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Multipass multiphoton absorption method and apparatusRelated Patent Categories: Radiation Imagery Chemistry: Process, Composition, Or Product Thereof, Imaging Affecting Physical Property Of Radiation Sensitive Material, Or Producing Nonplanar Or Printing Surface - Process, Composition, Or ProductMultipass multiphoton absorption method and apparatus description/claimsThe Patent Description & Claims data below is from USPTO Patent Application 20070087284, Multipass multiphoton absorption method and apparatus. Brief Patent Description - Full Patent Description - Patent Application Claims
STATEMENT OF PRIORITY [0001] This application claims the priority of U.S. Provisional Application No. 60/211,704 filed Jun. 15, 2000, the contents of which are hereby incorporated by reference. FIELD [0002] This invention relates to a method of enhancing the efficiency of a light source (e.g., a short pulse laser) used in a multiphoton absorption (e.g., curing) process, the method comprising passing the light through a photoreactive composition a plurality of times. BACKGROUND [0003] Molecular two-photon absorption was predicted by Goppert-Mayer in 1931. Upon the invention of pulsed ruby lasers in 1960, experimental observation of two-photon absorption became a reality. Subsequently, two-photon excitation has found application in biology and optical data storage, as well as in other fields. [0004] There are two key differences between two-photon-induced photoprocesses and single-photon induced processes. Whereas single-photon absorption scales linearly with the intensity of the incident light, two-photon absorption scales quadratically. Higher-order absorptions scale with a related higher power of incident intensity. As a result, it is possible to perform multiphoton processes with three-dimensional spatial resolution. Also, because multiphoton processes involve the simultaneous absorption of two or more photons, the absorbing chromophore is excited with a number of photons whose total energy equals the energy of an excited state of a multiphoton photosensitizer, even though each photon individually has insufficient energy to excite the chromophore. Because the exciting light is not attenuated by single-photon absorption within a curable matrix or material, it is possible to selectively excite molecules at a greater depth within a material than would be possible via single-photon excitation by use of a beam that is focused to that depth in the material. These two phenomena also apply, for example, to excitation within tissue or other biological materials. [0005] Major benefits have been achieved by applying multiphoton absorption to the areas of photocuring and microfabrication. For example, in multiphoton lithography or stereolithography, the nonlinear scaling of multiphoton absorption with intensity has provided the ability to write features having a size that is less than the diffraction limit of the light utilized, as well as the ability to write features in three dimensions (which is also of interest for holography). Such work has been limited, however, to slow writing speeds and high laser powers. Thus, there is a need for methods of improving the throughput and efficiency of multiphoton absorption systems. SUMMARY [0006] The present invention provides a method of increasing the efficiency of a multiphoton absorption process. The method includes: providing a photoreactive composition; providing a source of sufficient light for simultaneous absorption of at least two photons by the photoreactive composition; exposing (preferably, pulse irradiating, for example, using a near infrared pulsed laser having a pulse length of less than about 10 nanoseconds) the photoreactive composition to at least a first transit of light from the light source; and directing at least a portion of the first transit of the light back into the photoreactive composition using at least one optical element, wherein a plurality of photons not absorbed in the first transit are used to expose the photoreactive composition in a subsequent transit. [0007] It is an advantage of the invention that more efficient use of high power laser light in a multiphoton absorption process can be obtained by passing the laser light through the photoreactive composition a plurality of times. This can be accomplished by use of an appropriate optical element, such as a focusing mirror, a waveguide, or a cube corner reflective element, for example. [0008] Preferably, directing at least a portion of the first transit of the light back into the photoreactive composition includes directing at least a portion of the first transit of the light back into the photoreactive composition at the same location exposed to the first transit of light. Alternatively, directing at least a portion of the first transit of the light back into the photoreactive composition includes directing at least a portion of the first transit of the light back into the photoreactive composition at a location different from that exposed to the first transit of light. [0009] The present invention also provides a method of increasing the efficiency of a multiphoton absorption process that includes: providing a photoreactive composition; providing a source of sufficient light for simultaneous absorption of at least two photons by the photoreactive composition; focusing the light at a first focal point