Patent Application
20210341382
The present invention generally relates to the field of interferometry and noise reduction in interferometers and other measurement devices derived from or coupled to interferometers such as ellipsometers or biosensors.
It is known to use interferometric devices for ellipsometric measurements, such as in the European patent EP1893977B1 or in the French patent FR2685962. It is also known to use interferometric devices for surface plasmon resonance (SPR) measurements allowing detecting molecular targets, for example in international patent applications WO2017153378 of the inventors of the present application and WO2009080998A2, or the American U.S. Pat. No. 7,233,396B2. These devices often implement bulky and expensive equipment, such as acousto-optic modulators, photo-elastic modulators, or Fresnel rhombohedra.
In both cases, ellipsometric measurements or surface plasmon resonance measurements, a high phase resolution is necessary in order to improve the sensitivity of the measurements. This high resolution can be attributed by a temporal modulation of the phase of the signals passing through the measuring devices. Indeed, an intrinsic advantage of the various phase modulation systems relative to other systems for measuring stationary signals without phase modulator is the noise reduction which a frequency analysis allows in the recovery of amplitude and phase, in particular thanks to the use of synchronous detections at the modulation frequencies of the detected interferometric signal. The American U.S. Pat. No. 5,485,271A describes an interferometric ellipsometer incorporating an electro-optic phase modulator. Other techniques allow a phase modulation obtained by modulation of the birefringence of a component of the measuring device, such as those described in the American U.S. Pat. No. 7,339,681B2, where a liquid crystal cell is used, or in the American U.S. Pat. No. 8,004,676B1, where a photo-elastic modulator is used. Also, a phase modulation can be obtained by modulating the wavelength of the source beam, as in the international patent application WO2017/153378, proposed by the present inventors, which describes a compact interferometer as well as a biochemical sensor derived therefrom, but which requires a particular optical chip producing two reflections, from two distant layers to carry out this modulation, which limits the application thereof to particular samples and does not allow any optical excitation condition.
It is also known to use interferometers called common path interferometers, in which the reference beam and the signal beam linked to a sample are displaced as much as possible along the same path, to reduce the noise of the interferometric measurements, because of the good immunity of this type of interferometers vis-à-vis the environmental vibrations.
It is also known to use asymmetric interferometers where the optical path difference between the two interfering arms is large enough for a low wavelength modulation of the used light source to result in a sufficient phase modulation in order to be able to extract an amplitude signal and a phase signal from the interferometric signal as explained by Vaillant et al. in “An unbalanced interferometer insensitive to wavelength drift”. Sensors and Actuators A: Physical, 268, 188-192 (2017). In this type of devices, however, the geometric path of the two beams is not common, thus limiting the stability of the system.
Furthermore, the drawback of the aforementioned techniques lies in the space requirement of the implementation device thereof as well as in their cost, in order to obtain high measurement accuracy and sensitivity. An interesting approach to minimize the noise of the interferometers consists in using a polarisation interferometer which measures the phase shift between two orthogonal components of the field, because in this case the path followed by the two components of the field can be relatively common. Nevertheless, the phase modulation of one component relative to the other requires an optical separation of the beams and a particular appliance such as those which are previously mentioned, such as for example a photo-elastic modulator.
The technical problem that the inventors propose to solve is to simplify the implementation and improve the sensitivity of interferometric measuring devices of the polarisation interferometer type, and the integration of this type of device as well. within ellipsometers than within SPR devices and to reduce the space requirement and weight thereof by avoiding the use of conventional active phase modulators and the use of movable parts to determine the state of the phase shift within of said polarisation interferometer.
In order to solve this problem while overcoming the aforementioned drawbacks, the applicant has developed a laser device for polarisation interferometry, adapted to deliver a temporally phase-modulated laser beam comprising:
As stated later in the description, the geometric path for the two TE and TM components of the field is advantageously constant.
Advantageously, the longitudinal single-mode laser source of the laser device can be a semiconductor laser which can be wavelength-modulated by the electrical current for powering the laser over a tunability range of less than one thousandth of the wavelength. This low tunability is achieved by most consumer laser diodes.
Preferably, the semiconductor laser which can constitute the longitudinal single-mode laser source of the laser device may be a vertical cavity surface emitting laser diode called VCSEL. However, in the absence of VCSEL, more widely tuneable and typically more expensive lasers can be used.
