Scanning near field optical microscope based on the use of polarized light

An optical fiber probe which has a minute opening on the top of a sharpened tip is allowed to get close to a sample, and the probe is moved by a piezo actuator along x- and y-axis directions so that a minute spot beam emanating from the minute opening can scan over the sample. For circular polarization modulation to be incorporated in the process, a beam, before it is incident on the optical fiber probe, is given an optical delay changing at a frequency of p (Hz) by means of a piezo-optical modulator. A minute spot beam emanating from the minute opening passes through the sample to be received after the passage through an analyzer by a light receiving element. The output from the light receiving element is fed to a lock-in amplifier, p- and 2p-components are separated through lock-in rectification, and they are rendered images by a controller.

BACKGROUND OF THE INVENTION 
This invention relates to a measuring apparatus which measures the 
polarizing activity and the distribution thereof of a test substance with 
a high resolution, by detecting a beam which has interacted with a tiny 
area of substance at the tip of probe, and by utilizing the polarization 
characteristics the beam presents. 
It becomes important to observe, for various test samples, the distribution 
of their optical activities (circular dichroism and optical rotation) at 
very tiny areas, and to obtain quantitative evaluations of those optical 
activities. Such optical activities include natural optical activities, 
electro-optical activities, magneto-optical activities, and piezo-optical 
activities, and with the recent technical advance in the field of memories 
having a gigantic capacity such as a hard disk, opto-magnetic disk, etc., 
a demand for the equipment allowing a precise observation and measurement 
of magneto-optical effects becomes rapidly intense. 
For example, to observe the distribution of magneto-optic effects as one 
aspect of such optical activities with a high precision requires to 
observe magnetic sectors and barriers, and the well-known method used for 
this purpose includes polarized light microscopy, Lorenz transmission 
electronmicroscopy, spin-polarized scanning electronmicroscopy, and 
magnetic force microscopy. A recent article reports an observation of the 
magnetic barriers of a vertically magnetized membrane by the use of a 
scanning near field optical microscope (APPLIED OPTICS, Vol. 31, No. 22, 
1992, p. 4563, E. Betzig et al.). 
Here a scanning near field optical microscope will be briefly sketched. A 
widely available method consists of sharpening an optic fiber or a beam 
transmitting body, and preparing a minute opening at its tip having a 
diameter equal to or less than the wavelength of beam. By the same method 
with which a conventional scanning atomic force microscope or a scanning 
tunnel microscope adjusts the distance between a cantilever and a sample, 
the minute opening is placed so close to the surface of a sample that the 
distance in between is equal to or less than the wavelength of beam. By 
introducing, while maintaining above state, a beam into the optic fiber 
with such minute opening, radiating a tiny area of a sample with the beam 
emanating from the minute opening, and scanning the beam over the sample 
in a two-dimensional plane, a microscope can achieve a high resolution 
microscopy in accordance with the size of minute opening. In the example 
mentioned earlier where a scanning near field optical microscope was used 
for the observation of magnetic barriers, a linearly polarized beam 
emanating from a minute opening is allowed to radiate a sample, and the 
beam transmitting through the sample is received by an analyzer 
(cross-Nichol method). 
On the other hand, the method by which to quantitatively determine the 
circular dichroism or optical rotation of a sample, for example, on the 
basis of magneto-optical effects (methods dependent on other optical 
activities works on the essentially same principle) is described in detail 
in "Light and magnetism" published by Asakura Publishing Co. (written by 
Sato, K.). The optical rotation due to magnetism can be determined by 
perpendicularly intersecting polarizers (cross-Nichol method), 
Faraday-cell method, and a rotational analyzer. The use of a quarter-wave 
plate will allow the measurement of the circular dichroism of sample. 
Further, modulation of a circularly polarized beam (circular polarization 
modulation) will enable the measurement of both the magneto-optical 
rotation and magneto-optical circular dichroism with a high sensitivity. 
Here, circular polarization modulation will be briefly sketched with 
reference to FIG. 2. A linearly polarized beam having passed through a 
linear polarizer 101 is given, by a piezo-optical modulator 102 working on 
birefringence, an optical delay which changes at a frequency of p (Hz). 
Then, the same beam, after having been reflected from or passed through a 
sample 103 (the beam is reflected from a sample in FIG. 2), is allowed to 
pass through an analyzer 104 to reach a light receiving element 105 for 
registration. From p (Hz) component and 2p (Hz) component of the beam 
having passed through the analyzer 104, it is possible to determine the 
circular dichroism and optical rotation the beam has undergone, 
respectively. 
The principle underlying circular polarization modulation will be described 
by equations. For brevity, the direction along which a beam transmits is 
supposed to coincide with z-axis. Let's assume that in FIG. 2 the linear 
polarizer 101 has an angle of 45.degree. with respect to x-axis. The 
electric field E.sub.1 of the beam having passed through the linear 
polarizer 101 can be expressed by: 
EQU E.sub.1.sup..varies. (i+j) (1) 
given that i and j are the unit vectors of x- and y-axes respectively. 
Given that there is a delay of .delta. between x- and y-components of the 
electric field E.sub.2 of the beam which has passed through the 
piezo-optical modulator 102, 
EQU E.sub.2.sup..varies. {i+exp(i.delta.)j} (2). 
