Abstract:
The present invention relates to a device ( 1 ) for characterizing an interface of a structure ( 6 ), said structure ( 6 ) comprising a solid first material and a second material, the materials being separated by said interface. The device ( 1 ) comprises:
       means ( 2 ) for generating a first mechanical wave;   means ( 2 ) for forming Brillouin oscillations;   means ( 10 ) for detecting time variation of the Brillouin oscillations;   means ( 12 ) for responding to the time variation of the Brillouin oscillations to identify reflection of said first mechanical wave by said interface or transmission through said interface of a second mechanical wave interfering with the first mechanical wave; and   means ( 13 ) for determining the variation in amplitude of the Brillouin oscillations before and after reflection or transmission by said interface.       
 
     The invention also relates to a corresponding method of characterization.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to the field of measuring structural properties. More precisely, the present invention relates to characterizing a structure by means of a soundwave that is generated and detected by a light pulse. 
         [0002]    U.S. Pat. No. 5,748,318 discloses a system for characterizing thin films and interfaces between such films by measuring their mechanical and thermal properties. In the system described, light is absorbed in the thin layer or in a structure made up of a plurality of thin layers, and the modifications to light reflection and transmission are analyzed. The change in reflection or transmission is used to provide information about ultrasound waves produced in the structure. As a result, it is possible to determine the thicknesses of layers and also several optical properties of the structure. 
         [0003]    U.S. Pat. No. 5,748,318 is thus an example implementation of a pump-probe system known to the person skilled in the art. In such a system, the light source is a laser emitting short pulses (e.g. femtosecond pulses) at a fixed wavelength so as to produce a first beam that is split in a splitter into a “pump” beam and into a “probe” beam. Thereafter, the light path of the probe beam or the light path of the pump beam is varied by a mirror that is servo-controlled in position. It is then known that the properties of the structure under the effect of the emitted beams give rise to modification in the reflection (or transmission) properties of the probe wave. In particular, and in known manner, by observing the modification in reflection as a function of time, it is possible to determine echoes that are characteristic of interfaces of a structure. Analyzing the echo signal then makes it possible, by way of example, to deduce the thickness of the material, if the speed of propagation of the soundwave in the medium is known. 
         [0004]    In order to increase the number of characteristics that are extracted, and in particular both speed and thickness, the publication  Evidence of laser - wavelength effect in picosecond ultrasonics: possible connections with interband transition  (Physics Review Letters, Mar. 12, 2001, Volume 86, Number 12) describes the use of a pump-probe device as described above, but associated with a wavelength-tunable laser, thus enabling the wavelength of the emitted signals to be varied. 
         [0005]    As a result of these wavelength effects, it is possible to access both thickness characteristics and speed characteristics in certain types of structure. Specifically, as described in the publication  A novel approach using picosecond ultrasonics at variable laser - wavelength for the characterization of aluminum nitride films used for microsystem applications  (A. Devos, G. Caruyer, C. Zinck, and P. Ancey, World Congress on Ultrasonics (Paris Sep. 7-10, 2003), pp. 793-796 ISBN 2-9521105-0-6), for a structure that is transparent to the probe beam, an acousto-optical interaction appears within the material giving rise to the appearance of oscillations instead of simple pulses observed by echo. Those oscillations are known as “Brillouin” oscillations and they have a period that depends on the wavelength of the probe and on the speed of sound in the material. 
         [0006]    Another example of a device making use of Brillouin oscillations is also described in patent application FR 2 887 334. 
         [0007]    Nevertheless, the number of characteristics that are measurable using those methods remains limited, and it is not possible in particular to characterize accurately a surface or an interface. 
       OBJECT AND SUMMARY OF THE INVENTION 
       [0008]    The present invention seeks to solve the various technical problems set out above. In particular, the present invention seeks to propose a device enabling an interface of a structure to be characterized in reliable manner. More particularly, the present invention seeks to propose a device making it possible in reliable manner to characterize a surface of the structure, e.g. its roughness, or an interface between two layers of the structure, e.g. its acoustic transmission coefficient. 
