Method and apparatus for generating isolated ultrashort electromagnetic pulses.

Abstract

The invention relates to a method for generating isolated ultrashort electromagnetic pulses (IR2) comprising the following steps: - directing a first laser pulse (IL1) at oblique incidence and with p-polarization, having a normalized peak amplitude greater than or equal to 1, onto a surface (SC) of a target in the condensed state (CEC), thereby generating a plasma mirror (MPR) oscillating at relativistic speed on the surface of the target; and - directing a second laser pulse (IL2) of duration (τ2) less than or equal to the optical period (T1) of the first pulse onto said surface of the target, corresponding to said plasma mirror, the second laser pulse being spatially and temporally superimposed on the first laser pulse; thereby, the second laser pulse, reflected by the plasma mirror, undergoes a shift in its wavelength and a modification of its duration. Figure for the abstract: Fig. 1.

Description

Title of the invention: Method and apparatus for generating isolated ultrashort electromagnetic pulses.

[0001] The invention relates to a method and apparatus for generating ultrashort and isolated coherent electromagnetic pulses of high energy. It relates in particular, but not exclusively, to the generation of pulses of subfemtosecond duration (lfs=10 15 s) or "attosecond" (las=10 18 s) and an energy typically between 1 mJ and 1 J. Such pulses lend themselves to numerous applications, from deep ultraviolet (DUV) or extreme ultraviolet (XUV or EUV: 10 - 200 nm) lithography, used for the manufacture of integrated circuits, to fundamental research (study of ultra-fast processes in matter etc.) and to eye surgery.

[0002] To date, several technologies make it possible to generate electromagnetic pulses whose spectrum extends entirely or partly into the DUV range (200nm - 400nm), extreme ultraviolet or even into X-rays (wavelength less than 10 nm).

[0003] Excimer lasers use a gas of molecules in a cavity that is pumped by an electrical discharge. The discharge excites the molecules, which, upon de-exciting, emit coherent radiation in the ultraviolet. Depending on the gas used in the cavity, UV radiation ranging from 351nm (XeF) to 157nm (F2) can be produced. Excimer lasers currently produce nanosecond pulses with a repetition rate of the order of kHz with an energy per pulse of 1 mJ to 100mJ (corresponding to average powers of approximately 1 W to 100W). Their energy efficiency is of the order of 0.3%. This technology is widely used in DUV lithography. Its main drawback is that the minimum achievable wavelength remains relatively high (157 nm), thus limiting the minimum size of the engravings that can be produced.Furthermore, it does not allow ultra-short pulses to be obtained, i.e. with a duration (full width at half-height) of less than 100 ps (1 ps=10 12 s), or even much less.

[0004] Hot plasmas (-6000 K) created by focusing an intense laser (typically CO2) on matter emit thermal radiation that extends to the extreme ultraviolet. These sources make it possible to reach very short wavelengths, of the order of 10 nm, with an average power of the order of 100 W and an energy conversion efficiency of approximately 0.3%. This technology is widely used in EUV lithography, particularly for etching at 13.5 nm. However, it has major drawbacks: the light generated is incoherent and therefore difficult to manipulate, the hot plasma also emits energetic particles (electrons and ions) which damage EUV optics, the technology is very expensive and very energy-intensive. In addition, it does not allow for ultrashort coherent pulses.

[0005] Free electron lasers (FELs) use a linear accelerator to accelerate electron bunches to high energies (100 MeV - several tens of GeV) and make them oscillate in an undulator composed of a series of magnets (or coils) that deliver a strong magnetic field. This oscillating motion produces synchrotron radiation in the UV, EUV, and X-rays that is amplified by wave-electron coupling (laser effect). These lasers produce coherent, ultrashort pulses (a few femtoseconds), of relatively high energy (a few tens of pj). The main disadvantages of FELs are their extremely high cost and their modest efficiency, which limits them to fundamental research. Furthermore, they do not allow the production of pulses of at-tosecond duration (by "attosecond pulses" we mean pulses of duration less than 1 fs).

