Abstract:
Technologies are described for methods to fabricate lasers to amplify light. The methods may comprise depositing nanoparticles on a substrate. The length, width, and height of the nanoparticles may be less than 100 nm. The methods may further comprise distributing the nanoparticles on the substrate to produce a film. The nanoparticles in the film may be coupled nanoparticles. The coupled nanoparticles may be in disordered contact with each other within the film. The distribution may be performed such that constructive interference of the light occurs by multiple scattering at the boundaries of the coupled nanoparticles within the film. The methods may comprise exposing the film to a power source.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims priority to Provisional Application No. 62/331,735 filed May 4, 2016, titled “Ultrathin Film Lasing”, the entirety of which is hereby incorporated by reference. 
     
    
     STATEMENT OF GOVERNMENT RIGHTS 
       [0002]    The present invention was made with government support under contract numbers DE-AC02-98CH10886 and DE-SC0012704, awarded by the U.S. Department of Energy, and DMR1105392, awarded by the National Science Foundation. The United States government has certain rights in this invention. 
     
    
     FIELD OF THE INVENTION 
       [0003]    This disclosure relates generally to lasing in ultrathin films. 
       BACKGROUND 
       [0004]    In a laser, a gain medium is a material with properties that allow it to amplify light by way of stimulated emission. Stimulated emission is a process when an electron in an atom makes a transition from a higher to a lower energy state and produces an additional photon. 
       SUMMARY 
       [0005]    In some examples, methods for fabricating a laser to amplify light are generally described. The methods may comprise depositing nanoparticles on a substrate. The length, width, and height of the nanoparticles may be less than 100 nm. The methods may comprise distributing the nanoparticles on the substrate to produce a film. The nanoparticles in the film may be coupled nanoparticles. The coupled nanoparticles may be in disordered contact with each other within the film. The distribution may be performed such that constructive interference of the light occurs by multiple scattering at the boundaries of the coupled nanoparticles within the film. The methods may comprise exposing the film to a power source. 
         [0006]    In some examples, lasers are described. The lasers may comprise a power source effective to produce light. The lasers may comprise a substrate in optical communication with the power source. The substrate may include a film. The film may include nanoparticles. The length, width, and height of the nanoparticles may be below 100 nm. The nanoparticles in the film may be coupled nanoparticles. The coupled nanoparticles may be in disordered contact with each other within the film. A distribution of the coupled nanoparticles on the substrate may be effective to produce constructive interference of the light by multiple scattering at the boundaries of coupled nanoparticles within the film. 
         [0007]    In some examples, sensing devices are generally described. The sensing devices may comprise a substrate. The sensing devices may comprise a film on the substrate. The film may include nanoparticles. The length, width, and height of the nanoparticles may be below 100 nm. The nanoparticles in the film may be coupled nanoparticles. The coupled nanoparticles may be in disordered contact with each other within the film. A distribution of the coupled nanoparticles on the substrate may be effective to produce constructive interference of a first light by multiple scattering at boundaries of coupled nanoparticles within the film. The sensing devices may comprise a sensing element. The sensing element may be in optical communication with the film. The film may be effective to receive the first light and emit second light. A quantity of lumens of the second light may be greater than a quantity of lumens of the first light. The sensing element may be effective to detect the second light and generate a response. 
