Abstract:
Provided is single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method, and more particularly, single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method, capable of depositing a carbon nanotube thin film on a filter using a vacuum chamber and a membrane filter, etching the filter using an etchant so as to be transferred to an upper surface of the polymer, coating the polymer on the carbon nanotube to thereby produce a carbon nanotube saturable absorber, as a method of producing a carbon nanotube thin film to transfer the thin film to the polymer using a multi-filtration method in order to produce a passive saturable absorber to be used in laser oscillation.

Description:
TECHNICAL FIELD 
     The following disclosure relates to single-walled carbon nanotube saturable absorber production using a multi-vacuum filtration method, and more particularly, to single-walled carbon nanotube saturable absorber production using a multi-vacuum filtration method, capable of depositing a carbon nanotube thin film on a filter using a vacuum chamber and a membrane filter, etching the filter using an etchant so as to be transferred to an upper surface of the polymer, coating the polymer on the carbon nanotube thin film to thereby produce a carbon nanotube saturable absorber, as a method of producing a carbon nanotube thin film to transfer the thin film to the polymer using a multi-filtration method in order to produce a passive saturable absorber to be used in laser oscillation. 
     BACKGROUND 
     Generally, a carbon nanotube has optical non-linearity 1000 times higher than that of other materials, such that the carbon nanotube is appropriate for use as a saturable absorber. In addition, in the carbon nanotube, a band-gap thereof is determined according to a diameter of the carbon nanotube and a rolling vector of a graphite plate due to a small size of a nano scale and a specific carbon bond. Since this band-gap is significantly small (0.4 eV or less) as compared with other semiconducting materials, the optical non-linearity may be significantly increased. In addition, since the nanotube may be produced so as to have various sizes while changing the diameter, the band-gap may be changed, such that the carbon nanotube has variability with respect to band-widths in which mode-lock may be performed. Further, generally, when a saturable absorber is interlocked with an existing optical device to be used, damage cannot help but occur at an external environment and a periphery junction part. However, the carbon nanotube may minimize this damage due to excellent mechanical strength as described above. In addition, the carbon nanotube may be easily produced, have significantly low producing cost, as compared with a semiconductor saturable absorber mirror (SESAM), and be easily combined with a fiber laser system in a film form, or the like. 
     The saturable absorber used in passive mode-locking system is a non-linear optical medium of which absorption is decreased when light intensity is increased. In the case in which the saturable absorber is inserted into a cavity, a pulse width may be shortened while pulse shuttles in the cavity, such that an ultra-fast light pulse may be generated. 
     As conditions of the saturable absorber for a mode-lock, the saturable absorber should have an absorption rate more than a gain constant of a semiconductor laser and a recovery time faster than a carrier relaxation time of the semiconductor laser. In the existing fiber laser system, the SESAM has been mainly used for the passive mode-lock. However, in the SESAM, a wavelength band in which the mode-lock may be performed is determined according to a thickness of a stacked semiconductor layer, but production of the semiconductor appropriate for the wavelength region of 1.3 to 1.5 um, which is a wavelength generally used in the fiber laser, requires a complicated process, such that producing cost may be high. In addition, it may be almost impossible to vary the wavelength band in which the mode lock may be performed, and it is difficult to combine the SESAM with the fiber laser system, such that the SESAM has many limitations. 
     In order to overcome this problem, the saturable absorber using a carbon nanotube has been mainly produced. A method of producing the saturable absorber using the carbon nanotube may be classified into two types, that is, a composite type method and a spray type method. 
     The composite type method is a method of co-dispersing a liquid polymer and a single-walled carbon nanotube (SWNT) and curing the dispersant to thereby produce the saturable absorber. In this method, the curing should be performed at a constant temperature for 1 week, such that it takes a long time to produce the saturable absorber. In addition, since the polymer and the SWNT are not uniformly mixed but sporadically mixed, incident laser may be irregularly reflected. Further, it may be difficult to adjust a thickness or optical absorbance of the saturable absorber at a desired degree. 
     The spray type method is a method of producing a saturable absorber by directly spraying a single walled carbon nanotube on a thin film polymer to dispose the single-walled carbon nanotube on the polymer. This method has a disadvantage in that it may be difficult to obtain a uniform surface due to characteristics of the spray type method, such that scattering of the laser may be generated, and it may be difficult to obtain the desired optical absorbance similarly to the composite type method. 
     As the related art, an ultrafast carbon nanotube saturable absorber for solid-state laser mode-locking has been disclosed in KR 10-2010-0043446. The related art relates to a carbon nanotube saturable absorber obtained by forming a carbon nanotube solution by mixing a single-walled carbon nanotube (SWCNT) produced from an electric discharge with diclobenzene (DCB), mixing the carbon nanotube solution with polymethyl methacrylate (PMMA) to produce a SWCNT/PMMA composite, and forming a thin film with the SWCNT/PMMA composite by a spin coating method on a substrate. In this case, it may be difficult to adjust the thickness or optical absorbance of the saturable absorber at a desired degree. 
     RELATED ART DOCUMENT 
     Patent Document 
     
