Near-field terahertz imaging
View Patent ↗A T-ray imaging system employing an optical resonator that is adapted to be (i) positioned in the near-field proximity to a surface of a sample and (ii) pumped with pump light such that the pump light traverses a relatively thin layer of an electro-optically responsive material (EORM) located in the resonator's cavity. The imaging system has an optical detector that is adapted to detect at least a portion of the pump light reflected from the resonator, while the sample is illuminated with terahertz (THz) radiation such that the EORM is exposed to that radiation resulting in a detectable phase shift in the reflected pump light.
1. A method of detecting interrogation radiation, comprising:
pumping an optical resonator having an electro-optically responsive material (EORM) with pump light, wherein the pump light traverses the EORM; and
detecting at least a portion of the pump light output from the resonator, while exposing the EORM to the interrogation radiation, wherein:
the optical resonator is defined by first and second reflectors, wherein the EORM is interposed between said first and second reflectors; and
one of the first and second reflectors comprises a Bragg reflector.
2. The invention of claim 1 , wherein the interrogation radiation comprises THz radiation.
3. The invention of claim 1 , further comprising:
positioning the EORM in near-field proximity to a surface of a sample; and
repeating the steps of pumping and detecting such that, for at least some repetitions, the pump light traverses the EORM at different spatial locations with respect to said surface.
4. The invention of claim 3 , further comprising forming an image of the sample based on the detection results.
5. The invention of claim 3 , comprising the step of scanning a beam of the pump light over the resonator while keeping position of the resonator with respect to the sample substantially unchanged.
6. The invention of claim 3 , comprising the step of changing relative position of the resonator and the sample.
7. The invention of claim 1 , wherein:
pumping the optical resonator comprises the step of transmitting the pump light from outside the resonator through the first reflector and into the resonator interior.
8. The invention of claim 7 , further comprising:
reverberating the pump light between the first and second reflectors; and
transmitting the pump light from the resonator interior through the first reflector.
9. The invention of claim 7 , wherein each of the first and second reflectors is a Bragg reflector.
10. The invention of claim 1 , wherein:
the EORM is characterized by first and second principal polarization axes; and
the step of detecting comprises:
constructively interfering pump light polarized along the first and second principal polarization axes to form a first output beam;
destructively interfering the pump light polarized along the first and second principal polarization axes to form a second output beam; and
converting the first and second output beams into an electrical signal indicative of a phase shift between the pump light polarized along the first principal polarization axis and the pump light polarized along the second principal polarization axis.
11. The invention of claim 10 , wherein the step of detecting comprises introducing a constant π/2 phase shift between the pump light polarized along the first principal polarization axis and the pump light polarized along the second principal polarization axis.
12. A system, comprising:
an optical resonator having an electro-optically responsive material (EORM), wherein said resonator is adapted to be pumped with pump light such that the pump light traverses the EORM, wherein the EORM is adapted to be exposed to interrogation radiation; and
an optical detector adapted to detect at least a portion of the pump light output from the resonator having the EORM exposed to the interrogation radiation, wherein:
the optical resonator is defined by first and second reflectors, wherein the EORM is interposed between said first and second reflectors; and
one of the first and second reflectors comprises a Bragg reflector.
13. The invention of claim 12 , wherein the interrogation radiation comprises THz radiation.
14. The invention of claim 12 , wherein:
the EORM is adapted to be positioned in near-field proximity to a surface of a sample; and
the system is adapted to form an image of the sample by configuring the detector to serially detect the pump light that traversed the EORM at two or more different spatial locations with respect to said surface.
15. The invention of claim 14 , wherein the system is adapted to scan a beam of the pump light over the resonator, while keeping position of the resonator with respect to the sample substantially unchanged.
16. The invention of claim 12 , wherein:
each of the first and second reflectors is a Bragg reflector.
17. The invention of claim 12 , wherein, for the pump light, a ratio of the reflection coefficient of the second reflector to the reflection coefficient of the first reflector has a value between about 0.01 and 0.7.
18. The invention of claim 12 , further comprising:
a first source adapted to produce the interrogation radiation; and
a second source adapted to produce the pump light, wherein the system is adapted to function using at least one of pulsed pump light and pulsed interrogation radiation.
19. The invention of claim 12 , wherein:
the EORM is characterized by first and second principal polarization axes; and
the detector comprises:
a polarizing beam splitter adapted to: (i) constructively interfere pump light polarized along the first and second principal polarization axes to form a first output beam and (ii) destructively interfere the pump light polarized along the first and second principal polarization axes to form a second output beam; and
a balanced photodetector pair adapted to convert the first and second output beams into an electrical signal indicative of a phase shift between the pump light polarized along the first principal polarization axis and the pump light polarized along the second principal polarization axis.
20. The invention of claim 19 , wherein the detector comprises a quarter-wave plate adapted to introduce a constant π/2 phase shift between the pump light polarized along the first principal polarization axis and the pump light polarized along the second principal polarization axis.
21. A method of detecting interrogation radiation, comprising:
pumping an optical resonator having an electro-optically responsive material (EORM) with pump light, wherein the pump light traverses the EORM;
positioning the EORM in near-field proximity to a surface of a sample; and
detecting at least a portion of the pump light output from the resonator, while exposing the EORM to the interrogation radiation.
22. The invention of claim 21 , wherein the interrogation radiation comprises THz radiation.