Abstract:
A special “standing-laser-poling” method for volumetric domain inversion of nonlinear ferroelectric media, such as LiNbO 3 , is provided. Using the combination of a short-wavelength, high-field laser standing wave pattern and a back ground electric field, a short-period bulk domain inversion pattern can be naturally engraved within the nonlinear media.

Description:
FIELD OF THE INVENTION  
         [0001]    The invention relates to the general field of achieving short-period volumetric domain inversion of nonlinear media to facilitate frequency conversion processes, with particular reference to the field of second harmonic generation of laser lights.  
         BACKGROUND OF THE INVENTION  
         [0002]    Nonlinear optics is concerned with the optical properties of matter in intense radiation fields, such as those produced by a laser or a coherent source of EM wave. The optical nonlinearity of a material results from an anharmonic (and usually anisotropic as well) restoring force when an electron is perturbed by an electric field (or electromagnetic field). For example, in lithium niobate (LiNbO 3 ), the restoring force is stronger for perturbations along the direction of the inbuilt electric field than for perturbations opposed to the inbuilt field. Unlike the situation in linear optical materials at low light intensities, the electromagnetic polarization induced by nonlinear optical materials responds nonlinearly to the electric field of the light. This in turn can give rise to a variety of optical phenomena that can be used to manipulate light, e.g., optical harmonic generation, Raman scattering, parametric amplification, and intensity-dependent refractive indices (see, Neil Broderick&#39;s article: “November 2002: Lithium niobate,” Nature Magazine).  
           [0003]    Ferroelectric materials, to which lithium niobate belongs, have spontaneous polarization (i.e., inbuilt electric field). That is, these materials have internal electric dipole moments. The direction of these moments can be controlled to form certain desired domain configurations within the ferroelectric media, such as the aforementioned lithium niobate. In this connection, much effort and research have been involved in developing structures having particular domain patterns for optic frequency conversions, in particular the second harmonic generation (SHG) The most popular approach to this end has been the so-called quasi-phase-matching (QPM). It is a technique for phase matching nonlinear optical interactions in which the relative phase between the optic pump wave and the generated second harmonic wave is corrected at regular intervals using a structural periodicity built into the nonlinear media (see FIG. 1A and FIG. 1B). A comprehensive reference is made, for example, to the journal article by Dr. Martin M. Fejer et al.: Quasi-Phase-Matched Second Harmonic Generation: Tuning and Tolerances, IEEE Journal of Quantum Electronics, Vol. 28, No. 11, 1992.  
           [0004]    [0004]FIG. 1A and FIG. 1B show the effect of phase matching on the growth of second harmonic intensity with the propagating distance in a nonlinear crystal. In FIG. 1A, curve A corresponds to the theoretically perfect phase matching at every point along the light wave propagation direction. Curve C represents the situation of non-phase-matched interaction. Curve B 1  gives the desired first-order QPM by flipping the sign of the spontaneous polarization (Ps) every coherent length (l c ) of the interaction curve C. Here the coherent length means the distance over which the phases of the original optical radiation and the generated double-frequency optic radiation slip by 180 degrees and the direction of energy flow reverses (to be further elaborated at equation (1) in the following). Note that when l c  is very small, curve B 1  is approaching the ideal curve A. Additionally, in FIG. 1B, with curve A still representing perfect phase matching, curve B 3  reflects the less favorable, low-efficiency conversion situation of third-order QPM by flipping Ps every three coherent lengths.  
           [0005]    Much of the interest in second harmonic generation (SHG) is due to the increasing possibility that frequency conversion via domain periodic patterning will provide reliable, inexpensive, and compact sources of desired radiation having adequate energy for its purposes. In particular, much of the current attention is devoted to generating blue optical radiation, of wavelength in the range of about 400-450 nm from a near-IR pump laser, for the realization of next generation (15 GB) DVD (digital video disc) data pickup heads and projection TVs. However, these applications require short-pitched (i.e., short domain inversion period) frequency converters.  
           [0006]    Existing domain patterning methods can roughly be divided into two categories, namely, the shallow modulation and the deep (i.e., volumetric) domain inversion. The former can be accomplished by various ways. They are, for example, location-selective electron beam scanning (see [Fujimura M. et al., 1992]), ion beam scanning (see, [Mizuuchi K. and Yamamoto K., 1993]), focused laser beam scanning (see [Daneshvar K. and Kang D. H., 2000]), proton exchange of various details on the nonlinear crystal surface (see [Bortz M. L. et al. 1994], [Yamamoto K. and Mizuuchi K., 1992], U.S. Pat. No. 5,943,465 to Kawaguchi et al. (1999)). Although these shallow approaches can access the much-anticipated short-period (2-4 microns) domain patterning, the resultant active QPM regions are usually of less than 2 microns depth. This makes aligning the active regions with a single-mode optic fiber of normal core diameter of 8-10 microns very hard, and usually necessitates the construction of a waveguide. What&#39;s more, due to the resultant non-perpendicular domain walls, the efficiency of such second harmonic generation is always lower than that of the case with a volumetric domain inversion. This invention is in the more favorable category of volumetric domain inversion.  
