METHOD AND APPARATUS FOR SCATTERING-TYPE SCANNING NEAR-FIELD OPTICAL MICROSCOPY (S-SNOM)

Abstract

A method of scattering-type scanning near-field optical microscopy (s-SNOM) comprises placing an s-SNOM tip 11 at a near-field distance from a sample 1 and subjecting the s-SNOM tip 11 to a mechanical oscillation, which provides a primary modulation, illuminating the oscillating s-SNOM tip 11 with a sequence of illumination light pulses, wherein each of the illumination light pulses hits the s-SNOM tip 11 at a specific s-SNOM tip modulation phase i of the mechanical oscillation, collecting scattering light pulse amplitudes Si, each being created by scattering one of the illumination light pulses at the s-SNOM tip 11, using a scattering light detector device 30, collecting the s-SNOM tip modulation phase i associated to each of the collected scattering light pulse amplitudes Si, using a mechanical oscillation detector device 40, and calculating an s-SNOM near-field signal by demodulating a scattering light function S(i) of the scattering light pulse amplitudes Si in dependency on the s-SNOM tip modulation phases i, wherein each of the s-SNOM tip modulation phases pi is obtained by splitting an output signal of the mechanical oscillation detector device 40 into a first output signal portion X and a second output signal portion Y being phaseshifted relative to the first output signal portion X and calculating the s-SNOM tip modulation phase i of the primary modulation from the first and second output signal portions X, Y. Furthermore, a scanning near-field optical microscopy apparatus 100 is described.

Claims

1. A method of scattering-type scanning near-field optical microscopy (s-SNOM), comprising the steps of placing an s-SNOM tip at a near-field distance from a sample to be investigated and subjecting the s-SNOM tip to a mechanical oscillation, which provides a primary modulation, illuminating the s-SNOM tip which is oscillating with a sequence of illumination light pulses, wherein each of the illumination light pulses hits the s-SNOM tip at a specific s-SNOM tip modulation phase .sub.i of the mechanical oscillation, collecting scattering light pulse amplitudes S.sub.i, each being created by scattering one of the illumination light pulses at the s-SNOM tip, using a scattering light detector device, collecting the s-SNOM tip modulation phase .sub.i associated with each of the collected scattering light pulse amplitudes S.sub.i, using a mechanical oscillation detector device, and calculating an s-SNOM near-field signal with a demodulation device by demodulating a scattering light function S(.sub.i) of the scattering light pulse amplitudes S.sub.i in dependency on the s-SNOM tip modulation phases .sub.i, wherein each of the s-SNOM tip modulation phases .sub.i is obtained by the steps of splitting an output signal of the mechanical oscillation detector device with a phase shifter device coupled with the mechanical oscillation detector device, wherein the output signal is split into a first output signal portion and a second output signal portion being phase-shifted relative to the first output signal portion, and calculating the s-SNOM tip modulation phase .sub.i of the primary modulation from the first and second output signal portions.

2. The method according to claim 1, wherein the second output signal portion is created by passing a portion of the output signal of the mechanical oscillation detector device through an all-pass filter.

3. The method according to claim 1, wherein the second output signal portion is created by passing the output signal of the mechanical oscillation detector device through a self-calibrating phase shifter module implemented with phase-locked loops and voltage controlled oscillators.

4. The method according to claim 1, wherein the second output signal portion is created by a digital signal processing module and/or by employing field-programmable gate arrays.

5. The method according to claim 1, wherein the splitting of the output signal is executed such that the first and second output signal portions have equal maximum amplitudes.

6. The method according to claim 5, wherein the splitting of the output signal is executed such that the second output signal portion has a /2 phase-shift relative to the first output signal portion, and the s-SNOM tip modulation phase .sub.i is calculated by .sub.i=arctan (Y/X).

7. The method according to claim 1, wherein the demodulating of the scattering light function S() comprises extracting Fourier coefficients from the scattering light function S().

