Apparatus for carrying out polarization resolved Raman spectroscopy

Abstract

An apparatus for carrying out polarization resolved Raman spectroscopy on a sample (15), in particular a crystalline sample, comprises: at least one light source (11), in particular at least one laser, for providing excitation radiation to a sample (15), a spectrograph (31) for dividing light from the sample (15), in particular Raman scattered light from the sample (15), into at least one spectrum of spatially separated wavelength components and for directing at least a portion of the at least one spectrum to a detector (29), in particular a CCD detector, a polarization state control element (27) for the light from the sample (15), the polarization state control element (27) being arranged in a light path of at least one light beam (25) traveling from the sample (15) towards the detector (29), and the polarization state control element (27) comprising at least one polarization sensitive optical element (45, 47), in particular a Wollaston prism, the at least one polarization sensitive optical element being adapted to split the at least one light beam (25) into at least two, in particular orthogonally, polarized light beams (35a, 35b, 37a, 37b).

Claims

1. An apparatus for carrying out polarization resolved Raman spectroscopy on a sample, the apparatus comprising: at least one light source for providing excitation radiation to a sample, a spectrograph for dividing light from the sample into at least one spectrum of spatially separated wavelength components and for directing at least a portion of the at least one spectrum to a detector), a polarization state control element for the light from the sample, the polarization state control element being arranged in a light path of at least one light beam traveling from the sample towards the detector, and the polarization state control element comprising at least one polarization sensitive optical element, the at least one polarization sensitive optical element being adapted to split the at least one light beam into at least two polarized light beams.

2. The apparatus of claim 1, wherein the polarization state control element further comprises at least one beam splitter adapted to split the light beam into at least two split beams and at least two polarization sensitive optical elements arranged such that at least one of the at least two split beams is passing through one of the polarization sensitive optical elements and at least another one of the at least two split beams passes through the other one of the polarization sensitive optical elements.

3. The apparatus of claim 2, wherein a waveplate is arranged in at least one light path between the beam splitter and one of the polarization sensitive optical elements.

4. The apparatus of claim 1, wherein the polarization state control element comprises at least one beam splitter adapted to split the light beam into three split beams, and three polarization sensitive optical elements arranged such that a first one of the three split beams is passing through a first one of the polarization sensitive optical elements, a second one of the three split beams is passing through a second one of the three polarization sensitive optical elements, and a third one of the three split beams is passing through a third one of the three polarization sensitive optical elements.

5. The apparatus of claim 4, wherein a waveplate is arranged in at least one light path between the beam splitter and one of the three polarization sensitive optical elements.

6. The apparatus of claim 4, wherein a first waveplate is arranged in the light path between the beam splitter and the first polarization sensitive optical element.

7. The apparatus of claim 4, wherein a second waveplate is arranged in the light path between the beam splitter and the second polarization sensitive optical element.

8. The apparatus of claim 4, wherein no waveplate is arranged in the light path between the beam splitter and the third polarization sensitive optical element.

9. The apparatus of claim 1, wherein the polarization state control element is arranged in the light path between the sample and the spectrograph.

10. The apparatus of claim 1, wherein at least one beam splitter is arranged in the light path between the light source and the sample, the beam splitter being adapted to split a light beam from the light source into at least a first beam and a second beam.

11. The apparatus of claim 10, wherein a waveplate, preferably a half-wave plate or a quarter-wave plate, is arranged in the light path of at least the second beam in between the beam splitter and the sample, for generating a polarization manipulated second beam, and wherein, preferably, at least one beam combiner is provided to combine at least the first beam and the polarization manipulated second beam and to direct the combined beams onto the sample, preferably generating at least two spatially separated spots in the same focal plane on the sample.

12. The apparatus of claim 10, wherein the at least one beam splitter is adapted to split the light beam from the light source into a first beam, a second beam, and a third beam, wherein, preferably, a waveplate, preferably a half-wave plate or a quarter-wave plate, is arranged in the light path of the second beam in between the beam splitter and the sample, for generating a polarization manipulated second beam, and wherein, further preferably, a further waveplate, preferably a half-wave plate or a quarter-wave plate, is arranged in the light path of the third beam in between the beam splitter and the sample, for generating a polarization manipulated third beam.