Moreover, the passive phase delay element of the laser device may comprise a component having a birefringence, in particular a birefringent crystal having an optical axis oriented along one of said TE or TM polarisation components of the source laser beam Ssource.
The laser device may further comprise:
Advantageously, said reference electronic analysis unit is connected to the means for temporal modulation of the laser source so as to constitute a servo-control loop to stabilise the average phase shift Δref.
Optionally, in order to facilitate the alignment, it may be useful to use beam splitters having reflective or anti-reflective treatments on different interfaces to avoid having multiple parasitic reflections. Other means can nevertheless be employed alone or in addition to suppress the multiple parasitic reflections, such as the use of non-parallel faces, or/and by spatially discriminating the first reflection and the first transmission, this including the use of thick blades.
The present application also proposes a polarisation interferometer configured to measure characteristics of a sample, comprising:
More specifically, said opto-mechanical interface is a simple support, or an optical coupling system, which can include different optics, configured to transmit the temporally phase-modulated laser beam Smodulated towards the sample under the optical excitation conditions desired by the user so as to optically excite the sample so as to generate an output beam Ssample.
More specifically, the modulated electrical signal Isample representative of the output beam Ssample includes an amplitude term Asample proportional to the product of the amplitudes of the two electric transverse TE and magnetic transverse TM components of the output beam Ssample and a phase term Δsample including an optical phase shift increment Δ between the two electric transverse TE and magnetic transverse TM components induced by the sample. Consequently, the electronic analysis unit is configured to, by analysing said electrical signal Isample, extract said amplitude term Asample and said average phase term Δsample between the two electric transverse TE and magnetic transverse TM components of the output beam Ssample allowing determining the optical characteristics of said sample. When the laser device further comprises, as described above:
The present application also proposes an ellipsometer configured to determine an ellipsometric parameter Δellipsometry of a sample comprising an polarisation interferometer as previously described and in which:
The phrase “the sample is capable of being placed at the opto-mechanical interface of the polarisation interferometer”, should mean that the opto-mechanical interface of the polarisation interferometer is capable of receiving the sample.
In a first variant of the proposed ellipsometer, said ellipsometer latter may further comprise a first additional detection channel, said first additional detection channel, said first additional detection channel comprising:
In a second variant of the proposed ellipsometer, different from the first variant described above, said ellipsometer may further comprise a second additional detection channel, said second additional detection channel comprising:
The present application also proposes a biosensor of the surface plasm on resonance detection system type configured to determine characteristics of a sample consisting of a microfluidic layer MF, corresponding to the biological or biochemical medium to be analysed, the biosensor comprising:
Other advantages and features of the present application will result from the following description, given by way of non-limiting example and made with reference to the appended figures:
b are commented on in more detail in the following detailed description and examples, which illustrate the invention without limiting the scope thereof.
With reference to
In order to be more accurate, when it is stated that the phase modulation can be carried out by keeping a constant geometric path for the two components of the field, it should be understood that the geometric path travelled by the two components of the field does not change over time, in particular at the element where the phase shift between these components occurs, that is to say in the passive phase delay element which is therefore fixed and advantageously monolithic for a better stability.
Preferably, the laser source 1 is a semiconductor laser, for example a vertical cavity surface emitting laser diode VCSEL.
Moreover, the temporal modulation of the wavelength of the laser source 1 can be performed by temporally modulating the electrical current powering the laser source 1. The temporal modulation of the wavelength of the laser source 1 is typically carried out over a tunability range of less than one thousandth of the wavelength. Thus, it is not necessary to resort to a birefringence modulation or another type of modulator to cause this phase modulation between said components of the field.
In particular, the passive phase delay element 3 may for example comprise or consist of a component having a birefringence, such as a birefringent crystal. In this particular case, the birefringent crystal advantageously has an optical axis along one of the two orthogonal transverse polarisation components of the source laser beam. Conventionally, these two polarisation components are called TE and TM for “electric transverse” and “magnetic transverse” with reference to a certain predetermined plane of incidence. The geometric path followed by the components of the TE and TM field can then be entirely common. The phase modulation between the two components of the field is thus generated independently of the nature of a possible sample intercepting the beam, and of the optics used to excite the sample such as lenses, prisms or coupling gratings.
