JP4578710B2 - Method of creating domain-inverted structure by femtosecond laser irradiation - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method of creating a periodically poled structure in a ferroelectric material by interfering femtosecond laser pulsed light having a high energy density, in particular, a poled structure having a fine periodic interval, and the periodically poled structure. The present invention relates to a wavelength conversion optical device to which is applied.
[0002]
[Prior art]
In wavelength conversion using the nonlinear optical effect, it is necessary to achieve phase matching between the fundamental wave and the wavelength converted wave. However, by periodically reversing the polarization direction of the domain in the ferroelectric, Wavelength conversion can also be performed by matching. In such a quasi phase matching element, restrictions on applicable materials and fundamental wave wavelengths can be removed.
[0003]
In addition, since the optical density can be kept high over a long propagation distance in the waveguide type element, the “waveguide type pseudo phase conversion element” can realize wavelength conversion with high efficiency.
[0004]
When the domain polarization inversion period is Λ, the quasi-phase matching condition for generating the second harmonic is given by equation (1).
[0005]
Λ = (2m + 1) λ _p / 2 (n (λ _p ) −n (λ _sh )) (1)
Here, .lambda.p: wavelength of the fundamental wave, Ramudash: the second-harmonic _{wavelength, n (λ p), n} (λ sh) , the effective refractive index for each wavelength, m: is an integer.
[0006]
As a method of fabricating a periodic inversion structure in a ferroelectric,
(1) A method of diffusing Ti or Li ₂ O into a LiNbO ₃ crystal and heat-treating at 1000 to 1100 ° C. [ Non-patent Documents 1 and 2 ],
(2) A method in which pyrophosphoric acid is used for -Z of LiNbO ₃ , proton exchange is performed at a temperature of about 260 ° C., and then heat treatment is performed at 500 to 600 ° C. [ Non-patent Document 3 ], and KTiOPO ₄ ( KTP) method for ion exchange of crystals with Rb / Ba salt [ Non-Patent Document 4 ],
(3) A method of irradiating a focused electron beam on a −z plane of LiNbO ₃ or LiTaO ₃ crystal at room temperature [ Non-patent Document 5 ] and a method of irradiating an ion beam [ Non-patent Document 6 ],
(4) A method of applying a pulse voltage by providing a periodic electrode on one side of a z-cut LiNbO ₃ or LiTaO ₃ crystal and a uniform electrode on the opposite side [ Non-patent Document 7 ],
Has been reported.
[Non-Patent Document 1]
Nishihara et al. Opt. Lett. 16 (1991) 375
[Non-Patent Document 2]
J. et al. Webjorn et al. IEEE Photonics Tech Lett. 1 (1989) 316
[Non-Patent Document 3]
K. Mizuchi et al. Appl. Phys. Lett. 60 (1992) 1283
[Non-Patent Document 4]
C. van der Poel Appl. Phys. Lett. 57 (1990) 2074
[Non-Patent Document 5]
M.M. Yamada et al. Electron. Lett. 27 (1991) 1868
[Non-Patent Document 6]
K. Mizuchi et al. Electron. Lett. 29 (1993) 2064
[Non-Patent Document 7]
M.M. Yamada et al. Appl. Phys. Lett. 62 (1993) 435
[0007]
[Problems to be solved by the invention]
In the above methods (1) and (2) , the crystal needs to be treated at a high temperature, and is accompanied by a chemical reaction, so the process is complicated and it is difficult to reduce the polarization period interval. In the above method (3) , since electrons and ions have electric charges, a high voltage is required to process a ferroelectric material that is an electrical insulator, and a beam is scanned. In order to do so, the process takes a long time. In the method (4) , since it is necessary to provide an electrode, the process becomes complicated, and since the polarization inversion period is determined by the period interval of the electrode, the minimum period is limited to about 1 μm. There is a problem. Many methods have been developed to create a domain-inverted periodic structure, but no method using light has been developed so far.
[0008]
[Means for Solving the Problems]
The present invention reverses the ferroelectric polarization domain using a strong electric field of a high-density laser pulse. If the focused laser beam is used, the polarization in the ferroelectric can be reversed at room temperature without any chemical reaction. Since light has no electric charge, it is suitable for processing a ferroelectric material that is an electrical insulator.
