CN117631401A - Nonlinear optical device and preparation method thereof - Google Patents
Nonlinear optical device and preparation method thereof Download PDFInfo
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- 241001270131 Agaricus moelleri Species 0.000 claims abstract description 69
- 239000004038 photonic crystal Substances 0.000 claims abstract description 55
- 238000009826 distribution Methods 0.000 claims abstract description 46
- 239000013078 crystal Substances 0.000 claims abstract description 38
- 238000007493 shaping process Methods 0.000 claims abstract description 35
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- GQYHUHYESMUTHG-UHFFFAOYSA-N lithium niobate Chemical compound [Li+].[O-][Nb](=O)=O GQYHUHYESMUTHG-UHFFFAOYSA-N 0.000 description 4
- 238000012986 modification Methods 0.000 description 4
- 230000004048 modification Effects 0.000 description 4
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- 238000004364 calculation method Methods 0.000 description 3
- 238000001069 Raman spectroscopy Methods 0.000 description 2
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- CPLXHLVBOLITMK-UHFFFAOYSA-N magnesium oxide Inorganic materials [Mg]=O CPLXHLVBOLITMK-UHFFFAOYSA-N 0.000 description 1
- 239000000395 magnesium oxide Substances 0.000 description 1
- AXZKOIWUVFPNLO-UHFFFAOYSA-N magnesium;oxygen(2-) Chemical compound [O-2].[Mg+2] AXZKOIWUVFPNLO-UHFFFAOYSA-N 0.000 description 1
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Abstract
The invention belongs to the technical field of nonlinear beam shaping, and discloses a nonlinear optical device and a preparation method thereof. According to the distribution characteristics of the pump light beams and a quasi-phase matching mechanism, the shaping structure of the nonlinear photonic crystal with the three-dimensional conical pattern is designed; based on the shaping structure, the ferroelectric crystal is modified by femtosecond laser, and the nonlinear photonic crystal with a three-dimensional conical pattern is prepared and used as a nonlinear optical device; when the pump beam is incident to the nonlinear optical device, a flat-top second harmonic beam can be generated, and meanwhile, the flat-top beam and double frequency can be realized. The invention solves the problem that the existing nonlinear flat-top beam shaping technology is limited to two dimensions. The invention expands the nonlinear flat-top beam shaping technology from two dimensions to three dimensions, and can realize effective full-wave plane flat-top distribution and nonlinear frequency conversion at the same time.
Description
Technical Field
The invention belongs to the technical field of nonlinear beam shaping, and particularly relates to a nonlinear optical device capable of realizing flat-top beams and frequency doubling at the same time and a preparation method thereof.
Background
Uniform irradiation on a target is required in applications such as inertial confinement fusion, biomedical imaging, optical trapping, and spatial debris removal. Flat-top beams have a uniform intensity distribution, and have been widely studied in the linear optics field, but little in the nonlinear field, still have challenges.
Nonlinear photonic crystals have spatially varying second order nonlinear optical coefficients that can efficiently exchange energy in frequency conversion by quasi-phase matching. Related researches for generating nonlinear flat-top beams only exist in 1998, and Imeshev et al modulate a second-order nonlinear optical coefficient in ferroelectric crystal lithium niobate into a transverse uneven periodic distribution through a photoetching technology, so that flat-top distribution of second harmonic in one dimension is realized. However, in conventional techniques (such as electric field polarization and lithography), the modulation of the second order nonlinear optical coefficient in nonlinear photonic crystals is limited to two dimensions due to the lack of viable methods to prepare periodic distributions along the depth. One of the dimensions is required for quasi-phase matched frequency conversion. Therefore, the two-dimensional flat-top nonlinear photonic crystal still cannot realize full wave surface modulation under the condition that only one dimension is left.
Disclosure of Invention
The invention solves the problem that the existing nonlinear flat-top beam shaping technology is limited to two dimensions by providing a nonlinear optical device and a preparation method thereof.
