Nanoobject trap and method of confining a nanoobject

Abstract

A method of confining a nanoobject and a nanoobject trap for an optically levitated elongated nanoobject are disclosed. The nanoobject has an aspect ratio greater than two and said nanoobject trap comprises: a chamber for containing said nanoobject; means for generating a harmonic potential for confining said nanoobject in said nanoobject trap; and a feedback signal generator. The feedback signal generator comprises an optical detector for detecting oscillation of said nanoobject, said feedback signal generator being configured to generate a feedback signal in dependence upon said detected oscillation; and an all-pass filter for generating a phase change of in said feedback signal such that said feedback signal is substantially ninety degrees out of phase with a first harmonic of said detected oscillation. A modifier modifies said harmonic potential in dependence upon said feedback signal.

Description

[0001] M24B154 / JM - 1 -

[0002] NANOOBJECT TRAP AND METHOD OF CONFINING A NANOOBJECT

[0003] FIELD OF THE INVENTION

[0004] The field of the invention relates to trapping an elongated nanoobject.

[0005] BACKGROUND

[0006] Nanoobjects or particles may be levitated and confined in harmonic fields such as those generated by lasers. Counterpropagating laser beams may interfere to form a standing wave and a nano particle may be levitated and confined between nodes of the standing wave in an optical trap. The nano particle or object will oscillate along the direction of the optical axis between the nodes. Collisions with gas particles will dampen these oscillations. Where the pressure is reduced the damping effect is reduced and the particles or nanoobjects may escape from the trap. Elongated nanoobjects with a high aspect ratio may be particularly unstable and difficult to retain in a trap at low pressures.

[0007] It would be desirable to be able to maintain a nanoobject within a trap over a range of pressures.

[0008] SUMMARY

[0009] One aspect provides a nanoobject trap for an optically levitated elongated nanoobject having an aspect ratio greater than two, said nanoobject trap comprising: a chamber for containing said nanoobject; means for generating a harmonic potential for confining said nanoobject in said nanoobject trap; a feedback signal generator comprising an optical detector for detecting oscillation of said nanoobject, said feedback signal generator being configured to generate a feedback signal in dependence upon said detected oscillation, and an all-pass filter for generating a phase change in said feedback signal such that said feedback signal is substantially ninety degrees out of phase with a first harmonic of said detected oscillation; and a modifier for modifying said harmonic potential in dependence upon said feedback signal.M24B154 / JM - 2 -

[0010] A nanoobject may be confined or trapped within nodes of a harmonic potential. The velocity of the particle as it oscillates reaches a minimum at either end of the oscillation and a maximum towards the centre. Thus, the velocity is substantially 90° out of phase with the position of the nanoobject from the centre of oscillation. When the pressure of the gas in the chamber is relatively high, such as towards atmospheric pressure then contact with gas molecules damps the oscillations and the particle is held relatively stably between the two nodes. Where the pressure is reduced the damping decreases and there is a risk that the oscillations extend beyond one of the nodes and the particle or nanoobject escapes and is no longer trapped. One way to address this may be to modify the harmonic field such that it increases as the nanoobject approaches a node.

[0011] Such a modification that is dependent on the position of the nanoobject could be generated by detecting the position of the nanoobject using an optical detector which may detect side scattered light and generating a feedback signal based on the detected position and using the feedback signal to modify the harmonic potential. The modifier may be configured to modify a strength of the harmonic potential in dependence upon the feedback signal, the feedback signal may depend on the detected position.

[0012] A suitable modification may be generated by passing the detected signal through an all-pass filter configured to provide a phase offset or lag, this phase offset or lag can be set to be 90° with respect to the first harmonic of the oscillation such that the magnitude of the signal increases as the particle or nanoobject approaches a node.

[0013] Where the position detected is the position from the centre of the oscillation or the position relative to the antinode, then this detection signal can be used to generate a feedback signal to modify the harmonic potential, such that the harmonic potential is increased as the particle or nanoobject reaches the node providing a kick to the particle to push it back into the trap. In this way theM24B154 / JM - 3 -

[0014] amplitude of the oscillations is reduced and the particle is slowed which is viewed as cooling.

