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US-20260126479-A1 - Vapor Cells Having Optical Windows with Multilayer Coatings

US20260126479A1US 20260126479 A1US20260126479 A1US 20260126479A1US-20260126479-A1

Abstract

In a general aspect, vapor cells are disclosed that include a dielectric body and an optical window. The dielectric body has a cavity and an exterior surface that defines an opening to the cavity. The optical window includes a substrate and first and second multilayer coatings. In some aspects, the substrate has first and second substrate surfaces on opposite sides of the substrate, and the first and second multilayer coatings are disposed on, respectively, the first and second substrate surfaces. The first multilayer coating defines a first window surface that is bonded to the exterior surface of the dielectric body and extends across the opening. The second multilayer coating defines a second window surface that faces an exterior of a vapor cell. The first and second multilayer coatings apply, respectively, first and second stresses to the substrate, with the second stress counteracting the first stress.

Inventors

  • Rajesh PANDIYAN
  • Sean Michael O'Neill
  • James P. Shaffer

Assignees

  • Quantum Valley Ideas Laboratories

Dates

Publication Date
20260507
Application Date
20251106

Claims (20)

  1. 1 . A vapor cell, comprising: a dielectric body having a cavity and an exterior surface that defines an opening to the cavity; a vapor or a source of the vapor residing in the cavity, the vapor or the source of the vapor comprising alkali metal atoms; and an optical window comprising: a substrate having first and second substrate surfaces on opposite sides of the substrate, a first multilayer coating disposed on the first substrate surface and defining a first window surface of the optical window, the first window surface bonded to the exterior surface of the dielectric body and extending across the opening, the first multilayer coating applying a first stress to the substrate, and a second multilayer coating disposed on the second substrate surface and defining a second window surface of the optical window, the second window surface facing an exterior of the vapor cell, the second multilayer coating applying a second stress to the substrate that counteracts the first stress.
  2. 2 . The vapor cell of claim 1 , wherein the first multilayer coating or the second multilayer coating comprises a sequence of layers having respective layer stresses that alternate between tension and compression.
  3. 3 . The vapor cell of claim 1 , wherein the first and second stresses oppose each other and result in a net stress applied to the substrate of no greater than 100 MPa.
  4. 4 . The vapor cell of claim 3 , wherein the net stress is no greater than 10 MPa.
  5. 5 . The vapor cell of claim 1 , wherein each layer in the first and second multilayer coatings has a layer stress of no greater than 10 MPa.
  6. 6 . The vapor cell of claim 1 , wherein the alkali metal atoms have a target optical transition when in a vapor state; wherein the substrate is transparent to a laser wavelength that is matched to the target optical transition; and wherein the first and second multilayer coatings each comprise a first type of layer and a second type of layer, the first type of layer having a first index of refraction at the laser wavelength in a first range from 1.9 to 4.5, the second type of layer having a second index of refraction at the laser wavelength in a second range from 1.2 to 1.9.
  7. 7 . The vapor cell of claim 6 , wherein the substrate has a substrate index of refraction at the laser wavelength in a substrate range from 1.3 to 1.6.
  8. 8 . The vapor cell of claim 6 , wherein the first and second multilayer coatings each comprise a third type of layer that has a third index of refraction at the laser wavelength, the third index of refraction having a magnitude between the first and second indices of refraction.
  9. 9 . The vapor cell of claim 1 , wherein the first multilayer coating comprises an end layer that is formed of a material chemically inert to the alkali metal atoms; and wherein the end layer comprises: a bonding portion that is bonded to the exterior surface of the dielectric body, and a covering portion that extends across the opening to the cavity.
  10. 10 . The vapor cell of claim 1 , wherein the first multilayer coating comprises an end layer that is formed of amorphous silicon; and wherein the end layer comprises: a bonding portion that is bonded to the exterior surface of the dielectric body, and a covering portion that extends across the opening to the cavity.
  11. 11 . The vapor cell of claim 10 , wherein the dielectric body is a silicon dielectric body and comprises a silicon oxide layer that defines the exterior surface of the dielectric body.
  12. 12 . The vapor cell of claim 10 , wherein the dielectric body comprises silicon oxide.
  13. 13 . The vapor cell of claim 1 , wherein the first multilayer coating has a root mean square (RMS) surface roughness, R q , no greater than 1 nanometer.
  14. 14 . The vapor cell of claim 1 , wherein the alkali metal atoms have a target optical transition when in a vapor state, and the substrate is transparent to a laser wavelength that is matched to the target optical transition; and wherein the first and second multilayer coatings are each configured as an antireflection coating for the laser wavelength.
  15. 15 . A method of manufacturing a vapor cell, comprising: fabricating an optical window by: obtaining a substrate having first and second substrate surfaces on opposite sides of the substrate, forming a first multilayer coating on the first substrate surface, thereby defining a first window surface of the optical window, the first multilayer coating applying a first stress to the substrate, and forming a second multilayer coating on the second substrate surface, thereby defining a second window surface of the optical window, the second multilayer coating applying a second stress to the substrate that counteracts the first stress; obtaining a dielectric body that has a cavity and an exterior surface that defines an opening to the cavity; and bonding the first window surface of the optical window to the exterior surface of the dielectric body, the first window surface extending across the opening to the cavity, the second window surface on a side of the optical window opposite the first window surface.
