Three-photon light sheet imaging

Abstract

A light sheet imaging system, such as a light sheet microscope, comprises an illumination arrangement for generating a light sheet for three-photon excitation of a fluorescent sample, and a fluorescence collection arrangement for collecting fluorescence generated in the sample as a result of three-photon excitation by the light sheet. The light sheet may be a non-diffractive, propagation-invariant light sheet. The light sheet may be formed from and/or comprise a Bessel beam. A method of light sheet imaging comprises using a light sheet for three-photon excitation of a fluorescent sample, and collecting fluorescence generated in the sample as a result of three-photon excitation of the sample by the light sheet. Such a method may be used for light sheet microscopy.

Claims

1. A light sheet imaging system comprising: an illumination arrangement for generating a light sheet for three-photon excitation of a fluorescent sample; and a fluorescence collection arrangement for collecting fluorescence generated in the sample as a result of three-photon excitation by the light sheet, wherein the light sheet is a non-diffractive, propagation-invariant light sheet and the light sheet is formed from and/or comprises an Airy beam having an intensity maximum and one or more intensity side-lobes, wherein the intensity maximum follows a curved path or the light sheet is formed from and/or comprises a parabolic beam having an intensity maximum and one or more intensity side-lobes, wherein the intensity maximum follows a curved path.

2. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises a light sheet generating element for use in converting an optical beam into the light sheet.

3. A light sheet imaging system according to claim 2, wherein the light sheet generating element comprises at least one of a cylindrical lens, a cylindrical reflector and a cylindrical mirror or wherein the light sheet generating element comprises a scanner.

4. A light sheet imaging system according to claim 2, wherein the illumination arrangement comprises an optical source, wherein the illumination arrangement comprises an optical component which is configured to convert an optical beam generated by the optical source into a non-diffractive, propagation-invariant optical beam, and wherein the light sheet generating element is configured to convert the non-diffractive, propagation-invariant optical beam into the non-diffractive, propagation-invariant light sheet.

5. A light sheet imaging system according to claim 4, wherein the optical component comprises a fixed, static and/or passive optical component.

6. A light sheet imaging system according to claim 4, wherein the optical component comprises at least one of an axicon, a phase mask and a cylindrical lens which is tilted relative to a direction of propagation.

7. A light sheet imaging system according to claim 4, wherein the optical component is dynamic and/or re-configurable.

8. A light sheet imaging system according to claim 7, wherein the optical component comprises a diffractive optical component.

9. A light sheet imaging system according to claim 2, wherein the illumination arrangement comprises an optical source, wherein the illumination arrangement comprises an optical component, wherein the light sheet generating element is configured to convert an optical beam generated by the optical source into an intermediate light sheet, and wherein the optical component is configured to convert the intermediate light sheet into the non-diffractive, propagation-invariant light sheet.

10. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an input GRIN lens for coupling the light sheet into a sample and the fluorescence collection arrangement comprises an output GRIN lens for collecting fluorescence emitted from the sample, wherein the input and output GRIN lenses are arranged with their respective optical axes parallel to one another and the illumination arrangement further comprises a reflector, a mirror, or a deflector for deflecting light received from the input GRIN lens towards the sample.

11. A light sheet imaging system according to claim 10, wherein the illumination arrangement comprises an input optical fiber for coupling light to the input GRIN lens and the fluorescence collection arrangement comprises an output optical fiber for receiving light from the output GRIN lens, and wherein the input and output fibers are arranged side-by-side.

12. A light sheet imaging system according to claim 1, wherein the light sheet comprises at least one of: one or more wavelengths in the range 700 nm to 1800 nm; one or more wavelengths between 1030 nm and 1050 nm; one or more wavelengths between 1540 nm and 1560 nm; or one or more wavelengths between 1665 nm and 1685 nm.

13. A light sheet imaging system according to claim 1, wherein the light sheet is configured for three-photon excitation of exogenous fluorophores and/or the light sheet is configured for three-photon excitation of endogenous fluorophores.

