WO2018115446A1 - Appareil permettant de mesurer une activité optique et/ou l'anisotropie optique - Google Patents

Appareil permettant de mesurer une activité optique et/ou l'anisotropie optique Download PDF

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Publication number
WO2018115446A1
WO2018115446A1 PCT/EP2017/084396 EP2017084396W WO2018115446A1 WO 2018115446 A1 WO2018115446 A1 WO 2018115446A1 EP 2017084396 W EP2017084396 W EP 2017084396W WO 2018115446 A1 WO2018115446 A1 WO 2018115446A1
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Prior art keywords
optical
sample
radiation
adjustable
module
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PCT/EP2017/084396
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English (en)
Inventor
Nicola Felice CERULLO
Dario POLLI
Fabrizio PREDA
Julien RÉHAULT
Jan HELBING
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Politecnico Di Milano
Universität Zürich
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Publication of WO2018115446A1 publication Critical patent/WO2018115446A1/fr

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    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/21—Polarisation-affecting properties
    • G01N21/23—Bi-refringence
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/17—Systems in which incident light is modified in accordance with the properties of the material investigated
    • G01N21/19—Dichroism
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N21/00—Investigating or analysing materials by the use of optical means, i.e. using sub-millimetre waves, infrared, visible or ultraviolet light
    • G01N21/62—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light
    • G01N21/63—Systems in which the material investigated is excited whereby it emits light or causes a change in wavelength of the incident light optically excited
    • G01N21/65—Raman scattering
    • G01N2021/653—Coherent methods [CARS]
    • G01N2021/655—Stimulated Raman

Definitions

  • This disclosure relates to the field of measurement on optical activity and/or optical anisotropy.
  • optical activity is the ability of a chemical substance to rotate the direction of polarization of light to the right or left, independent of sample orientation.
  • Optical activity occurs in chiral materials, those lacking mirror symmetry.
  • ⁇ is a complex function of the frequency ⁇ , whose real and imaginary parts describe, respectively, the circular birefringence (CB, often also called optical rotatory dispersion ORD) and the circular dichroism (CD) .
  • CB circular birefringence
  • CD circular dichroism
  • the former is intimately dependent on the difference of the refractive indexes of the chiral material ex- perienced by LH and RH circularly polarized light ( ⁇ - ⁇ ( ⁇ )- ⁇ ( ⁇ )) , while the latter depends on the difference of the extinction coefficients for LH and RH circularly polarized light (ku-i(u))-kRH(u))) .
  • Linearly polarized light can be regarded as a superposition of LH and RH circularly polarized light.
  • CD differential absorption
  • CB difference in refractive index
  • CB is associated with the component of the chiral signal that is in phase with the incident light
  • CD gives rise to a 90° phase-shifted component
  • Spectropolarimeters are known ; these apparatuses typically use a lamp as an incoherent light source and are based on the following principle of measurement: a monochromator selects a particular wavelength of the incoming light, which then passes through a linear polarizer. A photoelas- tic modulator (PE ) induces a polarization modulation (usually at tens of kHz frequency) , between LH and RH light. A detector, typically a photom ultiplier tube (PMT) or an InGaAs photodiode, measures the intensity of light transmitted by the sample. For an isotropic sample of chiral mole- cules, this intensity is slightly different for LH and RH polarization states (typically by 1 part in 1 0 2 -
  • Spectropolarimeters have the disadvantages of being very expensive, often bulky (double prism monochromators require long optical path length) and measuring only CD (typically, fur- ther accessories must be inserted into the beam path in order to measure also CB).
  • Document US 3,737,235 discloses a polarization interferometer with beam polarizing compensator wherein no polarizer coupled with the sample and the detector, so as to provide a second polychromatic radiation having a linear polarization, is described or suggested.
  • the absence of the said polarizer coupled with the sample and the detector limits the measurable quantities to linear dichro- ism and circular dichroism , whereas, as explained later, the presence of such a polarizer in our setup allows one to measure not only linear dichroism and circular dichroism, but also linear bire- fringence and circular birefringence.
  • Document US2012/0268740 describes an apparatus for vacuum ultraviolet circular dichroism spectroscopy consisting of a broadband source followed by the sample under test, a compensator (which can be continuously rotated during measurement) , an analyzer and a spectrometer- detector.
  • the compensator imparts a circular polarization component to the beam .
  • the spectrometer detector comprises both dispersive/diffractive elements and a multi-element array detector, like a photodiode array or a charge coupled device.
  • the broadband light from the optical system is spatially separated by the dispersive/diffractive element, such that light of different wavelength illuminates the detector array at different locations.
  • the apparatus described by US2012/0268740 requires a multi-element detector and measures a signal as a function of the rotation angle of a compensator, for several angles, and so it performs slow measurements. Moreover, the apparatus described by US2012/0268740 is specifically designed for vacuum ultraviolet experiments (between 10 and 200nm) for integration into a beam line at a synchrotron radiation facility. It is also observed that according to US2012/0268740 the generated signals have very low intensity and it is difficult to extract them from the instrumental noise.
  • CD and CB apparatuses are often based on a time-domain measurement rather than a frequency dispersion device like a monochromator: light from the (incoherent) source, like a glow bar, is split into two replica of variable time delay by a Mi- chelson interferometer. Spectra are obtained by scanning the time delay and Fourier-transform ing the measured interferogram .
  • CD and CB are the same as in the UV-visible range: a linear polarizer is followed by a fast polarization modulator, which changes the state of polarization from LH to RH circular polarization at high frequency. Lock-in demodulation then separates the chiral from the achiral signal.
  • the sample is chiral and absorbs LH and RH polarized light differently, the intensity at the detector will be modulated accordingly and an interferogram can be recorded.
  • the two perpendicular polarized replica created by the interfer ⁇ ometer do not interfere and no interferogram is seen.
  • the Applicant notices that the known optical activity measurement techniques show lim itations due to the complexity and costs of the apparatuses employed and/or in their performances, such as, the measurement time, the inherent stability, the detectable properties and the light wavelength range.