[0015] The cooling or reduction in oscillatory movement of the nanoobject may be done by modifying the harmonic potential confining the nanoobject at a frequency that is at and / or is double a frequency of the oscillation. As the oscillation of concern for retaining the nanoobject within the trap is the translational oscillation then the modifying should be at double the translational oscillatory frequency so that the kick or increased force is applied to the nanoobject at either end of the oscillation. This doubling in frequency may be derived from the optical detector being configured to detect a second harmonic of the oscillation frequency of the nanoobject. Alternatively, it may be derived from the optical detector being configured to detect a first harmonic of the oscillation and the feedback signal generator comprising a frequency multiplier for doubling the frequency of the detected signal.

[0016] Directly detecting the second harmonic of the nanoobject’s oscillation is a low noise way of determining what is in effect a derivative of the position as forming derivatives with electronics creates noise. Furthermore, it requires fewer components and is therefore more cost effective.

[0017] It should be noted that an all-pass filter to generate the phase lag is used such that the overall phase lag between the first harmonic of the oscillation and the modifying signal is substantially 90 degrees. Where the detected signal is the second harmonic of the oscillation then the phase lag will be 180 degrees between the detected signal and the modifying signal. Where the other circuitry in the system that processes and transmits the feedback signal introduces some phase shift, then the phase shift of the all pass filter may be set such that overall the signal for modifying the harmonic potential is 90 degrees out of phase with the first harmonic of the frequency of oscillation of the nanoobject. A phase shift of between 80° and 100° may provide an acceptable modification signal, preferably between 85 and 95°.M24B154 / JM - 4 -

[0018] This method allows for effective feedback even at higher pressures of 1 -WmBar where the peaks of the particle motion in frequency space are relatively broad and conventional lock-in amplifiers cannot operate well.

[0019] In some embodiments, said nanoobject comprises an object with an aspect ratio greater than three, preferably greater than five.

[0020] A high aspect ratio particle is more sensitive to interaction with molecules and thus, were the trap to be used to measure pressure or torque then such a high aspect ratio particle would be effective. However, problems associated with such particles is that they are particularly unstable in these traps particularly at lower pressures where the gas no longer damps the motion. Thus, embodiments are particularly suitable for use with such nanoobjects as they can be held securely within the trap using the feedback signal.

[0021] A nanoobject is an object with dimensions of the order of nanometres, in some cases dimensions between 1 and 5,000 nanometres, at least one dimension being less than 1,000 nm.

[0022] Although, the nanoobject may have a number of forms, in some embodiments the nanoobject comprises a cylinder.

[0023] The nanoobject may be formed of silicon and may have a diameter of between 80 and 200 nm and a length of between 500 and 1 ,500 nm.

[0024] The chamber may comprise a vacuum chamber.

[0025] In some embodiments, said means for confining said nanoobject in a harmonic potential comprises at least one laser for generating a laser beam; andM24B154 / JM - 5 -

[0026] optical components for channelling and focussing said laser beam within said chamber, such that said nanoobject is levitated and confined during operation of said nanoobject trap.

[0027] In some embodiments said nanoobject trap comprises an optical trap.

[0028] In some embodiments, said optical components are configured to channel said laser beams to form counterpropagating beams that interfere to form a standing wave said nanoobject being confined between nodes of said standing wave.

[0029] In some embodiments said counterpropagating beams are generated by one laser, said nanoobject trap comprising a mirror for reflecting said laser beam to generate said counter propagating beam.

[0030] In some embodiments, the laser is a laser suitable for telecoms use with a wavelength of between 1 ,300 and 1 ,700 nm, in some cases 1 ,550 nm.

[0031] In some embodiments, said at least one laser comprises drive circuitry for generating pulses of circularly polarised light, such that said laser beam switches between a linearly polarised and a circularly polarised beam, said polarisation switching causing said nanoobject to rotate about the optical axis, said nanoobject comprising a nanorotor.

[0032] The switching between circularly polarised and linearly polarized light provides a rotational kick to the nanoobject at a frequency that is substantially equal to half the frequency of the pulses.

[0033] In some embodiments, the frequency of rotation may be between 0.1 and 1 ,000 MHz and be frequency locked to an external reference with extreme stability such that a linewidth of the order of microHz may be provided.M24B154 / JM - 6 -

[0034] A further aspect provides a sensor comprising a nanoobject trap according to one aspect, said sensor further comprises a detector for detecting rotation of said nanorotor and for determining a phase lag between said pulsed driving signal and said rotation.

[0035] In some embodiments, said detector comprises said optical detector and is configured to receive a signal indicative of the pulsed drive signal and is configured to determine a phase lag between said detected rotation and said pulsed drive signal.

