Abstract:
In apparatus for superresolution microscopy or microlithography, wherein a spot in the specimen to be examined or in the microlithographic medium is raised to an excited state by a first pulse of light, and a second pulse of light reduces the excitation in the peripheral parts of the spot to increase the resolution of the instrument, a method whereby the wavelength of the second pulse in the specimen or medium is the same as the wavelength of the first pulse, thereby allowing the cost and complexity of the apparatus to be lowered.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 11/795,094, filed Mar. 17, 2008 now U.S. Pat. No. 7,816,654, which claims priority from U.S. Provisional Patent Application Ser. No. 60/644,597, filed Jan. 16, 2005, from U.S. Provisional Patent Application Ser. No. 60/644,629, filed Jan. 16, 2005 and from U.S. Provisional Patent Application Ser. No. 60/669,302, filed Apr. 5, 2005, the entire disclosure of which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to imaging apparatus, and more specifically to fluorescence microscopes and microlithography exposure tools that resolve detail finer than the Abbe resolution limit. Still more specifically, the present invention relates to improvements in devices, such as stimulated emission depletion microscopy and microlithography exposure tools, which scan a specimen or microlithographic emulsion with a spot or line at a first wavelength, and by means of radiation at a second wavelength, reduce the excitation in the peripheral parts of the spot or line, thereby improving the resolution of the instrument. 
     BACKGROUND OF THE INVENTION 
     Hell and Wichmann in U.S. Pat. No. 5,731,588 and Baer in U.S. Pat. Nos. 5,777,342, 5,866,911, 5,952,668 and 6,259,104 proposed using stimulated emission to quench excitation in the peripheral parts of an excited spot in a scanning fluorescence microscope to improve resolution. In this method a spot was excited by a light pulse of one wavelength (the excitation wavelength) and quenched by a subsequent light pulse of a second wavelength (the quenching wavelength). The technique is often referred to as “STED” for “stimulated emission depletion” microscopy. This method has been experimentally verified as published in various papers from the laboratory of Stefan Hell. 
     Other techniques related to STED have been proposed that do not involve stimulated emission, but share with STED the need for providing an exciting beam to the specimen paired with a second beam to reduce excitation in the peripheral parts of the excited spot or line, thereby “sculpting” the excited spot or line to have a sharper central maximum. An example of such alternative techniques, which will be referred to herein as “STED-like,” is excited state depletion microscopy (Hell and Kroug, Appl. Phys. B. 60:495(95)), where the sculpting beam pulse occurs before the excitation pulse and renders molecules exposed to the sculpting beam temporarily insensitive to subsequent excitation by the excitation beam. Furthermore, various STED-like techniques have been proposed that employ proteins and dye molecules able to switch between a fluorescent and non-fluorescent forms by light pulses of appropriate wavelengths. 
     Although these STED and STED-like techniques promise greatly improved resolution, the requirement of providing two extremely short pulses of different wavelengths in the specimen, such that the pulses are synchronized with each other, has impeded the acceptance of the technique as a practical route to superresolution. Although various proposals have been made to create two synchronized pulsed output beams with different wavelengths from a single input laser (e.g., U.S. Pat. No. 5,866,911), such proposals require some frequency converting devices such as frequency multiplying crystals or optical parametric oscillators to convert the wavelength of one or both of these beams, so that even though the two beams had the same wavelength when they leave the laser, after conversion, when they reach the specimen, the beams had different wavelengths. Besides adding cost and complexity to the instrument, some frequency conversion devices are designed to be optimum at just one wavelength, limiting the versatility of the system. Furthermore the techniques require that the focusing optics be achromatized in order to deal with different excitation and quenching wavelengths. Another factor that has impeded the wider adoption of the technique is that because of the complexity of the apparatus to provide two synchronized pulses of different wavelengths to the specimen, the implementations have been in the form of special purpose integrated instruments not well suited to the modular architecture of current scanning microscopes. 
     OBJECTS AND ADVANTAGES 
     One object of the present invention is to implement STED and STED-like microscopy or microlithography in which the sculpting beam projected on the specimen has the same wavelength as the excitation beam. 
