Atomic force microscope that uses a single light source to drive cantilever vibration and detect cantilever movement

Abstract

Atomic force microscopes (AFMs) and methods for operating AFMs use a light source on a cantilever having a tip to induce cantilever oscillation and measure the cantilever motion. A drive system is used to modulate the light source so that the cantilever oscillates and the light source can measure the cantilever motion.

Description

[Technical Field]

[0001] This application claims the benefit of U.S. Provisional Patent Application No. 63 / 443,634, filed February 6, 2023, which is incorporated herein by reference. [Background technology]

[0002] In atomic force microscopes (AFMs), a light source such as a focused laser diode is often used in conjunction with an optical position-sensing detector to detect cantilever motion in an optical lever configuration. Many AFM operating modes, such as non-contact or intermittent contact topographic modes, typically require the cantilever to oscillate at one or more of its resonant frequencies. The most common approach is to drive the cantilever oscillation using a piezoelectric transducer coupled to the cantilever-mounting tip.

[0003] Because cantilevers are consumables that need to be replaced frequently and because resonant frequencies vary from cantilever to cantilever, the cantilever resonant frequency must be determined in order to select an appropriate frequency for driving the cantilever oscillation. This is often done by applying a frequency-swept drive voltage to a piezoelectric transducer and using an optical lever to observe the resonance as a peak in the vibration amplitude as a function of frequency. In addition to observing the cantilever resonance, such an approach often finds additional resonances of surrounding structures, making it difficult to determine which peaks are true cantilever resonances and which are resonances of the surrounding structures.

[0004] One well-accepted solution to this problem is to not use a piezoelectric transducer to drive the cantilever vibration, but instead use a modulated laser beam focused on the cantilever beam to excite the vibration. The absorption of the modulated light results in modulated heating of the cantilever, which in turn excites the vibration through modulated bending of the cantilever. This approach is used to excite only the cantilever and not the resonances of surrounding structures. The disadvantage of this approach is that it requires two light sources: one to measure the cantilever motion and a second to excite the cantilever vibration. Typically, using two light sources, including two laser diodes and associated focusing optics, adds cost and complexity to the AFM head design. Summary of the Invention

[0005] An atomic force microscope (AFM) and method for operating an AFM uses a light source on a cantilever having a tip to induce cantilever vibration and measure the cantilever motion. A drive system is used to modulate the light source so as to induce cantilever vibration and so that the light source can measure the cantilever motion.

[0006] An AFM according to one embodiment of the present invention includes a cantilever having a tip adapted to engage a sample of interest, a light source that emits light onto the cantilever, and a drive system operatively configured to modulate the light source to induce cantilever vibration and to enable the light source to measure the cantilever motion.

[0007] According to one embodiment of the present invention, a method of operating an AFM includes modulating a light source directed at a cantilever having a tip to induce cantilever vibration upon engagement of the tip with a target sample and to measure the cantilever movement, detecting light from the light source reflected from the cantilever to measure the cantilever movement, and using the measured cantilever movement to obtain AFM data of the target sample.

[0008] An AFM according to one embodiment of the present invention includes a cantilever having a tip adapted to engage a sample of interest, a light source adapted to emit light onto the cantilever, a drive system configured to operate the light source at a constant intensity for a first time period followed by a second time period during which the light intensity of the light source is reduced, the second time period inducing cantilever oscillation and the first time period being used to measure cantilever motion, a photodetector positioned to detect light reflected from the cantilever to measure cantilever motion, and a processing unit operatively coupled to the photodetector to receive the measured cantilever motion to obtain AFM data of the sample of interest.