14. A light sheet imaging system according to claim 13, wherein the light sheet is configured for three-photon excitation of a green fluorescent protein or a red fluorescent protein and/or wherein the light sheet is configured for three-photon excitation of NADH or a flavin.

15. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an optical source, and wherein the optical source is coherent.

16. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an optical source, and wherein the optical source comprises a laser and/or an optical parametric oscillator (OPO).

17. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an optical source, and wherein the optical source is tuneable.

18. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an optical source, and wherein the optical source is configured to generate a stream of optical pulses.

19. A light sheet imaging system according to claim 18, wherein at least one of: the stream of optical pulses has an average power of at least 1 mW, at least 5 mW, at least 10 mW, at least 50 mW or at least 100 mW; each optical pulse has a duration of 1 ps or less, 500 fs or less, or 100 fs or less; or each optical pulse has an energy of at least 10 nJ, at least 50 nJ, at least 100 nJ or at least 500 nJ.

20. A light sheet imaging system according to claim 18, wherein the illumination arrangement comprises a pulse compression arrangement for compressing the optical pulses.

21. A light sheet imaging system according to claim 20, wherein the pulse compression arrangement comprises at least one of: one or more elements or components for providing anomalous dispersion; or one or more elements or components for providing normal dispersion.

22. A light sheet imaging system according to claim 1, wherein the illumination arrangement comprises an optical source, and wherein the illumination arrangement comprises a wavelength conversion element or component for converting or adjusting a wavelength of light generated by the optical source.

23. A light sheet imaging system according to claim 1, wherein the light sheet imaging system comprises, or forms part of, a light sheet microscope.

24. A method of light sheet imaging comprising: using a light sheet for three-photon excitation of a fluorescent sample; and collecting fluorescence generated in the sample as a result of three-photon excitation of the sample by the light sheet, wherein the light sheet is a non-diffractive, propagation-invariant light sheet and the light sheet is formed from and/or comprises an Airy beam having an intensity maximum and one or more intensity side-lobes, wherein the intensity maximum follows a curved path or the light sheet is formed from and/or comprises a parabolic beam having an intensity maximum and one or more intensity side-lobes, wherein the intensity maximum follows a curved path.

25. A method of light sheet imaging according to claim 24, wherein the light sheet is configured for three-photon excitation of a green fluorescent protein or a red fluorescent protein and/or wherein the light sheet is configured for three-photon excitation of NADH or a flavin.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The present invention is described herein by way of non-limiting example only with reference to the drawings of which:

(2) FIG. 1 is a schematic of a three-photon light sheet fluorescence microscopy system;

(3) FIG. 2A is a two-dimensional transverse intensity profile of a Bessel beam of the microscopy system of FIG. 1;

(4) FIG. 2B is a one-dimensional transverse intensity profile of the Bessel beam of the microscopy system of FIG. 1;

(5) FIG. 2C is a schematic side view showing the arrangement of the detection objective above the light sheet formed using the Bessel beam of the microscopy system of FIG. 1;

(6) FIG. 3A is a simulated two-dimensional single-photon transverse intensity profile of a Bessel beam at the focal plane;

(7) FIG. 3B is a simulated two-dimensional three-photon transverse intensity profile of a Bessel beam at the focal plane;

(8) FIG. 3C shows simulated one-dimensional single- and three-photon transverse intensity profiles corresponding to the two-dimensional Bessel beam profiles of FIGS. 3A and 3B at the focal plane;

(9) FIG. 3D is a simulated two-dimensional single-photon transverse intensity profile of a Gaussian beam at the focal plane;

(10) FIG. 3E is a simulated two-dimensional three-photon transverse intensity profile of a Gaussian beam at the focal plane;

(11) FIG. 3F shows simulated one-dimensional single- and three-photon transverse intensity profiles corresponding to the two-dimensional Gaussian beam profiles of FIGS. 3D and 3E at the focal plane;