         [0009]    Thus, in one aspect, there is provided a device for characterizing an interface of a structure, said structure comprising a solid first material and a second material, which materials are separated by said interface. The device comprises:
       means for generating a first mechanical wave in the first material, e.g. means for generating pump radiation;   means for forming Brillouin oscillations, comprising means for generating probe radiation configured to propagate at least in part in the first material;   means for detecting the variation in time of the Brillouin oscillations in particular in the first material;   identification means configured to use the time variation of the Brillouin oscillations in particular in the first material to identify reflection of said first mechanical wave by said interface or transmission through said interface of a second mechanical wave interfering with the first mechanical wave; and   determination means configured to determine the variation in amplitude of the Brillouin oscillations in particular in the first material before and after reflection or transmission by said interface.       
 
         [0015]    By monitoring the variations in the amplitude of the Brillouin oscillations, it is possible to evaluate certain physical parameters of the structure. More precisely, since the Brillouin oscillations are formed by a soundwave propagating over a certain distance in the structure, it is possible to observe and make use of wave phenomena suitable for characterizing the structure, e.g. interference phenomena between a plurality of waves or indeed the effects of diffraction by surface irregularities, in particular. 
         [0016]    Preferably, the determination means are configured to determine the variation in amplitude of the Brillouin oscillations as a function of the wavelength of the probe radiation. Analyzing the Brillouin oscillations at different wavelengths makes it possible to refine the characterization of the structure, or indeed to determine additional characteristics. 
         [0017]    In an embodiment, the second material presents an acoustic impedance that is very different from the acoustic impedance of the first material, for example it is a gas, and: the identification means are configured to identify a reflection of said first mechanical wave by said interface, and the determination means are configured to determine the variation in amplitude of the Brillouin oscillations before and after reflection by said interface, in order to characterize the roughness of said interface. 
         [0018]    In this embodiment, transmission of the soundwave by said interface in the second material may be made to be negligible, in particular when the impedance of the second material is much greater than that of the first material (the soundwave is reflected with a change of sign at the interface) or indeed when the impedance of the first material is much greater than that of the second material (the soundwave is reflected at the interface without change of sign). Thus, the second material may be a gas, e.g. air, i.e. said interface may be a free surface of the first material. 
         [0019]    In this embodiment, the device makes it possible to characterize the surface properties of the structure, e.g. roughness, by comparing the amplitude of the Brillouin oscillations before and after reflection by said surface. The surface irregularities then give rise to a phenomenon of the soundwave being diffracted, which causes the reflected soundwave to disperse, and thus leads to variation in the amplitude of the measured Brillouin oscillations. 
         [0020]    Preferably, the means for generating probe radiation are configured to change the wavelength of the probe radiation as a function of the size of the roughness to be measured. Characterization of an article by monochromatic radiation is limited by the wavelength of said radiation. In the present circumstances, by varying the wavelength of the probe radiation, characterization makes use of soundwaves that have different frequencies due to repetitive patterns in the surface of the structure. Thus, varying the wavelength makes it possible to characterize different repetitive patterns in the surface of the structure, i.e. its roughness. 
         [0021]    In another embodiment, the second material is a solid thin layer, and: the identification means are configured to identify transmission by said interface of a second mechanical wave interfering with the first mechanical wave, and the determination means are configured to determine the variation in amplitude of the Brillouin oscillations before and after transmission by said interface of the second mechanical wave, in order to characterize the acoustic transmission coefficient of said interface. 
         [0022]    In this embodiment, interference takes place between two waves, one of which has been transmitted through an interface. Depending on the amplitude of the interference, as determined by the Brillouin oscillations, it becomes possible to evaluate the amplitude of the waves transmitted by the interface, and thus the transmission coefficient of the interface. 
         [0023]    Preferably, the means for generating a first mechanical wave in the first material are configured to form the first and second mechanical waves simultaneously respectively in the first material and in the second material. More precisely, the two mechanical waves are formed at the interface between the first and second materials, the first wave propagating in the first material and the second wave propagating in the second material. Since the second material is a layer that is thin, the second wave is quickly reflected by the second surface of the second layer and returns towards the interface between the first and second materials, with a fraction thereof passing through the interface, depending on the transmission coefficient of said interface, so as to form the second mechanical wave in the first material. Since the second mechanical wave was formed at the same time as the first mechanical wave, the two waves are mutually coherent and they can therefore interfere, thereby leading to variation in the amplitude of the measured Brillouin oscillations. 