[0036] In some embodiments, said sensor comprises a pressure sensor, said phase lag being indicative of a pressure within the chamber.

[0037] In some embodiments, said sensor comprises a torque sensor said phase lag being dependent on a torque exerted on said nanorotor.

[0038] In some embodiments, said torque is indicative of a flow rate of a gas.

[0039] A yet further aspect provides a method of stabilising an optically levitated elongated nanoobject having an aspect ratio greater than two, said method comprising: generating a harmonic potential for confining said nanoobject; detecting oscillation of said nanoobject; generating a feedback signal in dependence upon said detected oscillation; passing said feedback signal though an all-pass filter to generate a phase change in said feedback signal such that said feedback signal is substantially ninety degrees out of phase with a first harmonic of said detected oscillation; and modifying said harmonic potential in dependence upon said feedback signal, such that an amplitude of said oscillations is reduced.

[0040] In some embodiments, said harmonic potential for confining said nanoobject is generated by a laser beam, said method further comprising: channelling andM24B154 / JM - 7 -

[0041] focussing said laser beam within a vacuum chamber containing said nanoobject, said nanoobject being levitated and held by said harmonic potential.

[0042] In some embodiments, said method further comprises causing said levitated nanoobject to rotate about the optical axis by driving said laser to generate pulses of circularly polarised light, such that said laser beam switches between a linearly polarised and a circularly polarised beam, said polarisation switching causing said nanoobject to rotate and form a nanorotor.

[0043] In some embodiments, said method comprises performing said method with said vacuum chamber at a first higher pressure; and reducing said pressure within said vacuum chamber to a pressure that is lower than said first pressure.

[0044] As noted previously, the nanoobject or rotor may be particularly prone to exiting the trap at lower pressures where the damping of the oscillations due to interactions with gas molecules is reduced and thus, it may be advantageous to generate a suitable feedback signal at a higher pressure where the nanoobject is more stably held and then lower the pressure with the feedback signal in place keeping the particle stably within the trap as the damping is reduced.

[0045] In some embodiments, the method further comprises detecting rotation of said nanorotor; and determining a phase lag between said pulsed driving signal and said rotation.

[0046] In some embodiments, said method comprises determining at least one of a torque exerted on said nanorotor and a pressure within said vacuum chamber in dependence upon said phase lag.

[0047] Further particular and preferred aspects are set out in the accompanying independent and dependent claims. Features of the dependent claims may be combined with features of the independent claims as appropriate, and in combinations other than those explicitly set out in the claims.M24B154 / JM - 8 -

[0048] Where an apparatus feature is described as being operable to provide a function, it will be appreciated that this includes an apparatus feature which provides that function or which is adapted or configured to provide that function.

[0049] BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Embodiments of the present invention will now be described further, with reference to the accompanying drawings, in which:

[0051] Figure 1 schematically shows a sensor comprising a nanoobject trap according to an embodiment;

[0052] Figure 2 shows a nanoobject trap according to an embodiment;

[0053] Figure 3 schematically shows the rotational oscillations;

[0054] Figure 4 schematically shows a flow diagram illustrating steps in a method for confining a nanoobject within a trap; and

[0055] Figure 5 schematically shows a flow diagram illustrating steps in a method for measuring pressure using an optically trapped nanoobject.

[0056] DESCRIPTION OF THE EMBODIMENTS

[0057] Before discussing the embodiments in any more detail, first an overview will be provided.

[0058] If nanoparticles can be stably trapped in a harmonic potential such as one generated by one or more laser beams then external forces such as those due to gas pressure or flow that affect the nanoparticle may be measured with extreme accuracy. Elongated nanoparticles, may be particularly sensitive to interactions with gas molecules, but are challenging to trap securely.

[0059] Embodiments provide a feedback signal generated by detecting the oscillations of the nanoparticle, and use an all-pass filter and frequency doubling to modulate the driving signal of the harmonic potential and thus, provide an improved stability in the particle trap.M24B154 / JM - 9 -

[0060] Figure 1 schematically shows an apparatus according to an embodiment.

