NONLINEAR AND GAIN OPTICAL DEVICES FORMED IN METAL GRATINGS
In one aspect, an optical gain device is provided. The optical gain device comprises a metallic film and an optical gain medium. The metallic film has a plurality of slits therethrough. The plurality of slits are configured such that the film selectively and resonantly transmits light over a preselected frequency range. The optical gain medium is situated within or substantially near at least one slit of the plurality of slits.
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This application is a Divisional of U.S. application Ser. No. 12/044,038, filed on Mar. 7, 2008, to Gang Chen, et al., entitled “Nonlinear and Gain Optical Devices Formed in Metal Gratings,” currently allowed and commonly assigned with this application and incorporated herein by reference.TECHNICAL FIELD OF THE INVENTION
The invention is directed, in general, to nonlinear optical devices and methods of fabricating and using such devices.BACKGROUND OF THE INVENTION
Optical grating devices are of increasing importance to today's economy, and every reason exists to believe that the trend will continue. In classical optics, when a slit exists in a thin nonmetallic film, and the diameter of the slit is much less than the wavelength of incident light, the amount of light that traverses the slit can be very small.SUMMARY OF THE INVENTION
The invention provides an optical gain device in one aspect and a method of generating light in another aspect.
In one embodiment, an optical gain device comprises a metallic film. The film has a plurality of slits therethrough. The array is configured such that the film selectively and resonantly transmits light over a preselected frequency range. The optical gain device further comprises an optical gain medium, situated within or substantially near at least one slit of the plurality of slits.
In another embodiment, a method of generating light is provided. The method comprises providing a metallic film with a substantially regular array of transmission slits therethrough. The array is configured to selectively pass light in a band. The band includes wavelengths wider than the slits of the array. The method further comprises producing an electromagnetic field within the transmission slits by illuminating a region on one side of the film with light of a first wavelength, the region including a portion of the array. In some embodiments, an enhancement of a transmission of a second wavelength of light through the metallic film occurs in response to the step of illuminating the film with the light of the first wavelength.
The foregoing has outlined certain aspects and embodiments of the invention so that those skilled in the pertinent art may better understand the detailed description of the invention that follows. Additional aspects and embodiments will be described hereinafter that form the subject of the claims of the invention. Those skilled in the pertinent art should appreciate that they can readily use the disclosed aspects and embodiments as a basis for designing or modifying other structures for carrying out the same purposes of the invention. Those skilled in the pertinent art should also realize that such equivalent constructions do not depart from the scope of the invention.
For a more complete understanding of the present Application, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
Referring initially to
The device 100 has a plurality of transmission slits (“slits”) 113, 115 that traverse from a front of the metal film 107 to a back of the metal film 107. In some embodiments, a width of the slits 113, 115 is 50-500 nanometers, and a height of the slits 113, 115 is 5-10 microns, although the height of the slits 113, can be in the macroscopic range, e.g., greater than about 1 millimeter. The slits 113, 115, also have a regular periodicity, also known as a “pitch” 110 and may have about the same width. In one embodiment, the pitch 110 is one slit 113 per micron along the surface of the device 100, although other pitches are used in other embodiments.
Each slit 113, 115, further contains an optical dielectric or semiconducting media 116 which is either a non-linear optical material or an optical gain media. Generally, a nonlinear optical material is for nonlinear optical devices, and gain media is for an amplifier and laser.
The slits 113, 115 are typically narrower than the wavelength of the light that the non-linear optical media is configured to amplify or wavelength-convert. In other words, the slit width is sub-wavelength. The metallic film 107 is typically thicker than the skin depth of the metal for the same wavelength of light so that such light would not substantially penetrate through the metallic film 110 in the absence of the array of slits 113, 115.
If slits in a metallic film, such as the metallic film 107, are arranged to form a grating, the amount of light that is transmitted through such a structure can be several orders of magnitude higher than standard optical diffraction theory predicts. In certain circumstances, the fraction of light that is transmitted through a grating formed in a metallic film approaches 100%, e.g., the transmitted intensity may be greater than about 50% of the incident intensity and may be preferably greater than about 80% of the incident intensity.
It is believed that an enhanced transmission will occur due to waves of charge propagating along the surfaces of the metallic film. Charge wave excitations and associated electromagnetic fields that propagate along a metal surface or a metal-dielectric interface are known as surface plasmonic waves or “surface plasmons.”
