JP2007234721A - Vertical cavity surface emitting laser - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a vertical resonator type surface-emitting laser in a structure where the lamination of an addition element, or the like cannot be blocked when composing the vertical resonator type surface-emitting laser having reflecting mirrors composed of two-dimensional photonic crystal. <P>SOLUTION: In the vertical resonator type surface-emitting laser having an active layer between first and second reflecting mirrors, the first and second reflecting mirrors are composed of a two-dimensional photonic crystal having a two-dimensional periodic structure. A two-dimensional periodic structure 102 at least in one reflecting mirror is arranged in a two-dimensional period while a substance having a low refraction factor is buried into a substance 101 having a high refraction factor. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a vertical cavity surface emitting laser (Vertical Cavity Surface Emitting Laser).

As one of the surface emitting lasers, a vertical cavity surface emitting device in which both sides of the active region are sandwiched between two reflecting mirrors, an optical resonator is formed in a direction perpendicular to the substrate surface, and light is emitted in a direction perpendicular to the substrate surface. Lasers are known.
This vertical cavity surface emitting laser has been actively researched because it has many features as follows.
That is, the beam shape is close to a circle and optical coupling with an optical fiber is easy, wafer inspection can be performed without cleavage, operation at a low threshold value of around 0.1 milliampere, and high-speed modulation are possible. It has many features such as being advantageous for in-plane integration.
Although the vertical cavity surface emitting laser has many of these features, it has the following problems.
That is, a distributed Bragg reflector (hereinafter referred to as DBR) is generally used as a reflector for a vertical cavity surface emitting laser.
However, in order for such a DBR to obtain a high reflectivity necessary for laser oscillation, several tens of layers are required, and there is a problem that the thickness hinders current injection and heat dissipation.

Conventionally, as a configuration example of a vertical cavity surface emitting laser using a reflecting mirror other than a DBR, for example, Non-Patent Document 1 proposes the following.
Here, a calculation study is performed on a vertical cavity surface emitting laser having a configuration in which the DBR on one side is replaced with a diffraction grating (a kind of one-dimensional photonic crystal).
As a result, it has been reported that good resonator characteristics comparable to the conventional structure using DBR on both sides are obtained.

Further, in Non-Patent Document 2 and Non-Patent Document 3, studies are made on reflected light and transmitted light when light is incident on a two-dimensional photonic crystal slab from a direction perpendicular to the surface.
It has been reported that light of a certain frequency is reflected with an efficiency of almost 100%.
According to these, similarly to the diffraction grating, it is considered that a vertical cavity surface emitting laser can be configured using a two-dimensional photonic crystal slab as a reflecting mirror.
In addition, when a two-dimensional photonic crystal is used rather than a one-dimensional photonic crystal, the light in the in-plane direction can be controlled more efficiently, so that a higher-performance vertical cavity surface emitting laser can be realized. Conceivable.
Optics Express, Vol. 11,1799 Physical Review B, Vol. 65,235112 Optics Express, Vol. 13,6564

In consideration of the non-patent document 2 and the non-patent document 3 described above, a vertical cavity surface emitting laser as shown in FIG. It can also be configured.
That is, as shown in FIG. 2, when two-dimensional photonic crystals 210 and 212 are used as reflecting mirrors for a vertical cavity surface emitting laser, several reflecting mirrors conventionally formed of a thick multilayer film of about several μm can be obtained. It is possible to form a very thin film on the order of ten to several hundred nm.
As a result, problems such as current injection caused by the above-described DBR thickness and hindrance to heat dissipation can be reduced.
In addition, based on the above-described examination, it is considered that the light in the in-plane direction can be more efficiently controlled by the above configuration when the two-dimensional photonic crystal is used than the one-dimensional photonic crystal. A higher-performance vertical cavity surface emitting laser can be realized.

  However, when a photonic crystal layer is formed on the outermost surface, the stacking of additional elements may be hindered by innumerable holes and grooves such as the holes 211. Specifically, inconveniences such as hindering the stacking of electrodes, polarizers, and the like occur.

