Abstract:
An imaging system is provided that includes a optical pulse generator for providing an optical pulse having a spectral bandwidth and includes monochromatic waves having different wavelengths. A dispersive element receives a second optical pulse associated with the optical pulse and disperses the second optical pulse at different angles on the surface of the dispersive element depending on wavelength. One or more focal elements receives the dispersed second optical pulse produced on the dispersive element. The one or more focal element recombine the dispersed second optical pulse at a focal plane on a specimen where the width of the optical pulse is restored at the focal plane.

Description:
PRIORITY INFORMATION 
     This application claims priority from provisional application Ser. No. 61/048,284 filed Apr. 28, 2008, which is incorporated herein by reference in its entirety. 
    
    
     SPONSORSHIP INFORMATION 
     This invention was made with government support awarded by the U.S. Army Research Office under Contract No. W911NF-07-D-004 and the Department of Energy Computational Science Fellowship under Contract No. DE-FG02-97ER25308. The government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention is related to the field of multiphoton excitation microscopy/microfabrication, and in particular to axial resolution for two-photon wide-field illumination microscopy and microfabrication. 
     Multiphoton excitation fluorescence microscopy has recently gained popularity for cellular and tissue imaging. It provides intrinsic three-dimensional (3-D) resolution, allows deep imaging into tissues, achieves submicron optical resolution, and minimizes photodamage and photobleaching. Moreover, multiphoton excitation microfabrication has been also widely used since it can generate finer 3-D features than conventional two-dimensional (2-D) lithographic techniques. However, both systems use either laser scanning or laser writing technique to achieve this intrinsic optical sectioning capability based on spatially focusing laser light at the focal point of a high numerical aperture objective. It is common to implement laser scanning microscopy and laser writing microfabrication, but its major drawback is the longer image acquisition or fabrication time than that in the wide-field illumination based systems. This limits multiphoton fabrication in producing small prototypes although the submicron optical resolution and optical sectioning capabilities are very attractive for making 3-D structures. 
     Recently, the concept of temporal focusing was introduced, which disperses optical pulse into monochromatic waves at different angles on a grating surface and recombine them at focal plane. It is very useful in multiphoton depth-resolved wide-field illumination, since the original optical pulses are restored only at focal plane, and several applications in the nonlinear microscopy were proposed. However, temporal focusing has never been applied to 3-D lithographic microfabrication. In addition, depth discrimination capability for wide-field illumination system has not been fully evaluated both theoretically and empirically although it is one of the most important parameter in designing multiphoton wide-field illumination systems. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, there is provided an imaging system. The imaging system includes a optical pulse generator for providing an optical pulse having a spectral bandwidth and includes monochromatic waves having different wavelengths. A dispersive element receives a second optical pulse associated with the optical pulse and disperses the second optical pulse at different angles on the surface of the dispersive element depending on wavelength. One or more focal elements receive the dispersed second optical pulse produced on the dispersive element. The one or more focal element recombine the dispersed second optical pulse at a focal plane on a specimen where the width of the optical pulse is restored at the focal plane. 
     According to another aspect of the invention, there is provided a method of performing operations an imaging system. The method includes providing an optical pulse having a spectral bandwidth and includes monochromatic waves having different wavelengths and receiving a second optical pulse associated with the optical pulse. Also, the method includes dispersing the second optical pulse at different angles depending on wavelength. Moreover, the method includes recombining the dispersed second optical pulse at a focal plane on a specimen where the width of the optical pulse is restored at the focal plane. 