  • the Applicant addressed the technical problem of providing an apparatus alternative to the known ones and, according to a first aspect, the present invention relates to an optical apparatus as de ⁇ fined by the appended independent claims 1 and 9. Particular embodiments of the optical apparatus are defined by the dependent claims 2-8 and 1 0 - 1 5. A method for measuring optical activity and optical anisotropy is defined by the appended claim 1 6.
  • Figure 1 schematically shows an example of an optical apparatus configured to analyse optical activity and optical anisotropy of a sample
  • Figure 2 schematically shows a first optical apparatus which is a first particular embodiment of the apparatus of Figure 1 , employable to obtain Circular Dichroism spectra;
  • Figure 3 schematically shows a second optical apparatus which is a second particular embodiment of the apparatus of Figure 1 , employable to obtain Circular Dichroism and Circular Birefringence spectra;
  • Figure 4a shows the Circular Birefringence on spectra experimentally obtained using an apparatus analogous to the one of Figure 3;
  • Figure 4b shows the Circular Dichroism spectra experimentally obtained using an apparatus analo- gous to the one of Figure 3;
  • Figure 5 schematically shows a third optical apparatus which is a third particular embodiment of the apparatus of Figure 1 , employable to measure Stimulated Raman Optical Activity;
  • Figure 6 shows examples of time profiles of radiation pulses propagating into said third optical apparatus.
  • Figure 1 shows schematically an optical apparatus 1 00 which can be employed to analyse optical activity and optical anisotropy of a sample 1 .
  • the optical apparatus 100 can be employed to measure the following optical properties: Circular Dichroism (CD) and Circular Birefringence (CB, sometimes also called Optical Rotatory Dispersion, ORD), both connected to the optical activity.
  • Optical activity is present in chiral materi- als/samples lacking mirror symmetry.
  • the optical apparatus 1 00 can be employed to measure also the Stim ulated Raman Optical Activity (Stimulated ROA) .
  • optical apparatus 1 00 can be employed to measure Linear Birefringence (LB) and Linear Dichroism (LD) which are connected to optical anisotropy.
  • the sample 1 to be tested with the optical apparatus 1 00 can be an isotropic sample, e.g. ensem- bles of randomly oriented molecules such as chromophores in solution.
  • anisotropic samples e.g. molecular ensembles in which the molecules are to some extent aligned, such as in samples with "texture”
  • LD and LB signals are usually m uch larger than CD and CB.
  • optical activity spectra are by definition independent of the orientation of the sample, while LB and LD completely vanish by proper orientation of the sample 1 .
  • the optical apparatus 1 00 can be used at will to measure LB and LD or, by proper rotation of the sample or the optical bench, CD and CB spectra.
  • the optical apparatus 1 00 comprises an electromagnetic radiation source module 2 and an optical path 20 including an adjustable optical birefringent module 3 and a support 6 for the sample 1 under test.
  • the first optical path 20 is coupled to a detector 4 connected with a processing and control module 5.
  • the order of the components/elements of the optical apparatus 100 along the optical path 20 can be different from the one shown in the drawings.
  • the following axes/directions can be defined :
  • the first direction x and the second direction y define a plane which is orthogonal to the propagation direction z.
  • the electromagnetic radiation source module 2 (hereinafter, the source module 2) is configured to generate a polychromatic radiation having a linear polarization.
  • the source module 2 includes a radiation generation device 7 which can be an incoherent source (e.g. a lamp) or a coherent source (e.g. a laser) .
  • the source module 2 is structured to generate a polychromatic (i.e. m ulti-wavelength) radiation having a broad bandwidth , particularly, comprising light in the UV (1 80-400nm), visible (400-750 nm) and infrared (750 ⁇ -20 ⁇ ) range.
  • the bandwidth of the radiation produced by source module 2 can be chosen depending on the supposed property of the sample 1 under test.
  • the source module 2 also comprises a polarization control module 8 which allows selecting a controllable polarized radiation having a high purity.
  • it could be linear polarization with pre-established direction, selected by a polarizer with very high ex- tinction ratio (typically >10 5 ).
  • it could be a circular/elliptical polarization with selectable ellipticity and orientation of the axes.
  • the source module 2 is configured to produce an incident radiation RIN having, as an example, a linear polarization Pe lying in the plane x-y and forming a non-null angle ⁇ & with the first direction x and a non-null angle with the second direction y.
  • the adjustable optical birefringent module 3 is an optical device configured to produce first and second linearly polarized radiations reciprocally delayed by an adjustable time delay.
  • the adjustable optical birefringent module 3 is structured to receive the incident radiation RIN and produce a transmitted radiation RTR which is formed by a first transmitted radiation component Rxi propagating along the propagation direction z (having a linear polarization parallel to the first direction x) and a second transm itted radiation component R y i prop ⁇ agating along the propagation direction z (having a linear polarization parallel to the second direction y) .
  • the first transmitted radiation component R x i and the second transmitted radiation component R y i are replicas (particularly, attenuated replica) of the incident radiation RIN entering the optical birefringent module 3.
  • the second transmitted radiation component R y i is delayed, by an adjustable time delay x, with respect to the first transmitted radiation component Rxi .
  • the above defined delay x can have a negative or a positive value.
  • the adjustable optical birefringent module 3 includes at least one movable birefringent component. Particular embodiments of the adjustable optical birefringent module 3 will be described later.
  • the sample 1 can be any type of material (suitably housed) employed, as an example, in biological and pharmaceutical areas for which there is the need to investigate its optical activity.
  • CD and CB measurements in the UV spectral region can be performed in order to retrieve crucial information on the structural conformation of macromolecules, molecules, DNA/RNA and proteins, to study and characterize, e.g. their structure, stability (under heat, stress, denaturation), fold- ing, kinetics and interactions.
  • CD and CB measurements can be performed from the near-infrared to the long-wavelength infrared region, especially in the 3-10 ⁇ region, in resonance with the so-called fingerprint vibrational region of molecules (often called Vibrational Circular Dichroism, VCD) , to provide three-dimensional structural information, not only on small molecules, but also on large and complex biopolymers such as proteins, polypeptides and nucleic acids/DNA.
