TEMPERATURE DEPENDENCE OF LASER EMISSION FROM SCATTERING MEDIA CONTAINING LASER DYE

Abstract: A method and system provides for remotely sensing an environmental condition of an object (10). The method includes the steps of: providing the object (10) with an amplifying scattering medium (12), such as a dye and randomly distributed nanoparticles scatters, that exhibits a dependence of laser emission on at least one environmental condition; scattering medium with a pump laser beam (14A) for inducing a laser-like emission (14B) from the amplifying scattering medium; detecting a wavelength of an emission peak of the laser-like emission; and correlating the detected wavelength with a value of the environmental condition. The environmental condition may be temperature, and it is shown that there is a linear dependence in the 77K-380K range, with a slope of about 0.09 nm/K.

Description: TEMPERATURE DEPENDENCE OF LASER EMISSION FROM SCATTERING MEDIA CONTAINING ASER DYE CLAIM OF PRIORITY FROM A COPENDING PROVISIONAL PATENT APPLICATION: Priority is herewith claimed under 35 U.S.C. §119 (e) from copending Provisional Patent Application 60/113,471, filed 12/23/98, entitled "Temperature Dependence of Laser Emission from Scattering Media Containing Laser Dye" by Nabil M. Lawandy. The disclosure of this Provisional Patent Application is incorporated by reference herein in its entirety. FIELD OF THE INVENTION: This invention relates to optical emitters and, in particular, to optical emitters that employ dyes. BACKGROUND OF THE INVENTION: U.S. Patent No.: 5,448,582, issued 9/5/95 to Nabil M. Lawandy and entitled "Optical Sources Having a Strongly Scattering Gain Medium Providing Laser-Like Action" describes various embodiments of an optical gain medium that includes an emission and amplification phase in combination with scattering particles or sites. Laser dyes, such as Rhodamine, are disclosed as being suitable embodiments for the emission and amplification phase. OBJECTS AND ADVANTAGES OF THE INVENTION: It is an object and advantage of this invention to provide a technique to achieve a wavelength tuning of an optical gain medium comprising a dye in combination with scattering particles . It is another object and advantage of this invention to provide a technique to obtain a remote monitoring of an object using an amplification and scattering medium having an emission wavelength that varies as a function of an environmental condition at the object, such as temperature or pH. SUMMARY OF THE INVENTION The foregoing and other objects and advantages of the invention are realized by methods and apparatus in accordance with embodiments of this invention. A method in accordance with this invention provides for remotely sensing an environmental condition of an object, and includes the steps of: (a) providing the object with an amplifying scattering medium, such as a dye and randomly distributed nanoparticle scatters, that exhibits a dependence of laser emission on at least one environmental condition; (b) interrogating the object by irradiating the amplifying scattering medium with a pump laser beam for inducing a laser-like emission from the amplifying scattering medium; (c) detecting a wavelength of an emission peak of the laser-like emission; and (d) correlating the detected wavelength with a value of the environmental condition. In the presently preferred, but not limiting embodiment of this invention, the environmental condition is temperature. In this embodiment it is shown that there is a linear dependence of the wavelength shift as a function of temperature in the 77K-380K range, with a slope of about 0.09nm/K. Other environmental conditions, such as pH, can also be sensed through the use of this invention. BRIEF DESCRIPTION OF THE DRAWINGS The above set forth and other features of the invention are made more apparent in the ensuing Detailed Description of the Invention when read in conjunction with the attached Drawings, wherein: Fig. 1 depicts a fluorescence and laser emission from an amplifying scattering medium taken at 77 K and at room temperature (296 K) . Fig. 2 illustrates the peak position of the spectra of the radiation from an amplifying scattering medium as a function of the temperature. Fig. 3 depicts an exemplary embodiment of a remote monitoring system, specifically one adapted for remote temperature sensing. DETAILED DESCRIPTION OF THE INVENTION The disclosure of the above-referenced U.S. Patent No.: 5,448,582 is incorporated by reference herein in its entirety. The absorption and fluorescence spectral lineshapes of Rhodamine dyes are strongly affected by temperature changes and aggregation phenomena at high concentrations. These changes are known to directly affect the emission of dye laser systems. In accordance with an aspect of this invention a temperature dependence of laser emission is obtained from a matrix, such as polymer sheets, containing laser dyes and nanoparticle scatterers, optically pumped by a frequency doubled and Q-switched Nd:YAG laser. This invention relies on the inventor's earlier work that demonstrates that laser action can be obtained from disordered systems where scattering provides the requisite feedback for line narrowing. EXAMPLE An experiment utilized a pump laser with a pulse duration of 7ns and operating at a 1Hz repetition rate which was focused to a spot size of 1cm 2 on samples. The samples were comprised of Rhodamine 610 laser dye, at concentrations in the range of 2xl0 ~4 M to 5xl0 ~3 M, and Ti0 2 nanoparticle scatterers (approximately 250nm in diameter) that were dispersed in a polymer matrix comprised of a mixture of polymethyl methacrylate (75%) and hydroxyethyl methacrylate (25%) . The scatterer concentration was in the range of 2.5g - 10. Og of Ti0 2 for each kilogram of the polymer mixture, which corresponded to a particle density of the order of about 10 11 scatterers/cm . The samples used in the temperature experiments were 1mm thick and 1cm in diameter