Assumed that the unit vectors of right- and left-circularly polarized beams 
are expressed by following equations respectively: 
EQU r=(i+ij)/2.sup.1/2 
EQU I=(i-ij)/2.sup.1/2, 
then, E2 can be expressed by the following equation: 
EQU E2.sup..varies. {(1-i.exp(i.delta.))r+(1+i.exp(i.delta.))i}(3) 
Suppose that the complexly expressed amplitude reflections of right- and 
left-circulatory polarized beams are expressed by r+exp(i.theta.+) and 
r-exp(i.theta.-) respectively, then the electric field E.sub.3 of 
reflected beam can be expressed by: 
EQU E.sub.3.sup..varies. 
((1-i.exp(i.delta.))r+exp(i.theta.+)r+(1+i.exp(i.delta.))r.exp(i.theta.-)i 
}(4). 
The intensity I of the beam emanating from the analyzer having an angle of 
.phi. with respect to x-axis is expressed by: 
EQU I.sup..varies. {R+(.DELTA.R/2) sin .delta.+R sin (.DELTA..theta.+2.phi.) 
cos .delta.} (5) 
where 
R=(r+2+r-2)/2 
.DELTA.R=r+2-r-2 
.DELTA..theta.=.theta.+-.theta.-=-2.theta..sub.k 
.DELTA.R/R=4.eta..sub.k 
and where .theta..sub.k represents a Kerr's rotation angle and .eta..sub.k 
a Kerr's ellipticity. Assumed that .phi.=0, and .DELTA..theta. is 
sufficiently small, .delta..about.sin 2.pi.pt. Then, the equation can be 
resolved by the use of Bessel function into: 
EQU I.about.I(0)+I(p) sin 2.pi.pt+I(2p) cos 4.pi.pt+ (6). 
In this equation, I(o), I(p), and I(2p) represent factors respectively 
containing 0th-order, 1st-order and 2nd order Bessel functions, and 
EQU I(p).sup..varies. .eta..sub.k, I(2p).sup..varies. .theta..sub.k(7). 
Therefore, p(Hz) component gives the Kerr's ellipticity and 2p(Hz) gives 
the Kerr's rotation angle. For details, see the above-described "Light and 
magnetism." 
The above-described various methods employed for the observation of minute 
magnetic sectors have a number of problems as will be described later. For 
example, polarized light microscopy, operating in the same manner as 
conventional optical microscopy, has its resolution restricted by the 
diffraction limit of a beam used, and only achieves a resolution that 
allows distinguishing the width of about half the wavelength of beam used. 
Further, as it depends on the cross-Nichols method for detecting the 
optical activities of a sample, its detection sensitivity is low. Lorenz 
transmission electronmicroscopy has a resolution sufficiently high to 
distinguish about 10 nm intervals, but it is only applied to a thinly 
sectioned sample. Spin-polarized scanning electronmicroscopy has a problem 
in that it requires a large cost for installment. Magnetic force 
microscopy has a considerably high resolution that allows discrimination 
of several tens nm intervals, but it can be scarcely applied for the 
quantitative determination of the magnitude of a magnetic field or 
magnetization. Scanning near field optical microscope has its resolution 
determined principally by the diameter of opening of the probe, and has a 
considerably high resolution. The conventional minute spot scanning 
microscopy, however, usually depends, for the detection of optical 
activities of a sample, on the cross Nichols method, and presents 
following problems. It allows only a low sensitivity. Notwithstanding that 
the closer the minute spot beam emanating from a minute opening is to a 
linearly polarized beam, the higher the detection sensitivity, the minute 
spot beam emanating from a minute opening is usually elliptically 
polarized. This may form another cause for a lowered sensitivity. 
Among the apparatuses for quantifying various magneto-optical effects, 
there are some that allow the very sensitive quantification of a rotation 
angle through modulation, for example, by the use of a rotating analyzer. 
This method, however, can not be applied to a tiny area exceeding the 
typical level handled by a conventional optical microscope. 
As illustrated above by referring to the microscopic observation of 
magneto-optical effects as an example, the conventional methods whereby 
the distribution and quantification of optical activities of a sample have 
been obtained have more or less defects to be corrected, although some are 
advantageous in sensitivity, resolution and tolerance of sample handling, 
and others are advantageous in cost. What is mentioned above applies to 
the measurement not only of magneto-optical effects but also of optical 
activities at large. In view of this the object of the present invention 
is to provide an apparatus with which it is possible to observe/measure 
the optical activities of a sample with a high resolution and sensitivity, 
at a low cost, quantitatively, and without imposing any restrictions on 
the handling of sample. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a scanning near field 
optical microscope based on polarized light which requires only a low cost 
for production because of its being based on the constitution of scanning 
atomic force microscopy, and allows observation of a sample without 
imposing any restrictions on the preparation thereof. 
It is another object of the present invention to provide a scanning near 
field optical microscope where the size of a beam to radiate a sample or 
emanating from a sample is determined by the size of tip according to the 
principle underlying scanning near field optical microscope, and thus it 
allows the observation of a sample with a very high resolution without 
being affected by a diffraction limit. 
It is another object of the present invention to provide a scanning near 
field optical microscope which becomes possible to achieve a highly 
sensitive circularly polarized modulation by giving a periodically 
changing optical delay to a beam. 