When slits in a metallic film, such as the metallic film 107, are spaced regularly with a certain pitch to form a grating, light incident on one side of the metallic film can cause a complex, banded plasmon structure to arise and propagate on the surface of the film. The incident light can resonantly couple to such surface plasmons. Indeed, such surface plasmons can couple one side of the metallic film 107 to the other side of the metallic film 107 through the slits grating therethrough, thereby generating surface plasmons on the other side of the metallic film 107. The corresponding surface plasmons on the other side of the metallic film 107 can then, create electromagnetic waves (e.g., light) that radiates from other side of the metallic film 107.
In general, two types of resonant modes allow the surface plasmons on one side of the film to couple, through the grating to surface plasmons on the other side of the metallic film 107. The two modes are “plasmon modes” and “waveguide modes.” For the purposes of the present disclosure, a “resonant mode” is deemed to include a plasmon mode, a waveguide mode, or both.
Each mode has at least one associated resonant frequency band. Typically, light in such a frequency band is needed to efficiently excite the corresponding resonant modes. In a surface plasmon mode, the pitch of the slits primarily determines the resonant frequency; in a waveguide mode, the height of the slits primarily determines the resonant frequency.
Although surface plasmon modes and waveguide modes generally have different resonant frequency bands, the bands may overlap in some circumstances and embodiments. Surface plasmon modes and waveguide modes generally exhibit enhanced electromagnetic fields close to the surface and interfaces of the periodic slits (including in the slits themselves). The waveguide modes will also exhibit enhanced electromagnetic fields in or near the grating.
In various embodiments of the device 100, the optical media 116 is capable of amplifying, wavelength converting, or nonlinearly modifying a signal light incident on a first surface of the metallic film 110, in response to an optical pumping. The pumping light is also incident on the metallic film 107 when the device 100 behaves, such as a non-linear optical media or optical gain media. In some embodiments wherein the device 100 is configured as a laser, an optical pump can also be removed, as the light amplification starts with spontaneous emission of an optical media, and therefore an optical pump is not necessary.
Generally, in the device 100, there are various optical resonance modes which are either a surface plasmon mode or a waveguide mode. Typically, in the device 100, such resonant modes are used to enhance the intensity (i.e., the amplitude) of the electromagnetic field of incident light in the slits or 113, 115 that pierce the metallic film 107. In the slits 113, 115, the enhanced intensity of the electromagnetic fields is due to the resonant mode or resonant modes that optically couple the two surfaces of the metallic film 107. The high intensity electromagnetic field in the slits modifies the optical properties of the nonlinear optical or optical gain material in the slits thereby modifying optical characteristics of light passing through the slits 113, 115.
For instance, if the surface area slit of the slit 113 is ten square nanometers, and the entire area struck by the pump light 140 is one hundred square nanometers, the amplitude of an electromagnetic field can be approximately ten-fold (100 nanometers divided by 10 nanometers) in the slits. The electro-magnetic field intensity can be about a hundred times larger in the slits than at the surface of the metal film. Such one-hundred-fold increase in an electromagnetic field intensity can be significantly affect the optical properties of nonlinear when dealing with nonlinear optical materials, such as a nonlinear optical media 120.
In particular, the optical medium 116, situated in or proximate to the slits 113, 115 can be a nonlinear optical material that can wavelength convert, or nonlinearly modify the phase or amplitude of the incident light. These nonlinear effects can be self induced by the incident light itself or can be induced by light of a different wavelength from a separate optical pump. The nonlinear media 116 can be a material such a nonlinear optical inorganic (e.g. semiconducting or insulating materials, such as lithium niobate or a group 3-5 semiconductor) or organic (e.g. polymeric) materials deposited in the slits 113, 115, or a polymer with optically active quantum dots dispersed therein. Herein, “proximate” is defined to mean within one to several wavelengths of the pump light.
The optical medium 116 can also be optical gain medium, which under optical pumping can achieve population inversion. The incident light can then be amplified when passing through the slits. For the optically pumped amplifier, the pitch and the thickness of the metallic film 107 is calculated and fabricated such that the pumping wavelength is resonant with either surface plasmon or waveguide modes and the amplifying wavelength is resonant with excitation of a surface plasmon mode. For an optically pumped laser, the pitch and the thickness of the metal films is designed such that the pumping wavelength is resonant with excitation of either a waveguide or surface plasmon mode and the lasing wavelength is resonant with excitation of a waveguide mode.