The invention provides a preparation method of a nonlinear optical device, which comprises the following steps:
step 1, designing a shaping structure of a nonlinear photonic crystal with a three-dimensional conical pattern according to the distribution characteristic of a pump beam and a quasi-phase matching mechanism;
step 2, based on the shaping structure, utilizing femtosecond laser to modify the ferroelectric crystal to prepare a nonlinear photonic crystal with a three-dimensional conical pattern, wherein the nonlinear photonic crystal is used as a nonlinear optical device;
when the pump beam is incident to the nonlinear optical device, a flat-top second harmonic beam can be generated, and meanwhile, the flat-top beam and double frequency can be realized.
Preferably, the distribution characteristic of the pump beam satisfies gaussian distribution, the outer diameter of the shaping structure is the same as the outer diameter of the pump beam, and the radius of the inner circular mesopore of the corresponding cross section of the three-dimensional conical pattern forming the truncated area at the position of different interaction lengths is changed; quasi-phase matching is performed along the propagation direction y-axis, the flat-top beam is shaped in two dimensions of the x-axis and the z-axis, and the functional form of the interaction length is selected as follows:
wherein L (x, z) is the interaction length, L 0 Nonlinear photonic crystal quasi-phase matching grating length omega x And omega z Beam waist radii in x-direction and z-direction, respectively, of a pump beam with gaussian distribution; a is a cut-off value, and the value of a corresponds to the radius of the hole in the inner circle.
Preferably, the frequency doubling intensity of the flat-top second harmonic beam is expressed as follows:
wherein I is 2ω (x, z) is the doubling intensity, C is the material constant of the ferroelectric crystal, I 0 Is the square of the amplitude of the gaussian beam electric field complex amplitude.
Preferably, the focal energy of the femtosecond laser is larger than the ferroelectric crystal damage threshold of the focusing position, and the femtosecond laser is used to generate a modified region on the surface or inside of the ferroelectric crystal.
Preferably, the ferroelectric crystal is transparent to the wavelength of the femtosecond laser.
Preferably, the femtosecond laser is a near infrared femtosecond laser.
Preferably, when the femtosecond laser is used for modifying the ferroelectric crystal, the pulse energy of the femtosecond laser is adjusted along with the increase of the processing depth, focal spot distortion caused by aberration and absorption loss is compensated, and the overall uniformity of the prepared nonlinear photonic crystal is ensured.
Preferably, the ferroelectric crystal is modified by the femtosecond laser to ensure that the second-order nonlinear optical coefficient is as followsModulating in a three-dimensional space; in nonlinear photonic crystals hundreds of microns long, nonlinear conversion efficiency reaches 10 -2 An order of magnitude; the flatness coefficient of the flat-top second harmonic beam is more than 90%.
Preferably, the method for manufacturing a nonlinear optical device further includes:
and 3, performing characteristic measurement on the emergent flat-top second harmonic beam, and verifying the performance of the nonlinear optical device based on the obtained measurement result.
On the other hand, the invention provides a nonlinear optical device, which is prepared by adopting the preparation method of the nonlinear optical device.
One or more technical schemes provided by the invention have at least the following technical effects or advantages:
firstly, designing a shaping structure of a nonlinear photonic crystal with a three-dimensional conical pattern according to the distribution characteristic of a pump beam and a quasi-phase matching mechanism; and then based on the shaping structure, the ferroelectric crystal is modified by femtosecond laser to prepare the nonlinear photonic crystal with the three-dimensional conical pattern, and the three-dimensional nonlinear photonic crystal is used as a nonlinear optical device. When the pump beam is incident to the prepared nonlinear optical device, a flat-top second harmonic beam can be generated, and meanwhile, the flat-top beam and double frequency are realized. The invention prepares a nonlinear optical device capable of realizing flat-top beam and frequency doubling simultaneously, and the flat-top truncation of the frequency doubling wave surface emitted by the device can be respectively and flexibly adjusted in mutually perpendicular coordinate directions. The device can expand the nonlinear flat-top beam shaping technology from two dimensions to three dimensions, and can realize effective full-wave plane flat-top distribution and nonlinear frequency conversion at the same time. The invention provides a convenient and practical generation mode for generating the nonlinear flat-top beam and provides reference for nonlinear beams with other wave surface distribution. Has positive significance for promoting future development in optical manipulation, optical communication and super-resolution imaging.
