Abstract:
Phase matching high harmonic generation (HHG) uses a single, long duration non-collinear modulating pulse intersecting the driving pulse. A femtosecond driving pulse is focused into an HHG medium (such as a noble gas) to cause high-harmonic generation (HHG), for example in the X-ray region of the spectrum, via electrons separating from and recombining with gas atoms. A non-collinear pulse intersects the driving pulse within the gas, and modulates the field seen by the electrons while separated from their atoms. The modulating pulse is low power and long duration, and its frequency and amplitude is chosen to improve HHG phase matching by increasing the areas of constructive interference between the driving pulse and the HHG, relative to the areas of destructive interference.

Description:
PRIORITY 
       [0001]    This application claims benefit of U.S. Provisional Patent Application No. 60/875,175, filed Dec. 15, 2006. 
         [0002]    U.S. patent application Ser. No. 11/888,916 is incorporated herein by reference. 
     
    
     GOVERNMENT SUPPORT 
       [0003]    The present invention was made with the support of the U.S. government, which may have certain rights in this invention. Funding was provided by two sources: 
       U.S. Department of Energy grant # DE-FG52-06NA26151, and The National Science Foundation grant # EEC0310717. 
    
    
     BACKGROUND OF THE INVENTION 
       [0004]    1. Field of the Invention 
         [0005]    The present invention relates to phase matching in high harmonic generation (HHG) using a non-collinear pulse to modulate the field seen by the driving pulse. In particular, the present invention relates to such phase matching using a weak, long duration non-collinear modulation pulse intersecting the driving pulse. 
         [0006]    2. Description of the Prior Art 
         [0007]    High-order harmonic generation (HHG) driven by ultrashort laser pulses is a source of extreme-ultraviolet and soft X-ray light with the unique properties of ultrashort pulse duration and high spatial and temporal coherence. This source has made possible new ultrafast spectroscopic probes of atoms, molecules and materials. So far, however, most applications have used relatively long wavelengths, because the conversion rapidly decreases at shorter wavelengths. This decrease is not due primarily to the very high-order nonlinearity of the process—in fact, the atomic physics of HHG is non-perturbative, and has favorable scaling to shorter wavelengths. The major challenge is that, unlike low-order nonlinear processes such as second-harmonic generation, HHG is inherently associated with ionization of the nonlinear medium. In HHG, an electron is first ionized by the field of an intense femtosecond laser. Once free, the electron begins to oscillate in response to the laser field. A small fraction of the ionized electron can re-collide with its parent ion, recombining and liberating the excess energy as a short-wavelength photon. 
         [0008]    As in all nonlinear parametric processes in nature, efficient conversion of light from one frequency to another using nonlinear optics requires that the process be phase-matched. As the pump beam propagates, the nonlinear response of the medium coherently adds to the harmonic signal. The generated field continues to add constructively if the two waves travel with the same phase velocity through the medium, leading to a bright, phase-matched beam at the new wavelength. If the process is not phase-matched, coherent build-up is limited to a propagation distance over which the relative phase of the fundamental and the harmonic light slip by 180°. This distance is the coherence length L c =π/Δk, where Δk is the phase mismatch between the polarization wave and the harmonic wave. For HHG, dispersion of the free-electron plasma reduces L c  to the micrometer or even sub-micrometer range for up-conversion to very short wavelengths, which are only generated when the laser is very intense and thus the medium is already highly ionized. As a result, efficient harmonic generation is possible only at relatively low levels of ionization, below a ‘critical’ ionization level of around 5% for Argon or around 0.5% for helium, corresponding to photon energies of around 50 eV and around 130 eV respectively. Thus, new methods that can correct for this phase mismatch in ionized media (plasmas) are a ‘grand challenge’ in this area of laser science. 
         [0009]    In the absence of phase-matching, quasi-phase matching (QPM) techniques have been successfully demonstrated to compensate for this phase slip by periodically readjusting the relative phase of the fundamental and nonlinear response at an interval corresponding to the coherence length. In the visible region, this is achieved by periodically reversing the polarization of a non-centrosymmetric nonlinear-optical material. However, this implementation cannot be used for HHG, because HHG uses a low-pressure gas as the nonlinear medium. 