(12) FIG. 4A is a simulated two-dimensional single-photon transverse intensity profile of a Bessel beam at a distance of 50 μm from the focal plane;

(13) FIG. 4B is a simulated two-dimensional three-photon transverse intensity profile of a Bessel beam at a distance of 50 μm from the focal plane;

(14) FIG. 4C shows simulated one-dimensional single- and three-photon transverse intensity profiles corresponding to the two-dimensional Bessel beam profiles of FIGS. 4A and 4B at a distance of 50 μm from the focal plane;

(15) FIG. 4D is a simulated two-dimensional single-photon transverse intensity profile of a Gaussian beam at a distance of 50 μm from the focal plane;

(16) FIG. 4E is a simulated two-dimensional three-photon transverse intensity profile of a Gaussian beam at a distance of 50 μm from the focal plane;

(17) FIG. 4F shows simulated one-dimensional single- and three-photon transverse intensity profiles corresponding to the two-dimensional Gaussian beam profiles of FIGS. 4D and 4E at a distance of 50 μm from the focal plane;

(18) FIG. 5A is a plot of the simulated modulation transfer function against normalized spatial frequency for single- and three-photon Bessel and Gaussian beams at the focal plane;

(19) FIG. 5B is a plot of the simulated modulation transfer function against normalized spatial frequency for single- and three-photon Bessel and Gaussian beams at a distance of 50 μm from the focal plane;

(20) FIG. 6 is a schematic of a first alternative illumination and detection arrangement for use with the three-photon light sheet fluorescence microscopy system of FIG. 1;

(21) FIG. 7A is a schematic of a second alternative illumination and detection arrangement for use with the three-photon light sheet fluorescence microscopy system of FIG. 1;

(22) FIG. 7B is a schematic of a third alternative illumination and detection arrangement for use with the three-photon light sheet fluorescence microscopy system of FIG. 1;

(23) FIG. 8A is a schematic of an alternative sample chamber arrangement for use with the three-photon light sheet fluorescence microscopy system of FIG. 1;

(24) FIG. 8B is a detailed schematic of the alternative sample chamber arrangement of FIG. 7A; and

(25) FIG. 9 is a schematic of a three-photon light sheet fluorescence microscopy system for use in vivo.

DETAILED DESCRIPTION OF THE DRAWINGS

(26) Referring initially to FIG. 1 there is provided a three-photon light sheet fluorescence microscopy system for imaging a sample such as a tissue sample, one or more particles, cells, colloids, organisms, organelles or the like. The sample may have been treated or otherwise exposed to an exogenous fluorescent substance such as an exogenous fluorophore or the like for three-photon excitation by a light sheet generated by the microscopy system. Alternatively, the sample may include an endogenous fluorescent substance such as an endogenous fluorophore such as NADH and/or one or more flavins for three-photon excitation by a light sheet generated by the microscopy system.

(27) The illumination propagates along the x-direction, the beam is expanded along the y-direction to form the light sheet, and fluorescence is collected along the z-direction. The three-photon light sheet fluorescence microscopy system of FIG. 1 permits imaging of fluid-immersed samples on a horizontal surface such as a standard microscope slide or dish. This is enabled by positioning illumination and detection objectives at 45 degrees to the horizontal sample. By translating the sample and the detection objective along the z-direction as indicated by the “scan dir.” arrow, different “planes” of the sample are illuminated sequentially by the light sheet and the fluorescence generated in each plane is collected and imaged onto an image sensor such as a camera (CAM.sub.1) so that a three dimensional volumetric image can be recorded.

(28) The three-photon light sheet fluorescence microscopy system of FIG. 1 includes an illumination arrangement including a laser, an optical component MK.sub.1 and a light sheet generating element (LSG). The optical component MK.sub.1 may be a static optical component such as an axicon or a mask for the generation of a Bessel beam. Alternatively, the optical component MK.sub.1 may be a dynamic optical component configured so as to generate a Bessel beam. For example, the optical component may be a spatial light modulator or a digital micro-mirror configured so as to generate a Bessel beam by reflection therefrom. As shown in FIGS. 2A and 2B, a Bessel beam has a bright central spot surrounded by a series of concentric rings which decrease in intensity with distance from the central spot. The LSG can be a cylindrical lens or a scanning device such as an acousto-optical deflector (AOD) for progressively generating a light sheet.

