Glass with modified surface layer

1. A glass substrate comprising:

an alkali-containing bulk having an aluminosilicate composition comprising from about 7 mol % to about 26 mol % Al 2 O 3 and at least 8 mol % Na 2 O; and

an alkali-depleted surface layer,

wherein the alkali-depleted surface layer is amorphous and comprises a substantially homogenous composition,

wherein the alkali-depleted layer comprises a thickness in the range from about 10 nm to about 3000 nm, and

further wherein the alkali-depleted surface layer comprises about 0.5 atomic % alkali or less.

2. The glass substrate of claim 1 , wherein the alkali-depleted surface layer is substantially free of crystallites.

3. The glass substrate of claim 1 , wherein the alkali-containing bulk and the alkali-depleted surface layer comprise Al 2 O 3 and SiO 2 , wherein the alkali-depleted surface layer comprises an atomic structure comprising aluminum substantially in a 4-coordinated state.

4. The glass substrate of claim 3 , wherein the atomic structure of the alkali-depleted surface layer comprises silicon in a 4-coordinated state.

5. The glass substrate of claim 3 , wherein less than about 5 % of the aluminum in the alkali-depleted surface layer is in coordinated state other than the 4-coordinated state.

6. The glass substrate of claim 3 , wherein the alkali-depleted surface layer comprises a total amount of oxygen, wherein from about 3 % to about 30 % by fraction of the total amount of oxygen is in a 3-coordinated state.

7. The glass substrate of claim 6 , wherein the alkali-depleted surface layer is substantially free of non-bridging oxygens.

8. The glass substrate of claim 7 , wherein the alkali-containing bulk comprises non-bridging oxygens and bridging oxygens.

9. The glass substrate of claim 7 , wherein the alkali-containing bulk is substantially free of non-bridging oxygens.

10. The glass substrate of claim 3 , wherein the alkali-containing bulk further comprises an alkali-metal oxide selected from Li 2 O, K 2 O, Rb 2 O and Cs 2 O.

11. The glass substrate of claim 3 , wherein the alkali-depleted surface layer comprises Al 2 O 3 in the range from about 1 mol % to about 50 mol %.

12. The glass substrate of claim 3 , wherein the alkali-depleted surface layer comprises a binary Al 2 O 3 —SiO 2 composition.

13. A glass substrate comprising:

a thickness t;

an alkali-containing bulk having a bulk refractive index and an aluminosilicate composition comprising from about 7 mol % to about 26 mol % Al 2 O 3 and at least 8 mol % Na 2 O; and

an alkali-depleted surface layer comprising a thickness in the range from about 10 nm to about 3000 nm,

wherein the alkali-depleted surface layer comprises a layer refractive index that is less than the bulk refractive index, and

further wherein the alkali-depleted surface layer comprises about 0.5 atomic % alkali or less.

14. A method of forming a glass substrate with a modified surface layer comprising:

providing a glass substrate comprising a concentration of alkali, a glass transition temperature (Tg) and a surface layer, the glass substrate further comprising an aluminosilicate composition comprising from about 7 mol % to about 26 mol % Al 2 O 3 and at least 8 mol % Na 2 O;

reducing the concentration of alkali in the surface layer,

wherein the reducing comprises contacting a surface of the glass substrate with an electrode and subjecting the glass substrate to a thermal poling,

wherein the electrode comprises an anode in contact with an anodic surface of the glass substrate and a cathode in contact with the cathodic surface of the glass substrate,

wherein the thermal poling comprises applying voltage to the glass substrate such that the anode is positively-biased relative to the glass substrate to induce alkali depletion at the anodic surface of the glass,

wherein the anode is a non-porous film comprising an oxidation-resistant conductive film or a noble metal, and the cathode is a graphite film,

wherein the surface layer with reduced concentration of alkali comprises a substantially homogenous composition, and

further wherein the surface layer with reduced concentration of alkali comprises about 0.5 atomic % alkali or less and a thickness in the range from about 10 nm to about 3000 nm.

15. The method of claim 14 , wherein the thermal poling comprises heating the glass substrate and electrode to a temperature below Tg prior to applying voltage to the glass substrate.

16. The method of claim 14 , wherein the thermal poling comprises applying voltage in the range from about 100 volts to about 10,000 volts to the glass substrate for a duration in the range from about 1 minute to about 6 hours.

17. The method of claim 14 , wherein the glass substrate is subjected to the thermal poling under vacuum, in an inert gas environment, or a permeable gas environment.