         [0010]    Past experimental work used a periodically modulated hollow waveguide to modulate the intensity of the driving laser to implement QPM for high-harmonic generation. U.S. Pat. No. 6,151,115, incorporated herein by reference, is a useful background reference. Even a small modulation (around 1%) of the driving laser results in significant modulation in both the amplitude and phase of the harmonics. Although this past work succeeded in enhancing conversion efficiency into the soft X-ray region of the spectrum by about one order of magnitude, further optimization will require a more sophisticated approach. This is because optical loss of the driving laser, refraction, mode beating and group-velocity dispersion all result in a continuous variation of the coherence length along the direction of propagation, making it difficult to optimize the modulation period. Finally, modulation periods shorter than the waveguide diameter will not significantly influence the laser field, making it challenging to compensate for very short coherence lengths. 
         [0011]    Recently, Voronov et al. demonstrated that a weak counterpropagating pulse can be used to disrupt high-harmonic emission, with the objective of using this technique to implement QPM. This experiment used a simple focused-beam geometry in a low-pressure gas. The counterpropagating field induced both a standing amplitude and phase modulation on the driving laser field. Even though the counterpropagating field was weak, it distorted the field of the driving laser, essentially turning off phase-coherent high-harmonic production in the region where the two pulses overlapped. That work also demonstrated that if the HHG signal is deliberately suppressed by a non-optimum focusing geometry, a single counterpropagating pulse can recover much of the original harmonic signal that had previously been obtained in the optimum-focus geometry. However, this work only investigated harmonic emission in regimes where conventional phase-matching was already possible in the medium. Attempts to obtain enhancements significantly greater than what could otherwise be obtained were not successful. 
         [0012]    A pending U.S. patent application having some co-inventors with the present application teaches a technique for quasi-phase matching and quantum control of high harmonic generation in waveguides using a train of counterpropagating pulses. The counterpropagating pulse technique presents an advantage over previous QPM techniques in that varying the format of the counterpropagating light pulse allows for dynamic optimization of quasi-phase matching, and also because when short counterpropagating light pulses are used, shorter coherence lengths can be compensated for compared with using a structured waveguide. In this QPM technique, the counterpropagating pulses intersect with the driving pulse and suppress the HHG emission from out-of-phase regions. However, our calculations showed (OL 32, 2975) that the QPM efficiency factor in this technique is smaller than 0.15 (the QPM efficiency factor is the ratio between the generated signal under QPM and the generated signal under perfect phase matching condition). Thus, a need remains to devise a QPM method for HHG with larger QPM efficiency factor. In addition, the counterpropagating pulses QPM technique is limited to the case where the coherence length is larger than ˜10-100 microns. At keV energies, however, the coherence length is typically in the micron range. 
         [0013]    A need remains in the art for a method of phase matching high harmonic generation (HHG) using one or more non-collinear modulating pulses intersecting the driving pulse that can be implemented using light pulses with duration &gt;L c , and that allows for a maximum QPM efficiency factor. 
       SUMMARY OF THE INVENTION 
       [0014]    An object of the present invention is to provide a method of phase matching high harmonic generation (HHG) using long duration non-collinear modulating pulses that intersect with the driving pulse. 
         [0015]    The HHG phase matching method of comprises the steps of: providing a volume filled with a gas, for example a noble gas such as argon, focusing a femtosecond (note that “femtosecond pulse” is defined herein to include pulses from a fraction of a femtosecond to a plurality of femtoseconds, i.e. what is termed in the field an “ultrashort light pulse”) driving pulse into the gas to cause high-harmonic generation (HHG), for example in the X-ray region of the spectrum, via electrons separating from and recombining with gas atoms, and providing a long duration non-collinear pulse to intersect the driving pulse and modulate the field seen by the electrons while separated from their atoms. The modulating pulse is low power and long duration (many times longer than the coherence length of the conversion process absent the modulating pulse), and its frequency and amplitude is chosen to improve HHG phase matching by increasing the areas of constructive interference between the driving pulse and the HHG, relative to the areas of destructive interference. 
         [0016]    The volume may comprise a hollow cylindrical waveguide, a planar waveguide, or a free-space configuration (albeit enclosed to contain the HHG medium). It is often convenient to use a counterpropagating modulating pulse to provide sufficient interaction between the driving and modulating pulses. 
         [0017]    While the counterpropagating pulse is a long duration pulse (for example extending the length of the waveguide), it is advantageous to modulate the amplitude and wavelength of the pulse as it progresses (for example via pulse shaping). The intensity of the modulating pulse can be quite small—for example less than 10 10  W/cm 2 , or 10 −6  of the intensity of the driving pulse. Even though the modulating pulse is very long, its energy is still as little as 1/10 of that of the driving pulse, while resulting in many orders of magnitude increase in the x-ray yield. More than one modulating pulse may be used to fine-tune the response, but a single pulse of sufficient duration produces dramatic improvements. 
