JP2002314194A - Photonic crystal laser - Google Patents

Abstract

PROBLEM TO BE SOLVED: To reduce the threshold energy and the threshold gain of a defect type photonic crystal laser. SOLUTION: The photonic crystal has materials with different refractive index which are periodically arranged in lattice form. Parts of lattice points comprising cylindrical-shaped air holes (vacancy) 2 corresponding to the materials with low refractive index are removed for forming lattice defects. Gain medium 3 of the laser is arranged in the position of the lattice defects for forming a photonic crystal laser. The cylindrical holes 2 are made in a dielectric material such as silicon corresponding to the material with higher refractive index of the photonic crystal by etching or the like. Since current injection excitation or light excitation is efficiently aroused only in parts of a resonator by arranging the gain medium in it, the threshold energy can be reduced. Also, by optimizing the radius and the refractive index of the gain medium, the threshold gain can be reduced.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a laser device, and more particularly, to a photonic crystal laser using a lattice defect in a photonic crystal as an optical resonator.

[0002]

2. Description of the Related Art A laser utilizing an optical resonator due to a defect in a photonic crystal can control spontaneous emission light because the resonator has a high performance (that is, a large Q value). As a result, since the spontaneous emission light coupling coefficient can be increased (the coefficient is 1 in the limit), the realization of a zero threshold laser is expected.

A conventional defect type laser of this type will be described with reference to FIG. In FIG. 11, a gray background portion 101
Denotes a gain medium (semiconductor, dye, organic substance, etc.) of the laser, and a round white portion (white portion) 102 is formed by forming a cylindrical hole in the gain medium 101 by etching or the like. Usually, when the gain medium 101 is a compound semiconductor, its refractive index is about 3.5, and a lattice-like periodic refractive index modulation is realized by the air and the semiconductor in the cylindrical holes 102 arranged in a lattice. Thus, a photonic crystal laser is formed.

In this laser device, light of a specific range of wavelength cannot pass through the photonic crystal because of the structure in which the refractive index changes periodically. That is, a photonic band gap is formed. The width of the photonic band gap increases as the refractive index of the gain medium 101 increases. Then, as shown in FIG.
When the individual air cylindrical holes 102 are removed, defects 104 are formed. This defect 104 functions as an optical resonator for laser oscillation.

FIG. 12 shows the wavelength characteristic of the transmittance when the lattice constant (a) of the photonic crystal having the arrangement shown in FIG. 11 is 320 nm and the radius (r) of the air cylindrical hole 102 is 128 nm. Indicates the existence (work) of the resonator. FIG.
It can be seen from FIG. 7 that a photonic band gap, which is a region where light does not pass at a wavelength of 1000 to 1800 nm, exists. Then, a peak of transmission by the defective resonator (corresponding to the resonance wavelength) is seen around a wavelength of 1287.6 nm.

[0006] Since the gray ground portion 101 in FIG. 11 is made of a gain medium such as a semiconductor, when this portion is excited by light or current injection, laser oscillation occurs. FIG.
In the structure of No. 1, for example, 102 air cylinder holes are 9 ×
When nine and 104 resonators are installed at 5 × 5 positions, the oscillation threshold is 63.3 cm ⁻¹ and the oscillation wavelength is 1287.6 nm.

[0007]

However, for laser oscillation, it is sufficient to provide a gain medium only in the resonator. However, in the case of the above-described defect type photonic crystal laser as in the conventional example, it is apparent from FIG. As described above, the gain medium 101 exists outside the resonator 104. For this reason, there is a problem to be solved that the oscillation threshold energy (for example, oscillation threshold current) is large because the energy of photoexcitation or current injection is also consumed outside the resonator.

Further, in order to lower the oscillation threshold energy, it is necessary to set the oscillation wavelength at the center of the photonic band gap. Because around this center,
This is because the Q of the resonator is the highest. For this purpose, it is necessary to change the refractive index of the gain medium in the resonator and shift the resonance wavelength. However, in the conventional structure as shown in FIG. 11, since the refractive index is the same inside and outside the resonator and fixed. , Could not optimize it.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-described problems, and has as its object to achieve lower threshold energy (for example, lower threshold current), and furthermore, an oscillation threshold. It is an object of the present invention to provide a photonic crystal laser capable of lowering a value gain.

[0010]

In order to achieve the above object, a photonic crystal laser according to the first aspect of the present invention comprises a photonic crystal in which substances having different refractive indexes are periodically arranged in a lattice pattern. A photonic crystal laser using a lattice defect as an optical resonator by creating a lattice defect by removing a part of lattice points composed of a lower material,
A gain medium of a laser is arranged at the position of the lattice defect.