It is a further object of the present invention to provide a scanning near 
field optical microscope where it is possible to reduce the changes in 
polarization state due to external disturbing sources when the probe 
consists of a light transmitting body made of a material having a smaller 
photo-elasticity coefficient, and it can minimize the adverse effects due 
to stresses developed as a result of bending when the same material is 
applied for the preparation of a probe with a hook-like bent.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
To solve the above-described problem, this invention comprises: 
a light source; 
a probe having a sharpened tip; 
a means which maintains the tip so close to the surface of a sample that 
the interval therebetween is kept within an action distance which allows 
an inter-atomic interaction or interactions by way of other elements to be 
present between the tip and the surface; 
a means which obtains a polarized beam carrying the data of optical 
activities of a tiny area of the sample or of a medium, by allowing a beam 
emanating from the source to interact with the sample surface at the tip; 
a light receiving means which receives the polarized beam; 
a modulating means which is inserted on a light path between the source and 
the light receiving means, and gives an optical delay changing at a 
regular cycle; and 
a rectifying means which removes, from the output delivered by the light 
receiving means, components having frequencies as certain integer times 
high as that of the modulation frequency given by the modulating means. 
Further, the probe is allowed to have a light transmitting body. 
The means which maintains the tip so close to the surface of a sample that 
the interval therebetween is kept within an action distance which allows 
an inter-atomic interaction or interactions by way of other elements 
further comprises: 
a moving means which alters the distance between the tip and the surface; 
a distance determining means which determines the distance between the tip 
and the surface; and 
a control means which maintains the distance between the tip and the 
surface constant based on a signal delivered by the distance determining 
means. 
The distance determining means still further comprises: 
a vibrating means which vibrates the tip and the surface relative to each 
other in a horizontal or vertical direction; and 
a displacement detecting means which detects the displacement of the tip. 
The modulating means which is inserted on a light path between the source 
and the light receiving means, and gives an optical delay changing at a 
regular cycle acts as a means to generate a periodically changing stress 
in the light transmitting body. 
The present invention further comprises: 
a light source; 
a probe having a sharpened tip and a light transmitting body; 
a means which maintains the tip so close to the surface of a sample that 
the interval therebetween is kept within an action distance which allows 
an inter-atomic interaction or interactions by way of other elements to be 
present between the tip and the surface; 
a means which obtains a polarized beam carrying the data of optical 
activities of a tiny area of the sample or of a medium, by allowing a beam 
emanating from the source to interact with the sample surface at the tip; 
and 
a light receiving means which receives the polarized beam, wherein: 
the material constituting the light transmitting body has a 
photo-elasticity coefficient of 10.times.10.sup.-6 [mm.sup.2 
.multidot.N-1] or less. 
The material constituting the light transmitting body includes quartz glass 
containing lead oxide. 
Preferred embodiments of this invention will be described below with 
reference to attached figures. 
(1) EXAMPLE 1 
FIG. 1 illustrates the constitution of Example 1 of this invention. The 
basic constitution is the same with that of a conventional scanning near 
field optical microscope. The figure shows an apparatus which incorporates 
an optic fiber probe which is produced after an optic fiber acting as a 
light transmitting body has a minute opening prepared at its tip, wherein 
a minute spot beam emanating from the minute opening radiates a sample 
(illumination mode) and the beam after passing through the sample is 
registered (transmission type). Description will be given with reference 
to this example. 
Firstly, a conventional minute spot scanning microscopy will be described 
with regard to its constitution and operation. A light source 11 to 
produce a gas laser, solid compound-based laser, or semiconductor-based 
laser generates a light flux, which passes through a polarization 
adjusting element 17 consisting of a wavelength plate, and a piezo-optical 
modulator 10, to be incident, through the intervention of a fiber coupler 
9, on the input end of optic fiber probe 1. The optic fiber probe 1 is 
usually made of a single mode fiber, and its other end has its sharpened 
tip whose circumference is coated by a film composed of a metal like gold 
or aluminum, such that the tip has a minute opening whose diameter is 
equal to or less than the wavelength of the beam. The beam incident on the 
input end of optical fiber probe 1 emanates from the minute opening as a 
minute spot beam. The optic fiber may be made of a multi-mode fiber or a 
single hollow fiber, instead of a single mode fiber. Further, the optical 
fiber probe 1 has a part close to the tip bent like a letter L, and is 
attached on the surface of a piezo-electric element such as a bimorph or a 
quartz vibrator. It is possible to operate the apparatus in an AFM mode, 
or a dynamic mode often used in conjunction with scanning atomic force 
microscopy (AFM), by vibrating the optical fiber probe 1 vertically with 
respect to a sample by means of the piezo-electric element 18. 
A sample 2 is placed on a piezo actuator 15 which can move along x-, y- and 
z-axis directions, and a controller 16 controls the movement of piezo 
actuator 15. The controller, while maintaining constant the distance 
between the sample 2 and the tip of optical fiber probe 1, scans the beam 
over the sample 2 in x- and y-axis directions using piezo actuator 15. In 
this example, a method based on a device generally called an optical lever 
is used for determining the distance between the sample 2 and the tip of 
optical fiber probe 1. This method consists of converging a beam emanating 
from a laser source 12 onto the surface of a mirror placed close to the 
tip of optical fiber probe 1, of receiving the reflected beam with a 
bisected light receiving element 13, and of determining the difference 
between the intensities of beams received by respective bisected segments, 
thereby to monitor the vibration state (frequency, amplitude and phase of 
the vibration) of optical fiber probe 1. For example, when the optical 
fiber probe 1 comes close to the sample 2, its vibration state undergoes a 
change in the presence of forces resulting from inter-atomic interactions. 
