A quantum dot laser with a low threshold and a preparation method thereof

Abstract

The present invention discloses a quantum dot laser with a low threshold and a preparation method thereof, which includes a quartz substrate, a silica periodic grating, a quantum dot layer, and a silica protective layer stacked in sequence. The quantum dot layer is composed of closely packed mercury selenide quantum dots with a diameter of 4.8 nm to 6.6 nm, mercury sulfide quantum dots with a diameter of 4 nm to 5.2 nm, quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide with an effective exciton diameter of 4.8 nm to 6.4 nm, or quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide with an effective exciton diameter of 4 nm to 5 nm. The present invention realizes environmentally stable electron doping in quantum dots by controlling the size of the quantum dots, uses the doped quantum dots as the gain medium of the laser, and further realizes a low gain threshold, a low laser threshold, sustainable continuous laser output, and a designable output wavelength.

Description

Technical Field

[0001] The present invention relates to the field of semiconductor optoelectronic devices, and particularly to a low-threshold quantum dot laser and a method for preparing the same. Background Art

[0002] Benefiting from the quantum confinement effect, semiconductor quantum dots exhibit many advantages as optical gain materials, such as gain wavelengths adjustable with size, potential low gain thresholds, and optically gain thresholds insensitive to temperature. However, the band-edge states of quantum dots have at least twofold degeneracy, and at least a portion of the quantum dots in the population must contain two or more excitons to achieve optical gain. In addition, the multi-excitons induced to generate optical gain in colloidal quantum dots will be rapidly consumed by Auger recombination, thereby limiting the optical gain lifetime. For lasers under continuous optical pumping, the pumping rate must be greater than the rate of gain decay to maintain the optical gain state. The pumping power to meet this requirement is close to or even exceeds the thermal limit that general quantum dots can withstand. Except for achieving continuous laser output with a duration of sub-hour magnitude in a few types of colloidal quantum dots, most reported lasers rely on ultrafast pulsed pumping.

[0003] To reduce the gain threshold and achieve low-threshold quantum dot laser emission, electrons can be pre-doped into the conduction band of quantum dots to "bleach" the ground-state absorption, thereby reducing the gain threshold below the basic single-exciton limit and even achieving zero-threshold optical gain. However, the current electron doping methods used in quantum dot lasers for reducing the gain threshold do not have environmental stability, and doping quantum dots exposed to oxygen or water will cause their de-doping. Summary of the Invention

[0004] The present invention aims to solve at least to some extent the technical problems existing in the related art.

[0005] The object of the present invention is to provide a low-threshold quantum dot laser and a method for preparing the same, so as to obtain a laser emitter with a low gain threshold, a low laser threshold, sustainable continuous laser output, a designable emission wavelength, and environmental stability.

[0006] To achieve the above object, on the one hand, the present invention provides a low-threshold quantum dot laser, including a quartz substrate, a silica periodic grating layer, a quantum dot layer, and a silica protective layer stacked in sequence, and the quantum dot layer is composed of closely packed quantum dots.

[0007] Preferably, the quantum dots are mercury selenide quantum dots, mercury sulfide quantum dots, quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, or quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide.

[0008] Preferably, the quantum dots are mercury selenide quantum dots, the mercury selenide quantum dots are in zinc blende phase, and the diameter is 4.8 nm to 6.6 nm.

[0009] Preferably, the quantum dots are mercury sulfide quantum dots, the mercury sulfide quantum dots are in zinc blende phase, and the diameter is 4 nm to 5.2 nm.

[0010] Preferably, the quantum dots are quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide. The quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide are composed of a zinc blende phase mercury selenide core, a mercury cadmium selenide alloy layer coated on the surface of the mercury selenide core, and a cadmium selenide shell coated on the surface of the mercury cadmium selenide alloy layer. The cadmium element in the mercury cadmium selenide alloy layer gradually increases from inside to outside. The effective exciton diameter of the quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide is 4.8 nm to 6.4 nm.

[0011] Preferably, the quantum dots are quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide. The quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide are composed of a zinc blende phase mercury sulfide core, a mercury cadmium sulfide alloy layer coated on the surface of the mercury sulfide core, and a cadmium sulfide shell coated on the surface of the mercury cadmium sulfide alloy layer. The cadmium element in the mercury cadmium sulfide alloy layer gradually increases from inside to outside. The effective exciton diameter of the quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide is 4 nm to 5 nm.