[0009] Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention. [Brief explanation of the drawings]

[0010] [Figure 1] 1 illustrates an atomic force microscope (AFM) according to one embodiment of the present invention. [Figure 2] 2 is a pulse timing diagram for signals used in the AFM shown in FIG. 1, in accordance with one embodiment of the present invention. [Figure 3] 2 is a flowchart for operating the AFM shown in FIG. 1 in accordance with one embodiment of the present invention. [Figure 4]1 is a process flow diagram of a method of operating an AFM, according to one embodiment of the present invention.Like reference numerals may be used throughout this description to identify like elements. DETAILED DESCRIPTION OF THE INVENTION

[0011] An atomic force microscope (AFM) according to an embodiment of the present invention uses a single modulated laser diode focused on the cantilever to drive cantilever oscillation at the cantilever's resonant frequency by using low-duty-cycle modulation of the laser beam at a subharmonic of the cantilever resonance. In the simplest embodiment, the laser runs at constant intensity for N-1 cycles (first time interval) of cantilever oscillation, then turns off or reduces in intensity for part of a single cycle (second time interval), before turning on again at constant intensity to begin the next sequence; this sequence is repeated every N cycles. This allows F R A driving force is generated on the cantilever at a frequency of 1 / N, where F R is the resonant frequency of the cantilever.

[0012] Detection of cantilever motion is typically achieved by an optical sensor or detector in an optical lever configuration while the laser runs at constant intensity for N-1 cycles. A blanking function prevents detection of motion during individual cycles when the laser is turned off or its intensity is reduced. This can be achieved, for example, by using a lock-in detector or amplifier, a typical approach for detecting cantilever vibration, but turning off the input for exactly one cycle surrounding the moment the laser light is turned off. Because driving a cantilever at a subharmonic of the resonant frequency is inherently less efficient than driving it at the resonant frequency itself, cantilevers can be designed to increase actuation efficiency by using semi-reflective coatings of specific materials to increase absorption of light at the laser wavelength and selecting the thickness of the coating to increase mechanical excitation due to light absorption. The position at which the light is focused along the cantilever beam can be selected to provide a good compromise between good actuation efficiency and a good signal-to-noise ratio (SNR) when detecting cantilever motion. The amplitude of the cantilever oscillation can be controlled by adjusting the duration for which the light source is off. R A variation between 0 and 50% of one cycle of corresponds to a range from 0 to the maximum available amplitude for a given subharmonic factor N. In addition to, or instead of, varying the duration of the reduced intensity, the amount of intensity reduction can be adjusted to control amplitude, for example, by reducing the power to the light source.

[0013] Alternatively, the laser intensity during the constant intensity interval may be used to control the amplitude of the vibration. Higher intensity light generates larger vibrations. However, changing the light intensity also affects the sensitivity of the vibration measurement. A third way to change the amplitude of the vibration is to change the factor N, where a larger N results in a smaller vibration amplitude and a smaller N results in a larger amplitude. However, the factor N also affects the measurement SNR, with a smaller N resulting in a lower SNR. For this reason, it may be preferable to change the duty cycle of the interruption or the amount of reduction in light intensity during the interruption, since the method used to control the amplitude does not interfere with the measurement of the deflection.

[0014] 1, an AFM 100 includes a force-sensing cantilever 102 having a sharp tip 104 facing a target sample 106 and an optically reflective surface 108 facing away from the sample. Light from a light source 110, such as a laser, is focused onto the reflective surface 108 of the cantilever 102 using a focusing lens 112 and is reflected from the reflective surface to a position-sensitive photodetector 114. When the tip engages the sample 106, forces acting on the cantilever tip 104 cause the cantilever 102 to flex or bend, changing the angle of the reflective surface 108 on the cantilever, which in turn changes the angle of the reflected beam and the center position of the beam on the position-sensitive photodetector 114. This combination of the light source 110, focusing lens 112, the reflective surface 108 on the short cantilever 102, and the slightly distant position-sensitive photodetector 114 constitutes an "optical lever" system 116. Because the length of the cantilever 102 is very short compared to the optical path length between the cantilever and the photodetector 114, such an optical "lever" amplifies the motion of the cantilever tip 104, converting small movements of the tip into much larger movements of the center of the beam on the photodetector 114. This type of "optical lever" is the most commonly used method of detecting cantilever motion in AFMs. Alternatively, an optical interferometer may be used to detect cantilever motion. In either case, a light source such as a laser is used.