         [0024]    Preferably, the device further comprises adjustment means for adjusting parameters of a theoretical model giving the values for variation in amplitude of the Brillouin oscillations for different probe radiation wavelengths, in order to obtain the amplitude variation of the Brillouin oscillations as determined by the determination means, with the parameters as adjusted in this way serving to characterize the interface, and in particular the acoustic transmission coefficient of the interface. 
         [0025]    Using a theoretical model, it is possible to predict the form of the Brillouin oscillations that can be obtained from a structure having characteristics (dimensions, quality, materials, . . . ) that constitute the parameters of the theoretical model. Thus, by adjusting the parameters of the theoretical model in order to obtain the Brillouin oscillations as observed, it becomes possible to discover the characteristics of the structure under study, e.g. the thickness of a layer, the transmission coefficient of an interface, or indeed the speed of sound in the layer. Furthermore, since such adjustment is performed over a greater or lesser number of values, it is possible to select the accuracy desired for the results that are obtained. 
         [0026]    Preferably, the means for generating pump radiation and the means for generating probe radiation are identical or they are different. 
         [0027]    The means for generating pump radiation and the means for generating probe radiation may comprise one or two tunable laser sources. 
         [0028]    Alternatively, they may comprise two means for emitting a continuum of light. 
         [0029]    Alternatively, the means for generating pump radiation may comprise a fixed laser source, and the means for generating probe radiation may comprise a tunable laser source. 
         [0030]    In another aspect, the invention also provides a method of characterizing an interface of a structure, said structure comprising a solid first material and a second material, which materials are separated by said interface. The method comprises the following steps:
       forming a first mechanical wave in the first material, e.g. with pump radiation;   forming Brillouin oscillations with probe radiation propagating at least in part in the first material;   detecting the time variation of the Brillouin oscillations in particular in the first material;   identifying reflection of said first mechanical wave by said interface or transmission through said interface of a second mechanical wave interfering with the first mechanical wave on the basis of the time variation of the Brillouin oscillations, in particular in the first material; and   determining the variation in amplitude of the Brillouin oscillations, in particular in the first material before and after reflection or transmission by said interface.       
 
         [0036]    Preferably, the variation in amplitude of the Brillouin oscillations as a function of the wavelength of the probe radiation is determined. 
         [0037]    In an implementation, the second material presents acoustic impedance that is very different from the acoustic impedance of the first material, for example it is a gas, and: a reflection of said first mechanical wave by said interface is identified, and the variation in amplitude of the Brillouin oscillations before and after reflection by said interface is determined in order to characterize the roughness of said interface. 
         [0038]    Preferably, the wavelength of the probe radiation is selected as a function of the size of the roughness to be measured. 
         [0039]    In another implementation, the second material is a solid thin layer, and: transmission through said interface of a second mechanical wave interfering with the first mechanical wave is identified, and the variation in amplitude of the Brillouin oscillations before and after transmission by said interface of the second mechanical wave is determined in order to characterize the acoustic transmission coefficient of said interface. 
         [0040]    Preferably, the first and second mechanical waves are formed simultaneously respectively in the first material and in the second material. 
         [0041]    Preferably, the parameters of a theoretical model giving the values for variation in amplitude of the Brillouin oscillations for different wavelengths of the probe radiation are adjusted in order to obtain the variations in amplitude of the Brillouin oscillations as determined, with the parameters as adjusted in this way serving to characterize the interface and in particular the acoustic transmission coefficient of the interface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0042]    The invention and its advantages can be better understood on reading the following detailed description of a particular embodiment given by way of non-limiting example and illustrated in the accompanying drawings, in which: 
           [0043]      FIG. 1  is a diagrammatic view of a characterization device of the invention; 
           [0044]      FIG. 2  shows a first implementation of the invention; 
           [0045]      FIG. 3  shows an example of a result that can be obtained in the first implementation of the invention; 
           [0046]      FIG. 4  shows a second implementation of the invention; 
           [0047]      FIG. 5  shows an example of a result that can be obtained in the second implementation of the invention; and 
           [0048]      FIGS. 6 and 7  are examples of flow charts for implementations of methods of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0049]      FIG. 1  is a diagram showing an embodiment of a characterization device  1  of the invention. 
         [0050]    The device  1  thus comprises a short-pulse laser source  2 . The short pulses from the laser need to be matched to the desired time resolution. It is possible to envisage using pulses of the order of 1 picosecond (ps) or 0.1 ps. 