Turning now to
In the illustrated embodiment 150, the slits 151, 153 have a width of less than the lasing wavelength. They also have a height that can be between a fraction of the wavelength of an incident light to several wavelengths depending on the desired resonance. For example, the height can be from several tens of nanometers to several tens of microns. The pitch between the slits can be several tens of nanometers up to about 10 microns, depending on the resonance wavelength of interest.
Turning now to
The device 200 has a metal 210, 212, 214 on the sides of optical media 220. In particular, the nonlinear optical material 220 is situated within or proximate to the slits 213, 215 through a metal film. A back of the metal 210, 212, 214 is coupled to the transparent or translucent silicon dioxide substrate 109. In one embodiment, the substrate 109 is one millimeter thick silicon dioxide, although other thicknesses can be used.
Typically, the electromagnetic field intensity within the slits 213, 215 has an intensity that is proportional to the square of the pitch divided by slit width. In other words, the pitch of the slits 213, 215 divided by a horizontal width of a slit can be designed to enhance the intensity of the electromagnetic field within the slits 213, 215.
A signal light 235 is introduced into the nonlinear optical media 220, and a transmitted signal 270 is generated as a result of a modification of the signal light by the non-linear optical media 220 in the slits. The transmitted signal 270 can be modified as a result of absorption saturation, induced transparency, second harmonic generation of light, a sum-difference generation of light, or four-wave mixing that occurs due to the enhanced electromagnetic field 219 in the slits. Generally, the interaction of the signal light 235 with the nonlinear optical media 220 is controlled by the enhanced electromagnetic field 219 in the slits 213, 215.
The amount of enhancement of the electromagnetic field 219 strength within the slits 213, 215 may depend on the area of the slit 113. For example, the enhancement may depend on the ratio of the area of the slit openings over the area of the metal of the device 200 that is struck by light.
In one embodiment, the signal light 235 resonantly excites a resonant mode that enables the signal light 235 to propagate through the slits 213, 215. In such embodiments, the device 200 is a band-pass filter for the signal light 235, because only certain resonant frequencies can pass through the device 200.
The signal light itself 340 is then modified in amplitude or phase through its interactions with the non-linear optical media 220, which produce the transmitted light 370. The transmitted light 370 can be produced as a result of an absorption saturation, a self-phase modulation, an induced transparency, a second harmonic generation light, a sum or difference generation of light, or by four-wave mixing generation of light in the non-linear optical media 20 due to the amplitude-enhanced electromagnetic field 319 therein.
Turning now to
Turning now to
In some embodiments, both resonant mode and waveguide modes are used during operation of the devices. In other words, a separate pump can be used to nonlinearly modify the response of the nonlinear media in the device, wherein the pump light has a wavelength that can resonantly excites both surface plasmon and waveguide mode.
Turning now to
Turning now to
The pitch and thickness of the slits 513, 515 is designed such that the pump light resonantly excites either a plasmon or waveguide mode. However, the lasing wavelength is resonant with a waveguide mode.
Turning now to
In a step 710, a translucent or transparent substrate, such as the substrate 109 is provided.
In a step 720, a metal film, such as the metal film 107, is deposited on the translucent or transparent substrate, e.g., via a conventional metal evaporation and deposition process.
In a step 730, a first mask having a plurality of slits is formed on the metal film. In some embodiments, the plurality of slits has a pitch and width suitable to enable incident light to resonatingly excite a surface plasmon mode on the metal film if the film has the slit pattern of the mask. In other embodiments, the dimensions of the slits are calculated so as to enable incident light of a preselected wavelength to excite a waveguide mode within the final slits that will be produced in the metal film. In some further embodiments, the mask is configured to enable the etching of slits designed to accommodate both the optical excitation of waveguide modes therein and the optical excitation of surface plasmon modes on the resulting patterned metal film.
In a step 740, a set of slits are generated, such as through wet or dry etching of the metal film under the control of the mask. In some embodiments, the etching is performed before annealing of the deposited metal film to the transparent or translucent substrate.
In a step 750, the slits are filled with a material having a substantial optical non-linear property. In some alternative embodiments, the material is an optical gain material that can be pumped, e.g., to produce an inverted population of excited ions or charge carriers.
