MULTIPOINT PHOTO-ACOUSTIC MEASURING DEVICE

Abstract

A method comprises non-destructive contact-free physical characterization of a sample by repeated excitations of the surface of a sample with a sequence of pulses comprising at least one pump pulse by a first “pump” laser followed by a succession of L temporarily offset pulses by a second “probe” laser, and the analysis of the beam emitted by the surface of the sample by an activated photodetector, for the acquisition of signals delivered by the photodetectors during constant time windows.

Claims

1. A non-destructive and contact-free method for physical characterization of a sample, comprising repeatedly exciting a surface of a sample with a sequence of pulses comprising at least one pump pulse by a first “pump” laser followed by a succession of L pulses temporarily offset by a second “probe” laser, and analyzing a beam emitted by the surface of the sample by an activated photodetector for the acquisition of signals delivered by the photodetectors during constant time windows; wherein the pump and probe beams have a uniform spatial distribution of a “top hat” type along N dimensions, N being equal to one or two; the photodetector comprises an N-dimensional array of M photodetectors, with M greater than 2; and the method further comprises recording, for each sequence, an array M.sub.PD of M×L signal values delivered by each of the photodetectors before and after a probe pulse, and before the following pump pulse, and applying at least one digital processing to the array to establish a map of the sample zone analyzed by the photodetector in the form of an array M.sub.CC of the values of the physical characteristic observed for Q points of the zone analyzed, Q being between 1 and M.

2. The method of claim 1, wherein the digital processing involves applying a transformation array M.sub.TR to the array of M.sub.PD M×L signal values delivered by the photodetector to determine the array M.sub.CC.

3. The method of claim 2, further comprising recalculating the array M.sub.TR through supervised learning.

4. The method of claim 2, further comprising recording ea plurality of transformation arrays M.sub.TR, each corresponding to a particular physical characteristic.

5. The method of claim 2, further comprising automatically optimizing the focus of the optics of the “pump” and “probe” beams involving controlling a sequence of variation of focusing and measuring a quality factor of the signal produced by the photodetector, and selecting the focusing corresponding to a maximization of the quality factor over all of the recorded values.

6. A system for non-destructive and contact-free physical characterization of a sample, comprising two pulsed laser sources for emission, respectively, of a “pump” beam and a “probe” beam, as well as a detector, wherein the system further includes at least one device for shaping a beam to transform the distribution of a “pump” beam and a “probe” beam into a uniform spatial distribution of a “top hat” type along N dimensions, N being equal to one or two and wherein the detector comprises an N-dimensional array of M photodetectors, each configured to measure a number of photons before and after a probe pulse, and before the following pump pulse, with M greater than 2.

7. The system of claim 6, wherein the pump and probe beams are coaxial in a zone of interaction with the sample.

8. The system of claim 6, wherein the pump beam is perpendicular to a plane of a zone of interaction with the sample, and the probe beam forms an angle other than 90° with the plane of the zone of interaction with the sample.

9. The system of claim 6, further comprising a computer for controlling a recording, for each sequence, of an array M.sub.PD of M×L signal values delivered by the photodetectors and for applying at least one digital processing to the array to establish a map of a zone of the sample analyzed by the detector in the form of an array M.sub.CC of the values of the physical characteristic observed for Q points of the zone analyzed, Q being between 1 and M, as a function of at least one transformation array recorded in a computer memory device.

10. A computer memory device for customization of a system according to claim 6, comprising a recording of a digital transformation array M.sub.TR to an array of M×L signal values delivered by the photodetectors to determine an array M.sub.CC.

11. The method of claim 1, further comprising using the method to perform N-dimensional mapping of the thickness of a coating layer of a sample, with N equal to 1 or 2.

12. The method of claim 1, further comprising using the method to perform N-dimensional mapping of the Young's modulus of a sample, with N equal to 1 or 2

13. The method of claim 1, further comprising using the method to perform N-dimensional mapping of the adhesion strength of a sample, with N equal to 1 or 2.

14. The method of claim 1, further comprising using the method to perform N-dimensional mapping of the crystalline state of a sample, with N equal to 1 or 2.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Other features and advantages of the present disclosure will be evident from the following detailed description, given by way of non-limiting example and with reference to the accompanying drawings, in which:

[0051] FIG. 1 is a schematic view of the measurement system;

[0052] FIG. 2 is a schematic view of a particular normal incidence configuration;

[0053] FIG. 3 is a detailed schematic view of the array photodetector;

[0054] FIG. 4 is a schematic view of an array of photo-sensitive elements; and

[0055] FIG. 5 is a schematic view of the photodetector monitoring.