[0006] High-order harmonic generation in gases (HHG) is a technique that consists of focusing femtosecond laser pulses, generally in the near infrared, of relatively high intensity (typically ~1015 W / cm2) in a gas to obtain the emission of harmonic radiation from the laser, with harmonic orders that can reach several tens. In the time domain, the harmonic radiation appears in the form of attosecond pulse trains, but in some cases it is possible to obtain isolated attosecond pulses. Typical efficiencies obtained (infrared to UV to EUV) are of the order of 105 to 106 over the 100eV to 100eV range (i.e. 120 nm to 12 nm). Today, this type of source is produced with infrared and near-infrared lasers of a few mJ to a few tens of mJ, which makes it possible to achieve at best UV / EUV pulses of around ten pJ of energy.This type of EUV light source is relatively inexpensive, but remains mainly used in time-resolved fundamental physics experiments. The energy performance of these installations is very low, which makes them poorly suited to industrial applications.

[0007] Other techniques, exploiting the reflection of laser pulses on dense plasmas (“plasma mirrors”) are less mature.

[0008] The technique of generating harmonics on a relativistic plasma mirror - see for example (Vincenti 2019) - consists of focusing an intense femtosecond infrared or near-infrared laser, with high temporal contrast and power of a few tens of TW to a few tens of PW to achieve laser intensities I>1018 W / cm2 on an initially solid target. At the focal point, the laser completely ionizes the target and forms a dense plasma, which is reflective for the incident field. Under the effect of this field, the "plasma mirror" thus formed oscillates at speeds relativistic to the laser period. These oscillations cause a periodic temporal compression of the incident field which corresponds, in the frequency domain, to a spectrum of high-order Doppler harmonics of the laser frequency, which can extend to EUV or even X-rays, and in the time domain to a train of attosecond pulses. The conversion efficiencies obtained strongly depend on the laser conditions (intensity, angle of incidence) and plasma conditions (density, length of the plasma-vacuum interface).Under optimized conditions and for 100TW class lasers (~2J-20fs, max intensity of the order of 1019W / cm2), the conversion efficiencies are less than 10 4 below 80nm (i.e. energies of the order of 100pJ). For multi-PW class lasers (100J-20fs, intensities I>1020W / cm2), these conversion efficiencies are of the order of 0.5% at 80nm (i.e. energies of the order of a joule). These efficiencies decrease in power law with the frequency and therefore quickly become negligible at short wavelengths. This technology is currently at the experimental proof of concept stage by a few large laser laboratories at the world's leading edge in the field. It is not yet used for industrial purposes.

[0009] The relativistic flying mirror technique exists only as a theoretical concept. It consists of focusing a first very intense infrared laser in a gas - (Bulanov 2003) - or on a solid thin sheet - (Tamburini 2014) - a few tens of nm thick to generate a relativistic plasma mirror. A second counter-propagating laser is then focused on the mirror generated by the first pulse to be compressed in time and shortened in wavelength by the Doppler effect. For example, in the case of a thin sheet, a first very intense laser (multi-PW) is focused (I>1023W / cm2) on one side of the sheet to accelerate the ions and electrons of the sheet by radiation pressure and create a relativistic mirror. A second counter-propagating laser to the mirror is sent from the other side of the sheet. The laser reflected by the sheet is then compressed in time and converted by Doppler effect to UV / EUV.Unlike harmonic generation on relativistic plasma mirrors, this technique should allow the generation of isolated femtosecond or attosecond pulses. Unfortunately, the conversion efficiencies and the minimum accessible UV / EUV wavelengths remain very limited. In (Tamburini 2014), the authors mention a minimum wavelength of the order of 570nm from an 800nm ​​laser with conversion rates (infrared to 570nm) lower than 1%. One of the main limitations of the proposed techniques is that they do not allow, under realistic conditions, to have a mirror of sufficient optical quality (density, surface) to be able to properly reflect the second . pulse. Furthermore, these techniques are extremely complex to implement experimentally because they require spatio-temporal synchronization on femtosecond time scales and micrometric space scales of two ultrashort high-power laser pulses, all for low efficiencies, even in the ideal case. It is therefore unlikely that this technique can be implemented for industrial purposes.

[0010] The invention aims to overcome, in whole or in part, the aforementioned drawbacks of the prior art. More particularly, it aims to generate coherent and ultrashort isolated electromagnetic pulses, the spectrum of which can extend in whole or in part into the DUV, EUV or even X domains and having an energy greater than or equal to 1 mJ. No technique of the prior art makes it possible to obtain such a result. Furthermore, the invention aims to generate these pulses in a manner that is relatively simple to implement, making them suitable for industrial applications.