  16. 16 . The method of claim 15 , wherein the first multiplayer coating or the second multilayer coating comprises a sequence of layers having respective layer stresses that alternate between tension and compression.
  17. 17 . The method of claim 15 , wherein the first and second stresses oppose each other and result in a net stress applied to the substrate of no greater than 100 MPa.
  18. 18 . The method of claim 17 , wherein the net stress is no greater than 10 MPa.
  19. 19 . The method of claim 15 , wherein each layer in the first and second multilayer coatings has a layer stress of no greater than 10 MPa.
  20. 20 . The method of claim 15 , wherein the substrate is transparent to a laser wavelength that is matched to a target optical transition of alkali metal atoms when in a vapor state; and wherein the first and second multilayer coatings each comprise a first type of layer and a second type of layer, the first type of layer having a first index of refraction at the laser wavelength in a first range from 1.9 to 4.5, the second type of layer having a second index of refraction at the laser wavelength in a second range from 1.2 to 1.9.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Prov. Pat. App. No. 63/717,686, which was filed on Nov. 7, 2024, and entitled, “Bonding Vapor Cells That Have Optical Coatings.” The disclosure of the priority application is hereby incorporated herein by reference in its entirety. GOVERNMENT LICENSE RIGHTS This invention(s) was made with U.S. government support as part of the “Science of Atomic Vapors for New Technologies” (SAVaNT) program under Contract No. HR00112190080 to the Defense Advanced Research Projects Agency (DARPA). The U.S. government has certain rights in the invention(s). BACKGROUND The following description relates to vapor cells that have optical windows with multilayer coatings. Vapor cells can be manufactured by sealing a vapor within an enclosed volume. The vapor can be used as a medium to interact with electromagnetic fields that are incident on the vapor cell. Beams of light generated by lasers can be directed through the vapor to probe and measure the response of the vapor to the electromagnetic fields. In this way, a vapor cell can be used to determine properties of the electromagnetic fields. DESCRIPTION OF DRAWINGS FIG. 1A is a schematic diagram, in exploded perspective view, of an example vapor cell having a dielectric body and an optical window; FIG. 1B is a schematic diagram, in perspective view, of the example vapor cell of FIG. 1A, but in which the optical window is bonded to the dielectric body; FIG. 1C is a schematic diagram, in exploded perspective view, of the example vapor cell of FIG. 1A, but in which a source of vapor resides in cavity; FIG. 1D is a schematic diagram, in exploded perspective view, of the example vapor cell of FIG. 1A, but in which a source of vapor resides in a second chamber of a cavity of the example vapor cell; FIG. 1E is a schematic diagram, in cross section view, of the optical window of FIG. 1A, showing a substrate of the optical window and first and second multilayer coatings disposed on respective surfaces of the substrate; FIG. 1F is a schematic diagram, in exploded perspective view, of the example vapor cell of FIG. 1A, but in which the optical window includes covering and bonding portions; FIG. 2A is a schematic diagram, in exploded perspective view, of an example vapor cell having two optical windows; FIG. 2B is a schematic diagram, in perspective view, of the example vapor cell of FIG. 2A, but in which both optical windows are bonded to a dielectric body of the example vapor cell; FIG. 3 is a flowchart of an example processes for bonding an optical window to a dielectric body of a vapor cell; FIG. 4 is a table showing examples of thicknesses for multilayer films that are stacked on both sides of a glass substrate; FIG. 5 is a schematic diagram, in elevation view, of an example optical window having first and second sets of integrated antireflection layers; FIG. 6A is a graph showing, in simulation, an example spectrum for an antireflection coating that, when centered at 509 nm, has a wavelength range from 500-540 nm, and when centered at 852 nm, has a wavelength range from 800-900 nm; FIG. 6B is a graph showing an example of a reflectance spectra that is measured for a deposited antireflection coating; FIG. 7 is a graph of a deflection measurement for an example optical window having first and second multilayer coatings on opposite sides of a borosilicate glass substrate; FIG. 8 is a scanning electron micrograph of an example surface morphology of a 20×20 μm2 area of a 50-nm thick layer of SiO2 grown using a “dry” thermal oxidation process; FIG. 9A is a photograph of an example antireflection structure that is based on six antireflection layers/glass/six antireflection layers/a-Si//SiO2/Si; FIG. 9B is a schematic diagram, in perspective view, of an example wafer bonding architecture; FIG. 9C is a photograph of an example vapor cell that includes two cavities and a side pocket connected thereto and further includes an integrated antireflection structure that is based four antireflection layers/glass/four antireflection layers/a-Si//SiO2/glass; FIG. 10A is a photograph of an example antireflection structure that is based on six antireflection layers/glass/six antireflection layers/a-Si//SiO2/Si and in which the example antireflection structure is being subjected to a crack opening method using a razor blade; FIG. 10B is a photograph of the bulk fracture surfaces of an example pair of substrates bonded together at a bonding temperature of 250° C.; and FIG. 10C is a photograph of the bulk fracture surfaces of a second example pair of substrates bonded together at a bonding temperature of 250° C. DETAILED DESCRIPTION In a general aspect, vapor cells are described that include optical windows with multilayer coatings thereon. Bonding processes for the optical windows are also described, and these bonding processes may correspond to wafer bonding processes in certain cases. The vapor cells may be engineered to enhan