Photoacoustic remote sensing (PARS)

1. A photoacoustic remote sensing system (PARS) for imaging a subsurface structure in a sample with optical resolution, comprising:

a pulsed or intensity-modulated excitation beam configured to generate ultrasonic signals in the sample at an excitation location;

a continuous interrogation beam incident on the sample at the excitation location, a portion of the interrogation beam returning from the sample that is indicative of the generated ultrasonic signals;

an optical system that focuses the excitation beam at a first focal point and the interrogation beam at a second focal point, the first and second focal points being below the surface of and within the sample; and

an interferometer that detects the returning portion of the interrogation beam.

2. The PARS of claim 1 , wherein the first and second focal points are within 1 mm of the surface of the sample.

3. The PARS of claim 1 , wherein the first and second focal points are greater than 1 μm below the surface of the sample.

4. The PARS of claim 1 , wherein at least one of the first and second focal points is spaced below the surface of the sample at a depth that is greater than a focal zone of the respective at least one of the excitation beam and the interrogation beam.

5. The PARS of claim 1 , wherein the excitation beam and the interrogation beam have a lateral separation of less than 1 mm within the sample.

6. The PARS of claim 1 , wherein the excitation beam and the interrogation beam have a lateral separation of less than 0.5 mm within the sample.

7. The PARS of claim 1 , wherein the excitation beam has a focal point that is laterally within the focal zone of the interrogation beam.

8. The PARS of claim 1 , wherein the interrogation beam has a focal point that is laterally within the focal zone of the excitation beam.

9. The PARS of claim 1 , further comprising a processor that calculates an image of the sample based on the returning portion of the interrogation beam.

10. The PARS of claim 1 , wherein at least one of the first focal point and the second focal point has a focal diameter of less than 30 μm.

11. The PARS of claim 1 , wherein at least one of the first focal point and the second focal point has a focal diameter of less than 10 82 m.

12. The PARS of claim 1 , wherein at least one of the first focal point and the second focal point has a focal diameter of less than 1 μm.

13. The PARS of claim 1 , wherein the excitation beam is scanned through the sample while the interrogation beam is stationary.

14. The PARS of claim 1 , wherein the interrogation beam is scanned through the sample while the excitation beam is stationary.

15. The PARS of claim 1 , wherein each of the interrogation beam and the excitation beam are scanned through the sample concurrently.

16. An endoscopic device that uses a photoacoustic remote sensing confocal microscopy system (PARS) for imaging a subsurface structure in a sample with optical resolution, the endoscopic device comprising:

a fiber optic cable having an input end and a detection end;

a pulsed or intensity-modulated excitation beam coupled to the input end of the fiber optic cable, wherein in use the excitation beam generates ultrasonic signals in the sample at an excitation location that is adjacent to the detection end of the fiber optic cable, the fiber optic cable focusing the excitation beam at a first focal point that is below the surface of and within the sample;

a continuous interrogation beam coupled to the input end of the fiber optic cable and incident on the excitation location, the fiber optic cable focusing the interrogation beam at a second focal point that is below the surface of and within the sample, wherein a portion of the interrogation beam that is indicative of the generated ultrasonic signals is received by the detection end of the fiber optic cable and travels to the input end; and

an interferometer that receives the returning portion of the interrogation beam at the input end of the fiber optic cable.