Therefore, by adjusting the movement of piezo actuator 15 along z-axis 
direction in such a way as to allow the optical fiber probe 1 to make a 
vibration with a constant amplitude, it is possible to maintain constant 
the distance between the surface of sample 2 and the tip of optical fiber 
probe. Thus, while maintaining constant the distance between the sample 2 
and the tip of optical fiber probe 1, it is possible, by scanning the beam 
over the sample by moving the piezo actuator 15 in x- and y-axis 
directions, and by monitoring how much the piezo actuator 15 moves along 
z-axis direction, to obtain the image of surface texture of sample 2. 
As the tip of optical fiber probe 1 is positioned close to the surface of 
sample 2, the minute spot beam emanating from the minute opening transmits 
through the sample 2, is converged by a converging lens 3, has its path 
bent by a mirror 4, and passes through an analyzer 5 and filter 6 to be 
received by a light receiving element 7. The filter 6 placed in front of 
the light receiving element 7 is to cut off the laser beam 12 which acts 
as one arm of the optical lever. 
As the light source 11 usually consists of a laser source based on a gas or 
solid molecule, it often happens that a linearly polarized beam impinges 
on the optical fiber probe 1. But, the optical fiber probe 1 generally 
contains elements which may resolve the polarization state or retards the 
phase, of an incident beam, and thus the minute spot beam emanating from 
the minute opening often suffers a degraded polarization or becomes an 
elliptically polarized beam, notwithstanding that the incident light is a 
linearly polarized beam. When such polarized beam is radiated upon the 
sample 2, and its polarization state is monitored by a cross-Nichols 
method, the overall sensitivity will become low. To avoid such 
inconvenience, it is necessary to insert a wavelength plate or 
compensation plate, that is, an agent to cause an appropriate retardation, 
on the incident path of optical fiber probe 1, thereby to adjust the 
polarization state of incident light. Through this procedure it is 
possible to obtain a minute spot beam with a practically linear 
polarization. 
In spite of above fact, this invention adopts circular polarization 
modulation dependent on a cross-Nichols method which is principally very 
sensitive. The method adopted in this example whereby an optical delay is 
given to a minute spot beam emanating from the minute opening of optical 
fiber probe 1 in accordance with a modulation frequency of p (Hz) depends 
on the use of a piezo-optical modulator (PEM) 10 working on birefringence 
which incorporates an optically active crystal such as quartz or the like. 
A driver 14 to drive PEM not only activates the piezo-optical modulator 10 
but delivers a reference signal with respect to which a lock-in amplifier 
8 performs a lock-in rectification. A light flux emanating from the light 
source 11 passes through the piezo-optical modulator 10 to be given an 
optical delay there, and is incident through the intervention of fiber 
coupler 9 on the input end of optical fiber probe 1. 
Detection of optical activities using a modulated circularly polarized beam 
is so sensitive that, as long as any optical delay is given at all to a 
minute spot beam by means of an external modulating means, it is possible 
to detect optical activities. What should be noted here is that as long as 
the piezo-optical modulator 10 incorporates an optically active crystal, 
the crystal axis has to be taken into account. Namely, modulation 
efficiency will be higher if an incident polarized beam is adjusted 
according to the angle the beam forms with the axis of crystal. Thus, the 
polarized state of an incident beam is adjusted by means of a polarization 
adjusting element 17 placed on the input side of the piezo-optical 
modulator 10. Needless to say, the polarization adjusting element 17 may 
be so constituted as to allow an incident beam to pass through a half 
wavelength plate capable of rotating the beam, or to allow an incident 
beam pass through a quarter wavelength plate to convert it into a 
circularly polarized beam, and then to permit a specific component 
thereof, say, a linearly polarized beam to exit therefrom. This example 
uses a piezo-optical modulator 10 incorporating an optically active 
crystal, but any other modulator can be used with the same profit as long 
as it can give a periodic optical delay to an incident beam. 
Placement of the mirror 4 in front of the analyzer 5 is undesirable because 
the mirror may add an extra polarization characteristic, and ideally the 
light path should not be bent. However, for a beam converged by the 
converging lens 4 to be guided to the light receiving element 7, it is 
necessary with a conventional transmission type scanning near field 
optical microscope to bend the light path by means of the mirror 4 for the 
convenience of designing. The mirror incorporates a dichroic mirror 
instead of a conventional vapor-deposited aluminum mirror, thereby to 
lessen the difference in reflection of p- and s-polarized beams. Through 
this procedure it becomes possible to ignore the polarization 
characteristic given by the mirror 4. 
