Abstract:
A near-field optical microscope comprising an optical probe which is arranged in opposition to a sample and irradiates the sample with an evanescent wave, a plurality of photodetectors which detect respective components of light scattered in different directions, among components of the scattered light generated by scattering of the evanescent wave by the sample, and a processor which analyzes characteristics of the sample on the basis of respective detection signals of the plurality of photodetectors.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a microscope which utilizes near-field optics. 
     2. Related Background Art 
     The probe of a near-field optical microscope is mainly constructed of a probe wherein the distal end of an optical fiber is sharpened into a tip, which is covered with a metal film by vapor deposition or the like and then formed with a micro-opening. The optical fiber probe is caused to guide illuminating light, thereby to generate an evanescent wave at the tip micro-opening. When the evanescent wave is brought near to a sample, it is converted into scattered light by the interaction thereof with the sample. The microscopic configuration or structure of the sample, or the material distribution of a microscopic region can be observed by detecting the intensity profile of the scattered light. 
     The simplest scheme of the probe is a scheme wherein the optical fiber probe for guiding the illuminating light functions also as an optical fiber probe for detecting the scattered light. Herein, the intensity of the scattered light can be detected in such a way that the scattered light picked up at the micro-opening is guided by the optical fiber probe and then split from the entered illuminating light by a semitransparent mirror. 
     It is known, however, that since the near-field optical microscope in the prior art illuminates the sample and detects the scattered light through the micro-opening, it exhibits low conversion and coupling efficiencies, resulting in feeble light detection, which makes the signal-to-noise ratio (hereinbelow, expressed as “SNR”) of the detected light very inferior. 
     In view of the drawback, there has been proposed a system wherein a sample is placed on the totally reflective surface of a total reflection prism, and wherein scattered light based on the interaction between the sample and an evanescent wave on the totally reflective surface is picked up by the optical fiber probe. It has also been studied to heighten the intensity of the scattered light by raising the intensity of illuminating light for the total reflection prism. 
     Disadvantageously, however, this system is applicable only to samples each of which can be arranged on the total reflection prism. That is, it has a very narrow range of applications to configurations and materials. 
     Moreover, the prior-art system cannot distinguish the configuration or structure of the sample from the difference of the materials. In a case, for example, where a minute ruggedness coexists with a minute material difference on a flat surface, the same outputs are produced, and the ruggedness and the material difference cannot be identified. 
     In this manner, the prior-art examples have had the problems that the SNR is inferior on account of the feeble light detection, that measuring conditions and subjects to-be-measured are limited, and that subjects to-be-measured are further limited because the configuration or structure of the sample cannot be discriminated from the difference of the materials. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to solve the problems of the prior art as stated above, and to provide a near-field optical microscope with which samples to-be-analyzed are not limited, which is less expensive and which is capable of detection of good SNR. 
     A near-field optical microscope for accomplishing the above object of the present invention comprises an optical probe which is arranged in opposition to a sample, and which irradiates the sample with an evanescent wave; a plurality of photodetectors which detect respective components of light scattered in different directions, among components of the scattered light generated by scattering of the evanescent wave by said sample; and a processor which analyzes characteristics of said sample on the basis of respective detection signals of said plurality of photodetectors. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram showing the construction of the first embodiment of the present invention; 
     FIG. 2 is a schematic arrangement diagram showing the construction of the second embodiment of the present invention; 
     FIG. 3 is a schematic arrangement diagram showing the construction of the third embodiment of the present invention; and 
     FIG. 4 is a schematic plan showing a state where an optical detection system in the third embodiment of the present invention is projected on an XY-plane. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, the embodiments of the present invention will be described. 
     Embodiment 1: 
     The optical detection system of a microscope in Embodiment 1 of the present invention will be described with reference to FIG.  1 . 
     FIG. 1 is a schematic block diagram in the case where two photodetectors are arranged. 
     Referring to the figure, numeral  1  designates an optical head. The optical detection system includes an optical fiber probe  2 , light detecting sensor lenses  3  and  5 , photodetectors or photosensors  4  and  6 , a laser  7 , and a collimator lens  8 . Numeral  9  indicates illuminating light, and numeral  10  a sample. The microscope includes a processor  11 , a memory  12 , and an image monitor  13 . 
     A light beam emergent from the laser  7  is coupled with the optical fiber probe  2  by the collimator lens  8 , and is guided as the illuminating light  9  inside the probe  2 . Part of the illuminating light  9  is converted into an evanescent wave at a micro-opening which is formed at the tip of the optical fiber probe  2 . 