         
         KR 10-2010-0043446 A (Apr. 29, 2010) 
       
    
     SUMMARY 
     An embodiment of the present invention is directed to providing single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method capable of uniformly forming a surface of the carbon nanotube thin film unlike the existing composite type method or spray method to thereby produce a carbon nanotube saturable absorber having a low scattering degree by producing a carbon nano thin film separately from a polymer using a multi-vacuum filtration method and coating both surfaces of the carbon nanotube thin film with the polymer to produce the saturable absorber, and capable of adjusting the desired optical absorbance of the saturable absorber by a method of individually producing the carbon nanotube thin film several times and then overlapping the produced thin films. 
     In one general aspect, single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method includes: filling a carbon nanotube mixed solution  200  in which a carbon nanotube solution and distilled water are mixed in a vacuum chamber  100  installed with a membrane filter  20  and filtering the solution to deposit a carbon nanotube thin film  10  on the membrane filter  20  (S 10 ); separating the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon from the vacuum chamber  100  to dry the membrane filter  20  (S 20 ); floating the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon on an etchant  400  and dissolving the membrane filter  20  to thereby remove the membrane filter  20  (S 30 ); diluting the etchant  400  to replace the etchant  400  with distilled water  500  and then sinking a lower polymer film  30  under the carbon nanotube thin film  10  (S 40 ); removing the distilled water  500  while adjusting a position of the lower polymer film  30  to deposit the carbon nanotube thin film  10  on the lower polymer film  30  (S 50 ); and coating an upper polymer film  40  on the carbon nanotube thin film  10  deposited on the lower polymer film  30  (S 60 ). 
     In S 10 , the carbon nanotube mixed solution  200  may be prepared at a low concentration and filtered through the membrane filter  20  several times to deposit the carbon nanotube thin film  10  on the membrane filter  20 . 
     In S 20 , the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon may be dried at room temperature for 10 minutes or more. 
     In S 60 , the upper polymer film  40  may be coated by a spin coating method. 
     The etchant  400  may be 3M NaOH solution. 
     The lower polymer film  30  may be made of polydimethylsiloxane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a method of depositing a carbon nanotube thin film according to an exemplary embodiment of the present invention. 
         FIG. 2  is a schematic diagram showing a process of forming a polymer layer on a lower surface of the carbon nanotube thin film according to the exemplary embodiment of the present invention. 
         FIG. 3  is a schematic diagram showing a single-walled carbon nanotube saturable absorber produced by the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to the present invention. 
         FIG. 4  is a graph for describing a laser oscillation principle of the single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention. 
         FIG. 5  is a graph showing absorbance spectrum analysis of the single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention. 
         FIG. 6  is a photograph showing the single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention. 
         FIG. 7  is a photograph of a state in which single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention is attached to a distal end of a fiber in order to be used. 
     