         [0008]    The foregoing summary is illustrative only and is not intended to be in any way limiting. In addition to the illustrative aspects, embodiments, and features described above, further aspects, embodiments, and features will become apparent by reference to the drawings and the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0009]    The foregoing and other features of this disclosure will become more fully apparent from the following description and appended claims, taken in conjunction with the accompanying drawings. Understanding that these drawings depict only several embodiments in accordance with the disclosure and are, therefore, not to be considered limiting of its scope, the disclosure will be described with additional specificity and detail through use of the accompanying drawings, in which: 
           [0010]      FIG. 1  is a system drawing illustrating a system to make and use an ultrathin film for ultrathin film lasing; 
           [0011]      FIG. 2  is a drawing representing a scanning electron image of a film of ZnO coupled nanoparticles for ultrathin film lasing; 
           [0012]      FIG. 3  is a graph of normalized emission spectra for a film of ZnO coupled nanospheres, a film of ZnO coupled nanospheres which was annealed, and ZnO nanospheres dispersed in ethanol; 
           [0013]      FIG. 4  is a graph of emission intensity as a function of pump fluence for the film of ZnO coupled nanospheres and the annealed film of ZnO coupled nanospheres; 
           [0014]      FIG. 5 a    is a graph of a two dimensional time-resolved emission measurement of the film of ZnO coupled nanospheres; 
           [0015]      FIG. 5 b    is a graph of a two dimensional time-resolved emission measurement of the annealed film of ZnO coupled nanospheres; and 
           [0016]      FIG. 6 a    is a graph of a slice of a two dimensional time-resolved emission measurement of film of ZnO coupled nanospheres  300 ; 
           [0017]      FIG. 6 b    is a graph of a slice of emission spectra at 0-4 ps for individual lasing modes of the film of ZnO coupled nanospheres at a fluence of 77 μJ/cm 2 ; 
           [0018]      FIG. 6 c    is a graph of emission spectra at 0-4 ps for individual lasing modes of the film of ZnO coupled nanospheres at a fluence of 150 μJ/cm 2 ; 
           [0019]      FIG. 7  is an illustration of the film of ZnO coupled nanospheres and the annealed film of ZnO coupled nanospheres; 
           [0020]      FIG. 8  is an illustration of film of ZnO coupled nanoparticles interspersed with a material; 
           [0021]      FIG. 9  is an illustration of film of ZnO coupled nanoparticles utilized as a near field power source for an amplified spontaneous emission (ASE) material; 
       
    
    
       [0022]    all arranged according to at least some embodiments described herein. 
       DETAILED DESCRIPTION 
       [0023]    In the following detailed description, reference is made to the accompanying drawings, which form a part hereof. In the drawings, similar symbols typically identify similar components, unless context dictates otherwise. The illustrative embodiments described in the detailed description, drawings, and claims are not meant to be limiting. Other embodiments may be utilized, and other changes may be made, without departing from the spirit or scope of the subject matter presented herein. It will be readily understood that the aspects of the present disclosure, as generally described herein, and illustrated in the Figures, can be arranged, substituted, combined, separated, and designed in a wide variety of different configurations, all of which are explicitly contemplated herein. 
         [0024]    As used herein, any compound, material or substance which is expressly or implicitly disclosed in the specification and/or recited in a claim as belonging to a group or structurally, compositionally and/or functionally related compounds, materials or substances, includes individual representatives of the group and all combinations thereof. 
         [0025]      FIG. 1  is a system drawing illustrating a system  100  to make and use an ultrathin film for ultrathin film lasing, arranged in accordance with at least some embodiments presented herein. Nanoparticles  10  may be placed in a chamber  30 . The nanoparticles  10  may be nanoparticles of any shape including a rounded object such as a tube, rod, ellipsoid, ovoid, or sphere, with all three dimensions of length, width, and height below 100 nm. A cross section of the nanoparticles  10  may similarly be under 100 nm. Nanoparticles  10  may include, for example, zinc oxide, gallium arsenide, and nitrides or oxides of Group II-VI or Group III-V semiconductors. In an example, the nanoparticles  10  may be nanospheres of zinc oxide (ZnO) with radii of 35-50 nm. An organic solvent  20  may be placed into chamber  30 . Organic solvent  20  may be polar, non-polar, protic, or non-protic. Organic solvent  20  may include ethanol. Nanoparticles  10  may disperse within organic solvent  20  to form solution  40 . Solution  40  may be deposited on a center of a substrate  50 . Substrate  50  may have any thickness or conductivity, may be flexible, and may be transparent. Substrate  50  may include glass, silicon, ITO covered glass, or metal thin-films. For example substrate  50  may be borosilicate glass, soda lime glass, quartz, PYREX, or other suitable glass material. In some implementations, substrate  50  may have a thickness of 0.3, 0.5 or 0.7 millimeters, although in some implementations substrate  50  may be thicker (such as tens of millimeters) or thinner (such as less than 0.3 millimeters). In some implementations, substrate  50  may be non-glass, such as polycarbonate, acrylic, polyethylene terephthalate (PET) or polyether ether ketone (PEEK). In some implementations, substrate  50  may be non-transparent, such as a metal foil or stainless steel-based. 