         [0051]    In a first embodiment, this source is wavelength tunable by means of a suitable oscillator, e.g. of the titanium: sapphire type, capable of producing 120 femtosecond (fs) pulses at a repetition rate of 76 megahertz (MHz) or indeed 80 MHz, centered on a wavelength that is suitable over the range 700 nanometers (nm) to 990 nm, or indeed over the range 680 nm to 1070 nm. 
         [0052]    The source produces radiation that is split by a splitter  3  into pump radiation  4  and probe radiation  5 , both of which are to interact with the structure  6  for analysis. 
         [0053]    The probe radiation  5  is subjected to path length variation compared with the pump radiation  4 , e.g. by means of a movable mirror  7  that is servo-controlled in position, so as to reach the structure  6  with a time offset relative to the pump radiation. 
         [0054]    It is then focused on the structure  6  by an optical system  8 , and it is reflected in the form of a signal  9  to detector means  10 , e.g. of the photodetector type, in order to generate a signal that can be analyzed by processor means  11 , e.g. a conventional type of computer suitable for performing the processing of the invention. 
         [0055]    Alternatively, the probe signal may also be detected after transmission through the structure  6 . 
         [0056]    In order to ensure that the signals pass properly from the source to the structure, the optical system is adapted to accommodate the variation in wavelength from the source. The person skilled in the art knows how to adapt such an optical system depending on the selected sources and wavelength ranges, and only a few examples of suitable optical systems are described herein. 
         [0057]    The optical systems should preferably be broadband systems, both concerning the mirrors and the treated lenses. In order to achieve a sufficient signal-to-noise ratio, pump-probe devices make use of modulation of the pump radiation and demodulation of the probe radiation. The modulation must be performed outside the noise range of the laser, typically at several 100 kilohertz (kHz). It is performed by an acousto-optical modulator that acts like an electrically controlled grating, or indeed by an electro-optical modulator, or indeed by an optical chopper. The way the grating diffracts the pump radiation varies with wavelength. Thus, by changing the wavelength, the pump radiation is caused to vary in direction, which means that the device can lose its adjustment. It is therefore possible to use an acousto-optical modulator that can be controlled using an electrical signal of variable frequency. Varying deflection of the pump radiation is thus compensated by changing the pitch of the electrically generated grating. 
         [0058]    When using a half-wavelength that is obtained by optical doubling in a non-linear crystal, e.g. of the beta barium borate (BBO) type, doubling relies on a phase tuning condition in the crystal, which is associated with its orientation relative to the radiation. The change in wavelength needs to be taken up over this angle. This step is performed manually or automatically. 
         [0059]    The person skilled in the art readily understands that the pump and probe beams may also be generated by two distinct sources. Under such circumstances, the sources may themselves be movable in order to generate the variation in the optical path length of the probe radiation relative to the pump radiation. It is also possible to use a laser source of fixed wavelength and a source that is tunable. 
         [0060]    In a second embodiment, the source  2  serves to generate a light continuum extending over a broad range of wavelengths. Under such circumstances, the detector means  10  may comprise a spectrometer (not shown) for analyzing the intensity of the received light prior to transmitting the signal for analysis to the processor means  11 . It is also possible to use any system of filters in front of an ordinary photodetector. 
         [0061]    The plurality of wavelengths is then generated continuously, e.g. by a fixed wavelength femtosecond laser associated with an optical fiber. 
         [0062]    In general manner, any type of source suitable for generating short laser pulses corresponding to a discrete or continuous set of wavelengths may be used. 
         [0063]    Likewise, it is possible to use any means suitable for producing a time offset between the pump radiation and the probe radiation. This offset can thus be produced by varying the optical path length as described above, or by means enabling the arrival time of one pulse to be adjusted relative to another. 
         [0064]    The processor means  11  comprise identification means  12  and determination means  13 . 
         [0065]    The identification means  12  receive the variation over time in the signals detected by the detector means  10 , and in particular the Brillouin oscillations. The identification means  12  are configured to identify, from the received signal, a reflection of a mechanical wave on the interface of the structure, or indeed a transmission of a mechanical wave through the interface of the structure. 