     Another object of the invention is to provide a microscope that does not require components for frequency conversion or laser synchronization. 
     Another object of the invention is to provide a microscope in which only one wavelength of light is required for excitation and quenching, reducing the requirements for chromatic dispersion correction in the focusing optics. 
     Still another object of the invention is to provide a microscope that can be added in a modular fashion to an existing multiphoton microscope, with large cost savings for an owner of an existing multiphoton microscope. 
     Still another object of the invention is to provide the resolution advantages of STED and STED-like microscopy, while allowing tuning of the wavelength of excitation, to match the properties of particular fluorophores. 
     Yet other objects of the invention will become apparent in this disclosure. 
     SUMMARY OF THE INVENTION 
     The foregoing objects are achieved and the foregoing problems are solved in one illustrative embodiment of the invention, applied specifically to the field of fluorescence microscopy, although the principles embodied therein also apply to the other applications of the present invention such as microlithography, nanofabrication and optical information storage and retrieval. The specimen to be examined by the microscope is stained with a dye that can be excited to fluorescence via multiple photon absorption at a particular wavelength and where such fluorescence can be quenched by a single photon process by light of this same wavelength. Light from a short pulsed laser such as a mode locked titanium-sapphire laser is split into two portions, one of which is directed to a spot in the specimen as a pulse of no more than a few picoseconds in order to excite the dye at that spot by multiple photon absorption. The second portion of the pulse is stretched to a pulse duration from several to several hundred picoseconds, for example by the well known means of using prisms, diffraction gratings or large pathlengths in dispersive media (e.g. in optical fibers) to introduce group velocity dispersion, and this stretched pulse is shaped spatially into a pattern having a point of substantially zero intensity, and imaged on the excited spot following the exciting pulse. By causing this zero intensity point to substantially overlap with the central maximum of the excited spot, and by causing the intensity of the quenching beam to rise quickly with distance from this central point, the quenching thereby increases with distance from the central maximum of the excited spot, reducing its diameter, and thereby increasing resolution. The excited and quenched spot is scanned over the specimen and the fluorescence emanating from the spot, following each quenching pulse, is measured and used to form an image of the specimen, using imaging techniques well know in the art of scanning spot confocal and multiphoton microscopy. 
     According to one aspect of the invention, an apparatus, for irradiating a selected region of a target material in order to excite members of an excitable species contained within the selected region to a particular state of excitation, includes means for directing to said target material a first radiation and a second radiation. The means for directing the first radiation and the second radiation are configured such that the first radiation is adapted to excite the members to said particular state of excitation, and such that the second radiation is adapted to reduce the ratio of excited members in the particular state of excitation to members not in the particular state of excitation at the selected region. Further, the apparatus provides that the intensity distribution of the first radiation in the target material is significantly different from the intensity distribution of the second radiation. The apparatus includes elements to provide that at the target material, the first and the second radiation have the same wavelength. 
     According to another aspect of the invention, a method is provided for irradiating a selected region of a target material in order to excite members of an excitable species contained within the selected region to a particular state of excitation. The method includes directing to the target material a first radiation and a second radiation, such that the first radiation is adapted to excite the members to the particular state of excitation and such that the second radiation is adapted to reduce the ratio of excited members in the particular state of excitation to members not in the particular state of excitation at the selected region. Further, the method provides that the intensity distribution of the first radiation in the target material is significantly different from the intensity distribution of the second radiation. The method also causes the first and the second radiation to have the same wavelength at the target material. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The principles of the invention will be more particularly discussed with reference to the accompanying drawings in which: 
         FIG. 1  is a schematic cross-sectional view showing a microscope embodying the invention; 
         FIG. 2  is a schematic cross-sectional view showing a detail of light rays in a prism shown in  FIG. 1 ; 
         FIG. 3  is a three dimensional perspective view of three different positions of the prism shown in  FIG. 2 ; 
         FIG. 4  is a schematic cross-sectional view showing a magnified detail from  FIG. 1 ; and 
         FIG. 5  is a plan view showing the prisms of  FIGS. 1 ,  2 ,  3 , and  4  in which the real image plane is in the plane of the illustration; 
         FIG. 6  is a schematic cross-sectional view showing an embodiment of the instrument; 
         FIG. 7  is a schematic cross-sectional view of the four beam segments emanating from the hollow pyramid illustrated in the device of  FIG. 6 ; 
         FIG. 8  is a plan view of the segmented mirror in the device illustrated in the device of  FIG. 6 ; and 
         FIG. 9  is a magnified cross-sectional view of the hollow pyramid illustrated in the device of  FIG. 6 . 