Drawings
Fig. 1 is a schematic diagram of a structure and design principle of a nonlinear optical device according to an embodiment of the present invention.
FIG. 2 is a schematic diagram of a nonlinear photonic crystal fabricated using a femtosecond laser in accordance with an embodiment of the present invention.
FIG. 3 is a schematic diagram of the results of flat-top frequency doubling intensity distribution and normalized intensity contour distribution obtained by an embodiment of the present invention.
Fig. 4 is the normalized intensity obtained from a single column of pixels of a CCD camera in the center of the spot by generating a flat top doubling.
Detailed Description
In order to better understand the above technical solutions, the following detailed description will refer to the accompanying drawings and specific embodiments.
The embodiment of the invention provides a preparation method of a nonlinear optical device, which mainly comprises the following steps:
step 1, designing a shaping structure of a nonlinear photonic crystal with a three-dimensional conical pattern according to the distribution characteristic of a pump beam and a quasi-phase matching mechanism;
step 2, based on the shaping structure, utilizing femtosecond laser to modify the ferroelectric crystal to prepare a nonlinear photonic crystal with a three-dimensional conical pattern, wherein the nonlinear photonic crystal is used as a nonlinear optical device;
when the pump beam is incident to the nonlinear optical device, a flat-top second harmonic beam can be generated, and meanwhile, the flat-top beam and double frequency can be realized.
In addition, the preferred scheme can further comprise:
and 3, performing characteristic measurement on the emergent flat-top second harmonic beam, and verifying the performance of the nonlinear optical device based on the obtained measurement result.
Wherein, under the condition that the distribution characteristic of the pump beam meets Gaussian distribution, the diameter of the outer circle of the shaping structure is the same as that of the pump beam, and the radius of the inner circle center hole of the corresponding cross section of the three-dimensional conical pattern forming the truncated area at the position of different interaction lengths is changed; quasi-phase matching is performed along the propagation direction y-axis, the flat-top beam is shaped in two dimensions of the x-axis and the z-axis, and the functional form of the interaction length is selected as follows:
wherein L (x, z) is the interaction length, L 0 Nonlinear photonic crystal quasi-phase matching grating length omega x And omega z Beam waist radii in x-direction and z-direction, respectively, of a pump beam with gaussian distribution; a is a cut-off value, and the value of a corresponds to the radius of the hole in the inner circle.
The frequency doubling intensity of the flat-top second harmonic beam is expressed as follows:
wherein I is 2ω (x, z) is the doubling intensity, C is the material constant of the ferroelectric crystal, I 0 Is the square of the amplitude of the gaussian beam electric field complex amplitude.
The focal energy of the adopted femtosecond laser is larger than the ferroelectric crystal damage threshold value of a focusing position, and a modified area is generated on the surface or inside the ferroelectric crystal by the femtosecond laser.
The ferroelectric crystal is transparent to the wavelength of the femtosecond laser.
Specifically, the femtosecond laser is near infrared femtosecond laser. The femtosecond laser has flexible three-dimensional processing capability, the nonlinear coefficient can be adjusted in depth, and the two-dimensional flat-top distribution is completely realized on the generated beam cross section by the modulation of the second-order nonlinear coefficient of the three-dimensional written by the femtosecond laser focusing. Near infrared femtosecond laser can be focused deep in any transparent material.
When the femtosecond laser is used for modifying the ferroelectric crystal, the pulse energy of the femtosecond laser is adjusted along with the increase of the processing depth, focal spot distortion caused by aberration and absorption loss is compensated, and the overall uniformity of the prepared nonlinear photonic crystal is ensured.
Modifying the ferroelectric crystal by using the femtosecond laser to realize the modulation of a second-order nonlinear optical coefficient in a three-dimensional space; in nonlinear photonic crystals hundreds of microns long, nonlinear conversion efficiency reaches 10 -2 An order of magnitude; the flatness coefficient of the flat-top second harmonic beam is more than 90%.