[0011] According to the invention, this object is achieved by using a first intense laser pulse ("generator pulse") to generate an oscillating relativistic plasma mirror, on which a second laser pulse ("source pulse") is reflected. The source pulse has a duration (full width at half maximum) less than or equal to the optical period of the generator pulse, and preferably less than half of this optical period. Since the relativistic plasma mirror oscillates at the frequency of the first pulse, the source pulse does not see an oscillating mirror, but a mirror moving monotonically, at a relativistic speed (or, at most, performing a single back-and-forth movement).By choosing the appropriate time shift between the first and second pulses, the source pulse will be reflected by the plasma mirror when the latter moves at a quasi-constant, relativistic speed in a direction opposite to the propagation of this pulse. The source pulse then undergoes a decrease in its wavelength ("blue shift") by the Doppler effect and a strong time compression.

[0012] The technique of the invention is much easier to implement than that of the relativistic flying mirror, because it does not require that the two pulses be counter-propagating. On the contrary, it is preferable that they propagate coaxially, which greatly simplifies the optical assembly. Furthermore, the two pulses can come from the same initial laser pulse, which ensures their synchronization. Furthermore, the optical quality of the plasma mirror thus obtained is much better than that of a relativistic flying mirror.

[0013] Compared to the relativistic plasma mirror technique, in which a single laser pulse generates the plasma and is reflected from it, the invention makes it possible to obtain a much better conversion efficiency towards DUV / EUV. Indeed, the invention makes it possible in principle to convert almost all of the energy of the source pulse towards short wavelengths, whereas in the case of the prior art only a small fraction of this energy is converted. In fact, in some cases the invention makes it possible to achieve a conversion efficiency greater than 1 (relative to the energy of the source pulse), because the source pulse acquires energy from the plasma mirror at the moment of its reflection.

[0014] Furthermore, the invention makes it possible to obtain single ultrashort pulses (up to durations of a few tens of attoseconds), whereas the relativistic plasma mirror technique generates trains of such pulses. To date, other techniques, such as high order harmonic generation (HHG) in gases, make it possible to generate isolated attosecond pulses, but with very low energies and conversion efficiencies (nanojoules, efficiencies of the order of 106 for HHG).

[0015] As will be explained in more detail below, the invention can already be implemented using existing and proven technologies.

[0016] An object of the invention is therefore a method for generating isolated ultrashort electromagnetic pulses comprising the following steps: - directing onto a surface of a target in the condensed state a first laser pulse at oblique incidence and p polarization, said first laser pulse having a first wavelength, corresponding to a first optical period, and a normalized peak amplitude greater than or equal to 1, whereby a plasma mirror oscillating at relativistic speed is generated on the surface of the target; and - directing onto said surface of the target, in correspondence with said plasma mirror, a second laser pulse having a second wavelength, less than the first wavelength, and a duration less than or equal to the first optical period, the second laser pulse being spatially and temporally superimposed on the first laser pulse; whereby the second laser pulse, reflected by the plasma mirror, undergoes a shift in its wavelength and a modification in its duration.

[0017] According to particular embodiments of such a method:

[0018] The duration of the second laser pulse may be less than or equal to half of the first optical period, the time shift between the first and second laser pulses was chosen such that, during the entire duration during which the second laser pulse is incident on the plasma mirror, the latter moves away from the surface of the target; whereby the second laser pulse undergoes a blue shift of its wavelength and a time compression by Doppler effect.

[0019] The second laser pulse can propagate in a direction collinear with a propagation direction of the first laser pulse.

[0020] The first laser pulse may be incident on the target surface with a angle of incidence between 25° and 75°.

[0021] The second laser pulse may have, on the surface of the target, a normalized peak amplitude lower than that of the first laser pulse.

[0022] The second laser pulse may be incident on the surface of the target in s polarization.

[0023] The method may comprise a step of stabilizing the phase of the carrier of the first laser pulse relative to its envelope.

[0024] The method may comprise a step of controlling the shift of the wavelength of the second laser pulse reflected by the plasma mirror by adjusting at least one parameter chosen from: a phase of the carrier of the first laser pulse relative to its envelope; a delay between the first and second laser pulses; a peak amplitude of the first laser pulse.

[0025] The method may comprise a prior step of generating the first and second laser pulses from the same so-called initial laser pulse.

[0026] The initial laser pulse may be a near infrared pulse, said prior step comprising: - the conversion into wavelength of a majority fraction of the initial laser pulse towards the mid-infrared by an optical parametric amplifier; and - spectral broadening of a minority fraction of the initial laser pulse by propagation in an optically non-linear medium and its temporal compression by a dispersive delay line.