  In view of the above problems, the present invention provides a vertical resonator having a structure in which the stacking of additional elements and the like is not hindered in constructing a vertical cavity surface emitting laser having a reflecting mirror made of a two-dimensional photonic crystal. An object of the present invention is to provide a surface emitting laser.

In order to solve the above problems, the present invention provides a vertical cavity surface emitting laser configured as follows.
The vertical cavity surface emitting laser of the present invention is a vertical cavity surface emitting laser having an active layer between a first reflecting mirror and a second reflecting mirror.
The first and second reflecting mirrors are composed of a two-dimensional photonic crystal having a two-dimensional periodic structure;
The two-dimensional periodic structure in the at least one reflecting mirror is characterized by being arranged in a two-dimensional period in a state in which a substance having a low refractive index is embedded in a substance having a high refractive index.
The vertical cavity surface emitting laser of the present invention is characterized in that the two-dimensional photonic crystal has a defect disturbing periodicity in the two-dimensional periodic structure.
The vertical cavity surface emitting laser of the present invention is characterized in that a plurality of defects that disturb the periodicity are periodically formed.
Also, the vertical cavity surface emitting laser of the present invention is such that the defect that disturbs the periodicity is missing a part of the holes arranged in a two-dimensional period in a state of being embedded in a substance having a high refractive index. Or make the hole size or shape different from other holes,
Alternatively, it is characterized in that it is formed by filling the holes arranged in the two-dimensional period with substances having different refractive indexes.
Further, in the vertical cavity surface emitting laser of the present invention, the two-dimensional photonic crystal forms a photonic band structure between the frequency of the electromagnetic wave and the wave vector,
The electromagnetic wave mode is described outside a light line in the photonic band structure.

According to the present invention, a vertical cavity surface having a structure that does not hinder the stacking of additional elements and the like in constructing a vertical cavity surface emitting laser having a reflecting mirror made of a two-dimensional photonic crystal. A light emitting laser can be realized.
In particular, it is possible to provide a photonic crystal surface emitting laser having a flat surface shape and suitable for stacking.

As described above, the two-dimensional photonic crystal is considered to be able to control light in the in-plane direction more efficiently than the one-dimensional photonic crystal. The use of this was studied and the present invention was completed.
Details of these will be described below.
The embodiment of the present invention can be specifically configured as follows. A basic configuration excluding the reflecting mirror can be the same as that of a conventional general vertical cavity surface emitting laser.
As the active layer and the clad, a double hetero structure, a multiple quantum well structure, a quantum dot structure, or the like used in a normal vertical cavity surface emitting laser can be applied as it is.
The resonator length L based on the active layer thickness and the cladding layer thickness is NL = nλ / 2 (N: refractive index of the resonator medium, n: when the refractive index of the reflecting mirror is larger than the refractive index of the cladding layer). It is necessary to design so as to satisfy the relationship of a positive integer, λ: light wavelength.
Further, when the refractive index of the reflecting mirror is smaller than the refractive index of the cladding layer, NL = nλ. This is to increase the gain by positioning the antinode of the standing wave due to the resonant light in the active layer, and is derived from the phase shift relationship at the resonator end.
As the constituent material, GaAs / AlGaAs, InGaAsP / InP, AlGaInP / GaInP, GaN / InGaN / AlGaN, GaInNAs / AlGaAs, or the like can be used.
These are also not particularly different from conventional vertical cavity surface emitting lasers.

As an example of the present embodiment, a configuration using an n-type and p-type GaN layer for the clad layers on both sides and a non-doped GaN / InGaN multiple quantum well structure for the active layer can be employed.
Here, as a means for injecting carriers into the active layer, for example, a method of having a pair of electrodes of an anode and a cathode and injecting carriers into the active layer by injecting current from the electrodes can be employed.
As the electrode, a ring electrode as used in a normal vertical cavity surface emitting laser can be used, or electrodes having various shapes such as a circle and a rectangle can be used.
About the material of an electrode, it depends on the laser element material of the site | part which forms an electrode. For example, Au-Ge-Ni and Au-Sn can be used for n-type GaAs, and Au-Zn and In-Zn can be used for p-type GaAs.
A transparent electrode such as ITO can also be used.