[0015] The light source 110 is driven by a laser driver 118, which sets the light source intensity, which can be modulated by an input signal. In the prior art, a system called blueDrive™ by Oxford Instruments Asylum Research modulates the light source intensity at the cantilever's resonant frequency. The time-varying intensity of the modulated light causes time-varying thermal expansion of the material within the cantilever. If this expansion is not uniform throughout the cantilever, it causes time-varying bending of the cantilever, exciting it to oscillate at the resonant frequency. Specifically, if light causes non-uniform expansion throughout the thickness of the cantilever beam, the periodic bending of the cantilever causes it to oscillate. This non-uniformity in expansion can arise simply because the side of the cantilever facing the light source absorbs more light than the side of the cantilever facing away from the light source. The non-uniformity can be improved by selecting a reflective coating or other layer within the cantilever's structure to have greater or less optical absorption than the rest of the materials comprising the cantilever beam.

[0016] Because the methods described herein operate at subharmonic frequencies with low duty cycles, which can result in relatively inefficient cantilever vibration drive compared to prior art blueDrive systems, embodiments of the present invention use a silicon cantilever beam coated with a metallic reflective layer on the cantilever 102. This metallic reflective layer is selected to have higher optical absorption and lower optical reflectance than metals selected solely to maximize reflectance at wavelengths of light typically used in red or infrared light in the 600 nm to 1000 nm wavelength range from laser diodes. In certain embodiments, this metallic reflective layer is selected from among chromium, rhodium, platinum, platinum-iridium, nickel, and / or molybdenum materials, all of which have lower reflectance and higher absorption than aluminum, silver, or gold, which are commonly used on cantilevers to maximize reflectance. However, if sufficient vibration amplitude is achievable with the available system parameters and sufficient reflectance is present for the optical lever detection system to operate as desired, highly reflective materials or no reflective coating may be used. Almost all metallic reflective materials have a reflectivity of about 5 x 10 -6 Chromium has a higher coefficient of thermal expansion (CTE) than silicon, which has a value of 5E-6 / °C or 1 / °F. For example, nickel has a CTE of 13E-6 / °C, and platinum has a value of 9E-6 / °C. While this value is lower than the highest reflectivity materials such as aluminum (Al) (CTE = 24E-6 / °C) and gold (Au) (CTE = 14E-6 / °C), their higher absorption more than compensates for the reduced CTE. Additionally, the high modulus of elasticity of materials such as platinum, platinum-iridium, nickel, and chromium makes them more efficient at driving cantilever oscillation according to embodiments of the present invention than highly reflectivity materials such as aluminum and gold.

[0017] The choice of where to focus light onto the cantilever beam 102 also affects the performance of embodiments of the present invention. With the blueDrive system, two separate light sources are used, and each may be focused at a different point along the cantilever beam to maximize the efficiency of the drive light source (e.g., focused near the fixed base of the cantilever for the cantilever's lowest frequency vibration mode, or near a point of high curvature for the particular vibration mode of the cantilever being driven) and the motion-sensing light source (e.g., focused near the free end of the cantilever for the lowest frequency mode, or near a point of high slope for the particular mode being driven). Because embodiments of the present invention have a single light source, i.e., light source 110, a compromise may be necessary where light is focused at a point between the point of high curvature and the point of high slope. When the cantilever is driven to oscillate at a resonant frequency higher than its lowest resonant frequency, more complex vibrational mode shapes must be considered when selecting an appropriate compromise position of the light for simultaneous drive and sensing. Examples of complex vibrational mode shapes that can be used are described in Raman et al., "Cantilever dynamics in atomic force microscopy," Nano Today, February-April 2008, which is incorporated herein by reference.

[0018] The sample of interest 106 can be positioned on a sample z piezo device 120, which can move the sample in the z direction, i.e., toward or away from the tip 104. The sample z piezo device 120 is controlled by a sample z controller 122, which can move the sample z piezo device as needed.

[0019] To drive cantilever 102 to generate cantilever oscillations and sense cantilever motion using single light source 110, AFM 100 includes oscillator 124, divide-by-N circuit 126, divide-by-2 circuit 128, one-shot circuit 130, set-reset (SR) flip-flop 132, and lock-in amplifier 134. The operation of these components is described below with reference to Figure 2, which is a pulse timing diagram for signals used in AFM 100, according to an embodiment of the present invention.

