Tunable cross-coupling evanescent mode optical devices and methods of making the same

Tunable cross-coupling evanescent mode optical devices and methods of making the same are described. In one aspect, a tunable optical device includes a first layer, a second layer, a metal layer disposed between the first and second layers, and an electrode. The first layer is supportable of electromagnetic field modes over a range of wavelengths that includes a target wavelength range. The second layer is disposed between the metal layer and the electrode and has an index of refraction that is adjustable over a range of values. The metal layer is disposed between the first and second layers and has at least one corrugated metal film region with a corrugation periodicity enabling cross-coupling of evanescent modes of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer with different respective wavenumbers. The cross-coupling evanescent modes have a cross-coupling wavelength determined at least in part by the corrugation periodicity and the index of refraction of the first and second layers and is substantially unaffected by the electrode.

BACKGROUND

Surface plasmon resonance occurs when radiant energy is “coupled” (or transferred) to electrons in a metal. The wavelength of light at which coupling occurs depends on the characteristics of the metal that is illuminated and the optical properties of the surrounding environment. When there is a match or resonance between the energy and wavenumber of the light photons and the electrons at the metal surface, a transfer of energy occurs. The coupling of light into a metal surface produces a plasmon (i.e., a group of excited electrons which behave like a single electrical entity). The plasmon, in turn, generates an electro-magnetic field that typically extends on the order of about 100 nanometers (nm) above and below the metal surface and oscillates with optical frequencies.

U.S. Pat. No. 5,841,143 has proposed a waveguide integrated fluorescence sensor that includes a corrugated dielectric-metal-dielectric thin film stack that is fabricated on a p-n junction. Fluorescent light of a single wavelength is selectively transmitted through the corrugated thin film stack and converted into an electronic signal by the p-n junction. Wavelength filtering is enabled by evanescent mode (or evanescent wave) cross-coupling across the metal film. Such cross-coupling includes cross-coupling between surface plasmons across the metal film and cross-coupling between surface plasmons and waveguide modes across the metal film.

SUMMARY

The invention features tunable cross-coupling evanescent mode optical devices and methods of making the same. The invention enables the wavelength of light transmitted through a corrugated metal layer to be readily changed based on selective adjustment of the index of refraction of a variable index layer in the vicinity of the corrugated metal layer.

In one aspect of the invention, a tunable optical device includes a first layer, a second layer, a metal layer disposed between the first and second layers, and an electrode. The first layer is supportable of electromagnetic field modes over a range of wavelengths that includes a target wavelength range. The second layer is disposed between the metal layer and the electrode and has an index of refraction that is adjustable over a range of values. The metal layer is disposed between the first and second layers and has at least one corrugated metal film region with a corrugation periodicity enabling cross-coupling of evanescent modes of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer with different respective wavenumbers. The cross-coupling evanescent modes have a cross-coupling wavelength determined at least in part by the corrugation periodicity and the index of refraction of the first and second layers and is substantially unaffected by the electrode.

In another aspect, the invention features a method of making the above-described tunable optical device.

DETAILED DESCRIPTION

Referring toFIG. 1, in some embodiments, a tunable optical device10includes a waveguide layer12, a corrugated metal layer14, a layer16with an adjustable index of refraction, an optional photosensitive region18, and an electrode19. In some embodiments, tunable optical device10is formed by known thin film fabrication techniques.

Waveguide layer12is able to support electromagnetic field modes over a range of wavelengths that includes a target wavelength range. Waveguide layer12may be formed of any dielectric material, including silicon dioxide, silicon nitride, and lithium fluoride. In some embodiments, waveguide layer12has a thickness of about 200–500 nm.

Corrugated metal layer14is a continuous thin film layer that has a thickness between about 20 nm and 100 nm. Corrugated metal layer14may be formed of any thin metal film, including thin films of silver and gold. The corrugation periodicity (i.e., the spatial distance between adjacent corrugation peaks or adjacent corrugation valleys) of metal layer14is selected to enable cross-coupling of evanescent modes (or evanescent waves) of equal wavelength within the target wavelength range and localized on opposite sides of the metal layer14with different respective wavenumbers. As used herein, the term evanescent mode cross-coupling refers to cross-coupling between surface plasmons and cross-coupling between surface plasmons and waveguide modes. In one implementation, metal layer14has corrugations that are characterized by a peak-to-valley depth of about 50 nm and a corrugation periodicity of about 1.1 micrometer (μm). In this implementation, the cross-coupling wavelength range extends from about 850 nm to about 400 nm when the waveguide layer12has a refractive index of about 1.5 and the refractive index of a liquid crystal layer16is varied from about 1.5 to about 1.7. The cross-coupling wavelength range in this implementation may be extended by increasing the grating periodicity. In some embodiments, metal layer14is localized in separate discrete regions of tunable optical device10. In some embodiments, tunable optical device10includes multiple separate metal film regions each of which is characterized by a different respective corrugation periodicity.