[0009]
That is, according to the present invention, two femtosecond laser pulses having a high energy density of 0.1 TW / cm ² or more and interfering with each other are applied to the z-plane of the ferroelectric crystal, and the electric vector of the interference laser light is below the grating the short periodic domain inversion structure formed in the ferroelectric crystal surface having a spacing 5~0.5μm the ferroelectric crystal surface by irradiation so as to be parallel to the polarization direction This is a method for creating a domain-inverted structure by femtosecond laser irradiation.
[0010]
The present invention is also the above method for producing a domain-inverted structure , wherein an optical waveguide is formed on a ferroelectric crystal and the laser pulse is irradiated onto the optical waveguide .
[0011]
The present invention uses LiNbO ₃ , Mg-doped LiNbO ₃ , LiTaO ₃ , KTiOPO ₄ (KTP), β-Ba ₂ B ₂ O ₃ (BBO), or LiB ₂ O ₅ (LBO) as the ferroelectric. This is a method for producing a domain-inverted structure by femtosecond laser irradiation.
[0012]
The present invention also provides a harmonic generation element, a sum frequency generation element, or a difference frequency generation element using the short-period domain inversion structure created by the above method.
[0013]
The present inventors have previously developed a two-beam hologram exposure method using a femtosecond laser in place of the conventional laser beam irradiation method using a photosensitive material. Using the characteristics of energy density and coherence, holograms can be recorded with a pair of pulsed light branched from a single pulse on transparent organic, inorganic, semiconductor, or metal materials that are not inherently photosensitive. The method was realized and a patent application was filed (Japanese Patent Application No. 2000-314715).
[0014]
In this method, a femtosecond laser having a pulse width of 900 to 10 femtoseconds, a peak output of 1 GW or more, and a Fourier limit or an approximation thereof is used as a light source, and a pulse from the laser is divided into two by a beam splitter. The beam is temporally controlled via an optical delay circuit, and the reflection surface that rotates slightly is spatially controlled by using a flat mirror and a concave mirror, and the hologram is recorded on the substrate surface or inside the substrate. Ablation of the substrate material caused by high-density energy irradiation by collimating the plane of polarization, condensing at an energy density of 100 GW / cm ² or more, and matching the focused spots of the two beams in time and space Transparent material due to change in shape of substrate surface and / or change in refractive index of substrate material due to change in atomic arrangement structure of substrate material A method for producing a hologram by irreversibly two-beam laser interference exposure method characterized by recording a hologram on a semiconductor material or a metal material.
[0015]
By using the interference of the laser light by the two-beam laser interference exposure method, direct, but can record the periodic uneven structure on the surface of the material, the lower side of the concavo-convex structure by the method of the present invention, periodically poled Is formed. From the characteristic of the interference of laser light, it is possible to control the periodic interval in the range of 5 to 0.5 μm by changing the incident angle of light.
[0016]
Further, by changing the irradiation dose rate and pulse energy density of the pulse, it is possible to control the area of the polarization inversion. The wavelength conversion efficiency depends on the ratio of the inversion domain to the non-inversion domain, and becomes maximum when the value is 1: 1.
[0017]
Therefore, the intensity of the second harmonic of the fundamental wave incident on the waveguide was monitored, as its value is maximized, the number of times the illumination laser pulse, be determined pulse energy density, the element having the maximum wavelength conversion efficiency Can be produced.
[0018]
DETAILED DESCRIPTION OF THE INVENTION
FIG. 1 is a conceptual diagram of a system of a laser exposure apparatus used in the method of the present invention. In this system, a femtosecond laser light source, a beam splitter for splitting the pulse beam from the laser into two, an optical delay circuit for temporally controlling the convergence position of the pulsed light, and spatial control The optical system includes a plane mirror, a concave mirror, and a mechanism for finely rotating the mirror. By slightly moving the position of the mirror in the direction perpendicular to the mirror surface and parallel to and perpendicular to the incident beam, the optical path length can be changed to provide an optical delay circuit. The converging position of the condensing of the two beams incident on the base material and the size of the condensing spot can be controlled by an optical delay circuit and a mirror.
[0019]
The two-beam laser exposure system requires an optical system that can control the position on a micron scale, and as an apparatus with high-accuracy position control that can handle this, an optical delay circuit capable of fine control and a plane that can be rotated slightly The two beams are condensed on the substrate or inside the substrate by the optical system having both the mirror and the concave mirror and the function of detecting the presence or absence of the convergence of the two beams. This makes it possible to match the focused spot.