[0020] 1 and 2, oscillator 124, which may be a synthesized frequency generator, generates the resonant frequency F of cantilever 102. R This signal serves as a reference signal for a lock-in amplifier 134, which is used to measure the amplitude of the oscillations using the signal from the photodetector 114. To generate the subharmonic frequencies, a divide-by-N circuit 126, which may be a self-resetting counter circuit, generates a subharmonic frequency Fs, which is F R The maximum excitation of cantilever vibration for a fixed value of N is equal to F when light source 110 emits a pulse F with each pulse from one-shot circuit 130. This signal in turn triggers one-shot circuit 130, which has an adjustable pulse width. The pulse width of one-shot circuit 130 is controlled by an amplitude control signal applied to the pulse width input of the one-shot circuit. The output of one-shot circuit 130 in turn turns off or reduces the intensity of light source 110 during the pulse. The maximum excitation of cantilever vibration for a fixed value of N is equal to F when light source 110 emits a pulse F with each pulse from one-shot circuit 130. RThis occurs when the laser turns off for a duration of 1 / 2 cycle of the pulse. The amplitude of the oscillation can be reduced by selecting an intensity higher than 0 during the pulse or by using pulses shorter than 1 / 2 cycle. The light source 110, which may include a laser diode with electronic drive circuitry, can have a small effective time delay as it turns off at the start of the pulse and turns on again after the end of the pulse to stabilize. Therefore, the pulse length and timing can be adjusted to compensate for this effect. Coarse adjustment of the oscillation amplitude can be made by changing the value of N for the divide-by-N circuit 126; a higher N results in a lower amplitude.

[0021] As described above, oscillator 124, divide-by-N circuit 126, one-shot circuit 130, and laser driver 118 operate to not only generate cantilever oscillations but also to sense cantilever motion as tip 104 of cantilever 102 engages sample 106. These components can thus be considered to form a laser or subharmonic drive system that drives light source 110 to generate cantilever oscillations and measure cantilever motion.

[0022] Since the same light source is used to detect cantilever motion, care must be taken to ensure that the detection system operates properly with a modulated light source. Typically, in an AFM, the light source has a constant intensity. A solution that allows modulation of the light source 110 effectively disables the input of the lock-in amplifier 134 (i.e., sets the input signal level to 0) while the intensity of the light source is reduced or turned off. As used herein, reducing the intensity of the light source (i.e., the intensity of the light from the light source) can include turning the light source off. Ideally, the input of the lock-in amplifier 134 (the "lock-in input") is F R The light source 110 is disabled for a time interval having a duration of one full cycle of F, which time interval includes all times that the intensity of the light source 110 is reduced or turned off. RA complete cycle of is equal to or longer than the modulation pulse time (typically F R Since the pulse duration is typically less than 1 / 2 cycle of F, additional time may be used to disable the lock-in input during the time after the pulse when the laser diode has not yet fully stabilized at a constant intensity. Disabling the lock-in input is accomplished by a blanking input in the lock-in amplifier 134 (or a separate circuit before the input of the lock-in amplifier), which effectively forces the input signal to zero during the blanking signal pulse. R A blanking pulse having a duration of one complete cycle of the laser 110 can be generated by the SR flip-flop 132, where the output is a logic low signal when the R input is triggered and a logic high signal when the S input is triggered. When the R input of the SR flip-flop 132 is coupled to the divide-by-N circuit 126 and the S input of the SR flip-flop 132 is coupled to the divide-by-2 circuit 128, the pulse timing diagram of Figure 2 applies, and the blanking signal (i.e., the output of the SR flip-flop 132) goes low for one cycle or approximately (±10%) one cycle at the start of each modulation pulse of the laser 110. The lock-in amplifier 134 is configured so that its input is disabled when the blanking signal is a logic low signal.

[0023] Disabling the lock-in input with a blanking pulse for a single cycle every N cycles reduces the amplitude of the signal detected at the output of lock-in amplifier 134 by a factor of (N-1) / N. For a relatively large value of N, such as 10, this reduction in output is only 10%, which has only a slight effect on the detected signal-to-noise ratio. If N is reduced to 2, the minimum value for which embodiments of the present invention can typically function, then the detected amplitude will decrease by 50%, and the reduction in signal-to-noise ratio will be even greater. The output filter of lock-in amplifier 134 is F R The frequency band must be adjusted to filter all frequencies higher than F / N. Therefore, if N is large, the detection bandwidth can be greatly reduced. However, in most practical AFM setups, FR is 10-1000 times higher than the detection bandwidth required to properly operate the AFM, so a value of N=10 has little or no effect on the detection bandwidth, which can limit the operating speed of the AFM 100.

