Method for manufacturing of patterned SrB.SUB.4.BO.SUB.7 .and PbB.SUB.4.O.SUB.7 .crystals

Abstract

An SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal is configured with a plurality of domains with respective periodically alternating polarity of the crystal axis so that the disclosed crystal is capable of quasi-phasematching (QPM). The disclosed crystal is manufactured by a method including patterning a surface of a crystal block of SrB4O7 or PbB4O7, thereby providing patterned uniformly dimensioned regions with a uniform polarity sign on the surface. The method further includes generating a disturbance on the patterned surface, thereby inverting a sign of crystal polarity of every other region to form the SrB.sub.4O.sub.7 or SrB.sub.4O.sub.7 crystal with a plurality of domains with alternating polarity enabling a QPM mechanism.

Claims

1. A strontium tetraborate (SrB.sub.4O.sub.7) or lead tetraborate (PbB.sub.4O.sub.7) crystal comprising a plurality of uniform domains with respective periodically alternating polarity of the crystal axis, the uniform domains defining a volume periodic structure of the SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal enabling quasi phase-matching (QPM) use.

2. The SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal of claim 1, wherein the crystal is configured to be a nonlinear optical element with the QPM used for converting a fundamental frequency to a higher harmonic which is selected from the group consisting of a second harmonic, third harmonic generation, higher harmonic generations and optical parametric interactions.

3. The SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal of claim 1, wherein a thickness of each domain for a VIS-DUV light range is between 0.2 m and about 20 m.

4. The SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal of claim 1, further having a clear aperture with a diameter which varies from about 1 mm to about 5 cm.

5. The SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal of claim 1, wherein the uniform domains have parallel walls deviating from one another at less than 1 m over a 10 mm distance.

6. A method of fabricating a periodic structure in a strontium tetraborate (SrB.sub.4O.sub.7) or lead tetraborate (PbB.sub.4O.sub.7) nonlinear crystal, comprising: patterning a surface of a SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 block, thereby providing a plurality of alternating protected and unprotected uniformly dimensioned regions with a uniform polarity sign of the crystal axis; generating a disturbance on the surface, thereby inverting a sign of crystal polarity of every other region such as to provide the SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 block with a periodic volume structure including a plurality of uniformly dimensioned domains with an alternating polarity of the crystal axis, thereby obtaining the SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 nonlinear crystal enabling quasi phase-matching (QPM).

7. The method of claim 6, wherein the patterning step includes: metallizing the patterned surface, applying a layer of photoresist atop the metallized surface, applying a mask with a desired period atop the layer of photoresist, thereby providing a plurality of uniformly-dimensioned regions with exposed photoresist and covered photoresist which alternate one another, and removing the photoresist layer and metal off the uniformly-dimensioned regions with exposed photoresist, thereby forming uniformly-dimensioned patterned regions.

8. The method of claim 6, wherein the step of generating disturbance includes: generating an internal disturbance on the structured surface while utilizing a high temperature melt technique, thereby growing the SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal with a plurality of uniformly-dimensioned domains which have alternating polarity corresponding to the polarity of respective uniformly-dimensioned regions, wherein the high temperature technique is selected from a Czochralski method, Bridgeman, directional recrystallization, or top-seeded solution growth.

9. The method of claim 6, wherein the step of generating disturbance on the surface includes applying an external force to the protected regions of the patterned surface, thereby flipping the polarity of every other region.

10. The method of claim 9 further comprising utilizing a high temperature melt technique, thereby growing the SrB.sub.4O.sub.7 or PbB.sub.4O.sub.7 crystal with a plurality of uniformly-dimensioned domains with alternating polarity corresponding to the polarity of respective uniformly-dimensioned regions, wherein the high temperature technique is selected from a Czochralski method, Bridgeman, directional recrystallization, or top-seeded solution growth.

11. The method of claim 6, wherein the domains of the volume periodic structure of the SrB.sub.4O.sub.7 or SrB.sub.4O.sub.7 nonlinear crystal have respective parallel walls which deviate from one another at less than 1 micron (m) over a 10 mm distance.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The above and other aspects and feature will become more readily apparent in conjunction with the following drawings, in which:

(2) FIG. 1 is a graphical representation of the quasi-matching principle;

(3) FIG. 2 is an elevated view of the inventive SBO and PBO crystals;

(4) FIG. 3 is a flow diagram illustrating a preferred embodiment of the inventive method of fabricating the crystal of FIG. 2;

(5) FIGS. 4A-4D are diagrammatic representation of selective steps of the inventive method;

(6) FIG. 5 illustrates growing an SBO/PBO crystal with the domain structure inherited from the seed of FIG. 2; and

(7) FIG. 6 illustrates growing an SBO/PBO crystal with the domain structure originated at the seeding.