(29) The beam-shaping arrangement includes a series of lenses L1 to L6 for coupling light from the laser towards the sample via the optical component MK.sub.1 and the LSG. The beam-shaping arrangement further includes irises I.sub.1 and I.sub.2, a quarter waveplate λ/4, mirrors M1 and M2 and an illumination objective OBJ.sub.1.

(30) The three-photon light sheet fluorescence microscopy system of FIG. 1 includes a detection objective OBJ.sub.2, a third mirror M.sub.3, a fourth mirror M.sub.4, a filter F for blocking light at the one or more excitation wavelengths and an image sensor or camera CAM.sub.1.

(31) In use, light from the laser is expanded by lenses L.sub.1 and L.sub.2 to match the optical component MK.sub.1. Having passed through the optical component MK.sub.1, the Bessel beam is then passed through lens L.sub.3, where is it focused onto the iris I.sub.1 and through another lens L.sub.4, where it is shaped to the diameter of the LSG. The light sheet that is formed by the LSG is then passed through the lens L.sub.5, where it is focused onto the iris I.sub.2 and through another lens L.sub.6, which collimates the light sheet onto the quarter wave plate λ/4, which provides polarization control. From there, the light sheet is reflected from the first mirror M.sub.1 onto the second mirror M.sub.2 and from there onto the illumination objective OBJ.sub.1, which is positioned at 45 degrees to the horizontal sample stage. FIG. 2C shows a side view of the detection objective positioned adjacent the light sheet generated using the Bessel beam corresponding to the profiles of FIGS. 2A and 2B. Fluorescence generated in the sample passes through the detection objective OBJ.sub.2 onto the third mirror M.sub.3 and from there onto the fourth mirror M.sub.4, where it is directed to the image sensor or camera CAM.sub.1 via the filter F.

(32) The mirrors M.sub.1 to M.sub.4 can be used to facilitate the vertical alignment of the objectives with the sample. In one example, mirrors M.sub.2 and M.sub.3 can be fixed with respect to the axis of the objectives OBJ.sub.1 and OBJ.sub.2, respectively. Appropriate alignment of mirrors M.sub.1 and M.sub.4 allows a vertical translation of both objectives OBJ.sub.1 and OBJ.sub.2 and mirrors M.sub.2 and M.sub.3 with respect to the sample stage without altering the alignment. Axial translation stages on the objectives may facilitate fine-tuning the alignment.

(33) Various optional elements are included to control the polarization and to filter the light. Optionally, the irises, I.sub.1 and I.sub.2 may be included as shown in FIG. 1 to select the first diffraction orders to allow more flexibility and prevent unwanted illumination of the sample. Lenses L.sub.1 to L.sub.6 form telescopes to relay the light between the various components. The quarter waveplate λ/4 can be introduced to control the polarization of the light, e.g. by converting linear polarized light to circularly polarized light that produces a more uniform excitation of fluorescence. Light filters, F, e.g. to block excitation light and select fluorescence emission, can be introduced between OBJ.sub.2 and the camera, CAM.sub.1. If necessary or convenient, elements of the illumination path and the detection path can be brought closer than the mirrors to OBJ.sub.1 and OBJ.sub.2, respectively.

(34) The laser is configured to generate light at one or more wavelengths in the range 1000 nm to 1800 nm for three-photon excitation of the fluorophore with which the sample is treated. For example, the laser may be configured to generate light having wavelengths around 1675 nm for the three-photon excitation of exogenous fluorophores. Alternatively, the laser may be configured to generate light having wavelengths around 1040 nm for the three-photon excitation of endogenous fluorophores such as NADH and/or one or more flavins.