[0024] The output of the lock-in amplifier 134, a measured amplitude signal, is sent to a processing unit 136, where the measured amplitude signal may be used to obtain AFM data of the sample 106, such as image data or measurements, using existing methods. The processing unit 136 may be any device that has one or more processors and / or controllers that not only process the measured amplitude signal but also control various components of the AFM 100. One particular control signal provided by the processing unit 136 is an amplitude control signal to the one-shot circuit 130 for adjusting the output pulse width.

[0025] Those skilled in the art will appreciate that many variations of the system shown in FIG. 1 can be used to achieve subharmonic driving of cantilever vibration and sensing of cantilever motion with a single light source.

[0026] 3 shows a flowchart for operating AFM 100 in accordance with an embodiment of the present invention. In step 302, a type of cantilever is selected for use with AFM 100. Cantilevers are typically commercially produced, with each type having a known frequency range for vibration modes of interest.

[0027] Next, in step 304, a particular vibration mode to be used for the AFM 100 is selected. In the case of Photo-Induced Force Microscopy (PiFM), the second vibration mode is often selected. Once a mode is selected, the possible frequency range for that mode is known. The frequency range may be provided by the manufacturer or may have been determined in previous experiments.

[0028] Next, in step 306, a frequency sweep is performed through the target frequency range while driving the cantilever 102 at a relatively large amplitude using the subharmonic drive system of the AFM 100 to facilitate low-noise detection of cantilever motion. In this step, a low value of N, such as N=2, and a 50% duty cycle can be used to drive the large amplitude. Amplitude versus frequency data is recorded during the sweep.

[0029] After sweeping, the resonant peak frequency of the vibration mode is determined by observing which frequency has the highest amplitude in step 308. The width of the resonant peak and other characteristics of the resonant peak may also be determined to select an appropriate drive frequency.

[0030] Next, in step 310, an appropriate drive frequency is selected using the known resonant frequency and peak shape, as in commonly used AFM operation modes. In commonly used amplitude detection modes, the drive frequency is generally slightly higher than the resonant peak frequency to somewhat reduce the slope of the amplitude decrease in the frequency region directly above the resonant peak.

[0031] Next, in step 312, a target vibration amplitude is selected when the tip 104 is far from the sample surface. Many factors influence the selection of the amplitude. For PiFM, the target amplitude is often in the range of 0.2-2 nm RMS (root mean square).

[0032] Next, in step 314, the amplitude of the vibration is adjusted to match the target amplitude using the methods described above. For example, the duty cycle of the time that the laser 110 is turned off or the amplitude is reduced can be adjusted to reach the target amplitude. An initial value of N (e.g., N=8) may be used to provide an adequate SNR for measuring the amplitude during normal AFM operation.

[0033] Next, in step 316, the maximum duty cycle (time off=F RIf the target amplitude cannot be reached within 50% of one section in (N), repeatedly select a lower N value until the target amplitude can be reached, down to a minimum value of N=2. Since lowering N reduces the SNR of amplitude detection, it is optimal to use the largest N value that can reach the target amplitude up to a certain point. Beyond N=8, the improvement in SNR is small, so there is no need to use values ​​greater than 8. At this point, the AFM is ready for normal operation.

[0034] Next, in step 318, using existing methods, the tip is brought close to the sample surface ("approach").

[0035] After approximation, in step 320, the tip 104 engages the sample surface, which represents closed-loop control of the tip-sample using changes in the amplitude of the cantilever oscillation as a measure of changes in the tip-sample spacing. The tip-sample spacing can be adjusted in different ways depending on the AFM operating mode.

[0036] After application, in step 322, an AFM image can be acquired by scanning the position of the tip 104 relative to the surface of the sample, acquiring a PiF image, or performing other AFM measurements of interest, such as measuring a PiF spectrum at a specific point on the sample surface.

