DE29521543U1 - Near-field optical microscope - Google Patents

Description

Near-field optical microscope

The invention relates to a near-field optical microscope (NSOM = Near Field Scanning Optical Microscope) and a probe head therefor.

An NSOM according to the preamble of claim 1 or 13 is disclosed in Toledo-Crow et al, Applied Physics Letters, Volume 60, pages 2957 to 2959 (1992) and is shown schematically in Figure 1. In an NSOM, a waveguide tip, usually the tip of an optical fiber, is used to measure the optical coupling between the tip and a sample. In the known NSOM according to Toledo-Crow et al, the amplitude change of the tip vibration is used as the control value for controlling the distance between the sample and the tip. This has the advantage of offering a faster measurement compared to frequency measurement, since changes in the damping, i.e. in the magnitude of the vibration, can in principle be measured instantaneously. Toledo-Crow et al use optical means to measure the vibration amplitude, which includes a laser source, a Wollaston prism, a beam splitter, an objective lens, a polarization analyzer, and a light detector.

Despite its sensitivity and speed, this known design is quite complicated. It is expensive, it takes up a considerable amount of space and also requires alignment and adjustment of the optical components, which can be disturbed by mechanical shocks. Because of the alignment and adjustment required, it is difficult to operate such a device completely automatically, as is necessary, for example, in a satellite, in a hazardous environment such as a nuclear reactor, in a vacuum chamber or in a cryostat, and is also desired in a commercial turnkey system. The spatial requirements of the device can also be problematic in some applications, for example it would not be easy to accommodate a device of this type in the limited sample space of a magnetic cryostat. A further consequence of the space and alignment requirements is that the sample must be scanned in order to be able to build up an image, since with this setup the scanning of the

tip is impractical because then the optical control system would have to follow the scanning of the tip. For some samples this is unimportant, but it can be a problem for large or heavy samples, such as mechanical working parts, and for samples that cannot be held still, such as living organisms or plants.

Another NSOM which also uses optical means to measure the vibration of the tip is described by Betzig et al in Applied Physics Letters, Volume 60, pages 2484 to 2486 (1992). Embodiments are disclosed which use not only the amplitude but also optionally the phase of the vibration as a control value. The device of Betzig et al has similar advantages and disadvantages to that of Toiedo-Crow et al.

Therefore, it is an object of the present invention to provide a near-field optical microscope wherein vibration changes of the tip are measured by means that do not require optical or mechanical alignment and adjustment, wherein changes in the vibration state of the tip can be measured quickly, accurately and with high sensitivity, wherein the probe head is compact and lightweight, and wherein the probe head is robust.

The above objects are achieved according to the invention by the characterizing features of claim 1 and 13 respectively.

For NSOMs that wobble or vibrate the tip during operation, the tip or other cooperating member forms an oscillator. The Q factor of this oscillator is an important parameter for the performance of the NSOM. With a coupled oscillator arrangement according to the invention, the Q factor can be increased dramatically and, as described in more detail below, can be easily manufactured to pre-calculated optimal design values over a wide range.

The signal is purely electrical and can therefore be very easily recorded and processed. The instrument is very sensitive. The signal reacts very quickly to changes in the vibration frequency, amplitude and phase of the tip. Because there is no

optical components for the scanning device and no optical access to the probe head is required, the probe head is very compact, whereby the probe head is used here as a collective term for the components of the NSOM that form a structural unit with the tip.

The probe head of the inventive NSOM is a small, robust component that can withstand aggressive environments such as extremes of temperature. Furthermore, the nature of the construction means that the probe head is a physically separate part of the NSOM, connected only by electrical leads to the remaining parts of the NSOM. The electrical leads as well as the optical fiber are flexible and lossless. Consequently, the probe head can be installed in a cryostat, including a magnetic cryostat, in a vacuum chamber or in an isolated radioactive environment. Conventional outputs from a cryostat, from a vacuum chamber, etc. can be used to connect the probe head to the remaining parts of the NSOM.