13. The apparatus of claim 12, wherein at least one beam combiner is provided to combine the first beam, the polarization manipulated second beam and the polarization manipulated third beam and to direct the combined beams onto the sample, preferably generating three spatially separated spots in the same focal plane on the sample.

14. The apparatus of claim 1, wherein at least one beam combiner is provided to combine two or more light beams from two or more different light sources and to direct the combined beams onto the sample, preferably to separated spots in the same focal plane on the sample.

15. The apparatus of claim 14, wherein the light beam from at least one of the light sources is a polarization manipulated beam.

16. The apparatus of claim 1, wherein the sample is a crystalline sample, the at least one light source comprises at least one laser, the light from the sample is Raman scattered light, and the detector is a CCD detector.

17. The apparatus of claim 1, wherein the polarization sensitive optical element is a Wollaston prism.

18. The apparatus of claim 1, wherein the at least one polarization sensitive optical element is adapted to split the at least one light beam into at least two orthogonally polarized light beams.

19. The apparatus of claim 3, wherein the waveplate is a half-wave plate or a quarter-wave plate.

20. The apparatus of claim 4, wherein the beam splitter is adapted to split the light beam into three parallel beams.

Description

(1) One or more examples will hereinafter be described in conjunction with the following drawing figures, where like numerals denote like elements, and

(2) FIG. 1 shows schematically a first variant of an apparatus for carrying out polarization resolved Raman spectroscopy on a sample,

(3) FIG. 2 shows schematically a second variant of an apparatus for carrying out polarization resolved Raman spectroscopy on a sample,

(4) FIG. 3 shows schematically a third variant of an apparatus for carrying out polarization resolved Raman spectroscopy on a sample, and

(5) FIG. 4 shows schematically a fourth variant of an apparatus for carrying out polarization resolved Raman spectroscopy on a sample.

(6) The apparatus shown in FIG. 1 comprises at least one light source 11, which is preferably a laser, for providing a light beam 13 of excitation radiation to a sample 15. In particular, the light beam 13, which is in particular a laser beam, is split into two orthogonally polarized laser beams. Thus, preferably two orthogonally polarized laser beams 13 are incident on the sample 11. The two orthogonally polarized laser beams 13 are reflected by a dichroic beam splitting mirror 17 and further guided by mirrors 19 and 21 to an objective 23 which comprises a plurality of lenses to focus the laser beams 13 on the sample 15.

(7) The orthogonally laser beams 13 are slightly deviating in their propagation direction and thus, the objective focuses them to two spatially separated spots 303, 305 as shown in the window 301 obtained from a photo of the focal plane on the sample 15.

(8) The optical setup is arranged in a backscattering geometry such that the objective 23 also serves for collecting the light scattered from the sample 15, in particular from the two illuminated spots 303, 305. The backscattered light beam 25, at least the spectral components of the backscattered light beam that are different from the spectral components of the laser beam 13, can pass through the dichroic mirror 17 and enter a polarization state control element 27 for the light beam 25 scattered from the sample 15.

(9) The polarization state control element 27 is arranged in the light path of the light beam 25 collected by the objective 23 and traveling towards a detector 29 and before the light path passes through a spectrograph 31. The polarization state control element 27 comprises a beam splitter 33 adapted to split the light beam 25 into a first split beam 35 and a second split beam 37, which are traveling in parallel to each other after the first split beam 35 is reflected by mirror 39.

(10) The first beam 35 passes through a half-wave plate 41 while the second beam 37 passes through an optical element 43, for example a quarter-wave plate, which may also chance the polarization state of the second beam 37.

(11) Subsequently, the first beam 35 passes through a first polarization sensitive optical element, here a first Wollaston prism 45, which splits the first beam 35 into two orthogonally polarized beams 35a, 35b which are depicted in FIG. 1 as a single beam. The polarization of the beam 35a may for example be 0° and the polarization of the beam 35b may for example be 90°. The second beam 35 passes through a second polarization sensitive optical element, here a second Wollaston prism 47, which splits the second beam 37 into two orthogonally polarized beams 37a, 37b which are depicted in FIG. 1 as a single beam. The polarization of the beam 37a may for example be +45° and the polarization of the beam 37b may for example be −45°.