SPECIFIC DESCRIPTION

(8) Reference will now be made in detail to the disclosed inventive concepts. Wherever possible, same or similar reference numerals are used in the drawings and the description to refer to the same or like parts or steps. The drawings are in simplified form being far from precise scale.

(9) It generally known that the domain walls can be relatively planar/parallel, protruding seemingly without noticeable deflection. It can be suggested that SBO/PBO may be one of these materials. According to the data obtained by Applicants, this suggestion was verified by providing an SBO crystal with planar walls which protrude over a distance of at least several millimeters. In one of the experimental samples of the inventive SBO/PBO grown in accordance with the disclosed method, the deviation of protruding walls over a 10 mm distance did not exceed the detection limit (0.1 micron in that case). However, the data suggests that the deviation of twins in the disclosed crystal is less than one lattice period. As a consequence, indefinitely large apertures of QPM structures reaching several centimeters can be obtained in the inventive SBO/PBO utilizing the inventive method.

(10) Referring to FIG. 2, inventive crystal SBO or PBO 10 is configured with a periodic structure 12 of domains 14 and 17 having respective opposite polarities +/ which alternate one another. These domains have highly parallel walls. The periodic structure 12 allows the use of a QPM technique to generate high harmonic wavelength of the fundamental wave which includes second harmonic generation, third and higher harmonic generation, and optical parametric interactions. Recent experiments conducted by the Applicants resulted in crystal 10 provided with a volume periodic pattern which includes a sequence of uniformly dimensioned 3D-domains 14, 17 having respective positive and negative polarities which alternate one another and provide the crystal with a clear aperture having a diameter of up to a few centimeters. The domains each are configured with a uniform thickness corresponding to the desired coherence length l and ranging from about 0.2 m to about 20 m. The crystal 10 can be utilized as an optical element, such as a frequency converter incorporated in a laser which operates in a variety of frequency ranges. For example, crystal 10, configured to convert frequency in a laser outputting radiation in a DUV frequency range, has a coherence length l ranging between 0.2 to about 5 nm. The volume pattern may extend through the entire thickness of crystal block 10 between faces +C and C, or terminate at a distance from one of these faces.

(11) FIGS. 3 and 4A-4D illustrate the inventive multi-step method. The initial stage 32 of FIG. 3 includes preparing a single monocrystal block SBO/PBO 10 of FIG. 4A in accordance with techniques well known to one of ordinary skill worker. The following stage is designed to provide one (or both) surfaces +C, C, or any other polar surface with a surface periodic pattern having the desired period. This stage utilizes alternative techniques all resulting in a plurality of parallel, uniformly dimensioned adjacent regions 30, 32 (FIG. 4) which are formed, for example, on face +C and thus have the uniform polarity. However, the structure of every other region is different from the flanking regions. In other words, structurally regions 30 differ from regions 32. Several techniques can be utilized to realize such a configuration.

(12) For example, one of the polar faces is provided with a photoresist layer, as shown in 34 of FIG. 3. Thereafter, using any of the known methods, such as standard lithography or e-beam, the mask with the desired period is applied upon the photoresist layer, as shown in 36 of FIG. 3. Thereafter, the photoresist layer is removed from the unprotected areas in step 38 leaving thus crystal block 10 of FIG. 4B with the surface pattern defined by alternating protected and unprotected regions 30, 32, as shown in step 40. As a result, the surface to be patterned includes a plurality of photoresist exposed and covered by the mask alternating areas.

(13) Alternatively, the step 40 of providing protected/unprotected regions 30, 32 can be conducted by applying a metal coating to one of the polar faces in step 42. Thereafter, a photoresist layer is provided atop the metal coating in step 44. The steps 36 and 38, discussed above, follow to provide the structured polar face with protected and unprotected regions 30, 32 in step 40 as shown in FIG. 4B.

(14) The following stage of the inventive method includes fabricating crystal 10 having a volume periodic pattern which includes domains 14 and 17 having different crystal polarity. There are several premises suggested and verified by Applicants that are critical to the manufacturing of crystal 10 of FIG. 4D which is structurally identical to FIG. 2. In accordance with one premise, any disturbance impacting regions 30 or 32 or the entire patterned face +C of crystal block 10 of FIG. 4C leads to the change of the polarity of the impacted regions. According to the second premise, the volume pattern to be formed from the surface pattern inherits all of the parameters of the surface pattern. In other words, the number of regions 30, 32 and each region's length and width remain unchanged as the volume pattern protrudes through the body of crystal 10 of FIG. 4D, but the polarity of every other region flips. Thus, regions 30, 32 of crystal 10 grow into respective domains 14, 17 extending toward the opposite crystal polar face C with every other domain, for example 14, having the polarity flipped opposite to that of domains 17.