[0012] Preferably, the product of the grating period and the effective refractive index of the silica periodic grating is equal to the wavelength of the band-edge emission peak of the quantum dots in the quantum dot layer.

[0013] Preferably, the low-threshold quantum dot laser further includes a pump source for exciting the quantum dot layer.

[0014] According to another aspect of the present invention, a method for preparing a low-threshold quantum dot laser is provided, including the following steps:

[0015] S1. Prepare quantum dots;

[0016] S2. Based on a quartz substrate, use reactive ion etching to form a silica periodic grating layer on the quartz substrate;

[0017] S3. Disperse the quantum dots obtained in step S1 in a toluene solution to obtain a toluene dispersion of the quantum dots, and then spin-coat the toluene dispersion of the quantum dots on the silica periodic grating layer obtained in step S2 to obtain a quantum dot layer;

[0018] S4. On the quantum dot layer obtained in step S3, a silica protective layer is obtained by spin-coating spin-on glass.

[0019] Preferably, the quantum dots in step S1 are mercury selenide quantum dots, mercury sulfide quantum dots, quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, or quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide. When the quantum dots are mercury selenide quantum dots, they are prepared by the thermal injection method, and the diameter of the quantum dots is controlled by controlling the reaction temperature and growth time.

[0020] When the quantum dots are mercury sulfide quantum dots, they are prepared by the thermal injection method, and the diameter of the quantum dots is controlled by controlling the reaction temperature and growth time.

[0021] When the quantum dots are quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, they are composed of a mercury selenide core, a cadmium mercury selenide alloy layer, and a cadmium selenide shell. The mercury selenide core is prepared by the thermal injection method, and the diameter of the mercury selenide core is controlled by controlling the reaction temperature and growth time. The cadmium mercury selenide alloy layer and the cadmium selenide shell are prepared by the successive ionic layer adsorption reaction method. At the same time, by controlling the injection rates and amounts of mercury ions, cadmium ions, and selenium ions, the element ratio and thickness of the cadmium mercury selenide alloy layer are controlled. The diameter of the effective exciton is controlled by controlling the diameter of the mercury selenide core and the thickness of the cadmium mercury selenide alloy layer.

[0022] When the quantum dots are quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide, they are composed of a mercury sulfide core, a cadmium mercury sulfide alloy layer, and a cadmium sulfide shell. The mercury sulfide core is prepared by the thermal injection method, and the diameter of the mercury sulfide core is controlled by controlling the reaction temperature and growth time. The cadmium mercury sulfide alloy layer and the cadmium sulfide shell are prepared by the successive ionic layer adsorption reaction method. At the same time, by controlling the injection rates and amounts of mercury ions, cadmium ions, and sulfur ions, the element ratio and thickness of the cadmium mercury sulfide alloy layer are changed. The diameter of the effective exciton is controlled by controlling the diameter of the mercury sulfide core and the thickness of the cadmium mercury sulfide alloy layer.

[0023] Beneficial effects:

[0024] 1. The present invention uses a size control method for electron doping in the conduction band energy level. The electron doping in the quantum dots obtained by this method is stable in the atmospheric environment. The obtained quantum dot lasers have the characteristics of low gain threshold, low laser threshold, designable emission wavelength, and environmental stability.

[0025] 2. The present invention uses a core / alloy / shell structure to construct a gentle interfacial potential barrier and passivate the surface of quantum dots to suppress Auger recombination. The larger volume also helps the quantum dots absorb pump photons more fully, which can further reduce the laser threshold to obtain sustainable continuous laser emission.

[0026] 3. The present invention uses closely packed quantum dots to form a quantum dot layer, improving the net modal gain of the quantum dot layer, which is beneficial to increasing the laser emission intensity. Brief Description of the Drawings

[0027] Figure 1 Schematic diagram of the structure of a low-threshold quantum dot laser according to an embodiment of the present invention.