         [0018]    The driving pulse is intense and short—preferably femtoseconds in duration. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  (Prior Art) is a schematic diagram illustrating high-harmonic emission generation (HHG) in without phase matching. 
           [0020]      FIG. 2  is a schematic diagram illustrating how the coherence length within the HHG medium is modified by modifying the field seen by the driving pulse. 
           [0021]      FIG. 3  is a diagram illustrating HHG with phase matching accomplished by a modulating pulse. 
           [0022]      FIG. 4  is a schematic diagram illustrating how the combination of the medium dispersion and the continuously-varying shift in the phase of the emitted harmonics, which is induced by the modulating long pulse, results in modification of the effective coherence length within the nonlinear medium increasing the total conversion efficiency of light into higher harmonics. 
           [0023]      FIG. 5  is a block diagram illustrating a preferred embodiment of apparatus for accomplishing HHG with phase matching accomplished by a modulating long pulse. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0024]      FIG. 1  (Prior Art) is a schematic diagram illustrating high-harmonic emission generation (HHG) without phase matching. Driving pulse  102  comprises a femtosecond laser pulse, which enters the HHG medium  120 . Diagram  104  indicates the regions within the HHG medium in which constructive and destructive interference of HHG emissions will occur, due to the short coherence length of the HHG beam. The areas indicated by a plus (+) are areas of constructive interference, while the areas indicated by a minus (−) are areas of destructive interference. The length of one positively interfering area (+) in the absence of a modulating pulse is one coherence length. 
         [0025]    Briefly, an ultrashort light pulse  102  is focused into a medium  120  (for example a noble gas) to generate high-order harmonics  106  in the vacuum-ultraviolet to x-ray regions of the spectrum (generally termed “x-ray HHG” herein). However, particularly for conversion to very short wavelengths, the high-harmonic process is not well phase-matched, for a variety of reasons. The most significant is usually the presence of a plasma, generated either through pre-ionization of the medium or through the intense laser-matter interaction itself, that affects the speed of propagation of the driving laser pulse  102 . The “polarization” of the medium follows the propagation of the driving laser pulse, while the generated signal  106  travels at the (different) speed of light of the signal in the medium. 
         [0026]    Note that the curve labeled  106  in  FIG. 1 , as well as the curve labeled  306  in  FIG. 3  graph the amplitude of the respective HHG signal as it propagates. Thus, the x-axis for these curves corresponds to propagation distance through the medium, while the y-axis corresponds to the amplitude of the signal. 
         [0027]    Hence, as indicated by output HHG signal  106 , at first the HHG signal increases, but as the coherence length is reached destructive interference causes the signal to decrease. This process is repeated with a period of two coherence lengths, with the HHG beam being amplified in portions  108  of the waveform and the HHG beam being attenuated in portions  110  of the waveform. Hence, output HHG beam  106  will contain little HHG signal. The largest HHG signal would be obtained if the waveguide length corresponded to an odd number multiple of coherence lengths, but the HHG signal never gets very large. 
         [0028]      FIGS. 2-4  are schematic diagrams illustrating the operation of the present invention.  FIG. 2  illustrates the changes in coherence length within the medium that are caused by the addition of a modulating pulse.  FIG. 3  shows the x-ray HHG output signal resulting from this change in coherence length.  FIG. 4  illustrates the process by which the combination of the medium dispersion and the continuously-varying shift in the phase of the emitted harmonics, which is induced by the modulating long pulse, results in modification of the effective coherence length within the nonlinear medium resulting in an increase in the total conversion efficiency of light into higher harmonics. 
         [0029]    In HHG, the emitted high-order harmonic light in general exhibits a phase shift relative to the driving laser  102 . This phase shift is a result of the re-scattering mechanism that results in high-harmonic emission. An electron ionized from an atom or molecule by the driving laser field begins to oscillate as a free electron under the influence of the laser field. But then it can re-encounter the atom or molecule from where it originated, and this collision results in emission of high harmonic light. The phase of the electron quantum wave function acquired by the electron along its femtosecond “boomerang” path under the influence of the laser field can be very large, reaching hundreds of radians. It is also related to the intensity of the laser. Thus, inducing a shallow sinusoidal (or other oscillatory waveform) modulation in the laser intensity along the propagation direction leads to sinusoidally-modulated phase-shift in the high harmonic generation at any particular wavelength of emission. A convenient way to induce such a sinusoidal modulation is by interfering the driving laser pulse  102  with a weak and long modulating pulse  302  that propagates in a different (non-collinear) direction. This modulating pulse  302  can also have a different wavelength from the driving laser pulse  102 . The periodicity of the modulation is determined by the periodicity of the interference intensity grating between the driving laser and the modulating pulse, and therefore can be controlled, for example, by changing the wavelength or propagation direction of the modulated pulse. The spatial period (measured along the propagation direction of the driving laser) of this interference intensity grating should ideally correspond to an integer multiple of twice the coherence length of the high-harmonic process in the absence of the modulating pulse. The amplitude of the phase-shift modulation is determined by the amplitude of the intensity grating and therefore can be controlled by the intensity of the modulating beam. For example, the intensity of the modulated pulse is tuned such that the absolute value of mth-order Bessel function of the amplitude of the induced phase shift by the modulated pulse is largest. 