     According to another aspect of the invention, there is provided a method of forming an imaging system. The method includes providing a optical pulse generator for providing an optical pulse having a spectral bandwidth and includes monochromatic waves having different wavelengths. Also, the method includes positioning a refractive dispersive element to receive a second optical pulse associated with the optical pulse and disperses the second optical pulse at different angles on the surface of the dispersive element depending on wavelength. Furthermore, the method includes arranging one or more focal element to receive the dispersed second optical pulse produced on the dispersive element. The one or more focal element recombines the dispersed second optical pulse at a focal plane on a specimen where the width of the optical pulse is restored at the focal plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram illustrating depth-resolved wide-field illumination multiphoton excitation microscopy; 
         FIG. 2  is a schematic diagram illustrating depth-resolved wide-field illumination microfabrication; 
         FIG. 3A-3B  are graphs illustrating results for intensity square along an optical axis with different groove frequency of a grating; 
         FIGS. 4A-4B  are graphs illustrating results for intensity square along an optical axis with different Gaussian beam 1/e radius; 
         FIGS. 5A-5B  are graphs illustrating results for intensity square along an optical axis with different spectral bandwidth of optical pulse; 
         FIG. 6  is a schematic diagram illustrating a 3D lithographic microfabrication system based on standing wave two-photon excitation wide-field illumination; and 
         FIGS. 7A-7B  are schematic diagrams illustrating wavefronts associated using standing wave two-photon excitation wide-field illumination and the system shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention proposes the design of a 3-D multiphoton lithographic system which generates 3-D microstructures with multiphoton wide-field illumination. This is the first time a microfabrication technique with multiphoton wide-field illumination is introduced. A mathematical model has been derived based on diffraction theory, to predict the axial resolution for both multiphoton wide-field illumination microscopy and microfabrication based on numerical simulation. Finally, the design parameters to improve axial resolution are reviewed. Future works include combining this optical model with a photopolymerization process model to predict fabrication efficiency and resolution. 
       FIG. 1  shows a microscopic imaging system  2  based on depth-resolved widefield illumination. The microscopic imaging system  2  includes a 4-f imaging system and a reflective diffraction grating structure  8  acting as a dispersive element. If a mirror is located at the image plane instead of the reflective diffraction grating structure, microscopic imaging system  2  works as conventional widefield illumination microscopy without depth-resolving capability. In order to enable depth discrimination, the temporal focusing is introduced. Ultrafast optical pulses  4  are introduced having spectral bandwidth, and includes monochromatic waves with different wavelengths. A beam expander  6  controls the illumination field of view size at the Specimen  16  and a reflective diffraction grading structure  8  disperses the ultrafast optical pulses  4  at different angles, depending on the wavelength. Each monochromatic wave diffracts with different angles on the surface of the reflective diffraction grating structure  8 , depending on wavelength, propagates through a tube lens  10 , beam splitter  14 , and an objective  12 . 
     These monochromatic waves are recombined only at the focal plane on the specimen  16 , and the width of the ultrafast optical pulses  4  is also restored at that plane. Far from the focal plane, the optical pulses  4  become broadened since it combines out of phase. It causes fluorescence yield to drop since broadened optical pulse has low instantaneous intensity. Therefore, temporal focusing enables optical section by preferentially exciting only the focal plane. By moving specimen  16  along optical axis, images at different planes of the specimen  16  can be also obtained. Fluorescence 2-D images are acquired by locating the intensified charged-couple device (iCCD)  18  at the conjugate plane of the object plane. Note in other embodiments of the invention the reflective diffraction grating structure  8  can also serve as a transmission dispersive element. 
