Cutting elements comprising sensors, earth-boring tools comprising such cutting elements, and methods of forming wellbores with such tools

1. An earth-boring tool, comprising:

a bit body defining at least one pocket therein;

at least one cutting element secured within the at least one pocket of the bit body, the at least one cutting element comprising a hard material, a substrate, and a window laterally surrounded by the hard material, wherein the window comprises a material having a different composition or structure than the hard material and the same thickness as the hard material; and

at least one signal generator and at least one sensor, wherein the at least one signal generator and the at least one sensor are each disposed within a void defined within the substrate of the at least one cutting element, wherein the at least one signal generator is configured to provide an electromagnetic or acoustic signal through the window to an interface between a surface of the hard material and a subterranean formation in contact with at least a portion of the surface, and wherein the at least one sensor is configured to receive a return electromagnetic or acoustic signal through the window from the interface.

2. The earth-boring tool of claim 1 , wherein the at least one signal generator is configured to provide electromagnetic radiation to the interface, and wherein the at least one sensor comprises a spectrometer configured to receive electromagnetic radiation from the interface.

3. The earth-boring tool of claim 2 , wherein the spectrometer comprises an infrared spectrometer.

4. The earth-boring tool of claim 1 , wherein the window comprises at least one exterior surface transparent to at least one wavelength of electromagnetic radiation.

5. The earth-boring tool of claim 1 , wherein the window comprises a single-crystal diamond window.

6. The earth-boring tool of claim 5 , wherein the hard material comprises polycrystalline diamond.

7. The earth-boring tool of claim 1 , wherein the earth-boring tool comprises a transceiver comprising both a signal generator and a sensor.

8. A method of forming a wellbore, comprising:

rotating a bit body of an earth-boring tool within a wellbore and cutting formation material with at least one cutting element mounted within at least one pocket of the bit body, the at least one cutting element comprising:

a generally planar volume of hard material bonded to a substrate;

a window laterally surrounded by the hard material, wherein the window comprises a material having a different composition or structure than the hard material and the same thickness as the hard material;

a transmitter disposed within a void defined within the substrate, the transmitter comprising at least one transmitter selected from the group consisting of an electromagnetic and an acoustic transmitter; and

a sensor disposed within the void defined within the substrate, the sensor comprising at least one sensor selected from the group consisting of a spectrometer and an acoustic receiver;

transmitting a signal through the window to an interface between a surface of the at least one cutting element and the formation material, the signal comprising at least one signal selected from the group consisting of an electromagnetic signal and an acoustic signal; and

measuring a response received at the sensor from the interface through the window as the at least one cutting element is used to cut formation material, the response comprising at least one response selected from the group consisting of an electromagnetic response and an acoustic response.

9. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises determining a composition of the formation material.

10. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises determining a thickness of the at least one cutting element.

11. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises measuring an amplitude of the response.

12. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises measuring a period of time between the transmission of the signal and reception of the response at the sensor.

13. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises measuring an infrared or Raman signal at the sensor.

14. The method of claim 8 , wherein measuring a response received at the sensor as the at least one cutting element is used to cut formation material comprises measuring a wavelength between about 2.5 μm and about 12.0 μm.

15. A cutting element for an earth-boring drilling tool, the cutting element comprising:

cutting element body comprising a diamond table bonded to a substrate, and a window laterally surrounded by the diamond table wherein the window comprises a material having a different composition or structure than the diamond table and the same thickness as the diamond table, wherein the substrate defines a void within the substrate and wherein the diamond table defines a cutting surface;

at least one transmitter disposed within the void within the substrate, the at least one transmitter oriented and configured to dispense an electromagnetic or acoustic signal through the window to an interface between the cutting surface and a surface of a subterranean formation engaged by at least a portion of the cutting surface; and

at least one sensor disposed within the void within the substrate, the at least one sensor oriented and configured to measure an electromagnetic or acoustic signal returned through the window from the interface between the cutting surface and the surface of the subterranean formation engaged by the at least a portion of the cutting surface.

16. The cutting element of claim 15 , wherein the at least one sensor is configured to measure an electromagnetic or acoustic signal reflected from the interface.

17. The cutting element of claim 15 , wherein the at least one sensor is configured to measure an electromagnetic or acoustic signal retransmitted from the interface.

18. The cutting element of claim 15 , wherein the window comprises single-crystal diamond.