(35) The laser generates a stream of optical pulses with each pulse having a duration of the order of 100 fs or less and an energy of the order of 100 nJ at the light sheet. The illumination arrangement may include one or more elements or components for the compensation of dispersion associated with propagation of light via the various optical components between the laser and the sample. The laser may generate a light sheet with sufficient peak power to generate three-photon excitation of the fluorophore at relatively low average powers of only a few mWs and may provide enhanced image contrast or improved SBR at average intensity levels which are surprisingly low relative to the average intensity levels which can give rise to phototoxicity. Moreover, as described below with reference to FIGS. 3A to 3F, 4A to 4F, 5A and 5B, theoretical studies have demonstrated that use of a light sheet for three-photon fluorescence light sheet microscopy generated using a Bessel beam has the potential to provide high image contrast and superior image resolution across the light sheet compared with the use of a Gaussian beam.

(36) FIGS. 3A and 3B show the two-dimensional transverse single-photon and three-photon intensity profiles of a Bessel beam at a focal plane of an ideal lens respectively for a numerical aperture (NA) of 0.2 and the corresponding one-dimensional transverse single-photon and three photon intensity profiles are plotted in FIG. 3C. Similarly, FIGS. 3D and 3E show the two-dimensional transverse single-photon and three-photon intensity profiles of a Gaussian beam at the focal plane of the ideal lens respectively for a NA of 0.2 and the corresponding one-dimensional transverse single-photon and three-photon intensity profiles are plotted in FIG. 3F. All units are in μm.

(37) Comparing FIGS. 3C and 3F, it may be seen that the three-photon intensity profile of the Bessel beam is surprisingly similar to the three-photon intensity profile of the Gaussian beam at focal plane of the ideal lens. More importantly, as shown in FIGS. 4A-4F the three-photon intensity profile of the Bessel beam is essentially propagation-invariant for distances of up to 50 μm from the focal plane of the ideal lens. The profiles of FIGS. 3A-4C suggest that a light sheet generated using a Bessel beam may have a three-photon intensity at the focal plane which is comparable to, or only marginally less than, the three-photon intensity at the focal plane of a light sheet generated using a Gaussian beam and three-photon intensities in regions either side of the focal plane which may actually be significantly greater than the three-photon intensities in regions either side of the focal plane of a light sheet generated using a Gaussian beam. In other words, the profiles of FIGS. 3A-4C suggest that a light sheet generated using a Bessel beam has the potential to provide a fluorescence image with an enhanced contrast or signal-to-background ratio (SBR) across an extended field of view compared with a Gaussian beam.

(38) FIG. 5A is a plot of the modulation transfer function (MTF) against normalized spatial frequency for the one- and three-photon Bessel and Gaussian beams of FIGS. 3A, 3B, 3C and 3D at the focal plane of the ideal lens for a numerical aperture (NA) of 0.2. Similarly, FIG. 5B is a plot of the modulation transfer function against normalized spatial frequency for the one- and three-photon Bessel and Gaussian beams of FIGS. 4A, 4B, 4C and 4D at a distance of 50 μm from the focal plane of the ideal lens for a NA of 0.2. In each case, the spatial frequency is normalized to the maximum transmitted spatial frequency 2 NA/λ, for a peak excitation wavelength λ of 800 nm. In each case, the one- and three-photon Bessel and Gaussian beams were assumed to comprise a stream of optical pulses that would typically have a Gaussian spectrum but no constraint on pulse duration is assumed. As shown in FIGS. 5A and 5B, the MTF for a three-photon Bessel beam is only marginally lower than the MTF for a three-photon Gaussian beam at the focal plane whereas the MTF for the three-photon Bessel beam is significantly higher than the MTF for the three-photon Gaussian beam at a distance of 50 μm from the focal plane. This demonstrates that the use of a Bessel beam for three-photon light sheet microscopy has the potential to provide fluorescence images across an extended field of view with both an axial and a lateral spatial resolution which are comparable to, or only marginally less than, those provided by a Gaussian beam at the focal plane.