In some implementations, the corrugations of metal layer16are characterized by a sinusoidal surface relief. In these implementations, the corrugation may be achieved by patterning one of the underlying layers (e.g., variable index layer16or photo-sensitive region18). For example, a photoresist layer may be spun on a planar surface of variable index layer16or photosensitive region18. The photoresist is exposed to two interfering laser beams of the same wavelength. This causes a sinusoidal variation in the photoresist exposure. Upon development, the photoresist layer will have a sinusoidal surface relief with dimensions and periodicity corresponding to the desired corrugation peak-to-valley depth and the desired corrugation periodicity. The surface relief pattern may be transmitted into the variable index layer16or the photosensitive region18by etching (e.g., ion beam milling or dry chemical etching). If the sinusoidal surface relief is etched into the photosensitive region18, the variable index layer16is formed on the etched photosensitive region18and the metal layer is formed on the variable index layer16. The variable index layer16is sufficiently thin that the sinusoidal spatial variations are transferred from photosensitive region18to the overlying metal layer14. If the sinusoidal surface relief is etched into the variable index layer16, a metal film is simply formed on the variable index layer16to form the corrugated metal layer14. Waveguide layer12is formed over the corrugated metal layer14.

Variable index layer16may be formed of any dielectric or electro-optic material that has an index of refraction that may be varied controllably over a range of refraction index values that includes refraction index values that are different from the refraction index of waveguide layer12. In some implementations, variable index layer16is formed of an electro-optic material. Exemplary electro-optic materials that may be used to form variable index layer16include: lithium niobate; lithium tantalite; potassium dihydrogen phosphate; potassium dideuterium phosphate; aluminum dihydrogen phosphate; aluminum dideuterium phosphate; barium sodium niobate; and liquid crystal. In these implementations, a refraction index adjuster20(e.g., a voltage source) is operable to apply a voltage between corrugated metal layer14and electrode19. The applied voltage is sufficient to create across variable index layer16an electric field of sufficient strength to controllably adjust the refractive index of variable index layer16. In some implementations, variable index layer16has a thickness between about 50 nm and 100 nm.

The optional photosensitive region18may be formed of any material that responds to electromagnetic fields within the target wavelength range with a detectable photo-response. In some implementations, photosensitive region18includes a conventional semiconductor p-n (or n-p) junction. In these implementations, a photo-response detector22(e.g., an electronic circuit) is operable to measure electrical responses of photo-sensitive region18to evanescent mode fields that are transmitted across corrugated metal layer14(e.g., measure an electrical current generated in the photosensitive region18or measure a change in voltage or resistance across the photosensitive region18).

Electrode19may be formed of any electrically conducting material (e.g., a metal or indium-tin-oxide). Electrode19is configured so that it does not affect evanescent mode cross-coupling across metal layer14. For example, in the illustrated embodiment, photosensitive region18prevents electrode19from influencing evanescent mode cross-coupling across metal layer14. In some embodiments, electrode19is disposed between variable index layer16and photosensitive region18. In these embodiments, electrode19is formed of a material (e.g., indium-tin-oxide for visible and ultraviolet wavelengths) that is substantially transparent to light within the target wavelength range.