(12) The optical system of the apparatus of FIG. 1 further comprises a lens system 49, which may also be a slit lens, and which is adapted to focus the polarized beams 35a, 35b, 37a, 37b leaving the Wollaston prisms 45, 47 through a slit 51 of the spectrograph 31. The lens system 49 may also include an edge filter.

(13) The spectrograph 31 comprises a collimation lens system 53 having a focus in the slit. The collimation lens system 53 transfers the beams 35a, 35b, 37a, 37b into collimated beams which may pass through an optional edge filter (not shown) and a transmission grating 55.

(14) The grating 55 divides each of the beams 35a, 35b, 37a, 37b into a respective spectrum of spatially separated wavelength components and directs at least a portion of each spectrum via a focusing lens system 57 on an array of pixels 59 of the detector 29 to detect the spectrum of each polarized beam 35a, 35b, 37a, 37b.

(15) Window 307 shows the spectra measured on the array of pixels 59. As can be seen, eight different spectra can be measured simultaneously. The spectra are separated from each other along the y-axis. The spots that are spatially distributed along the x-axis are associated with the different wavelengths contained in each spectrum.

(16) The eight spectra are due to the fact that the sample 15 is illuminated in 2 spots with 2 beams of different polarization. Therefore, the backscattered beam 25 comprises in fact two backscattered Raman beams due to the 2 spots 303, 305 and the different polarizations of the incident light. Sample responses from different incident laser polarization are therefore collected simultaneously. Furthermore, the beam splitter 33 splits each of the two backscattered Raman beams such that they pass through each Wollaston prism 45, 47. Hence, each of the polarized beams 35a, 35b, 37a, 37b comprises in fact two beams due to their origin from 2 spots illuminated with different laser polarization. Hence, in total 8 beams leave the Wollaston prisms and pass through the entrance slit 51 of the spectrograph 31. This is shown in window 309 which is derived from a photo of the plane of the slit 51 and which shows that eight beams travel through slit 51 where they are spatially separated from each other. This explains the eight spectra that are detectable on the array of pixels 59.

(17) The optical setup of the apparatus of FIG. 2 corresponds in substance to the optical setup of the apparatus of FIG. 1. Correspondingly, the apparatus of FIG. 2 can also detect eight spectra simultaneously as explained above.

(18) The apparatus of FIG. 2 comprises a laser 11 as light source, emitting light at a wavelength of 785 nm. The laser 11 may for example be a diode laser. Beam splitter 63 splits laser beam 61 into two beams 65, 67. The beam 65 travels through half-wave plate 69 which changes the polarization of the beam 65. The beam 71 emerging from the half-wave plate 69 and the beam 67 are combined by mirror 73 and beam splitter 75 to laser beam 13.

(19) As explained with respect to FIG. 1, beam 13 therefore consists of in fact two beams with different polarization. The beams 67 and 71 will be combined such that their propagation axes slightly deviate from each other. This will produce two spots (similar to spots 303, 305 in FIG. 1) in the sample focal plane. An adjustment of the propagation axis of the beam 67 is for example possible by tilting the mirror 73.

(20) The optical setup of the apparatus of FIG. 3 is similar to the optical set-up of the apparatuses of FIGS. 1 and 2 as described before. However, the light emitted by laser 11 is passing through some optical elements 77, 79, 81, 83 with line filter 79 and linear polarizers 81, 83, reflected by mirrors 85 and split, by use of beam splitters 87 into three beams 89, 91, 93. The split beam 89 passes through waveplate 95 and the split beam 91 passes through waveplate 97 while the beam 93 does not pass through a waveplate. This results in three beams with polarization of 0°, 45° and 90° which are, by use of beam splitters 99 and 101, combined at a slightly different angle in their propagation axes resulting in three spots 313, 315 and 317 as shown in window 311 in a focal plane on the sample 15.