By the term “common geometric path”, it should be understood that the light beams of the components of the TE and TM field are spatially superimposed. Such a configuration allows, for example, a pooling of the noise undergone by the different beams, making the device more stable, this despite the difference in the optical paths travelled by the TE and TM components. It is recalled that the optical path is defined by the product of the refractive index encountered by the geometric path.
The optics used to excite the sample, mentioned in the previous paragraph, allow, for example, defining the angle(s) of incidence and more generally the illumination conditions on the sample. The term “exciting the sample” means to generating, using the laser device, an electromagnetic field, within the sample.
Different elements can also be added to this device, in particular in order to stabilise or even control the average phase shift existing between the two components of the field or more generally to control the polarisation state emanating from the laser device. In particular, with reference to
This modulated electrical signal Iref is received and analysed by a reference electronic analysis unit 6a. The modulated electrical signal Iref includes a temporally phase-modulated interferometric term and having an amplitude Aref proportional to the product of the amplitudes of the two electric transverse TE and magnetic transverse TM components of the reference portion Sreference.
Indeed, the modulated electrical signal Iref represents the interferometric signal detected by the reference photo-detector, which can be written in the form:
I
ref
∝E
TE
2
+E
TM
2+2mETEETM cos(Δmod+Δref)=ETE2+ETM2+Aref2 cos(Δmod+Δref)
where
ETE and ETM are the amplitudes of the TE and TM components of the reference portion Sreference,
M is a coefficient less than or equal to 1, and
A2ref=2mETEETM, and
Δmod is a temporally modulated phase term, preferably sinusoidally, but not necessarily, depending on the choice of the current modulation function. The analysis of this type of modulated signal Iref is in particular detailed in the references Al Mohtar, Abeer, et al. “Generalized lock-in detection for interferometry: application to phase sensitive spectroscopy and near-field nanoscopy.” Optics express 22.18 (2014): 22232-22245 and the U.S. Pat. No. 9,518,869B2 which proposes the use of a modified synchronous detection called generalized synchronous detection to perform the analysis. The use of generalized synchronous detection effectively allows extracting the amplitude Aref and phase Δref information, where Δref characterises, in our case, the phase shift between said components TE and TM.
Thus, the reference electronic analysis unit 6a is capable of extracting from this electrical signal Iref, comprising a phase-modulated interferometric term, the average phase shift Δref between the two electric transverse TE and magnetic transverse TM components of the reference portion Sreference, and extracting said amplitude term Aref. This method thus allows extracting said phase shift Δref without ambiguity over the definition interval thereof. Otherwise, other extraction methods can be considered for particular temporal modulation functions, such as methods based on successive constant phase shifts, or the use of a ramp modulation called serrodyne ramp modulation. Particular attention should be given to the fact that the current modulation also causes a temporal modulation of the intensity of the laser, which causes a modulation of the intensity terms ETE2, ETM2 and Aref2 around their average values. This additional modulation can interfere with the measurement of the phase shift if it is not taken into account in the processing. A generalized synchronous detection, like the one mentioned, allows processing signals whose amplitude is also temporally modulated and allows overcoming this difficulty, ideally by adjusting the phase modulation depth. Otherwise, this modulation on said intensity terms ETE2, ETM2 and Aref2 can be neglected at the cost of a certain error, or else the measured intensity Iref can be corrected to compensate for this modulation, knowing the used modulation function. The laser device thus constituted is named D′.
Furthermore, by stabilising the average phase shift Δref, the reference electronic analysis unit 6a can provide a correction coefficient to the means 2 for temporal modulation of the laser source 1 so as to adjust the temporal modulation of the laser source 1 and to stabilise the average wavelength λ thereof. Thus, as indicated in dotted lines in
I
sample
∝=I
0
+A
sample
2 cos(Δmod+Δsample).
In particular also, the reference electronic analysis unit can be configured to, by analysing said electrical signal Isample, extract said amplitude term Asample and said average phase term Δsample between the two electric transverse TE and magnetic transverse TM components of the output beam Ssample allowing determining optical characteristics of said sample, specifically via the optical phase shift increment Δ, calculated, in the case where the interferometer is of the I′ type, that is to say in the case where it comprises a laser device with a reference arm D′, by: Δ=Δsample−Δref to within an easily determinable additive constant, for example by calibration on a sample with a known Δ.