         [0026]    Substrate  50  may be rotated by spin coater  60 . Spin coater  60  may spin substrate  50  at 8,000 to 12,000 rpm. Solution  40  may be distributed over surface of substrate  50  by centrifugal force during spin coating. Spin coated solution  40  and substrate  50  may form a sample  65 . 
         [0027]    Sample  65  may be heated by heater  80 . Heater  80  may heat sample  65  to a temperature between about 250° C. to about 350° C. Organic solvent  20  in solution  40  may evaporate out of solution  40  when heated. Heating sample  65  may produce sample  75 . Sample  75  may include substrate  50  coated with film  70 . Film  70  may have a thickness of between 100 nm to 200 nm. Film  70  may include 3-4 layers of nanoparticles  10 . A thickness of film  70  may be less than a wavelength (λ) of light emitted from film  70  upon lasing. A thickness of film  70  may be −λ emission /4. Film  70  may include coupled nanoparticles  90  distributed on the substrate. Coupled nanoparticles  90  may be nanoparticles  10  in film  70  which may be in random, disordered, and non-uniform contact with each other within film  70 . Coupled nanoparticles  90  may include nanoparticles that have surface areas in contact with surface areas of other nanoparticles. Coupled nanoparticles  90  may alternatively, or additionally, include nanoparticles that are not directly in contact but are electromagnetically connected with each other. Coupled nanoparticles  90  may produce a multiple scattering effect within film  70 . Coupled nanoparticles  90  may be distributed on the substrate such that random, disordered, and non-uniform contact with each other within film  70  is effective to produce constructive interference of a light by multiple scattering of the light at the boundaries of coupled nanoparticles  90  within film  70 . Multiple scattering produced by film  70  of coupled nanoparticles  90  may provide coherent interference of the light within film  70  and may be able to build an emission gain by trapping the light. 
         [0028]    Other processes such as nanoimprint lithography, dip coating, inkjet printing, printing from solution, and matrix-assisted pulse laser evaporation of nanoparticles  10  may be used to produce film  70 . Lasing may be induced in film  70  when film  70  is exposed to a power source such as a light source pump  92  or an electric pump  95 . For example, film  70  may be effective to receive light  91  from light source pump  92  and emit spectra of light  97 . A quantity of lumens of emitted spectra of light  97  may be greater than a quantity of lumens of light  91 . In another example, film  70  may be effective to receive electricity  94  from electric pump  95  and emit spectra of light  97 . Film  70  may be an ultrathin film of coupled nanoparticles  90  which when exposed to a power source such as a light source pump  92  or an electric pump  95  may emit light  97  and display room-temperature lasing. In an example, light source pump  92  may be effective to produce a 280 nm ultrafast pump pulse at about 100 fs to create a density of electron hole pairs above the optical bandgap in coupled nanoparticles  90  in film  70 . 
         [0029]    In an example, an ultrathin film was fabricated. A dilute solution of zinc oxide nanoparticles, with an average diameter of 35 nm were dispersed in ethanol. The solution was deposited on a glass substrate and spin-coated at 10,000 rpm. The spin-coated sample was then heated on a heating plate to 300° C. to remove the organic solvent. The resulting film was about 120 nm thick. Additional films were fabricated by the same process with varying thicknesses from 80 nm to 160 nm. Some of the additional films were annealed for a duration of about 5 minutes at 800° C. The various films fabricated were then evaluated with an ultrafast broadband optical Kerr spectrometer to produce time-resolved emission studies. 
         [0030]      FIG. 2  is a scanning electron micrograph of a film of ZnO coupled nanoparticles for ultrathin film lasing arranged in accordance with at least some embodiments presented herein. Those components in  FIG. 2  that are labeled identically to components of  FIG. 1  will not be described again for the purposes of clarity. Film  70  may include nanoparticles  10 . As shown in  FIG. 2 , film  70  may include 3-4 layers of nanoparticles  10 . Scale bar  200  may be 100 nm in length. As indicated by scale bar  200 , a thickness of film  70  may be about 100 nm and nanoparticles  10  may be nanoparticles with dimensions of about 35-50 nm in any direction. 