     
    
    
     DESCRIPTION OF THE INVENTION 
       FIG. 1  shows a cross sectional view of an embodiment of the invention for scanned fluorescence microscopy. U.S. Pat. Nos. 5,777,342, 5,866,911, 5,952,668 and 6,259,104 contain useful background to the following discussion, and are incorporated herein by reference. Light from a femtosecond pulsed titanium-sapphire laser is directed on rotatable half-wave plate  1 , and the emerging beam is directed onto a polarizing beamsplitter  2 . Beam  3 , which is one of the beams emerging from the beam splitter  2  will be used for multiphoton excitation. Beam  3  is directed onto a chromatic dispersion compensating assembly, as discussed in U.S. Pat. No. 6,178,041, the disclosures of which are hereby incorporated by reference, consisting of four diffraction gratings  4 ,  5 ,  6  and  7 . The beam emerging from grating  7  is focused by lens  8  onto one end of an optical fiber  9 , which conducts the light to the scan head  10 . Negative chromatic dispersion produced by the gratings  4 ,  5 ,  6  and  7  compensate for positive dispersion in the fiber  9 , so as to maintain a short pulse width in the pulse directed on the specimen, as discussed in U.S. Pat. No. 6,178,041. Within the scan head  10 , light leaving the exit end of fiber  9  is collimated by lens  11  and reflected by mirror  12  (which may be a dichroic beam splitter) onto scanning means  13  and then, onto objective  14 , which focuses the excitation pulse into a spot in specimen  15 . 
     Beam  3 ′ emerging from beamsplitter  2  will be used to produce the quenching beam. Beam  3 ′ is directed successively to mirrors  16  and  17 , the position of which can be moved in the direction of the axis of the beam  3 ′ to induce a variable path length, and thereby change the time delay between the offset of the excitation pulse and the onset of the quenching pulse. The beam emerging from mirror  17  is reflected by mirror  18  to an assembly consisting of four diffraction gratings  19 ,  20 ,  21  and  22  that apply negative chromatic dispersion, and the beam emerging from grating  22  is focused by lens  23  onto the end of optical fiber  24 . The negative dispersion produced by the assembly of gratings  19 ,  20 ,  21  and  22  is greater or smaller than the positive dispersion introduced by fiber  24 , so that the pulse experiences a net stretching. As an alternative, the gratings  19 ,  20 ,  21  and  22  could be put in a configuration that produces positive dispersion. The stretched pulse conducted by fiber  24  is split by a fiber optical 1:4 splitter  25  equally into four fibers. The light emerging from these fibers  26 ,  27 ,  28  and  29  is collimated by four identical lenses, of which only lenses  30  and  30 ′ are in the plane of this drawing and are shown. The beam emerging from lens  30  is reflected by mirror  31  onto lens  32 , and the converging beam leaving lens  32  enters equilateral prism  33  where it experiences total internal reflection to be directed parallel to the axis of the cone of excitation light emerging from the end of fiber  9 , and it is focused to a spot in the same plane as the end of fiber  9 . The ends of fibers  27 ,  28  and  29  also are focused to spots of light in this same plane, so that the spots are at the corners of a square with the tip of fiber  9  at the center. 