     In this embodiment, the optical fiber probe  2  is set to be substantially perpendicular to the plane of the sample  10  (an XY-plane). 
     The evanescent wave interacts with the sample  10  in the near-field region of the microscope, thereby to be partly converted into scattered light. The components of the scattered light pass through the sensor lenses  3  and  5  to enter the respectively corresponding photodetectors  4  and  6 , in which the light components are photoelectrically converted. The two photodetectors  4  and  6  are set at inclination angles θ 1  and θ 2  relative to a Z-axis within a ZX-plane, and their distances are set at l 1  and l 2 , respectively. Here in this embodiment, the extent of the scattered light which arrives at the photodetectors  4  and  6  is determined by the pupil positions of the sensor lenses  3  and  5 . Therefore, the distances l 1  and l 2  of the respective photodetectors  4  and  6  extend from the tip of the fiber probe  2  to the pupil positions of the correspondent sensor lenses  3  and  5  for picking up the scattered light. The difference between the outputs of the photodetectors  4  and  6  is taken and is subjected to A/D (analog-to-digital) conversion by the processor  11 . 
     The processor  11  stores the difference signals of the photodetectors  4  and  6  in the memory  12  while mapping them in accordance with the outputs of an XYZ-encoder not shown. If necessary, the processor  11  can read out the stored data from the memory  12  and display them on the image monitor  13 . 
     In this embodiment, the two photodetectors  4  and  6  are located at symmetric positions at which both the inclination angles and the distances are equal as θ 1 =θ 2  and l 1 =l 2 . 
     Owing to such a construction, it is realized to detect the symmetry of the configuration or structure or the material of the sample  10  at a good SNR. 
     Concretely, it is assumed that the configuration or structure or the material of the sample  10  has a Z-axial symmetry substantially within the ZX-plane. In the case where the probe  2  lies on the symmetry axis (Z-axis), the in-phase components of the element of the configuration or structure or the material are removed, and only the different-phase and antiphase components of the element are differentially detected as the difference signal of the outputs of the two photodetectors  4  and  6  located at the symmetric positions. That is, the superfluous DC (direct current) components in the detection signals of the photodetectors are eliminated, and the AC (alternating current) components can be detected with a doubled amplitude. 
     In actuality, it may be considered that, with reference to the symmetry of the sample  10 , the magnitude of deviation therefrom can be detected. More specifically, while the sample  10  is scanned in an X-direction, the magnitude of deviation is outputted larger at a position remoter from the symmetry axis. In case of a smaller output, it can be judged that the fiber probe  2  lies nearer to the symmetry axis. 
     When the optical detection system is constructed in this manner, it is permitted to obtain information which has hitherto been undetectable. 
     Further, if the material of the sample  10  contains asymmetric components (such as a minute material difference) with the configuration or structure thereof being symmetric, only the asymmetric components are directly produced as a difference output. To the contrary, if the configuration or structure of the sample  10  is asymmetric with the material thereof being symmetric, only the asymmetric components are outputted. It is therefore permitted by the present invention to analyze those characteristics of feeble light which have hitherto been buried in the unnecessary components and been indiscernible. 
     As described above, according to the present invention, the signals which have heretofore been difficult to be detected can be derived at the good SNR by the spatial difference detection. Accordingly, the precise information of the sample can be obtained, and the range of applications of a near-field optical microscope apparatus can be widened. 
     Embodiment 2: 
     FIG. 2 shows the construction of the optical detection system of a microscope in Embodiment 2 of the present invention. 
     This embodiment is so constructed that two optical detection subsystems are arranged adjacently on an identical side, not at the opposite side positions as in Embodiment 1. The constituents of the optical detection system having the same numerals and symbols as in Embodiment 1 shall be omitted from description. 
     Referring to FIG. 2, the first optical detection subsystem includes a photodetector or photosensor  23  and a sensor lens  21  arranged at a distance l 1  and an inclination angle θ 1 , while the second optical detection subsystem includes a photodetector  24  and a sensor lens  22  arranged at a distance  12  and an inclination angle θ 2 . In this embodiment, l 1 =l 2  holds and (θ 1 −θ 2 ) is very small. For this reason, the difference between the outputs of the first and second optical detection subsystems becomes an output produced by subjecting substantially the same scattered light to a spatial differentiation and an angular differentiation. Accordingly, the gradients of the spatial and angular distributions of a sample  10  can be directly found, and the minute spatial and angular distributions of the configuration or structure of the sample  10  or the material thereof can be detected at good SNRs. 