The nonlinear optical device can be prepared by adopting the preparation method of the nonlinear optical device.
The present invention is further described below.
The structural schematic diagram and the design principle of the nonlinear optical device provided by the invention are shown in fig. 1, and the period length of the quasi-phase matching grating determines the intensity and efficiency of frequency doubling. Thus, a quasi-phase matched grating structure with laterally varying period lengths can be used to convert the desired uniform frequency doubled intensity distribution. The interaction length in the transverse period and the longitudinal period is changed through the three-dimensional conical pattern nonlinear photonic crystal, the conversion efficiency of the edge is higher, and therefore flat-top frequency doubling can be generated. In areas without periodicity, the frequency doubling conversion efficiency is negligible. In the x-z section perpendicular to the y-axis of the transmission direction, the fundamental frequency beam is intercepted by a three-dimensional nonlinear photonic crystal, and the doubling frequency has a flat intensity curve in a certain range of x epsilon-a, a and z epsilon-a, a by taking the intersection point of the optical axis and the section as the origin. The ratio of the flat uniform radius of the target flat-top beam to the beam waist radius of the gaussian beam is defined as the cut-off factor. The chopping of the beam is described here by a chopping value a, the magnitude of which is the product of the chopping factor and the beam waist radius. For a given pump beam profile, the driving force is controlled by the interaction length L. The three-dimensional nonlinear photonic crystal for shaping the nonlinear flat-top beam can be flexibly designed and processed according to the distribution characteristics of the pump beam and a quasi-phase matching mechanism. It is noted that the proper form of L depends on the beam waist radius of the pump beam and the flat extent of the doubled beam (i.e. the beam cutoff), and that the particular shaping structure is only suitable for a given pump beam. The normal incidence and the reverse incidence of the pump beam along the y-axis with the periodic distribution are applicable, and the interaction length in the transmission process is not changed, so that no influence is caused on the generated flat-top frequency doubling.
As shown in fig. 1 (a), when the fundamental frequency beam is incident on the two-dimensional beam-shaped nonlinear photonic crystal, frequency multiplication achieves a flat-top distribution in only one dimension. I.e. the wavefront of the doubled (green beam) can only achieve a single-dimensional flat-top distribution in the x-direction.
In the present invention, the three-dimensional nonlinear photonic crystal for beam shaping is designed in a conical pattern, as shown in (b) of fig. 1, the fundamental frequency beam is incident in the y-axis direction, and the polar axis z of the crystal is perpendicular to the transmission direction. After three-dimensional nonlinear photon crystal, frequency doubling (green beam) can finally obtain two-dimensional flat-top distribution, so that the wave surface is completely flat on the x-z plane. The three-dimensional conical pattern nonlinear photonic crystal designed by the invention can be increased by one dimension, and the flat-top distribution shaping of the complete space wavefront is realized.
The invention designs a three-dimensional cone patterned nonlinear optical device for generating nonlinear flat-top beams based on a quasi-phase matching principle. When the device is prepared, femtosecond laser is focused in a transparent material, material modification is induced, the second-order nonlinear optical coefficient is modulated in a three-dimensional space, and then the conical patterned nonlinear photonic crystal is obtained by processing. The double frequency wave surface flat top cut-off can be flexibly adjusted in the mutually perpendicular coordinate directions respectively.
The invention changes the interaction length through the transverse and longitudinal distribution of the cone, and the conversion efficiency of the edge is higher, thereby generating a flat-top second harmonic beam. In nonlinear photonic crystals of hundreds of microns, the nonlinear conversion efficiency is 10 -2 The conversion efficiency is improved by two orders of magnitude compared with the prior art. Truncated flat top doubles, measured in mutually perpendicular coordinate directions. The flatness of the section is over 90 percent, and the optimal flatness is 97.1 percent. The device provides a three-dimensional nonlinear beam shaping technology, and can realize effective full-wave plane flat-top distribution and nonlinear frequency conversion at the same time. The three-dimensional nonlinear photonic crystal in the device can be kept stable at normal temperatureThe conical pattern structure distribution has the characteristic of fitting pump light, can reduce light energy loss, and can more effectively meet the advantage of uniform radiation. In addition, the design principle of the device is also applicable to non-Gaussian pump beams.