Next, a two-dimensional photonic crystal used as a reflecting mirror will be described.
A two-dimensional photonic crystal is a structure in which a period of refractive index is provided in the in-plane direction formed by two axes in spatial coordinates, and a periodic refractive index in the remaining one direction. There is no change.
One type of two-dimensional photonic crystal is a so-called two-dimensional photonic crystal slab in which holes 302 are periodically provided in a slab 301 such as a semiconductor as schematically shown in FIG.
This two-dimensional photonic crystal slab has features such as being relatively easy to fabricate and being able to be used as an optical waveguide by utilizing optical confinement due to a refractive index difference in the vertical direction. Most actively studied among crystals.

Conventionally, when light is incident on the surface of a two-dimensional photonic crystal slab from a substantially vertical direction and used as a reflecting mirror, in order to obtain good characteristics, the refractive index difference between the slab and the material adjacent to the top and bottom of the slab Light confinement has been considered important.
For example, in Non-Patent Document 3, when the refractive index difference between the two-dimensional photonic crystal slab and the substrate becomes small, the reflectance peak due to resonance becomes very small, and when the refractive index difference becomes zero, the resonance disappears. There is a description that it will end up (that is, it will not have high reflectivity).

However, as will be described below, when a two-dimensional photonic crystal is used as a reflecting mirror, it is actually not necessary to have a slab structure having a refractive index difference.
That is, even when there is no refractive index difference in the direction perpendicular to the surface, it can be operated as a reflecting mirror having a high reflectance.
For example, even if the material vertically adjacent to the two-dimensional photonic crystal constituting the reflector is the same as the material having a high refractive index among the materials constituting the two-dimensional photonic crystal, the reflector having high reflectivity is used. It is possible to obtain

These will be further described with reference to the schematic diagram of the two-dimensional photonic crystal in the present embodiment shown in FIG.
In FIG. 1, 101 is a high refractive index material, and 102 is a cylinder made of a low refractive index material.
Here, the high-refractive-index substance 101 is two-dimensionally arranged in a state where the cylinders 102 made of the low-refractive-index substance are embedded.
This structure is the same as the structure in which the material adjacent to the top and bottom of the two-dimensional photonic crystal slab is replaced with a high refractive index material constituting the slab.
Since this structure has a structure in which a low refractive index substance is embedded in a high refractive index substance, this structure is hereinafter defined as an embedded two-dimensional photonic crystal.

Here, the material used for the high refractive index substance is not particularly limited, but a material such as a semiconductor or a dielectric that transmits light having a laser oscillation wavelength is preferable.
In addition, when oscillating by light excitation, either a semiconductor or a dielectric can be used, but when oscillating by current injection, a semiconductor is preferable.
The low refractive index material is not particularly limited, but when it is desired to make a large difference in refractive index from the high refractive index material, a material such as air having a refractive index as small as possible is preferable.

4 and 5 show transmission spectra calculated using the transfer matrix method.
FIG. 4 shows the calculated transmission spectrum when light is incident on the two-dimensional photonic crystal slab, and FIG. 5 shows the embedded two-dimensional photonic crystal when light is incident in the direction perpendicular to the plane.
For convenience of calculation, both two-dimensional photonic crystals are assumed to have an infinite width in the in-plane direction.
The high refractive index material of the embedded photonic crystal has an infinite thickness in the light traveling direction, that is, in the direction perpendicular to the photonic crystal plane.
In these figures, the horizontal axis represents the normalized frequency (λ / a), and the vertical axis represents the transmittance. 4 and 5 were calculated with the refractive index of the high refractive index substance being 3.46 and the refractive index of the low refractive index substance being 1.0.
This value roughly matches when Si or GaAs is used as the high refractive index material and air is used as the low refractive index material.
The calculation was performed with the lattice constant a and the radius r = 0.2a, and the height t of the cylinder was changed. In the case of a two-dimensional photonic crystal slab, the height of the cylinder coincides with the thickness of the slab.