    
    
     
       
         
               
             
               
               
               
             
               
               
             
               
               
               
             
           
               
                   
               
               
                 [Detailed Description of Main Elements] 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                  10: Carbon nanotube thin film 
                 20: Membrane filter 
               
               
                   
                  30: Lower polymer film 
                 40: Upper polymer film 
               
               
                   
                 100: Vacuum chamber 
               
             
          
           
               
                   
                 200: Carbon nanotube mixed solution 
               
             
          
           
               
                   
                 300: Petri dish 
                   
               
               
                   
                 400: Etchant 
               
               
                   
                 500: Distilled water 
               
               
                   
                   
               
             
          
         
       
     
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method will be described in detail with reference to the accompanying drawings. 
       FIGS. 1 and 2  are schematic diagrams showing the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to an exemplary embodiment of the present invention. 
     As shown in  FIGS. 1 and 2 , the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to the exemplary embodiment of the present invention is configured to include filling a carbon nanotube mixed solution  200  in which a carbon nanotube solution and distilled water are mixed in a vacuum chamber  100  installed with a membrane filter  20  and filtering the solution to deposit a carbon nanotube thin film  10  on the membrane filter  20  (S 10 ); separating the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon from the vacuum chamber  100  to dry the membrane filter  20  (S 20 ); floating the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon on an etchant  400  and dissolving the membrane filter  20  to thereby remove the membrane filter  20  (S 30 ); diluting the etchant  400  to replace the etchant  400  with distilled water  500  and then sinking a lower polymer film  30  under the carbon nanotube thin film (S 40 ); removing the distilled water  500  while adjusting a position of the lower polymer film  30  to deposit the carbon nanotube thin film  10  on the lower polymer film  30  (S 50 ); and coating an upper polymer film  40  on the carbon nanotube thin film  10  deposited on the lower polymer film  30  (S 60 ). 
     First, S 10  is a step of allowing a single-walled carbon nanotube (hereinafter, referred to as the carbon nanotube) to form a uniform layer. To this end, the membrane filter  20  made of porous cellulose is installed in the vacuum chamber  100 , and distilled water is fully filled therein. The individualized carbon nanotube solution is dropped in the vacuum chamber  100  fully filled with the distilled water and captured on the membrane filter  20  while being continuously circulated by applying vacuum, such that the carbon nanotube thin film  10  is deposited on the membrane filter  20 . 
     In this case, a filter having a pore smaller than a length of the carbon nanotube needs to be used as the membrane filter  20 . Further, in S 10 , the carbon nanotube thin film  10  may be deposited on the membrane filter  20  by preparing the carbon nanotube mixed solution  200  at a low concentration and passing the solution through the membrane filter  20  several times. That is, as shown in  FIG. 1 , a process of filtering the carbon nanotube mixed solution  200  uniformly dispersed in the distilled water to allow the carbon nanotube to be uniformly distributed on the membrane filter  20  is performed several times at a low concentration, such that the carbon nanotube does not aggregate, thereby making it possible to uniformly producing the carbon nanotube thin film  10 . 
     In addition, the carbon nanotube thin film  10  is cured through the separating of the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon from the vacuum chamber  100  to dry the membrane filter  20  (S 20 ). In this case, it is preferable that the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon is dried at room temperature for 10 minutes or more. 
       FIG. 2  is a schematic diagram showing a process of forming a polymer layer on a lower surface of the carbon nanotube thin film according to the exemplary embodiment of the present invention and corresponds to processes of S 30  to S 50 . 
     Then, in S 30 , the membrane filter  20  including the carbon nanotube thin film  10  deposited thereon is floated on the etchant  400  filled in a Petri dish  300 , such that the membrane filter  20  was dissolved and removed. That is, when the membrane filter  20  is maintained at a state in which it contact the etchant  400  to thereby be floated by surface tension for 10 minutes, the membrane filter  20  is sufficiently dissolved and removed, such that only the carbon nanotube thin film  10  is floated on the etchant  400 . Here, the etchant  400  may be 3M NaOH solution, and a preferable concentration thereof is 10 wt. %. 
     Further, in S 40 , the etchant  400  is diluted with distilled water in a state in which the carbon nanotube thin film  10  is floated on the etchant  400  as described above to allow the etchant to be completely replaced with the distilled water  500 , and the lower polymer film  30  is sunk under the carbon nanotube thin film  10 , such that the lower polymer film  30  is positioned on a bottom of the Petri dish  300 . 
     In this case, the lower polymer film  30  may be made of polydimethylsiloxane, wherein since polydimethylsiloxane, which is silicon rubber or silicon resin, is a flexible material, it is easy to input the lower polymer film between the carbon nanotube thin film  10  and the Petri dish  300  to allow the lower polymer film to be positioned on the bottom of the Petri dish  300  as shown in  FIG. 2 . 
     S 50  is a step of slowly removing the distilled water  500  while adjusting the position of the lower polymer film  30  so that the carbon nanotube thin film  10  is accurately deposited on the lower polymer film  30  to deposit the carbon nanotube thin film  10  on the lower polymer film  30 . 
     S 60  is a step of picking out from the Petri dish  300  in a state in which the carbon nanotube thin film is deposited on the lower polymer film  30  to coat an upper polymer film  40  on the carbon nanotube thin film  10 . That is, as shown in  FIG. 3 , in S 60 , the polymer films  30  and  40  are positioned on both surfaces of the carbon nanotube thin film  10  like a sandwich. 
     In this case, the upper polymer film  40  may be coated using a spin coating method. 
     The spin coating method is a method of disposing the lower polymer film  30  including the carbon nanotube thin film  10  deposited thereon on a spin coater, dropping a polymer solution onto the carbon nanotube thin film  10 , and then rapidly rotating the carbon nanotube thin film  10  to form a polymer thin film. The upper polymer film  40  may be formed on the carbon nanotube thin film  10  by the spin coating method. 
     Therefore, in the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to the exemplary embodiment of the present invention, a surface of the carbon nanotube thin film may be uniformly formed unlike the composite type method and the spray type method, such that the carbon nanotube saturable absorber having a low scattering degree may be produced. 
     In addition, the saturable absorber may be produced while adjusting the desired optical absorbance by a method of individually producing the carbon nanotube thin film several times and then overlapping the produced thin films. 
     Further, in light passing through the saturable absorber, absorption rate, a is changed as the following Equation. 
     