and were placed in contact with a cold finger within an evacuated dewar. The thermometer was a temperature dependent resistor that was calibrated with liquid nitrogen and with an ice water mixture. Experiments were performed to measure the emission spectra of the samples over the range of temperatures from 77K- 380K. In order to record the data a SPEX-640 monochromator/spectrometer was used, with the integration time set to 10s, and with a 50 micron entrance slit. The width of the spectral emission depended on the pumping energy and exhibited a spectral width of about 30nm for very low pump fluences (fluorescence spectra) , and a collapsed linewidth of about 5nm when the pump fluence was near 5mJ/cm 2 (laser spectra) . Fig. 1 shows the fluorescence and laser spectra at liquid nitrogen temperature (77K) and at room temperature (296K) . It should be noticed that both the fluorescence and laser emission spectra shift to the red when the temperature is increased (fluorescence and laser emission spectra are put on the same scale only for easier visualization) . More particularly, this data indicates that there is a red shift of about 2nm on both the fluorescence and laser emission spectra, between the two above-mentioned temperatures. However, although the fluorescence spectra are partially overlapping, the laser spectra are completely separated from each other. This separation in the laser spectra thus provides a mechanism to achieve high resolution remote temperature sensing. That is, if an object of interest includes the amplifying scattering medium of this invention, such an object can be interrogated with a laser, and the resulting emission detected and analyzed so as to correlate the wavelength of the emission peak with the local temperature of the object of interest. Fig. 2 shows the peak position of the laser emission as a function of the temperature. The results show a linear dependence of the emitted laser emission wavelength as a function of temperature in the 77K-380K range, with a slope of about 0.09nm/K. It has also been observed that this slope may change if one changes the concentration of the dye and/or scatterers. It is believed that one can attribute the temperature shifts on the spectral line shapes to two main effects: the first is the redistribution of vibronic populations in the multilevel dye system, and the second is associated with re-emission at longer wavelengths. Referring to Fig. 3, in accordance with an exemplary embodiment of this invention an object 10 has a coating or region 12 comprised of the amplifying scattering medium 12 in accordance with this invention. A remotely located interrogation laser 14, such as a Nd:YAG laser, provides an interrogation beam 14A at a first wavelength which impinges on the coating 12. In response, the coating 12 emits a laser-like emission 14B that has an emission peak centered at a second wavelength. The emission 14B is received and detected by a detector 16, and the wavelength of the emission peak is determined. The detector 16 could comprise any suitable device for resolving the wavelength of the second emission 14B, such as a grating or a prism having an optical output coupled to a photodetector . The detector 16 then correlates the detected wavelength of the emission peak with the temperature (T) in the environment of the object 10. Circuitry such as a lookup table, or any suitable technique, can be used for correlating the emission wavelength and the temperature. In this case the lookup table may store a plurality of temperatures, with an individual temperature being stored at an address that corresponds to a particular emission wavelength. By applying a value of a detected emission wavelength to the lookup table, the corresponding temperature is retrieved. The detector 16 may thus also comprise a digital data processor. The temperature could be one induced in the object from the object's external environment, or a temperature generated in the object's internal environment. In conclusion, the temperature dependence of laser emission from amplifying polymer sheets containing dyes and randomly distributed Tiθ2 nanoparticle scatters has been demonstrated. The results show a linear dependence of the radiation wavelength shift as a function of temperature in the 77K-380K range, with a slope of about 0.09nm/K. These results demonstrate the applicability of using amplifying scattering media in the form of, by example, coatings or paints as high resolution, remote optical thermometers with high signal to noise properties. It should be appreciated that the teachings of this invention are not limited to only the specific dye and scattering materials listed above, and that other dyes and scattering materials can be employed. Neither is the teaching of this invention limited to only the disclosed concentrations of dye and scattering particles, or to only the disclosed wavelengths, ranges of temperature, ranges of wavelengths, specific embodiments of excitation sources (e.g., only Nd:YAG lasers), etc. Other materials besides dyes may also employed in the gain material, such as certain semiconductor materials provided in a particulate form. Furthermore, other external environmental factors may have an effect on the emission wavelength, such as pH, and the teaching of this invention is thus not intended to be limited to only the remote sensing of temperature. For example, it is known that the dye Nile Red experiences a shift in emission wavelength in response to a change in pH . Certain dyes are also believed to experience a shift in emission wavelength in response to a change in a gaseous environment, such as a type of gas present and/or a concentration of a particular gas. Thus, while the invention has been particularly shown and described with respect to preferred embodiments thereof, it will be understood by those skilled in the art that changes in form and details may be made therein without departing from the scope and spirit of the invention.