DETAILED DESCRIPTION

[0056] The present disclosure combines the use of a pump beam for excitation and a probe beam for detection, which are generated by a synchronous or asynchronous system with a device for shaping the beams in a line or in a square array, then an acquisition means of the probe beam before and after disturbance by the sample. The acquisition is carried out by probe signal discretization of n points by a photodetector with an element array with subtraction from the signal after disturbance of the signal before disturbance, then by the digitization of the differential signal and the temporal reconstruction of the sample response.

[0057] The equipment for mapping a physical characteristic of a sample according to the present disclosure, described by way of example, comprises a system (100) of two laser sources producing two beams (110, 120), a pump beam (120) and a probe beam (110) offset temporally with possible adjustment of the delay.

[0058] The pump beam (120) has a form factor in a line or in a square array form, with a non-Gaussian form (“top hat beam”).

[0059] The probe beam (110) has a form factor in a line or in a square array form, identical to the pump beam.

[0060] A “top hat” type of beam exhibits an almost uniform fluence (energy density) in a circular disc. It is generally formed of diffractive optical elements from a Gaussian beam. It may be obtained by combining a beam profiler (“laser beam profiler”) with a picosecond laser or a femtosecond laser.

[0061] The system further includes a differential photodetector (200) with an element array (201 to 205) in line or in a square array form measuring at each of its points the difference of the signals before and after disturbance of the incident probe beam on the sample. It optionally allows, as a variant, the output of the respective average values of the outward and return signals in order to balance the respective averages of the outward and return signals on a single reference value. The electrical signals delivered by the elements (201 to 205) are transmitted to a pre-processing circuit (210) then to a digital analog digitizer (220) making it possible to record the response signals measured by the photodetector (200).

[0062] A preferred configuration illustrated in FIG. 1 involves placing the pump (111 to 113) and probe (121 to 123) beams at any angle of incidence with respect to the sample interaction plane (150), the angle between the two beams being between 0° and 180°. A particular case of this configuration appears when the angle of incidence of the probe (121 to 123) with respect to the normal of the sample (150) is 0°.

[0063] In this case it is necessary to add a certain number of optical elements.

[0064] Similarly, when the angle between the probe (121 to 123) and pump (111 to 113) beams is 0°, it is necessary to add an optical element (130) allowing the beams to recombine with each other.

[0065] The system comprises a synchronous or asynchronous device (100) making it possible to generate two pulsed laser beams (110,120) offset temporally, a Probe beam (110) and a Pump beam (120). The wavelengths of the beams are independent; for the Probe beam (110) the wavelengths are comprised within the spectral band of the photodetector (200).

[0066] The Pump beam (120), coming from an asynchronous or synchronous device, passes through an optical device (125) for shaping the beam (120). At the output of the device, the beam has a shape adapted to the sensor array of the photodetector and a uniform distribution of energy over its entire surface (so-called “top hat” shape of the signal).

[0067] The Probe beam (110), coming from an asynchronous or synchronous device (100), passes through a sampling optic, which makes it possible to sample a fraction of power to send it to one of the element arrays of the differential photodetector (200) with an element array (201 to 205). Initially, the probe beam passes through an optical beam shaping device (115). At the output of the device, the beam has a shape adapted to the sensor array of the photodetector (200) and a uniform distribution of energy over its entire surface (so-called “top hat” shape of the signal).

[0068] Then, in the case of the configuration with a normal incidence of the probe beam (110) on the sample (150) illustrated in FIG. 2, the probe beam passes through semi-reflecting optics.

[0069] Then, in the case of an angle of 0° between the pump (120) and probe (110) beams, the beams are recombined in a recombination optic making it possible to give them a single orientation on the sample (150).

[0070] Finally the two beams (110, 120) are focused on the sample via an optical focusing device. In the case of a configuration with non-normal incidence of the probe beam on the sample, the probe beam passes through two different optical focusing devices, the paths before and after the sample not being similar, the semi-reflecting optics are not used in this configuration.

[0071] The probe beam reflected or transmitted (151 to 153) by the sample (150) is then injected into the differential photodetector (200) with an element array (201 to 205). The element array (201 to 205) of the photodetector (200) discretizes the signal into n signals.

