Patent Application
20070217466
The present invention relates to a two-dimensional photonic crystal surface-emitting laser, which can be employed in an optical pickup whose spot size is equal to or smaller than the diffraction limit or a laser-beam processing having a high level of energy efficiency.
In the field of optical storage devices, reducing the spot size of the laser beam used for recording (or writing) information into a recording medium or restoring (or reading) the information from the recording medium is required to increase the recording density on the recording medium. Simply focusing the laser beam by a beam-focusing unit including one or more focusing lenses cannot make the spot size of the laser beam equal to or smaller than the diffraction limit determined by the wavelength of the laser beam and the numerical aperture of the beam-focusing unit. Accordingly, in recent years, many techniques for achieving a spot size smaller than the diffraction limit have been researched. Such techniques are called the super-resolution technique.
Non-Patent Document 1 discloses a laser beam suitable for reducing the spot size.
Non-Patent Document 2 discloses a method and device for producing a radial-polarized ring laser beam.
In this device, the He—Ne laser 21 generates a linearly polarized laser beam 1 (
After passing through the polarization-converting plate 27, the laser beam travels through the second focusing lens 28 and other components and reaches the half mirror 31, which reflects the laser beam toward the aperture 32. The aperture 32 has a ring-shaped transparent area 322, which allows the passage of light, and the blocking areas 321 and 323, which block the laser beam outside the transparent area 322. The aperture 32 gives the laser beam a ring-shaped cross section (
A radially polarized laser beam can be suitably used in the field of metal-processing using a laser beam as well as in the super-resolution technique. Non-Patent Document 3 discloses the result of a calculation, which proves that an irradiation of a radially polarized laser beam onto a metal makes the processing speed higher than in the case of using a circularly or linearly polarized laser beam having the same energy level. According to that document, this is because metals have higher energy-absorbing efficiencies for radially polarized light than other kinds of polarized light.
[Non-Patent Document 1] S. Quabis et al., “Focusing light to a tighter spot”, Optics Communications, vol. 179, pp. 1-7
[Non-Patent Document 2] R. Dorn et al. “sharper Focus for a Radially Polarized Light Beam”, Physical Review letters, vol. 91, No. 23, pp. 233901-1-233901-4
[Non-Patent Document 3] V. G. Niziev et al., “Influence of beam polarization on laser cutting efficiency”, Journal of Physics D-Applied Physics, vol. 32, No. 13, pp. 1455-1461
Problems to be Solved by the Invention
In the radial-polarized ring laser beam generator disclosed in Non-Patent Document 2, the four sections 271-274 of the polarization-converting plate 27 are designed to change the direction of polarization of the laser beam to a common direction within each of the four segments 361-364 of the beam's cross section (
The radial-polarized ring laser beam generator disclosed in Non-Patent Document 2 uses a complex optical system including a large number of optical components to produce a radially polarized beam from a linearly polarized beam generated by the laser light source. Accordingly, this radial-polarized ring laser beam generator is very costly.
Thus, an objective of the present invention is to provide a two-dimensional photonic crystal surface-emitting laser that can generate a radially polarized laser beam having a ring-shaped cross section (i.e. a radial-polarized ring laser beam) without using a complex optical system and thereby contribute to the reduction of the device cost.
Means for Solving the Problems
To solve the previously described problems, the present invention provides a two-dimensional photonic crystal surface-emitting laser, which is characterized by:
The two-dimensional photonic crystal surface-emitting laser according to the present invention can generate a radial-polarized ring laser beam by itself. Since there is no need to use a complex optical system for converting the polarization, the total production cost of the radial-polarized ring laser beam generator is reduced. The laser beam thereby produced is radially polarized at any point within its cross section. Accordingly, the diameter of the laser beam can be reduced to achieve a spot size smaller than the diffraction limit. The spot size thereby achieved can be smaller than that achieved by the device disclosed in Non-Patent Document 2. Thus, a super-resolution laser beam having a diameter equal to or smaller than the diffraction limit is obtained.
A device for generating a super-resolution laser beam having a diameter equal to or smaller than the diffraction limit can be constructed by combining a two-dimensional photonic crystal surface-emitting laser according to the present invention and one or more focusing lenses for focusing the laser beam generated by the aforementioned laser into a laser beam having a diameter equal to or smaller than the diffraction limit. This super-resolution laser beam generator can be used as a light source in an optical pickup for recording information on an optical storage medium with high density or restoring information recorded on the optical storage medium with high density.