[0028] Figure 2 Schematic diagram of the quantum dot structure of the present invention; Figure 2 In (a), it is a schematic diagram of the mercury selenide quantum dot structure in Embodiment 1 of the present invention; Figure 2 In (b), it is a schematic diagram of the mercury sulfide quantum dot structure in Embodiment 2 of the present invention; Figure 2 In (c), it is a schematic diagram of the quantum dot structure of a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide in Embodiment 3 of the present invention; Figure 2 In (d), it is a schematic diagram of the quantum dot structure of a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide in Embodiment 4 of the present invention.

[0029] Figure 3 Schematic diagram of the electron doping mechanism based on quantum dot size control of the present invention.

[0030] Figure 4 Schematic diagram of the gain threshold reduction mechanism of the doped quantum dots of the present invention; Figure 4 In (a), it is a schematic diagram of the gain threshold of quantum dots when all the quantum dots in the quantum dot layer of the present invention are undoped quantum dots; Figure 4 In (b), it is a schematic diagram of the gain threshold of the doped quantum dots when the 1S e energy level of all the quantum dots in the quantum dot layer of the present invention is occupied by one doped electron; Figure 4 In (c), it is a schematic diagram of the gain threshold of the doped quantum dots when the 1S e energy level of all the quantum dots in the quantum dot layer of the present invention is occupied by two doped electrons.

[0031] Figure 5 Energy level position diagram of the quantum dots of the present invention; Figure 5 In (a), it is the energy level position diagram of the mercury selenide quantum dots in Embodiment 1 of the present invention; Figure 5 In (b), it is the energy level position diagram of the mercury sulfide quantum dots in Embodiment 2 of the present invention.

[0032] Figure 6Energy level doping probability diagram of the quantum dots of the present invention; Figure 6 Among them, (a) is the energy level doping probability diagram of mercury selenide quantum dots in Example 1 of the present invention; Figure 6 Among them, (b) is the energy level doping probability diagram of mercury sulfide quantum dots in Example 2 of the present invention.

[0033] Figure 7 Gain threshold diagram of the quantum dots of the present invention; Figure 7 Among them, (a) is the gain threshold diagram of mercury selenide quantum dots in Example 1 of the present invention; Figure 7 Among them, (b) is the gain threshold diagram of mercury sulfide quantum dots in Example 2 of the present invention.

[0034] Figure 8 Pulse laser threshold diagram of the quantum dots of the present invention; Figure 8 Among them, (a) is the pulse laser threshold diagram of mercury selenide quantum dots in Example 1 of the present invention; Figure 8 Among them, (b) is the pulse laser threshold diagram of mercury sulfide quantum dots in Example 2 of the present invention.

[0035] Figure 9 Continuous laser threshold diagram of the quantum dots of the present invention; Figure 9 Among them, (a) is the continuous laser threshold diagram of mercury selenide quantum dots in Example 1 of the present invention; Figure 9 Among them, (b) is the continuous laser threshold diagram of mercury sulfide quantum dots in Example 2 of the present invention.

[0036] Figure 10 Quantum dot pulse laser threshold diagram of the present invention; Figure 10 Among them, (a) is the quantum dot pulse laser threshold diagram of the core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide in Example 3 of the present invention; Figure 10 Among them, (b) is the quantum dot pulse laser threshold diagram of the core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide in Example 4 of the present invention.

[0037] Figure 11 Quantum dot continuous laser threshold diagram of the present invention; Figure 11 Among them, (a) is the quantum dot continuous laser threshold diagram of the core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide in Example 3 of the present invention; Figure 11 Among them, (b) is the quantum dot continuous laser threshold diagram of the core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide in Example 4 of the present invention.

[0038] Figure 12 Value diagram of the saturation absorption coefficient of quantum dots with different diameters in the theoretical calculation process of the present invention.

[0039] Figure 13 Value diagram of the exciton lifetime of the bare core quantum dots of the present invention;Figure 13 In (a) is a graph showing the values of the single exciton lifetimes of bare core quantum dots with different diameters and doping levels during the theoretical calculation process of the present invention; Figure 13 In (b) is a graph showing the values of the biexciton lifetimes of bare core quantum dots with different diameters and doping levels during the theoretical calculation process of the present invention.