In operation, incident radiation24couples into waveguide layer12by exciting surface plasmon modes supported at the metal/waveguide layer interface28or by exciting waveguide modes in the waveguide layer12. Both surface plasmons and waveguide modes are evanescent modes whose electromagnetic fields30decay rapidly with increasing distance away from the metal layer/waveguide layer interface32and the center of the waveguide layer26, respectively. Evanescent modes can also be supported at the variable index layer/metal layer interface32(surface plasmons) and in the variable index layer16(waveguide modes). If the media on opposite sides of the metal film have different values of refractive index, evanescent modes with the same wavelength and different wavenumbers, kz, are supported on opposite sides of the corrugated metal layer. The presence of the corrugation allows wavenumber matching and, as a result, evanescent modes on one side of the corrugated metal layer can cross couple with evanescent modes with equal wavelength on the opposite side of the corrugated metal layer. In particular, surface plasmons with the same wavelength on opposite sides of the corrugated metal layer can cross couple. Also, surface plasmons and TM waveguide modes of equal wavelength and localized to opposite sides of the corrugated metal layer can cross couple. TE waveguide modes have an orthogonal polarization with respect to surface plasmons and, therefore, cannot interact with surface plasmons. As a consequence of the corrugation-induced cross coupling, radiative energy at the desired wavelength is transmitted across the otherwise opaque corrugated metal layer. The corrugation periodicity of the corrugated metal layer14and the indices of refraction of layers12,16determine the wavelength at which the surface plasmons cross-couple across corrugated metal layer14.

Referring toFIG. 2, in one illustrative example, at the wavelength λ, two surface plasmon states at opposite sides of the corrugated metal layer14couple when the wavenumbers (kz) of the surface plasmon states that are parallel to the metal interfaces are matched via the corrugated metal layer14. In particular, the presence of the corrugation contributes a wavenumber contribution, Δkz, which is equal to +/−2πn/Λ, where Λ is the corrugation periodicity and n is an integer having a value of 1 or more. Since the surface plasmon dispersion curves36,38for surfaces28,32of metal layer14are diverging, there is one and only one wavelength, λ, where wavenumber matching is accomplished. In the illustrated embodiment, if an electric field is applied across the variable index layer16, the refractive index of this layer changes. For the purpose of this discussion, without loss of generality, it is assumed that the refractive index increases; although in some implementations, the refraction index of layer16may decrease in response to an applied electric field. As the refraction index increases, the surface plasmon dispersion curve38“rotates” (as shown by arrow40) towards larger kzvalues. The cross-coupling wavelength is shifted to a longer wavelength λ′ (i.e., lower energy; as shown) because the corrugation period has not changed. Thus, the radiation wavelength that is allowed to pass through the metal film and, hence, be detected varies as a function of the voltage applied across the variable index layer.

As explained in detail below, in some embodiments, the tunable optical device10may be implemented as an optical sensor that may be tuned to detect desired wavelengths of light by adjusting the voltage applied between the corrugated metal layer14and electrode19. In other embodiments, the optional photosensitive region18may be omitted and the resulting tunable optical device may be used as a tunable optical wavelength filter.

As shown inFIG. 3, in some embodiments, the tunable optical device ofFIG. 1may be implemented as a fluorescence sensor41. In these embodiments, a fluorescent material42is placed adjacent to the waveguide layer12of the fluorescence sensor41. The fluorescent material42may be in a gaseous, liquid, or solid state. Alternatively, a film of fluorescent material42may be deposited on a surface of waveguide layer12. In operation, excitation radiation44is applied to the fluorescent material42. The excitation radiation44may be delivered from either an external source or from light confined to the waveguide layer12via a waveguide mode. The excitation radiation44includes wavelengths that excite atoms or molecules of interest in the fluorescent material12. The waveguide layer12supports the propagation of waveguide modes that generate a strong electromagnetic field in the vicinity of the fluorescent material42, enhancing the intensity of fluorescent light that is generated by the fluorescent material42.

Fluorescent light flows away from the fluorescent material42and into waveguide layer12. The wavelength content of the resulting evanescent modes is the same as that of the fluorescent emission spectra of the fluorescing atoms or molecules in the fluorescent material12. Surface plasmons at interface46between the waveguide layer12and corrugated metal layer14are excited at all fluorescent emission wavelengths. TM waveguide modes in the waveguide layer12also are excited at all fluorescent wavelengths. Based on the refractive index to which variable index layer16is adjusted, surface plasmons or TM waveguide modes (or both) of a desired cross-coupling wavelength are supported at both metal interfaces46,48and layers12,16respectively. Thus, only fluorescent light at the desired wavelength is transmitted through the corrugated metal layer14to the photosensitive region18.

Referring toFIG. 4, in some embodiments, a tunable optical wavelength filter50may be implemented by forming the multi-layer stack12–16on an electrode52that is substantially transparent to radiation within the target wavelength range. In operation, only radiation54that has a wavelength corresponding to the tuned evanescent mode cross-coupling wavelength is able to pass though tunable optical wavelength filter50.

Other embodiments are within the scope of the claims.