Furthermore, the two-dimensional photonic crystal surface-emitting laser according to the present invention can be used in a laser-beam processing as a light source for casting light onto an object to be worked. Since the light generated by this laser is radially polarized, the energy of the laser beam is efficiently supplied into the metal, so that the metal can be worked (cut, engraved, etc.) at high speeds. In the case of using the present laser in a laser-beam processing, it is unnecessary to focus the laser beam to makes its diameter equal to or smaller than the diffraction limit.
FIGS. 2(a-c) are schematic diagrams showing the construction of a conventional device for producing a radial-polarized ring laser beam.
FIGS. 3(a) and 3(b) show the states of polarization of the laser beam observed while the beam is passing through the device shown in
FIGS. 5(a) and 5(b) are perspective views of two-dimensional photonic crystals 45 and 45′ used in the surface-emitting laser of the present embodiment.
FIGS. 6(a) and 6(b) show the results of calculations of the electromagnetic field distribution within the two-dimensional photonic crystals used in the two-dimensional photonic crystal surface-emitting laser according to the present invention.
The two-dimensional photonic crystal surface-emitting laser (which is simply called the “surface-emitting laser” hereinafter) according to the present invention has a two-dimensional photonic crystal located on one side of an active layer. A pair of electrodes is provided on both sides of the active layer and the two-dimensional photonic crystal. It is possible to add a spacer or similar member between the active layer, the two-dimensional photonic crystal and the electrodes.
The active layer may be the same as those conventionally used in conventional Fabry-Perot laser light sources. The two-dimensional photonic crystal in the present invention consists of a plate-shaped body material in which areas whose refractive index differs from that of the body material (which is called the “modified refractive index areas” hereinafter) are periodically arranged. The modified refractive index areas can be arranged in a square or triangular lattice pattern or some other pattern. An example of the modified refractive index area is a hole. This form is preferable in that it creates a large difference in refractive index between the modified refractive index area and the body material and is easy to manufacture. Alternatively, the modified refractive index area may be created by embedding some member into the body material. This form of modified refractive index area is suitable for preventing a heat deformation of the modified refractive index area, which can take place if the two-dimensional photonic crystal needs to be adhered to another layer at a high temperature during the manufacturing process. The modified refractive index area consisting of an embedded member is also suitable for the case where a new layer is to be epitaxially grown after the photonic crystal is created during the manufacturing process.
Minimally, the electrode located on the side closer to the active layer (i.e. the first electrode) must be a ring electrode having a hole at its center. Both the circumference and the hole of the ring electrode may be circular, square, hexagonal or any other form.
The surface-emitting laser according to the present invention operates as follows: When a voltage is applied between the first and second electrodes, an electric current flows into the active layer and causes an emission of light in that layer. This light forms a standing wave within the two-dimensional photonic crystal and is thereby amplified. As a result, a laser beam is generated to the direction perpendicular to the surface of the two-dimensional photonic crystal.
In the surface-emitting laser according to the present invention, since the first electrode located closer to the light-emitting layer is ring shaped, the electric current flowing into the light-emitting layer also has a ring-shaped field distribution in which the current in a circumferential zone around the center of the light-emitting layer is stronger than that at the center. Accordingly, the field distribution of the light emitted from that layer is also ring shaped. The light having such a field distribution creates a resonance mode in the two-dimensional photonic crystal, where the magnitude of the envelope of the amplitude of the electromagnetic waves is zero at the center of the crystal surface. The resultant laser beam has a ring-shaped cross section where the field strength is zero at its center. The direction of polarization of this laser beam is radial.
The present invention has published a paper titled “Nijigen Fotonikku Kesshou Menhakkou Leezaa No Denkyoku Kouzou Oyobi Jissou Houhou No Kaizen (Improvements of Electrodes Structure of Two-Dimensional Photonic Crystal Surface-Emitting Laser and Its implementation Method)” (Wataru KUNISHI et al., Preprints of the Symposia of the 66th Meeting of the Japan Society of Applied Physics in Autumn 2005, vol. 3, Symposium No. 9p-H-4), which discloses a two-dimensional photonic crystal surface-emitting laser using a window-shaped electrode having a central window (or hole). The ring electrode of the present embodiment differs from the window-shaped electrode disclosed in the aforementioned paper in that the former electrode is the first electrode, which is located closer to the active layer, whereas the latter is the second electrode, which is located farther from the active layer. Another difference exists in that the former electrode is intended to produce a radial-polarized ring laser beam, whereas the latter is intended to provide a passage for the laser beam. For the same reason as described in the aforementioned paper, it is desirable to use a window-shaped electrode as the second electrode of the present invention.