[0027] The method may comprise a prior step of shaping the temporal intensity and / or polarization profile of the second laser pulse.

[0028] Another object of the invention is an apparatus for generating isolated ultrashort electromagnetic pulses comprising: - a laser system configured to generate a first laser pulse having a first wavelength corresponding to a first optical period and a second laser pulse having a second wavelength, less than the first wavelength, and a duration less than or equal to the first optical period; - an optical system configured to: - directing the first laser pulse, at oblique incidence and having p-type polarization, onto a surface of a target in the condensed state; - directing the second laser pulse onto said target surface, such that it is spatially and temporally superimposed on the first laser pulse; the laser system and the optical system also being configured such that the first laser pulse has, on the surface of the target, a normalized peak amplitude greater than or equal to 1.

[0029] Other characteristics, details and advantages of the invention will emerge from reading the description given with reference to the appended drawings given by way of example and which represent, respectively:

[0030] [Fig. 1], a block diagram of a method according to one embodiment of the invention;

[0031] [Fig.2], the first and second laser pulses and the main parameters characterizing these pulses;

[0032] [Fig.3A], a graph of the second laser pulse positioned relative to a optical cycle of the first laser pulse;

[0033] [Fig.3B], the spectra of the first and second laser pulses;

[0034] [Fig.3C], obtained from a simplified simulation, a graph of the second laser pulse positioned relative to an optical cycle of the first laser pulse after reflection by the plasma mirror;

[0035] [Fig.3D], the spectra of the pulses of [Fig.3C];

[0036] [Fig.4A], obtained from a first principles simulation, a graph of a second laser pulse before and after reflection by a plasma mirror;

[0037] [Fig.4B], the spectra of the pulses of [Fig.4A]; and

[0038] [Fig.5], the basic diagram of an apparatus according to an embodiment of the invention.

[0039] [Fig. 1] and [Fig. 2] illustrate the principle underlying the invention.

[0040] A first laser beam FL1 carrying a generator laser pulse IL1 and a second laser beam FL2 carrying a source pulse IL2 propagate along a common propagation direction DPI forming an angle 0; - strictly less than 90° - with the normal to the surface SC of a solid CEC target (more generally in the condensed state, the use of a liquid target also being possible). The chemical nature of the target is of secondary importance, since it is only intended to generate a non-equilibrium plasma in which the movement of the ions can be ignored as a first approximation and only that of the electrons is really important.

[0041] The generating laser pulse IL1 has a wavelength Xi (and therefore an optical period Ti) which is typically in the infrared range.

[0042] The laser beam FL1 is focused on the target at an intensity sufficient to generate a relativistic plasma mirror. Typically, this is satisfied for a normalized peak amplitude ai of the pulse greater than or equal to 1, the normalized amplitude being defined by

[0043] a _ eEr “] — meü>|C

[0044] where e and me are the charge and mass of the electron, respectively, EL the peak amplitude of the pulse (expressed in V / m), coi the angular frequency of the laser and c the speed of light. For a wavelength Xi=3 pm, ai=l corresponds to an intensity of approximately 1.5 1017 W / cm2.

[0045] The laser pulse IL1 having a p-type polarization, the component of the electric field perpendicular to the surface sets the plasma electrons into oscillation with a period Ti and with a relativistic peak velocity. Provided that the electron density ne of the plasma is such that the plasma frequency _ lnee2 (£o being the permittivity ^P y vacuum electric field) is greater than that - co2 - of the IL2 source pulse, we thus obtain a relativistic plasma mirror MPR on which the said IL2 source pulse is reflected. This condition is satisfied for IL2 source pulses in the near infrared or visible (or even in the near or mid-ultraviolet) as long as the target is in the condensed state (solid or liquid) and the amplitude of the IL1 pulse field is sufficiently high.

[0046] In order for the component of the electric field of IL1 perpendicular to the surface of the target to have sufficient amplitude, the angle of incidence 0; must preferably be greater than or equal to 25°. Furthermore, it is preferable that 0; does not exceed 75° to avoid the focal spot being spread too much on the target, reducing the amplitude of the electric field.

