Resonance-enhanced detection methods using scanning probe microscopes and a range of modulation frequencies.

Abstract

Scanning probe microscopes and methods for resonance-enhanced detection using scanning probe microscopes use a light source modulated over a range of frequencies to illuminate the interface between the microscope's probe tip and a sample with modulated electromagnetic radiation from the light source. The vibrational response of a driven cantilever is then detected in response to the modulated electromagnetic radiation at the interface between the probe tip and the sample. The amplitude of the cantilever's vibrational response over the entire range of modulation frequencies is measured to derive a photo-induced force microscope (PiFM) value.

Description

[Technical field]

[0001] This application may benefit from U.S. Provisional Patent Application No. 62 / 949,404, filed December 17, 2019, which is incorporated by reference herein. [Background technology]

[0002] In scanning probe microscopy (SPM), it is often desirable to use the resonance of the cantilever probe to amplify the signal from the measured force. This typically involves providing a tip-sample force interaction modulated at the frequency of the resonance, which can add energy over multiple cycles to produce a much larger deflection than can be achieved by the same force modulation at a non-resonant frequency. However, as the probe interacts with the sample, the resonance frequency and Q of the probe can shift. In particular, the resonance frequency is shifted due to different local material properties of the sample surface and imperfect maintenance of the tip-sample distance. When specialized versions of SPM such as photo-induced force microscopy (PiFM) are used, the absorption of light by localized regions of the sample can also affect the tip-sample interaction force, and therefore shift the resonance frequency and Q. When a static excitation frequency is used, such a fixed frequency can shift from the peak frequency, and as a result, the amplification factor of the cantilever response is not constant. Such varying amplification factors mean that the sensitivity of the measurement can change with time, with position on the sample surface, or with the wavelength and intensity of light applied to the interface. Such changes in response due to changes in resonance introduce errors in the reported data that are not easily corrected for in the acquired data. Summary of the Invention [Means for solving the problem]

[0003] Scanning probe microscopes and resonance-enhanced detection methods using scanning probe microscopes involve illuminating an interface between a microscope probe tip and a sample with modulated electromagnetic radiation from a light source modulated over a range of frequencies. The vibrational response of a driven cantilever is then detected in response to the modulated electromagnetic radiation at the interface between the probe tip and the sample. The amplitude of the vibrational response of the cantilever over the entire range of modulation frequencies is measured to derive a photo-induced force microscope (PiFM) value.

[0004] A method for resonance-enhanced detection using a scanning probe microscope according to one embodiment of the present invention includes driving a cantilever into oscillation with a probe tip of a scanning probe microscope, modulating a light source over a range of frequencies and illuminating an interface between the probe tip and a sample with modulated electromagnetic radiation from the light source, detecting a vibrational response of the cantilever in response to the modulated electromagnetic radiation at the interface between the probe tip and the sample, and measuring the amplitude of the vibrational response of the cantilever over the entire range of modulation frequencies to derive a photo-induced force microscope (PiFM) value.

[0005] A scanning probe microscope according to one embodiment of the invention includes a cantilever having a probe tip, a dither drive unit coupled to the cantilever for driving the cantilever to oscillate, a light source for illuminating an interface between the probe tip and a sample, a signal generator coupled to the light source for modulating the light source over a range of frequencies and illuminating the interface between the probe tip and the sample with modulated electromagnetic radiation from the light source, a detector for detecting a vibrational response of the cantilever in response to the modulated electromagnetic radiation at the interface between the probe tip and the sample, and an amplitude measurement system for measuring the amplitude of the vibrational response of the cantilever over a range of modulation frequencies to derive a photo-induced force microscope (PiFM) value.