         [0030]    In  FIGS. 2 and 4 , diagram  104  indicates the original coherence lengths from  FIG. 1 . Diagram  204  indicates the coherence lengths when a modulating pulse  302  affects the field seen by the driving pulse  102  according to the present invention. As diagram  204  shows, the areas of constructive interference are increased and the areas of destructive interference are decreased when the modulating pulse is used.  FIG. 3  shows how the output HHG signal  306  is amplified because of the increased coherence lengths. 
         [0031]    As indicated on the left side of  FIG. 4 , without the modulating pulse  302 , the medium dispersion leads to linear growth in the phase shift  202  between the driving laser and the harmonic signal  106  which results in equal in-phase (+) and out-of-phase (−) coherent zones as shown in diagram  104 , and thus a periodic increase and decrease of the HHG signal  106 . The modulated pulse  302  induces a sinusoidal phase shift  203  in the phase of the emitted harmonics. The combination of the linear and sinusoidal phase shifts results in a stair-step type phase shift,  206 , which leads to an increase of the effective length of the in-phase coherent zones and a decrease in the length of the effective out-of-phase coherent zones as shown in diagram  204 , allowing the growth of the HHG signal  306 . Calculations show that a single, long duration (many coherence lengths) modulating pulse can give rise to a phase matching efficiency factor of 0.3—significantly larger than is possible if the modulating pulse is used primarily to shut-off effective harmonic generation, as in Quasi Phase Matching. 
         [0032]    Note that the curves labeled  202 ,  203 , and  206  in  FIG. 4  graph the phase changes applied to the HHG signal as it propagates. Thus, the y-axis for these curves corresponds to phase shift. 
         [0033]    Expanding on this concept, complex phase shift structures can be induced by using multiple modulated pulses of varying frequencies, pulse shapes, and polarizations. It is possible to consider each modulated pulse as contributing a single Fourier component to the phase structure. That is, the interference between each modulated pulse and the driving laser leads to a sinusoidal phase-shift in the phase of the emitted harmonics. Thus, a complex phase structure in the HHG process will be induced by interfering multiple modulated pulses with the driving pulse, with each wave inducing a single Fourier component of the structure. Using multiple modulated pulses, 1D, 2D and even 3D lattices, quasi-lattices, random lattices, or other complex structures can be induced. Such structures may be used for enhanced phase matching or other spatio-temporal manipulation of coherent x-ray beams. 
         [0034]    The periodic modulation of phase is used to correct for the periodic phase slip between the polarization of the medium created by the driving laser (that generates the harmonics), and the propagation of the harmonic radiation itself. 
         [0035]    The profile of modulating pulse  302  (i.e. pulse shape) might be varied over the duration of the pulse to control the amount of phase shift of the polarization so that it optimizes phase matching at every point along the propagation of the driving pulse. The intensity of the modulating pulse affects the magnitude of the phase shift, while the phase or wavelength of modulating pulse  302  can be varied over its duration to optimize phase matching conditions. This can be done either by careful calculation, or it can be done adaptively, by varying the pulse shape and parameters in real time and optimizing the output. 
         [0036]    The intensity of modulating pulse  302  can be very low (i.e. even as low as 10 −6  or less of the driving pulse intensity). This means that, even though the modulating pulse must be long in duration, so that the driving and modulating pulses intersect over an extended propagation distance in the medium to phase-match over this length, the total energy required for modulating pulse  302  is quite moderate. 
         [0037]    Driving pulse  102  is intense and ultrashort—preferably ˜femtoseconds in duration. As an example, the driving pulse could be a 20 fs beam at about 5.5×10 15  W/cm 2 , with a wavelength of 0.8 μm, about a 1-10 kHz repetition rate, and optionally chirped or shaped in time. In this example, the modulating beam is counter propagating and the interaction with the driving pulse occurs in a preformed cylindrical waveguide, 5 to 10 cm long, filled with doubly ionized Ne ions at 70 Torr pressure. The modulating pulse is on the order 10 cm long (around the length of the waveguide) with a wavelength of 1.6 mm, and intensity of 2.35×10 9  W/cm 2 . 