[0047] The source pulse IL2 has a duration r2 (defined at half-height in intensity - FWHM from the English "Full Width at Half Maximum") which is less than the optical period Ti of the generating pulse (which necessarily means that the wavelength X2 and the optical period T2 of the source pulse are less than Xi and Ti respectively). Preferably, the duration r2 is less than or equal to Ti / 2 (and consequently X2 <Xi / 2). Si le retard At entre les deux impulsions (plus précisément, le retard entre les pics de leurs enveloppes) et la phase CEP de la porteuse PORT1 de l’impulsion génératrice IL1 relative à son enveloppe ENVI (en anglais « Carrier -Envelope Phase ») sont choisis de manière opportune, l’impulsion source IL2 « voit » un miroir plasma ayant une composante de vitesse, approximativement constante, dans la direction opposée à sa direction de propagation DPI.

[0048] According to a preferred embodiment of the invention, At and CEP are chosen such that the plasma mirror moves away from the surface (heads towards the vacuum) for the entire duration of the source pulse. Consequently, the IR2 pulse obtained by reflection of the source pulse will undergo a “blue” shift (i.e. towards small wavelengths) and a time compression, and in some cases an amplification by energy transfer from the generator pulse via the plasma mirror. The generator pulse will also be reflected by the MPR plasma mirror; it will undergo a spectral broadening and a deformation of its time profile (in [Fig.l] the reflected generator pulse is designated by IR1). The two reflected pulses IR1 and IR2 propagate along the same DPR propagation direction and are spatially and temporally superimposed; however, their spectra are sufficiently different that it is possible to separate them, by filtering or by exploiting the fact that IR2 diverges more than IR1 due to its longer wavelength (as will be explained later, it is also possible to use a generator pulse and a source pulse with orthogonal polarizations, which also allows separating the IR1 and IR2 pulses by means of a polarizer).

[0049] [Fig.3A] is an idealized plot of the electric field of the source pulse IL2 and 1.5 optical cycles of the pulse IL1. The source pulse IL2 is a pulse of a single optical cycle (t2=T2) and its wavelength X2 is smaller than Xi by a factor of 6. [Fig.3B] is a plot of the spectra SIL2, SIL1 of these two pulses. [Fig.3C] is an idealized plot of the electric field of the reflected pulses IR1 (over a fraction of its duration) and IR2. [Fig.3D] is a plot of the spectra SIR2, SIR1 of these two pulses. The comparison between [Fig.3A] and [Fig.3C] shows the time compression of the source pulse and that between [Fig.3B] and [Fig.3D] the broadening and the "blue" shift of its spectrum. In these figures, the spectra are represented as a function of the angular frequency co normalized to co2, the angular frequency of the source pulse.

[0050] While the previous figures were obtained using a simplified model (perfectly reflecting mirror oscillating at the period of the generating pulse), [Fig.4A] and [Fig.4B] illustrate the results of a first-principles kinetic simulation, of the "particle-in-cell" (PIC) type, of the reflection of a near-infrared laser (X2=lpm) on a relativistic plasma mirror accelerated by a mid-infrared laser (Xi=3 qm). The normalized laser amplitude is ai = 7 for the generating pulse and a2=l for the source pulse. Simulations indicate that the source pulse reflected by the plasma mirror, IR2, is temporally compressed from an initial duration of 3.3fs (i.e. one optical cycle in FWHM intensity) - IL2 curve - to a duration of 80as (FWHM intensity). The intensity of the reflected pulse is more than 60 times that of the incident source pulse and contains 2.6 times more energy than it, this additional energy being transferred from the generating pulse via the motion of the relativistic plasma mirror. The spectrum of the reflected pulse, SIR2, is very spectrally broadened compared to that - SIL2 - of the initial pulse and spreads from a few nanometers (X-rays) to Ipm. It is flat over a good part of the spectral range, giving very high generation efficiencies. About 30% of the initial energy is below 100nm, i.e. in the EUV and X-ray spectral regions.

[0051] [Fig. 5] illustrates an apparatus for generating isolated ultra-short electromagnetic pulses according to an embodiment of the invention. This apparatus consists of mainly of an SL laser system to generate the IL1 generator pulses and IL2 sources and of an SO optical system to direct these pulses onto a CEC condensed state target.

[0052] The laser system SL comprises a laser oscillator OL which delivers so-called initial laser pulses ILI, typically of femtosecond or picosecond duration, generally in the near or mid-infrared. A beam splitter SF separates these pulses into two components. It is advantageous for the laser oscillator to have a stabilized carrier phase relative to the envelope (CEP), which can be obtained by techniques known from the prior art. Indeed, the source pulse is reflected on the plasma mirror during a determined fraction of the optical cycle of the generator pulse. It is therefore very important that the position of the field under the envelope of the generator pulse is identical at each shot. This parameter is given by the CEP of the generator pulse, which in turn depends on that of the initial pulse. Note that the CEP of the source pulse is not, in itself, an important parameter to control.