In a step 760, the first mask is stripped off by a conventional process to expose an optical device having a metal layer with a regular array of slits therein, wherein the slits are filled with a nonlinear optical dielectric or semiconductor material, e.g., as in the structure 100 of
In some further embodiments, in a step 770, a second mask is applied to the optical device 100. The second mask is applied at the top and bottom of a device, such as the nanostructure device 100 of
Then, in some embodiments, in a step 780, a set of holes are generated, i.e., by dry or wet etching under the control of the second mask. The holes are substantially perpendicular to the set of slits in the nanostructure device 150, such as the electrodes 171 and 173 in
Then, in some embodiments, in a step 790, the set of holes are filled with an electrode metal to generate the device 150. The electrodes are insulated from the metallic film. However, the electrodes form contact with the media in the slit, e.g., to enable the electrical pumping of said medium in embodiments for amplifiers and/or lasers.
Turning now to
The transmitted signal 270 can arise from a combination of the signal light 235 and an enhancement of the strength of the pump light 240 within the slits 213, 215 of
For this scheme, the current traveling between the electrodes produces an inverted population of charge carriers, i.e., electrons or holes, in the media located in the slits in the device 150. Under such electrical pumping, the media in the slits can amplify incoming light to generate light 670 outgoing from the array of slits in the device 150. The outgoing light 670 is then coupled to an optical multiplexer 880 and is then selected as an output 890. In other embodiments, pump light may be used to pump the media in the slits rather than a current for a laser embodiment of the device 150 of
Turning now to
In a step 910, a metallic film is provided with a substantially regular array of slits therethrough, the array is configured to selectively pass light in a wavelength band. The band is limited to wavelengths of light that are longer than the widths of the slits of the array.
In a step 920, a region of one side of the metal film is illuminated with light to produce electromagnetic fields in the slits of the array wherein the electromagnetic field within the transmission slits has a greater amplitude that the electromagnetic amplitude of the light striking the metallic field. That is, the illuminating step 920 produces an amplitude-enhance electromagnetic field inside the slits.
In a step 930, light is output from the slits via a substantial non-linear optical effect in response to the illumination of the slits at the step 920. That is, a nonlinear optical media in the transmission slits generates the light being output as a result of the amplitude-enhanced electromagnetic field in the slits.
In some embodiments, the method further comprises amplifying further comprises receiving pump light in the slits. The slits of the array of slits can include a Fabry-Perot cavity. The pump light can have a wavelength configured to resonantly excite surface plasmon modes on the film or a waveguide mode in the slits. In some embodiments, a transmission of a second wavelength of light is enhanced on the metallic film that occurs in response to the step of illuminating a region of one side.
Those skilled in the art to which the invention relates will appreciate that other and further additions, deletions, substitutions and modifications may be made to the described embodiments without departing from the scope of the invention.
1. An optical gain device, comprising:
- a metallic film, the film having a plurality of slits therethrough, the plurality of slits configured such that the film selectively and resonantly transmits light over a preselected frequency range; and
- an optical gain medium, situated within or substantially near at least one slit of said plurality of slits.
2. The optical gain device of claim 1, wherein the film is configured to resonantly transmit light in a visible or near infra-red wavelength range.
3. The system as claim in claim 1, further comprising a plurality of electrodes electrically connected to drive current through the optical gain medium in one or more of the slits.
4. The system as in claim 3, wherein said one or more slits are Fabry-Perot cavities.
5. The system as in claim 4, wherein at least one of the electrodes is coupled to a voltage driver.
6. A method of generating light, comprising:
- providing a metallic film with a substantially regular array of slits therethrough, the array configured to selectively pass light in a band, the band including wavelengths larger than a width of the slits of the array;
- producing an electromagnetic field within the slits by illuminating a region of said metallic film with light, the region including the array; and
- optically amplifying light in the slits while performing the producing.
7. The method as recited in claim 6 wherein the optically amplifying further comprises receiving pump light in the slits.
8. The method as recited in claim 6 wherein one of the slits of the array of slits includes a Fabry-Perot cavity.
9. The method as recited in claim 8, wherein said pump light has a wavelength configured to resonantly excite surface plasmon modes on the film or a waveguide mode in the slits.
10. The method as recited in claim 6, further comprising enhancing a transmission of a second wavelength of light on the metallic film that occurs in response to the step of illuminating a region of one side.
International Classification: H01S 3/08 (20060101); H01S 3/091 (20060101);