         [0031]      FIG. 3  is a graph of normalized emission spectra for a film of ZnO coupled nanospheres  300 , an annealed film of ZnO coupled nanospheres  310 , and ZnO nanospheres dispersed in ethanol  320 , arranged in accordance with at least some embodiments presented herein. An emission spectra may be the spectrum of frequencies of electromagnetic radiation emitted when an atom makes a transition from a high energy state to a low energy state. The collection of transitions may lead to different wavelengths and may comprise the emission spectra. Normalized emission spectra for a film of ZnO coupled nanospheres  300  may be shown for spectra near fluence thresholds of 60 μJ/cm 2  and 68 μJ/cm 2 . Annealed film of ZnO coupled nanospheres  310  may display a broader linewidth due to emission from interfacial defect states which may contribute to a lower energy tail of the band-edge emission. 
         [0032]      FIG. 4  is a graph of emission intensity as a function of pump fluence for the film of ZnO coupled nanospheres  300  and annealed film of ZnO coupled nanospheres  310 , arranged in accordance with at least some embodiments presented herein.  FIG. 4  illustrates emission counts increase exponentially for ZnO coupled nanospheres  300  at a fluence threshold range  400  while emission counts increase only linearly as fluence increases for annealed film of ZnO coupled nanospheres  310 . Fluence threshold range  400  may be 60 μJ/cm 2  to 68 μJ/cm 2 . At fluence threshold range  400  emission counts of the film of ZnO coupled nanospheres  300  increased by 3 orders of magnitude from 10 5  to 10 8 . 
         [0033]      FIG. 5 a    is a graph of a two dimensional time-resolved emission measurement of film of ZnO coupled nanospheres  300 , arranged in accordance with at least some embodiments presented herein.  FIG. 5 b    is a graph of a two dimensional time-resolved emission measurement of annealed film of ZnO coupled nanospheres  310 , arranged in accordance with at least some embodiments presented herein.  FIG. 5 a    shows high intensity and short duration of the emitted light from film of ZnO coupled nanospheres  300 , illustrating lasing in film of ZnO coupled nanospheres  300 . The lasing is characterized by an ultrafast, picosecond time scale emission process that is at least an order of magnitude faster than films under non-lasing conditions.  FIG. 5 b    illustrates no lasing and emission of light spread over time in annealed film of ZnO coupled nanospheres  310 . The random lasing displayed in  FIG. 5 a    is counterintuitive to commonly cited criteria of strongly scattering particles and an optically thick sample, in which the mean free path for scattering exceeds the thickness of the material. Predictions of the optimal size of a particle of ZnO to optimize resonant scattering at the band-edge emission wavelength is ˜260 nm.  FIGS. 5 a  and 5 b    illustrate lasing occurring in film of ZnO coupled nanospheres  300  of randomly scattered nanoparticles and lasing did not occur in annealed film of ZnO coupled nanospheres  310  where the nanoparticles are ordered and aligned through annealing. 
         [0034]      FIG. 6 a    is a graph of a two dimensional time-resolved emission measurement of a film of ZnO coupled nanospheres  300 , arranged in accordance with at least some embodiments presented herein.  FIG. 6 b    is a graph of a slice of emission spectra at 0-4 ps for individual lasing modes of film of ZnO coupled nanospheres  300  at a fluence of 77 μJ/cm 2 , arranged in accordance with at least some embodiments presented herein.  FIG. 6 c    is a graph of a slice of emission spectra at 0-4 ps for individual lasing modes of film of ZnO coupled nanospheres  300  at a fluence of 150 μJ/cm2, arranged in accordance with at least some embodiments presented herein.  FIG. 6 a    shows the lasing modes of film of ZnO coupled nanospheres  300  at 77 μJ/cm 2  as a function of time in picoseconds and wavelength of the emission.  FIG. 6 b    shows distinct temporal characteristics of individual lasing modes of film of ZnO coupled nanospheres  300  at a fluence of 77 μJ/cm 2 .  FIG. 6 c    shows distinct temporal characteristics of individual lasing modes of film of ZnO coupled nanospheres  300  at a fluence of 150 μJ/cm2. The showing of discrete lasing modes in the absence of an external optical cavity indicates random lasing in film of ZnO coupled nanospheres  300 . Sub-diffraction length scales of film of ZnO coupled nanospheres  300  may confine the incoming light fields within film of ZnO coupled nanospheres  300  and selectively outcompete extended modes that allow the emitted photons to diffuse throughout film of ZnO coupled nanospheres  300 . The size of nanospheres in film of ZnO coupled nanospheres  300  may allow for a close-packing arrangement with minimum void sizes in the form of air pockets. For example, decreasing the nanosphere diameter from 200 nm to 35 nm may result in a decrease in void size by more than 90 percent and may allow more emitted photons to interact within film of ZnO coupled nanospheres  300 . More emitted photons interacting within film of ZnO coupled nanospheres  300  may increase a coherent scattering process for light to be amplified, and may accelerate excitation delay via amplified stimulated emission. 
