as shown in fig6 , the device of the invention includes a short - pulse laser source 1 . the short pulses of the source must be adapted to match the desired time resolution . pulses of about 1 ps or of about 0 . 1 ps are imaginable . in a first embodiment , the source is wavelength - tunable via a tunable oscillator of the titanium - sapphire type that can generate pulses of 120 fs at a repetition rate of 76 mhz centered on a wavelength that is tunable in the range 700 nm to 990 nm . this source generates a signal that is split by a splitter 2 into a pump signal p and a probe signal s , both of which are designed to interact with the structure 5 to be analyzed . the probe signal s is subjected to a variation in optical path length relative to the pump signal p , e . g . via a moving mirror 3 that is position servo - controlled . said probe signal is then focused on the structure 5 by an optical system 4 , and is reflected towards detection means 6 , e . g . of the photodetector type that are designed to generate a signal that can be analyzed by computation and processing means 7 . naturally , the probe signal can also be detected in transmission through the structure 5 . in order to enable the signals to go along the proper path from the source to the structure , the optical system is adapted to the variation in wavelength coming from the source . the person skilled in the art is capable of adapting said optical system depending on the chosen sources and wavelength ranges , and merely a few examples of usable optical systems are given herein . the optical systems should preferably be broadband as regards both mirrors and treated lenses . in order to achieve a signal - to - noise ratio that is sufficient , pump - probe experiments use modulation of the pump beam and demodulation of the probe . the modulation should be performed outside the noise range of the laser , typically a few 100 khz . it is performed by an acousto - optical modulator that acts as an electrically controlled grating . the diffraction of the pump beam by the grating varies with varying wavelength . thus , by changing wavelength , the pump beam sees its direction change so that it is possible that the device might lose its setting . it is thus possible to use an acousto - optical modulator that can be controlled with an electrical signal of variable frequency . the deviation of the beam is thus compensated by changing the pitch of the grating that is generated electrically . when a half - wavelength is used that is obtained by optical doubling in a non - linear crystal , e . g . of the beta barium borate ( bbo ) type , the doubling is based on a phase tuning condition being satisfied in the crystal , which condition is related to its angular position relative to the beam . the change of wavelength must be made up on that angle . this is performed manually or automatically . at the outlet of the detector , the processor means 7 receiving the signal are constituted by a computer of known type that enables the processing of the invention to be implemented . the person skilled in the art can easily understand that the pump and probe beams can also be generated by two distinct sources . in which case , the sources can themselves be moving sources in order to generate the variation in the optical path length of the probe signal relative to the pump signal . it is also possible to use a fixed - wavelength laser source , and a tunable source . in a second embodiment , the source 1 makes it possible to generate a continuum of light extending over a wide wavelength range . in which case , the detector means 6 can comprise a spectrometer ( not shown ) serving to analyze the intensity of the light received before transmitting the signal to be analyzed to the processor means 7 . any system of filters in front of a usual photodetector can also be used . the plurality of wavelengths is then achieved continuously , e . g . by a fixed - wavelength femtosecond laser associated with an optical fiber . in general , it is understood that the type of source used is not limiting to the present invention and that any type of source 1 making it possible to generate short laser pulses corresponding to a discrete or continuous set of wavelengths can be used . similarly , in all of the embodiments , it is possible to use any means suitable for generating a time shift between the pump first beam and the probe second beam . this shift can thus be generated by varying optical path length as described above , or indeed by means making it possible to adjust the time of arrival of one pulse relative to another . at the processor means 7 , a theoretical model is stored for variation of the jump amplitude as a function of wavelength . this theoretical model is obtained from a simple physical model making it possible to understand the origin of the jumps observed in the signal as a function of wavelength . a transparent layer of the structure 5 acts as an optical resonator of the fabry - perot type for the probe light . in the presence of a deformation pulse , the layer behaves as if its thickness were slightly smaller or slightly larger depending on the sign of said deformation . if a jump appears on reflection of the acoustic pulse off the free surface , it is because it becomes extensive whereas it was compressive . since the thickness changes slightly , the reflectivity of the interferometer system constituted by the transparent layer changes accordingly . it is possible to establish an analytical expression for the change in reflectivity induced by such a mechanism . firstly , the reflectivity of a transparent thin layer of finite thickness e is written : r = r 01 + r 12 ⁢ ⅇ 2 ⁢ ⅈ ⁢ ⁢ k 1 - r 01 ⁢ r 12 ⁢ ⅇ 2 ⁢ ⁢ ⅈ ⁢ ⁢ ke , where r 01 ( or r 12 ) designates the electromagnetic reflection coefficient between