       FIG. 2  is schematic diagram illustrating the depth-resolved wide-field illumination multiphoton excitation microfabrication  24 . The basic concept of temporal focusing is same, and pattern forming device  28  such as a digital micromirror device (DMD) with digital light processing (DLP), spatial modulator, or a microelectromechanical system (MEMS) mirror array can be inserted in order to generate a 2-D pattern at the focal plane. In other embodiments of the invention, the. It is very similar to microscopic imaging system  2  shown in the  FIG. 1 . However, the DMD with DLP or MEMS mirror array  28  is incorporated to generate a 2-D pattern formed beam  40  projected to the focal plane in the specimen  38 . By moving the specimen  38 , different 2-D patterns are made with mirrors for the different depth. A beam expander  30  controls the field of view size of the 2-D pattern formed beam  40  using ultrafast optical pulses  29 , and a reflective diffraction grading structure  32  disperses the 2-D pattern formed beam  40  at different angles, depending on the wavelength. Note in other embodiments of the invention the reflective diffraction grating  32  structure can also serve as a transmission dispersive element. Each monochromatic wave is directed to a different angle on the surface of the reflective diffraction grating structure  32 , depending on wavelength, propagates through a tube lens  34  and an objective  36 . In other embodiments of the invention, a digital micromirror device (DMD) with digital light processing (DLP), a spatial modulator, or a microelectromechanical system (MEMS) mirror array can be inserted after dispersion by reflective diffraction grating structure  32  or respective transmission dispersive element for pattern forming. 
     These monochromatic waves are recombined only at the focal plane in the specimen  38 , and the width of the 2-D pattern formed beam  40  is also restored at focal plane. By moving the specimen  38  along an optical axis, different patterns at different planes of the specimen  38  can be fabricated. 
     The desired 2-D pattern formed beam  40  can be generated by using the DMD as an intensity based spatial light modulator, see insert  26 , positioning at a conjugate plane of the specimen  38 . 2-D patterned illumination is delivered to focal plane instead of an uniform illumination used in imaging system  2 . Due to the axial discrimination of this process, the pattern will be generated only at the focal plane in the specimen  38 . When the specimen  38  moves to next depth section, the mirrors can be reconfigured and new patterns can be created at a different plane. 
     In order to optimize the design of the 3-D two-photon lithographic microfabrication system, it is important to thoroughly understand the image formation theory underlying this approach. One can derive an optical model of light distribution near the focal plane based on diffraction theory. This optical model allows one to accurately predict the axial resolution that can be achieved. Further, this optical model allows one to examine the effects of different design parameter choices in optimizing performance. 
     First, it is assumed that the input beam profile is Gaussian with a width of S (1/e beam radius). The spectral distribution of the input beam is also assumed to be Gaussianwith a bandwidth of Ω. The transverse field has the form: 
                       U   0     ⁡     (     x   ,   y   ,   Δω     )       =       A   0     ⁢   exp   ⁢       {     -         x   2     +     y   2         S   2         }     ·   exp     ⁢     {     -       Δ   ⁢           ⁢     ω   2         Ω   2         }               (   1   )               
where (x, y) are the lateral coordinate, and A 0  is the amplitude. Δω=ω−ω 0  where ω is the angular frequency, and ω 0  is the center frequency. To examine how this wave propagates through an optical system as shown on  FIGS. 1 and 2 , one can further assume that the lenses are perfectly chromatic-aberration-corrected with no dispersion. Also, it is assumed that the system is completely diffraction limited. With these assumptions, one can write the field at grating surface as:
 