The tip of the NSOM is formed from the end portion of an optical fiber. The tip terminates in an optically transparent aperture. The tip is preferably provided with a reflective coating around its optically transparent aperture.

In an advantageous embodiment, the first oscillator means is a bimorph, i.e. the first part of the piezoelectric material is divided into at least two connected parts with different piezoelectric properties. Typically, there are two connected longitudinal halves with dimensional ratios similar to those of a conventional bimetallic strip. The two halves are either made of different materials or of the same material but are connected to each other with different relative crystal orientations.

In a preferred embodiment of the NSOM of the invention, both the first and the second oscillator means consist of a part of piezoelectric material. Of course, the second part can also be a bimorph.

In a particularly preferred embodiment of the NSOM of the invention, the first and second oscillator means are formed by a tuning fork, the tuning fork having first and second prongs in the conventional manner. This is a particularly attractive construction from a practical and commercial point of view, since the technology associated with tuning forks, particularly quartz tuning forks used in the watch industry, is very mature. In particular, such tuning forks produce an almost exactly defined frequency and are very reliable and inexpensive.

In a further group of embodiments of the NSOM of the invention, the second oscillator means is formed by an oscillator circuit, typically an electronic circuit. The oscillator circuit is then coupled to the first oscillator means either electronically, e.g. by a wire, or optoelectronically, e.g. by an optical fiber with associated semiconductor lasers and detectors acting as the optoelectronic converters.

In a first design of the wobble means in an NSOM of the invention, the wobble means is a physically separate component called a wobble block made of piezoelectric material. Electrical leads are brought into contact with the piezoelectric wobble block, which is wobbled, i.e., set in an oscillatory motion, by applying an electrical input signal driving the wobble, such as a sinusoidal voltage from a conventional signal generator. In this design of the wobble means, it is desirable that the piezoelectric wobble block act purely as a driver element and is not sensitive to the interaction between tip and sample. Consequently, in this design of the wobble means, it is preferred that the material used for the wobble means be made of a ceramic, amorphous, polycrystalline or other similar material which has poor mechanical oscillatory properties, particularly over the frequency ranges used in operation. This ensures that oscillations of the mechanical parts of the coupled oscillator arrangement, e.g. resonance oscillations of the tuning fork, do not cause any or at least no strong oscillation of the wobble block.

In a second design of the wobble means in an NSOM of the invention, a separate wobble block can be dispensed with by using the piezoelectric oscillator material also for generating the wobble motion. In this case, the wobble means is built together with at least the first part of piezoelectric material and comprises at least one electrical contact arranged on at least the first part of piezoelectric material. In order to induce wobble movements, an electrical signal is applied analogously to the first design of the wobble means described above. This electrical input signal driving the wobble motion deforms at least the first part of piezoelectric material to induce the desired oscillatory movement of the tip. In embodiments of this type with first and second oscillator means made of piezoelectric material, in particular tuning fork and double bimorph embodiments, it is preferred that the wobble means comprise an electrical contact arranged on the first part of piezoelectric material, i.e. on the first prong of the tuning fork, and a further electrical contact arranged on the second part of piezoelectric material, i.e. on the second prong of the tuning fork. The symmetrical effect of the wobble driver is thereby ensured.

In the first design, the wobble means does not have to be part of the probe head. In other words, since the wobble means only has to produce relative motion between the tip and the sample, the wobble means can be used to wobble the sample instead of the tip. The second design of the wobble means with integral wobble and oscillator means, on the other hand, is not suitable for sample wobbling.