Claims: CLAIMS What is claimed is: 1. A method of remotely sensing an environmental condition of an object, comprising steps of: providing the object with an amplifying scattering medium that exhibits a dependence of laser emission on at least one environmental condition; interrogating the object by irradiating the amplifying scattering medium with a pump laser beam for inducing a laser-like emission from the amplifying scattering medium; detecting a wavelength of an emission peak of the laser-like emission; and correlating the detected wavelength with a value of the environmental condition. 2. A method as in claim 1, wherein the environmental condition is temperature. 3. A method as in claim 1, wherein the environmental condition is pH. 4. A method as in claim 1, wherein the environmental condition is a gaseous environment. 5. A method as in claim 1, wherein the medium is comprised of a dye and randomly distributed nanoparticle scatters . 6. A method as in claim 2, wherein there is a linear dependence of the wavelength shift as a function of temperature in the 77K-380K range, with a slope of about 0.09nm/K. 7. A method as in claim 1, wherein the amplifying scattering medium is provided within a monolithic polymer- containing structure. 8. A method as in claim 1, wherein the amplifying scattering medium is provided within a polymer-containing matrix. 9. A method as in claim 1, wherein the amplifying scattering medium is provided as a coating. 10. A method as in claim 1, wherein the amplifying scattering medium is comprised of a Rhodamine laser dye and nanoparticle scatterers. 11. A method as in claim 10, wherein the Rhodamine laser dye is provided in a concentration in the range of about 2xl0 ~4 M to 5xl0 ~3 M, and wherein the nanoparticle scatterers are comprised of Ti0 2 nanoparticle scatterers that are dispersed with said laser dye in a polymer matrix. 12. A method as in claim 11, wherein said polymer matrix is comprised of a polymer mixture comprised of polymethyl methacrylate and hydroxyethyl methacrylate. 13. A method as in claim 11, wherein a concentration of said Ti0 2 nanoparticle scatterers is in the range of about 2.5g - lO.Og of Ti0 2 for each kilogram of the polymer matrix. 14. A method as in claim 10, wherein a particle density of said nanoparticle scatterers is of the order of 10 11 scatterers/cm 3 . 15. A method as in claim 10, wherein said nanoparticle scatterers are approximately 250nm in diameter. 16. A method as in claim 10, wherein said Rhodamine laser dye is comprised of Rhodamine 610. 17. A method as in claim 5, wherein said dye is comprised of Nile Red. 18. A system for remotely sensing an environmental condition of an object, comprising: an object comprising an amplifying scattering medium that exhibits a dependence of laser emission on at least one environmental condition; a laser source for interrogating the object by irradiating the amplifying scattering medium with a pump laser beam for inducing a laser- like emission from the amplifying scattering medium; a detector for detecting a wavelength of an emission peak of the laser-like emission; and circuitry for correlating the detected wavelength with a value of the environmental condition. 19. A system as in claim 18, wherein the environmental condition is temperature. 20. A system as in claim 18, wherein the environmental condition is pH . 21. A system as in claim 18, wherein the environmental condition is a gaseous environment. 22. A system as in claim 18, wherein the medium is comprised of a dye and randomly distributed nanoparticle scatters . 23. A system as in claim 19, wherein there is a linear dependence of the wavelength shift as a function of temperature in the 77K-380K range, with a slope of about 0.09nm/K. 24. A system as in claim 18, wherein the amplifying scattering medium is provided within a monolithic polymer- containing structure. 25. A system as in claim 18, wherein the amplifying scattering medium is provided within a polymer-containing matrix. 26. A system as in claim 18, wherein the amplifying scattering medium is provided as a coating. 27. A system as in claim 18, wherein the amplifying scattering medium is comprised of a Rhodamine laser dye and nanoparticle scatterers. 28. A system as in claim 27, wherein the Rhodamine laser dye is provided in a concentration in the range of about 2xl0 "4 M to 5xl0 ~3 M, and wherein the nanoparticle scatterers are comprised of Ti0 2 nanoparticle scatterers that are dispersed with said laser dye in a polymer matrix. 29. A system as in claim 28, wherein said polymer matrix is comprised of a polymer mixture comprised of polymethyl methacrylate and hydroxyethyl methacrylate. 30. A system as in claim 28, wherein a concentration of said Ti0 2 nanoparticle scatterers is in the range of about 2.5g - lO.Og of Ti0 2 for each kilogram of the polymer matrix. 31. A system as in claim 27, wherein a particle density of said nanoparticle scatterers is of the order of 10 scatterers/cm . 32. A system as in claim 27, wherein said nanoparticle scatterers are approximately 250nm in diameter. 33. A system as in claim 27, wherein said Rhodamine laser dye is comprised of Rhodamine 610. 34. A system as in claim 22, wherein said dye is comprised of Nile Red.

Inventors: LAWANDY NABIL M, LAWANDY, NABIL, M.
Applicants: UNIV BROWN RES FOUND, BROWN UNIVERSITY RESEARCH FOUNDATION
Application year: 1999
Main classification: H01S3
IPC codes: G01K11