[0006] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrated by way of example of the principles of the invention. [Brief description of the drawings]

[0007] [Figure 1] 1 shows an example of three resonance curves taken at different positions on a non-uniform sample. [Diagram 2] 1 shows a scanning probe microscope according to an embodiment of the present invention. [Diagram 3] 1 shows a scanning probe microscope according to an alternative embodiment of the present invention. [Figure 4] 3 shows a scanning probe microscope according to another embodiment of the present invention. [Diagram 5] 1 is a process flow diagram of a resonance-enhanced detection method using a range of modulation frequencies according to one embodiment of the present invention. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0008] The need for monitoring and correcting for resonance changes in SPMs has long been recognized, and various forms of solutions have been reported in the existing literature and patents. One solution involves periodically or continuously measuring the frequency response of the cantilever resonance frequency or the phase of the cantilever oscillation, and adjusting the appropriate modulation frequency in the SPM to track the changing resonance frequency. Another solution is to apply a range of frequencies (by sweeping or using band excitation) that covers the expected changes in resonance frequency, determine the peak frequency, and measure the response as a function of modulation frequency to report the signal magnitude at the peak. The main disadvantage of both approaches is that the frequency response of a particular cantilever resonance sometimes exhibits a level of complexity that makes identification of a single peak frequency difficult or impossible. Resonant frequency response curves can vary from an ideal Lorentzian-shaped single peak with a well-defined Q to complex response curves with multiple maxima and minima and an overall shape that is far from the ideal Lorentzian. In this case, determining the "peak frequency" to track or report the response is problematic.

[0009] Figure 1 shows an example of three resonance curves taken at different positions on a non-uniform sample. As can be seen from these three curves, the resonance peak frequency and the overall structure of the curves all vary between the three positions. Of the three curves, curve C is closest to the expected peak shape, while curve B is particularly problematic, with multiple maxima that may lead to errors in identifying one particular resonance peak. The phase of the vibration is not shown in Figure 1, but is similarly complex, making it impossible for a phase-locked loop to automatically select and lock one precise resonance peak.

[0010] Existing peak tracking methods include (1) R. Arinero et al, “Image processing for resonance frequency mapping in atomic force modulation microscopy,” Rev. Sci. Instrum. 78, 023703 (2007), (2) G. Ramer et al, “Implementation of Resonance Tracking for Assuring Reliability in Resonance Enhanced Photothermal Infrared Spectroscopy and Imaging,” Appl. Spectrosc. 71, 2013-2020 (2017), (3) C. Prater and K. Kjoller, US Patent Number 8,680,467, “High frequency deflection measurement of IR absorption with a modulated IR source,” filed 9 / 19 / 11, issued 3 / 25 / 14, and (4) Prater and K. Kjoller, US Patent Number 10,228,388,“Method and apparatus for resolution and sensitivity enhanced atomic force microscope based on Such references focus on finding and reporting the peak frequency of the resonance curve and then reporting the magnitude of the response at the peak frequency.

[0011] Other existing methods include the band excitation method described in S. Jesse et al., “The band excitation method in scanning probe microscopy for rapid mapping of energy dissipation on nanoscale”, Nanotechnology 18 435503 (2007). Such references have essentially the same interest in looking for peak frequencies and responses at the peak frequencies; the band excitation method simply applies a range of frequencies simultaneously and measures the response as a function of frequency within that range. Although the band excitation method has certain advantages over the peak tracking method, which sweeps or tracks frequencies, it shares the same disadvantages as the peak tracking method when the frequency response of a resonance has a complex behavior, since its output is not integrated over the region of interest.

[0012] Embodiments of the present invention address these disadvantages by solving the problem in a different way. Instead of looking to apply a range of frequencies to track changing resonant frequencies or to look for a response at a single "peak" frequency, the response over a range of frequencies is integrated and reported. This has the effect of summing the response at all frequencies in the region of interest, which is particularly advantageous when the frequency response curve is not a well-behaved single Lorentzian peak. A range of frequencies can be applied by sweeping a single frequency over the target range or by band excitation, which applies a range of frequency components simultaneously. When using swept frequencies, general lock-in signal detection can be used with an integrator at the output. In the case of band excitation, a single amplitude envelope measurement may be made.