the media 0 and 1 ( or 1 and 2 ), the subscripts 0 , 1 , and 2 corresponding generally to a succession of layers 0 , 1 , and 2 . the deformation acoustic pulse carries a very small variation in thickness of the layer ( referenced δe ), which allows us to write the effect on the reflectivity δr in the following form : more precisely , the quantity obtained experimentally is the relative change of reflectivity in intensity : δ ⁢ ⁢ r r = 2 ⁢ ⁢ ⁢ ( δ ⁢ ⁢ r r ) = 2 ⁢ ⁢ ⁢ ( 1 r ⁢ ∂ r ∂ e ) ⁢ δ ⁢ ⁢ e thus , the physical effect that concerns us results from the derivative of the complex reflectivity of the transparent layer . on the basis of this result , we can trace the expected changes as a function of the probe wavelength as shown in fig5 . in particular , it can be noted that , by exploring the signs of the jumps over well - chosen wavelength ranges , it is possible to determine at least one zeroing wavelength . in accordance with the invention , the wavelength of the probe radiation is caused to vary and the zeros of the amplitude of the jumps in the signal are detected , as are the associated changes of signs , as shown in fig4 a and 4 b . it should be noted that , since the variation in the amplitude of the jumps as given in fig4 a and 4 b represents experimental results , said variation is very sensitive to wavelength , as confirmed by the theoretical slope at the time of zeroing in fig5 . in accordance with the invention , the heavy wavelength dependency can also be used to identify the reflectivity jumps more clearly . thus , a step of identifying an amplitude jump can itself include a sub - step of varying the wavelength . such an identification step can easily by implemented by a test as a function of a threshold of amplitude variation over a given time , optionally with the wavelength being varied . thus , by comparing the zeros of the jumps of the signal obtained after detection with the theoretical model , it is possible to determine a wavelength λ 0 characteristic of zeroing . however , depending on the model adopted , the relative change of reflectivity in intensity is a function only of the wavelength , of thickness of the transparent layer , and of the speed of propagation of the wave ( or of the optical index of the layer ), and more precisely , of the product of the optical index multiplied by the thickness of the layer n . e . determining the zeroing characteristic wavelength λ 0 thus makes it possible to obtain a value characteristic of the product of the index multiplied by the thickness ( n . e ) 0 . this additional information on the characteristics of the layer then makes it possible to increase considerably the precision of the results obtained . for a brillouin oscillation zone , the period of oscillation of the signal is measured . using the known formula : t ⁡ ( λ , n , e ) = λ 2 ⁢ ⁢ nv ⁢ ⁢ cos ⁢ ⁢ θ it is then possible to obtain a first item of information on the index and on the thickness as a function of the wavelength . by then placing ourselves at the zeroing characteristic wavelength , the product ( n . e ) 0 is set . it is also possible to measure the oscillation time for the brillouin oscillations t ⁡ ( e , v ) = 2 · e v corresponding to one go - and - return of the wave in the layer , which gives a third item of information on the index and on the thickness . the person skilled in the art can then easily understand that the data constituted by the product ( n . e ) 0 of the invention considerably increases the precision of the results on the index and thickness values . in particular , the applicant has shown that , for a structure of the aln / ai / ti / si type , e . g . in a resonator of the bulk acoustic wave ( baw ) type , the method of the present invention makes it possible to cause the uncertainty in thickness to go from 6 % in the absence of determination of the product ( n . e ) 0 to 0 . 17 % with such determination . it can also be noted that it is possible to refine the physical model shown in fig5 by involving new effects . in particular , it is also possible to affect the reflectivity of the layer via the optical index ( photo - elastic effect ). the reflectivity jump detected can then be written as the sum of two synchronous contributions in wavelength , i . e . the inversions and the zeroing take place simultaneously . furthermore , we have described an example in which the comparison with the theoretical model is made relative to a point of zeroing of the jump amplitude as a function of wavelength , but naturally any point or characteristic of the physical model can be used . it can thus be the maxima , the decreasing of the maxima , the spacing between two zeros , etc . the present invention relates particularly but not exclusively to any transparent layer on a substrate or on an absorbent layer . more generally , the probe signal must be suitable for “ seeing ” the two end interfaces of any given layer . this is thus achievable on a layer that is relatively absorbent but that is fine enough for the probe signal to reach the end that is further away . preferably , the pump signal should also be absorbed in depth into the structure . an implementation is given for a layer of aln in a baw resonator , but naturally the device of the invention operates for any type of structure as defined above . the method can , for example , be used for the “ loading ” layer of sio 2 which is deposited on the upper electrode of a component . the applicant has also been able to characterize “ high - k ” thin layers of oxides of the srtio 3 and batio 3 types by using the method and the device of the invention . the invention is described above by way of example . naturally , the person skilled in the art is capable of implementing various variants of the invention without going beyond the ambit of the patent .