       
         
           
             
               α 
               ⁡ 
               
                 ( 
                 I 
                 ) 
               
             
             = 
             
               
                 
                   α 
                   0 
                 
                 
                   1 
                   + 
                   
                     I 
                     / 
                     
                       I 
                       sat 
                     
                   
                 
               
               + 
               
                 
                   α 
                   vs 
                 
                 . 
               
             
           
         
       
     
     In the Equation, I indicates intensity of incident light pulse, α 0  and α ns  indicate linear limits of saturable absorption and non-saturable absorption, respectively. Further, I sat  indicates saturation intensity. As may be seen by the Equation, since the absorption rate of the saturable absorber is changed according to the intensity of the incident pulse, when pulse components generated in a cavity pass through the saturable absorber, only a component having a high pulse intensity may be selectively transmitted. Optical pulses having a significantly short pulse width in a time axis may be generated by this phenomenon. 
     To this end, production of uniform thin film type carbon nanotube film is necessary. The uniform thin film type carbon nanotube film may be produced by the method in  FIG. 1 , and the saturable absorber that is necessary for fiber type mode locked femtosecond laser oscillation may be produced by the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to the present invention. 
     Further,  FIG. 6  is a photograph showing the single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention. The single-walled carbon nanotube saturable absorber according to the present invention is configured so that the carbon nanotube thin film  10  is positioned between the lower and upper polymer films  30  and  40  and formed at inner portions of the polymer films  30  and  40  as shown in  FIG. 6 . 
     The single-walled carbon nanotube saturable absorber formed as described above is attached to a distal end of a fiber as shown in  FIG. 7  to serve as the saturable absorber, such that laser oscillation may be performed. The reason is that when the laser passes through the saturable absorber, instantly, optical loss is smaller than optical gain, such that the laser is oscillated as may be seen by  FIG. 4 . 
       FIG. 5  is a graph showing optical absorbance of the single-walled carbon nanotube saturable absorber according to the exemplary embodiment of the present invention at each wavelength. 
     With the single-walled carbon nanotube saturable absorber production via a multi-vacuum filtration method according to the exemplary embodiment of the present invention, a surface of the carbon nanotube thin film may be uniformly formed unlike the composite type method and the spray type method, such that the carbon nanotube saturable absorber having a low scattering degree may be produced. 
     In addition, the saturable absorber may be produced while adjusting the desired optical absorbance by a method of individually producing the carbon nanotube thin film several times and then overlapping the produced thin films. 
     The present invention is not limited to the above-mentioned exemplary embodiments but may be variously applied, and may be variously modified by those skilled in the art to which the present invention pertains without departing from the gist of the present invention claimed in the claims.