8. The method according to claim 1, including at least one of the features the output signal is passed through a band-pass filter before the splitting of the output signal, the sequence of illumination light pulses is used for creating a sampling clock signal for sampling the scattering light pulse amplitudes S.sub.i and the s-SNOM tip modulation phases .sub.i, and a sample image is collected by repeating the steps of placing the s-SNOM tip, illuminating the oscillating s-SNOM tip, collecting scattering light pulse amplitudes S.sub.i, collecting the s-SNOM tip modulation phases .sub.i, and calculating the s-SNOM near-field signal with a plurality of tip positions relative to the sample.

9. The method according to claim 1, further comprising introducing at least one secondary modulation to the s-SNOM method and providing at least one secondary modulation detector device, providing at least one secondary modulation phase .sub.i for each of the collected scattering light pulse amplitudes S.sub.i by phase shifting of a portion of an output signal of the at least one secondary modulation detector device, and calculating the s-SNOM near-field signal by demodulating the scattering light function S(.sub.i) of the scattering light pulse amplitudes S.sub.i in dependency on the s-SNOM tip modulation phase .sub.i and the at least one secondary modulation phases .sub.i.

10. A scanning near-field optical microscopy (s-SNOM) apparatus, being configured for scattering-type scanning near-field optical microscopy, comprising a scanning near-field optical microscope including an s-SNOM tip being configured for a placement at a near-field distance from a sample to be investigated, while being subjected to a mechanical oscillation, which provides a primary modulation, an illumination device being arranged for illuminating the oscillating s-SNOM tip with a sequence of illumination light pulses, wherein each of the illumination light pulses hits the s-SNOM tip at a specific s-SNOM tip modulation phase .sub.i of the mechanical oscillation, a scattering light detector device being arranged for collecting scattering light pulse amplitudes S.sub.i, each of which being created by scattering one of the illumination light pulses at the s-SNOM tip, a mechanical oscillation detector device being arranged for collecting the s-SNOM tip modulation phase .sub.i associated with each of the collected scattering light pulse amplitudes S.sub.i, and a demodulation device being arranged for calculating an s-SNOM near-field signal by demodulating a scattering light function S(.sub.i) of the scattering light pulse amplitudes S.sub.i in dependency on the s-SNOM tip modulation phases .sub.i, wherein the mechanical oscillation detector device is coupled with a phase shifter device, the phase shifter device is configured for splitting an output signal of the mechanical oscillation detector device into a first output signal portion and a second output signal portion being phase-shifted relative to the first output signal portion, and the demodulation device is configured for calculating the s-SNOM tip modulation phases .sub.i of the primary modulation from the first and second output signal portions.

11. The s-SNOM apparatus according to claim 10, wherein the phase shifter device comprises an all-pass filter, a self-calibrating phase shifter module implemented using a phase-locked loops and voltage controlled oscillators, a digital signal processing module or a field-programmable gate arrays.

12. The s-SNOM apparatus according to claim 10, wherein the phase shifter device is configured for splitting the output signal such that the first and second output signal portions have equal maximum amplitudes.

13. The s-SNOM apparatus according to claim 12, wherein the phase shifter device is configured for splitting the output signal such that the second output signal portion has a /2 phase-shift relative to the first output signal portion, and the demodulation device is configured for calculating the s-SNOM tip modulation phase .sub.i by .sub.i=arctan (Y/X).

14. The s-SNOM apparatus according to claim 10, wherein the demodulation device is configured for demodulating the scattering light function S() by extracting Fourier coefficients from the scattering light pulse amplitudes S().

15. The s-SNOM apparatus according to claim 10, including at least one of the features a band-pass filter is arranged between the mechanical oscillation detector device and the phase shifter device, a sampling clock source is coupled with the scattering light detector device and the mechanical oscillation detector device, wherein the sampling clock source is arranged for creating a sampling clock signal for sampling the scattering light pulse amplitudes S.sub.i and the s-SNOM tip modulation phases Q.sub.i, and a scanner device is arranged for scanning the s-SNOM tip and the sample relative to each other and for collecting a sample image.