The invention will now be described in more detail using the figures as an example. The figures show:

Figure 1 is a schematic block diagram of a Toledo-Crow et al.

known NSOM with optical scanning means;

Figure 2 is a schematic block diagram of an NSOM according to the invention;

Figure 3 is a schematic detailed view of the probe head of the NSOM according to

a preferred embodiment of the invention with two views, one rotated 90 degrees with respect to the other;

Figure 4 is a schematic and highly exaggerated figure showing a

Charge distribution in a piezoelectric tuning fork, where the dashed lines represent the planes of zero strain: Figure 4A at rest; Figure 4B oscillating in a resonance mode; and Figure 4C off resonance;

Figure 5 highly schematic diagram of another embodiment with

double bimorph;

Figure 6 highly schematic diagram of another embodiment of a

first oscillator means comprising a piece of piezoelectric material and a second oscillator means comprising an electrical oscillator circuit;

Figure 7 schematic diagrams of various inventive

Tuning fork arrangements:

Figure 7A A tuning fork suitable for use with a separate sweep block, with a pair of contacts for the scanning signal arranged on one side surface of the tuning fork (visible in the view on the right) and a ground contact arranged on the rear surface of the tuning fork (visible in the view on the left);

Figure 7B A tuning fork suitable for use with a separate sweep block, with a pair of threaded contacts for the scanning signal (but no ground contact), the white areas indicating exposed quartz and the two grey shades indicating the two threaded contacts;

Figure 7C A tuning fork with an integral wobbler. The tuning fork has a pair of contacts for the scanning signal located on one side surface of the tuning fork (seen on the right in the view) and a ground contact located on the rear surface of the tuning fork as in Figure 7A (seen on the left in the view). However, there are additionally two wobbler contacts for receiving the AC electrical input signal driving the wobbler;

Figure 8 different properties of a NSOM with a voice! according to

Figure 7B and an optical fiber attached thereto according to Figure 3 each in the form of a measurement of the scanning signal as a function of the sweep frequency:

Figure 8A Scanning signal as a function of sweep frequency in the absence of tip-sample interaction;

Figure 8B Scanning signal as a function of sweep frequency for different levels of tip-sample friction force ('friction'), showing the effect that the interaction between tip and sample has on the tip oscillation. The points are measured and the lines are fitted to calculations from a driven harmonic oscillator model;

Figure 8C Sample signal as a function of sweep frequency showing the effect of sticking the tip to the tuning fork on the oscillator properties of the tuning fork! The right vertex shows the tuning fork response before sticking the tip to the tuning fork, and the left vertex shows the corresponding response after sticking the tip, i.e. in a composite probe head.

Figure 3 shows the probe head according to a first embodiment of the invention. A crystal quartz tuning fork 5 is connected to an optical fiber 20. The

Tuning fork 5 is attached to a cylindrically shaped wobble block 50 made of ceramic piezoelectric material, as shown by the broken lines. The tuning fork 5 is a coupled oscillator having a first oscillator formed by the tine 30 and a second oscillator formed by the tine 31.

The end section of the optical fiber 20 forms the tip 10. The tip 10 protrudes beyond the end of the tuning fork by a length 'p', which is preferably selected to be between 0.5 and 1.0 millimeters. The tip 10 is preferably tapered and has a narrowed end. The taper can be formed, for example, by a conventional optical fiber drawing process. The tip preferably has a reflective aluminum coating 11. The aluminum can be applied, for example, by a conventional vapor deposition process. The end of the tip section forms an optically transparent opening 12 through which photons can pass.

The optical fiber 20 is bonded with a bonding agent 13, e.g. adhesive or varnish, along the length of one side of the tuning fork.

Contact construction

The tuning fork 5 has a pair of scanning contacts 40, 41. In Figure 3 and also in Figure 7A, both scanning contacts 40, 41 are arranged on the same side surface of the tuning fork, which can be seen in the right-hand view, while a ground or earth contact 43 is arranged on the rear surface of the tuning fork, which can be seen in the left-hand view.

Figure 7B shows an alternative arrangement for the contacts, which is also suitable for a probe head with a separate wobble block 50 as shown in Figure 3. In this arrangement, the sensing contacts 40, 41 run on both sides of the tuning fork through a somewhat complex geometric arrangement. The unshaded, white areas represent exposed quartz, and the two different hatched areas represent the two threaded contacts 40 and 41, respectively. No ground contact 43 is provided. This arrangement from the electronics industry

known contact arrangement is particularly effective in converting deformation-induced piezoelectric charge into a scanning signal.