[0040] Figure 14 For the mercury selenide quantum dots with a diameter of 5.8 nm in Example 1 of the present invention, the number of excitons per quantum dot on average at the initial stage is 0.39, and it is a graph showing the change of the photon number density in the cavity with time calculated under this condition.

[0041] Figure 15 It is a graph showing the values of the absorption cross-sectional areas of quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide and quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide with different effective exciton diameters at an excitation wavelength of 400 nm during the theoretical calculation process of the present invention. Detailed implementation manners

[0042] To make the objectives, technical solutions, and advantages of the present invention clearer, the technical solutions in the present invention will be clearly and completely described below in conjunction with the accompanying drawings in the present invention. Obviously, the described embodiments are part of the embodiments of the present invention, rather than all of them, and they should not be construed as limiting the present invention. Based on the embodiments in the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts fall within the scope of protection of the present invention. In the description of the present invention, it should be understood that the terms used are only for the purpose of description and cannot be construed as indicating or implying relative importance.

[0043] Next, in conjunction with Figures 1 - 15 a low-threshold quantum dot laser and a method for preparing the same provided by the present invention will be described.

[0044] Example 1: As Figure 1 shown, the present invention provides a low-threshold quantum dot laser, including a quartz substrate 1, a silica periodic grating 2, a quantum dot layer 3, and a silica protective layer 4 arranged successively upward.

[0045] Its working principle is as follows: The low-threshold quantum dot laser is excited by photons with an energy greater than or equal to the energy gap between the second hole energy level 1P h and the second electron energy level 1P e of the quantum dots in the quantum dot layer 3, and the 1S e energy level and 1P eThe energy levels are doped with different numbers of electrons in the unexcited state, and the laser emission of the low-threshold quantum dot laser can be output from the upper surface of the silica protection layer 4 or the lower surface of the quartz substrate 1.

[0046] Furthermore, the product of the grating period of the silica periodic grating 2 and the effective refractive index is equal to the wavelength of the band-edge emission peak of the quantum dots in the quantum dot layer 3.

[0047] Such a setting can provide optical feedback for the gain medium of the laser, that is, the quantum dot layer 3, so as to obtain laser emission.

[0048] Furthermore, the low-threshold quantum dot laser further includes a pump source for exciting the quantum dot layer 3.

[0049] The pump source can emit one of pulsed light and continuous light, and the emitted light wavelength is 400 nm. Since the first electron energy level 1S of the quantum dots in the quantum dot layer 3 e may be occupied by two doped electrons, resulting in the complete blocking of the exciton transition from the first hole energy level 1S h to the first electron energy level 1S e level, therefore, the emission photon energy of the pump source should be greater than or equal to the energy gap between the second hole energy level 1P h and the second electron energy level 1P e in the quantum dot layer 3. The pump photons are injected from the upper surface of the silica protection layer 4 or the lower surface of the quartz substrate 1 to excite the low-threshold quantum dot laser, so that the laser emission of the laser is output from the upper surface of the silica protection layer 4 or the lower surface of the quartz substrate 1.

[0050] Furthermore, the quantum dot layer 3 is composed of closely packed quantum dots. By closely packing the quantum dots, the net mode gain of the quantum dot layer can be improved, which is beneficial to improving the laser emission intensity.

[0051] Furthermore, the quantum dots are mercury selenide quantum dots 5, and its specific structure is as shown in Figure 2 in (a) below. The mercury selenide quantum dots 5 are in the zinc blende phase and have a diameter of 4.8 nm to 6.6 nm.