         [0035]      FIG. 7  is an illustration of film of ZnO coupled nanospheres  300  and annealed film of ZnO coupled nanospheres  310 , arranged in accordance with at least some embodiments presented herein. As shown in  FIG. 7 , film of ZnO coupled nanospheres  300  may include randomly scattered nanoparticles  10  and the randomly scattered nanoparticles may provide a geometry (density, size, film morphology) which is conducive to lasing. Randomly scattered nanoparticles  10  may form random sized and shaped boundaries  330  between the nanoparticles  10  within film of ZnO coupled nanospheres  300 . Random boundaries  330  formed by coupled nanoparticles  10  and coupled nanoparticles  10  may provide coherent interference within the film and may be able to build an emission gain by trapping light. In annealed film of ZnO coupled nanospheres  310 , nanoparticles  10  are aligned by fusing of nanoparticles  10  during annealing. Fusing of nanoparticles  10  during annealing may prevent lasing by causing changes in the scattering profile of annealed film of ZnO coupled nanospheres  310  and introducing interfacial trap states. 
         [0036]      FIG. 8  is an illustration of a film of ZnO coupled nanoparticles  10  and a material  800  interspersed among ZnO coupled nanoparticles  10 , arranged in accordance with at least some embodiments presented herein. Those components in  FIG. 8  that are labeled identically to components of  FIGS. 1-7  will not be described again for the purposes of clarity. In an example, an ultrathin film  810  including ZnO coupled nanoparticles was made as described above in reference to  FIGS. 1 and 7 , and material  800  interspersed among ZnO coupled nanoparticles  10  was fabricated by atomic layer disposition (ALD). An exemplary ALD cycle may include flowing a material precursor into a reaction chamber that contains the ultrathin film  810  including ZnO coupled nanoparticles  10 . The chamber may then be evacuated and a molecule reactive to the precursor may be introduced to the chamber. The ALD cycle may be repeated to fill boundaries  330  between coupled ZnO nanoparticles  10  to intersperse material  800  among coupled ZnO nanoparticles  10 . In an embodiment, the material  800  forms a coating or film on the coupled ZnO nanoparticles  10 . 
         [0037]    The precursor may be any suitable precursor known in the art. In certain embodiments, the precursor is a halide, alkoxide, or an alkyl of a metal or metalloid, such as aluminum, titanium, hafnium, or silicon. In certain embodiments, the precursor may contain aluminum, such as for example aluminum trichloride, dimethylaluminum propoxide, tri-i-butylaluminum, triethylaluminum, triethyl(tri-sec-butoxy)dialuminum, trimethylaluminum, aluminum s-butoxide, aluminum ethoxide, aluminum i-propoxide, or dimethylaluminum i-propoxide. The molecule reactive to the precursor may be an oxidizing agent, such as for example oxygen, water, hydrogen peroxide, or ozone. 
         [0038]    A refractive index of material  800  may be different from a refractive index of coupled nanoparticles  10  and material  800  may contribute to multiple scattering within film  810 . Material  800  may have a higher bandgap than ZnO. Material  800  may be a dielectric. When material  800  is a dielectric, material  800  may prevent a short circuit between a first and second conductor placed on either side of film  810 . Material  800  may include aluminum oxide, silicon oxide, titanium oxide, hafnium oxide, or any other dielectric material. Material  800  may increase multiple scattering within film  810 . 