Here, an optical resonator is characterized in that a gain medium of a laser is buried in some holes in a group of holes corresponding to the substance having a lower refractive index. it can.

Further, the lattice constant of the photonic crystal a, the radius of the material of the lower of the refractive index r, radius r _d of the gain _medium, the refractive index of the gain medium and n _d,
n _d is, the following equation _{n d ≧ 6.425 -15.814 (r d} /a)+13.0
98 (r d _{/ a)} ² so as to satisfy the can, characterized in that setting the value of n _d.

Further, silicon, InP lattice-matched to InGaAsP, or a dielectric material of SiC, SiN, or GaN is used as a material having a higher refractive index in the photonic crystal, and InGaAsP, In is used as the gain medium.
GaN, InGaSb, AlInAs, GaAsSb,
Or InPAs, or various dyes (for example,
Rhodamine 6G) or Si to which a rare earth such as Eb (erbium) is added, or amorphous Si to which a rare earth is added, is used as the material having the lower refractive index of the photonic crystal. .5
It can be characterized in that oxides of SiO ₂ and Al _x O _y are used to a certain extent.

(Function) According to the present invention, by disposing the gain medium only in the resonator, a lower threshold energy (for example, a lower oscillation threshold current) can be obtained. In the present invention, a low oscillation threshold gain can be obtained by optimizing the refractive index and radius of the gain medium.

[0015]

Embodiments of the present invention will be described below in detail with reference to the drawings.

(First Embodiment) FIG. 1 shows a configuration of a defect type photonic crystal laser according to an embodiment of the present invention. In one embodiment of the present invention, wavelengths 1100-130
A laser near 0 nm is taken up.

This defect-type photonic crystal laser uses, for example, silicon having a refractive index of 3.5 as a substance 1 surrounding a cylindrical hole (hole) 2 of air arranged in a lattice shape, and uses a defect lattice resonator in a cavity. for example those in which a gain medium 3 made of InGaAsP, the radius of the gain medium 3 _{r d,} the refractive index _{n d.}

In order to match the resonance wavelength of the resonator with the emission wavelength of the gain medium 3, the composition ratio of InGaAsP is changed as required. When changing the refractive index of the gain medium 3 (in the vicinity of 1.5 to 2.0), for example, a polymethine dye is used, and the type and dilution ratio of a solvent such as alcohol are changed.

The lattice constant (a) of the photonic crystal is 32
0 nm, radius (r) of air cylinder hole 2 is 128 nm
(Same as the above conventional example), radius r of gain medium 3_dAnd refraction
Rate n _dCan change the resonance wavelength of the resonator
This is shown in the characteristic diagrams of wavelength versus transmittance in FIGS. 2 and 3.
You. Here, one dielectric substance surrounding the cylindrical hole 2 of air (or
Or indirect bandgap semiconductor such as silicon
Low efficiency, which does not become a laser medium)
3.5.

FIG. 2 is a refractive index _{n d} of the gain medium 3, 1.
8, 2.4, 3.4 (radius r _d = 160 nm)
In shows how the change in the resonance wavelength constant), 3 if the radius _{r d} of the gain medium 3 is varied with 100,128,160nm the resonant wavelength of (constant refractive index _n d = 3.4) The state of change is shown. The position of the resonance wavelength (laser oscillation wavelength) in the photonic band gap is an important parameter that determines the magnitude relation of the threshold gain. As shown in FIGS. by varying the rate n _d and radius r _d, it can be set the resonance wavelength in a desired position (ideally in the middle of the band gap). The laser oscillation is at this resonance wavelength.

[0021] Normally, larger radius r _d of the gain medium 3 is, the oscillation wavelength is positioned at the center of the photonic band gap, the threshold gain is reduced.

Next, the defect type photonic crystal laser threshold gain of the present invention, the radius r _d of the oscillation wavelength and the gain medium _3, the relationship between the refractive index n _d shown in FIGS. 4-8.

[0023] 4, 5, originating threshold gain and the oscillation wavelength and shows the relationship between the radius _{r d} of the gain medium 3, 4 is the refractive index _n d = 1.8, FIG. 5 is a refractive index _{n d} = 3.4. Here, a curve connecting a plurality of black circles or black square dots as experimental data shows the characteristics of the transmission threshold gain, and a curve connecting a plurality of white circles or white square dots as the experimental data indicates the oscillation wavelength. It shows the characteristics, and is the same in the case of FIGS. 6 to 9 described later.