  • VCD Vibrational Circular Dichroism
  • the sample 1 under test when reached by the transmitted radiation RTR i.e. the first and second transmitted radiation components Rxi and R y i
  • the transmitted radiation RTR i.e. the first and second transmitted radiation components Rxi and R y i
  • the output radiation Rou has a polarization state depending on the optical anisotropy of the sample.
  • Detector 4 is configured to convert a polychromatic radiation resulting from the optical path 20 into an electrical signal.
  • the detector 4 is an optical-to-electrical converter to be coupled to the sample 1 and configured to convert at least a portion of the output radiation Rou into a detected electrical signal S(T) (e.g. an electrical current ⁇ ( ⁇ ), or an electrical voltage V(x)), proportional to the output radiation Rou.
  • S(T) e.g. an electrical current ⁇ ( ⁇ ), or an electrical voltage V(x)
  • the detected electrical signal S(v) represents an interference electrical signal (that is also called interferogram) , having a behavior depending on said optical activity and/or said optical anisotropy of the sample 1 , that oscillates as a function of the said adjustable time delay ⁇ between the first radiation component R x i and the second radiation component R y i imparted by the adjustable optical birefringent module 3.
  • the detector 4 is a photodiode or a photom ultiplier with suitable bandwidth.
  • the detector 4 can be a bolometer or a MCT (Mercury Cadmium Telluride) detector or other suitable I R-detector.
  • the detector 4 is a single-element device having a single continuous detection surface which is illuminated by the polychromatic radiation.
  • the detector 4 has, particularly, a single output for the detected electrical signal S( ).
  • the processing and control module (5) is configured to process the electrical signal S(T) and provide an electromagnetic spectrum which depends on the optical activity and/or the optical anisotro- py of the sample 1 . Particularly, the processing and control module 5 is configured to perform a
  • the processing and control module 5 is also configured to vary said adjustable time delay ⁇ within a delay range which is a multiple of the duration of the optical cycle of the central wavelength of the polychromatic incident radiation and which de- termines the resolution of the retrieved electromagnetic spectrum .
  • the processing and control module 5 is provided with an analog-digital converter 14 and a computing device 15 (CU) .
  • the computing device 15 (as an example, a computer apparatus or an oscilloscope) can include a module configured to compute a Fourier Transform of an input digital signal.
  • the processing and control module 5 can cooperate with an actuator 9 acting on the adjustable optical birefringent module 3 to vary the time delay ⁇ .
  • the actuator 9 may include a motor, which allows the adjusting of the movable component of the optical birefringent module 3.
  • the processing and control module 5 acts on the actuator 9 to vary, preferably, in con- tinuous manner the adjustable time delay ⁇ introduced by the optical birefringent module 3 within a delay range, defined by a minimum and a maximum value.
  • the processing and control module 5 generates control signal Sc to control the actuator 9.
  • the optical birefringent module 3 varies the adjustable time delay ⁇ within a given delay range, preferably in continuous manner.
  • the processing and control module 5 either generates a control signal Sc to control the actuator 9 and/or it receives a suitable signal that uniquely identifies the actuator position at every measurement, and hence allows determining the delay ⁇ with suitable precision.
  • the electromagnetic radiation source module 2 generates the in ⁇ cident radiation RIN which propagates along the optical path 20 comprising the adjustable optical bi- refringent module 3 and the sample 1 .
  • the adjustable optical birefringent module 3 provides a transmitted radiation RTR reaching the sample 1 which provides an output radiation Rou.
  • the output radiation Rou is detected by the detector 4 which converts it into the electrical signal S(T).
  • the electrical signal S(T) is processed by the processing and control module 5.
  • the highly polarized incident radiation RIN generated by the electromagnetic radiation source module 2 reaches the adjustable optical birefringent module 3 which produces the transmitted radiation RTR having the two delayed replicas Rxi and R y i of the incident radiation RIN.
  • the adjustable optical birefringent module 3 is designed to introduce a delay ⁇ up to, as an example, hundreds or thousands optical cycles, where the optical cycle is evaluated with reference to the central carrier wavelength of the incident radiation RIN.
  • the adjustable optical birefringent module 3 is so as to allow controlling with very high precision (on the order of attoseconds) the relative delay ⁇ between the two transmitted radiation components Rxi and R y i .
  • the adjustable optical birefringent module 3 introduces the said delay ⁇ by means of a linear translation of preferably one birefringent optical element.
  • the relative strength of these two replicas Rxi and R y i can be tuned by changing the direction of the polarization of the incident radiation RIN (angle ⁇ in Figure 1 ) with respect to the optical axes of the birefringent materials inside the adjustable optical birefringent module 3.
  • the first transmitted radiation component Rxi and the second transmitted radiation component R y i reach the sample 1 : if such sample does not show optical activity (i.e. it is not a chiral sample) and it is not anisotropic (i.e. it does not show LD nor LB) the polarization states of the two perpendicular incom ing electrical fields Rxi and R y i are not rotated during their propagation through the sample 1 .
  • the output radiation Rou shows only the following two components:
  • a second output radiation component RY y 2 propagating along the propagation direction z (having polarization parallel to the second direction y) which has been induced by the second transmitted radiation R y i after propagation in the sample 1 .
  • the sample 1 shows an optical activity (i.e. it is a chiral sample) and/or it is anisotropic (i .e. it shows LD and/or LB)
  • the output radiation Rou which shows not only the aforementioned RXx ⁇ and RY y 2, but also two more components:
  • a third output radiation component RX y 2 propagating along the propagation direction z (having po- larization parallel to the second direction y) which is due to the interaction of the chiral (and/or anisotropic) sample 1 with the incident first transm itted radiation R x i ;
  • a fourth output radiation component RYx2 propagating along the propagation direction z (having polarization parallel to the first direction x) which is due to the interaction of the chiral and/or anisotropic sample 1 with the incident second transmitted radiation R y i .
  • the output radiation Rou reaches the input port of the detector 4 on which the radiation compo- nents having the same polarization direction interfere.
  • the third output radiation component RXy2 having polarization parallel to the second direction y interferes with the second output radiation component RY y 2
  • the fourth output radiation component RY X 2 having polarization parallel to the first direction x interferes with the first output radiation component RX X 2.