within the photoreactive composition, wherein a first portion of light is absorbed by the photoreactive composition and a second portion of light transits the photoreactive composition; and focusing the second portion of light at a second focal point within the photoreactive composition. [0010] Preferably, focusing the second portion of light at a second focal point further includes reflecting the second portion of light through the photoreactive composition. Alternatively, focusing the second portion of light includes focusing the second portion of light at a plurality of focal points. Preferably, reflecting the second portion of light includes reflecting multiple transits of the second portion of light through the photoreactive composition without focusing. [0011] Preferably, reflecting multiple transits of the second portion of light includes selectively directing the second portion of light between a plurality of optical elements, wherein at least one optical element of the plurality of optical elements is capable of selectively reflecting the light through the photoreactive composition without focusing, and at least one optical element of the plurality of optical elements is capable of selectively focusing the light at a focal point within the photoreactive composition. [0012] If desired, reflecting the second portion of light through the photoreactive composition and focusing the second portion of light is repeated one or more times to create a plurality of focal points. Furthermore, if desired, reflecting the second portion of light involves reflecting multiple transits of the second portion of light through the photoreactive composition without focusing. [0013] Preferably, the photoreactive composition includes a curable species that is cured proximate the first focal point and proximate the second focal point. If desired, the first focal point and the second focal point are at the same location within the photoreactive composition. [0014] The present invention also provides a method of increasing the efficiency of a multiphoton absorption process. The method includes: providing a photoreactive composition disposed on a reflective substrate; providing a source of sufficient light for simultaneous absorption of at least two photons by the photoreactive composition; exposing the photoreactive composition to the light from the light source at a first focal point; and reflecting the light back into the photoreactive composition by the reflective substrate. Preferably, the method further includes directing the light to an optical element for reflecting the light back into the photoreactive composition at a second focal point. More preferably, in this method, reflecting the light by the reflective substrate and reflecting the light by an optical element are repeated one or more times to create a plurality of focal points. [0015] A photoreactive composition of the present invention includes a reactive species, which is preferably a curable species, such as monomers, oligomers, reactive polymers, and mixtures thereof, although non-curable species are also possible. Preferred examples of a curable species include addition-polymerizable monomers and oligomers, addition-crosslinkable polymers, cationically-polymerizable monomers and oligomers, cationically-crosslinkable polymers, and mixtures thereof. [0016] Preferably, the photoreactive composition also includes a multiphoton photosensitizer. A photoreactive composition may or may not include an electron donor compound. A photoreactive composition can optionally include a photoinitiator. [0017] A preferred photoreactive composition includes about 5% to about 99.79% by weight of the at least one reactive species, about 0.01% to about 10% by weight of the at least one multiphoton photosensitizer, up to about 10% by weight of the at least one electron donor compound, and about 0.1% to about 10% by weight of the at least one photoinitiator, based upon the total weight of solids. [0018] The present invention also provides an apparatus for multiphoton absorption. The apparatus includes: a photoreactive composition; a light source providing sufficient light for simultaneous absorption of at least two photons by the photoreactive composition; a plurality of optical elements, wherein the photoreactive composition is located between at least two of the plurality of optical elements, wherein at least one optical element of the plurality of optical elements is capable of selectively reflecting the light through the photoreactive composition without focusing, and at least one optical element of the plurality of optical elements is capable of selectively focusing the light at a focal point within the photoreactive composition. [0019] The optical element is preferably one or more of concave spherical mirrors, concave aspheric mirrors, planar mirrors, digital micromirror devices, polarizers, lenses, retroreflectors, gratings, phase masks, holograms, diffusers, Pockels cells, wave-guides, wave plates, birefringent liquid crystals, prisms, and combinations thereof. [0020] The light source preferably includes a pulsed laser. Preferably, the wavelength of the light is about 300 nm to about 1500 nm, more preferably, about 600 nm to about 1100 nm, and most preferably, about 750 nm to about 850 nm. Continue reading about Multipass multiphoton absorption method and apparatus... 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