The photonic crystal laser oscillates with a threshold gain of about 20-500 cm ^-1 . Of course, the lower the value, the better. In either case, the threshold gain is the radius r _d of the gain medium 3 is increased, decrease it, seen from FIGS. However, as shown in FIG. 5, when the refractive index n _d of the gain medium 3 is 3.4,
Laser, the radius r _d of the gain medium 3 is large, the oscillation wavelength is present two, i.e., it can be seen that oscillates at multimode. If single-mode oscillation is important for application, the conditions for oscillating in multiple modes must be avoided.

[0025] 6 to 8, originating threshold gain and the oscillation wavelength and shows the relationship between the refractive index _{n d} of the gain medium 3, respectively 6 radius _r d = 80 nm of the gain medium 3,
FIG. 7 shows the case where r _d = 160 nm, and FIG. 8 shows the case where r _d = 190 nm. Again, that there are combinations of radius r _d and a refractive index n _d of threshold gain becomes a minimum value, can be seen from Figures 6-8. Furthermore, as shown in FIGS. 7 and 8, when the refractive index n _d and radius r _d are both large, 4, similarly to FIG. 5, requires careful oscillates in a multimode.

The radius of the gain medium (horizontal axis: obtained from FIGS. 4 to 8 (and from similar data not shown))
Here, an radius (r _d) with a lattice constant (a), a by which) the optimum refractive index and the minimum threshold gain of the gain medium 3 expressed in normalized value (r d _{/ a)} in FIG. 9 . Here, for comparison with the conventional structure of FIG. 11, the value on the vertical axis (minimum transmission threshold gain) is not normalized, and the lattice constant a of the air cylindrical hole 2 is
= 320 nm and a value for the radius r = 128 nm. When the refractive index n _d of the gain medium 3 is represented by the following formula (1) (when the code), the threshold gain is minimized. _{n d ≧ 6.425 -15.814 (r d} /a)+13.098(r d / a) 2 ... (1)

As is apparent from FIGS. 6-8, when the refractive index n _d of the gain medium 3 is less than this value, because the threshold gain increases rapidly, to the exclusion of the range in claim 3 I have. Note that under optimum conditions, the oscillation threshold gain is 4
4.7cm ^-1 _(r d = _190nm, when n d = 1.6) is, is significantly improved than 63.3Cm ^-1 of threshold gain of the conventional defect photonic crystal laser .

(Second Embodiment) FIG. 10 shows a sectional structure of another embodiment of the current injection type defect type photonic crystal laser according to the present invention. The gain medium portion 3 has a multilayer structure, and has a small band gap of InGaAsP32.
And a quantum well composed of InP31 having a large band gap.

An n-InP layer 51 is formed between the gain medium portion 3 and the n-InP substrate 5. A p-InP electrode layer (electrode portion) 6 is laminated on the upper surface of the gain medium portion 3, and a p-InGaAsP layer 7 having a small band gap and giving a good ohmic is laminated on the electrode layer 6. . Further, SiO ₂ for covering the surface of the i-InP dielectric 1 formed on the substrate 5 to prevent current loss
Of the insulating film 8 is formed. 2 is a penetrated air cylindrical hole similar to that of the above-described embodiment. When a current is injected into the electrode layer 6, laser light is generated from the optical resonator including the gain medium portion 3.

The other design conditions such as the arrangement, dimensions and the like, and the operational effects and the like are almost the same as those of the above-described first embodiment of the present invention.

(Other Embodiments) In the first and second embodiments of the present invention described above, the gain medium of the laser is arranged at the position of the lattice defect from which the air cylindrical holes (holes) have been removed.
A similar effect can be obtained even if an optical resonator is configured by embedding the gain medium of the laser in some of the air cylinder holes without removing the air cylinder holes.

In the first embodiment of the present invention, silicon is used as a dielectric, InGaAsP is used as a gain medium, and an air cylindrical hole is used. However, InGaAs is used as a dielectric.
InP lattice-matched to sP, or SiC, SiN,
InGaN, InG
It is also possible to use aSb, AlInAs, GaAsSb, InPAs and the like, and further use Si containing a rare earth such as various dyes (for example, rhodamine 6G) or Eb (erbium), or amorphous Si containing a rare earth. Further, instead of the air cylindrical hole, an oxide such as SiO ₂ or Al _x O _y having a refractive index of about 1.5 may be used.

The oscillation wavelength in the above embodiment is 11
Although the wavelength is set to 00 to 1300 nm, it goes without saying that the present invention can be applied to other wavelength regions if the constituent materials are changed.

[0034]

As described above, according to the present invention,
By arranging the gain medium of the defect type photonic crystal laser only in the resonator, current injection pumping or optical pumping can be efficiently performed only in this portion. (Current) can be realized.