Another arrangement is shown in Figure 7C. Two contact pairs, namely a first sensing contact pair 40, 41 and a second additional contact pair 44, 45 are provided. As described below, the contact pair 44, 45 provides the sweep means, potential differences being applied between the contacts 44 and 45 to cause the deformation of the tuning fork tines.

The sensing contacts are located at the base of the tines on the inner and outer sides respectively, as these are the planes of maximum stretch during vibration and thus provide the largest sensing signal. In other words, the tuning fork performs a dual function of wobbling and sensing. The obvious advantage of this design is that a separate wobble block as shown in Figure 3 can be dispensed with, which in turn results in an even more compact and simple NSOM. In this design, it is desirable to keep the wobbling drive circuit and the sensing circuit as physically far apart as possible to avoid capacitive coupling between the two. In fact, the coupling between the circuits should ideally be entirely piezoelectric.

Tuning fork construction

The design of the tuning fork 5 itself is also crucial for the performance of an NSOM according to the invention and will now be explained in more detail. Both the dimensions and the material properties are important. An ideal tuning fork for NSOM applications would have a high resonance frequency to allow fast scanning of samples, a low tine stiffness i.e. spring constant and would produce a large piezoelectric response to a small deformation.

The parameters important for the design of a tuning fork for such applications are the dimensions of the tines 30, 3I ₁ , namely their length T, width 'w' and thickness 't', and the properties of the material used, namely the

Elastic modulus �E', density ρ, piezoelectric stress tensor dy and the crystal axes along which the tuning fork is formed.

In the application of quartz, the X and Y directions shown in the figures represent the 'a' and 'c' axes of the hexagonal lattice basis of quartz (using standard notation).

A convenient starting point for determining tuning fork dimensions is to select a desired value for the frequency 'f. Typically, a frequency of 10 to 100 kHz is a good compromise between resolution and scanning speed. However, this selection is extremely application dependent.

For example, in some research applications it will be acceptable to wait many hours for a single image, whereas a fast measurement within minutes would be desirable for a commercial production environment. Furthermore, the overall size of a typical image can vary greatly, so the number of image elements can also vary accordingly over orders of magnitude.

A desired stiffness 'k^' at resonance should be chosen for the tines. A value of 'k^' on the order of 1 Newton per meter or less is generally desirable. This value is chosen because the typical effective spring constant for an atom bound in a solid is on the order of 10 Newton per meter and, at least in imaging applications, one wants to avoid the tip breaking atomic bonds in the solid. Such a value of 'kgjf' ^{also ensures} that the tip remains atomically sharp when scanning over a sample surface and that it does not pick up 'dust', ie atoms of the sample.

Using the theory of elastic deformation of materials, the following formulas were derived:

2π&iacgr;&Lgr;

we

where 'k' is the static stiffness and is related to the aforementioned 'k^' by the equation ke _ff =k/Q, where 'Q' is the Q-factor or the sharpness of the resonance, which is defined by the ratio f/Äf of the resonance frequency T and its width 'Äf.

From the above formula it is clear that the length T is fixed by the material parameters E and ρ and the desired grid properties f and k.

The electric field induced by the deformation of the tuning fork is also an important parameter and is now discussed. The following expression was derived for the local electric field 'F, which has the value '5F at every point (y,z) in the plane (Y,Z) defined by the contact electrodes 40, 41:

where 'du' is the longitudinal piezoelectric constant of the crystal, 'ε ₀ ' is the dielectric constant in vacuum and 's _s ' is the static relative dielectric constant of the piezoelectric material. It was assumed that the piezoelectric crystal has a trigonal or hexagonal symmetry. However, corresponding expressions can be derived in an analogous manner for other crystal classes.

Now the scanning signal voltage can be approximated by the potential falling across the width ‘w’ caused by the electric field averaged over the total area of each contact electrode. It is given by:

·

·

From the above formula it is clear that the width V of the tuning fork can be freely chosen within a certain range. In the embodiments described so far, 'w' was chosen so that a typical wobble-induced displacement of 0.1 nanometer results in an induced voltage of the order of microvolts. With a voltage of this magnitude, the sampling signal can be measured perfectly with conventional instruments. The magnitude of the voltage also means that typical sources of interference do not cause any problems.