By virtue of an apparatus having above constitution, a minute spot beam 
emanating from the minute opening interacts with the surface of sample 2, 
is converted, through that interaction, into a transmissive beam, passes 
through the sample 2 being given, during passage, optical activities 
including circular dichroism and optical rotation, and passes through the 
analyzer 5 to be incident on the light receiving element 7 so that it may 
be registered there. This signal is rectified by the lock-in amplifier 8 
which uses the reference signal (has a frequency of p) delivered by the 
PEM driver 14 for rectification, and the rectified output is fed to the 
controller 16. When 2p component is submitted to lock-in rectification, it 
gives an optical rotation, and when p component is submitted to lock-in 
rectification, it gives a circular dichroism. Thus, when these signals are 
submitted to the controller 16 to be converted into images in synchrony 
with the scanning movement of piezo actuator 15 as in a conventional 
scanning near field optical microscope, they will visualize the 
distribution of optical activities of sample. Incidentally, if only p 
component is required, the analyzer 5 placed in front of the light 
receiving element 7 may be omitted. 
Further, when not only the distribution of optical activities but also the 
absolute quantities of those activities are desired, the ratio of p 
component to the direct current component will give the ellipticity and 
the ratio of 2p component to the direct current component will give the 
optical rotation. For the latter purpose it is not necessary to move the 
piezo actuator 15 so that the beam can scan over the sample along x- and 
y-axis directions, but to adjust the piezo actuator 15 such that a desired 
spot of sample 2 is placed close to the optical fiber probe 1 for 
measurement. By this process it is possible to quantitatively determine 
the optical activities of a very tiny area of sample as with a 
conventional method based on the modulation of a circularly polarized 
beam. 
Assumed that the optical fiber probe 1 gives an optical delay of .pi./2 as 
a result of mechanical stresses as does a quarter wavelength plate, the 
electric field E.sub.2 of a beam as described by equation (2) in the 
"Description of the Related Art" comes to be expressed by the following 
equation because of an optical delay given by the optical fiber probe 1. 
EQU Ex.sup..varies. {i+i.exp(i.delta.)j} (2') 
As a result, the intensity I of light emanating from the analyzer becomes: 
EQU I.sup..varies. {R+(.DELTA.R/2) cos .delta.+R.multidot.sin 
(.DELTA..theta.+2.phi.) sin .delta.} (5') 
This equation, when resolved by a Bessel function, gives: 
EQU I.about.I(0)+I(p) sin 2.pi.pt+I(2p) cos 4.pi.pt+ (6) 
EQU I(p).sup..varies. .theta..sub.k, I(2p).sup..varies. .eta..sub.k(7') 
What is worthy of notice here is that what p and 2p components represent in 
this equation is opposite to what the same expressed in equation (7) in 
"Description of the Prior Art" represent. 
The present invention has an above-described constitution and operates in 
an above-described manner, and combines scanning near field optical 
microscope with circular polarization modulation thereby to make it 
possible not only to observe the distribution of optical activities of 
sample 2 with a high sensitivity and resolution, but also to 
quantitatively determine the optical rotation and ellipticity of a desired 
tiny area of the sample. Further, as this method is based on scanning near 
field optical microscope, it allows the production of a smaller apparatus 
with a lower cost than is possible with other similar observation means 
dependent on conventional techniques. The sample 2 may be in the 
atmosphere, in a liquid, or in a vacuum for measurement, and does not need 
to be thinly sectioned. Thus, this method does not impose any special 
restrictions on the preparation of sample. 
(2) EXAMPLE 2 
Next, Example 2 according to this invention will be described by means of 
attached figures. FIG. 3 shows the constitution of Example 2 of this 
invention. In Example 1 light is passed through a sample for measurement. 
In this example, measurement is performed using light reflected from a 
sample. As the basic constitution and operation are the same with those of 
Example 1, parts achieving the same functions are represented by the same 
symbols, and their explanation will be omitted. 
The fates a beam undergoes, after having emanated from a light source 11, 
till it exits from the minute opening of optical fiber probe 1 as a minute 
spot beam are practically the same with those of the beam in Example 1. 
However, in this example, a piezo-optical modulator 10 is not placed on a 
light path between the light source 11 and the optical fiber probe 1. As 
described above, the optical fiber probe 1 often has a property of 
retarding a beam. This is especially true when the optical fiber probe 1 
is bent like a letter L so as to be operable in a dynamic AFM mode because 
the bent part causes a retardation in a beam. To avoid such inconvenience, 
a polarization adjusting element 17 is placed on the input side of optical 
fiber probe 1 and the polarization state of an incident beam is modified 
by that polarization adjusting element 17 so as to adjust the polarization 
characteristics of minute spot beam emanating from the minute opening. The 
polarization adjusting element 17 usually consists of a wavelength plate 
like a half-wavelength plate or a quarter-wavelength plate, but the use of 
a compensatory plate will allow a more precise adjustment. 
A minute spot beam reflected from the sample 2 after having interacted with 
the latter is converged by a converging lens 3. The converging lens 2 may 
be positioned at any place on the light path as long as it efficiently 
converge the minute spot beam reflected from the sample 2, or it, instead 
of being made of a lens, may be made of a light-converging mirror like a 
parabolic mirror, as long as it has a light converging activity. The beam, 
after being converged by the converging lens 3, is given an optical delay 
with a frequency of p (Hz) while it passes through a piezo-optical 
modulator 10, and then passes through an analyzer 5 and filter 6 to be 
received by a light receiving element 7. As far as only the determination 
of circular dichroism of a beam from p component is required, the use of 
analyzer 5 may be omitted. As the remaining constitutions and operations 
are the same with those of Example 1, description of them will be omitted. 
Also with this example one can visualize the distribution of optical 
activities of sample 2 and quantitatively determine those optical 
activities with a high sensitivity and resolution. 