The transmission spectrum of the two-dimensional photonic crystal slab in FIG. 4 is a combination of a slowly changing background and a sharply changing peak (resonance).
The gradual change of the background is due to the formation of a kind of Fabry-Perot resonator due to Fresnel reflection caused by the difference in refractive index at the slab interface.
Since the resonator length changes as the slab thickness changes, the background also changes.
As a result of adding resonance caused by a periodic structure of refractive index (that is, a two-dimensional photonic crystal), a spectral shape as shown in FIG. 4 is obtained.
In a certain frequency region, the transmittance is almost 0, that is, the reflectance is almost 1, and it functions as a reflector having a high reflectance.

In the case of the embedded two-dimensional photonic crystal shown in FIG. 5, the background has a transmittance of almost 1 in all regions, and resonance is added thereto as in the two-dimensional photonic crystal slab.
When attention is paid to the graphs of t = 0.3a and t = 0.5a, there is a frequency region in which the transmittance is almost 0, that is, the reflectance is almost 1, like the two-dimensional photonic crystal slab.
Since an actual embedded photonic crystal has a finite thickness, it forms a Fabry-Perot resonator, and the background changes slowly rather than flatly.
That is, the actual embedded photonic crystal reflector shows a spectrum slightly different from that in FIG.
However, what is essential here is that the difference in refractive index perpendicular to the photonic crystal plane is not essential, and even in the buried type, it has a frequency region showing high reflectivity, that is, high reflectivity reflection. It can be used as a mirror.

The radius r and height t of the cylinder are not limited to the values used this time.
As a method of manufacturing the embedded photonic crystal, a pattern can be formed on the surface by lithography or etching, and then bonded to the same kind of wafer, or by using an embedded regrowth method.
There is also a method of writing a structure inside a substance using a femtosecond laser or the like.

Next, examples of the present invention will be described.
[Example 1]
In Example 1, a vertical cavity surface emitting laser configured by applying the present invention will be described.
FIG. 6 shows the configuration of a vertical cavity surface emitting laser according to this embodiment.
In FIG. 6, 601 is an n-type GaAs substrate, 602 is a lower clad layer containing a two-dimensional photonic crystal, 605 is an active layer, and 606 is an upper clad layer containing a two-dimensional photonic crystal.
Reference numeral 608 denotes a lower electrode, 609 denotes an upper electrode, 610 denotes a two-dimensional photonic crystal, 611 denotes a cylindrical hole, and 612 denotes a two-dimensional photonic crystal.

The upper and lower sides of the active layer 605 are sandwiched between a reflecting mirror made of a buried two-dimensional photonic crystal 612 and 610. The vertical cavity surface emitting laser of this embodiment operates by current injection.
The vertical cavity surface emitting laser according to the present embodiment includes a reflecting mirror and a lower cladding layer 602 made of a lower photonic crystal 610 made of n-type (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P and air on an n-type GaAs substrate 601. Is formed.
Further, an active layer 605 having a strained quantum well structure of In _0.56 Ga _0.44 P / (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P is provided thereon.
The number of well layers is 3, the thickness of each of the In _0.56 Ga _0.44 P layer and the (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P layer is 6 nm.
On the active layer, a reflecting mirror made of an upper photonic crystal 612 made of p-type (Al _0.5 Ga _0.5 ) _0.5 In _0.5 P and air and an upper cladding layer 606 are formed. The distance between the active layer and the reflector is approximately 500 nanometers. Further, an n-electrode 608 made of Ni / Au / Ge is formed below the vertical cavity surface emitting laser, and a p-electrode 609 made of Au—Zn is formed on the top.
In the configuration of FIG. 2 described above, the reflecting mirror made of the upper photonic crystal 212 is located on the outermost surface, and it is difficult to provide an electrode directly on the reflecting mirror due to the unevenness. However, according to the configuration of this example, Since the outermost surface becomes flat, the electrode can be easily formed directly on the upper reflecting mirror.