16. The s-SNOM apparatus according to claim 1, wherein the scanning near-field optical microscope is configured for introducing at least one secondary modulation, at least one secondary modulation detector device is coupled with at least one further phase shifter device being arranged for providing at least one secondary modulation phase .sub.i for each of the collected scattering light pulse amplitudes S.sub.i by phase shifting of a portion of an output signal of the at least one secondary modulation detector device, and the demodulation device is configured for calculating the s-SNOM near-field signal by demodulating the scattering light function S() of the scattering light pulse amplitudes S in dependency on the s-SNOM tip modulation phase .sub.i and the at least one secondary modulation phases .sub.i.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0043] Further details and advantages of the invention are described in the following with reference to the attached drawings, which schematically show in

[0044] FIG. 1: an illustration of an s-SNOM apparatus according to preferred embodiments of the invention;

[0045] FIGS. 2 and 3: further details of a phase shifter device of the s-SNOM apparatus according to FIG. 1;

[0046] FIG. 4: an illustration of obtaining the scattering light function S.sub.i(.sub.i);

[0047] FIG. 5: modulation channels for tapping (A) and reference arm (B) modulation after calibration;

[0048] FIG. 6: examples of measuring the scattering light function S.sub.i(.sub.i) and demodulation results obtained with the invention;

[0049] FIG. 7: a configuration of the phase shifter device including PLLs; and

[0050] FIG. 8: a general illustration of the signal processing according to further embodiments of the invention.

PREFERRED EMBODIMENTS OF THE INVENTION

[0051] Features of preferred embodiments of the invention are described in the following with particular reference to phase-domain sampling of scattering light pulse amplitudes and demodulating the scattering light function. Details of an s-SNOM apparatus, like the driving of the s-SNOM tip or the analysis of the near-field signal for obtaining sample features, are not described as far as they are known per se from prior art (see e. g. [1]). With regard to the implementation of s-SNOM measurements, employing an interferometer for scattering light detection and the optional introduction of at least one secondary modulation, [1] is introduced to the present disclosure by reference. The invention is not restricted to the particular configurations of the illustrated s-SNOM apparatus, but rather can be varied, e. g. with regard to the configuration of the phase shifter device and/or the introduction of further modulations. For instance, there can be more than one optical signal channel (see e. g. FIG. 8), or the phase shift between the first and second output signal portions can be selected in a range deviating from 90. Signal conditioning can be used if necessary (e. g. filtering, amplification, subtraction, etc.).

[0052] The s-SNOM method can be applied with various modulation conditions. For a standard application, there is one single modulation by the motion of the s-SNOM tip (so called self-homodyne mode, shd mode). Alternatively, there is a primary modulation by the motion of the s-SNOM tip and a secondary modulation by a reference arm (so called pseudo heterodyne mode, pshet mode). FIG. 1 and the following description of the s-SNOM apparatus 100 refer in exemplary, not-restricting manner to the shd mode. For applying the pshet mode, the configuration of the s-SNOM apparatus 100 is modified by introducing at least one further modulation as mentioned below (see section Data processing-pseudo-heterodyne mode and FIG. 8).

s-SNOM Apparatus

[0053] FIG. 1 schematically shows the s-SNOM apparatus 100, which comprises a scanning near-field optical microscope 10 including an s-SNOM tip 11, an illumination device 20, a scattering light detector device 30, a mechanical oscillation detector device 40, a demodulation device 50, a phase shifter device 60 and a control device 70. The control device 70, like e. g. a computer unit, is arranged for controlling the components of the s-SNOM apparatus 100. Accordingly, it is coupled at least with the scanning near-field optical microscope 10, the illumination device 20, the scattering light detector device 30, the mechanical oscillation detector device 40, the demodulation device 50 and the phase shifter device 60. In practice, the control device 70 can be replaced by or completed with separate control units of the components 10 to 60.