The design formulas for other coupled oscillator arrangements, especially those for which only a single piece of piezoelectric material can be used, can be easily derived in an analogous manner.

Two examples of tuning fork probe heads are described in more detail below.

example 1

A quartz plate is chosen with a thickness of 100 micrometers and an orientation of (XYt)-5°, this designation corresponding to the IEEE standard for piezoelectricity ANSI/IEEE Standard 176 (1987). Such an orientation results in high signal sensitivity. A tuning fork shape is cut out of this quartz plate with the dimensions: w=100 micrometers, t=300 micrometers and 1=2.75 millimeters. The material parameters are E=6xlO ¹⁰ N/m ² , p=2650 kg/m ³ and d ₁₁ =2.31xl0 ¹² Coulomb per Newton.

The resulting fork has a frequency f=30000 Hz and Iw=1 N/m. After manufacturing the probe head, i.e. after bonding the optical fiber to the tuning fork, a sharpness of Q=2000 was obtained. The manufacturing process according to the invention can optionally be used to reduce Q from its initial value given after manufacture to a lower, predetermined value.

It is the optical fiber 20 that primarily causes the reduction of the Q factor of the probe head from that of the simple tuning fork. Consequently, for AFM applications where no optical fiber is required, the Q factor of the probe head approaches that of the free tuning fork and can have a value of Q=10,000 or more in air.

The sensitivity of the tuning fork can be specified by the voltage induced at the contacts by an incremental deformation at the end of the tine of the tuning fork. The example tuning fork has a value of 6V/5t=93 millivolts per nanometer.

A typical amount of deformation at the tine ends caused by the wobble means is 5 picometers. This corresponds to an induced piezoelectric voltage at the contacts 40, 41 of approximately 10 microvolts. It has been assumed here that the voltage is used as the sensing signal. However, other sensing signals, such as impedance, can be used.

For a better understanding of the sampling signal, reference is made to Figure 4. Figure 4 shows, in a highly exaggerated manner, the piezoelectrically induced charge distribution over the tines of a tuning fork at different deformations. Figure 4A shows the distribution for zero bending, i.e. at rest. Figure 4B shows the situation for equal and opposite bending of the two tines of the tuning fork, as occurs when the tuning fork is driven at its primary resonance frequency, i.e. wobbled. Figure 4C shows the situation when the two tines bend together.

For example, when the contact arrangement of Figure 7A is used, the sensing signal is almost zero for equal and opposite bending (Figure 4B) because the contributions from the two contacts cancel. The sensing signal then increases in response to non-mirrored deformation of the tines 30, 31. Such non-mirrored deformation is produced by the interaction between sample and tip, since the interaction predominantly attenuates the tine to which the tip is attached. With such a contact arrangement, the signal is

hence a measure of the differential bending of the two tines, with the maximum signal occurring at exactly the same bending of the tines, as shown in Figure 4C. It is this mechanism that is responsible for the coupling of the mechanical oscillations of the two tines, which in turn is the reason for the large Q factors. The scanning signal is typically used to keep the interaction between tip and sample at a constant level during scanning by feeding it to suitable control software or hardware.

Example 2

Quartz tuning forks are mass produced for the electronics industry. Quartz tuning forks with a main oscillation frequency f=2 ¹⁵ =32768 Hertz are readily available. Such a tuning fork was used to build an NSOM according to the invention. The tuning fork has the dimensions: l=3.9 millimeters, t=600 micrometers and w=400 micrometers. In the finished NSOM, the probe head had a quality factor of Q=3000. This results in a convenient value for the stiffness of =7 N/m.

Probe head reaction

Figure 8 shows the signal as a function of sweep frequency for an embodiment using a probe head with a tuning fork as shown in Figure 7B. The tuning fork is attached to a ceramic sweep block 50 as described in connection with Figure 3. An optical fiber 20 is attached to the tuning fork in the manner shown in Figure 3, for example.