         [0039]      FIG. 9  is an illustration of film  300  of ZnO coupled nanoparticles  10  utilized as a near field power source for an amplified spontaneous emission (ASE) material, arranged in accordance with at least some embodiments presented herein. Those components in  FIG. 9  that are labeled identically to components of  FIGS. 1-8  will not be described again for the purposes of clarity. Film  300  of ZnO coupled nanoparticles  10  may be formed on a material  900 . Material  900  may be an amplified spontaneous emission (ASE) material. Material  900  may include quantum dots. Constructive interference of light due to multiple scattering may be induced in film  300  when film  300  is exposed to a power source such as light source pump  92  or electric pump  95 . Film  300  may produce lasing and emit light  910 . Light source  92  may be in a vertical direction relative to film  300  and emoted light  910  may be in a horizontal direction relative to film  300 . Constructive interference of light due to multiple scattering in film  300  may act as a near field power source to ASE material  900 . ASE material  900  may optically amplify energy supplied by film  300  acting as a near field power source and produce lasing and emit light  920 . Emitted light  920  may be in a horizontal direction relative to ASE material  900  or in a vertical direction relative to ASE material  900 , either by scattering form the film  300  of ZnO coupled nanoparticles  10  or by a patterned grating structure. ASE material  900  may be of a thickness less than a wavelength (λ) of light emitted from ASE material  900  upon lasing. 
         [0040]    Among other possible benefits, a system in accordance with the present disclosure may produce films that may be used for integrated photonic applications including fiber-optic communication, biomedical applications, and photonic computing. The disclosed films may be utilized in photonic integrated circuits used in fiber-optic communications systems and quantum computing. The disclosed films may be utilized in lasing devices, for communication devices, and for sensing or detection devices. The disclosed film may be utilized in devices for cleaning with ultraviolet light. The disclosed system may provide low cost, high-efficiency light amplification processes. The disclosed system may provide a laser that is not based on a cavity. The disclosed system may provide lasing in arbitrarily thick samples and may permit fabrication of lasers on any underlying substrate. The disclosed system may provide a laser with broad modes and a low photon lifetime. The disclosed system may provide a laser with a low threshold and a high gain, such as for example, a gain of micro joules per cm 2  compared to milli joules per cm 2 , a gain of two orders of magnitude higher than previous lasers. The disclosed film may display improved lasing properties in films significantly thinner than films previously prepared. Previous films have been prepared by amplified spontaneous emission (ASE) in thin films of nanometer-sized quantum confined nanostructures and prepared by random lasing in larger, micron-sized particles. In ASE, the resulting emission spectrum may be derived from the gain profile of the medium while in random lasing; constructive light interference via multiple scattering may lead to distinct modes in the lasing spectrum. In ASE band engineering strategies may be employed to weaken many-body interactions and reduce losses via non-radiative Auger recombination and reabsorption. In random lasing systems, materials may be chosen that balance scattering losses and gain, by combining strongly scattering nanostructures with a gain medium (such as a laser dye), or by using bifunctional materials that scatter and deliver optical gain simultaneously. Some approaches tune the particle size to optimize resonant scattering at the band-edge emission wavelength for crystalline ZnO spherical particles due to their high refractive index of n=2.3 in ultraviolet light and strong photoluminescence. In these approaches, predictions of an optimal size of ˜260 nm radius for crystalline ZnO spherical particles have been achieved, but losses remain high with lasing threshold in the few mJ/cm 2  range. The disclosed film is significantly thinner than previously prepared films of ZnO and includes ZnO nanoparticles with a radius of ˜35 nm. The disclosed film also exhibits a much lower lasing threshold of &lt;75 μJ/cm 2  than previously prepared films of ZnO. 
         [0041]    While various aspects and embodiments have been disclosed herein, other aspects and embodiments will be apparent to those skilled in the art. The various aspects and embodiments disclosed herein are for purposes of illustration and are not intended to be limiting, with the true scope and spirit being indicated by the following claims.