[0020]
In the system shown in FIG. 1, a femtosecond pulse laser of a titanium sapphire mode-locked laser is regenerated and amplified to a maximum energy of ˜1 mJ and a time width of ˜100 fs, and the femtosecond laser beam obtained is ˜100 μm in diameter on the surface of a LiNbO ₃ crystal. The laser beam is condensed to an extent (the energy density of this laser beam is 100 TW / cm ² ).
[0021]
When the z-plane of a single-domain LiNbO ₃ crystal is irradiated with a laser pulse in an arrangement in which the electric vector of the laser beam is parallel to the polarization direction , the energy density is 100 GW / cm ² or more, preferably 10 TW / cm ^{When it is 2} or more, polarization inversion is observed with one pulse irradiation.
[0022]
The size of the polarization domain increases as the energy density increases. Moreover, when irradiated superimposed laser pulses in the same location, as the number of times of irradiation pulses is increased, the size of the domain increases.
[0023]
The inversion structure can be confirmed by utilizing the fact that, when immersed in a hydrofluoric acid mixed solution, the −z plane is etched, but the + z plane is not etched. When laser light is irradiated on a surface other than the z plane, no polarization inversion is observed. This shows that a domain having a reverse polarization is induced by the strong electric field of the high-density energy pulse.
[0024]
Similar to the method described in the above Japanese Patent Application No. 2000-312715, a femtosecond laser having a pulse width of 900 to 10 femtoseconds, a peak output of 1 GW or more, and a Fourier limit or approximating it can be used as a light source, and a pulse from the laser can be used. The beam is split into two by a beam splitter, the two beams are temporally controlled via an optical delay circuit, and the micro-rotating reflecting surface is spatially controlled using a flat mirror and a concave mirror, By condensing the polarization plane parallel to the surface or the inside of the substrate and matching the focused spots of the two beams in time and space, a high density energy of 0.1 TW / cm ² or more can be obtained. Irradiate the ferroelectric.
[0025]
At this time, under the condition that a hologram diffraction grating can be formed, when the interference femtosecond laser is irradiated on the z-plane of the ferroelectric LiNbO ₃ crystal so that the electric vector of the interference laser light is parallel to the polarization direction of the crystal, A periodic domain-inverted domain having a depth of about 5 μm can be formed below the formed surface concavo-convex diffraction grating.
[0026]
That is, the regenerated and amplified pulse is separated into two beams, re-associated on the surface of the LiNbO ₃ crystal, the electric vector of the laser light is parallel to the polarization direction , and the energy sum of the two beams is 0.1 TW / cm ^2. At this time, a periodically domain-inverted domain having a depth of about 5 μm is formed below the diffraction grating. It is considered that polarization reversal occurs due to an electric field of irradiation laser light, local heat, strain, or shock wave.
[0027]
The fringe interval d of the interference hologram diffraction grating is given by d = λ / 2 sin θ (λ is a laser wavelength = 800 nm, 2θ is an angle formed by two beams). Therefore, d can be controlled between 0.5 and 5 μm.
[0028]
Even if Mg-doped LiNbO ₃ , LiTaO ₃ , KTiOPO ₄ (KTP), β-Ba ₂ B ₂ O ₃ (BBO), or LiB ₂ O ₅ (LBO) is used as the ferroelectric crystal, the same periodic polarization inversion structure is used. Can be formed.
[0029]
According to the method of the present invention, light is optically dispersed in a single-domain nonlinear optical crystal substrate such as LiNbO ₃ , LiTaO ₃ , KTP, BBO, and LBO by a known technique such as selective diffusion of impurities or ion implantation or ion exchange. A waveguide can be produced.
[0030]
As for the orientation of the crystal substrate and the propagation direction of the optical waveguide, the quasi phase matching can be realized in any orientation as long as the polarization orientation of the nonlinear crystal and the light propagation orientation, that is, the orientation of the waveguide do not match. From the viewpoint of efficiency, it is desirable to take a combination in which the waveguide propagation direction is orthogonal to the crystal main axis having the maximum nonlinear constant.