(39) FIG. 6 shows a first alternative illumination and detection arrangement for replacing the mirror M.sub.1 and all of components in the optical path downstream of the mirror M.sub.1 in the three-photon light sheet fluorescence microscopy system of FIG. 1. A sample (not shown) is located in a sample chamber SC and remains fixed as the light sheet is sequentially directed along different “planes” of the sample by a scanning mirror SM via a pair of relay lenses RL and an illumination objective O1. The fluorescence emitted from the generally “planar” illuminated region of the sample is imaged onto the image sensor or camera CAM by a detection objective O2, an electrically tuneable lens ETL, and a tube lens TL. The scanning mirror SM and the electrically tuneable lens ETL are synchronized so that the “plane” of illumination and the plane of detection (i.e. the imaged plane) correspond. This not only allows the sample to be kept stationary whilst image stacks are taken, but also allows fast scanning of the sample. This may be particularly important when monitoring one or more live and/or mobile samples.

(40) FIG. 7A is a schematic of a second alternative illumination and detection arrangement for use with the three-photon light sheet fluorescence microscopy system of FIG. 1 in which the illumination objective O1 is oriented horizontally along a horizontal illumination axis and the imaging objective O2 is arranged vertically along a vertical imaging axis. Conversely, as shown in FIG. 7B, the illumination objective O1 may be oriented vertically along a vertical illumination axis and the imaging objective O2 may be arranged horizontally along a horizontal imaging axis.

(41) FIGS. 8A and 8B show an alternative sample chamber arrangement SC2 for use with the three-photon light sheet fluorescence microscopy system of FIG. 1 in which the illumination and imaging objectives are arranged along orthogonal axes below a sample 14. The alternative sample chamber arrangement SC2 includes a transparent membrane 10 for containing a fluid 12 surrounding the sample 14. Such an alternative sample chamber arrangement SC2 may permit the fluid 12 and the sample 14 to be replaced without re-alignment of objective O1 and O2. The sample 14 may be positioned using optical trapping and/or acoustic trapping.

(42) FIG. 9 shows a three-photon light sheet fluorescence microscopy system for imaging a sample (not shown) in vivo. The three-photon light sheet fluorescence microscopy system of FIG. 9 includes an illumination arrangement which includes a femtosecond laser, a dispersion compensation element DC, an input fiber coupling lens L.sub.1, an input optical fiber OF.sub.1, an illumination GRIN lens arrangement GRIN.sub.1 and a micro-prism beam deflector BD.

(43) The three-photon light sheet fluorescence microscopy system of FIG. 9 further includes a fluorescence collection arrangement which includes detection GRIN lens arrangement GRIN.sub.2, an output optical fiber OF.sub.1, an output fiber coupling lens L.sub.2, a filter F for blocking light at the excitation wavelength, a tube lens TL and an image sensor or camera CAM.

(44) The illumination GRIN lens arrangement GRIN.sub.1 includes at least one cylindrical GRIN lens for the generation of a light sheet at a position below the detection GRIN lens arrangement GRIN.sub.2.

(45) In use, a sample (not shown) is either treated with, or otherwise exposed to, an exogenous fluorescent substance such as a green fluorescent protein or a red fluorescent protein or the like for three-photon excitation by a light sheet generated by the microscopy system. Alternatively, the sample (not shown) may include an endogenous fluorescent substance such as an endogenous fluorophore such as NADH and/or one or more flavins for three-photon excitation by a light sheet generated by the microscopy system.