For a better understanding of the invention, a ferroelectric crystal, in particular a lithium niobate crystal, will be described below by taking as an example a pump beam having a gaussian distribution.
A method of fabricating a nonlinear optical device, comprising the steps of:
step 1: according to the distribution characteristics of the pumping light beam and a quasi-phase matching mechanism, a shaping structure of the nonlinear photonic crystal with the three-dimensional conical pattern is designed.
Namely, step 1 mainly designs the structural distribution of the nonlinear photonic crystal with the three-dimensional conical pattern, and determines the positions of the three-dimensional modified domains of the constituent units.
Step 1 utilizes the maximum nonlinear coefficient d of lithium niobate crystal 33 . If the frequency doubling generation process is phase matched, the frequency doubling generation process is performed at a low conversion efficiency limit (eta 0 And < 1), the conversion efficiency eta is as follows:
η＝η 0 ＝I 2ω /I 1ω , (1)
wherein I is 1ω And I 2ω The fundamental frequency intensity and the doubling frequency intensity are respectively. The intensity of the fundamental beam is gaussian in the x-z plane perpendicular to the propagation direction, expressed as:
wherein I is 0 Representing the square of the amplitude of the complex amplitude E of the gaussian beam electric field.
Wherein eta 0 Is defined as follows:
η 0 ＝C 2 L 2 I 1ω , (3)
where C is the material constant. For a Gaussian distribution of pump beams, the functional form of the interaction length is chosen as follows:
wherein a is a cut-off value, L 0 Nonlinear photonic crystal quasi-phase matching grating length omega x And omega z The beam waist radii of the Gaussian pump beam in the x and z directions, i.e., the 1/e electric field radius, respectively. By combining formulas (1) to (4), the intensity of the flat-top frequency doubling can be obtained as follows:
the diameter of the outer circle of the structure is determined at different propagation distances, i.e. the same as the outer diameter of the pump beam, in this case the outer circle diameter is 100 μm. However, the three-dimensional conical pattern constituting the truncated areas varies the diameter of the holes in the cross-section at locations of different interaction lengths. And (3) designing a quasi-phase matching pattern according to the condition judgment of the formula (4), and then calculating the positions of different interaction lengths to correspond to the diameters of the inner circular mesopores of the cross sections.
In order to make the designed shaping structure have a cross-sectional size perpendicular to the y-axis of the transmission direction identical to the actual cross-sectional size of the pump beam in this example, the outer contour radius of the structure of the three-dimensional conical pattern nonlinear photonic crystal is determined, i.e. identical to the beam waist radius of the gaussian pump beam, in this example the beam waist radius ω of the gaussian pump beam x And omega z 50 μm. However, the three-dimensional conical pattern that constitutes the hollows of the truncated areas varies in cross-sectional radius at different interaction lengths. This requires calculation of the radius of the three-dimensional conic pattern cross section according to the condition of equation (4).
The method for calculating the radius of the cross section of the three-dimensional conical pattern is as follows: in this example, the wavelength of the pump beam is 1030nm, the wavelength of the doubling frequency is 515nm, the refractive indexes of the sample corresponding to the pump beam and the doubling frequency are 2.149 and 2.2305 respectively, and the sample can be obtained by quasi-phase matchingThe period to the second order nonlinear coefficient change is 6.3 microns. Along with the length L of the nonlinear photonic crystal quasi-phase matching grating 0 The number of quasi-phase matched grating periods increases along the y-axis. The length L of the nonlinear photonic crystal quasi-phase matching grating corresponding to any section can be determined by knowing the period value and the corresponding period number 0 . At the part of the beam cross section larger than the cut-off value a, the interaction length L is equal to the length L of the corresponding nonlinear photonic crystal quasi-phase matching grating 0 . In the part of the beam cross section smaller than the cut-off value a, the position of any point on the beam cross section can be determined to correspond to the actual interaction length L according to the intensity of the pump beam and the intensity of the target flat-top frequency doubling. The magnitude of the cutoff value a at the different interaction lengths L is then in fact the radius of the holes in the inner circle of the cross-section of the three-dimensional conical pattern.