[0013] 2, there is shown a PiFM apparatus 200, a scanning probe microscope according to an embodiment of the present invention. The PiFM apparatus 200 includes a probe tip 202 coupled to a cantilever 204, a light source 206 in the form of a modulated tunable laser 206 for illuminating an interface between the probe tip and a sample 208 with electromagnetic radiation, a dither control system 210 for setting a constant distance between the probe tip and the sample, and a laser modulation control system 212 for modulating the tunable laser over a range of modulation frequencies to output integrated amplitude values ​​for the entire range of modulation frequencies as PiFM data usable to image the sample. In one embodiment, the modulated tunable laser 206 is an infrared (IR) laser.

[0014] The dither control system 210 of the PiFM device 200 includes a dither drive unit 214, a dither piezo device 216 coupled to the cantilever 204, a position feedback laser 218, an optical position detector 219, a first lock detector or amplifier 220, a sample Z servo controller 222, and a sample Z piezo device 224. For an example of amplitude modulation (AM) detection (also referred to as "gradient detection"), the dither drive unit 214 is configured to provide a signal having a desired drive frequency to the dither piezo device 216, which causes the cantilever 204 to oscillate. The optical position detector 219 optically measures the oscillation of the cantilever 204 using light from the position feedback laser 218 reflected from the cantilever. The signal from the optical position detector 219 is input to a first lock detector 220 which provides an amplitude error signal to the sample Z servo controller 222. The amplitude error signal indicates that the spacing between the tip 202 and the sample 208 deviates from the desired spacing. In response, the sample Z servo controller 222 provides a control signal to the sample Z piezo 224 so that the spacing between the tip 202 and the sample 208 can be adjusted. In particular, the dither frequency is fixed at a point slightly above the resonant frequency of the dither vibration mode of the cantilever 204. If the tip-sample spacing falls below the target value, the dither mode frequency is shifted downwards, effectively moving the dither drive frequency above resonance and decreasing the dither amplitude (and vice versa). The sample Z servo controller 222 determines the amplitude error and applies a correction to the voltage on the sample Z piezo 224 to return the tip-sample spacing to the target value. As the sample 208 is raster scanned relative to the tip 202, topography data is obtained from the output to the sample Z servo controller 222, and mechanical phase data (related to the mechanical stiffness and damping of the sample) is provided at the phase output of the lock detector 220.

[0015] The dither control system 210 can be substituted for a frequency modulation (FM) detection scheme without affecting the rest of the PiFM device 200. FM is preferred because the AM scheme can have excessively slow control loop response speed and imaging speed when operating with very high cantilever Q.

[0016] The laser modulation control system 212 of the PiFM device 200 includes a sweep frequency generator 226, an electronic mixer 228, a second lock detector or amplifier 230, and an integrator 232. The sweep frequency generator 226 operates to generate a modulated drive signal for the tunable laser 206 at a range of sweep modulation frequencies that includes the resonant frequency. The sweep frequency range may be a range of plus or minus 5% of the resonant frequency of the cantilever 204 or a wider range. As an example, in FIG. 1, if the resonant frequency is 247 kHz, the sweep frequency range may be 230 kHz to 260 kHz, which corresponds to a range of + / - 6% (or 12%). The electronic mixer 228 operates to mix the dither drive signal at the first frequency from the dither driver 214 and the modulated drive signal at the second frequency from the sweep frequency generator 226 to generate an output signal at a sum frequency that coincides with the cantilever resonant frequency. The second lock detector 230 uses the sum frequency signal from the mixer 228 as a reference against the cantilever vibration response signal from the optical position detector 219 to generate an output indicative of the amplitude of the cantilever vibration for the current modulation frequency. The integrator 232 operates to integrate the output of the second lock detector 230 as the modulation frequency applied to the tunable laser 206 is swept over a range of modulation frequencies. The output of the integrator 232 is PiFM data for each measured position of the sample, which is used to image the sample 208. Because the mixer 228, the second lock detector 230 and the integrator 232 operate to measure an integrated amplitude value, such components are considered to be part of an amplitude measurement system.