[0061] Nanoobject 10 is optically levitated within a harmonic potential formed by a laser beam output by laser 30 and focused by lenses 20. There is a mirror 40 that reflects the laser beam and interference between the counter propagating beams generates a standing wave and provides the harmonic potential that levitates and confines the nanoobject 10. Nanoobject 10 oscillates between nodes of the standing wave and provided the amplitude of the oscillations is not too large, then the nanoobject is confined within the trap. The lenses 20 and nanoobject 10 are within a vacuum chamber (not shown) that controls the pressure surrounding the nanoobject.

[0062] In order to maintain the nanoobject confined within the harmonic potential there is a feedback signal generated by optical detector 70 that detects light 62 that is scattered by reflection with the nanoobject 10. The nanoobject crosses the antinode in the centre of the oscillation twice in one period of the oscillation and by detecting this, the detected signal may have a frequency of twice the frequency of oscillation of the nanoobject. This signal is transmitted to all pass filter 60 that is configured to provide a phase shift to the signal. The all pass filter 60 is configured such that the signal that it outputs for transmission to laser drive circuitry 50 has a 90 degree phase lag compared to the first harmonic of the oscillation or in this case as the detected signal is the second harmonic, 180 degrees phase lag compared to the detected oscillation. This signal is received by drive circuitry 50 which uses it to modify the drive signal sent to laser 30 such that there is a boost to the harmonic potential of the standing wave nodes as the nanoobject reaches either end of its oscillation. In this way, the amplitude of the nanoobject’s oscillations are reduced and the nanoobject is cooled and is more securely confined within the trap.

[0063] In some embodiments the nanoobject may initially be levitated and confined within the trap with the vacuum chamber at a first pressure that may be several mbar. At this pressure the oscillation of the nanoobject is damped by interaction with the gas molecules. A feedback signal is generated by detecting theM24B154 / JM - 10 -

[0064] oscillations of the nanoobject at this first pressure and this feedback signal is applied by the drive circuitry 50 to the laser 30. The vacuum chamber may then be evacuated to a lower pressure and although damping of the nanoobject’s oscillations may be reduced, increases in the amplitude of the oscillations due to this reduced damping will be mitigated by the modifying of the laser beam by the feedback signal.

[0065] In some embodiments, the drive circuitry 50 may control the laser to provide pulses of circularly polarised light which pulses will cause the nanoobject or nanorotor 10 to rotationally oscillate about the optical axis. This rotational oscillation is shown in in Figure 3. These rotational oscillations are also affected by damping of gas molecules and monitoring the rotational oscillations allow pressure and / or gas flow to be accurately measured.

[0066] In embodiments, optical detector 70 in addition to detecting the translational oscillations of nanorotor 10 may detect changes in the rotational oscillations of the nanorotor by determining a phase lag between the rotation of the nanorotor and the pulses of circularly polarised light that generate this rotation. This phase lag will be indicative of the interactions between the gas molecules and the nanorotor and may provide an extremely accurate way of determining gas pressure and / or gas flow. Furthermore, this will be accurate across a large range of pressures making it a particularly sensitive pressure and / or flow meter.

[0067] Figure 2 shows the levitated particle or nanoobject 10 within a pressure controlled or vacuum chamber 80 with lenses 20 providing the focussing of the beam and reflected beam to the antinode at the centre of the oscillations of the nanoobject 10.

[0068] Figure 3 schematically shows how nanorotor 10 has a translational oscillation along, in this case, a vertical optical axis, and rotational oscillations about this axis. The rotational oscillations are caused by pulses of circularly polarised light. The rotation can be frequency locked to an external reference with extremeM24B154 / JM - 11 -

[0069] stability. The upper right-hand graph shows how the frequency is stable in the mHZ range at a rotational frequency of 1 MHZ. This provides a very low linewidth that is set by the drive circuitry for generating the pulses of circularly polarised light, with the pulses having a frequency of the order of MHZ providing a variation in frequency in the order of pHZ.

[0070] Detector 70 measures the phase of the rotational oscillations and receives an input signal 32 indicative of the pulses that provide the circular polarisation and cause the rotation of nanorotor 10. Comparison of this input signal with the measured phase can be used to accurately determine the phase lag between the two. This phase lag is indicative of interactions of the nanorotor with gas molecules and as such, with suitable calibration, can be used to provide a very accurate measurement of pressure and / or gas flow rate. Furthermore, these measurements can be made across a wide pressure range. The lower righthand graph shows how variations in the phase shift can be mapped to different pressures in the chamber.