[0054] The scanning near-field optical microscope 10 generally has an operation unit 12 including a tip drive unit and a control unit (not shown in detail). The operation unit 12 is fixed in space, e. g. arranged on a support 13. The sample 1 to be investigated is arranged on a substrate holder, which is coupled with a scanning device 14. The scanning device 14 is an x-y-z-table allowing a scanning movement of the sample 1 relative to the support 13 in a horizontal plane (perpendicular to the plane of drawing) while compensating for height variations of the sample in the z direction. The s-SNOM tip 11 is coated with a metallic material, like e. g. S.sub.i coated with PtIr alloy or gold and coupled via a deflectable cantilever with the operation unit 12, allowing a tip oscillation in a vertical direction (z-direction) perpendicular to the horizontal plane. The s-SNOM tip 11 is operated in tapping mode with typical oscillation amplitude in the range from 20 nm to 100 nm. With a practical example, the scanning near-field optical microscope 10 is an AFM microscope of the type NeaSnom (manufacturer: NeaSpec, Germany).

[0055] The illumination device 20 comprises a first light source 21 for creating illumination light pulses 2 and a second light source 22 for creating an indicator light beam 3. The control device 70 is provided for controlling the operation of the first and second light sources 21, 22. The first light source 21 is a pulsed laser combined with an optical parametric amplifier with a repetition rate of e. g. 200 kHz, a pulse duration of e. g. 30 fs and a centre wavelength of e. g. 633 nm, like e. g. the LC Pharos femtosecond laser combined with the LC ORPHEUS-3H tunable OPA (manufacturer: Light Conversion, Lithuania). The illumination light pulses 2 are directed to the oscillating s-SNOM tip 11, wherein preferably a whole range covered by the tip oscillation is homogeneously illuminated. Scattering light pulses 4 created at the s-SNOM tip 11 are collected with the scattering light detector device 30. Due to the near-field interaction with the sample 1, amplitudes S.sub.i of the scattering light pulses 4 depend on the distance of the s-SNOM tip 11 from the sample 1 (or correspondingly from the phase .sub.i of the tip oscillation). The scattering light detector device 30 is a photoreceiver, like a photodiode, which is fast enough to yield separated impulses for the individual optical scattering light pulses 4. In practice, collecting the scattering light pulses 4 can be executed using an interferometer (not shown), as disclosed e. g. in [1].

[0056] The second light source 22 is a continuous wave laser with a wavelength of e. g. 650 nm, like e. g. a laser diode. The indicator light beam 3 is directed to a reflecting section on the cantilever and/or an upper side of the oscillating s-SNOM tip 11 and deflected to the mechanical oscillation detector device 40. Due to the oscillation of the s-SNOM tip 11, the direction of the deflected indicator light beam 3 varies. For sensing the oscillation, the mechanical oscillation detector device 40 comprises a positions-selective photodetector, like a camera or preferably a quadrant photodetector.

[0057] The demodulation device 50 includes a data-acquisition card, like e. g. NI USB-6356 (manufacturer: National Instruments Corp., USA) and a calculation circuit. For calculating the s-SNOM tip modulation phases .sub.i and the s-SNOM near-field signal by demodulating the scattering light function S.sub.i(.sub.i) of the scattering light pulse amplitudes S.sub.i in dependency on the s-SNOM tip modulation phases .sub.i, the demodulation device 50 is coupled at least with the scattering light detector device and the mechanical oscillation detector device 40, and optionally further with the control device 70.

[0058] The output signal 31 of the scattering light detector device 30 is directly input to the demodulation device 50 for obtaining the scattering light pulse amplitudes S.sub.i. The scattering light detector device 30 further can be used as a clock source providing an optical trigger signal 32 (clock signal). With the illustrated example, the output signal 31 can be used for providing both of the scattered optical signal to be sensed and the optical trigger. For illustrative reasons, the optical trigger signal 32 is shown as a separate signal. Alternatively, the optical trigger signal 32 can be provided by a separate clock source, including a photodetector sensing the illumination or scattering light pulses 2, 4.