[0017] In operation according to one embodiment, a pulsed or amplitude modulated tunable infrared (IR) laser 206 of the PiFM device 200 radiates light to the interface between the probe tip 202 and the sample 208. In this embodiment, the PiFM device 200 operates in sideband mode, and the laser modulation frequency is the difference frequency between the dither drive frequency and the resonance frequency of the vibration mode used for PiFM detection. Instead of a fixed frequency signal generator, the PiFM device 200 in this embodiment uses a sweep frequency generator 226 that sweeps over a frequency range wide enough to cover the expected shifts in the cantilever resonance frequency over the course of an experiment. Nonlinear interactions between the illuminated tip 202 and the sample 208 result in frequency mixing of the dither frequency and the laser modulation frequency that generates a sum frequency that coincides with the cantilever resonance. Such an identical frequency is generated by an electronic mixer 228 using signals from the dither drive unit 214 and the sweep frequency generator 226. The cantilever deflection signal from the optical position detector 219, which contains the PiFM component at the sum frequency, is applied to the input of the second lock detector 230, and the reference signal for such lock detector is the electronically generated sum frequency from the mixer 228. During each frequency sweep by the sweep frequency generator 226, the amplitude output of the second lock detector 230 is integrated to provide a single integrated PiFM value at the end of each sweep, which is a single PiFM data point usable for PiFM imaging. In one embodiment, the sweep frequency generator 226 may be configured to generate an optional trigger output after each frequency sweep, which is used to clear the integrator 232 before the start of each frequency sweep.

[0018] In some embodiments, various experimental factors may be selected to optimize the measured PiFM data. By way of example, tip-sample position, time, wavelength of applied light, tip-sample distance, sample temperature, intensity of applied light, and / or morphology of the tip and its coating can be selected to derive optimized PiFM data. Additionally, in some embodiments, an alternative to using the sum frequency is also to use the difference frequency generated by mixer 228. Also, in some embodiments, an alternative to using the difference frequency is to use the sum frequency.

[0019] It should be noted that the time constant for the output of the second lock detector 230 must be significantly shorter than the sweep repetition rate (typical values ​​are 5 to 50 times shorter), unless it is intended to use the output time constant as an integration mechanism. Furthermore, the sweep repetition rate must be significantly slower than the time constant of the cantilever 204 amplitude change, determined by the frequency and Q of the cantilever oscillation at the PiFM signal frequency (typical values ​​are 5 to 50 times shorter). Such factors limit the speed at which PiFM data can be collected.

[0020] 3, there is shown a PiFM device 300 according to an alternative embodiment of the present invention. In this embodiment, an envelope detector 340 having a bandpass filter 342 is used in place of the second lock detector 230, thereby eliminating the need for the electronic mixer 228. All other components of the PiFM device 300 are similar to the PiFM device 200 shown in FIG.

[0021] A bandpass filter 342 is used to only allow cantilever vibration response signals from the optical position detector within a predefined range of frequencies that coincide with the range of sweep frequencies applied to the tunable laser 206. The bandpass filtered signal is then applied to an envelope detector 340 to detect the desired amplitude signal. The output of the envelope detector 340 is then sent to the integrator 232 which integrates the amplitude signal during each frequency sweep to generate the PiFM data.

[0022] 4, a PiFM device 400 using a bandpass excitation approach is shown according to another embodiment of the present invention. In this embodiment, the sweep frequency generator 226 is replaced by a digital signal generator 440 (also known as a digital waveform generator) that generates a waveform signal by inverse Fourier transform from a frequency spectral density function (i.e., covering the range of frequencies of interest) specified by a user. In this embodiment, a bandpass filter 442 and an amplitude detector 444, such as an envelope detector, are used instead of the second lock detector 230, and the electronic mixer 228 and the integrator 232 are eliminated. The output of the amplitude detector 444 is continuously sampled to provide PiFM data.