[0053] A first component of each pulse is converted to longer wavelengths and amplified by an amplifier device such as an optical parametric chirped pulse amplifier (OPCPA) to form a generator pulse ILI; for example (von Grafenstein 2020) describes an OPCPA operating in the mid-infrared and which may be suitable for implementing the invention. A second component, intended to form a source pulse IL2, undergoes spectral broadening by propagation in a non-linear medium MNL followed by optical compression in a dispersive delay line LRD - see for example (Nisoli 1998) - so as to reduce its duration to a few optical cycles, or even ideally to a single optical cycle.The dispersive delay line LRD also allows the delay At between the generator pulse and the source pulse to be adjusted (alternatively, a separate delay line can be used).

[0054] The generation of the generator and source pulses from the same laser oscillator is a preferred feature of the invention, even if it is not essential. Indeed, the delay At between the two pulses must be controlled with a precision of the order of a fraction of T2, that is to say of a few femtoseconds or less. This can be achieved without any particular difficulty using a conventional delay line, but it would be much more difficult to synchronize two separate laser sources with such precision.

[0055] The optical system SO recombines the pulses ILI and IL2 so as to make them collinear and to focus them on or near the target. In its simplest form it can comprise two concave mirrors, M1 and M2, one of which (M2) is pierced in its center.

[0056] In general, the generating pulse will have to be strongly focused - ideally close to the diffraction limit - to allow it to reach relativistic intensities. On the other hand, it is not necessary to strongly focus the source pulse: on the contrary, it is preferable not to do so to avoid it disturbing the dynamics of the plasma mirror, which ideally should be controlled exclusively by the generating pulse. Furthermore, the larger the spot formed by the IL2 beam on the plasma mirror - without exceeding the dimensions of the mirror itself, which is defined by the spot formed by the IL1 beam on the target - the larger the "waist" of the reflected IR2 pulse will be and the more it will be collimated, which will allow its efficient focusing.

[0057] The laser system may be more complex than that shown in [Fig.l]. For example, it may also comprise a device for controlling the polarization of the source pulse, such as a birefringent plate or a series of such plates. Indeed, the polarization of the source pulse has little influence on the conversion efficiency and it is transmitted to the reflected pulse. This makes it possible, for example, to obtain an EUV pulse of arbitrary polarization (e.g. elliptical or circular) by simply changing the polarization of the source pulse, which is very difficult, if not impossible, with the techniques known from the prior art using a single generator beam (and for which the generation efficiency towards EUV / X strongly depends on the polarization of this beam). Furthermore, it may be useful to rotate the polarization plane of the source pulse so that it is, on the target, in “s” polarization.This makes it possible to avoid a component of the electric field of the source pulse normal to the target and thus to minimize the impact of this same pulse on the dynamics of the plasma mirror.

[0058] The laser system may also comprise a system for temporal shaping ("Pulse Shaping" in English) of the source pulse - for example, an acousto-optic modulator - in order to control the temporal shape of the EUV pulse. As with polarization, the temporal shape of the source pulse has no influence on the conversion efficiency and is therefore transmitted to the reflected pulse. The temporal shaping of EUV pulses is very difficult, if not impossible, to obtain with the techniques known from the prior art.

[0059] According to a preferred embodiment of the invention, the laser oscillator OL may be an Ytterbium laser, which delivers pulses of very high energies (from a few hundred mJ to a few J) and of typical durations of the order of a few hundred fs to a few tens of ps with a wavelength of 2.5 pm. Most of this energy, typically about 90%, is directed to an OPCPA - such as that described in (von Grafenstein 2020) cited above - which converts the pulses in the range wavelength of 3 to 100 mJ (mid-infrared). With a conversion rate of up to 6-7%, we can expect to obtain mid-infrared pulses of a few tens to a few hundred mJ and a few optical cycles, with durations typically in the order of tens to a few hundred optical cycles. These pulses are intense enough to be able to generate relativistic plasma mirrors. The other part of the Ytterbium laser energy, typically about 10%, is compressed in a non-linear medium such as a gas cell, a hollow fiber or a thin plate in order to obtain a pulse at 1 to 2 optical cycles of a few mJ to a few hundred mJ synchronized with the mid-infrared pulse. Reflection from the plasma mirror generated by this last pulse converts a large part of the 1% source pulse in the UV / EUV as a single attosecond pulse.