[0059] Coupling of the demodulation device 50 with the mechanical oscillation detector device 40 is provided via the phase shifter device 60 (see FIGS. 2, 4 and 6). The output signal 41 of the mechanical oscillation detector device 40 is input to the phase shifter device 60 for generating first and second output signal portions X, Y as described below.

[0060] The demodulation device 50 uses analogue-to-digital converters (ADCs) to sample the electronic signals, including the optical scattering light pulse amplitudes S and the modulation signals X, Y. The ADCs are synchronized with the optical pulses by the use of an adequate sampling clock, in particular the optical trigger signal 31/32. The sampling clock therefore has the same frequency as the optical pulse train. Demodulating the optical scattering light pulse amplitudes S with the demodulation device 50 is described with further details below.

[0061] As schematically shown e. g. in FIGS. 1 to 3, the phase shifter device 60 is arranged for receiving the output signal 41 of the mechanical oscillation detector device 40 and for splitting the output signal 41 into a first output signal portion X and a second output signal portion Y being phaseshifted relative to each other. The phase shifter device 60 of this embodiment comprises a bandpass filter 61, a first buffer 62, an all-pass filter 63 (giving control of the phase) and a second buffer 64.

[0062] The band-pass filter 61 is configured for removing most of the broadband noise included in the output signal 41. As an example, the band-pass filter 61 passes signal components in a frequency range of 30 kHz centred on the mechanical oscillation frequency. For calibration purposes, the frequency range of the band-pass filter 61 can be set with the control device 70. The filtered signal is split into two branches after passing the band-pass filter 61. The embodiments of FIGS. 1 to 3 are described with reference to the self-homodyne mode (see below). For SNOM in pseudoheterodyne mode, another phase shifter device is used for the reference arm modulation. It is correspondingly adapted for reference arm modulation V.sub.ref, wherein the band-pass filter is replaced by a broader band-pass filter with switchable gain.

[0063] The first branch for providing the first output signal portion X without phase shift (0) includes the first buffer 62, which is adapted for adjusting the amplitude of the first output signal portion X, e. g. with an amplifier (optionally having an adjustable gain, not shown). The second branch for providing the second output signal portion Y with a phase shift includes the all-pass filter 63 and the second buffer 64, which is adapted for adjusting the amplitude of the second output signal portion Y, again with an amplifier having an adjustable gain. Details of the all-pass filter 63 for a 90 phase shift are illustrated in an exemplary manner in FIG. 3. Alternatively, another phase shift amount can be introduced. For calibration purposes, the transition frequency of the all-pass filter 63 can be set with the control device 70 and/or manually with control elements. Setting the amplitudes of the first and second output signal portions X, Y is executed with the control device 70 as well.

[0064] The phase shift between the X and Y channels preferably is set to /2 and their amplitudes is set with the buffers 62, 64 to be equal. With these parameters, the two channels correspond to the in-phase and quadrature components of the modulation signal, and a plot of Y vs X shows a circle. Example results of the X-Y-plot for the single modulation channel of shd mode and for reference arm modulation of pshet mode after calibration are shown in FIGS. 5A and 5B, resp. Advantageously, the phase .sub.i can be calculated for the i-th sampled optical pulse as

[00001]${}_i=\mathrm{arc}\tan \left(Y_i/X_i\right).$

[0065] Alternative choices of the phase shifts and amplitudes are possible, as detailed below.

[0066] Preferably, the phase shifter device 60 is calibrated externally before the measurement with the sample 1. The calibration is performed each time the frequency of the modulation is changed significantly. The calibration steps preferably comprise adjusting the band-pass filter 61 to maximize the amplitude of output X, adjusting the all-pass filter 63 to obtain a /2 phase shift between Y and X, and using the output buffer 64 with adjustable gain for adjusting the amplitude of Y to match the amplitude of X.