Photonic crystals 610 and 612 have a structure in which cylindrical vacancies 611 are periodically arranged in a triangular lattice pattern in the in-plane direction.
Since the active layer 605 has an AlGaInAs / InP multiple quantum well structure, it emits light in a wavelength band centered at about 1.53 micrometers when excited by light or current.
The structure of the photonic crystal is designed to have a high reflectivity when light in this wavelength band is incident on the surface in a direction substantially perpendicular to the surface, and in particular, light in the vicinity of 1.53 micrometers is incident on the surface perpendicularly. If so, an apparent near 100 percent reflection occurs.
FIG. 9 shows the photonic band structure of this photonic crystal.
In FIG. 9, a mode in the band near the dotted circle 901 is used.
In this vicinity, since the band has an extreme value, its inclination is very small and becomes zero. That is, the group velocity of light tends to be 0 or very small. By using the light mode described here, it is possible to form a standing wave having a uniform mode in a two-dimensional plane.
Since this mode is above (outside) the light line 902 in the photonic band structure, that is, on the high frequency side, it becomes a radiation mode and easily couples with external light.

[Example 2]
In Example 2, a vertical cavity surface emitting laser having a different form from that of Example 1 will be described.
FIG. 7 shows the configuration of the vertical cavity surface emitting laser of this example.
In FIG. 7, 701 is an n-type GaN substrate, 702 is a lower clad layer containing a two-dimensional photonic crystal, 705 is an active layer, and 706 is an upper clad layer containing a two-dimensional photonic crystal.
Reference numeral 708 denotes a lower electrode, 709 denotes an upper electrode, 710 denotes a two-dimensional photonic crystal, 711 denotes a cylindrical hole, and 712 denotes a two-dimensional photonic crystal provided with a defect.
As shown in FIG. 7, a two-dimensional photonic crystal is provided on an n-type GaN substrate 701, and a reflecting mirror and a lower cladding layer 702 are formed by a lower photonic crystal 710 made of n-type Al _0.3 Ga _0.7 N and air. ing.
Further, an active layer 705 having a multiple quantum well structure of In _0.02 Ga _0.98 N / In _0.1 Ga _0.9 N is provided thereon.
The number of well layers is 3, and the thicknesses of the In _0.02 Ga _0.98 N layer and the In _0.1 Ga _0.9 N layer are 5 nm and 2.5 nm, respectively.
On the active layer, a reflecting mirror made of an upper photonic crystal 712 made of p-type Al _0.3 Ga _0.7 N and air and an upper cladding layer 706 are formed.
Further, an n-electrode 708 made of Ti / Al is formed below the vertical cavity surface emitting laser, and a p-electrode 709 made of Ti / Pt / Au is formed on the top. The vertical cavity surface emitting laser of this embodiment operates by current injection.

FIG. 8 shows the configuration of the photonic crystal of this example.
The formed photonic crystal 801 has cylindrical vacancies 802 arranged in a square lattice, and the light of the wavelength and mode handled in this embodiment is in the photonic band gap of this photonic crystal and hardly exists. .
However, as shown in the drawing, the photonic crystal has a structure in which defects 803, which are regions where cylindrical vacancies are missing, are periodically arranged.
Periodically provided defects are those that are provided at intervals of m periods of the photonic crystal within the plane (m is a natural number), and each defect basically has the same size and shape. It is what you have.

If there is only one defect, it functions independently as a point defect. However, since a plurality of defects are periodically arranged at a certain distance as shown in FIG. 8, a plurality of defect modes are combined with each other to form a defect mode 1002 (FIG. 10).
Therefore, only light having a wavelength and a mode corresponding to the defect mode due to the combination of the modes of a plurality of defects can be present in the photonic crystal among the light emitted from the active material in the photonic band gap.
This defect mode can generate light having a uniform mode over a wide area in the photonic crystal plane.
Thereby, a surface emitting laser having a large area and a single mode or a uniform mode can be realized.
This is an important advantage particularly when the light is condensed by a lens.