       FIG. 2  shows a detail of the passage of rays of quenching light in prism  33 . The rays would form an image of the end of fiber  26  at plane  40 , but total internal reflection from prism face  41  causes the image instead to be formed at position  42 , adjacent to a chisel edge  43  of the prism.  FIG. 3  shows a three-dimensional perspective view of different positions of prisms such as prism  33 , showing the chisel edge  43 . Each of these prisms could be made by placing them so one face is horizontal, and making two cuts at 45° to the edges of the horizontal face, so the cuts almost touch, producing the chisel edge  43 . 
     When four of such prisms are positioned on a horizontal surface so that these four chisel edges, such as edge  43 , make the sides of a square, then they will be in the relative positions required in the device shown in  FIG. 1 . The magnified detail of  FIG. 1  shown in  FIG. 4 , shows how the end of fiber  9  can be tapered, by gradually removing the cladding, so at the tip only the core plus the minimum cladding needed to maintain propagation remains, and this tip just fits into this square, the edges of which are the chisel edges, such as edge  43 .  FIG. 5 , shows a plan view, such that the plane of the figure is parallel to the real image plane containing the tip  44  of fiber  9 , and the spot images  45 ,  46 ,  47  and  48  of the ends of fibers  26 ,  27 ,  28  and  29 .  FIG. 5  shows the relationship between these spot images and the tip of excitation fiber  9 . 
     Lenses  11  and  13  and mirror  12  reimage these spot images  45 ,  46 ,  47  and  48  to the real image plane of the specimen  15 , such that these reimaged spot images in the specimen are at the corners of a square. The reimaged spots diagonally opposite each other have a 180° phase difference, so that the light exactly cancels at the central point of the square, which is also the central point that the tip of fiber  9  that is imaged to in the specimen. This phase adjustment can be made, for example, by axially moving fiber  26  with respect to fiber  29 , until the desired cancellation is produced. As discussed in the cited references, it is important that light from one pair of diagonally opposite reimaged spot images does not interfere with light from the remaining pair. One way that this can be accomplished, as discussed for example in U.S. Pat. No. 6,259,104, is to make the phase relationship between adjacent pairs of reimaged spot images have a 90° phase difference, and again the required phase relationships can be produced by axially moving the tips of the fibers  26 ,  27 ,  28  and  29  relative to each other. The resulting phase can be described as adding 90° of phase shift as one moves clockwise in the plane of focus of the specimen from each of the four reimaged spot images to the next one. The cited references describe other ways of providing that light from opposite reimaged spot images cancels but light from one of these opposite pairs cannot interfere with light from the remaining pair. 
     The dimensions are chosen so that the distance between the centers of the reimaged spot images and the center of the image in the specimen of the tip of fiber  9  is about 2.3 dimensionless optical units (a separation of one optical unit equals the inverse of the product of numerical aperture of the objective lens and the wavevector k=2π/λ). This geometry will produce a doughnut shaped intensity distribution in the real image plane of the specimen, with a theoretically zero intensity central spot, and where the intensity rises rapidly with distance from that central point. 
     The styryl dye RH414 appears suitable for multiphoton excitation and for quenching at the same wavelength, in this case 760 nm. The paper by Dyba and Hell (Appl. Optics. 42:5123 (03)) shows that light of this wavelength will lead to multiphoton fluorescent excitation. An earlier paper by the same authors (Phys. Rev. Lett. 88:163902 (02)) showed that the same dye was susceptible to STED quenching by light around the same wavelength. There are other possible candidate dyes including DiD, and it appears possible that the ability to be multiphoton excited and quenched by light of the same wavelength will turn out to be a relatively common property of dyes, not presently known only because it has not been looked for. 
     The above discussion has not dealt with the issues of measurement of light emitting from the excited spot, and creation of an image based on scanning of the excited spot. That is because the process is the same as in well known confocal (e.g. Baer, U.S. Pat. No. 3,705,755), multiphoton (e.g. Denk et al, U.S. Pat. No. 5,034,613, the disclosures of which are hereby incorporated by reference) and the already cited STED patents. As in multiphoton and STED imaging, the light emanating from the excited spot can either be descanned and imaged on a confocal pinhole, or imaged on a detector without descanning or passage through a confocal pinhole, for greater collection efficiency. 