[0023] In operation, the digital signal generator 440 provides a modulation signal to the tunable laser such that the light from the tunable laser applied to the tip-sample interface is simultaneously modulated with a band of frequencies matching the range of target frequencies. The cantilever deflection signal from the light position detector 219 is then passed through a bandpass filter 442 to an amplitude detector 444. The amplitude output from the amplitude detector 444 is PiFM data that can be used to image the sample 208.

[0024] In the embodiment described above in FIG. 2, the PiFM device 200 operates in a sideband PiFM mode, where the modulation frequency applied to the light source, e.g., a tunable IR laser 206, is the difference frequency between the dither frequency and a range of frequencies that overlap the expected resonance frequency of the cantilever 204 for the vibrational mode excited by the PiFM force. In another embodiment, the PiFM device 200 can operate in a direct PiFM mode, where the light source is modulated with a range of frequencies that overlap the resonance. This embodiment does not rely on mixing between the dither and light source modulation frequencies, the electronic mixer 228 is eliminated, and the light source modulation frequency serves as the reference frequency for the second lock detector 230.

[0025] In the embodiments of the PiFM device described herein, the components of the PiFM device may be implemented in hardware, and possibly in software or some combination of hardware and software. Although not described herein, such embodiments may utilize a computer system to control the various components of the PiFM device.

[0026] A method for resonance-enhanced detection using a scanning probe microscope, such as PiFM device 200, 300 or 400, according to one embodiment of the present invention is described with reference to the flow chart of FIG. 5. In block 502, a cantilever having a probe tip of the scanning probe microscope is driven to oscillate. The cantilever can be driven by a dither drive unit of the scanning probe microscope. In block 504, a light source is modulated over a range of frequencies to illuminate an interface between the probe tip and the sample with modulated electromagnetic radiation from the light source. In some embodiments, the light source is modulated by a sweep frequency generator. In other embodiments, the light source is modulated by a sweep frequency generator. In block 506, a vibration response of the cantilever is detected in response to the modulated electromagnetic radiation at the interface between the probe tip and the sample. The vibration response of the cantilever is detected using an optical position sensitive detector. In block 508, the amplitude of the vibration response of the cantilever over the entire range of modulation frequencies is measured to derive a photo-induced force microscope (PiFM) value. The amplitude of the vibration response of the cantilever over the entire range of modulation frequencies can be measured using an amplitude measurement system as described herein.

[0027] In general, it will be readily understood that the components of the embodiments as described herein and illustrated in the accompanying drawings could be arranged and designed in a wide variety of different configurations. Thus, the detailed description of various embodiments as illustrated in the drawings is not intended to limit the scope of the disclosure but is merely representative of the various embodiments. Although various aspects of the embodiments are presented in the drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0028] The present invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The described embodiments are to be considered in all respects as illustrative and not restrictive. The scope of the invention is therefore indicated by the appended claims rather than by this detailed description. All changes which come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0029] References to features, advantages, or similar language throughout this specification do not imply that all features and advantages that may be realized by the present invention must be present in any single embodiment of the present invention. Rather, language referring to features and advantages is understood to mean that the particular feature, advantage, or characteristic described in connection with an embodiment is included in at least one embodiment of the present invention. Thus, discussions of features and advantages and similar language throughout this specification may, but do not necessarily, refer to the same embodiment.

[0030] Furthermore, the described features, advantages, and characteristics of the invention may be combined in any suitable manner in one or more embodiments. Those skilled in the relevant art will recognize in light of the description herein that the invention may be practiced without one or more of the specific features or advantages of a particular embodiment. In other cases, additional features and advantages may be recognized in a particular embodiment that are not present in all embodiments of the invention.

[0031] References throughout this specification to "one embodiment" or similar language mean that a particular feature, structure, or characteristic described in connection with the illustrated embodiment is included in at least one embodiment of the invention. Thus, references throughout this specification to "in one embodiment" and similar language may, but do not necessarily, all refer to the same embodiment.

[0032] In the foregoing description, specific details of various embodiments are provided. However, some embodiments may be practiced with less than all of such specific details. In other instances, specific methods, procedures, components, structures and / or functions are described in more detail than would be possible in enabling various embodiments of the invention, for the sake of brevity and clarity.