  • detector 4 generates the electrical signal Sf 1 ) that is not constant and rep- resents the interferogram (oscillating as a function of the delay ⁇ ) generated by the interference between the radiation components.
  • the processing and control module 5 performs the complex (comprising both the real and imaginary parts) Fourier Transform with respect to the delay ⁇ of the electrical signal Sft), so obtaining a complex spectrum as a function of the radiation wavelength.
  • this spectrum carries information on the optical activity of a chiral sample (CD,
  • ⁇ is a complex function of the frequency ⁇ , whose real and imaginary parts describe, respectively, the circular birefringence (CB) and the circular dichroism (CD) .
  • the circular birefringence (CB) is intimately dependent on the difference of the refractive indexes of the material experienced by LH and RH circularly polarized radiation (n_H((o)-nRH(u))), while the circular dichroism (CD) depends on the difference of the extinction coefficients for LH and RH circularly polarized light (ku-i(u>)-kRH((d)) .
  • Linearly polarized radiation can be regarded as a superposition of LH and RH circularly polarized radiation.
  • CD differential absorption
  • CB refractive index
  • Both correspond to the generation of a polarization component perpendicular to the incoming polarization direction, which is called chiral signal.
  • CB is associated with the component of the chiral signal that is in phase with the incident radiation, while CD gives rise to a 90° phase-shifted component.
  • optical apparatus 100 Particular embodiments of the optical apparatus 100 will be described in the following. Identical or analogous elements, components or devices are indicated in the figures of the present description with the same reference numbers or symbols.
  • first optical apparatus 110 first optical apparatus 110
  • Figure 2 schematically shows a first optical apparatus 1 1 0, which is a first embodiment of the optical apparatus 100 and can be used to measure a CD (Circular Dichroism) signal and/or LD signals.
  • the polarization control module 8 is a first linear polarizer employed to obtain the linearly polarized incident radiation RIN.
  • the adjustable optical birefringent module 3 comprises a first adjustable wedge pair 10 and an optical element 1 1 .
  • the first adjustable wedge pair 1 0 comprises a first optical wedge 12 and a second optical wedge 13.
  • Both first 12 and second 13 optical wedges are made of a birefringent material and , as an example, show a fast polarization direction (also called fast axis) parallel to the first direction x and a slow polarization direction (also called slow axis) parallel to the second direction y.
  • the first optical wedge 12 and the second optical wedge 13 are optical prisms.
  • At least one of the two optical wedges 12 and 13 is movable along the second direction y by means of the actuator 9.
  • the first optical wedge 12 is movable and the second optical wedge 13 is fixed.
  • the time delay ⁇ between the two replicas is adjusted by changing the position (i.e. the insertion) of the first optical wedge 12 along the second direction y.
  • the second (fixed) wedge 13 is used to m inimize the spatial separation of the different wavelengths (prism effect due to dispersion, i.e. variable index of refraction with wavelength) and of the two replicas R x i , R y i (due to different refraction because of birefringence, i.e. different index of refraction for the two polarizations) .
  • the optical element 1 1 is a birefringent plate having the optical axis perpendicular both to the propagation direction z and to the optical axis of the two optical wedges.
  • the optical element 1 1 causes a delay between the replicas (i.e. the components Rxi , R y i), which is of opposite sign with respect to that induced by the wedges pair 10, thus permitting both positive and negative delays ⁇ between the two radiation components having perpendicular polarizations.
  • the optical element 1 1 exchanges the fast and slow polarization directions with respect to the first optical wedge pair 1 0, reversing the relative delay ⁇ between the components Rxi and R y i .
  • the slow axis of the optical element 1 1 must be oriented along the first direction x and its fast axis along the second direction y.
  • the CD and/or LD signals can be measured while the CB and/or LB signals cannot be detected.
  • the two radiations having polarizations parallel to the first direction x and the second direction y at the output of sample 1 do not interfere at the detector if the interaction of the light with the sample does not cause a rotation of the polarization state. Linear and circular birefringence can thus not be measured.
  • the presence of (linear or circular) dichroism causes a projection of part of the two incoming or- thogonal polarizations onto a common polarization plane, so that CD and LD can be measured.
  • the optical intensity and therefore the electrical signal S(T) provided by the detector 4 shows the following expression :
  • the analog-to-digital converter 1 4 ( Figure 1 ) of the processing and control module 5 converts the electrical signal S(T) into a digital signal which is processed by the computing device 1 5 to provide a Fourier Transform with respect to the variable delay ⁇ .
  • the polarization control module 8 produces a linear polarization
  • the chiral signals are extracted by computing the sine Fourier transform of (1 ) (in the following indicated with sFT) with respect to ⁇ .
  • the equation (1 ) is not valid anymore: if the mayor axes of the polarization ellipse are x and y, the CD signal will be multiplied by a cosine term , so that the chiral signal will be extracted by computing a cosine Fourier (in the following indicated with cFT) with respect to 1 .
  • a mixture of cosine and sine Fourier transforms m ust be employed.
  • the optical apparatus 100 and the first embodiment of figure 2 make possible extending time-domain Fourier-Transform chiral spectroscopy to the visible and UV spectral range since the interferometer is based on the adjustable optical birefringent module 3 in which the two replicas of the radiation are not separated in space (that would cause in- creasing instabilities at shorter wavelengths, thus destroying the interference pattern) but rather in polarization.
  • the two replicas always travel via the same optics and follow the same optical path 20, so that the interferometer can be called "common mode".
  • Figure 3 schematically shows an example of a second optical apparatus 120, which is a second embodiment of the optical apparatus 1 00.
  • the second optical apparatus 120 is suitable to simultaneously provide CD and CB spectra and/or LB and LD spectra.
  • the second optical apparatus 1 20 can be analogous to the first optical apparatus 1 10 but it also comprises a second polarizer 16 which is placed , as an example, between the sample 1 and the detector 4.
  • the second polarizer 16 is oriented in such a way to select one of the two orthogonal polarizations of the output radiation Rou providing a selected output radiation Roup.