[0072] Afterwards, these signals are subtracted from the probe signal measured before reflection or transmission of the probe beam on the sample. The signals are conditioned and then digitized by the multi-channel A/D digitizer (220). In addition, the digitizer (220) is synchronized with the laser sources (100) to allow a temporal reconstruction of the n responses of the sample (150) by digital processing of the measurements.

[0073] In the case of a normal incidence of the probe channel on the sample (150), the probe beam reflected or transmitted (151 to 153) by the sample (150) is reflected by the semi-reflecting optics in the differential photodetector (200) with an element array. The element array (201 to 205) of the photodetector discretizes the signal into n signals. Then these signals are subtracted from the probe signal measured before reflection or transmission of the probe beam on the sample (150). The signals are conditioned and then digitized by the multi-channel A/D digitizer (220). In addition, the digitizer is synchronized with the laser sources to allow a temporal reconstruction of the n responses of the sample by digital processing of the measurements.

[0074] The differential photodetector (200) with an element array (201 to 205) comprises an array of photosensitive elements, which may range up to 64 elements distributed in a row or arranged in an 8×8 square. The spectral response of these photo elements (201 to 205) makes it possible to cover a spectral band ranging from 190 to 1700 nm. The functions covered by the photodetector are: [0075] Current/voltage conversion: This function (trans-impedance assembly) makes it possible to convert weak currents generated by the photosensitive elements into a usable voltage. There are as many current/voltage conversion modules as there are photosensitive elements. [0076] Subtraction: This function makes it possible to recover the useful signal, i.e., the response of the sample by taking the difference of the signal before sampling and the signals after sampling; only the disturbances generated by the sample remain in the signal. [0077] Signal conditioning: used to format the signals before acquisition by the digitizer.

[0078] The probe (110) and pump (120) beams may be offset spatially on the sample (150) in order to measure transverse physical phenomena.

[0079] The scanner system may be made with two movable mirrors or by means of two lenses, the first of which is off-centered with respect to the second.

[0080] Photodetector

[0081] FIGS. 3 to 5 illustrate the diagram of the photodetector (200).

[0082] The array photodetector (200) may be equipped with channels for monitoring the optical powers making it possible to visualize the average optical power of the probe before and after sampling in order to balance the differential channels of the detector and to optimize the signals. The advantage of line/square sensors is that the elements have a common cathode.

[0083] The current passing through the cathode is the sum of all the currents generated by each photosensitive element (201 to 205). Current flowing through the resistor R (206) creates a voltage across its terminals. This voltage is amplified using an amplifier (207) to obtain a voltage proportional to the optical power on the in-line sensor. Only one monitoring will therefore be necessary for the line/square sensor.

[0084] The photodetector (200) is equipped with an array of n photosensitive elements on the return sample path and a single photosensitive element or an array of n elements on the sampling channel before the sample. The currents generated are then converted into voltage before obtaining the difference between the signals.

[0085] The signals may be multiplexed at the photodetector output in order to limit the number of digitizer channels.

[0086] Signal Processing

[0087] The digitized signals are recorded in a table made up of the values of each of the elements (201 to 205), for the different moments of the probe pulses, which may be represented in the form of a set of light intensity curves measured by the photodetector or the number of photons counted by the photodetector with respect to time. These have a maximum value corresponding to the zero offset between the pump pulse and the first probe pulse, then generally decreasing values.

[0088] These curves are processed to extract characteristic information such as singular points or the slope of certain segments.

[0089] This digital array is processed by a transformation array associating the digital values from the photodetector (200) with the values of the physical characteristic studied. This transformation array may be built empirically, or by supervised learning. It may be reassessed regularly as a function of the results of the measurements taken.

[0090] This transformation array may be recorded on a device to allow the customization of mapping equipment, for example, by access to an online memory or in the form of a physical memory that may be inserted into a connector provided for this purpose in the equipment.

[0091] Applications

[0092] The equipment according to the present disclosure is suitable for different applications: [0093] Non-destructive testing of structural patterns of surfaces or successive layers of samples. [0094] Imaging of living cells, which are inherently moving, which, in particular, prohibits the use of long “poses.” [0095] Characterization and mapping of nanoparticles on a substrate, number, size, and distribution. [0096] Imaging of phenomena of homogeneity variation of the physical properties of a sample following a brief and non-reproducible event. For example, development of the thermal conductivity of thin layers during laser machining. [0097] Surface wave imaging without moving the probe relative to the pump.