LT6045B - Semiconductor saturable absorber mirror - Google Patents

Abstract

A SESAM device comprising a AIAs/GaAs or a AIOx/GaABragg mirror and asaturable absorber, formed from bismuth containing bulk semiconductor layers orquantum dots, grown on GaAs substrate. Such bismuth containing semiconductor materiais can be, for example GaAsBi, GaAsSbBi or GalnAsBi. The introduction of Bi atoms in such A3B5 group semiconductor materiais pushes the valence band energies up, while slightly influencing the positions of conduction band energies. Therefore theband width of wave lengths for which the SESAM device can be used is widened.

Description

The present invention relates to semiconductor devices and relates to semiconductor saturable absorber mirrors and can be used in synchronous mode lasers to generate short optical pulses.

TECHNICAL LEVEL

The proposal describes a semiconductor saturable absorber mirror (carbon.

Semiconductor Saturable Absorber Mirror-SESAM) and its fabrication technology can be used to produce passive semiconductor optical components and laser systems for short-pulse generation. Such short pulses are applied in optical communication technologies, materials processing, materials science and other fields.

The mode synchronization method is often used to generate short and ultra short laser pulses. Modulation synchronization is the coherent superposition of longitudinal resonator modes. In general, mode synchronization is accomplished by introducing time-dependent resonator losses (that is, at a particular resonator location, providing conditions at which amplification exceeds losses at certain time intervals. Mode synchronization can be active and passive. Active mode synchronization is implemented using electro-optical, aco , or lasers with a high lifetime of the laser medium by modulating the gain factor. Generally, short-pulse generation modes of active mode synchronization are too slow and require rapid external controls. In this case, passive mode synchronization has the advantage of allowing better performance of such a laser. This can be accomplished by placing it on a laser resonator [slsotinanf absorber. The absorption of such absorbers, as well as the losses of the resonator, depends on it of the same impulse intensity.

Saturated absorbers can usually be considered as a material or a structure of a two-atom or band-level system in which the energy of the laser radiation absorbed depends on the free states that can be occupied by the charge carriers excited by the absorption of photons of the appropriate wavelength. By absorbing light quanta with wavelengths corresponding to the difference in energy between the excited and non-excited levels (conduction and valence band), the charge carriers are transported to higher energy levels. If the radiation is sufficiently intense, the number of charge carriers in the excited state increases until the given free photon energy is occupied in the free state conduction band and further excitation by absorption of photons is no longer possible due to the Pauli prohibition principle. It is also important to note that most absorbers are formed such that excited charge carriers recombine in a non-radiation manner.

The following briefly discusses the processes of spontaneous pulse generation initiation and pulse stabilization when a saturating absorber is placed in a laser resonator. By placing the saturating absorber in the laser resonator, it is possible to observe that when the active medium begins to accumulate, the intensity of the incident absorber is low, the absorber is opaque and all incident radiation is absorbed. In such a laser resonator, pulse generation would not occur until spontaneous pulse generation had begun when the charge reversal and the random noise signal had exceeded the intensity required to brighten the absorber. As the intensity of radiation increases, such an absorber becomes clearer and when saturated produces maximum resonator quality. In this way, spontaneous passive mod synchronization begins.

At the onset of laser generation, the absorber stabilizes the laser pulses over time, with the least suppression of the most intense part of the pulse and the greatest suppression of the front and rear pulse fronts with higher absorption losses. In this way, the pulse, bypassing the laser resonator many times, shortens in time until a steady pulse generation is achieved. The pulses then stop short due to the temporal dispersion of the optical elements of the resonator and usually the limited wavelength band amplified by the laser. Such saturable absorbers must have a shorter than the duration of the pulses generated by the absorption recovery time and large absorption cross-section (about 10 to ¹⁶ cm ² and higher). Particularly suitable materials for the realization of such absorbers are semiconductors.

Semiconductor scavengers absorption coefficient dependence on the light output intensity can be approximated using bleachable for Mule: A (/) = - ^ - + a _nonsat · where A (l) the absorption coefficient / - luminous flux sat ¹⁺ ISAT intensity of a ₀ - linear absorption coefficient, a _nO nsat is the unsaturated part of the absorption coefficient, and / _S is the saturation flux intensity.

In practice, said absorbers are used in semiconductor medium based self-absorbing absorber mirrors (SESAM) (the schematic is shown in Fig. 1), which generally consists of a substrate (1), a Breg mirror (2), a saturating absorber (3) consisting of one or more semiconductors. bulk layers or quantum wells, and additional optical coatings (4) are formed to prevent resonance effects and better localize the electromagnetic field within the device itself (for maximum efficiency and avoiding optical interruptions). The first constructions of SESAM are described in U. Keller et al. in IEEE J. Sel.Top. Ouantum Electron. 2, 435 (1996) and is within the current state of the art known to those skilled in the art. SESAMs are formed by layering by molecular beam epitaxy, which should also be well known to those skilled in the art.