  • the polarizer 1 6 preferably is directly coupled with the sample 1 , which is in the support 6, and the detector 4 as to provide a second polychromatic radiation (R ou p) having linear polarization.
  • this condition can be fulfilled by rotating the second polarizer 1 6 so as to minimize the amplitude of the interference pattern in the S(T) (i.e. the interfer- ogram) when the sample 1 is removed or replaced by an achiral sample such as pure solvent.
  • the polarization control module 8 produces a linear polarization at the entrance of the adjustable optical birefringent module 3; in case it produces an elliptical polarization with major axes along x and y, the roles of sFT and cFT are inverted. For any other input polarization state an appropriate phase shift must be included.
  • the (absolute) magnitude of the CD and CB signals can be retrieved by suitable calibration via a second measurement where the second polarizer 1 6 is tilted by a well-defined angle Bcai (different from 0° and 90°) with respect to the vertical direction (as indicated in Fig. 3).
  • Bcai different from 0° and 90°
  • the two replicas generated by the adjustable optical birefringent module 3 interfere at the detector 4 and the much weaker chiral and anisotropy signals can be neglected, giving : i 0 £?
  • the amplitude of the ⁇ ( ⁇ ) interference term in equation (3) proportional to sin2 sin ⁇ cos ⁇ s maximized and the calibration procedure is facilitated (stronger signals are retrieved) .
  • CD ⁇ 2 ⁇ ⁇ 1 sFr[/ h ,(r)]/cFr[l Cfli ( 3 ⁇ 4 ft al )] (4b)
  • equations (2a), (2b) , (4a) and (4b) still hold if the sample or the optical bench are properly oriented in space. If this is not the case, equations (4a) and (4b) will yield the LD and LB spectra, respectively, instead of CB and CD.
  • the Applicant has realized (employing discrete optical components) the second optical apparatus 120 and has tested it with the two enantiomers of a chiral sample (Nickel Tartrate) in the near- infrared spectral region.
  • Figure 4a shows the CB spectra experimentally obtained : the spectrum for the RH enantiomer is, as expected, the mirror image of the one obtained for the LH enantiomer.
  • Figure 4b shows the CD spectra experimentally obtained using the same chiral sample of Figure 4a: the spectrum for the RH enantiomer is, as expected, the mirror image of the one obtained for the LH enantiomer.
  • the signal recorded using the second polarizer 1 6 in a tilted (angle ⁇ different from 0° and 90°) orientation can also be used for calibrating the optical delay ⁇ of the interferometer, precisely determ ining the zero-delay position.
  • This calibration relies on the fact that in this case the signal must be an even function (symmetric in ⁇ )_ so that a proper algorithm can flatten its Fourier Transform phase around zero, i.e. to minim ize the imaginary part of the Fourier Transform . This can be achieved, as an example, finding the peak of the self-convolution of the in- terferogram .
  • interferograms in equations (1 ) and (2a, 2b) lie over a background offset.
  • maximization of the modulation depth is needed.
  • the "modulation depth” is the ratio between the amplitude of the fringe pattern in the interferogram and its offset.
  • equation (1 ) shows that the offset is 1 and the amplitude of the fringe pattern is proportional to sin d cos d ⁇ so that the modulation depth for the CD spectrum is s ⁇ $ cos tf ⁇ CD
  • the first transmitted radiation component Rxi of the transmitted radiation RTR (having polarization parallel to the first direction x) is more intense than the other component R y i and generates a chiral signal (i.e. the third output radiation component RX y ⁇ ) having a polarization along the second direction y (Fig 3) .
  • the third output radiation component RX y 2 and the local oscillator RY y 2 are transmitted by the second polarizer 1 6, while RXx2 and RYx2 are blocked.
  • the third output radiation component RX y 2 interferes with the second output radiation component
  • RY y 2 which acts as a local oscillator for self-heterodyne amplification (sometimes also called ho- modyne amplification) as a function of delay ⁇ .
  • self-heterodyne amplification it is meant the (non-linear) mixing of the weak chiral electric field emitted by the sample 1 with the electric field of the local oscillator: their interference at the de- tector 4 causes the appearance of a m ixing term (double product of the two fields) that bears in ⁇ formation on the optical activity of the sample and is proportional to the local oscillator (that acts as an amplifier with tunable gain).
  • the self-heterodyne amplification analogously occurs with the m ixing of the LB signal and/or the LD signal and the local oscillator.
  • the above described optical apparatus 100 (together with the first and second embodiments 1 1 0 and 120) performs a high-frequency detection because the information on the CD/CB/LD/LB spectra is contained in the high-frequency oscillating pattern of the interferogram .
  • the information is distributed in the interferograms as variations in the amplitude, period and sign of the fringes.
  • the spacing between a maximum and the adjacent m inimum of the interferogram corresponds to a path-length difference, generated by the interferometer of the optical apparatus 1 00, of approximately half the central wavelength of the spectrum generated by the elec- tromagnetic radiation source module 2.
  • gear ratio depends only on the apex angle of the first wedge 12 and the birefringence (in particular the difference ⁇ 9 in the group index of refraction experienced by the ordinary and extraordinary waves) of the optical glass employed, and is typically -50 for small apex angles (a few degrees) and standard birefrin- gent materials (with birefringence ⁇ 9 «0. ⁇ ) .
  • a motor speed of 10 mm/second at 200- nm central wavelength corresponds to a 1 -kHz modulation (i.e. 1000 fringes per second) .
  • This enables the optical apparatus 100 (together with the first and second embodiments 1 1 0 and 120) to remove low-frequency fluctuations of the light source, thus improving the signal-to-noise ratio and reaching sensitivities as high as those achieved by spectropolarimeters, or even higher.
  • the actuator 9 has a motor which allows adjusting the movable component of the optical birefringent module 3 with a speed higher than 1 0 m m/second.
  • the optical apparatus 100 can also be operated in combination with a polarization modulator (not shown) , placed, as an example, between the first polarizer 8 and the adjustable optical birefringent module 3.
  • a polarization modulator (not shown)
  • modulation of the signal sign can also be induced purely mechanically by periodically changing the orientation of the first polarizer 8 between -9 to 9.
  • the position of the sample 1 shown in the figures 1 , 2 and 3 is only an example.