[0060] Several variants of this scheme are possible.

[0061] First of all, the source pulse can be previously doubled in frequency to 500nm (or even tripled, or even quadrupled), then compressed to a few cycles. This makes it possible not to have to generate a pulse of 1-2 optical cycles as at Ipm but of 2-4 optical cycles at 500nm.

[0062] Instead of using a Ytterbium laser, a 2pm Holmium laser could for example be considered. Its main advantage is that pulses in the mid-infrared can be derived from it with a very high conversion rate (greater than 10%). The source pulse can be compressed to 1 - 2 optical cycles while keeping its central wavelength at 2pm or be previously doubled to Ipm.

[0063] According to another embodiment, the laser oscillator OL can be a Ti:Sapphire oscillator emitting at 800 nm. Ti:Sapphire lasers are those which, to date, produce the most powerful light pulses, reaching several Petawatts (PW), with energies of a few hundred J for durations of the order of 10 fs. As in the previous case, an OPCPA can be used to generate a generator pulse in the mid-infrared with an efficiency of the order of a few percent. Thus, an initial pulse of 10 J at 800 nm can for example be converted into a generator pulse of 100 mJ to 100 mJ. At the same time, a smaller part of the energy of the 800 nm (a few tens or hundreds of mJ) can be compressed to a duration of one to a few optical cycles. This makes it possible to obtain EUV pulses of a few tens to a few hundred mJ after reflection on the plasma mirror.

[0064] As before, the compressed pulses can be pre-frequency doubled to 400nm, to increase the wavelength difference between the generator pulse and the source pulse.

[0065] According to another embodiment, the high energy 800 nm pulse (up to a few kJ) generated by a TkSapphire laser can be directly used as a generator pulse. The source pulse can be obtained by frequency conversion to the 3rd or 4th harmonic of the initial pulse at 800 nm (i.e. 266 or 200 nm), spectrally broadened and then compressed.

[0066] In addition to being able to achieve very short DUV / EUV wavelengths, the technique of the invention actually makes it possible to very simply adjust (or "tune") the wavelength of the reflected pulse by varying the amplitude and the sign of the mirror speed at the instant of reflection. These two physical quantities vary during an optical cycle of the generating pulse

[0067] At each optical cycle of the generating pulse, the plasma mirror is first pushed towards the target and then pulled towards the vacuum. If the source pulse is reflected when the mirror is pulled towards the vacuum (negative speed), it will be shifted "towards the blue". If the source pulse is reflected when the mirror is pushed towards the target, it will be shifted "towards the red". Several parameters make it possible to control in practice the amplitude and the sign of the speed of the mirror: - The delay between the generator pulse and the source pulse. This delay controls the instant at which the source pulse is reflected onto the plasma mirror, and therefore the amplitude and sign of the mirror velocity at that instant. - The CEP of the generating pulse. Changing the CEP changes the instant at which the plasma mirror is pulled into the vacuum. This has an effect equivalent to changing the delay. - The amplitude of the generating pulse, which can be increased or decreased by acting on the energy of this pulse or on the size of its focal spot. Increasing the amplitude of the laser field of the generating pulse increases the amplitude of the mirror speed, and therefore the importance of the blue or red shift undergone by the source pulse. - The source and generator pulses may not propagate in a co-linear manner, but their propagation directions may form an angle strictly less than 180° - preferably less than 90° and even more preferably not greater than 10° - to spatially separate the pulses after interaction.

[0068] So far, only the case where the source pulse is reflected while the plasma mirror moves towards the vacuum (away from the target), and therefore undergoes a blue shift and a time compression, has been considered. The invention is however not limited to this case. It is for example possible to adjust the delay Δt and / or the CEP so that the source pulse is reflected while the plasma mirror moves towards the target. This results in a red shift of the reflected pulse, up to the THz domain. It will in particular be possible to obtain THz pulses with durations shorter than the optical cycle. Several methods for generating such THz pulses exist, but the invention makes it possible to achieve unparalleled energies.

[0069] Furthermore, it is also possible to ensure that the rising edge of the source pulse is reflected while the plasma mirror is moving away from the target and its falling edge when it is approaching it, or vice versa. Under these conditions, the source pulse undergoes both a blue shift and a red shift. This results in a reflected pulse having an extremely broad spectrum, which can be spread over a large number of octaves (for example from 20 nm to 20 pm, or 10 octaves). Such pulses cannot be generated by any technique known from the prior art.

