Abstract:
Methods and apparatus for producing sub-diffraction limited images utilizing an exponential scaling effect. An exemplary system provides an optical source that focuses an optical beam onto a target. The focused optical beam has sufficient optical intensity to induce an exponential signal response within the target. A detection device detects the exponential signal response. A scanning device scans the focused optical source and another device records the detection of the exponential signal response for purposes of producing a sub-diffraction limited image. The system further includes a display device that displays at least a portion of the recorded detection.

Description:
PRIORITY CLAIM 
       [0001]    This application clams priority to provisional patent application 61/255,414 filed on Oct. 27, 2009 and 61/305,354 filed on Feb. 17, 2010 and are incorporated herein by reference. 
     
    
     FIELD OF THE INVENTION 
       [0002]    This invention relates generally to producing sub-diffraction limited imaging utilizing non-linear exponential scaling of laser-induced thermal radiation. 
       BACKGROUND OF THE INVENTION 
       [0003]    Improvement in image resolution with non-linear signal response has been investigated through the square law power scaling of two-photon absorption in biological [W. Denk, J. Strickler, W. Webb, Science 10248 (1990) 73-76] and IC applications [E. Ramsay, D. T. Reid, Optics Communications 10221 (2003) 10427-433]. The square power dependence of the two-photon signal narrows the image point response by square root of two. Image resolution is improved over the diffraction limit by the same factor. 
         [0004]    Two-photon absorption in silicon is produced using a near IR wavelength pulsed laser (1275 nm). The peak pulse power is sufficiently high to observe second order absorption of two photons at once. The combined energy is equivalent to a single photon at 10637.5 nm, well above that silicon bandgap energy. The resultant photo-carriers are collected as the laser is scanned. Recent efforts have demonstrated both increased transverse and axial resolution. This combination has produced dramatic 3-D images of the junction areas of various devices [V. Pouget, et al., 35th ISTFA, (2009) 10268-71]. 
         [0005]    Even higher peak laser powers can induce absorption of three photons, four photons, and so on. The related image resolution would improve as square root of the number of photons absorbed for each created photo-carrier. However, the required laser powers become somewhat problematic from a practical standpoint. 
       SUMMARY OF THE INVENTION 
       [0006]    The current invention provides methods and apparatus for producing sub-diffraction limited images utilizing this exponential scaling effect. 
         [0007]    An exemplary system provides an optical source that focuses an optical beam onto a target. The focused optical beam has sufficient optical intensity to induce an exponential signal response within the target. A detection device detects the exponential signal response. A scanning device scans the focused optical source and another device records the detection of the exponential signal response for purposes of producing a sub-diffraction limited image. 
         [0008]    In one aspect of the invention, the system further includes a display device that displays at least a portion of the recorded detection. 
         [0009]    In another aspect of the invention, the optical source is a laser. 
         [0010]    In still another aspect of the invention, the exponential signal response within the target is thermal radiation or a semiconductor device leakage current. 
         [0011]    In yet another aspect of the invention, the scanning device is a laser scanning or confocal type microscope. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]    Preferred and alternative examples of the present invention are described in detail below with reference to the following drawings: 
           [0013]      FIG. 1  is a laser scanning microscope (LSM) formed in accordance with the prior art; 
           [0014]      FIG. 2  is an LSM formed in accordance with an embodiment of the present invention; 
           [0015]      FIG. 3  illustrates predicted result of the present invention compared to prior art results; 
           [0016]      FIG. 4  shows experimental resulting image produced by the present invention; 
           [0017]      FIG. 5  is an LSM formed in accordance with an alternate embodiment of the present invention; and 
           [0018]      FIGS. 6   a, b  show prior art and present invention image results of the same circuit component. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0019]      FIG. 1  shows a laser scanning microscope (LSM)  100  formed in accordance with the prior art. LSMs are a form of confocal microscope. The following short description of LSM  100  is meant as a framework for understanding the current invention, not as a complete reference on the apparatus. 
         [0020]    LSM  100  includes a laser source  102 , which emits an optical beam  104  that is initially collimated. The collimated optical beam  104  passes through a beam splitter  106  on its way to a scanner assembly  108 . The scanner assembly  108  scans the optical beam  104  in angle along both directions transverse to the path of the optical beam  104  as it exits the scanner assembly  108 . A first lens  110  serves to both focus the optical beam  104  and to act as a field lens. An optical objective  112  focuses the optical beam  104  onto a target  114  in such a manner as to focus the optical beam  104  on the target  114  and to cause transverse angular deviations in the optical beam  104  induced by the scanner assembly  108  to be transformed into transverse spatial deviations on the target  114 . 
         [0021]    Some portion of the optical beam  104  striking the target  114  will be reflected back (as an optical beam  116 ) through the optical components described above until it strikes the beam splitter  106  where some of the returning optical beam  116  will be deviated towards a second lens  120 . The second lens  120  focuses the returning optical beam  116  onto an imaging detector  122  which produces an electrical detection signal  130  that is in turn passed to data acquisition and control (DAQC) electronics  124 . There are many known forms for the DAQC electronics  124 ; however, the basic functionality is to record the detection signal  130  as the scanner assembly  108  is directed to scan based on a scan control line (signal)  132 . A raster scan is typically used for the scan control line  132 . The recorded signal can then be sent to a video display  126  via a display line  134 . In this fashion the variations in the amount of the optical beam  104  reflected by the target  114  will appear as an image to a user of the LSM  100 . 