The measured signal is the amplitude of the alternating voltage sensed or picked up by the contacts 40, 41, this signal being induced by the bending of the tuning fork tines as shown in Figure 4.

In Figure 8A, the scanning signal is measured while the sweep frequency is changed. The measurement was performed in air with the tip away from any sample, i.e. in the absence of tip-sample interaction. In the figure, the dots represent measured data, while the lines are from a

driven harmonic oscillator model were calculated and fitted. The resonance frequency was measured as 33683 Hertz and the Q factor as 1410. The frequency resolution is 1.94 Hertz. A maximum signal of approximately 7.5 microvolts was measured.

Investigations have shown that it is advantageous to choose the free length 'p' of the tip projection so that the resonant frequency of this free end is higher than that of the tuning fork. This ensures that tip-sample interaction forces are effectively transmitted to the relevant tine of the tuning fork. This requirement is met by making the length 'p' sufficiently short. For example, a 'p' of less than 1 millimeter is preferred for an optical fiber of 100 micrometers diameter.

Figure 8B shows the effect of tip-sample interaction on the probe head response. The highest, sharpest peak corresponds to the situation shown in Figure 8A, when there is no sample-tip interaction. The response is then measured with progressively increasing interaction. In the figure from right to left, the tip-sample friction force or 'friction' was measured at 0, 0.6, 1.2 and 3 nanonewtons, which progressively dampen the oscillations. The dots represent measured data, while the lines are calculated and fitted from a driven harmonic oscillator model. The frequency resolution is 1.94 hertz.

It can be seen that the signal measured at the maximum frequency for no interaction, which in Figure 8B is approximately 31,700 Hz, is sensitive to the interaction between tip and sample. In addition, the frequency shift of the response peak caused by the friction force is accompanied by a decrease in the Q factor, so that the signal measured at the maximum frequency for no interaction remains measurable over a wide range of the tip-sample friction force. Consequently, for an NSOM according to the invention, a simple sweep drive signal with a constant frequency is in many cases completely sufficient, i.e. in most cases it is not necessary to take into account the dependence of the resonance frequency on the strength of the interaction.

In typical operation, the signal is used to maintain a constant distance between the tip and sample. Control distances in the range of 0 to 200 nanometers are common.

Figure 8C shows the effect that bonding the optical fiber to the tuning fork has on the oscillator properties. The right vertex shows the tuning fork response before bonding the fiber to the tuning fork. The measurement system was not accurate enough to measure the Q factor or the true maximum signal value because the resonance is too sharp. However, the Q factor was found to be at least 5000, although this is probably a significant underestimate. The left vertex shows a similar response after bonding the fiber to the same tuning fork, i.e. in a composite probe head. The Q factor has dropped to a value of 3560. The frequency resolution is 1.94 hertz.

Control of the distance between tip and sample

The usual way to regulate the distance between sample and tip in NSOM is to use the signal amplitude in a feedback loop with a piezoelectric shifter controlling the tip-sample distance. Such an operating mode is also suitable for an NSOM according to the invention.

However, another way of regulating the distance between sample and tip has been developed, which will now be described in more detail in connection with a contact arrangement according to Figure 7A. The tuning fork, or more precisely the probe head, since the resonant frequency of the combined tuning fork and tip piece is different from that of the free or bare tuning fork, is driven to resonance by the application of an alternating voltage of the appropriate frequency across the contacts 44, 45.

The response of a tuning fork can be considered as the electrical equivalent of a capacitance connected in series with a capacitance, an inductance and a resistance. In addition, this impedance reaches a maximum at the tuning fork’s resonance frequency. Since the interaction between tip and

• ·.· · · ··· · * '•••••J&idigr;*"II

sample affects the deformation of the tuning fork and induces detuning as shown for example in Figure 8B, the measurement of the impedance of the tuning fork is sensitive to the interaction between the tip and the sample. Thus it is possible to use the impedance to regulate the distance between the sample and the tip. The distance between the sample and the tip can for example be kept at a value that corresponds to a certain impedance value.