         [0066]    An identification of Brillouin oscillations before and after reflection or transmission is then transmitted to the determination means  13 , which are configured to determine the variation in amplitude of said Brillouin oscillations before and after reflection or transmission. Thus, the determination means  13  can calculate the ratio of the maximum amplitude of the Brillouin oscillations before reflection or transmission to the maximum amplitude of the Brillouin oscillations after reflection or transmission. The determination means  13  may also take account of a phase shift before and after reflection or transmission. 
         [0067]    Preferably, when the means for generating the probe radiation can emit probe radiation at different wavelengths, the determination means are configured to determine the variation in amplitude, before and after reflection or transmission of the Brillouin oscillations as a function of the wavelength of the probe radiation. 
         [0068]    Finally, the characterization device  1  may comprise adjustment means  14  for adjusting parameters of a theoretical model. The adjustment means  14  include a theoretical model predicting the variation in amplitude of the Brillouin oscillations as a function of certain structural characteristics (thicknesses, materials, etc.) of the structure being analyzed, and by modifying these parameters it can cause the results that are obtained experimentally to coincide with the results from the theoretical model. Under such circumstances, when the structure of the theoretical model corresponds to the structure being analyzed, it becomes possible to know the structural characteristics of the structure being analyzed by using the parameters obtained by the adjustment means  14 . 
         [0069]    There follows a description of two embodiments and uses of the  FIG. 1  characterization device  1 . 
         [0070]    In one embodiment, the device  1  is used to characterize an interface I between a substrate and a thin layer. Thus (see  FIG. 2 ), the structure  100  being analyzed comprises a substrate  101  that is preferably transparent and of determined thickness, having a thin layer  102  arranged thereon, which layer is preferably absorbent. The structure  100  is arranged in the device  1  so that the probe radiation S passes initially through the substrate  101  and subsequently the thin layer  102 . The structure  100  may thus be a sample of a solar panel comprising a glass substrate with an electrode arranged thereon. 
         [0071]    The thin layer  102  is selected so as to absorb the pump radiation P. Thus, the soundwave produced by the pump radiation is formed at the interface I between the substrate  101  and the thin layer  102 . 
         [0072]    Two soundwaves are then observed: a first soundwave  103  that propagates in the substrate  101  towards the free surface (that receives the probe radiation S), and a second soundwave  104  that propagates in the thin layer  102 . Since the thin layer  102  presents thickness that is very small, the soundwave  104  propagating therein is reflected by the opposite surface of the thin layer  102  and returns towards the interface I through which it passes to a greater or lesser extent depending on the transmission coefficient of said interface I. 
         [0073]    The fraction of the second soundwave  105  passing through the interface I is coherent with the first soundwave  103 , and can thus interfere therewith. Depending on the phase difference between the two soundwaves  103  and  105 , and depending on their respective amplitudes, the interference is pronounced to a greater or lesser extent. The characterization device  1  makes it possible to observe the Brillouin oscillations due to the first soundwave  103  and then due to the interference between the first and second soundwaves  103  and  105 . It is thus possible to determine the amplitude of the fraction of the second soundwave  105  that has interfered with the first soundwave  103 , and to deduce therefrom the transmission coefficient of the interface I. 
         [0074]    In particular, the processor means  11  serve firstly to identify (using the means  12 ) the moment in the observation of the Brillouin oscillations at which the fraction of the second soundwave  105  interferes with the first soundwave  103 , and then secondly to measure the variation in the amplitude of the Brillouin oscillations due to the interference (by using the means  13 ). 
         [0075]    Preferably, the processor means  11  can perform this analysis for different wavelengths, and then use the means  14  to compare the results obtained with a theoretical model in order to refine the structural parameters of the sample. 
         [0076]      FIG. 3  shows an example of a result obtained by a theoretical model: the figure shows how the ratio A 2 /A 1  (where A 1  is the amplitude of the Brillouin oscillations corresponding to the first soundwave  103  and A 2  is the amplitude of the Brillouin oscillations corresponding to interference between the first soundwave  103  and the fraction of the second soundwave  105 ) varies as a function of the wavelength of the probe radiation. In particular, the amplitude of the curve that is obtained (difference between the smallest ratio A 2 /A 1  and the largest ratio A 2 /A 1 ) enables the interface I between the two materials to be characterized, and the wavelength differences between two successive extremums enables the thickness of the thin layer to be characterized. 