  • the sample 1 can be also placed between the electromagnetic radiation source module 2 and the adjustable optical birefringent module 3.
  • the components of the adjustable op- tical birefringent module 3 can be placed according to a different order from the one shown in the drawings.
  • the sample 1 can be placed between two of the components of the adjustable optical birefringent module 3.
  • Figure 5 shows a third optical apparatus 130 which represents a third embodiment of the optical apparatus 1 00.
  • the third optical apparatus 130 can be used for measuring Raman Optical Activity
  • ROA spectra and is based on Stimulated ROA (called SROA in the following) and comprises, inter alia, an optical pump path 30 including a pulsed pump radiation source module 17 and an op ⁇ tional third polarizer 27.
  • the pulsed pump radiation source module 1 7 can be a coherent source, such as a pulsed laser producing a pump radiation Rp which is a pulsed radiation with spectrum narrower or equal to the width of the Raman features to be measured.
  • the third polarizer 27 is oriented to selected a linear polarization parallel , according to the example shown Figure 5, to the first direction x.
  • the third polarizer 27 is configured to transmit on the sample 1 a pump transm itted radiation Rp x , having linear polarization along the first direction x.
  • the propagation direction z and the further propagation direction r can be collinear.
  • the radiation generation device 7 it can be analogous to the pulsed pump radiation source module 1 7 and is configured to produce an incident radiation F1 ⁇ 2 having pulsed form , but possibly with a broader spectral bandwidth, which ultimately defines the accessible spectral re- gion to observe the SROA signals.
  • the adjustable optical birefringent module 3 shown in Figure 5 comprises, as an example, a second adjustable wedge pair 21 and a first fixed wedge pair 22 optically coupled to the optical element 1 1 .
  • the second adjustable wedge pair 21 and the first fixed wedge pair 22 are structurally analogous to the first adjustable wedge pair 1 0 of Figure 2.
  • the second adjustable wedge pair 21 comprises a third optical wedge
  • the third optical wedge 23 shows the optical axis parallel to the second direction y and the fourth optical wedge 24 shows the optical axis parallel to the propagation direction z.
  • the third optical wedge 23 is fixed with respect to the fourth optical wedge 24 and the whole second adjustable wedge pair 21 is movable by a corresponding actuator 9 along the second direction y.
  • the first fixed wedge pair 22 comprises a fifth optical wedge 25 showing an optical axis parallel to the propagation direction z and a sixth optical wedge 26 showing an optical axis parallel to the second direction y.
  • the transm itted radiation RTR exiting the adjustable optical birefringent module 3 and reaching the sample 1 plays the role of a Stokes beam in a standard stim ulated Raman scattering measurement. Its frequency detuning with respect to the pump pulse determ ines the frequencies of the vibrational modes accessible in the SROA measurement.
  • the sample 1 is placed so as to be reached by the transmitted radiation RTR and the pump transmitted radiation Rp x . Their interaction with the sample 1 produces the output radiation Rou.
  • the second polarizer 1 6 shown in figure 5 allows one to select the desired output radiation Roup, which shows polarization directed parallel to the second direction y, in accordance with the example of figure 5.
  • ROA allows characterizing chiral molecules. This phenomenon is based on inelastic Raman scattering of radiation which monitors the vibrational response of a molecule. Spontaneous ROA spectra can be measured as the difference between the Raman spectra of the sample under illum ination with RH and LH circularly polarized incident and/or scattered light. ROA signals are typically extremely small, of the order of 1 0 3 in comparison with spontaneous Raman scattering, i.e. the achiral background.
  • Spontaneous ROA can be used as an analytical tool in stereochemistry and biochem istry. Thanks to its high sensitivity to three-dimensional molecular structure, ROA spectroscopy can also be used to investigate the conformation of biological molecules such as proteins and DNAs in aqueous solutions where vibrational CD is limited by strong solvent absorption.
  • the x component of the transm itted radiation RTR interacts on the sample 1 with the pump transm itted radiation Rpx to generate a chiral SROA signal polarized along the second direction y, while the y-component of the pulsed incident radiation RIN will serve as local oscillator signal to amplify the chiral SROA signal.
  • the third optical apparatus 130 is based on a coherent interaction of two pulsed radiations sim ultaneously interacting with the sample 1 , thus amplifying the resulting signal by orders of magnitude with respect to the spontaneous ROA apparatuses.
  • the second polarizer 16 allows one to remove the achiral major component included into the out- put radiation Rou exiting the sample 1 .
  • the transmitted chiral signal and the local oscillator signal included into the selected output radiation Roup interfere on the detector 4, producing the electrical signal S'(T) defining an interferogram as a function of the delay T introduced by the adjustable optical birefringent module 3.
  • the processing and control module 5 performs a Fourier Transform of this interferogram with re- spect to the delay T yielding a spectrum , i.e. the SROA signal as a function of optical frequency.
  • the fourth optical wedge 24 and the fifth optical wedge 25, that are additional components with respect to the embodiments of Figure 2 and 3, allow scanning the delay ⁇ without changing the temporal-overlap between the major component of the Stokes pulse and the pump pulse.
  • Figure 6 shows the temporal profiles of the pump beam pulse Pp (polarized along the first direction x) , the first replica Psi of the Stokes beam (polarized along the first direction x) and the second replica Ps2 (polarized along the second direction y) .
  • the time delay ⁇ can be varied (preferably in a continuous manner) between a maximum and a minim um value.
  • the retrieved signal does not vary over the period of the carrier frequency (a few femtoseconds) , but it rather presents a slow variation of its profile over the course of the pulse envelope following its overlap with the pump pulse (over the picosecond time scale) .
  • this achiral signal constitutes a (large but slowly varying) offset in the retrieved interferogram , so that the Fourier Transform of the achiral signal is centred at low frequencies, out of the spectral region of interest given by the Fourier Transform of the aforementioned chiral signal and thus it does not perturb the SROA signal.
  • the modulation depth of the interferogram obtainable with the third optical apparatus 130 of figure 5 is proportional to cot( ) and the signal-to- noise ratio can be optim ized by adjusting the angle d.