Operation with integrated wobbler

An NSOM with a contact arrangement according to Figure 7C can be swept by applying to the contacts 44, 45 an AC input signal driving the wobbler, originating from an external driver circuit (not shown), with a frequency close to the resonant frequency of the tuning fork.

However, another way of generating the wobble motion for an NSOM has been developed using a contact arrangement as shown in Figure 7C, which will now be described in more detail. The weak piezoelectric signal detected by the sensing electrodes 40, 41 is fed to a voltage amplifier 46 with adjustable gain and/or phase. The output of the amplifier is fed back to the contacts 44, 45, causing the fork to vibrate at its natural frequency. The tuning force is thus used as its own oscillator. The gain and phase of the amplifier are adjusted so that the signal feeds itself in a closed loop.

This way of generating the wobble motion is particularly advantageous when the resonance sharpness 'Q' exceeds a few thousand, as it then becomes increasingly difficult to match an external drive frequency to that of the tuning fork. Temperature or pressure changes, as well as changes in the interaction between tip and sample, for example as shown in Figure 8B, can then cause a shift in the resonance frequency of a disturbing magnitude. In addition, the response times for such a system can be made less than the oscillation period 1/f to enable rapid scanning.

Further embodiments

Figures 5 and 6 show two further embodiments of NSOM according to the invention. For analogous parts, the same reference numerals as in Figures 3 and 7 have been retained. Only some of the main parts are shown, since these figures are mainly intended to illustrate the relevant principles.

Figure 5 shows an embodiment which includes an optical fiber 20 according to Figure 3, but a double bimorph coupled oscillator arrangement. Each arm is a bimorph bar comprising a first strip 30a or 31a of some kind of piezoelectric material glued together with a second strip 30b or 31b of piezoelectric material. The individual arms are connected by electrically conductive wires (dashed lines) which serve to couple the mechanical oscillations of the respective arms. Sensing contacts are not shown in order not to clutter the figure. In this embodiment, the wobble block 50 is placed below the sample S and the part 60 which holds the bimorphs together is a simple mounting block. The relative motion between tip and sample is thus generated in this embodiment by the vibration of the sample (and not the tip). Such an arrangement is particularly suitable for an operating mode in which, in the absence of interaction between tip and sample, the arms 30, 31 remain still, whereas the interaction between tip and sample causes the arms to overshoot, i.e., causes them to oscillate. To achieve this, the sweep frequency should be adapted to the natural oscillation frequency of the coupled oscillator arrangement.

Figure 6 shows an embodiment which includes an optical fiber 20 bonded to the piezoelectric arm 30 of Figure 3. The arm 30 is made from a single piece of piezoelectric material. This is the first oscillator 30. Instead of using a second piece of piezoelectric material to form the second oscillator, an electrical oscillator circuit 31 is provided. The two oscillators 30, 31 are connected by electrically conductive wires (dashed lines) which serve to couple the mechanical oscillations of the arm 30 and the electrical oscillations in the circuit 31. The sensing contacts 40, are shown. A comparable embodiment could be made so that

the first oscillator is a bimorph and not a single piece of piezoelectric material.

The figures further show that different combinations of individual characteristics can be freely chosen to a large extent, and that the combinations described up to this point are by no means the only possible ones.

19

Claims (18)