[0070] Bibliographic references:

[0071] (Vincenti 2019): H. Vincenti “Achieving Extreme Light Intensities using Optically Curved Relativistic Plasma Mirrors” Physical Review Letters 123, 105001 (2019).

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[0073] (Tamburini 2014): M. Tamburini, A. Di Piazza, T. V. Liseykina, and C. H. Keitel, « Plasma-Based Génération and Control of a Single Few-Cycle High-Energy Ultrahigh-Intensity Laser Puise » Phys. Rev. Lett., vol. 113, no. 2, p. 025005, Jul. 2014.

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Claims

Claims

1. Method for generating isolated ultrashort electromagnetic pulses, of duration less than 100 ps, ​​(IR2) comprising the following steps: - directing onto a surface (SC) of a target in the condensed state (CEC) a first laser pulse (IL1) at oblique incidence and in p polarization, said first laser pulse having a first wavelength (Xi), corresponding to a first optical period (Tl), and a normalized peak amplitude greater than or equal to 1, whereby a plasma mirror oscillating at relativistic speed (MPR) is generated on the surface of the target; and - directing onto said surface of the target, in correspondence with said plasma mirror, a second laser pulse (IL2) having a second wavelength, less than the first wavelength, and a duration (t2) less than or equal to the first optical period (Tl), the second laser pulse being spatially and temporally superimposed on the first laser pulse;whereby the second laser pulse, reflected by the plasma mirror, undergoes a shift in its wavelength and a modification in its duration.;

2. Method according to claim 1 in which the duration (r2) of the second laser pulse (IL2) is less than or equal to half of the first optical period, the time shift between the first and the second laser pulse was chosen such that, during the entire duration during which the second laser pulse is incident on the plasma mirror (MPR), the latter moves away from the surface of the target; whereby the second laser pulse (IL2) undergoes a blue shift of its wavelength and a time compression by Doppler effect.

3. Method according to one of the preceding claims in which the second laser pulse (IL2) propagates in a direction (DPI) collinear with a direction of propagation of the first laser pulse.

4. Method according to one of the preceding claims in which the first laser pulse (IL1) is incident on the surface of the target with an angle of incidence (0i) between 25° and 75°.

5. Method according to one of the preceding claims in which the second laser pulse (IL2) has, on the surface of the target, a normalized peak amplitude lower than that of the first laser pulse (ILI).

6. Method according to one of the preceding claims in which the second laser pulse (IL2) is incident on the surface of the target in s polarization.

7. Method according to one of the preceding claims comprising a step of stabilizing the phase (CEP) of the carrier of the first laser pulse (IL1) relative to its envelope.

8. Method according to one of the preceding claims comprising a step of controlling the shift of the wavelength of the second laser pulse reflected by the plasma mirror by adjusting at least one parameter chosen from: a phase (CEP) of the carrier of the first laser pulse relative to its envelope; a delay (At) between the first and the second laser pulse; a peak amplitude of the first laser pulse.

9. Method according to one of the preceding claims comprising a prior step of generating the first and second laser pulses from the same so-called initial laser pulse (ILI).

10. Method according to claim 9 wherein said initial laser pulse (ILI) is a pulse in the near infrared, said preliminary step comprising: - the conversion into wavelength of a majority fraction of the initial laser pulse towards the mid-infrared by an optical parametric amplifier (OPCPA); and - the spectral broadening of a minority fraction of the initial laser pulse by propagation in an optically non-linear medium (MNL) and its temporal compression by a dispersive delay line (LRD).

11. Method according to one of the preceding claims comprising a prior step of shaping the temporal intensity and / or polarization profile of the second laser pulse.

12. Apparatus for generating isolated ultrashort electromagnetic pulses, of duration less than 100 ps, ​​comprising: - a laser system (SL) configured to generate a first laser pulse (ILI) having a first wavelength (Xi) corresponding to a first optical period (Tl) and a second laser pulse (IL2) having a second wavelength, less than the first wavelength, and a duration less than or equal to the first optical period; - an optical system configured to: - directing onto a surface (SC) of a target in the condensed state (CEC) the first laser pulse (IL1), at oblique incidence and having a p-type polarization; - directing the second laser pulse (IL2) onto said target surface, such that it is spatially and temporally superimposed on the first laser pulse; the laser system and the optical system also being configured such that the first laser pulse has, on the surface of the target, a normalized peak amplitude greater than or equal to 1.

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