[0052] The present invention obtains electron-doped quantum dots based on a size control method, and uses the electron-doped quantum dots to reduce the gain threshold of the quantum dot population. As shown in Figure 3 for small-sized quantum dots 16, the strong quantum confinement effect gives them a large bandgap, and the two-fold degenerate lowest electron energy level 1S e is far above the ambient Fermi level 15, and 1S eThere will be no electron doping at the energy level; the quantum confinement effect in the medium-sized quantum dots 17 is weaker than that in the small-sized quantum dots 16, the bandgap of the quantum dots becomes narrower, and the 1S e energy level shifts down to near the ambient Fermi level 15, and 1S e about 50% of the energy level is occupied by electrons. If 1S e there is a doped electron at the energy level, then from 1S h the energy level to 1S e the ground state absorption of the energy level is partially blocked; the quantum confinement effect in the large-sized quantum dots 18 is the weakest, its bandgap is the narrowest, and 1S e the energy level is below the ambient Fermi level 15, which will lead to 1S e the probability that the energy level is occupied by electrons is much greater than 50%. If 1S e there are two doped electrons at the energy level, the ground state absorption is completely bleached. As Figure 4 shown in (a) below, assuming that the quantum dots in the quantum dot layer are all uniformly doped and excited, when none of the quantum dots in the quantum dot layer are doped, the gain threshold is for all the quantum dots in the quantum dot layer where 1S e there is one excited electron at the energy level, that is, the gain threshold in terms of excitons per quantum dot on average is , at this time, under the action of the external photon 19, the probabilities of stimulated emission and stimulated absorption are equal, and the number of outgoing photons 20 is the same as that of the external photon 19; as Figure 4 shown in (b) below, when all the 1S e energy levels of the quantum dots in the quantum dot layer are occupied by one doped electron, the gain threshold is for half of the quantum dots in the quantum dot layer where 1S e there is one excited electron at the energy level, that is, the gain threshold is ; as Figure 4 shown in (c) below, when all the 1S e energy levels of the quantum dots in the quantum dot layer are occupied by two doped electrons, the gain threshold is that there are no excited electrons in all the quantum dots in the quantum dot layer, that is, the gain threshold is .

[0053] The calculation model for the selection of the diameter range of the mercury selenide quantum dots 5 is as follows:

[0054] Step 1: Determine the positions of the energy levels of the mercury selenide quantum dots 5 with different diameters.

[0055] To determine the electron doping situation in the zinc blende-phase mercury selenide quantum dots 5 with different diameters, it is necessary to calculate the energy level positions of the quantum dots varying with the diameter, as well as the relative positions of each energy level and the ambient Fermi level. First, based on the two-band k.p. model, calculate the 1S e energy level, 1P e energy level, and 1De Energy level relative to 1S h Energy difference between energy levels:

[0056] (1)

[0057] In the formula, is the band gap of the semiconductor bulk material, is the Kane coefficient, is the free electron mass. For the 1S e energy level, 1P e energy level and 1D e energy level, they are respectively: , and , where is the radius of the quantum dot. For the mercury selenide quantum dot 5, , are taken as -0.20 eV and 20.0 eV respectively. Taking the energy difference of 0.8 eV between the valence band top of the mercury selenide bulk material and the environmental Fermi level as a reference, the relative positions of each energy level of the mercury selenide quantum dot 5 with different diameters to the environmental Fermi level are determined. In this process, the small shift of the 1S energy level with the quantum dot diameter is considered through the ratio of the effective electron mass to the effective hole mass h energy level. For the mercury selenide quantum dot 5, , are taken as 0.05 m 0 and 0.67 m 0 respectively. Taking the position of the environmental Fermi level relative to the vacuum level (0 eV) as -4.3 eV, and it can be ignored with the change of the quantum dot material and size. In this way, the positions of each energy level of the mercury selenide quantum dot 5 relative to the vacuum level with the change of diameter are obtained, as shown in Figure 5 in (a).

[0058] Step 2. Calculate the electron doping situation in the mercury selenide quantum dot 5 with different diameters.

[0059] Based on the Fermi-Dirac statistical rate distribution of electrons in each energy level of the quantum dot under the thermal equilibrium state, the probabilities that the 1S e energy level, 1P e energy level and 1D e energy level are occupied by electrons can be calculated:

[0060] (2)

[0061] In the formula, represents the energy level of the quantum energy level . Take the 1S e energy level, 1P e energy level or 1De The energy of the energy level, is the quantum energy level The probability of being occupied by electrons, is the Fermi energy, is the Boltzmann constant, is the thermodynamic temperature. For the 1S e energy level, 1P e energy level, 1D e energy level of mercury selenide quantum dot 5, the curves of the electron occupancy rate changing with the quantum dot diameter are as shown in Figure 6 (a) below. Due to this distribution law, there are quantum dot branches with the number of electrons doped on the 1S e energy level (doubly degenerate) being 0, 1, and 2 respectively. The proportions of these three branches in the quantum dot population can be expressed by the following formula:

[0062] (3)

[0063] In the formula, represents the proportion of quantum dots doped with e electrons on the 1S n energy level in the population. In addition, there are also quantum dot branches with the number of electrons doped from 0 to 6 on the 1P e energy level (six-fold degenerate) in the quantum dot population:

[0064] (4)

[0065] In the formula, represents the proportion of quantum dots doped with m electrons on the 1P e energy level in the population.