A major problem remains the development of quality SESAMs suitable for longer wavelengths (for example, corresponding to optical communication windows at 1.3 and 1.5 micrometers). At 1.3 µm wavelengths, SESAMs with an absorber from InGaAs quantum pits can be used (such a solution is described by R. Fluck et al., Opt. Lett. 21, 1378 (1996). This requires that the ln portion reach about 40 percent). As the indium concentration increases, the reserve energy gap decreases, so the optical absorption edge is shifted to longer wavelengths, but at the same time, the mismatch between the layer and the GaAs substrate grid significantly increases (3.5% for 40% ln), resulting in [tension, surface quality deterioration, light loss]. Lattice mismatches and lower layer stresses can be obtained by growing Breg mirrors and InGaAs absorbers on InP substrates (R. Fluck et al., Appl. Phys. Lett. 72, 3273 ( 1998)) However, Breg mirrors grown in the InP system have several drawbacks, including a smaller difference than in the case of GaAs / AIAs, between the layers of mirror materials. low refractive indexes and poor thermal conductivity.

Alternatively, an absorbent consisting of GalnNAs quantum wells was grown on a GaAs substrate and a GaAs / AIAs Breg mirror (a solution described by V. Liverini et al., Appl. Phys. Lett. 84, 4002 (1998) and PCT Application No. VV003055014). . Substitution of nitrogen with a portion of the arsenic atoms in the V-group crystalline subgroup accelerates the reduction of the bonding energy gap and increases the redshift of the absorption edge. In an embodiment of such a SESAM, the device was able to use a solid state 1.3 IJm wavelength laser mode synchronization to produce optical pulses of several picoseconds. The generation of shorter pulses was prevented by the relatively long lifetime of the unbalanced charge carriers in the absorber material. Also, SESAMs with GalnNAs absorbers are not suitable for longer wavelengths, since the introduction of nitrogen increases the density of the effective states in the conduction band and the associated optical intensity of saturation.

Solutions to overcome the above disadvantages have been proposed in Chinese patent no. CN 1953217, which describes a quantum pit derivative composed of GaNAs barrier layers and GalnNAsSb quantum pit layer, and in the scientific publication NK Metzger et al. In the latter case, SESAM mirrors are also grown using GaAs epitaxial technology, but their absorber consists of layers of the five-membered compound GalnNAsSb. In such a subgroup of the V group, adjacent As atoms are smaller than N atoms and heavier than Sb atoms. Nitrogen atoms contain s-electrons in the outer layer and interact with the conduction band levels of the compound, while Sb atoms have the greatest influence on the valence band structure of the compound. Both of these interactions cause the conductivity and valence bands to move towards each other and reduce the gap energy of the resin faster. However, they also have the undesirable side effect of increasing the densities of the conduction and valence band effect states, which occur in SESAM by increasing the intensity of the laser beam needed to fill those states and achieve optical clarity. Another disadvantage of this solution is the insufficiently short order of absorber recovery time of -12 ps. Although they have allowed the generation of 230 fs Cr ¹⁵⁺ laser pulses at 1528 nm, further reductions in laser pulse durations require absorbers with lower charge lifetimes.

THE SUBSTANCE OF THE INVENTION

The present invention seeks to solve the above problems and to provide a SESAM device which has a saturating absorber which has better characteristics than conventional InGaAs absorbers formed or grown on InP substrates.

Proposed SESAM device consisting of AlAsI GaAs or AIO _x / GaAs Brego mirrors grown on a GaAs substrate and a saturated absorber layer made from bismuth-containing semiconductor bulk or quantum wells, thereby increasing the energy levels of the valence band and helping to reduce the conductivity band energy levels Therefore, the use of a semiconductor material containing the absorber layer Bi can improve the performance of the SESAM device, such as the rate of action, by achieving short-pulse generation at higher wavelengths of IR at lower doping concentrations, and can better tailor its characteristics to specific applications.

BRIEF DESCRIPTION OF THE DRAWING FIGURES

In order to better understand the invention and evaluate its practical applications, the following illustrative drawings are provided. The drawings are given by way of example only and in no way limit the scope of the invention.

1. SESAM principal scheme;

FIG. Two-ternary semiconductor junior: GaAsi. _x Bi _x and Galn _x Asi. _x dependence of the reserve space on the structure of the compound;

3. Pulse-induced optical brightening dependence on photon energy measured in three GaAsi. _x Bi _{x in} layers with different parts of bismuth;

FIG. Temporal Dynamics of Induced Transfiguration GaAsi. _x Bi _{x in the} layer with 6% Bi grown on GaAs / AIAs Breg mirror measured in reflectance mode. The wavelength of the light was 1.1 micrometers, with an average storage pulse beam power of -190 mW;

FIG. A clearing of the induced temporal dynamics GaAsi_ _x Bi _x layer to 6% Bi, grown on a GaAs / aias Bragg mirrors, the measured reflectance mode. The wavelength of the light was 1.1 micrometers, with an average storage pulse beam power of -190 mW;

Dependence of Induced Transparency on Average Power of GaAs- | _x Bi _{x in the} layer with 6% Bi grown on GaAs / AIAs Breg mirror measured in reflectance mode. The wavelength of the light was 1.1 micrometer. The dotted curve represents the theoretical approximation using the saturable absorption formula.