[0071] Figure 4 shows steps of a method of confining a nanoobject according to an embodiment. In a first step S10 a laser beam is channelled and focussed within a chamber to levitate and confine a nanoobject. At step S20 translational oscillations of the nanoobject are detected and a feedback signal is generated. This feedback signal is generated to be double the frequency of the oscillation of the nanoobject, that is the second harmonic of the oscillation. At step S30 an all-pass filter is used for generating a phase change in the feedback signal such that the feedback signal is substantially ninety degrees out of phase with a first harmonic of the detected oscillation and at step S40 the laser beam is modified in dependence on the feedback signal such that at either end of a translational oscillation the nanoobject receives an increased force pushing it back towards the centre position. In this way the nanoobject is more securely confined. These steps of the method may initially be performed with the chamber at a first higher pressure and then when a feedback signal has been established the chamber may be evacuated to a lower pressure. At lower pressures the oscillationM24B154 / JM - 12 -

[0072] amplitude tends to increase, the presence of the feedback signal acts to counter this and the nanoobject is held within the trap.

[0073] Figure 5 shows further steps in a method where the levitated nanoobject is rotated and is a nanorotor and is used to detect pressure. At step S50 pulses of circularly polarised light are provided by the laser such that the levitated nanorotor rotates about the optical axis. At step S60 a phase lag between the rotation of the nanorotor and the pulse of the circularly polarised light is determined and at step S70 a pressure within the chamber is determined from this detected phase lag.

[0074] In summary, dielectric particles such as those made of silicon can be levitated and their oscillatory movements controlled using optical fields that generate a harmonic potential. Cylindrically shaped particles levitated in a lineally polarised laser field will be harmonically confined in all degrees of freedom including alignment. Levitation in a circularly polarised laser field will cause rotation about the optical axis, the frequency set by the balance between optical torque and damping due to gas collisions. Hence rotation is sensitive to local pressure in the vicinity of the nanoparticle or rotor.

[0075] The stability of the rotational frequency is limited by thermal fluctuations.

[0076] However, by driving the rotations with pulses of circularly polarised light it is possible to frequency lock the rotation to an external reference. In this way a nanorotor with 1 MHz frequency and a one microhertz linewidth has been produced. However, the phase lag between the rotation of the nanorotor and the drive does depend on environmental parameters and this dependency can be used to detect these environmental parameters. For example, the pressure or the gas flow rate may be determined by monitoring the phase lag. The rotation of the nanoobject may be compared to the drive signal using a lock in amplifier enabling precise determination of the phase shift and hence the local pressure. The levitated nanoobject may initially be levitated and the feedback signal determined at pressures between 1 to 100 mbar. Once the feedback signal forM24B154 / JM - 13 -

[0077] maintaining it stably in position has been produced then the pressure may be reduced and the active stabilization due to the feedback signal will keep the nanorotor trapped. The optical detector using back or side scattered light collected by an optical fibre may be used to detect the motion of the nanoobject to provide the feedback signal for active stabilization. Pressure measurements down to 10’12mbar are thereby enabled.

[0078] It should be noted that optically driven rotation requires nanoobjects with anisotropic susceptibility, i.e. non-spherical objects or particles, hence nanoobjects with an aspect ratio of two or more are selected. In addition, the frequency-locking requires particles where one of the dimensions should be significantly smaller than the wavelength of the light. 1 ,550 nm lasers are used in the telecoms industry and as such are relatively inexpensive and have a suitable wavelength when used with nanorotors with a diameter of less than 250 nm.

[0079] The material used for the nanoobject also plays a role. Where low laser powers are used, high optical susceptibilities and low absorption are preferable, hence silicon nanorods are preferentially selected as the material for the nanorotor.

[0080] Although illustrative embodiments of the invention have been disclosed in detail herein, with reference to the accompanying drawings, it is understood that the invention is not limited to the precise embodiment and that various changes and modifications can be effected therein by one skilled in the art without departing from the scope of the invention as defined by the appended claims and their equivalents.M24B154 / JM - 14 -

[0081] REFERENCE SIGNS

[0082] 10 nanoobject

[0083] 20 lens

[0084] 30 laser

[0085] 32 pulse signal input

[0086] 40 mirror

[0087] 50 laser drive circuitry

[0088] 60 all pass filter

[0089] 62 scattered light

[0090] 70 optical detector

[0091] 80 pressure controlled chamber

Claims

M24B154 / JM - 15 -CLAIMS1. A nanoobject trap for an optically levitated elongated nanoobject having an aspect ratio greater than two said nanoobject trap comprising:a chamber for containing said nanoobject;means for generating a harmonic potential for confining said nanoobject in said nanoobject trap;a feedback signal generator comprising:an optical detector for detecting oscillation of said nanoobject, said feedback signal generator being configured to generate a feedback signal in dependence upon said detected oscillation; andan all-pass filter for generating a phase change in said feedback signal such that said feedback signal is substantially ninety degrees out of phase with a first harmonic of said detected oscillation; anda modifier for modifying said harmonic potential in dependence upon said feedback signal.