Data ProcessingSelf-Homodyne Mode

[0067] For the standard SNOM in self-homodyne (shd) mode, there is a single modulation of the scattering light pulse amplitudes S.sub.i provided by the motion of the s-SNOM tip 11. FIG. 4 schematically illustrates the sampling process for obtaining the scattering light pulse amplitudes S.sub.i. The optical signal 31/S is sampled simultaneously with the modulation components, i. e. the first and second output signal portions X, Y. Vertical lines indicate sampling triggers. Accordingly, for the i-th illumination light pulse 2, the scattering light pulse amplitude S.sub.i and the associated amplitudes of the first and second output signal portions X.sub.i, Y.sub.i are collected. The current modulation phase .sub.i of the i-th scattering light pulse is obtained from the first and second output signal portions X.sub.i, Y.sub.i, which are sampled synchronously with the scattering light pulse amplitudes. The measurements from individual pulses are recorded, resulting in a data set S.sub.i(.sub.i), e. g. with i=1 to 50 000 for a given location of the sample surface. It is noted that the total number of points can be selected depending on signal quality, wherein a lower limit may be 10 000 or even 5000 pts, while there is no technical upper limit.

[0068] The acquired data set is transformed into the demodulated signal, for example the near-field signal (SNOM signal), as follows. After calculating the phases .sub.i of the modulations, Fourier coefficients u of the optical signals S.sub.i are extracted. The resulting amplitudes are the near-field signal to be obtained, and they correspond to the results from conventional lock-in detection.

[0069] FIG. 6 shows a shd mode dataset at various stages of processing. The raw data is obtained from the s-SNOM apparatus 100, as shown in FIG. 1. Using the s-SNOM apparatus 100, the tapping phase .sub.i is calculated as .sub.i=arc tan (Y.sub.i/X.sub.i). The signal S.sub.i vs .sub.i is shown in FIG. 6A. Demodulation is performed by extracting the Fourier coefficients u.sub.n from S(), obtained from S.sub.i e.g. by a binning algorithm, according to the equation:

[00002]$\begin{array}{cc}{u}_n=\frac{1}{2}\underset{2}{}S\left(\right)\exp \left[-\mathrm{in}\right]d& \left(1\right)\end{array}$

where u.sub.n is the coefficient for order n and i is the index of the illumination light pulses. In the case of shd-SNOM, |u.sub.n| is the SNOM signal of order n (ie: at frequency f=n).

[0070] The inventors have found that the demodulation can be performed numerically despite the irregular sampling of the phases .sub.i. Direct Fourier transform (DFT) and binning ([4]) can be employed for numerically executing the demodulation. The DFT algorithm performs the integration in eq(1) numerically using known techniques, such as Trapezoidal integration. For shd-SNOM, the inventors have found this algorithm to be simple and robust. In the binning algorithm, the dataset is first regularized to a constant spacing by binning. Every data point is assigned a bin according to its phase .sub.i, like in a histogram. The average of the signal S is then computed for every bin. The results are shown in FIG. 6A using crosses. As the data is regularly spaced, the Fourier coefficients are then extracted by standard Fast Fourier Transform (FFT) algorithms. The resulting amplitudes are shown in FIG. 6B. Advantageously, this algorithm is faster and yields higher quality data than DFT. Furthermore, it can be generalized to higher dimensions.

[0071] Auxiliary calculations can be performed at any point. For example, based on a stochastic analysis, one can remove improper sample points. As another example, one can compute the difference between two signals S.sub.d=S.sub.AS.sub.B and analyze S.sub.d alongside the other existing channels. In another case, it is possible to group the data using some criterion (e. g. the value of a further modulation M.sub.i, see FIG. 8) and perform the analysis separately on the subset of data.

Data ProcessingPseudo-Heterodyne Mode

[0072] pshet-SNOM employs a secondary modulation, which is introduced by using an interferometer for sensing the scattered light and modulating the length of an interferometer arm (see e. g. [1]). This length modulation is sensed by a secondary modulation detector device, sensing e. g. a position of an interferometer mirror or a driving voltage of an actor moving the mirror, and a further phase-shifter device. The secondary modulation detector device is completely unrelated to the mechanical oscillation detector device.