           [0007]    One popular approach to the volumetric domain patterning of a ferroelectric material for quasi-phase-matching (QPM) is by applying an electric field to that material to change the direction of spontaneous polarization at desired locations. This is commonly referred to as the electric field poling or electro-poling (see FIG. 2, wherein a top metal pattern  22  and a bottom metal sheet  21  on the ferroelectric material  23  are biased by the voltage source  20 , leading to domain inverted region  12  and non-inverted region  13 ). There is a long list of prior art related to variations of this method (e.g., U.S. Pat. No. 5,714,198 to Byer et al. (1998), U.S. Pat. No. 5,615,041 to Field et al. (1997)). In this connection, ferroelectric materials to be electro-poled are often sold in bulk form having spontaneous polarization in a single direction, e.g., the dominant spontaneous polarization extends throughout the material from one face to the opposite one. To achieve efficient quasi-phase-matching, namely the first order QPM-SHG (see FIG. 1A and FIG. 1B), adjacent domains are made to be of reversed directions of polarization. This has been routinely accomplished by the large, pulsed electric field in the range of 20-26 kV/mm (e.g., for LiNbO 3 ), with the width of each domain being about equal to one “coherent length” or period l c , of the desired nonlinear wave interaction within. Here the coherent length means the distance over which the phases of the original optical radiation and the generated double-frequency optic radiation slip by 180 degrees. That is,  
           Δ k·l   c =π  (1)  
           [0008]    where Δk is the difference of wave numbers (k=2π/λ, λ is wave length) between the pump laser and its radiated second harmonic wave within the patterned nonlinear media, and is often called the “mismatch”. With access to smaller period l c , more mismatch is allowed in generating second harmonic light. Hence, it is highly desirable to have short inversion period l c  in order to have wide 1 st -order QPM-SHG operating window and thus high frequency conversion efficiency.  
           [0009]    Two major problems with electric field poling are that it is difficult to provide short period (i.e., small l c ) domain inversion patterning and high-resolution domain wall between adjacent domains. The reasons are found to be electric field diffusion in the ferroelectric materials and the hardly avoidable fringe field (see [Kintaka K. et al., 1996]). This is, when a large electric field is applied between two contact electrodes (say, in the vertical direction, see FIG. 2) across a ferroelectric crystal (say, placed horizontally), the electric field distribution tends to broaden (or diffuse) horizontally within the crystal. When one broadened electric field distribution gets too close to another adjacent one, electric arc will occur. This practically limits the formation of volumetric domain inversion to a minimum period of about 6 microns on a piece of, e.g., LiNbO 3  crystal of about 500 microns thickness. As indicated by equation (1), this large period means hard QPM-SHG frequency conversion. In particular, other sources of mismatch arising from electro-poling fabrication error or changes of fundamental wavelength and temperature, etc. can add more detuning effects which further reduce the QPM bandwidth and the SHG conversion efficiency (see [Fejer M. M. et al, 1992] and [Wu J. et al., 1995]).  
           [0010]    It is for this reason, Karlsson et al. (U.S. Pat. No. 5,986,798) teaches a doping scheme to increase the resistivity of crystals in an attempt to suppress the electric field diffusion difficulty. However, all existing efforts have not brought satisfactory results. In fact, this is why, up to the present day, we hardly see less-than-6-micron-pitch bulk SHG crystals.  
           [0011]    From the application point of view, this poses a severe limit to a lot of frequency converting schemes. For example, 1 st -order SHG blue light practically cannot be generated from a bulk lithium niobate of large inversion pitch, and thus people are only left with the less desirable 3 rd -order options.  
         SUMMARY OF THE INVENTION  
         [0012]    It is an object of the invention to provide a method to make short period and efficient frequency converting nonlinear media, including those for frequency doubling and those further as photonic crystals.  
           [0013]    Another object of the invention is to provide frequency conversion media for very uniform domain inversion periods and deep active regions and sharp domain walls.  
           [0014]    Yet another object of the invention is to provide patterned nonlinear media of suppressed detuning effects, such as in the second harmonic generation.  