Protection claims 1. Near-field optical microscope, which comprises: a tip extending in a first direction (y) and designed as an end section of an optical fiber for interacting with a sample to be examined (S) in front of it; a displacement means for setting and regulating the distance in the first direction (y) between the tip and the sample; a scanning means for scanning the tip relative to the sample in a plane (x,z) lying substantially perpendicular to the first direction (y); a woofer means for generating oscillations of the tip relative to the sample which deflect substantially in the plane (x,z); and a sensing means for obtaining a signal responsive to changes in said oscillations and thereby sensitive to the interaction with the sample, characterized in that the sensing means comprises a coupled oscillator arrangement (5) consisting of at least a first and a second oscillator (30, 31) coupled thereto, the first oscillator (30) having an elongate first part of piezoelectric material extending in the first direction (y) which, with its one side facing the sample - distal end is held by a holding means (50; 60) and the other end, closest to the sample, is connected to the tip (10) in such a way that the tip (10) projects beyond the other end of the first part of piezoelectric material in the direction of the sample (S) to be measured, and that force components exerted on the tip, originating from its interaction with the sample, cause a change in the oscillations of the tip relative to the sample, and thus a bending or differential bending of the first part of piezoelectric material, which can be measured by a scanning contact arrangement (40, 41; 40, 41, 43) arranged at least on the first part of piezoelectric material and emitting a corresponding electrical signal. 2. Near-field optical microscope according to claim 1, characterized in that the tip (10) is designed as a tapered end section of the optical fiber (20) which ends in an optically transparent opening (12). 3. Near-field optical microscope according to claim 2, characterized in that the tip (10) has a reflective coating around its optically transparent opening (12). 4. Near-field optical microscope according to claim 1, 2 or 3, characterized in that the tip (10) projects beyond the other end of the first part of piezoelectric material in the direction of the sample (S) to be measured by a length of preferably less than 1 millimeter, in particular by a length between 0.5 and 1.0 millimeter. 5. Near-field optical microscope according to one of the preceding claims, characterized in that the optical fiber is attached along a side of the first part of piezoelectric material extending in the first direction (y). 6. Near-field optical microscope according to one of the preceding claims, characterized in that the second oscillator (31) comprises an elongated second part of piezoelectric material extending in the first direction (y) which is held with its one end remote from the sample by the or a further holding means (50; 60). 7. Near-field optical microscope according to one of the preceding claims, characterized in that the first and/or the second oscillator (30) are each designed as bimorph strips, which are each divided into at least two interconnected parts (30a, 30b; 31a, 31b) with different piezoelectric properties. 8. Near-field optical microscope according to claim 6 or 7, characterized in that the first and second oscillators (30, 31) are formed by a tuning fork (5) with a first prong (30) forming the first oscillator or a second prong (31) forming the second oscillator, as well as a bridging part forming the holding means (60). 9. Near field optical microscope according to one of claims 1 to 5, characterized in that the second oscillator (31) is an oscillator circuit which is electronically or optoelectronically coupled to the first oscillator (30). 10. Near field optical microscope according to one of the preceding claims, characterized in that the wobble means (50) is designed as a wobble block (50) made of piezoelectric material, which is driven by an electrical input signal applied thereto, causing the oscillations of the tip relative to the sample. 11. Near field optical microscope according to claim 10, characterized in that the wobble block (50) consists of a ceramic, amorphous, polycrystalline or similar piezoelectric material. 12. Near field optical microscope according to one of claims 1 to 9, characterized in that the wobbling means (50) consists of a drive contact arrangement (44, 45; 44 ₇ 45) arranged on at least the first part of piezoelectric material (30), to which an electrical drive signal can be applied which sets the first part of piezoelectric material connected to the tip (10) in an oscillatory movement deflecting substantially in the plane (x, z) in order to generate the oscillations of the tip. 13. Probe head for a near-field optical microscope with an optical fiber, one end of which forms a near-field optical probe tip, characterized by a tuning fork (5) with a first and second prong (30, 31), the optical fiber (20) being attached to the first prong (30) of the tuning fork. 14. Probe head according to claim 13, characterized in that the optical fiber (20) is glued with a connecting means (13) along the first prong of the tuning fork. 15. Probe head according to claim 14, characterized in that the connecting means is adhesive or varnish •» · 16. Probe head according to claim 13, 14 or 15, characterized in that the near-field optical probe tip (20) extends beyond the end of the first prong (30) by
a length of preferably less than 1 millimetre, in particular a
Length between 0.5 and 1.0 millimeters. 17. Probe head according to one of claims 13 to 16, characterized by a block (50) made of piezoelectric material to cause the tuning fork (5) to resonate. 18. Near-field optical microscope with a probe head according to one of claims 13 to 17.