         [0077]    Alternatively, the structure being analyzed may be a multilayer structure having a plurality of thin layers on the transparent substrate. Under such circumstances, the successive interferences with the waves transmitted by the various thin layers can also be used as interpreted in order to characterize the interfaces between the various thin layers. 
         [0078]    In another embodiment, the device  1  is used for characterizing the roughness of a surface of a given layer. Thus (see  FIG. 4 ), the structure  200  being analyzed mainly comprises a preferably transparent given layer  201  that presents a free surface  202  of roughness that is to be determined, and an absorbent layer (not shown) arranged on the surface opposite from the free surface  202 . The structure  200  is arranged in the device  1  in such a manner that the probe radiation S passes firstly through the free surface  202  for analysis, then through the given layer  201 , and then through the absorbent layer. 
         [0079]    Alternatively, instead of a free surface  202 , it would be possible to provide a layer of a second material presenting acoustic impedance that is very different from the acoustic impedance of the given layer  201  (i.e. much greater than or much less than said impedance), in order to obtain almost complete reflection of the soundwave at the interface. 
         [0080]    The absorbent layer is selected so as to absorb the pump radiation. Thus, the soundwave produced by the pump radiation is formed at the interface between the given layer  201  and the absorbent layer. 
         [0081]    A first soundwave  203  is then observed that propagates from the absorbent layer towards the free surface  202  of the given layer  201 , followed by a second soundwave  204  that is due to the first soundwave  203  being reflected on the free surface  202  and propagating towards the absorbent layer. 
         [0082]    The waveform of the second soundwave  204  then depends on the quality of the free surface  202  of the given layer, and in particular on its roughness, that enables reflection to take place more or less correctly. Thus, depending on the roughness of the free surface  202 , the second soundwave  204  becomes spatially dispersed to a greater or lesser extent. 
         [0083]    The characterization device  1  enables the Brillouin oscillations due to the first soundwave  203  and then to the second soundwave  204  to be observed. It is thus possible to estimate the roughness of the free surface  202  that reflected the soundwave. 
         [0084]    In particular, the processor means  11  make it possible firstly to use the means  12  to identify the moment in the observation of the Brillouin oscillations at which the first soundwave reflects on the free surface  202  of the given layer in order to form the second soundwave, and then secondly to measure the variation in the amplitude of the Brillouin oscillations due to the reflection (by using the means  13 ). 
         [0085]    The processor means can preferably perform this analysis at different wavelengths. 
         [0086]      FIG. 5  gives an example of the results obtained for different roughnesses of the free surface (5 nm, 10 nm, and 20 nm) and for different wavelengths of the probe radiation. It can be seen in particular that by shortening the wavelength of the probe radiation S, it is possible to analyze higher frequency soundwaves that are more sensitive to the same irregularities of the free surface. 
         [0087]      FIG. 6  is a flow chart  15  of an implementation of a method of the invention for characterizing an interface between a first material and a second material. 
         [0088]    In a first step  16 , a first mechanical wave and a second mechanical wave are formed, and then in a second step  17 , Brillouin oscillations are formed in the first material. Thereafter, in a third step  18 , transmission of the second mechanical wave in the first material is identified, and during a fourth step  19 , the variation in the amplitude of the Brillouin oscillations before and after transmission of the second mechanical wave is determined. The fourth step  19  may be performed in particular as a function of the wavelength of the probe radiation. Finally, in a preferred last step  20 , the parameters of a theoretical model are adjusted to the values that have been determined. 
         [0089]      FIG. 7  shows a flow chart  21  of an implementation of a method of the invention for characterizing the roughness of a surface of a first material. 
         [0090]    In a first step  22 , a first mechanical wave is formed in the first material and then in a second step  23 , Brillouin oscillations are formed in the first material. Then, in a third step  24 , reflection of the first mechanical wave on the free surface of the first material is identified, and during a fourth step  25 , the variation in the amplitude of the Brillouin oscillations before and after reflection of the first mechanical wave is determined. The fourth step  25  may in particular be performed as a function of the wavelength of the probe radiation. 
         [0091]    Thus, the subject matter of the invention makes it possible in reliable and non-destructive manner to obtain characteristics of a structure, in particular the roughness of a surface reflecting a soundwave, or indeed the transmission coefficient at an interface between two materials. Furthermore, the use of a probe of variable wavelength makes it possible to refine the above characteristics, and to obtain easily values that are more accurate and more complete.