Although in Example 1 the piezo-optical modulator 10 is placed on the input 
side of optical fiber probe 1, it is needless to say, the device in 
question may be put between the analyzer 5 and sample 2 as in this 
example. However, as the piezo-optical modulator 20 usually incorporates 
an optically active crystal, it is possible to efficiently give an optical 
delay to an incoming beam by directing the beam to the modulator 20 such 
that the polarization plane of the beam has a specific angle with respect 
to the crystal axis. If the piezo-optical modulator 10 is positioned in 
such a way as to receive light reflected from the sample 2, the 
polarization plane of the light will not take an optimum angle with 
respect to the crystal axis, and thus the modulation efficiency and 
detection sensitivity will often be worsened and lowered. 
(3) EXAMPLE 3 
Next, Example 3 according to this invention will be described with 
reference to attached figures. FIG. 4 represents the constitution of 
Example 3 of this invention. Although in Examples 1 and 2 an illumination 
mode is adopted whereby a beam emanating from a minute opening is allowed 
to illumine a sample, in this example a collection mode is adopted whereby 
a minute spot beam is detected through a minute opening. Further, a beam 
is allowed to pass through the sample 2. Parts having the same 
constitution or achieving the same functions as the corresponding parts of 
Example 1 are represented by the same symbols, and their explanation will 
be omitted. 
A beam emanating from a light source 11 is given an optical delay during 
passage through a polarization adjusting element 17 and piezo-optical 
modulator 10, and this is the same as in Example 1. The beam, after having 
been given an optical delay, is converged by a converging lens 19 into a 
convergent beam, and is incident through a side wall onto a triangular 
prism 2 carrying a sample 2 to be converged to the bottom surface. When 
the incident angle of the converged beam with respect to the bottom 
surface of prism exceeds a critical angle, that beam is totally reflected 
by that bottom surface, and the side of bottom surface facing the sample 2 
gives rise to an evanescent beam. When an optical fiber probe having a 
minute opening at its tip is allowed to approach the sample 2 by the same 
method as used in Example 1, the evanescent beam present on the surface of 
sample 2, through interaction with the optical fiber probe 1, is converted 
into a transmissive beam which enters into the minute opening, transmits 
through the optical fiber probe 1, and exits from the other end of the 
same probe. The beam emanating from the other end of probe is collimated 
by a collimator 20, and is allowed to pass through an analyzer 5 and 
filter 6 to be received by a light receiving element 7. As the 
constitution of other elements and their operation are the same with those 
of Example 1, their explanation will be omitted. With this example, it is 
also possible to obtain the distribution of optical activities of sample 2 
or to quantitatively determine the optical activities thereof with a high 
sensitivity and resolution. 
This example uses the triangular prism 21 in such a way as to totally 
reflect an incident beam to produce an evanescent beam, but the total 
reflection may be produced by means of a dark-field illumination and, 
needless to say, the method is not limited to any specific ones as long as 
an evanescent beam is produced on the surface of sample 2. It is also 
needless to say that, as in Example 1, the optical fiber probe 1 is 
allowed to illumine a tiny area of sample 2, and to receive a beam having 
undergone an interaction for measurement. In this case, the optical fiber 
probe must have, at the other end, a beam separating element like a beam 
splitter which separates a beam into an illumination component and a 
detection component. 
In Examples 1, 2 and 3 described above, the optical fiber probe 1 has a 
minute opening at its tip, and radiates or transmits a minute spot beam 
through the minute opening for radiation or for measurement. It is 
needless to say, however, the probe is not limited to any specific ones as 
long as it has a light transmitting body acting as a wave guide channel, 
being made of a comparatively transparent material to the wavelength of 
beams often used for measurement like quartz or lithium niobate, and has a 
minute opening at its tip which is equal or less in size than the 
wavelength of the beam. 
In Examples 1, 2 and 3 described above, the piezo-optical modulator 10 is 
installed, besides the optical fiber probe 1, to give an optical delay. 
However, many of the above-described light transmitting bodies can give an 
optical delay through photo-elasticity effects in the presence of an 
external force. Namely, instead of the piezo-optical modulator 10, a 
periodic force may be applied to a part of light transmitting body of the 
probe, and the resulting photo-elasticity effects may be utilized to give 
an optical delay changing in a periodic manner to a beam during the 
passage of beam through the light transmitting body. Further, by adjusting 
the intensity of an external force, it is possible to alter the magnitude 
of optical delay. As a result, it is possible to reduce the overall size 
of apparatus. 
Further, the method whereby the polarization state of a minute spot beam is 
adjusted by a polarization adjusting element 17 placed on the input side 
of a probe consisting of a light transmitting body is effective for the 
measurement combined not only with the modulation of a circularly 
polarized beam, but also with a cross-Nichols method. For example, 
although the optical fiber probe 1 may be bent in a form like a letter L 
so that the distance between the optical fiber probe 1 and sample 2 may be 
dynamically changed according to a dynamic AFM mode, the optic fiber probe 
1 will then present an optical anisotropy which causes a retardation in a 
beam passing therethrough. As a result, even if a linearly polarized beam 
is fed to the optical fiber probe 1, only an elliptically polarized beam 
will emanate from the minute opening. This inconvenience can be avoided by 
inserting a polarization adjusting element 17 in such a way as to cancel 
out the retardation, thereby allowing the minute spot beam to approximate 
a linearly polarized beam. Therefore, when this arrangement is applied to 
a cross-Nichols method, the overall detection sensitivity will become 
higher than is possible with a similar apparatus dependent on the 
cross-Nichols method which uses an elliptically polarized beam emanating 
from the minute opening without any special treatment therefor. 