     In consequence, the resolving power of the microscope is heightened for finding the discrepancy between the gradient of the scattered light based on the minute distribution of the configuration or structure of the sample  10  and that of the scattered light based on inhomogeneity in the material in the microscopic region of the sample  10 . Therefore, the analytical capability of the microscope for distinguishing the former from the latter is enhanced by the present invention. 
     As compared with Embodiment 1, this embodiment is suited to analyze the sample  10  having the minute spatial and angular distributions, rather than the sample  10  thought to be symmetric. 
     Embodiment 3: 
     FIG. 3 is a schematic arrangement diagram of the construction of the optical detection system of a microscope in Embodiment 3 of the present invention. 
     Unlike the foregoing embodiments, this embodiment has four photodetectors or photosensors arranged therein. The constituents of the optical detection system having the same numerals and symbols as in the foregoing embodiments shall be omitted from description. 
     Referring to FIG. 3, numerals  31 ,  32 ,  33  and  34  designate the photodetectors. Although the photodetectors may well be respectively furnished with sensor lenses as in the foregoing embodiments, the present invention can be performed even with the sole photodetectors each of which has an aperture equivalent to the pupil of the sensor lens. When such an optical detection system is employed, the number of components decreases, and also the number of portions to-be-adjusted decreases, so that reduction in cost is attained. The optical detection system, however, is less immune against stray light and a ghost image. 
     The photodetectors  31  and  32  are arranged at symmetric opposite-side positions within a ZX-plane under the conditions of distances l 1 =l 2  and inclination angles θ 1 =θ 2  as in Embodiment 1. The photodetectors  33  and  34  are arranged at opposite side positions symmetric relative to the ZX-plane, within a plane which is inclined an angle a relative to a YZ-plane. The distances of the photodetectors  33  and  34  are equal to those of the photodetectors  31  and  32  as expressed by l 3 =l 4 =l 1 , but the inclination angles thereof are different from those of the photodetectors  31  and  32  as expressed by θ 3 =θ 4 . It is the same as in the foregoing embodiments that the optical detection subsystems are arrayed in centripetal fashion. 
     FIG. 4 is a schematic plan showing the construction of the optical detection system in the third embodiment as is projected on an XY-plane. 
     Referring to the figure, the outputs of the photodetectors  31  and  32  arrayed symmetrically in an X-direction have their difference taken by a differential amplifier  41  included in a processor  11  not shown, whereby information  44  on balance in the X-direction is outputted. On the other hand, the outputs of the photodetectors  33  and  34  arrayed in a Y-direction have their difference similarly taken by a differential amplifier  42 , whereby an output  45  is obtained. Herein, although the photodetectors  33  and  34  are arrayed symmetrically relative to the ZX-plane, their positions have the same X-directional components relative to the YZ-plane. Therefore, the output of the differential amplifier  42  is not the information on balance in the mere Y-direction, but it becomes the information  45  on oblique balance. Further, the differential output  46  between the outputs  44  and  45  (produced by a differential amplifier  43 ) becomes information on overall directional balance. Since the information  46  has the oblique component, the processing is equivalent to the shift of a balance point. It can accordingly be said that this embodiment offers the optimum arrayal in a case where a sample  10  is presupposed to have a directionality in the X-direction. 
     Thus, this embodiment indicates that the present invention can be optimized even for a directional sample. As understood from the fact, in a case where a sample has directionalities in a plurality of directions, the optical detection system can assume an arrangement of heightened detecting capability by increasing the number of detection subsystems in accordance with the number of the directions. It is accordingly obvious that the range of subjects to-be-analyzed is enlarged by the present invention. 
     Needless to say, even in a case where the directionalities of the sample lie in the elements of the configuration or structure and the material of this sample, the elements can be discerned by applying the technique explained before. 
     In the above description of the embodiments, the constructions and effects have been revealed as to the examples each having a small number of optical detection subsystems. In substance, however, the present invention shall not be restricted to the mere examples. 
     In addition, although signal processing is somewhat complicated, a detection component based on a differentiation can be enhanced by slightly shifting distances l or inclination angles θ from symmetric positions. It is the characterizing feature of the present invention that the optical detection system of a microscope can be optimized to the supposed spatial and angular gradients of the characteristics of a sample desired to be analyzed. 
     Besides, it is to be understood that the present invention is not restricted to the mere arrangement of optical detection subsystems at symmetric and opposite side positions, but that even the directionality or orientation of the characteristics of a sample or the direction of the ruggedness of a sample can be detected on the basis of information of shifting phase obtained from an optical detection subsystem arranged at the midpoint between the symmetric and opposite side positions.