         [0022]    In one embodiment of the current invention as shown in  FIG. 2 , an LSM  100 - 1  provides sub-diffraction limited imaging. In this instance, the laser source  102  is assumed to be of sufficient optical power to significantly raise the temperature of the target  114  at the focal spot of the optical beam  104 . The LSM  100 - 1  includes a color beam splitter  200  which diverts some of the returning optical beam  116  into a thermal optical beam  202  while allowing a significant portion of the returning optical beam  116  (as well as the optical beam  104 ) to pass through without reflection. In operation the wavelength of the laser source  102  is chosen to be short compared to typical thermal radiation. In one non-limiting example, the laser wavelength of the optical beam is 532 nanometers produced by a double YAG laser. The color beam splitter  200  then transmits the 532 wavelength optical beam and reflects longer wavelengths. 
         [0023]    The thermal optical beam  202  passes through a third lens  204  that focuses onto a thermal detector  206 . If the target  114  is made sufficiently hot by the laser source  102 , i.e., in the range of 200 C and above, an InGaAs detector is used as the thermal detector  206 . The InGaAs detector has a typical wavelength response of approximately 1.0 to 1.7 micron. Lesser heating requires detection at longer wavelengths in order to produce sufficient response by the thermal detector  206 . InSb detectors work in the 102 to 106 micron range. HgCdTe detectors detect out to 112 micron. Several other options for the thermal detector  206  based on target temperature will be obvious to those versed in the art of thermal detectors. 
         [0024]    The thermal detector  206  produces an electrical thermal detection signal  210  which sent to the DAQC electronics  124 . The DAQC electronics  124  process the electrical thermal detection signal  210  in a fashion similar to the detection signal  130  so as to produce an image on display  126  of the variations in the electrical thermal detection signal  210 . 
         [0025]    An image of the electrical thermal detection signal  210  provides some general interest for thermal analysis of the target  114 . However, a primary interest for the current invention is the ability to produce sub-diffraction limited images. The optical intensity of the laser spot focused on the target  114  is not uniform, but falls off in a Gaussian or Airy pattern (dependent on the exact details of the optics of the LSM  100 - 1 ) in directions transverse to the central axis of the optical beam  104 . This fall off is referred to as the optical spot size of the optical beam  104  on the target  114  and is limited by diffraction. A perfect imaging system is said to have diffraction limited resolution as defined by a fraction of this spot size, e.g. the Rayleigh criteria. 
         [0026]    The temperature rise induced by the laser  102  is proportional to said optical intensity. However, the thermal radiation induced by the temperature rise depends exponentially on said temperature. Thus, halving the temperature rise can reduce the induced thermal radiation by a factor of 10 or more. Thus the spot size of the thermal radiation induced by the laser heating will be narrower than the diffraction spot size, allowing sub-diffraction limited imaging using the electrical thermal detection signal  210 . 
         [0027]      FIG. 3  shows a theoretical spot size of electrical thermal detection signal  210  (indicated as Laser-Thermal) in comparison to the Airy spot size for a typical high numerical aperture objective and a Two-Photon signal. A factor of three reduction in spot size, relating to a remarkable factor of three improvement in image resolution over the diffraction limit is predicted. The Two-Photon signal is the spot size related to the techniques described by Ramsay and Pouget. 
         [0028]      FIG. 4  shows an experimental result of the predicted image improvement. Spots as small as 0.36 micron are clearly seen—much smaller than the ˜1 micron laser spot size indicated in circle  280 . 
         [0029]    A second embodiment of an LSM  100 - 2  is shown in  FIG. 5 . In this embodiment, the target  114  is an integrated circuit. Contained within a typical integrated circuit are a number of junctions whose leakage current varies exponentially with temperature. This leakage current is monitored with a current sensor  300  (of which there are many forms) attached to the integrated circuit (the target  114 ) via current sensing lines  302  in such a way to produce a current sense signal  304 . The current sense signal  304  is passed to the DAQC electronics  124  and displayed as an image in a fashion similar to the two prior signals  130  and  210 . Due to the exponential response of leakage current with heating, the resultant leakage current image will also be sub-diffraction limited. 
         [0030]    An experimental example is shown in  FIGS. 6   a,b . The image in  FIG. 6(   a ) is derived from the electrical detection signal  130  of the reflected optical beam  104  and the image in  FIG. 6(   b ) is derived from the current sense signal  304 . A white line  340  in the images is part of a diode structure with a p-n junction. The p-n junction supplies the current sense signal  304  as shown in  FIG. 6(   b ). The improved resolution is clear. 
         [0031]    The above two embodiments clearly demonstrate that any signal that responds exponentially to the laser intensity or the resultant thermal signature can be utilized to produce the sub-diffraction imaging described herein. The possibilities include, but are not limited to:
       Chemical reaction rates   Biological growth rates   Electron emission   Transport rates across barriers (chemical barriers, optical barriers in near-field transmission with temperature expansion affecting barrier spacing)   Fluorescent emission rates   Ionization rates       
 
         [0038]    While the preferred embodiment of the invention has been illustrated and described, as noted above, many changes can be made without departing from the spirit and scope of the invention. Accordingly, the scope of the invention is not limited by the disclosure of the preferred embodiment. Instead, the invention should be determined entirely by reference to the claims that follow.