[0037] 4, a flowchart of a method for operating an AFM, such as AFM 100, is shown in accordance with one embodiment of the present invention. In block 402, a light source directed at a cantilever having a tip is modulated to induce cantilever vibration and measure cantilever motion due to engagement of the tip with a target sample. In block 404, light from the light source reflected from the cantilever is detected to measure cantilever motion. In block 406, the measured cantilever motion is used to acquire AFM data of the target sample.

[0038] 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 merely to illustrate various embodiments. While various aspects of the embodiments are presented in the drawings, the drawings are not necessarily drawn to scale unless specifically indicated.

[0039] The present invention may be embodied in other specific forms without departing from its spirit or essential characteristics. The described embodiments are to be considered in all respects as merely illustrative and not restrictive. The scope of the present invention is, therefore, indicated by the appended claims, not by this detailed description. All changes that come within the meaning and range of equivalency of the claims are to be embraced within their scope.

[0040] Throughout this specification, references to features, advantages, or similar language do not imply that every feature and advantage that can be realized by the present invention must be included in every single embodiment of the present invention. Rather, terms referring to features and advantages are understood to mean that the particular feature, advantage, or characteristic described with respect to an embodiment is included in at least one embodiment of the present invention. Thus, throughout this specification, discussions of features and advantages and similar language can, but do not necessarily, refer to the same embodiment.

[0041] 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 art will recognize, based on the description herein, that the invention can be practiced without one or more particular 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.

[0042] Throughout this specification, references to "one embodiment," "an embodiment," or similar language mean that a particular feature, structure, or characteristic described in connection with the indicated embodiment is included in at least one embodiment of the invention. Thus, throughout this specification, "in one embodiment," "in an embodiment," and similar language can, but do not necessarily, all refer to the same embodiment.

[0043] In the foregoing description, specific details of various embodiments have been provided. However, some embodiments may be practiced with less than all such specific details. In other instances, specific methods, procedures, components, structures and / or functions have not been described in any more detail than is necessary to enable various embodiments of the invention, for the sake of brevity and clarity.

[0044] Although method operations are 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 partially concurrently with other operations. In other embodiments, instructions or sub-operations of separate operations may be implemented in an intermittent and / or alternating manner.

[0045] It should also be noted that at least a portion 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. As an example, a computer program product embodiment includes a computer-usable storage medium having a computer-readable program stored thereon.

[0046] Alternatively, embodiments of the present invention may be implemented entirely in hardware, or in an implementation containing both hardware and software elements. In software embodiments, the software may include, but is not limited to, firmware, resident software, microcode, etc.

[0047] Although specific embodiments of the present invention have been described and illustrated, the invention is not limited to the specific forms or arrangements of parts so described and illustrated, and the scope of the present invention is to be defined by the claims appended hereto and their equivalents.

Claims

1. In an atomic force microscope (AFM), a cantilever having a tip adapted to be engaged with a target sample; a light source that emits light onto the cantilever; a drive system operatively configured to modulate a light source so as to induce cantilever oscillation and so as to enable the light source to measure cantilever motion.

2. 10. The atomic force microscope (AFM) of claim 1, wherein the drive system is configured to modulate the light source at a subharmonic frequency of a cantilever resonance frequency to induce the cantilever oscillation.

3. 3. The atomic force microscope (AFM) of claim 2, wherein the light source is part of an optical lever system that uses a position-sensitive photodetector to detect the cantilever motion.

4. 3. The atomic force microscope (AFM) of claim 2, wherein the drive system is configured to operate the light source at a constant intensity for multiple cycles of the cantilever resonant frequency, followed by periodic partial cycles in which the light intensity from the light source is reduced or turned off, thereby modulating the light source at a subharmonic frequency of the cantilever resonant frequency.

5. 5. The atomic force microscope (AFM) of claim 4, wherein the drive system is configured to control the amplitude of the cantilever oscillation by varying the time length of the partial cycle when the light intensity is reduced or turned off.

6. 5. The atomic force microscope (AFM) of claim 4, wherein the drive system is configured to control the amplitude of the cantilever oscillation by varying the amount of decrease in light intensity during the partial cycle when the light intensity is decreased.