[0073] For pshet-SNOM, the analysis proceeds in an analogue manner, but with two dimensions. The raw data is obtained from the s-SNOM apparatus 100 being adapted for the pshet mode. The phases are calculated as .sub.i=arctan (Y.sub.i/X.sub.i) for the mechanical oscillation and of .sub.i=arctan (Y.sub.i/X.sub.i) for the reference modulation. The coefficients are extracted using the two-dimensional equivalent of equation (1):

[00003]$\begin{array}{cc}{u}_{n,m}=\frac{1}{4{}^2}\underset{2,2}{}S\left(,{}^{}\right)\exp \left[-i\left(n+m{}^{}\right)\right]dd{}^{}& \left(2\right)\end{array}$

where u.sub.n,m is the coefficient of orders n, m. In the case of pshet SNOM, the coefficients |u.sub.n,m| are equivalent to the pshet side-bands amplitudes (ie: at f=n+mM).

[0074] In principle, multiple available algorithms can be used. A 2D equivalent of the DFT algorithm is possible but inefficient. The binning algorithm is straightforward to generalize. The points are partitioned into a 2D histogram using the values of .sub.i and .sub.i. The average for each bin is computed. The coefficients u.sub.n,m are obtained using the 2D Fourier transform.

ALTERNATIVE EMBODIMENTS

[0075] According to an alternative embodiment of the invention, the phase shifter device 60 is provided by a self-calibrating phase shifter module 65 as shown in FIG. 7, instead of the components 61 to 64 of FIG. 3. The phase shifter module 65 includes a first PLL circuit 66, a second PLL circuit 67 and a third PLL circuit 68, each including a VCO 66A, 67A and 68A, resp. The output signal 41 of the mechanical oscillation detector device 40 with the oscillation frequency f of the s-SNOM tip 11 is input to the first PLL circuit 66. Optionally, the output signal 41 may be frequency filtered with a band-pass filter 61 (see e. g. FIG. 3). The VCO 66A of the first PLL circuit 66 creates a signal component with 4f, on the basis of which a first square wave signal with f+90 and a second square wave signal with f are created, using a divide by 4 counter 66B combined with an adder 66C. The square wave signals are used for creating a cosine wave and a sine wave with the second and third PLL circuits 67, 68. After amplification, the cosine and sine waves are output as the first and second output signal portions X, Y with exact 90 phase shift.

[0076] The inventors have found, that the inventive s-SNOM method can be executed with one or more optical signal channels and with one or more modulations. FIG. 8 illustrates a generalized version of the demodulation device 50 being configured for receiving e. g. two optical scattering signals 31, 31 and two modulations signals 41, 41, a clock signal TRIG and at least one further auxiliary modulation signal component MOD. The optical scattering signals 31, 31 comprise scattering light pulse amplitudes collected via two different optical signal channels. The modulation signal 41 represents the primary modulation introduced by the mechanical oscillation of the s-SNOM tip and sensed by the mechanical oscillation detector device 40. The modulation signal 41 represents the secondary modulation introduced by a variation of an interferometer arm length and sensed by a secondary modulation detector device 40. With the demodulation device 50, e. g. the pshet mode can be executed. As a further example, the first optical scattering signal 31 comprises the output signal of the scattering light detector device 30 as described above, and the second optical scattering signal 31 comprises the output signal of a second photo-sensor of the scattering light detector device 30 being arranged for collecting scattering light along a different optical path (e. g. having a different direction and/or using different optical elements in the optical path). Using the first and second optical scattering signals may have advantages for improving the sensitivity and reliability of the s-SNOM apparatus 100.

[0077] For the multiple modulations, independent phase shifter devices 60, 60 are provided, each being tailored to one of the modulations. Each phase shifter device 60, 60 may contain signal conditioning electronics, like the band-pass filter 61 (see FIG. 1). If multiple signals are present, the collected data are processed and analyzed in the same way as described above.

[0078] The features of the invention disclosed in the above description, the drawings and the claims can be of significance individually, in combination or sub-combination for the implementation of the invention in its different embodiments.