In the above-described examples, the polarization adjusting element 17 is 
installed, besides the optical fiber probe 1, to change the polarization 
state of a beam before the beam is incident on the optical fiber probe 1. 
It is needless to say, however, that, when a method is employed which 
consists of directly applying a force onto part of fiber section of the 
optical fiber probe 1 and of controlling the intensity of that force, it 
is possible to alter the polarization state of a beam during its passage 
through the optical fiber by way of photo-elasticity effects resulting 
from the external force, and thus to allow the optical fiber probe 1 also 
to act as a polarization adjusting element 17. 
Incidentally, a typical glass material has a photo-elasticity coefficient 
of about 2-4 (10.sup.-6 .multidot.mm.sup.2 .multidot.N-6), and stresses 
therein cause a retardation in a beam, and hence a beam, when passing 
through such a glass material, undergoes a change in its polarized state. 
This can occur in the optical fiber probe 1 of Examples 1, 2 and 3 
described above. When the optical fiber (particularly its core section) 
acting as a light transmitting body is exposed to a vibration externally 
applied, or falls to vibration of itself having the optical fiber probe 
fixed, and stresses develop within, a beam passing therethrough has its 
polarization state altered as a result of photo-elasticity effects, the 
external vibration acts as a noise source to lower the S/N ratio, or only 
a slight shift of fixation may destroy the reproducibility of 
measurements. When the distance between the optical fiber probe 1 and 
sample 2 is controlled according to a dynamic AFM mode, the optic fiber 
probe 1 must be bent like a hook as with an AFM cantilever. To take such a 
form the fiber had to be bent while being heated, and the bent part has 
residual stresses, and when a beam passes through this part, it receives 
an optical delay as a result of photo-elasticity effects. This not only 
applies to a part bent in the form of a hook, but also to the general form 
of optical fiber probe 1 which is often not symmetrically configured with 
respect to the axis of light transmission, and results in the development 
of residual stresses. 
To meet above inconvenience, the probe is prepared whose light transmitting 
body is made of a material having a photo-elasticity coefficient of 
1.times.10.sup.-6 (mm.sup.2 .multidot.N-6) or less. By preparing such a 
probe it is possible to ignore the effects of stresses caused by external 
vibrations and fixations, and to suppress the effects of residual stresses 
developing as a result of asymmetrical configuration with respect to the 
optical axis to a negligible level. For example, crown glass FK51 or FK52 
provided by Shot Co. has a photo-elasticity coefficient of about 
0.7-1.times.10.sup.-6 (mm.sup.2 .multidot.N-6). Further, flint glass SF57 
provided by Shot Co. which is composed of quartz containing a large amount 
of lead oxide has a photo-elasticity coefficient of about 
0.02.times.10.sup.-6 (mm.sup.2 .multidot.N-6), and it can give the same 
coefficient as small as 0.005.times.10.sup.-6 (mm.sup.2 .multidot.N-6), 
depending on the ingredients contained therein. It is also possible to 
prepare an optical fiber probe from these glass materials. Thus, a light 
transmitting body made of a material whose photo-elasticity coefficient is 
1.times.10.sup.-6 (mm.sup.2 .multidot.N-6) or less is utilized to form a 
probe, and with that probe it is possible to make a measurement where 
changes in polarization characteristics due to external vibrations are 
effectively suppressed and a satisfactory S/N ratio is maintained. 
Further, even when the optical fiber probe 1 has its stem bent like a hook 
as in Examples 1, 2 and 3, it is possible to greatly reduce the changes in 
polarization state which otherwise a beam would have suffered during 
passage of the bent part. As is evident from above, preparing a probe from 
a light transmitting body made of a material having a smaller 
photo-elasticity coefficient is very useful for a measurement dependent on 
the use of a polarized beam including a cross-Nichols method as well as 
circular polarization modulation. 
Incidentally, without resorting to the preparation of a probe from a light 
transmitting body made of a material having a smaller photo-elasticity 
coefficient, it is possible to suppress the effects of residual stresses 
which may develop during the preparation of probe, by annealing the light 
transmitting body thereby to remove residual stresses therein. 
(4) EXAMPLE 4 
In Examples 1 to 3, the light transmitting body consists of an optical 
fiber which has its tip sharpened, and has a minute opening on the top of 
it. However, the method by which to illumine a tiny area of sample by a 
beam emanating from the tip of probe is not limited to the radiation 
through a minute opening, but there are a number of variants: an 
evanescent beam can be produced by plasmon deposited on the surface of a 
minute ball, or a grating whose lattice pitch is so shortened that the 
diffraction angle is lost. Further, the method whereby the tip of probe 
can detect a beam emanating from a tiny area of sample also has a number 
of variants: an evanescent beam may be produced on the surface of sample 
by total reflection or by reflection from a surface-coated plasmon. 