[0066] Step Three: Calculate the gain threshold of mercury selenide quantum dots 5 with different diameters.

[0067] After completing the analysis of the electron doping situation in the quantum dots, the gain threshold of mercury selenide quantum dots 5 with different diameters can be further calculated. When the total stimulated absorption rate of the quantum dot population is equal to the total stimulated emission rate , the corresponding average number of excitons per quantum dot is the optical gain threshold of the band-edge energy level, and are given by the following formula:

[0068] (5)

[0069] (6)

[0070] In the formula, is the transition probability of photons in a single spin-allowed transition, , and are respectively the proportions of quantum dots containing 0, 1, and ≥2 excitons in the population when the energy level is doped with electrons. For the sake of convenience, , and are collectively referred to as 1S e exciton distribution in the branch where the number of doped electrons at the n energy level is

[0071] (7)

[0072] In the formula, is the proportion of quantum dots containing N excitons in the quantum dot population with an average of excitons per quantum dot. Considering the proportions of the three branches with 0, 1, and 2 doped electrons at the 1S e energy level in the quantum dot population, the exciton distributions in the corresponding branches , and can be expressed as follows:

[0073] (8)

[0074] (9)

[0075] (10)

[0076] In addition, under the condition that all quantum dots in the quantum dot population are double-doped with electrons at the 1S e energy level, there are also seven branches with 0 - 6 doped electrons at the 1P e energy level, and the exciton distributions in the corresponding branches can be expressed as follows:

[0077] (11)

[0078] (12)

[0079] (13)

[0080] In the formula, , and are respectively when 2 electrons are doped at the 1S e energy level, and e electrons are doped at the 1P When there are 0 electrons, the proportion of quantum dots containing 0, 1, and ≥2 excitons in the population. For the sake of convenience in explanation, , and are collectively referred to as 1S e The exciton distribution in the branch where the number of doped electrons at the energy level is 2 and the number of doped electrons at the 1P e energy level is m . The curve of the gain threshold of mercury selenide quantum dot 5 varying with the quantum dot diameter is as shown in Figure 7 in (a).

[0081] Step 4: Calculate the pulsed laser threshold of mercury selenide quantum dots 5 with different diameters.

[0082] The calculation of the pulsed laser threshold of quantum dots is more complex than that of the gain threshold. It is necessary to establish the total dynamic equation describing the quantum dot - optical field coupling system, and then under the conditions of different initial average number of excitons per quantum dot , solve this total dynamic equation to obtain the function of the number of photons in the cavity varying with time , and then obtain the pulsed laser threshold by analyzing . We first established the dynamic equation describing the evolution of the number of excitons in the undoped quantum dot branch during the laser process:

[0083] (14)

[0084] (15)

[0085] (16)

[0086] In this equation, is the photon density in the cavity, and are the lifetimes of single excitons and biexcitons of undoped quantum dots respectively. We also established the dynamic equation describing the evolution of the number of excitons in the branch with single electron doping at the 1S e energy level during the laser process:

[0087] (17)

[0088] (18)

[0089] (19)

[0090] Among them, and are the lifetimes of single excitons and biexcitons of quantum dots with single electron doping at the 1S e energy level respectively. We also established the description of 1S eDouble-electron doping at the energy level and 1P e Doped at the energy level electrons The kinetic equation for the evolution of the exciton number in the branch with

[0091] (20)

[0092] (21)

[0093] (22)

[0094] Wherein and are the single-exciton and biexciton lifetimes of the quantum dots with double-electron doping at the 1S e energy level and doped with e electrons at the 1P energy level, respectively.