2. A nanoobject trap according to claim 1 , wherein said optical detector is configured to detect a second harmonic of said oscillation frequency of said nanoobject.

3. A nanoobject trap according to claim 1 or 2, wherein said optical detector is configured to detect a first harmonic of said oscillation, said feedback signal generator comprising a frequency multiplier for doubling said frequency of said detected signal.

4. A nanoobject trap according to any preceding claim, wherein said modifier is configured to modify a strength of the harmonic potential in dependence upon said feedback signal.M24B154 / JM - 16 -5. A nanoobject trap according to any preceding claim, wherein said nanoobject comprises an object with an aspect ratio greater than three , preferably greater than five.

6. A nanoobject trap according to claim 5, wherein said nanoobject comprises a cylinder.

7. A nanoobject trap according to any preceding claim, wherein said means for confining said nanoobject in a harmonic potential comprises:at least one laser for generating a laser beam; andoptical components for channelling and focussing said laser beam within said chamber, such that said nanoobject is levitated and confined during operation of said particle trap.

8. A nanoobject trap according to claim 7, wherein said optical components are configured to channel said laser beam to form counterpropagating beams that interfere to form a standing wave said nanoobject being confined between nodes of said standing wave.

9. A nanoobject trap according to claim 7 or 8, wherein said at least one laser comprises drive circuitry for generating pulses of circularly polarised light, such that said laser beam switches between a linearly polarised and a circularly polarised beam, said polarisation switching causing said nanoobject to rotate about the optical axis, said nanoobject comprising a nanorotor.

10. A sensor comprising a nanoobject trap according to claim 9, said sensor further comprising a detector for detecting rotation of said nanorotor, and for determining a phase lag between said pulsed driving signal and said rotation.

11. A sensor according to claim 10, wherein said detector comprises said optical detector and is configured to receive a signal indicative of the pulsed driveM24B154 / JM - 17 -signal and is configured to determine a phase lag between said detected rotation and said pulsed drive signal.

12. A sensor according to claim 10 or 11 , said sensor comprising a pressure sensor, said phase lag being dependent on said pressure.

13. A sensor according to claim 10 or 11 , said sensor comprising a torque sensor, said phase lag being dependent on a torque exerted on said nanorotor.

14. A method of stabilising an optically levitated elongated nanoobject having an aspect ratio greater than two, said method comprising:generating a harmonic potential for confining said nanoobject; detecting oscillation of said nanoobject;generating a feedback signal in dependence upon said detected oscillation;passing said feedback signal though an all-pass filter such that said feedback signal is substantially ninety degrees out of phase with a first harmonic of said detected oscillation; andmodifying said harmonic potential in dependence upon said feedback signal, such that an amplitude of said oscillations is reduced.

15. A method according to claim 14, wherein said harmonic potential for confining said nanoobject is generated by a laser beam, said method further comprising: channelling and focussing said laser beam within a vacuum chamber containing said nanoobject, said nanoobject being levitated and held by said harmonic potential.

16. A method according to claim 15, said method further comprising: causing said levitated nanoobject to rotate about the optical axis by driving said laser to generate pulses of circularly polarised light, such that said laser beam switches between a linearly polarised and a circularly polarised beam, said polarisation switching causing said nanoobject to rotate and form a nanorotor.M24B154 / JM - 18 -17. A method according to claim 16, said method further comprising: detecting rotation of said nanorotor; and determining a phase lag between said pulsed driving signal and said rotation.

18. A method according to claim 17, said method comprising determining at least one of a torque exerted on said nanorotor and a pressure within said vacuum chamber in dependence upon said phase lag.

19. A method according to any one of claims 14 to 18, said method comprising:performing said method within a vacuum chamber at a first higher pressure; andreducing said pressure within said vacuum chamber to a pressure that is lower than said first pressure.

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