Step 2: based on the shaping structure, the ferroelectric crystal is modified by femtosecond laser, and the nonlinear photonic crystal with the three-dimensional conical pattern is prepared, wherein the three-dimensional nonlinear photonic crystal is used as a nonlinear optical device.
Step 2, focusing the femtosecond laser into the ferroelectric crystal to process the beam shaping nonlinear photonic crystal designed in the step 1 and used for the three-dimensional Gaussian conversion top.
The femtosecond laser writing system produces a three-dimensional conical pattern nonlinear photonic crystal in a sample of z-cut 5% magnesium oxide doped lithium niobate crystal. The light source is a fs-pulse laser (pulse width: 190fs; repetition rate: 200 kHz), from a regeneratively amplified Yb: and the wavelength emitted by the KGW laser system is 1026nm. The sample is placed on a high-precision displacement bracket. The laser was focused by a 50 x microscope objective (na=0.42) and the spot size of the processing focal plane was about 1 μm. During processing, the femtosecond laser is polarized along the x-direction of the crystal. The key to preparing a three-dimensional nonlinear photonic crystal is to maintain the overall uniformity of the structure. In order to compensate for focal spot distortion caused by aberrations and absorption losses, the pulse energy needs to be adjusted with increasing depth. The pulse energy can be continuously adjusted by an attenuator consisting of a half-wave plate and a polarizing plate. Starting at a depth of 120 microns below the crystal + z surface, towardAnd (5) processing 20 layers. The pulse energy from bottom layer to top layer is controlled according to the depth from 10 muJ to 2.5 muJ. The scanning speed of the writing process was 100. Mu.m.s -1 。
Fig. 2 is a schematic diagram of a beam shaping three-dimensional nonlinear photonic crystal prepared by femtosecond laser according to the present invention, and the length of a scale is 30 micrometers, wherein (a) in fig. 2 is a side view of a beam shaping structural unit, (b) in fig. 2 is a front view of the beam shaping structural unit, and (c) - (f) in fig. 2 are three-dimensional nonlinear photonic crystals for nonlinear flat-top beam shaping under different cut-off values. After laser processing, a three-dimensional conical pattern nonlinear photonic crystal and a partial profile of its x-y cross section were observed by a double frequency confocal microscope, as shown in fig. 2. Fig. 2 (a) and (b) show the units of the three-dimensional conical pattern nonlinear photonic crystal. According to the quasi-phase matching condition, the period is Λ when the wavelength of the pump beam is 1030nm y =6.3 μm. It can be seen that the unit thickness along the y-axis is much smaller than the period length in (a) of fig. 2. As shown in FIG. 2 (b), it is a circle having a radius of 50 μm in the x-z plane, which corresponds exactly to the cross section of the beam.
The inner diameter of which is calculated according to the interaction length formula described in the above, and which changes as the interaction length increases. Fig. 2 (c), (d), (e) and (f) show different conical pattern quasi-phase matched gratings after processing according to the design algorithm described in step 1 to produce flat-top doubling with cut-off points of a=0, a= ±0.5 ω, a= ±0.75 ω and a= ±0.8 ω, respectively. Femtosecond laser pulse modification can lead to permanent changes of second-order nonlinear coefficients and refractive indexes of crystals. Obviously, in the achievement of the invention, the area of the crystal modified by the femtosecond laser pulse presents a clear and complete periodic structure, has uniform morphology and good consistency in all directions.
Step 3: and carrying out characteristic measurement on the emergent flat-top second harmonic beam, and verifying the performance of the nonlinear optical device based on the obtained measurement result.
Step 3, the pump beam is incident to the nonlinear photonic crystal prepared in step 2, and the performance of the nonlinear optical device is verified by measuring the characteristics of the emergent target flat-top beam.