           [0015]    The invented method uses short-wavelength (e.g., 0.2-4 microns), pulsed high field laser standing wave patterns to realize short-period volumetric domain inversion in the nonlinear media, such as lithium niobate (LiNbO 3 ). It is this short laser wavelength that will automatically force the domain inversion period within the nonlinear media to be about half the laser wavelength. Note, however, that although the nodes of a standing wave do not change their positions in either time or space, the standing wave amplitude and direction between them do change in time as cos (ωt), where ω(=2πf, f is frequency) is the high-field laser angular frequency. Thus, in order to periodically establish domain inversion within the nonlinear media (e.g., LiNbO 3 ) using standing laser wave pattern, an extra background field E back  is needed. For example, a uniform DC electric field can be applied. That is, suppose the threshold electric field to cause domain inversion in that nonlinear media is E th , then to realize the invented method, it is necessary to make both E back  and the peak standing wave amplitude E o  less than E th , respectively; while requiring E back  plus E 0  (i.e., when the two point in the same direction) to be greater than or equal to E th .  
           [0016]    For the existing electro-poling methods, a mask-patterned electric field of about 20-26 kV/mm is applied on LiNbO 3  for about 50 μs to several seconds each time. (The required electric field strength is known to be lower if the treated nonlinear crystal is properly heated.) For the invented standing-laser-poling method, if the chosen laser wavelength is about 1 μm, then existing high-power (1-10 MW) pulsed lasers such as YAG can be employed. Since the laser frequency is in the 1014 Hz range, the invented standing-laser-poling is in fact achieved by repeated poling actions within the applied, say, 10 ms laser pulse duration.  
           [0017]    When short-pitch periodic poling can be achieved, another very important category of applications is the manufacturing of so-called photonic crystals. Just as a process-patterned silicon crystal would direct electron flows in a desirable way, a properly patterned photonic crystal would do the same to light. By changing the size, distribution and periodicity of its ferroelectric domains, the properties of a PPLN (periodically poled lithium niobate) crystal, now known as a “designer” material, can be engineered to match the requirement of a given light-manipulation application (see, e.g., Neil Broderick&#39;s article: “November 2002: Lithium niobate,” Nature Magazine)  
           [0018]    A thorough prior art search concerning laser action and SHG (as well as other frequency converting) crystals has been conducted. All known prior arts are proved to have nothing to do with the invented “standing-laser poling” approach on creating short-pitched frequency converters. Namely, the typical laser setups of those prior arts (including fundamental laser, periodically-inverted crystal, photo-detector, etc.) are either merely for the routine second harmonic generation (SHG) of light wave itself through the already-electrically-poled crystal, or, in extra, as means for monitoring the domain inversion of the crystal (see, e.g., Karlsson et al. U.S. Pat. No. 5,986,798 (1999)). The former is just obtaining the routine quasi-phase-matched (QPM) SHG result once the crystal is properly poled (i.e., periodically domain inverted). The latter is a common practice in checking out the integrity of the poled crystal on a regular basis. None has anything to do with poling the crystal itself using large standing laser action and background DC field, as the current invention teaches.  
           [0019]    Chemla et al. (U.S. Pat. No. 4,860,296 Aug. 22, 1989) teaches a multiple-layer heterostructure which is incorporated within the laser resonant cavity to enhance the laser output. Although relating to standing laser wave, it is not relevant as regards to the crystal-poling purpose and subsequent frequency conversions aimed for by the current invention. Besides, having a standing wave within the resonant cavity is just about how a common laser works. In addition, as a side remark, any attempt to manufacture frequency converters using a multi-layer (along the wave propagation direction) approach will prove itself very time consuming and uneconomical. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0020]    [0020]FIG. 1A and FIG. 1B show the effect of phase matching on the growth of second harmonic intensity with propagation distance in a nonlinear crystal.  
         [0021]    [0021]FIG. 2 illustrates the setup of existing electro-poling method to volumetrically pattern the nonlinear crystal.  
         [0022]    [0022]FIG. 3 shows the setup of the invented standing-laser-poling method.  
         [0023]    [0023]FIG. 4A and FIG. 4B illustrates the detailed combined action of the invented standing laser field and the background field. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0024]    In order to create domain inversion of a desired period  11  within the nonlinear media  38 , a standing wave electric field pattern  36  of periodic is needed. This would require a high-field laser  30  of wavelength λ 2·l c  together with a background (e.g., constant uniform DC) electric field  42  as illustrated in FIG. 3 and FIG. 4A and FIG. 4B. The background field  42  is realized, for example, by two biased parallel metal plates  32  and  34 . Here is an example. If an inversion period of l c ≈0.5 μm is desired, then a high-field laser of wavelength λ 1 μm is employed to form a laser standing wave pattern  44 , by using a beam splitter  33  and mirrors  35 . The desired poling standing wave pattern  36 , of period of about 0.5 μm, emerges from the combination of standing wave  44  and background field  42 . Though oscillating temporally in amplitude, the poling standing field pattern  36  pulls the nonlinear media to render domain inversion in a patterned fashion every time when reaching its peak field.  