     Many alternative methods have been described for producing a sculpting beam with the required zero intensity central point (or line, in the case of line scanning microscopes) and any of these could be applied to the present invention, provided the wavelength of the sculpting beam was the same as the excitation wavelength. Although the preceding description described a fiber to couple the excitation laser to the microscope scan head, the present invention would also be applicable in instruments where the path between the laser and the scan head did not involve passage through an optical fiber. 
     The same principle of multiphoton excitation and quenching by a lengthened pulse of the same wavelength could be used for scanned beam microlithography and microfabrication. For microlithography, the goal is to quench the peripheral parts of an excited spot in the photosensitive film or emulsion layer, before they have lead to the lasting change. The two-photon-activatable photoacid generator molecule, BSB—S 2 , has been used for two-photon microfabrication (Zhou et al Science 296: 1106(02)) using light of 705 to 800 nm. This substance also has a relatively long lived singlet state that might be susceptible to single-photon-induced quenching in this general wavelength range, and also has a sufficiently long lived excited singlet state, that quenching should be possible before the acid triggering the lasting change is released. 
     The device of  FIG. 1  allows a common wavelength to be used both for excitation and for reducing the excitation in the peripheral parts of the excited spot or line. However because the axial positions of the fibers  26 ,  27 ,  28  and  29  must be adjusted to create phase differences of 90°, 180°, and 270° and because the particular distance to create these phases is proportional to the wavelength, the fibers must be repositioned whenever that common wavelength is changed. The device shown in  FIG. 6  creates phase differences that do not require mechanical readjustment when the wavelength is changed, and for that reason is a more preferred embodiment of the present invention. Those elements in  FIG. 6  that are identical to elements in  FIG. 1  have been given the same reference numerals. There are some differences. For example, in  FIG. 1 , the transmitted component  3  from beam splitter was passed through a chromatic dispersion means and then conducted by an optical fiber in its passage to the scan head  10 . In the device of  FIG. 6 , in contrast, there is no intervening optical fiber, and consequently there is no need for the chromatic dispersion means which is required in the  FIG. 1  device to compensate for the opposite chromatic dispersion of the fiber, to maintain the short excitation pulse width. The excitation beam instead is conducted by mirrors  51  and  52 , passed through a small transparent hole in segmented mirror  54 , which will be explained more fully in the next paragraph, and focused by lenses  53  and  55  onto the real image plane  33 . The system of conduction by a fiber and compensation by a chromatic dispersion means, as shown in  FIG. 1 , has reportedly been found to be difficult to adjust, and so a light conduction method such as shown in  FIG. 6 , avoiding such a fiber would probably be preferable in a system where the common wavelength is adjustable. 
     The excitation-reducing beam in the  FIG. 6  device, which as in the  FIG. 1  device is formed from the reflected output  3 ′ from beam splitter  2 , is passed through the same chromatic dispersion means and optical fiber as in the  FIG. 1  device. After collimation by lens  56 , the beam is directed onto a hollow pyramid  57 , which separates the beam into four quarter-pie-shaped segments  58 ,  59 ,  60  and  61 , separated by a non-illuminated x-shaped region, as shown in  FIG. 7 . Pyramid  57  is made of four triangles of a transparent material such as glass, with parallel faces, and edges beveled to fill the space at edges where the triangles touch. The cross-sectional plane of  FIG. 6  bisects two opposite triangles of pyramid  57   
     Each of the pie-shaped segments of the beam is directed to a separate segment of segmented mirror  54 , shown schematically in  FIG. 8 , with an exaggerated spacing between the segments. The central hole  64  of the segmented mirror is formed by making a small bevel at the right angle vertices of each of the four mirror segments. Beam segment  58  for example would be directed on mirror segment  62  and beam segment  60  would be directed on mirror segment  63 . The segmented mirror  54  reflects the beam segments onto lens  55 , which forms a real image of the segmented mirror and its central hole on real image plane  33 . Therefore an image of the central spot of the excitation beam is formed at plane  33 , surrounded by images of the four beam segments.  FIG. 6  is meant to show the relationship between the elements but not their relative sizes. The elements should be arranged so that the images in the plane of focus of the specimen of the “intensity-weighted central point” of the beam segments at plane  33  are about 2.3 optical units from the center of the excitation spot. The “intensity-weighted central point,” roughly speaking, would be the center of mass of a flat piece of pie that had a mass per unit area at each point proportional to the light intensity of the pie shaped segment. 