[0031]
Next, an interference femtosecond laser is irradiated to the optical waveguide produced on the nonlinear optical crystal substrate so that the propagation direction of the waveguide and the hologram fringe are orthogonal to form periodic domain inversion. If the fringe period Λ is matched with the equation (1), the pseudo phase condition is satisfied, and wavelength conversion by second order harmonic generation can be performed. Depending on the combination of crystal orientation, wavelength, and refractive index, wavelength conversion of sum frequency, difference frequency, etc. is possible by appropriately selecting the fringe interval. By changing the angle ψ formed by the waveguide and the fringe, the effective grating period Λ / cos (ψ) sensed by the light propagating in the waveguide can be controlled to satisfy the quasi phase matching condition.
[0032]
Even if a waveguide is not formed on the dielectric crystal, if a focused beam or parallel beam is incident on a dielectric crystal substrate having a domain-inverted structure with a lens or the like, wavelength conversion such as second harmonic generation occurs. Can be done. Also in this case, it is possible to realize the wavelength conversion by realizing the pseudo phase matching by either the method of adjusting the angle formed by the two beams to be interfered with or the method of adjusting the angle between the beam to be converted and the fringe. .
[0033]
Introduce a fundamental wave into the waveguide, monitor the wavelength-converted light intensity, and finely adjust the energy density of the laser pulse, the number of paths, and the angle between the waveguide and the diffraction grating fringe to maximize the intensity. Thus, an optimal periodic polarization inversion structure can be formed.
[0034]
After producing the domain-inverted structure, the waveguide is activated in parallel, and a second domain-inverted structure is produced adjacent to the first structure. By repeating this procedure, the domain-inverted region can be expanded and the wavelength conversion efficiency can be improved. Further, if the fringe interval of each domain-inverted structure is slightly changed, the wavelength width of the fundamental wave to be quasi-phase matched can be widened.
[0035]
【Example】
[Example 1]
A two-beam laser interference exposure system shown in FIG. 1 was used. That is, the laser is a regenerative amplification titanium sapphire laser, the oscillation center wavelength is ˜800 nm, the pulse width is ˜100 femtoseconds, the pulse energy is ˜4 mJ / pulse, and the peak output is ˜40 GW.
[0036]
The laser beam is divided into two by the half mirror HF1, and is condensed on the surface or inside of the sample S1 by the lens L1 and the lens L2. An optical circuit was installed in the optical path for the beam B1 or the beam B2, and the condensed spots of the two beams were matched temporally and spatially.
[0037]
The size φ of the focused spot is ˜100 μm, and the peak power density is calculated to be ˜400 TW / cm ² . The incident angle θ of the beam B1 and the beam B2 on the sample S1 was adjusted to change the fringe interval of the diffraction grating. The sample S1 is placed on the XY stage, the sample S1 is finely moved, and a hologram having a very small area can be recorded at a designated position of the sample S1. Using this two-beam laser interference exposure apparatus, a transmission hologram was recorded on a LiNbO ₃ single crystal.
[0038]
The size of the LiNbO ₃ single crystal was 10 × 10 × 1 mm, and the laser beam was incident so that the electric vector direction of the laser light was parallel to the crystal axis Z (polarization axis). The laser output was -200 mJ / pulse, the beam B1 was 100 mJ, the beam B2 was 100 mJ, the beam diameter was condensed to about 50 μm (energy density: 10 TW / cm ² ), and a hologram was recorded with one pulse. The angle θ formed by the beams B1 and B2 was 30 degrees. As shown in FIG. 2, diffraction gratings each having a grating interval of 1 μm were obtained. From the AFM measurement, the depth of the hologram fringe formed on the surface was about 200 nm.
[0039]
Further, a cross section of a sample etched with hydrofluoric acid (50% hydrofluoric acid + 50% nitric acid) at 60 ° C. for 1 hour was observed with an optical microscope. Fine etching marks were observed down to the depth t = about 5 μm below the surface grating. FIG. 3 is a cross-sectional view showing a polarization inversion structure formed on the lower side of the diffraction grating. This confirmed the formation of a domain-inverted structure with a fine period by a femtosecond laser under the diffraction grating .
[0040]
[Example 2]
As shown in FIG. 4, a plurality of optical waveguides 2 having the same width and depth for propagating light in the Y direction are formed on the X plane of the LiNbO ₃ crystal substrate 1 by the Ti diffusion method, and light input / output is performed. Both ends of the waveguide were polished for use. Irradiation was performed on each waveguide while changing one beam of B1 and B2 focused to a beam diameter of about 50 μm, one pulse at a time to a beam crossing angle θ = 5 to 80 degrees.