PREFERRED OPTIONS

In a preferred embodiment of the invention, the SESAM absorber layer (3) consists of a material containing bismuth atoms. Such bismuth-containing semiconductors may be GaAsBi, GaAsSbBi, or GalnAsBi. In these A ₃ B ₅ semiconductor compounds, the introduction of Bi atoms raises the energy levels of the valence band, with little effect on the position of the energy levels of the conduction band. Therefore, the defective density of the conduction band remains small, similar to that of the GaAs conduction band, resulting in low saturation flux of the absorber.

It should be emphasized that the use of GaAsBi and other bismid absorber layers provides unique opportunities for the production of infrared SESAMs on GaAs substrates. Experimental studies of GaAsBi have shown that the bonding energy gap of this compound decreases very rapidly during the introduction of Bi, at 90 meV for each percentage of Bi introduced. Therefore, e _g = 0.8 eV can be achieved when only 10% Bi is introduced into GaAs, and GaAsBi with a 15% Bi reserve energy gap even reduces to e _g = 0.4 eV. For comparison, the energy gap of the ternary compound InGaAs decreases much slower as the proportion of In grows, and as much as 30% In is introduced into GaAs, the energy gap decreases to only 1 eV. The dependence of the GaAsBi and GalnAs compounds on the structure dependence of the structure is shown in Figs. 2. Bi concentration may be selected according to the desired SESAM characteristics.

In order to better understand the advantages and operating principles of the present invention, the following example is provided below. According to the features of the present invention, GaAsi was formed. _X Bi _x layers with different fractions of bismuth, the measured optical transduction dependences of the accumulation pulse are shown in Figs. 3. Layers were grown on semi-insulating GaAs trays, all 1.5 microns thick. Femtosecond (150 fs, 200 kHz repetition) optical pulses generated by the ORPHEUS (Light Conversion) optical parametric amplifier were used in the measurements; optical acquisition - optical probing technique was used. The average energy of the accumulation pulses was 1 pJ, and the average energy of the probing pulses measured in pass mode was 0.05-0.1 pJ. Based on the onset of transparency, the gap energy of the layer can be determined, the layer with 11% Si effectively brightens at 0.8 eV photon energy corresponding to 1500 nm.

It is noteworthy that the lifetimes of the charge carriers in bismides can be regulated during the process of molecular fiber epitaxial growth of these layers. By increasing the flow of Bi molecules, some of the Bi atoms fall outside the crystalline lattice

Vgroup element subassembly nodes, and, for example, inter-nodes or jllllgroup element subassembly nodes, where they form deep defect levels, Bij or BiGa, respectively. These defects act as high-speed stacking centers that can reduce their lifespan to less than 1 ps. Bismuth dimers, trimers and clusters of larger Si atoms have the same effect. The density of all these defects, as well as the lifetime of the stacks, can be changed by lowering the growth temperature to less than 300 ° C or by further annealing the grown layers at temperatures above 600 ° C.

An exemplary temporal dynamics of the induced a clearing GaAsi_ _x Bi _x layer to 6% Bi, grown on a GaAs / aias Bragg mirrors, the measured reflection mode, is shown in Figure. Accordingly, the dependence of the induced leaching on the average power of the acquisition beam measured in reflection mode is given in Figs. 5.

Using the example described above, one of ordinary skill in the art can provide more different SESAM designs for infrared operation, but the scope of the invention should include all variations of SESAM devices that use bismuth atoms in the absorber material that raise valence band energy levels at low levels. affecting the position of the energy levels of the conduction band, which extends the wavelength range for which the SESAM device described above can be used.

Claims (6)

DEFINITION OF INVENTION 1. A saturated Breg mirror having a saturable absorber consisting of semiconductor bulk layers or quantum wells, characterized in that the absorber layer is composed of a semiconductor material containing bismuth atoms. 2. A saturated Breg mirror according to claim 1, wherein the absorbent material may be GaAsBi, GaAsSbBi or GalnAsBi. Breg saturating mirror according to one of claims 1 to 2, characterized in that the percentage of bismuth atoms in the GaAsBi absorber material is between 5 and 15 percent or varies between any values in the range of 5 to 15 percent. Breg saturated mirror according to one of claims 1 to 3, characterized in that said GaAsBi absorber is grown at a temperature of less than 300 ° C to increase the proportion of the bismuth atoms in the absorber layer. 5. A saturated Breg mirror according to one of claims 1 to 4, characterized in that said GaAsBi absorber is additionally annealed at a temperature higher than 600 ° C to increase the defect density in the absorber layer. 6. The Breg saturating mirror according to one of claims 1 to 5, characterized in that it is adapted to operate in one of the applications of IR laser radiation generation, data transmission, IR temporal resolution spectroscopy, radiation pulse characterization.