     As shown in  FIG. 9 , which is a magnified detail of  FIG. 6  to better show pyramid  57 , the pyramid can rotate about a point  70 , which is the interior vertex point. Pyramid  57  can rotate through small angles on two perpendicular axes of rotation, both of them perpendicular to the optical axis, and one of them perpendicular to the plane of  FIGS. 6 and 9 . It will be appreciated from  FIG. 9  that if the pyramid rotates a small angle on the axis perpendicular to the figure, that the angle of incidence of the collimated beam from lens  56  will increase on one pyramid face and decrease on other. Because of Snell&#39;s law, the angle of refraction will also change, so that after the rotation, the distance the beam travels through the glass of face  71  will be different from the distance the opposite beam travels through face  72 , which will create a phase difference between the beams, that increases with the tilt of the pyramid. 
     For any chosen phase difference, for example the 180° phase difference required for the spots of excitation-reducing light on opposite sides of the central spot of excitation, and for any chosen wavelength, whatever relative delay between the opposite faces that is produced by tilting of the pyramid to an arbitrary angle, there is a compensatory translation of the segments of segmented mirror, that when combined with the delay caused by mirror rotation (which at a very small angle approximates translation), will restore the net delay to create a 180° phase difference. It is possible to imagine a graph where the relative delay caused by the pyramid tilt is plotted on one axis and the compensatory movement of the mirror segments to make the net phase difference 180° is plotted on the other axis. The resulting graph will be a sloping line. A change of wavelength of the beam will change the speed of light through the glass, and consequently change the slope such a plotted line. Because they have different slopes, the lines corresponding to two chosen wavelengths will intersect at some point. The coordinates of this intersection point indicates a pair of relative delays caused by pyramid tilt and by mirror segment adjustment, such that the net delay is 180° for both of these wavelengths. If the pyramid is tilted at the angle needed to produce such a delay and the mirror segments are set at the corresponding positions, not only will the phase delay be exactly 180° for both of the chosen wavelengths (which are chosen to bracket the wavelength band desired for the quenching light), but additionally it will be nearly 180° for all the wavelengths in between. This is the well-known principle of achromatic correction, and it is particularly simple to implement in this case. The result is that not only is the arrangement of  FIG. 6  simpler than the arrangement of  FIG. 1 , but it also is substantially achromatic for phase delay. 
     It should be understood that to calibrate this system, three separate degrees of freedom are required to produce the required phase delays, but rotation of the pyramid only provides two such degrees of freedom. One possible solution is to cement three of the four triangular faces together, and allow the fourth face to pivot about the common vertex point  70 . It is possible to perform such a calibration of the delays during fabrication of the pyramid, and then in use, the two available degrees of freedom could be used for fine adjustment. 
     In addition to allowing the common wavelength to be changed without need to recalibrate the phase adjustments, an achromatic scheme also insures that when the pulses are spectrally broadened as a side effect of pulse compression and stretching, that each spectral component in the broadened pulse has the same phase delay, and consequence they each form a zero intensity point at the central point. Even in a fixed wavelength instrument, this may contribute to a slight improvement in resolution. 
     Although this disclosure has described single wavelength STED microscopy with just one illustrative STED microscope design, it will be obvious to one skilled in the art that provided a suitable fluorophore or photosensitizer is used, that the present invention could be applied to nearly any of the many other designs for STED and STED-like microscopes or microlithography exposure tools described in the cited references, including but not limited to devices with multiple simultaneously imaged spots, and a line-shaped rather than spot-shaped area subject to resolution enhancement, and to devices still to be proposed. The breadth of the present invention should not be limited by the particular illustrative examples but rather by the following claims.