The radial-polarized ring laser beam generated by the surface-emitting laser according to the present invention can be focused by one or more focusing lenses to make its diameter smaller than the diffraction limit, as described in Non-Patent Document 1.
(1) Embodiment of the Two-Dimensional Photonic Crystal Surface-Emitting Laser According to the Present Invention
An embodiment of the surface-emitting laser according to the present invention is described with reference to FIGS. 4, 5(a) and 5(b).
The substrate 41 is considerably thicker than any other layers. This is to make the distance between the lower surface 461 of the contact layer 46 and the active layer 43 adequately smaller than the distance between the upper surface 411 of the substrate 41 and the active layer 43.
A first electrode 471 is provided at the center of the lower surface 461 of the contact layer 46. The first electrode 471 is a ring electrode having a central hole 4711 in which no electrode material is present. As explained earlier, the hole 4711 is intended to create a ring-shaped field distribution of the electric current flowing into the active layer 43. A second electrode 472 having a central window 4721 is located on the upper surface 411 of the substrate 41. The window 4721 is intended to be a passage for the laser beam emitted from the surface-emitting laser of the present embodiment.
The surface-emitting laser of the present embodiment operates as follows: When a voltage is applied between the first and second electrodes 471 and 472, holes and electrons flow into the active layer 43, in which the holes recombine with the electrons, causing an emission of light. Since the first electrode 471 located closer to the active layer 43 is ring shaped, the density distribution of the holes and electrons inside the active layer 43 is also ring shaped, and so is the emitted field. A specific wavelength component of light generated in the active layer 43 is intensified due to interference within the two-dimensional photonic crystal 45, causing a laser oscillation. The laser light thus generated is emitted from the surface of the substrate 41 through the window 4721 to the outside. The laser beam thus emitted is a radial-polarized ring laser beam.
The first electrode 471, which is a circular ring in
FIGS. 6(a) and 6(b) each show an electromagnetic field distribution within the two-dimensional photonic crystal of the surface-emitting laser, calculated by a finite difference time domain (FDTD) method, where (a) uses the two-dimensional photonic crystal 45 and (b) uses the two-dimensional photonic crystal 45′. The diagrams (a-1) and (b-1) at the centers of FIGS. 6(a) and 6(b) each show the entire photonic crystal. The other pictures surrounding the two diagrams are enlarged views of the sections indicated by the squares in (a-1) and (b-1). In the surrounding pictures, the gray shading (dark/light) shows the magnetic field strength (strong/weak) and the arrows indicate the oscillating directions of the electric field. Both FIGS. 6(a) and 6(b) show that the electric field is oscillating in the radial direction, from the center (indicated by “X” in FIGS. 6(a) and 6(b)) to the circumference of the two-dimensional photonic crystal.
(2) Reference Example
In many two-dimensional photonic crystal surface-emitting lasers, the active layer 63 is designed to generate a TE-polarized light whose electric field oscillates in a direction parallel to that layer because this design can achieve a large gain. In this case, the laser oscillation is produced by generating a TE-polarized light in the active layer and causing a diffraction and interference of that light within the two-dimensional photonic crystal. The resultant laser beam emitted from the two-dimensional photonic crystal has a ring-shaped cross section in which the electric field of the light oscillates in the tangential direction. In contrast, in the reference example, the laser oscillation is produced by generating a TM-polarized light in the active layer and causing a diffraction and interference of that light within the two-dimensional photonic crystal. The resultant laser beam emitted from the two-dimensional photonic crystal has a ring-shaped cross section in which the magnetic field of the light oscillates in the tangential direction. Since the oscillating direction of the magnetic field is perpendicular to that of the electric field, the oscillating direction of the electric field in this laser beam, i.e. the direction of polarization, coincides with the radial direction. Thus, it is possible to produce a radial-polarized ring laser beam without using the ring-shaped first electrode.
An example of the active layer for generating a TM-polarized light is GaInAs/GaInAsP.
FIGS. 10(b-1)-10(b-4) are photographs each showing a cross section of the laser beam observed after the beam has passed through a polarizing filter that allows the passage of only a component of light polarized in the direction indicated by the arrows 711-714. Any of these photographs shows that the light is passing only at specific portions 731-734 where the direction of the radial polarization (i.e. the direction indicated by the arrows 72) coincides with the direction 711-714 of the component of light that can pass through the polarizing filter. These photographs prove that the laser beam hereby obtained is radially polarized.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-077470 (P) | Mar 2006 | JP | national |
