Labeling and authenticating using a microtag

1. An identifier, comprising:

a silicon film;

wherein the silicon film has a varying optical index of refraction;

wherein the varying optical index of refraction reflects one or more spectral peaks when illuminated with light;

wherein the one or more spectral peaks includes a reference peak; and

wherein the silicon film is fragmented and then oxidized.

2. The identifier as in claim 1 , wherein the identifier has an identifying data value associated with the one or more spectral peaks.

3. The identifier as in claim 2 , wherein the identifying data value includes an error correction code data.

4. The identifier as in claim 2 , wherein the identifying data value comprises a digital signature value.

5. The identifier as in claim 2 , wherein the identifying data value includes a channel code data.

6. The identifier as in claim 1 , wherein the identifier is added to an object for identification.

7. The identifier as in claim 1 , wherein the identifier comprises a rugate microtag.

8. The identifier as in claim 7 , wherein the varying optical index of refraction is generated using etching.

9. The identifier as in claim 8 , wherein the etching comprises electrochemical etching.

10. The identifier as in claim 9 , wherein the electrochemical etching creates small pores.

11. The identifier as in claim 1 , wherein the varying optical index of refraction comprises a sine-wave refractive index profile.

12. The identifier as in claim 1 , wherein the varying optical index of refraction comprises a plurality of composite sine-wave refractive index profiles.

13. The identifier as in claim 1 , wherein the reference peak position is a highest energy peak.

14. The identifier as in claim 1 , wherein the reference peak position is a lowest energy peak.