The Gaussian pump beam has a beam waist radius of 50 μm, a wavelength of 1030nm, a pulse duration of 270fs and a repetition frequency of 300Hz. The same applies to normal incidence and reverse incidence of the pump beam along the y-axis direction of the three-dimensional patterned nonlinear photonic crystal with periodic distribution. The y-axis of the sample is set along the propagation direction of the input light and the z-axis is set along the vertical direction. The image acquisition used a 30-ten-thousand-pixel digital CCD camera with a resolution of 640×480 and a pixel size of 7.4 μm×7.4 μm. When power measurement is required, the CCD will be replaced by a power meter. The minimum resolution accuracy of the power meter is 0.1 μw. The pump light is respectively and independently incident on the structures (c) - (f) in fig. 2.
Fig. 3 is a schematic diagram of the results of the flat-top frequency doubling intensity distribution and the normalized intensity contour distribution obtained by the present invention, specifically, the result display of the frequency doubling intensity distribution when the pump beam power is 0.082mW, fig. 3 (a) is the flat-top frequency doubling with the cutoff value of 0, fig. 3 (b) is the flat-top frequency doubling with the cutoff value of 0.5, fig. 3 (c) is the flat-top frequency doubling with the cutoff value of 0.75, and fig. 3 (d) is the flat-top frequency doubling with the cutoff value of 0.8. I.e. the intensity distribution of the doubling and the normalized intensity contour distribution are shown in fig. 3 (a) - (d). It can be seen that the flat top area on the cross section of the doubled light beam is gradually enlarged with the increase of the cut-off value.
Fig. 4 (a) - (b) are normalized intensities obtained from a single column of pixels of a CCD camera in the center of the spot by generating flat-top doubling, wherein the dashed line represents the theoretical predicted value of the truncated beam plane portion. The normalized intensity shown in fig. 4 (a) is a vertical pixel column along the x-axis (i.e., the lateral axis), the normalized intensity shown in fig. 4 (b) is a vertical pixel column along the z-axis (i.e., the longitudinal axis), and the normalized intensity shown in fig. 4 (c) is the input fundamental wavelength 1030nm, and the output flat-top frequency doubling power after beam shaping by the cutoff value 0.5 is related to the pump power. When the pump power is 1.027mW, the output frequency doubling power reaches 0.0125mW, and (d) in FIG. 4 is the microscopic Raman signal of the processing region and the crystal region.
The beam shaping structures with cutoff values a= ±0.5 ω, a= ±0.75 ω, and a= ±0.8 ω can produce flat doubling in two dimensions with normalized peak intensities of 0.363, 0.125, and 0.085 in the horizontal direction and 0.359, 0.121, and 0.081 in the vertical direction, respectively. The intensity of the target beam obtained in the invention is close to the theoretical value. The flatness coefficient can generally be used to determine how similar an actual beam is to an ideal flat top beam. According to ISO13694, the calculation is dividing the average irradiance value by the maximum irradiance value. The uniformity of the beam was evaluated according to (a) and (b) in fig. 4. The flatness coefficients of the frequency doubles with cutoff values of a= ±0.5 ω, a= ±0.75 ω, and a= ±0.8 ω are 0.971, 0.933, and 0.948 in the horizontal direction, and 0.966, 0.931, and 0.909 in the vertical direction, respectively. The closer the flatness coefficient of the beam is to 1, the better the flatness and uniformity of the beam are indicated.
Numerical calculation and experimental measurement are carried out on the flat-top distribution double-frequency power with the cutoff value of a plus or minus 0.5 omega. The relationship between the output power of the frequency doubling and the pump power is shown in fig. 4 (c). This relationship conforms to a quadratic curve. When the pump input power is 1.027mW, the power conversion efficiency of the three-dimensional beam shaping nonlinear photonic crystal with the cutoff value of a= ±0.5ω is about 1.22×10 -2 . Increasing the interaction length of quasi-phase matching, increasing the fundamental input energy, and adjusting the duty cycle helps to increase conversion efficiency. In the present invention, the conversion efficiency is affected by a minute change in refractive index caused by scattering and diffraction. Fig. 4 (d) shows raman spectra of the processed region and the unprocessed crystal region. The raman signal of the modified region of processing is reduced, which indicates that the laser radiation alters the physical structure of the region, possibly forming amorphous components.