7. 5. The atomic force microscope (AFM) of claim 4, wherein the drive system is configured to control the amplitude of cantilever oscillation by changing the value of the constant intensity outside of the partial cycle when the light intensity is reduced or turned off.

8. 3. The atomic force microscope (AFM) of claim 2, wherein the drive system is configured to allow the amplitude of cantilever oscillation to be roughly adjusted by selecting the particular subharmonic frequency at which the light source is modulated.

9. 4. The atomic force microscope (AFM) of claim 3, wherein the drive system is configured to measure the amplitude of the cantilever motion using a lock-in amplifier acting on a signal generated by the position-sensitive photodetector, the lock-in amplifier having a reference signal at a cantilever oscillation frequency.

10. 10. The atomic force microscope (AFM) of claim 9, wherein the drive system is configured to operate the light source at a constant intensity for multiple cycles of the cantilever resonant frequency, followed by periodic partial cycles in which the light intensity from the light source is reduced or turned off, to modulate the light source at a subharmonic frequency of the cantilever resonant frequency.

11. 11. The atomic force microscope (AFM) of claim 10, wherein the input signal to the lock-in amplifier is set to zero by a blanking signal during a time interval comprising a partial cycle in which the light intensity is reduced or turned off.

12. 12. The atomic force microscope (AFM) of claim 11, wherein the time interval during which the input signal is set to zero has a duration of about one cycle of the cantilever resonant frequency.

13. 10. The atomic force microscope (AFM) of claim 1, wherein the cantilever has a reflective coating comprising chromium, rhodium, platinum, platinum-iridium, nickel, or molybdenum.

14. 1. A method of operating an atomic force microscope (AFM), comprising: modulating a light source directed at a cantilever having a tip to induce cantilever oscillation upon engagement of the tip with a sample of interest and to measure the cantilever motion; detecting light from the light source reflected from the cantilever to measure the cantilever motion; and using the measured cantilever movement to obtain AFM data of the target sample.

15. 15. The method of operating an atomic force microscope (AFM) of claim 14, wherein modulating the light source comprises modulating the light source at a subharmonic frequency of a cantilever resonant frequency to induce the cantilever oscillation.

16. 16. A method of operating an atomic force microscope (AFM) as described in claim 15, wherein modulating the light source at a subharmonic frequency of the cantilever resonant frequency comprises operating the light source at a constant intensity for multiple cycles of the cantilever resonant frequency, followed by periodically performing partial cycles in which the light intensity from the light source is reduced or turned off.

17. 17. The method of operating an atomic force microscope (AFM) of claim 16, further comprising controlling the amplitude of the cantilever oscillation by varying the length of time of the partial cycle when the light intensity is reduced or turned off, varying the amount of reduction in light intensity during the partial cycle when the light intensity is reduced, or varying the value of the constant intensity outside of the partial cycle when the light intensity is reduced or turned off.

18. 15. The method of operating an atomic force microscope (AFM) of claim 14, further comprising measuring the amplitude of the cantilever motion using a lock-in amplifier operating on a signal produced by a position sensitive photodetector, the lock-in amplifier having a reference signal at the cantilever oscillation frequency.

19. 20. A method of operating an atomic force microscope (AFM) as described in claim 18, wherein modulating the light source at a subharmonic frequency of the cantilever resonant frequency includes operating the light source at a constant intensity for multiple cycles of the cantilever resonant frequency, followed by periodically executing partial cycles in which the light intensity from the light source is reduced or turned off, and wherein the input signal to the lock-in amplifier is set to 0 by a blanking signal during time intervals that include the partial cycles in which the light intensity is reduced or turned off.

20. In an atomic force microscope (AFM), a cantilever having a tip adapted to be engaged with a target sample; a light source that emits light onto the cantilever; a drive system configured to operate the light source at a constant intensity for a first time interval followed by a second time interval during which the light intensity from the light source is reduced, the second time interval being used to induce cantilever oscillation and the first time interval being used to measure cantilever motion; and a photodetector positioned to detect light reflected from the cantilever to measure the cantilever motion; a processing unit operatively coupled to the photodetector to receive the measured cantilever movement to obtain AFM data of the target sample.

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