(15) Referring specifically to step 46 of FIG. 3, the formation of the volume periodic pattern begins with profiling the surface of single crystal or crystal block 10 of FIG. 4B after the latter is patterned in step 40. Creating the surface profile with the desired period can be accomplished by a variety of techniques. For example, it is possible to apply a matrix with the desired pattern of formations of the same polarity to the patterned surface +C. The formation may include, for example, alternating with the desired period grooves and ridges which create respective indentations on the surface of crystal block 10. With crystal block 10 provided with formations structurally distinguishing regions 30, 32 from one another, alternative techniques for flipping the polarity of one group of regions 30, 32 can be implemented.

(16) One of these technique provides for crystal block 10 with the profiled surface of step 46 to be used as a seed for growing large-size SBO/PBO crystal 10, as shown step 56 of FIG. 3 and FIGS. 4D and 5 by utilizing, for example, the Czochralski method. The latter is one of many high temperature crystal-growth methods from a melt. Other methods, such as Bridgeman, directional recrystallization, top-seeded solution growth etc, all are part of the disclosed subject matter. Turning to FIG. 4, regions 30 of crystal block 10 have respective formations, such as grooves of step 46. Due to the elevated temperatures associated with any temperature crystal-growth method, the formations may melt away. For that reason, the temperature at the interface between seed/crystal 10 and the boule should be controlled to be at or below the known melting temperature.

(17) Importantly, however, that as the crystal-growth method continues, the seeding stress at the interface between seed 10 and growing boule (FIG. 6) flips the polarity of either regions 14 or regions 17 in the growing boule. As can be seen, seed 10 has pattern of regions 30, 32 with the same polarity, as indicated by uniformly facing arrows, but growing crystal 10 has the domains with respective opposite polarities. Once crystal 10 of step 56 is ready, it may be used as an optical element, such as frequency converter, in step 100. Alternatively, crystal 10 provided in step 56 can be further used as a seed for even larger crystals.

(18) Alternatively, crystal 10 of step 46 with the profiled surface may be impacted by an externally generated disturbance in step 48 as illustrated in FIG. 4C. For example, it can be a stress produced mechanically, electrically, or utilizing ion implantation (or in-diffusion), UV or X-ray radiation by any well-known method of generating the stress. Regardless of the origin and nature of the disturbance, it provides flipping the polarity of one group of regions 30 or 32, as shown in step 52. As in the previously disclosed operation, crystal 10 may be used as an optical element of step 100 or used as the seed in step 56. But in contrast to the procedure disclosed immediately above, crystal block used in the Czochralski method has regions 30 and 32 having respective opposite polarities due to the external force applied previously. The growing boule inherits the structure of crystal block 10. This particular technique is illustrated in FIG. 5. Note that both techniques for flipping polarity as disclosed above start with profiling the patterned surface in step 46. Yet there is an alternative technique which is disclosed immediately below.

(19) The crystal block of step 40 is immediately impacted by a force generated by an external source, as indicated in step 50 and illustrated in FIG. 6. The force affects either protected regions 30 or unprotected regions 32. The application of the external force continues until selective regions 30 or 32 have their initial polarity flipped, as indicated by step 52 and shown in FIG. 4D. Thus crystal block 10 of FIG. 4D is provided with domains 14, 17 having alternating polarity. Thereafter, crystal block 10 of FIG. 4D may be either used as a seed in step 56 and FIG. 4C, if the size of crystal 10 of FIG. 4D does not have the desired dimensions, or used as a ready to operate optical element of step 100. The Applicants demonstrated that such a patterned crystal produced from a seed in either approach disclosed above can grow in direction of polar axis c, non-polar axis a, any direction in between c and a, as well as in a direction close to a/c plane.

(20) An experimental SBO crystal with a 5 cm clear aperture was recently grown utilizing the inventive method. This particular dimension provides unique favorable conditions for using large diameter laser beams at a pump wavelength incident on the selected crystal surface without costly beam guiding optics. The length of SBO crystal 10 along axis A, coinciding with a direction of beam propagation, is limited by the dimensions of the patterned seed, which can be extended in the b crystallographic direction (FIG. 4D) to make the corresponding area larger. In contrast to the known prior art, the walls of respective domains 14 and 17 are ideally parallel to one another.

(21) The experimental crystals fabricated by any of the above disclosed method steps and incorporated in the lasers as the frequency converter demonstrated an output power at 266 nm ranging from 1 W to 10 W.

(22) It is to be understood that while the invention has been described in conjunction with the detailed description thereof, the foregoing description is intended to illustrate and not limit the scope of the invention, which is defined by the scope of the appended claims. For example, while the disclosure is dedicated to providing a periodic structure having alternating domains of uniform width, it is perfectly possible to use the disclosed method to construct aperiodic structures or non-planar structures such as photonic crystals. Other aspects, advantages, and modifications are within the scope of the following claims.