material.

3. The method of claim 2, wherein the response of the second non-centrosymmetric material is measured by measuring at least one of transmission, reflection, polarization rotation, and diffraction of the additional EM radiation by the polariton in the second non-centrosymmetric material.

4. The method of claim 3, wherein the at least one of the transmission, reflection, polarization rotation, and diffraction of the additional EM radiation is spectrally resolved.

5. The method of claim 2, wherein the response of the second non-centrosymmetric material is one of sum-frequency generation and difference-frequency generation caused by the interaction of the additional EM radiation and the polariton in the second non-centrosymmetric material.

6. The method of claim 2, wherein the response of the second non-centrosymmetric material is measured by using the additional EM radiation to image the polariton in the second non-centrosymmetric material.

7. The method of claim 2, wherein the measured response of the second non-centrosymmetric material is indicative of the amplitude and phase of the polariton in the second non-centrosymmetric material.

8. The method of claim 1, wherein the measuring comprises:

directing additional EM radiation to interact with the EM radiation in the sample from the polariton; and

measuring a response of the sample to the interaction of the additional EM radiation and the EM radiation in the sample from the polariton.

9. The method of claim 8, wherein the second-mentioned measuring comprises measuring at least one of diffraction, reflection, a change in absorption, and a change in polarization of the additional EM radiation by the sample caused by the presence of the EM radiation in the sample from the polariton.

10. The method of claim 9, wherein the measured at least one of diffraction, reflection, the change in absorption, and the change in polarization is spectrally resolved.

11. The method of claim 8, wherein the response is one of sum-frequency generation and difference-frequency generation caused by the interaction of the additional EM radiation and the EM radiation in the sample from the polariton.

12. The method of claim 1, wherein the EM radiation from the polariton reflects from a reflecting surface and back into the non-centrosymmetric material to form a second polariton, and wherein the measuring comprises:

directing additional EM radiation to interact with the second polariton; and

measuring a response of the non-centrosymmetric material to the interaction of the additional EM radiation and the second polariton.

13. The method of claim 1, wherein the measured response is indicative of absorption by the sample at the frequency of the EM radiation from the polariton.

14. The method of claim 1, wherein the measured response is indicative of the refractive index of the sample at the frequency of the EM radiation from the polariton.

15. The method of claim 1, wherein the measured response is indicative of the complex refractive index of the sample at the frequency of the EM radiation from the polariton.

16. The method of claim 1, further comprising repeating the positioning, directing, and measuring for a reference sample, and comparing the measurements of the first-mentioned sample and the reference sample.

17. The method of claim 1, further comprising temporally shaping EM radiation to form the at least one temporal pulse.

18. The method of claim 1, wherein the directing comprises directing a spatially periodic pattern of the at least one temporal pulse of EM radiation onto the non-centrosymmetric material.

19. The method of claim 18, further comprising selecting the period of the spatially periodic pattern to produce a selected frequency for the polariton.

20. The method of claim 18, wherein the directing further comprises crossing at least two beams of the at least one temporal pulse of EM radiation to form the spatially periodic pattern.

21. The method of claim 18, wherein the directing further comprises passing EM radiation through a mask to produce the spatially periodic pattern on the non-centrosymmetric material.

22. The method of claim 19, further comprising repeating the selecting, directing, and measuring for multiple frequencies of the polariton.

23. The method of claim 1, further comprising focusing the EM radiation from the polariton prior to it reaching the sample.

24. The method of claim 1, wherein the sample is one of a liquid, solid, and gas.

25. The method of claim 1, wherein the sample is separated from the non-centrosymmetric material.

26. The method of claim 1, wherein the frequency of the polariton is in the range of about 0.1 to 20 THz.

27. The method of claim 26, wherein the frequency of the polariton is in the range of about 1 to 10 THz.

28. An apparatus for characterizing a sample, comprising:

a non-centrosymmetric material;

a sample assembly configured to support the sample adjacent to the non-centrosymmetric material;

a light source which during operation directs at least one temporal pulse of coherent EM radiation into the non-centrosymmetric material, the at least one temporal pulse having an intensity and bandwidth sufficient to generate a polariton in the non-centrosymmetric material and cause EM radiation from the polariton to propagate into the sample, wherein the polariton has a frequency less than or equal to the bandwidth of the pulse.

29. The apparatus of claim 28 further comprising:

a second non-centrosymmetric material positioned to receive EM radiation from the sample in response to the EM radiation from the polariton in the first-mentioned non-centrosymmetric material, wherein during operation the EM radiation from the sample propagates into the second non-centrosymmetric material to form another polariton;

wherein during operation the light source directs a probe beam of EM radiation to interact with the polariton in the second non-centrosymmetric material; and

a detector positioned to measure a response of the second non-centrosymmetric material to the interaction of the probe beam and the polariton in the second non-centrosymmetric material.

30. The apparatus of claim 29, further comprising a computer coupled to the light source and the detector, wherein during operation the computer analyzes the measured response to characterize the sample.

31. The apparatus of claim 28 wherein:

during operation the light source directs a probe beam of additional EM radiation to interact with the EM radiation in the sample from the polariton, and the apparatus further comprises

a detector positioned to measure a response of the sample to the interaction of the probe beam and the EM radiation in the sample from the polariton.

32. The apparatus of claim 31, further comprising a computer coupled to the light source and the detector, wherein during operation the computer analyzes the measured response to characterize the sample.

33. The apparatus of claim 28, wherein the sample assembly comprises a reflecting surface configured to reflect the EM radiation from the polariton back into the non-centrosymmetric material to form a second polariton, and wherein during operation the light source directs a probe beam of additional EM radiation to interact with the second polariton, and the apparatus further comprises a detector positioned to measure a response of the non-centrosymmetric material to the interaction of the additional EM radiation and the second polariton.

34. The apparatus of claim 28, wherein during operation the light source causes the at least one temporal pulse to form a periodic spatial pattern in the non-centrosymmetric material.

35. The apparatus of claim 34, wherein during operation the light source directs a pair of excitation beams to interfere in the non-centrosymmetric material to form the periodic spatial pattern, the pair of excitation beams comprising the at least one temporal pulse.

36. The apparatus of claim 34, wherein the light source comprises a laser source, a phase mask, and one or more imaging lenses, wherein during operation the laser source directs a beam to the phase mask, the phase mask diffracts the beam into multiple orders, and the one or more imaging lenses recombines the multiple orders in the non-centrosymmetric material to form the periodic spatial pattern.

37. The apparatus of claim 34, further comprising a computer coupled to the light source to control the period of the periodic spatial pattern.