The I′ type polarisation interferometer which is illustrated in
Indeed, the parameter tan Ψ is obtained, as conventionally in ellipsometry by the formula:
where rTM and rTE are the sample reflection coefficients carried by the TM and TE components of the output beam Ssample. Thus, the parameter tan Ψ can be obtained, depending on the used experimental configuration, either by its square given by the equation:
if the TE component is recovered by the first additional detection channel, or by the equation:
if the TM component is recovered by the first additional detection channel.
A laser device is made as illustrated in
The temporal modulation is typically carried out by a modulation of the injection current of the used longitudinal single-mode laser source. The modulation is preferably sinusoidal but other modulations can be used in order to carry out an interferometric detection with discrete or continuous phase shift. In the sinusoidal case, the modulation of the injection current i(t) is, as mentioned, of the type: i0+β sin(Ωt). In the case of the type of aforementioned VCSEL, i0 is typically in the range of 4 mA. The current modulation induces an optical power modulation which is approximately equal to: P(t)=P0+γ sin(Ωt)=P0 (1+μ sin(Ωt)), where P0 is the power DC component, and μ*P0, the AC amplitude of the modulation, Ω is the pulse of the modulation. This power modulation induces a wavelength modulation approximately equal to λ(t)=Δ0+δ sin(Ωt), where λ0 is the average wavelength and δ is the wavelength modulation depth. In the presence of such a modulation (of current, but also of power and wavelength), a phase modulation is created between the TE and TM components as soon as the beam passes through the aforementioned birefringent YVO4 crystal. The induced phase modulation is written: a sin (Ωt) in the sinusoidal case, with the phase modulation depth given by:
where Δl is the optical path difference between the two components of the field within the delay element 3. In practice, it is interesting to work with a phase modulation a=3.83 rad as explained in another framework by Vaillant et al. in “An unbalanced interferometer insensitive to wavelength drift”. Sensors and Actuators A: Physical, 268, 188-192. In the above reference, this choice of phase modulation depth allows analysing the resulting interferometric signal more simply and simply extracting the amplitude information Asample and the desired phase term Asample.
In our case, in order to obtain the temporal phase modulation, the birefringence (ne−no) and the length L of the YVO4 crystal are such that the optical path difference given by the product L(ne−no) is at least in the order of magnitude of a millimetre, which corresponds to a cumulative phase shift between the TM component and the TE component in the range of 10,000 radians of visible light. This cumulative optical path difference is made with the previously mentioned components.
An ellipsometer is made as previously described and illustrated in
In order to implement the aforementioned ellipsometer, one further uses:
The ellipsometric parameter Δellipsometry is obtained by the formula Δellipsometry=Δsample−Δref to within an additive constant, with Δsample the extracted phase parameter of the electrical signal Isample from the analysis photo-detector 8, and corresponding to the phase shift between the TE and TM components of the output beam Ssample induced by the sample.
An ellipsometer is made as illustrated in
The ellipsometric parameter Δellipsometry is obtained as in Example 2. The parameter (tan Ψ)2 can be obtained, according to the used experimental configuration, either by the equation (tan ψ)2
if the TE component is recovered by the first additional detection channel, or the reverse if the TM component is recovered by the first additional detection channel. In practice, the coefficient of proportionality between (tan Ψ)2 and A2sample/Itan ψ can be predetermined simply by a calibration experiment on a known sample. In this example, the coefficient of proportionality is previously determined by measuring the parameter tan Ψ on a known sample.
From the Δellipsometry and tan Ψ parameters, it is possible to determine, as conventionally in ellipsometry, the complex optical index or the thickness of the known thin layer or other unknown parameters linked for example to the roughness.
An ellipsometer is made as illustrated in
The ellipsometric parameter Δellipsometry is obtained as in Examples 2 and 3. As described above, the parameter tan Ψ is directly obtained by its square:
With Itan Ψ_TE and Itan Ψ_TM the signals from the TE photo-detector and TM photo-detector. The coefficient of proportionality is equal to unity if the beams are shared in identical proportions. In practice, the coefficient can be predetermined simply by a calibration experiment, for example on a known sample. From the Δellipsometry and tan Ψ parameters, it is possible to determine, as conventionally in ellipsometry, the complex index and the thickness of layers within the measured sample.