[0041]
When a laser is incident from the light source 4 having a wavelength of 840 nm through the polarizer 5 from these waveguide end faces, the light emitted from the waveguide end face 6 is red in the waveguide irradiated at the beam crossing angle θ = 17 degrees. When measured with an optical power meter 8 through an outer cut filter, emission of green light having a wavelength of 420 nm was obtained, and it was confirmed that the device functions as a wavelength converter.
[0042]
When the domain inversion formed on the waveguide 2 is immersed in a hydrofluoric acid solution and etched, irregularities with a period of less than 3 μm consisting of an inversion part with a width of less than 1 μm and a depth of about 5 μm and a non-inversion part with a width of about 2 μm are obtained. Observed and confirmed that a periodic domain inversion was actually formed.
[0043]
[Example 3]
Similar to Example 2, a plurality of optical waveguides having the same width and depth for propagating light in the Y direction are formed on the X surface of the LiNbO ₃ substrate by the Ti diffusion method, and are used for light input / output. Both ends of the waveguide were optically polished. While monitoring the second harmonic output beam incident wavelength tunable laser with a wavelength of 840nm to these waveguides, by changing the number of irradiation times of the pulse, it was creation of the second harmonic generation element. According Set femtosecond laser beam crossing angle theta = 17 degrees, focused beam diameter of about 50μm on each waveguide was subjected to irradiation of 1 to 10 times the pulse, increasing the number of times of irradiation pulses, green It was confirmed that the light output intensity increased and eventually began to decrease.
[0044]
When the waveguide A irradiated with the pulse having the maximum second harmonic emission intensity three times and the waveguide B irradiated with the pulse ten times are immersed and etched in a hydrofluoric acid solution, an inversion portion and a non-inversion portion The ratio was approximately 4: 5 for waveguide A, but 2: 1 for waveguide B.
[0045]
In the quasi-phase matching, when the domain inversion period is constant, the ratio of the inversion part to the non-inversion part (Duty ratio) affects the second harmonic generation efficiency, and the maximum first in the case of Duty ratio = 1: 1 . It is known that second harmonic generation efficiency can be obtained. Phenomenon in which the output intensity is dependent on the number of times of pulse irradiation can be interpreted for Duty ratio with pulse irradiation times is increased. Therefore, the control is capable of adjusting a Duty ratio by number of times easily irradiation pulse can be obtained easily better second harmonic generation efficiency.
[Brief description of the drawings]
FIG. 1 is a conceptual diagram of a two-beam laser exposure system used in the method of the present invention.
FIG. 2 is an explanatory diagram of a diffraction grating recorded by a two-beam laser exposure system.
FIG. 3 is a cross-sectional view showing a domain-inverted structure formed on the lower side of the recorded diffraction grating shown in FIG. 2;
4 is a conceptual diagram of fabrication of a waveguide type quasi phase matching wavelength conversion element by a two-beam femtosecond laser beam interferometry in Example 2. FIG.
FIG. 5 is a conceptual diagram illustrating a relationship between a domain inversion region, a crystal axis, and a guided light propagation direction of a waveguide type quasi phase matching wavelength conversion element according to a third embodiment.

Claims (4)

Two femtosecond laser pulses having a high energy density of 0.1 TW / cm ² or more and interfering with each other are in the z plane of the ferroelectric crystal, and the electric vector of the interference laser light is parallel to the polarization direction of the crystal. and characterized by creating a short-period domain inversion structure with the spacing 5~0.5μm the ferroelectric crystal surface on the lower side of the diffraction grating formed in the ferroelectric crystal surface by irradiation so as To create a domain-inverted structure by femtosecond laser irradiation. 2. The method for producing a domain-inverted structure according to claim 1, wherein an optical waveguide is formed in a ferroelectric crystal, and the laser pulse is irradiated onto the optical waveguide. The ferroelectric material is LiNbO ₃ , Mg-doped LiNbO ₃ , LiTaO ₃ , KTiOPO ₄ (KTP), β-Ba ₂ B ₂ O ₃ (BBO), or LiB ₂ O ₅ (LBO). Item 3. A method for producing a domain-inverted structure by femtosecond laser irradiation according to Item 1 or 2. A harmonic generation element, a sum frequency generation element, or a difference frequency generation element using the short-period domain inversion structure created by the method according to claim 1 .

Images