  • Rotating the second polarizer 1 6 by an angle ⁇ and recording an interferogram under otherwise identical conditions enables to properly phase the interferograms (i.e. finding the position of the ze- ro path-length difference) and to measure the achiral SRS signal for absolute amplitude calibration.
  • the above method is compatible with high-frequency modulation of the pump-pulse intensity at MHz frequency combined with lock-in detection, which is known to significantly enhance the sensitivity of achiral SRS spectroscopy because the Fourier Transform operator is linear (i.e. the Fourier Transform of the difference of two signals is the difference of the two Fourier Transforms, where the difference is computed by the lock-in).
  • a high-frequency modulator (not shown) can be placed at the input or at the output of the third polarizer 27 and a lock-in amplifier (not shown) can be placed between detector 4 and the processing and control module 5.
  • the sequence order of the optical components of the adjustable optical birefringent module 3 can be modified with respect to the one shown in Figure 5.
  • the adjustable optical birefringent module 3 can be provided with a single optical wedge pair, analogous to the optical wedge pair 1 0 ( Figure 3) , which can be placed between the sample 1 and the second polarizer 16.
  • Optical apparatus (1 00) comprising:
  • a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;
  • an adjustable optical birefringent module (3) configured to produce first and second radiations (Rxi , R y i ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations
  • a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radia- tion, an output radiation (Rou) having a linearly polarized component (RX y 2 or alternatively RY X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;
  • a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX y 2 or alternatively RYx2) with a further linearly polarized component ( ⁇ ) of the second polychromatic radiation (Rou);
  • processing and control module (5) configured to process the interference electrical signal (S(-z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) .
  • adjustable optical birefringent module (3) comprises:
  • an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R x i , R y i) from the first polychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;
  • the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay.
  • the detector 4 is configured to receive said output radiation (Rou) comprising said linearly polarized component (RX y 2 or alterna- tively RY X 2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RY y 2 or alternatively RX X 2) having the function of local oscillator signal for self-heterodyne amplification ; the linearly polarized component (RXy2 or alternatively RYx2) and the additional linearly polarized component (RY y 2 or alternatively RXx2) having parallel polarizations.
  • said output radiation comprising said linearly polarized component (RX y 2 or alterna- tively RY X 2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RY y 2 or alternatively RX X 2) having the function of local oscillator signal for
  • the apparatus (1 10) according to the embodiment 1 wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Circular
  • CB Cirrcular Birefringence, also called Optical Rotatory Dispersion
  • LB Linear Birefringence
  • LD Linear Dichroism
  • an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transm itted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.
  • RPX linearly polarized pulsed pump radiation
  • said adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9) .
  • Method for measuring optical activity or optical anisotropy comprising :
  • Optical apparatus comprising:
  • a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;
  • an adjustable optical birefringent module (3) configured to produce first and second radiations (R x i , Ryi ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations
  • a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radia- tion, an output radiation (Rou) having a linearly polarized component (RXy2 or alternatively RY X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;
  • a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX y 2 or alternatively RY X 2) with a further linearly polarized compo- nent (RYy2 or alternatively RX X 2) of the second polychromatic radiation (Rou) ;
  • processing and control module (5) configured to process the interference electrical signal (S(z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) ;
  • the detector 4 is configured to receive said output radiation (Rou) comprising said linearly polarized component (RX y 2 or alternatively RYx2) corresponding to a chiral signal and/or a signal connected to optical anisotropy of the sample (1 ) and an additional linearly polarized component (RYy2) having the function of local oscillator signal for self-heterodyne amplification ; the linearly po ⁇ larized component (RXy2 or alternatively RYx2) and the additional linearly polarized component (RY y 2 or alternatively RX X ⁇ ) having parallel polarizations;
  • said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization;
  • adjustable optical birefringent module (3) comprises:
  • an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R x i , R y i ) from the first poly- chromatic radiation (RIN) reciprocally delayed by the adjustable time delay;
  • the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay.
  • V The apparatus (1 1 0) according to the embodiment I , wherein the apparatus is structured to provide an electromagnetic spectrum representing at least one of the following measures: a measure of CD (Circular Dichroism) associated with the sample (1 ) , a measure of VCD (Vibrational Circular
  • CB Cirrcular Birefringence, also called Optical Rotatory Dispersion
  • LB Linear Birefringence
  • LD Linear Dichroism
  • an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transm itted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.
  • RPX linearly polarized pulsed pump radiation
  • said adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9).
  • Optical apparatus (1 00) comprising :
  • a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;
  • an adjustable optical birefringent module (3) configured to produce first and second radiations (Rxi , R y i ) reciprocally delayed by an adjustable time delay and having reciprocally orthogonal linear polarizations
  • a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radiation, an output radiation (Rou) having a linearly polarized component (RX Y 2 or alternatively RY X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ; - a detector (4) configured to convert a second polychromatic radiation (Rou) resulting from the optical path (20) into an interference electrical signal (S(z)) representative of an interference of said linearly polarized component (RX Y 2 or alternatively RY X 2) with a further linearly polarized component (RY/2 or alternatively RX X ⁇ ) of the second polychromatic radiation (Rou);
  • processing and control module (5) configured to process the interference electrical signal (S(-z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical ani- sotropy of the sample (1 ) ;
  • adjustable optical birefringent module (3) comprises:
  • an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (R x i , R y i) from the first polychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;
  • the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ;
  • said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (R ou p) having a linear polarization.
  • the apparatus (120) according to the embodiments IX and XI I, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum representing a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associ- ated with the sample (1 ), a second electromagnetic spectrum representing a measure of LB (Linear
  • an additional optical path (30) including an additional radiation source apparatus (1 7, 27) config- ured to generate a linearly polarized pulsed pump radiation (Rpx) to be transm itted to the sample
  • adjustable optical birefringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9).