A biosensor as illustrated in
an opto-mechanical interface 70: support;
an analysis polarizer 8′: LPVISE050-A polariser (Thorlabs);
an analysis photo-detector 8: silicon photodiode, for example the reference PDA36A-EC (THORLABS);
an electronic analysis unit 6b: electronic acquisition card, for example the reference NI USB-6363 (National Instrument);
the preferably removable biochip 11 disposed on the interface 70 and comprising a prism 110 on which a gold metal layer ME (45 nm thick), which is capable of receiving the thiolated PEG layer constituting the sample to be analysed, is deposited.
In this specific case, the analysis photo-detector 8 is an imager allowing a multi-point measurement. The biosensor is used in a configuration called Kretschmann configuration via the prism (110). The parameter Δ=Δsample−Δref which can be determined thanks to the polarisation interferometer of the biosensor as previously described is generally not accessible with other types of SPR measuring devices.
It should be noted that the above SPR type measurements, performed with a laser device according to Example 1, can also be carried out using a laser device as proposed in the present application having other characteristics, for example, with a laser source operating at any other wavelength, such as in the mid-infrared or near infrared, for example with a VCSEL operating at a wavelength of about 850 mm with the same phase modulation, i.e. 3.84rad, and by adapting the current modulation in mA to carry out this phase modulation, as well as the components of the system to operate at this wavelength.
Other embodiments can be considered. For example, another embodiment can comprise multi-angle measurements, where, in both cases of applications to ellipsometry or the detection by surface plasmon resonance, measurements according to several angles of incidence are performed, or conversely, the beam at the output of the measured sample is split after interaction with the sample according to several different angles. In the case of the plurality of angles of incidence, a cylindrical lens can for example be placed upstream of the interface receiving the samples to be tested in order to obtain a beam focused in the plane of incidence, thus giving a plurality of angles of incidence illuminating the sample, the latter reflecting the extended beam received in several directions sensed by a linear detector (of the diode array type for example).
Also, as mentioned in Example 6, the analysis photo-detectors 8, photo-detector for complete ellipsometry 10, and photo-detector TE and photo-detector TM 101 and 102 can be two-dimensional sensors allowing imaging samples to be measured and obtaining two-dimensional maps of the characteristics of these samples. In this case, all types of two-dimensional sensors can be used, such as CCD or CMOS sensors, or photo-detectors having a reduced number of detection areas such as quadrant photodiodes which can also help in centring the beam.
Also, ellipsometric measurements in transmission can be carried out in the case of a sufficiently transparent sample. A plurality of laser optical sources can also be employed to extend the analysis spectral range. Also, the ellipsometric analysis can be extended to obtain additional information on the sample from the ellipsometric parameters determined from a model which can take into account in particular the density or roughness of a layer.
Also, the SPR device being able to integrate an ellipsometric measurement, the latter can be used to determine the characteristics of the layers composing the biochip, for example the thickness of the gold deposit, or the thickness (or the density) of a functionalisation layer, or else the molecular layers from the analyte passing through the microfluidic layer clinging to the surface. Thus the biochip can be prepared for the measurement of any biochemical species (pathogens, proteins, bacteria, biomarkers) by using the ellipsometric measuring device at each step of the functionalisation process, which is typically carried out on the SPR biochips to allow the detection of a target in particular using antibodies, DNA or aptamers.
The examples of optomechanical interfaces 70 given in this description are not limiting. Thus, in addition to a simple support as in Example 5, or a coupling prism provided or not with a resonant element as in Example 5, coupling gratings or lenses could be used to optically excite a resonance of the sample. In particular, the SPR devices typically require a coupling element as in the examples given in this description. The coupling element allows obtaining, if necessary, a plurality of excitation angles. The privileged excitation of the SPR devices is an excitation at a supercritical angle known as the Kretschmann configuration. Thus the essential role of the opto-mechanical interface is to define the angle(s) of incidence and more generally the illumination conditions on the sample.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1873212 | Dec 2018 | FR | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/FR2019/053109 | 12/17/2019 | WO | 00 |