                       U   1     ⁡     (       x   1     ,     y   1     ,     Δ   ⁢           ⁢   ω       )       =         U   0     ⁡     (       x   1     ,       y   1     ⁢   Δω       )       ⁢   exp   ⁢     {     ⅈ   ⁢     Δω   c     ⁢   sin   ⁢           ⁢     α   ·     x   1         }               (   2   )               
where c is the speed of light, and (x1, y1) are the lateral coordinates at the grating plane. The grating effectively introduces a phase chirp along one direction α=sin −1 (2πcG/ω 0 ) is the incident angle to the grating with groove frequency G such that the center wavelength of the input beam propagates along the optical axis. Since the grating and the microscope focal plane is conjugated by a 4-f imaging system, the field can be readily propagated along the optical path. Ignoring the field aperture of the microscope, the field at back aperture of the objective is:
 
                       U   2     ⁡     (       x   2     ,     y   2     ,   Δω     )       =       -   ⅈ     ⁢       exp   ⁢     {     ⅈ   ⁢           ⁢   k   ⁢           ⁢   2   ⁢           ⁢     f   1       }         λ   ⁢           ⁢     f   1         ⁢       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢         U   1     ⁡     (       x   2     ,     y   2     ,   Δω     )       ⁢   exp   ⁢     {       -   ⅈ     ⁢           ⁢   2   ⁢           ⁢   π   ⁢           x   1     ⁢     x   2       +       y   1     ⁢     y   2             f   1     ⁢   λ         }     ⁢     ⅆ     x   1       ⁢           ⁢     ⅆ     y   1                       (   3   )               
where f 1  is the focal length of the tube lens, k=ω/c, λ=2π/k, (x 2 , y 2 ) are the lateral coordinate of the back aperture plane. The field near the focal plane can be calculated as:
 
                       U   3     ⁡     (       x   2     ,     y   2     ,   Δω     )       =       -   ⅈ     ⁢       exp   ⁢     {     ⅈ   ⁢           ⁢   k   ⁢           ⁢       (       2   ⁢           ⁢     f   2       +     z   3       )     1       }         λ   ⁢           ⁢     f   1         ⁢       ∫     -   ∞     ∞     ⁢       ∫     -   ∞     ∞     ⁢         U   2     ⁡     (       x   2     ,     y   2     ,   Δω     )       ⁢   exp   ⁢     {       -   ⅈ     ⁢           ⁢   π   ⁢         x   2   2     +     y   2   2         f   2       ⁢       z   3       f   2         }     ⁢   exp   ⁢     {       -   ⅈ     ⁢           ⁢   2   ⁢           ⁢   π   ⁢           x   1     ⁢     x   3       +       y   1     ⁢     y   3             f   2     ⁢   λ         }     ⁢     ⅆ     x   2       ⁢           ⁢     ⅆ     y   2                       (   4   )               
Since the tube lens and the objective are chromatic-aberration-corrected, the effective optical path lengths (phase terms) are same for the different wavelength λ or wave vector k at the focal plane.
 
exp{ i 2 k ( f   1   +f   2 )}=const  (5)
 
The temporal evolution of the field and the time averaged intensity close to the focal plane can be expressed as:
 
                       U   3     ⁡     (       x   3     ,     y   3     ,     z   3     ,   t     )       =       ∫     -   ∞     ∞     ⁢         U   2     ⁡     (       x   2     ,     y   2     ,   Δω     )       ⁢     exp   ⁡     (       -   ⅈω     ⁢           ⁢   t     )       ⁢     ⅆ   ω                 (   6   )                 I   ⁡     (       x   3     ,     y   3     ,     z   3       )       =       1   T     ⁢       ∫   0   T     ⁢                U   3     ⁡     (       x   3     ,     y   3     ,     z   3     ,   t     )            2     ⁢           ⁢     ⅆ   t                   (   7   )               
Since multiphoton excitation is nonlinear process, excitation efficiency is proportional to N th  power of the intensity if N-photon excitation process happens.
 
     
       
         
           
             
               
                 
                   