Besides above a method may be employed whereby a cantilever, instead of a 
light transmitting body, which is produced after a semiconductor such as 
silicone or a metal has been sharpened, is utilized as a probe. Then, for 
example, the surface of sample and the sharpened tip of probe are 
illumined by a dark-field illumination as often used in microscopy, to 
cause a multiple scattering to occur between the sample and cantilever 
tip, and the scattered beam is measured by an external optical system. 
Next, Example 4 of this invention wherein the probe does not consist of a 
light transmitting body will be described with reference to attached 
figures. FIG. 5 shows the constitution of Example 4 of this invention. A 
probe 22 may consist of a metal probe as used in a scanning tunnel 
microscope or a silicone cantilever as used in a scanning atomic force 
microscope, or other various materials, but, needless to say, it is not 
limited to any specific materials as long as the material has a high 
scattering efficiency. The sharpened tip of probe 22 is allowed to get 
close to a sample 2. The example depicted in the figure uses an optical 
lever to move the tip towards the sample, and as this tool is the same as 
those described earlier, explanation thereof will be omitted. The sample 2 
is placed on a piezo actuator 15 and moves in x-, y- and z-axis 
directions. 
A light flux emanating from a light source 11, after having passed through 
a polarization adjusting element 17 and piezo-optical modulator 10, is 
converged by a collimator 20 and illumines the sample 2 and the tip of 
probe 22. This illuminating beam is given a periodically changing optical 
delay through the piezo-optical modulator 10. The light source 11 usually 
generates a laser, and there is no element intervened in the light path 
that may disturb the polarization state of a beam passing along the light 
path like the optical fiber probe as encountered in foregoing examples, 
but the polarization adjusting element 17 is used to adjust the 
polarization direction of an illuminating beam according to the condition 
of sample 2. 
As the tip of probe 22 gets so close to the sample 22 that the distance 
therebetween is equal to or less than the wavelength of an illuminating 
beam, a multiple scattering takes place as a result of interaction between 
the surface of sample 2 and the tip. This multiply scattered beam carries 
the optical information regarding the surface condition of sample 2 
depending on the size of tip of the probe 22. A converging lens converges 
this scattered beam which is then passed through an analyzer 5 and filter 
6 to be received by a light receiving element 7. As the subsequent 
processes are the same as those of foregoing examples, explanation thereof 
will be omitted. Adjusting the angles of light source 11 and of light 
receiving element 7 with respect to the sample 2 so that a beam emanating 
from the light source 11 to illumine the sample 2 and probe 22 may no 
enter the light receiving element, will enable a selective pick-up of a 
multiply scattered beam, and achievement of a measurement with a 
satisfactory S/N ratio. As is evident from above, even if a probe does not 
consist of a light transmitting body, it is possible to constitute a 
scanning near field optical microscope based on circular polarization 
modulation, and to observe optical activities of a sample with a high 
sensitivity. 
When a beam from the light source 11 radiates from one direction onto the 
sample 2 as in FIG. 5, it is needless to say that entry of the radiating 
beam from the light source 11 into the light receiving element 7 should be 
avoided as much as possible. Further, this example is based on a 
transmission type apparatus, but, needless to say, what is mentioned above 
also applies to a reflection type apparatus. 
Examples 1 to 4 described above use an optical lever to detect the 
displacement of optical fiber probe 1 or probe 22. However, needless to 
say, the method is not limited to an optical lever or any other specific 
methods, as long as the method permits detection of the minute 
displacement of optical fiber probe 1 or probe 22. For example, when a 
dynamic AFM mode is employed, the probe may be applied on a quartz 
vibration detector, and the changes in vibration of probe be followed 
after voltages delivered from the quartz vibration detector have been 
monitored. 
In Examples 1 to 4 described above, the method by which to control the 
distance between the optical fiber probe 1 or probe 22 and the sample 2 is 
based on a dynamic AFM mode. However, needless to say, the method is not 
limited to any specific modes, as long as the distance between the optical 
fiber probe 1 or probe 22 and the sample 2 can be rendered to any small 
size at will: the distance in question may be adjusted according to a 
static AFM mode, or by light interference, or on the basis of a shearing 
force produced as a result of interaction between the two elements here 
concerned, or through the utilization of a tunnel current. If the latter 
method is used, the optical fiber probe 1 or probe 22 may not be bent like 
a letter L. 
Although, in Examples 1 to 4 described above, the distance between the 
optical fiber probe 1 or probe 22 and the sample 2 is measured and 
actively controlled on the basis of the measurement, the method is not 
limited to this. For example, the probe and sample are allowed to move 
relative to each other to cause a current to flow therebetween, thereby 
maintaining constant the distance between the probe and sample surface by 
virtue of the viscosity of fluid as in a hard disk drive where the 
magnetic head floats by a certain definite amount (e.g., about 100 nm) 
over the disk by virtue of an air bearing. Thus, needless to say, the 
method does not require necessarily measurement of the distance between 
the probe and sample surface and a continuous monitoring of the proximity 
of the two elements here concerned, but any method, as long as it allows 
the probe and sample surface to be close and constant to each other, can 
be used with the same profit. 
In Examples 1 to 4 described above, the light source 11 generates a laser 
composed of a beam of a single wavelength. However, a Xenon lamp may be 
used as the light source 11, and light therefrom may be used after it has 
passed through a spectroscope so that an appropriate component might be 
selected. In this case, measurement can be performed using beams with 
different wavelengths.