[0033] Although method operations have been illustrated and described herein in a particular order, the order of each method operation may be changed such that certain operations are performed in reverse order or certain operations are performed at least in part concurrently with other operations. In other embodiments, instructions or sub-operations of separate operations may be implemented in an intermittent and / or alternating manner.

[0034] It should also be noted that at least some of the operations for the methods described herein may be implemented using software instructions stored on a computer usable storage medium for execution by a computer. By way of example, an embodiment of a computer program product includes a computer usable storage medium for storing a computer readable program.

[0035] A computer-usable or computer-readable storage medium may be an electronic, magnetic, optical, electromagnetic, infrared, or semiconductor system (or apparatus or device). Examples of non-transitory computer-usable and computer-readable storage media include semiconductor or solid state memory, magnetic tape, mobile computer disks, random access memory (RAM), read-only memory (ROM), rigid magnetic disks, and optical disks. Currently, examples of optical disks include compact disks with read only memory (CD-ROM), compact disks with read / write (CD-R / W), and digital video disks (DVDs).

[0036] Alternatively, an embodiment of the invention may be implemented entirely in hardware, or in an implementation containing both hardware and software elements. In a software embodiment, the software may include, but is not limited to, firmware, resident software, microcode, etc.

[0037] Although specific embodiments of the invention have been described and illustrated, the invention is not limited to the specific forms or arrangements of parts so described and illustrated, The scope of the invention is to be defined by the claims appended hereto and their equivalents.

Claims

1. In a method for resonance-enhanced detection using a scanning probe microscope (200, 300, 400), Driving (502) a cantilever (204) to oscillate together with a probe tip (202) of a scanning probe microscope (200, 300, 400); modulating a light source (206) over a range of frequencies and illuminating (504) an interface between the probe tip (202) and the sample (208) with the modulated electromagnetic radiation from the light source (206); detecting (506) a vibrational response of the cantilever (204) in response to modulated electromagnetic radiation at an interface between the probe tip (202) and the sample (208); and measuring (508) the amplitude of the vibrational response of the cantilever (204) over a range of modulation frequencies to derive a photo-induced force microscope (PiFM) value representative of the sum of the vibrational response of the cantilever (204) over a range of modulation frequencies.

2. 2. The method of resonance-enhanced detection using a scanning probe microscope of claim 1, wherein the step (504) of modulating the light source (206) comprises applying a frequency sweep modulation signal to the light source (206) such that the frequency of the modulated electromagnetic radiation sweeps through the range of frequencies, and the step (508) of measuring the amplitude of the vibration response of the cantilever (204) over the entire range of modulation frequencies comprises integrating the amplitude of the vibration response of the cantilever (204) over the entire range of modulation frequency amplitudes to derive the PiFM value.

3. 3. The method of resonance-enhanced detection using a scanning probe microscope of claim 1 or 2, wherein the vibrational response of the cantilever (204) comprises a response to a force or force gradient excited by electromagnetic radiation modulated at an interface between the probe tip (202) and the sample (208).

4. 4. The method of resonance-enhanced detection using a scanning probe microscope of claim 3, wherein the step of measuring (508) the amplitude of the vibration response of the cantilever (204) comprises mixing a modulation frequency of the light source (206) with a cantilever dither frequency to derive a sum or difference frequency used to detect the force or force gradient.

5. 5. The method of claim 4, wherein measuring the amplitude of the vibration response of the cantilever includes using a sum or difference frequency as a reference for a lock detector.

6. 2. The method of resonance-enhanced detection using a scanning probe microscope of claim 1, wherein modulating the light source comprises applying a modulation signal to the light source such that the modulated electromagnetic radiation simultaneously has the range of frequencies, and measuring the amplitude of the vibrational response of the cantilever over the entire range of modulation frequencies comprises simultaneously measuring the amplitude of the vibrational response of the cantilever in response to the modulated electromagnetic radiation to derive the PiFM value.