         [0038]      FIG. 5  is a block diagram illustrating a preferred embodiment  400  of apparatus for accomplishing HHG with phase matching using a modulating pulse according to the present invention. In this embodiment, both the driving pulse  102  and the modulating pulse  302  are derived from pulses from the same laser  402 , although this is not necessary. HHG is accomplished in a waveguide  408  containing HHG medium  120 . 
         [0039]    Laser output  404  is divided into beams  406  and  412  by beam splitter  422 . Beam  406  is reflected toward waveguide  408  by mirror  424  and forms driving pulse  102 . A compressor (not shown) would likely be used in this path to shorten pulse  102 . Beam  412  is frequency converted in block  416 , and pulse shaped in block  418 . It is reflected from mirror  426  to form modulating beam  302 . Mirror  426  has a hole allowing HHG beam  306  to pass through to where it is used. 
         [0040]    The intense ultrashort driving pulse  102  incident from the left originates from an ultrashort pulse laser system  402 . This beam generates coherent x-rays as it interacts with the gas or plasma  120 , so that the x-rays also propagate from left to right along with the driving pulse  102 . Modulating pulse  302 , incident from the right, is also generated by a laser, but this might be a totally separate laser, or a separate “beamline” from the same ultrafast laser (as shown in this diagram). If a separate laser is used, frequency conversion  416  may not be needed. The two pulses must be synchronized to the extent that they “collide” within medium  120 . The function of modulating beam  302  is to slightly perturb the coherent x-ray generation process that is driven by the driving pulse  102 . The result of this slight perturbation is that the efficiency of the conversion process is increased by orders of magnitude. 
         [0041]    This particular geometry at first looks like a difficult one to make work. The two beams  102 ,  302  are traveling in opposite directions, and the frequency conversion process driven by driving pulse  102  occurs as the beam propagates through the medium  120 . Thus, if the modulating pulse  302  is to make a difference, it must be very long in duration since the two pulses must interact continuously as driving pulse  102  propagates through the medium. Fortunately, weak modulating pulse  302  is extremely efficient at perturbing the generation process. This is a direct result of the physics of the x-ray upconversion process. The result is that modulating pulse  302 , although it must be relatively long in duration so that it overlaps with driving pulse  102  over an extended range, does not actually need all that much energy in it. A counterpropagating pulse with as little as 1/10 the energy of the driving laser pulse can result many orders of magnitude increase in the x-ray HHG yield  306 . 
         [0042]    Communication between intense driving pulse  102  and the modulating non-collinear weak pulse  302  is a direct result of the physics of the x-ray upconversion process. Coherent x-rays are generated when an atom (or ion or molecule) is illuminated by an intense laser, and the electromagnetic field of the laser gets strong enough to rip an electron out of the atom. Since the electromagnetic field of light is an oscillating wave, the liberated electron will respond by oscillating as well. Sometimes this electron can be driven back into the atom. When the electron slams into the atom, it can emit a high-energy photon. 
         [0043]    This process is in some sense similar to what occurs in an x-ray tube in a doctor&#39;s office, and different in one critical way. In an x-ray tube, electrons are accelerated into a target, and when these electrons hit atoms in the target, the result is x-ray emission. In the case of a conventional medical x-ray tube, however, each one of these collision events is completely random, and the result is that the x-rays are emitted in random directions. In the case of the present invention, each atom is acting in response to the same intense laser, and the result is that all the atoms respond in the same way and radiate in unison with each other. The result is that the emission is much more directional. This process would result in a perfect, directional beam of x-rays if only the laser light and the x-rays traveled at exactly the same speed. Both x-rays and laser light travel at approximately the speed of light, but the index of refraction, which characterizes the speed of light, is slightly different at the (relatively long) laser wavelength and the (relatively short) x-ray wavelength. 
         [0044]    In the scheme of the present invention, modulating pulse  302  traveling in the opposite direction of the HHG propagation just slightly perturbs the trajectory that the electron takes. This results in a modulation of the exact phase of the emission of the x-rays, and this modulation can be used to correct for the slightly different index of refraction of the two colors (of the driving pulse and HHG). 
         [0045]    While the exemplary preferred embodiments of the present invention are described herein with particularity, those skilled in the art will appreciate various changes, additions, and applications other than those specifically mentioned, which are within the spirit of this invention.