(21) The path of the backscattered light corresponds in substance to the path as described above with respect to FIG. 1. As a result, due to the three spots 313, 315, 317 caused by light of different polarization and the splitting of the beams from these spots in the polarization state control element 27, twelve polarization resolved beams travel through the slit 51, which may be equipped with a shutter, as shown in window 312. Consequently, twelve polarization resolved Raman spectra can be detected on the array of pixels of detector 29 as shown in window 319.

(22) As an option, the optical setup of the apparatus of FIG. 3 may comprise a polarization state control element 27 that includes at least one beam splitting element which is adapted to split a light beam coming from the sample into three split beams. A first waveplate, such as a half-wave plate, may be arranged in the light path between the beam splitting element and a first polarization sensitive optical element, in particular a first Wollaston prism. A second waveplate, in particular a quarter-wave plate, may be arranged in the light path between the beam splitting element and a second polarization sensitive optical element, in particular a second Wollaston prism. No waveplate may be arranged in the light path between the beam splitting element and a third polarization sensitive optical element, in particular a third Wollaston prism (not shown). As described before, each Wollaston prism splits each incident beam into two separate linearly outgoing beams.

(23) Thus, per incoming beam, the three Wollaston prisms generate six outgoing beams that travel towards the spectrograph 31. The three beams originating from Raman scattering in the spots 313, 315, 315 may therefore result in 18 beams which leave the three Wollaston prisms. Consequently, 18 spectra can be detected on the array of pixels of detector 29 as shown in window 319.

(24) The optical setup of the apparatus of FIG. 4 corresponds in substance to the optical setup of the apparatus of FIG. 3. However, the apparatus of FIG. 4 comprises three lasers 11a, 11b and 11c, illuminating linearly polarized light at 780 nm, 785 nm and 790 nm. The polarization of the light from laser 11c is changed by 45° by waveplate 95, and the polarization of the light from laser 11b is changed by 90° by waveplate 97. The light from the three lasers is combined by use of mirror 85 and beam splitters 99, 101 such that the laser beams propagate slightly into different directions, resulting again in three spots of laser light with different polarization (0°, 45°, 90°) as indicated in window 323 (obtained from a picture of the focal plane of the objective 23). The spots are illuminated by light at different wavelengths due to the different lasers 11a, 11b, 11c.

(25) Information obtained from the measured spectra can be used in an algorithm as described, for example, in the previously mentioned article of Ramabadran et al. The algorithm can be based on Raman tensor scattering theory. Thereby, crystallographic maps of the sample can be obtained, for example by using the same principles as in X-ray crystallography.

LIST OF REFERENCE SIGNS

(26) 11 light source, laser

(27) 11a laser

(28) 11b laser

(29) 11c laser

(30) 13 laser beam

(31) 15 sample

(32) 17 dichroic mirror

(33) 19 mirror

(34) 21 mirror

(35) 23 objective

(36) 25 light beam

(37) 27 polarization state control element

(38) 29 detector

(39) 31 spectrograph

(40) 33 beam splitter

(41) 35 first split beam

(42) 35a polarized beam

(43) 35b polarized beam

(44) 37 second split beam

(45) 37a polarized beam

(46) 37b polarized beam

(47) 39 mirror

(48) 41 half-wave plate

(49) 43 optical element

(50) 45 first Wollaston prism

(51) 47 second Wollaston prism

(52) 49 lens system

(53) 51 slit

(54) 53 collimation lens system

(55) 55 grating

(56) 57 focusing lens system

(57) 59 array of pixel

(58) 61 laser

(59) 63 beam splitter

(60) 65 beam

(61) 67 beam

(62) 69 half-wave plate

(63) 71 beam

(64) 73 mirror

(65) 75 beam splitter

(66) 77 optical element

(67) 79 optical element, line filter

(68) 81 optical element, polarizer

(69) 83 optical element, polarizer

(70) 85 mirror

(71) 87 beam splitter

(72) 89 beam

(73) 91 beam

(74) 93 beam

(75) 95 waveplate

(76) 97 waveplate

(77) 99 beam splitter

(78) 101 beam splitter

(79) 301 window

(80) 303 spot

(81) 305 spot

(82) 307 window

(83) 309 window

(84) 311 window

(85) 313 spot

(86) 315 spot

(87) 317 spot

(88) 319 window

(89) 321 window

(90) 323 window