7. 7. The method of resonance-enhanced detection using a scanning probe microscope of claim 6, wherein applying the modulation signal to the light source (206) comprises generating a modulation waveform that includes a sum or difference of multiple frequency components within the frequency range.

8. 8. A method of resonance enhanced detection using a scanning probe microscope according to claim 7, wherein the modulated signal is generated by an inverse Fourier transform of the spectral density of the range of applied frequencies.

9. 7. The method of resonance-enhanced detection using a scanning probe microscope of claim 6, wherein measuring (508) the amplitude of the vibrational response of the cantilever (204) comprises measuring the amplitude of the vibrational response of the cantilever (204) using a bandpass filter (442) and an amplitude detector.

10. The method of resonance enhanced detection using a scanning probe microscope according to any one of claims 1 to 9, wherein modulating (504) the light source (206) comprises modulating a tunable infrared (IR) laser.

11. In the scanning probe microscope (200, 300, 400), a cantilever (204) having a probe tip (202); a dither drive unit (214) coupled to the cantilever (204) for driving the cantilever (204) to oscillate; a light source (206) for illuminating the interface between the probe tip (202) and a sample (208); a signal generator (226, 440) coupled to the light source (206) for modulating the light source (206) over a range of frequencies and illuminating an interface between the probe tip (202) and the sample (208) with modulated electromagnetic radiation from the light source (206); a detector (219) for detecting a vibrational response of the cantilever (204) in response to the modulated electromagnetic radiation at an interface between the probe tip (202) and the sample (208); and an amplitude measurement system that measures the amplitude of the vibration response of the cantilever (204) over an entire range of modulation frequencies to derive a photo-induced force microscope (PiFM) value that represents the sum of the vibration response of the cantilever (204) over an entire range of modulation frequencies.

12. 12. The scanning probe microscope of claim 11, wherein the signal generator (226, 440) is configured to apply a frequency sweep modulation signal to the light source (206) such that the frequency of modulated electromagnetic radiation sweeps through the range of frequencies, and the amplitude measurement system is configured to integrate the amplitude of the vibration response of the cantilever (204) over the entire range of amplitudes of the modulation frequencies to derive the PiFM value.

13. 13. A scanning probe microscope as claimed in claim 11 or 12, wherein the vibrational response of the cantilever (204) comprises a response to a force or force gradient excited by the modulated electromagnetic radiation at an interface between the probe tip (202) and the sample (208).

14. 14. The scanning probe microscope of claim 13, wherein the amplitude measurement system includes a mixer that mixes the modulation frequency of the light source (206) with a cantilever dither frequency to derive a sum or difference frequency used to detect the force or force gradient.

15. 15. The scanning probe microscope of claim 14, wherein the amplitude measurement system includes a lock detector (230) coupled to the detector (219) to receive a detected vibration response of the cantilever (204), the lock detector (230) also coupled to the mixer to receive a sum or difference frequency used as a reference for lock detection.

16. 12. The scanning probe microscope of claim 11, wherein the signal generator (226, 440) is configured to apply a modulated signal to the light source (206) such that the modulated electromagnetic radiation simultaneously has the range of frequencies, and the amplitude measurement system includes an amplitude detector that simultaneously measures an amplitude of the vibrational response of the cantilever (204) in response to the modulated electromagnetic radiation to derive the PiFM value.

17. 17. A scanning probe microscope as claimed in claim 16, wherein the signal generator (226, 440) is configured to generate a modulated waveform signal comprising a sum or difference of a plurality of frequency components within the frequency range.

18. 20. A scanning probe microscope as claimed in claim 17, wherein the signal generator (226, 440) is configured to generate the modulated waveform by an inverse Fourier transform of a spectral density of the range of applied frequencies.

19. 17. The scanning probe microscope of claim 16, wherein the amplitude measurement system includes an amplitude detector and a bandpass filter (442) that measures the amplitude of the vibration response of the cantilever (204).

20. A scanning probe microscope according to any one of claims 11 to 19, wherein the light source (206) is a tunable infrared (IR) laser.

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