Note that both of the reflectors positioned above and below the active layer can be composed of photonic crystals in which defects are introduced, one is a photonic crystal in which no defects are introduced, and the other is a photonic crystal in which defects are introduced. It can also be composed of a nick crystal.
In addition, the defect may have any shape as long as it is a local defect that disturbs the periodicity of the two-dimensional photonic crystal.
As described above, the holes may be missing, or the holes may be larger (or smaller) than the others.
The shape of the holes may be different from the others. Further, the pores may be filled with substances having different refractive indexes.

It is a schematic diagram which shows the structure of the two-dimensional photonic crystal in embodiment of this invention. It is a schematic diagram for demonstrating the subject of the vertical cavity surface emitting laser using the two-dimensional photonic crystal in this invention. It is a schematic diagram which shows the structure of the two-dimensional photonic crystal slab used for description of embodiment of this invention. It is a figure which shows the transmission spectrum (calculated value) of the two-dimensional photonic crystal slab used for description of embodiment of this invention. It is a figure which shows the transmission spectrum (calculated value) of the two-dimensional photonic crystal used for description of embodiment of this invention. It is a schematic diagram which shows the structure of the vertical cavity surface emitting laser in Example 1 of this invention. It is a schematic diagram which shows the structure of the vertical cavity surface emitting laser in Example 2 of this invention. It is a schematic diagram which shows the structure of the two-dimensional photonic crystal in Example 2 of this invention. It is a schematic diagram explaining the photonic band structure in Example 1 of this invention. It is a schematic diagram explaining a defect mode in Example 2 of this invention.

Explanation of symbols

101: High refractive index material 102: Cylinder 203 made of low refractive index material 203: Photonic crystal layer 205: Active layer 207: Photonic crystal layer 210: Two-dimensional photonic crystal 211: Cylinder hole 212: Two-dimensional photonic crystal 301: Slab 302: Cylindrical hole 601: n-type GaAs substrate 602: lower clad layer containing two-dimensional photonic crystal 605: active layer 606: upper clad layer containing two-dimensional photonic crystal 608: lower electrode 609: upper electrode 610: two-dimensional photonic crystal 611: cylindrical hole 612: two-dimensional photonic crystal 701: n-type GaN substrate 702: lower cladding layer 705 including two-dimensional photonic crystal 705: active layer 706: including two-dimensional photonic crystal Upper clad layer 708: Lower electrode 709: Upper electrode 710: Two-dimensional photonic connection 711: cylindrical holes 712: it provided the defect two-dimensional photonic crystal 803: defect 902: light line 1001: the photonic band gap 1002: Failure Mode

Claims (6)

In a vertical cavity surface emitting laser having an active layer between a first reflecting mirror and a second reflecting mirror,
The first and second reflecting mirrors are composed of a two-dimensional photonic crystal having a two-dimensional periodic structure;
A vertical structure characterized in that the two-dimensional periodic structure in the at least one reflecting mirror is arranged in a two-dimensional period with a substance having a low refractive index embedded in a substance having a high refractive index. Cavity type surface emitting laser.   2. The vertical cavity surface emitting laser according to claim 1, wherein the substance having a high refractive index is a semiconductor.   3. The vertical cavity surface emitting laser according to claim 1, wherein the two-dimensional photonic crystal has a defect that disturbs periodicity in the two-dimensional periodic structure.   4. The vertical cavity surface emitting laser according to claim 3, wherein a plurality of defects that disturb the periodicity are periodically formed. The defect that disturbs the periodicity loses a part of the holes arranged in a two-dimensional period in a state of being embedded in a material having a high refractive index, or the size or shape of the holes is different from that of other holes. Different,
The vertical resonator type according to any one of claims 1 to 4, wherein the vertical cavity type is formed by filling the holes arranged in the two-dimensional period with substances having different refractive indexes. Surface emitting laser. The two-dimensional photonic crystal forms a photonic band structure between the frequency of the electromagnetic wave and the wave vector,
6. The vertical cavity surface emitting laser according to claim 1, wherein the electromagnetic wave mode is described outside a light line in the photonic band structure.

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