  • Optical apparatus comprising :
  • a radiation source module (2) configured to generate a first polychromatic radiation (RIN) ;
  • an adjustable optical birefringent module (3) configured to produce first and second radiations
  • a support (6) structured to support a sample (1 ) adapted to provide, in response to an input radiation, an output radiation (ROU) having a linearly polarized component (RXy2 or alternatively RY X 2) depending on optical activity and/or optical anisotropy of the sample (1 )) ;
  • ROU output radiation
  • a detector (4) configured to convert a second polychromatic radiation (ROU) resulting from the optical path (20) into an interference electrical signal (S( z)) representative of an interference of said linearly polarized component (RX y 2 or alternatively RY X 2) with a further linearly polarized component (RX/2 or alternatively RYx2) of the second polychromatic radiation (ROU) ;
  • processing and control module (5) configured to process the interference electrical signal (S(z)) and provide an electromagnetic spectrum depending on said optical activity and/or said optical anisotropy of the sample (1 ) ;
  • said apparatus further comprising a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization.
  • a polarizer (1 6) to be coupled with the sample (1 ) and the detector (4) so as to provide a second polychromatic radiation (Roup) having a linear polarization.
  • an adjustable wedge pair (1 0; 21 ) including a first optical wedge (12; 23) and a second optical wedge (13; 26) configured to produce said first and second radiations (Rx1 , Ry1 ) from the first pol- ychromatic radiation (RIN) reciprocally delayed by the adjustable time delay;
  • the apparatus further comprises an actuator module (9) connected to the processor and control module (5) and mechanically coupled with at least one between said first optical wedge (12 ; 23) and second optical wedge (13; 26) to modify said time delay;
  • ROU output radiation
  • the apparatus (120) according to the embodiments A, D and F, wherein the apparatus is further structured to provide at least one of the following spectra: a first electromagnetic spectrum repre- senting a measure of CB (Circular Birefringence, also called Optical Rotatory Dispersion) associated with the sample (1 ) , a second electromagnetic spectrum representing a measure of LB (Linear Birefringence) associated with the sample (1 ) , a third electromagnetic spectrum representing a measure of LD (Linear Dichroism) associated with the sample (1 ) .
  • CB Cirrcular Birefringence, also called Optical Rotatory Dispersion
  • LB Linear Birefringence
  • LD Linear Dichroism
  • an additional optical path (30) including an additional radiation source apparatus (1 7, 27) configured to generate a linearly polarized pulsed pump radiation (RPX) to be transmitted to the sample (1 ) ; wherein said optical path and the additional optical path are structured to cause Stim ulated Raman Optical Activity in said sample (1 ) and said interference electrical signal (S(-r)) representing the Stimulated Raman Optical Activity.
  • RPX linearly polarized pulsed pump radiation
  • said adjustable optical bire- fringent module (3) further comprises a fixed optical wedge pair (22) optically coupled with said adjustable optical wedge pair (21 ) ; wherein both first optical wedge (12; 26) and a second optical wedge (13; 24) are movable by said actuator module (9) ..
  • the described optical apparatus 1 00 shows several advantages. As an example, it does not require a PE (photoelastic modulator) or a lock-in amplifier, thus it can be much less expensive than the known ones. In addition, it does not require a mono- chromator, thus it is much more compact.
  • PE photoelastic modulator
  • lock-in amplifier a lock-in amplifier
  • the described optical apparatus 1 00 (and the corresponding embodiments) illuminates the sample with all the light colours sim ultaneously, so that it measures the entire spec- trum at once (m ultiplex advantage) .
  • the optical apparatus 100 is based on the heterodyned temporal interferometry technique, which combines heterodyne amplification of the chiral signal with a local oscillator signal and Fourier Transform detection of a temporal interferogram using a single- channel detector (i.e. a single-element device having only one continuous detection surface) .
  • the optical apparatus 1 00 allows extending the measurements to the UV and visible region thanks to the use of a common-path interferometer (while apparatuses employing polarization-division Mi- chelson interferometer require active stabilization) .
  • the optical apparatus 1 00 can afford much higher sensitivities due to the intrinsic higher stability of its interferometer path.
  • the above teachings make it possible and practical to perform in the time-domain CD and CB (and/or LD and LB) measurements without an expensive polarization modulator in a wide spectral range from the far UV to the far I R.
  • the optical apparatus 1 00 and, particular, the embodiments of figure 2 and 3 can be employed to provide electromagnetic spectra representing a measure of VCD (Vibrational Circular Dichroism) , in the relevant spectral range (e.g. I R) .
  • VCD Vehicle Circular Dichroism
  • the optical apparatus 1 00 perm its to measure the LB and LD signals.
  • the high intrinsic stability and precision of the described devices allows extending this measurement principle to a new form of chiral Raman spectroscopy.
  • Stimulated Raman signals can be fast acquired, so that SROA spectra could require acquisition times of only a few minutes, compared to hours required with current technology.

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Abstract

La présente invention concerne un appareil optique (100) comprenant : un module de source de rayonnement (2) configuré de sorte à générer un premier rayonnement polychromatique (RIN) ; un trajet optique (20) couplé au module source (2) et comprenant : un module biréfringent optique réglable (3) pour produire des premier et second rayonnements (Rxi, Ryi) retardés réciproquement par un retard temporel réglable, un support (6) structuré pour supporter un échantillon (1) conçu pour fournir, à la suite d'un rayonnement d'entrée, un rayonnement de sortie (Rou) ayant une composante à polarisation linéaire (RXy2 ou, en variante, RYx2) en fonction de l'activité optique et/ou de l'anisotropie optique de l'échantillon (1)) ; un détecteur (4) configuré de sorte à convertir un second rayonnement polychromatique (Rou) résultant du trajet optique (20) en un signal électrique d'interférence (S (τ)) représentant une interférence de ladite composante à polarisation linéaire (RXy2 ou, en variante, RYx2) avec une autre composante à polarisation linéaire (RYy2 ou, en variante, RXx2) du second rayonnement polychromatique (Rou) ; un module de traitement et de commande (5) configuré de sorte à traiter le signal électrique d'interférence (S (τ)) et à fournir un spectre électromagnétique en fonction de ladite activité optique et/ou de ladite anisotropie optique de l'échantillon (1).
PCT/EP2017/084396 2016-12-22 2017-12-22 Appareil permettant de mesurer une activité optique et/ou l'anisotropie optique WO2018115446A1 (fr)

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