                     
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     The axial optical resolution for the inventive system is simulated by using the mathematical model derived herein. Axial resolution in two-photon excitation microscopy is defined as the full width at half maximum (FWHM) of the squared average intensity. For the two-photon excitation, the two-photon-induced photochemical reaction will depend quadratically on the time averaged intensity, for example, I 2 . From this calculation, one can examine the grating frequency, G, on the axial resolution, as shown in  FIG. 3A . As G decreases, optical resolution also increases (worsens) since decreasing dispersion angle underfills the back aperture of the objective, as shown in  FIG. 3B . So, optical resolution is inversely proportional to numerical aperture (NA). 
     One can investigate how the field of view (FOV) is related to S. As S increases as shown in  FIG. 4A , optical resolution also increases (worsens) since increasing beam diameter results in less pulse width broadening outside the focal plane, as shown in  FIG. 4B . Therefore, optical resolution is proportional to FOV area, or S 2 . 
     The axial resolution of the system can be estimated as a function of spectral bandwidth. In order to minimize the effect of NA, one can use different groove frequencies in the grating, depending on Δλ=λ−λ 0 , as shown in  FIG. 5A . As bandwidth increases, excitation pulse width shortens inversely resulting in better axial resolution, as shown in  FIG. 5B . It is seen that the axial resolution decreases (improves) inversely with spectral bandwidth Δλ or temporal pulse width τ p  for the transform-limited optical pulse. 
       FIG. 6  show a 3D lithographic microfabrication system based on standing wave two-photon excitation wide-field illumination (SW-TPE-WI). It is similar to the 3D lithographic microfabrication system of  FIG. 1  except for using two opposite directional wide-field illumination instead of one directional illumination. For purposes of clarity  FIG. 6  focuses at the point where dispersion occurs using the reflective diffraction grating structure  8 . Note reflective diffraction grating structure  8  can also perform transmission dispersion. This technique results in lower background excitation beyond the focal plane as well as higher axial resolution than that system of  FIG. 1 .  FIG. 6  shows a single beam  44  with optical pulses separates into two identical beams  40 ,  42  at the 50:50 beam splitter (BS), and they propagate through two different optical paths and recombine, using objectives # 1  and # 2 , at the focal plane using mirrors M 2 -M 7 . Note that the two beams are symmetric around the focal plane, not an optical axis. Tubes TL 1  and TL 2  are used to focus both beams to the their objectives # 1  and # 2  for illumination. A specimen is between position at the focal plane of both objectives # 1  and # 2 . 
     With standing wave technique, all the wave fronts in different wavelength beams in objectives # 1  and # 2  are overlapped (or in phase) at the focal plane, as shown in  FIG. 7A .  FIG. 7B  shows wavefronts  52  of the objective  12  used in the system of  FIG. 1  that does not generate interference pattern  54  in the axial direction, and an optical pulse being restored at the focal plane when waves pass through focal plane. However,  FIG. 7A  shows two beams  56 ,  60  with same wavelength travel from the different directions (downward and upward), and the interference patterns exist where they are overlapped. This pattern  62  is also perpendicular to an optical axis (parallel to focal plane). Interference pattern peaks for all the wavelengths included in the optical pulse are designed to be exactly overlapped at the focal plane, which means very high instantaneous intensity peak happens only at the focal plane when all the wavelength components are overlap together. Since the period between interference pattern peaks depends on the wavelength, interference pattern peaks cannot be overlapped any more in the space other than focal plane. Additionally, it also enhances axial optical resolution since standing wave  58  provides the interference period of equal or smaller than half of wavelength. 
     The invention proposes a 3D two-photon lithographic microfabrication, based on depth-resolved wide-field illumination which is also useful in the microscopy. A mathematical model was derived for calculating axial resolution in the case of depth-resolved wide-field illumination with temporal focusing. With numerical simulations, dominant design parameters to affect axial resolution can be obtained as follows: 
                     FWHM   z     ∝       FOV   ·     τ   p       NA             (   9   )               
Importantly, the simulation will allow one to better design the physical instrument by evaluating how various parameters, such as grating frequency, beam waist, and laser bandwidth, will affect patterning resolution at the focal plane.
 
     Optical simulation alone is not sufficient to predict the resolution of the structures to be patterned in the microfabrication system. Since photopolymerization is a highly nonlinear kinetic process, it is impossible to predict the characteristics of the structures to be produced without coupling the optical model with a model of photopolymerization. Comprehensive photo-chemical models will be created of two-photon photopolymerization processes induced by either spatial or temporal focusing. The availability of this model will allow one to find optimal experimental parameters to improve fabrication throughput and resolution. Finally, one can compare the axial point spread function (PSF) measurements with the simulation results, and fabricate microstructures for biological applications with the inventive microfabrication system. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof various changes, omissions and additions to the form and detail thereof may be made therein, without departing from the spirit and scope of the invention.