(46) The laser generates a stream of pulses at a wavelength in the range 1000 to 1800 nm. The dispersion compensation element DC is used to compensate for dispersion in the input fiber coupling lens L.sub.1, the input optical fiber OF.sub.1, the cylindrical illumination GRIN lens arrangement GRIN.sub.1, the micro-prism beam deflector BD and the sample (not shown) to ensure that the pulses having a duration of the order of a hundred fs or less and a peak power of the order of 100 nJ or less at the light sheet for the three-photon excitation of the fluorophore in the sample (not shown). The input optical fiber OF.sub.1 may be configured to provide anomalous dispersion and the dispersion compensation element DC may comprise a normally dispersive element to pre-chirp the optical pulses generated by the laser for compression of the pulses at the light sheet. For example, the input optical fiber OF.sub.1 may be a hollow-core photonic bandgap fiber. The dispersion compensation element DC may comprise or be formed from a normally dispersive material such as glass, may comprise a normally dispersive optical fiber and/or may comprise one or more chirped mirrors.

(47) The fluorescence generated in the sample is collected by the detection GRIN lens arrangement GRIN.sub.2 and imaged onto the camera CAM via the output optical fiber OF.sub.1, the output fiber coupling lens L.sub.2, the filter F and the tube lens TL. One of ordinary skill in the art will understand that the compact arrangement provided by the input optical fiber OF1, the illumination GRIN lens arrangement GRIN.sub.1 and the micro-prism beam deflector BD, makes the three-photon light sheet fluorescence microscopy system of FIG. 8 particularly suitable for in vivo imaging applications.

(48) One of ordinary skill in the art will understand that in each of the illumination and detection arrangements described with reference to FIGS. 1-9, a sample is illuminated by one or more light sheets for three-photon excitation of the sample along an illumination axis and fluorescence is collected along a detection axis which is generally orthogonal to the illumination axis. Other illumination and detection arrangements are also possible in which the illumination axis and the detection axis are arranged orthogonally. Additionally or alternatively, a sample may be illuminated by a first light sheet travelling in a first direction along an illumination axis and a second light sheet travelling in a second direction along the illumination axis, wherein the second direction is opposite to the first direction.

(49) At least some of optical components of the illumination arrangement of the three-photon light sheet fluorescence microscopy system of FIG. 1 may be re-arranged. For example, the order in which the optical component MK.sub.1 and the light sheet generating element LSG appear in the optical path may be reversed.

(50) The light sheet may be formed from and/or may comprise a superposition or an array of beams such as an array of Bessel beams.

(51) Rather than using a Bessel beam, it may be possible to use a different non-diffractive and/or propagation invariant beam such as an Airy beam and/or a parabolic beam to create the light sheet. One of ordinary skill in the art will appreciate that such non-diffractive and/or propagation invariant beams may be produced by replacing the component MK1 of FIG. 1 with a suitably configured spatial light modulator and/or a digital micro mirror. Alternatively, an Airy beam may be produced using a cylindrical lens which is tilted with respect to a direction of propagation of the light from the laser. One of ordinary skill in the art will also appreciate that some non-diffractive and/or propagation invariant beams such as an Airy beam or a parabolic beam may be considered to propagate along a curved path in the sense that such beams may have a central intensity maximum and one or more intensity side-lobes, wherein the intensity maximum follows a curved path which may, for example, be parabolic. For such beams, the resulting light sheet may be curved and references to a “plane” as used in the description above should be understood to encompass such a curved sheet. One of ordinary skill in the art will also understand that linear deconvolution can be used to compensate for artefacts in the image of the fluorescence generated when using a curved light sheet such as a light sheet formed from an Airy beam, thereby effectively “cancelling” the curvature of the Airy beam light sheet.

(52) The light sheet may be formed from and/or may comprise a superposition or an array of Airy beams.

(53) The light sheet may be formed from and/or may comprise a superposition or an array of parabolic beams.

(54) An optical source other than a laser may be used. For example, a different coherent optical source such as an optical parametric oscillator (OPO) may be used.

(55) The optical source may comprise a wavelength conversion element or component for converting or adjusting a wavelength of the generated light. For example, the optical source may comprise a photonic crystal rod for soliton self-frequency shifting. The optical source may be tuneable.