System and method for analyzing light by three-photon counting

1. A system for measuring one or more characteristics of light of a photon energy E ph from a light source, that can be determined from measuring three-photon absorption events, the system comprising:

a) a detector having a band gap material characterized by gap energy between 2.1 and 3 times E ph ;

b) an optical element configured to concentrate a beam of light from the light source on the detector;

c) a signal amplifier that amplifies an output signal indicative of when three photons produced by the light source undergo a three-photon absorption event in the band gap material; and

d) an analyzer that analyzes the output signal to count or measure a rate of the three-photon absorption events, and determines the one or more characteristics of the light from the light source.

2. A system according to claim 1 , also including the light source.

3. A system according to claim 2 , wherein the light source and optical element are configured for producing a light intensity I on the detector, and the band gap material has a thickness of at least 0.5 times a minimum of an absorption length for three-photon absorption of light of photon energy E ph and intensity I in the band gap material, and a Rayleigh length for the light beam concentrated on the band gap material.

4. A system according to claim 1 , wherein the band gap material has a thickness no greater than twice a diffusion length of electrons in the conduction band of the band gap material.

5. A system according to claim 2 , wherein the optical system concentrates the light from the light source so that more than half of the power falls within a spot smaller than 10 wavelengths in diameter, on the detector.

6. A system according to claim 2 , wherein the light source produces few enough photons of energy greater than half the gap energy of the band gap material, so that any two-photon or one-photon absorption in the detector contributes less to the output signal than three-photon absorption, by at least a factor of 10.

7. A system according to claim 2 , comprising a filter that filters out enough photons of energy greater than half the gap energy of the band gap material, so that any two-photon or one-photon absorption in the detector contributes less to the output signal than three-photon absorption, by at least a factor of 10.

8. A system according to claim 2 , wherein more than 90% of the three-photon absorption events produced in the band gap material by photons from the light source are due to entangled three-photon states, and the one or more characteristics comprise a number of entangled three-photon states emitted by the light source.

9. A system according to claim 1 , wherein the one or more characteristics comprise a magnitude and direction of asymmetry in a shape of pulses produced by the light source, and the analyzer is configured to find the magnitude and direction of asymmetry from the output signal.

10. A system according to claim 9 , also comprising the light source, wherein the pulses produced by the light source have full width at half maximum intensity shorter than 1 picosecond.

11. A system according to claim 1 , wherein the one or more characteristics comprise a complete pulse shape of pulses produced by the light source, and the analyzer is configured to find the complete pulse shape from the output signal.

12. A system according to claim 1 , also including the light source, wherein the light source is a pulsed light source, and one or both of the signal amplifier and the analyzer is gated so that data is used only from a time interval around each pulse, that is shorter than an interval between pulses.

13. A system according to claim 1 , wherein the one or more characteristics comprise a third order coherence function of the light source, and the analyzer is configured to find the third order coherence function from the output signal.

14. A system according to claim 13 , wherein the third order coherence function has a sensitivity for pulsed light better than 500 femtojoule per pulse at a timescale shorter than 1 femtosecond.

15. A system according to claim 13 , also including the light source, wherein the light source comprises a polariton laser.

16. A system according to claim 1 , wherein the response time of the band gap material and the signal amplifier are short enough so that the time resolution of the output signal is shorter than 1 nanosecond.

17. A system according to claim 1 , wherein the signal amplifier comprises a photomultiplier tube.

18. A system according to claim 1 , wherein the signal amplifier comprises an avalanche photo diode.

19. A system according to claim 1 , also including a light source modulator, configured to modulate the intensity of the beam of light concentrated on the detector by a modulation frequency, wherein the analyzer is configured to find a component of the output signal modulated at 3 times the modulation frequency, due to three-photon absorption.

20. A system according to claim 1 , wherein the detector comprises an anti-reflection coating suitable for light of photon energy E ph .

21. A system according to claim 1 , wherein the detector comprises a resonant cavity for light of photon energy E ph .

22. A system according to claim 21 , wherein the resonant cavity comprises one or more of a photonic crystal, a Fabry-Perot cavity, and a plasmonic cavity.

23. A system for measuring one or more characteristics of light of a photon energy E ph from a light source, that can be determined from measuring three-photon absorption, the system comprising:

a) a detector having a band gap material characterized by gap energy between 2.1 and 3 times E ph , comprising an anti-reflection coating suitable for light of photon energy E ph ;

b) an optical element configured to concentrate light from the light source on the detector;

c) a signal amplifier that amplifies an output signal indicative of when three photons produced by the light source undergo a three-photon absorption event in the band gap material, the output signal being usable for counting or measuring a rate of three-photon absorption events and determining the one or more characteristics of light from the light source.

24. A method of measuring one or more characteristics of light of a photon energy E ph from a light source, that can be determined from measuring three-photon absorption events, the method comprising:

a) producing light from the light source;

b) concentrating the light on a detector having a band gap material characterized by band gap between 2.1 and 3 times E ph ;

c) collecting an electron excited to the conduction band of the band gap material by a three-photon absorption event, caused by light from the light source;

d) amplifying a current of the collected electron to produce an indication of the three-photon absorption event in an output signal; and

e) analyzing the output signal to count or measure a rate of the three-photon absorption events and determine the one or more characteristics of the light from the light source.

25. A method according to claim 24 , wherein the light source is a pulsed light source, and the one or more characteristics comprise a magnitude and direction of asymmetry of the shape of pulses produced by the light source.

26. A method according to claim 24 , wherein the one or more characteristics comprise a third order coherence function of the light source.

27. A method according to claim 24 , wherein the one or more characteristics comprise a number of entangled three-photon states emitted by the light source.

28. A method according to claim 24 , wherein the band gap material has a thickness of at least 0.5 times a minimum of an absorption length for three-photon absorption of the light concentrated on the semiconductor detector, and a Rayleigh length for the light concentrated on the semiconductor detector.

29. A method according to claim 24 , wherein more than half of the power of the light concentrated on the detector falls within a spot smaller than 10 wavelengths in diameter.