In summary, the pump beam is incident to the nonlinear optical device prepared by the invention, so that the target beam (with high flatness and high nonlinear conversion efficiency) can be obtained, and the frequency doubling of the flat-top distribution is realized on the full-wave surface.
Finally, it should be noted that the above-mentioned embodiments are only for illustrating the technical solution of the present invention, and not for limiting the same, and although the present invention has been described in detail with reference to examples, it should be understood by those skilled in the art that modifications and equivalents may be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention, and all such modifications and equivalents are intended to be encompassed in the scope of the claims of the present invention.
Claims (10)
1. A method of manufacturing a nonlinear optical device, comprising the steps of:
step 1, designing a shaping structure of a nonlinear photonic crystal with a three-dimensional conical pattern according to the distribution characteristic of a pump beam and a quasi-phase matching mechanism;
step 2, based on the shaping structure, utilizing femtosecond laser to modify the ferroelectric crystal to prepare a nonlinear photonic crystal with a three-dimensional conical pattern, wherein the nonlinear photonic crystal is used as a nonlinear optical device;
when the pump beam is incident to the nonlinear optical device, a flat-top second harmonic beam can be generated, and meanwhile, the flat-top beam and double frequency can be realized.
2. The method of manufacturing a nonlinear optical device according to claim 1, wherein the distribution characteristics of the pump beam satisfy gaussian distribution, the outer diameter of the shaping structure is the same as the outer diameter of the pump beam, and the radius of the holes in the inner circle of the corresponding cross section of the three-dimensional conical pattern constituting the truncated area is varied at positions of different interaction lengths; quasi-phase matching is performed along the propagation direction y-axis, the flat-top beam is shaped in two dimensions of the x-axis and the z-axis, and the functional form of the interaction length is selected as follows:
wherein L (x, z) is the interaction length, L 0 Nonlinear photonic crystal quasi-phase matching grating length omega x And omega z Beam waist radii in x-direction and z-direction, respectively, of a pump beam with gaussian distribution; a is a cut-off value, and the value of a corresponds to the radius of the hole in the inner circle.
3. The method of manufacturing a nonlinear optical device according to claim 2, wherein the frequency doubling intensity of the flat-top second harmonic beam is expressed as follows:
wherein I is 2ω (x, z) is the doubling intensity, C is the material constant of the ferroelectric crystal, I 0 Is the square of the amplitude of the gaussian beam electric field complex amplitude.
4. The method of manufacturing a nonlinear optical device according to claim 1, wherein the focal energy of the femtosecond laser used is larger than a ferroelectric crystal damage threshold of a focusing position, and a modified region is generated on the surface or inside of the ferroelectric crystal by using the femtosecond laser.
5. The method of manufacturing a nonlinear optical device according to claim 1, wherein said ferroelectric crystal is transparent to a wavelength of said femtosecond laser.
6. The method of manufacturing a nonlinear optical device according to claim 1, wherein the femtosecond laser is a near infrared femtosecond laser.
7. The method for manufacturing a nonlinear optical device according to claim 1, wherein when the ferroelectric crystal is modified by the femtosecond laser, the pulse energy of the femtosecond laser is adjusted with the increase of the processing depth, focal spot distortion caused by aberration and absorption loss is compensated, and the overall uniformity of the manufactured nonlinear photonic crystal is ensured.
8. The method of manufacturing a nonlinear optical device according to claim 1, wherein the ferroelectric is irradiated with the femtosecond laserThe crystal is modified, so that the second-order nonlinear optical coefficient is modulated in a three-dimensional space; in nonlinear photonic crystals hundreds of microns long, nonlinear conversion efficiency reaches 10 -2 An order of magnitude; the flatness coefficient of the flat-top second harmonic beam is more than 90%.
9. The method of manufacturing a nonlinear optical device according to claim 1, further comprising:
and 3, performing characteristic measurement on the emergent flat-top second harmonic beam, and verifying the performance of the nonlinear optical device based on the obtained measurement result.
10. A nonlinear optical device prepared by the method of any one of claims 1-9.
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