         [0025]    The required laser pulse power can be calculated as follows. To realize the invented method, it is necessary to have both the background DC field E dc  and the peak standing wave amplitude E 0  less than the threshold field E th  for domain inversion, respectively, while requiring their sum to be greater than or equal to E th . Take LiNbO 3  for an example. If the required threshold electric field to cause domain inversion within LiNbO 3  is 26 kV/mm along a chosen crystal axis and facet, then it can be arranged, for example, such that E dc ≈14 kV/mm, and the peak laser electric field E dc ≈14 kV/mm, say. There are simply many workable combinations of E dc  and E 0  values to carry out the invented method.  
         [0026]    The corresponding peak laser power is about E 0 H 0 A, where the magnetic field intensity H is equal to B/μ 0 , in which μ 0  (=4π10 −7  Henry/m) is the magnetic permeability, B is the magnetic flux density, and A is the laser beam cross-sectional area. Assuming TEM wave for the high field laser beam  30  (see the setup in FIG. 3), such that B is equal to E 0 /C (C=3108 m/s), and for a beam diameter of D≈1 cm, then the desired laser power is about 35 MW. There are quite a few existing choices for pulsed lasers of this power level and of wavelengths in the 0.2-4 μm range. For example, according to US Naval Research Laboratory&#39;s Plasma Formulary (1987) P.50, at least several high-power pulse-type lasers are available:  
                               TABLE 1                                       Wavelength   Pulsed power level           Type   (μm)   (W)                           Color Center   1-4   &gt;10 6              Holmium   2.06   &gt;10 7              Iodine   1.315   &gt;10 12             Nd-glass, YAG   1.06    ˜10 14             Ruby   0.6943    10 10             Kr—F   0.26   &gt;10 9              Xenon   0.175   &gt;10 8                         
 
         [0027]    In addition, existing compact diode-pumped solid-state (DPSS) lasers, with its high repetition rate (&gt;1 kHz) and high power (≧1 MW), can either be directly applied for the invented purpose or further power amplified by proper pulse compression using existing laser rods (see, [Pasmanik G. A., 2000]).  
         [0028]    The created short-period standing wave pattern does not diffuse and broaden within the nonlinear crystal as happens to the existing DC electro-poling method. This is because if the field diffusion problem can also occur with a laser (EM) wave, then any pump laser wave frequency can never be doubled after passing through a frequency doubling crystal. In other words, there would have been no second harmonic generation should the existence of any EM waveform can never be allowed within a domain inverted nonlinear crystal.  
         [0029]    The nonlinear media, or crystal, that the invented method can adopt may be LiNbO 3 , LiTaO 3 , KTiOPO 4  (KTP), KH 2 PO 4  (KDP), 2-methyl-4-nitroaniline (mNA) (see [Suhara T. et al., 1993]), β-BaB 2 O 4  (BBO), LiB 3 O 5  (LBO) (Ding Y. J. et al., 1998), silica fiber (see [Pruneri V. and Kazansky P. G., 1997]), and all other nonlinear materials still under development. Further, the completed frequency-converting nonlinear media can also be employed in the general field of frequency conversion other than the second harmonic generation, for example, the sum and difference frequency generations of interest in the telecommunications area. It should also be noted that, to the best knowledge of the inventor, any thick (&gt;10 μm) volumetric domain inversion with period less than 2 μm, on the most popular LiNbO 3 , is practically inaccessible by existing electro-poling methods. The invention thus makes it possible that a large wave number difference (i.e., mismatch), between a pump laser and its second harmonic, so long as it&#39;s less than 1.9 micron −1  according to equation (1), can still accomplish frequency doubling on a converter crystal of about 1.6 micron inversion period.  
         [0030]    While the invention has been particularly shown and described with reference to the preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention. For example, the power of the domain-shaping standing laser wave can be made to be time varying in all kinds of ways. The background DC field can be made slowly varying in both shape and magnitude. Or, the invented patterned optic element can be incorporated into a resonant cavity to generate frequency-altered laser light. Further, due to the large variety of nonlinear physical property of the employed nonlinear crystals, e.g., lithium niobate, from different sources, the needed threshold field for domain inversion can vary in a wide range. It is particularly so if the crystal is being heated while applying the invented method. Lastly, the invented method can be applied on more than one dimension of the nonlinear crystal, for example, for the light focusing purpose.