Further, according to the present invention, by optimizing the radius and the refractive index of the gain medium, there is an effect that a low threshold gain can be realized.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of a photonic crystal laser in which a gain medium of the present invention exists only in a defective optical resonator.

FIG. 2 shows the wavelength dependence of the transmittance of the resonator of the photonic crystal laser of the present invention (r _d = 160 nm, n _d = 1.8,
FIG. 4 is a characteristic diagram illustrating the case of (2.4, 3.4).

FIG. 3 shows the wavelength dependence (r _d = 100, 128, 160n) of the transmittance of the resonator of the photonic crystal laser of the present invention.
FIG. 9 is a characteristic diagram showing a case where m, _nd = 3.4).

FIG. 4 is a characteristic diagram showing the relation between the oscillation threshold gain and the oscillation wavelength of the photonic crystal laser of the present invention and r _d (when n _d = 1.8).

FIG. 5 is a characteristic diagram showing the relation between the oscillation threshold gain and the oscillation wavelength of the photonic crystal laser of the present invention and r _d (when n _d = 3.4).

FIG. 6 shows the relation between oscillation threshold gain, oscillation wavelength, and n _d of the photonic crystal laser of the present invention (r _d = 80 nm).
FIG. 9 is a characteristic diagram showing the case of FIG.

FIG. 7 shows the relationship between the oscillation threshold gain and oscillation wavelength of the photonic crystal laser of the present invention and n _d (r _d = 160 nm).
FIG.

FIG. 8 shows the relation between the oscillation threshold gain and the oscillation wavelength of the photonic crystal laser of the present invention and n _d (r _d = 190 nm).
FIG.

[9] In the optimum refractive index and the gain medium normalized the radius of the photonic minimum oscillation threshold gain and the gain medium crystal laser according to the present invention (r d _{/ a)} (wherein, r _d is the radius of the gain medium, (a) is a characteristic diagram showing a photonic crystal lattice constant).

FIG. 10 is a schematic sectional view showing a sectional structure of another embodiment of the current injection type defect type photonic crystal laser according to the present invention.

FIG. 11 is a schematic diagram showing a configuration of a conventional photonic crystal laser in which a gain medium exists outside an optical resonator due to a defect.

FIG. 12 is a characteristic diagram showing a relationship between the transmittance and the wavelength of a resonator in a conventional photonic crystal laser.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Dielectric (silicon etc.) which comprises a defect type photonic crystal laser 2 Cylindrical hole (or oxide) of air arrange | positioned at the lattice shape opened in the dielectric 3 Gain of the laser which exists only in a defect optical resonator Medium (InGaAsP, etc.) 5 n-InP substrate 6 p-InP electrode layer (electrode part) 7 p-InGaAsP layer 8 SiO ₂ insulating film 31 InP layer 32 InGaAsP layer 51 n-InP layer 101 Conventional defect type photo Gain medium constituting nick crystal laser 102 Cylinder holes of air arranged in a lattice formed in gain medium 104 Optical resonator due to defects a Lattice constant of air cylinder hole 2 b Radius of air cylinder hole 2 _d Gain medium 3 normalized radius of the radius n _d optimal refractive index and the gain medium having a refractive index r d _/ a gain medium 3 of the gain medium 3

Claims (4)

[Claims] 1. A photonic crystal in which substances having different refractive indices are periodically arranged in a lattice form, by removing a part of lattice points made of a substance having a lower refractive index to form a lattice defect. 1. A photonic crystal laser, wherein a laser gain medium is arranged at the position of the lattice defect. 2. The optical resonator according to claim 1, wherein a gain medium of a laser is buried in some holes in a group of holes corresponding to the substance having a lower refractive index. 2. The photonic crystal laser according to 1. 3. The photonic crystal has a lattice constant of a,
The radius of the material having a lower the refractive index r, radius _{r d} of the gain medium, the refractive index of the gain medium and _{n d, n d} is the formula _{n d ≧ 6.425 -15.814 (r d} / a) +13.098 _(r d /
so as to satisfy a) ^2, photonic crystal laser according to claim 1 or 2, characterized in that setting the value of n _d. 4. A material having a higher refractive index of the photonic crystal, such as silicon, InP lattice-matched to InGaAsP, or a dielectric material of SiC, SiN, or GaN.
GaSb, AlInAs, GaAsSb, or InP
As or various dyes (for example, rhodamine 6
S) with rare earth elements such as G) and Eb (erbium)
i, or amorphous Si to which rare earth is added, as the material having the lower refractive index of the photonic crystal, an air cylindrical hole or SiO having a refractive index of about 1.5
_2, Al _x O _y the photonic crystal laser according to claim 3, characterized in that an oxide of.