[0095] As shown in (a) of Figure 6 , for mercury selenide quantum dots 5 with a diameter less than or equal to 5.8 nm, since the probability of the 1P e energy level being occupied by electrons is extremely small, it can be considered that only undoped, single-electron-doped at the 1S e energy level, double-electron-doped at the 1S e energy level and undoped at the 1P e energy level branches of quantum dots exist in the quantum dot population. Considering the influence of the laser behavior of these three branches on the number of cavity photons, the following kinetic equation for describing the evolution of the number of photons in the cavity can be established:

[0096] (23)

[0097] In the formula, is the density of quantum dots in the gain medium, is the photon lifetime in the cavity. For mercury selenide quantum dots 5 with a diameter less than or equal to 5.8 nm, by combining Equation (23) and Equations (14) to (22), a set of total kinetic equations for describing the quantum dot - optical field coupling system is formed. As shown in (a) of Figure 6 , for mercury selenide quantum dots 5 with a diameter greater than 5.8 nm, since the probability of the 1S e energy level being occupied by electrons is close to 100%, and the electron occupancy rate at the 1P e energy level begins to gradually increase, it can be considered that only seven branches with double-electron doping at the 1S e energy level and doped with 0 to 6 electrons at the 1P e energy level exist in the population, and the evolution of the number of photons in its cavity is described by the following kinetic equation:

[0098] (twenty four)

[0099] For this equation, there exists as well as For HgSe quantum dots 5 with a diameter greater than 5.8 nm, the combination of equations (24) and (20) to (22) constitutes another set of overall dynamic equations describing the quantum dot-light field coupling system.

[0100] In order to start cavity photon accumulation, it is necessary to set the initial value of the total kinetic equation. At the initial moment, the exciton distribution of each doping degree quantum dot branch is determined by the initial average number of excitons per quantum dot. Combining equations (7) to (13), we can calculate that the photon number density at the initial moment is set to 1 photon per cubic centimeter, that is, In addition, the photon lifetime in the cavity is set to 10 ps, the transition probability of a single spin-allowed photon in the transition The quantum dot saturation gain coefficient Determine that there is the following relationship:

[0101] (25)

[0102] Where, the refractive index of the gain medium is Assuming it is 1.5, is the speed of light in a vacuum, and the density of quantum dots in the gain medium The volume of quantum dots and quantum dot packing density related to, exist For the quantum dot layer 3, is 0.4 and does not change with the change of the volume of quantum dots in the quantum dot layer 3. Saturation gain coefficient and The reciprocal of is approximately linear. We assume that the diameter of the quantum dots is 3nm and 12nm respectively. 500cm -1 and 100 cm -1 , other diameter quantum dots G 0 With these two values as reference, the inverse of the relative quantum dot volume presents a linear distribution. The values of the saturation absorption coefficient of quantum dots with different diameters are as follows Figure 12 The single exciton Auger lifetime and biexciton Auger lifetime of quantum dots with different diameters and doping conditions are calculated based on the volume scale of quantum dots and the statistical scale of the number of carriers. The following is a volume scale model used to calculate the biexciton Auger lifetime of undoped quantum dots. :

[0103] (26)

[0104] In the formula, is a proportionality coefficient, taking 1 ps / nm 3 . According to the statistical model of the number of carriers, the number of Auger recombination paths is proportional to the product of the number of all possible interband transitions and the number of carriers that can receive the energy released by a given interband transition. Therefore, for an exciton state with electrons and holes, the Auger lifetime can be expressed as:

[0105] (27)

[0106] In the formula, is a constant, representing the recombination time of a single Auger recombination path. Through formula (27), the ratios of the single-exciton Auger lifetime and the biexciton Auger lifetime of quantum dots with various doping levels to can be determined. Also, based on calculated by formula (26) and combined with the above proportional relationship, the single-exciton Auger lifetime and the biexciton Auger lifetime of quantum dots with different doping levels can be determined. For the single-exciton radiative lifetime and the biexciton radiative lifetime of quantum dots with different doping levels, first assume that the single-exciton radiative lifetime of undoped quantum dots is 50 ns and does not change with the change of the quantum dot diameter, and it is considered that the single exciton of undoped quantum dots is purely radiative, that is, and the single-exciton lifetime of undoped quantum dots have a relationship. Also, based on the statistical scaling of the radiative lifetime with the number of carriers at the 1S energy level, it is determined that the single-exciton radiative lifetime of doped quantum dots is 1 / 2. In addition, regardless of whether the quantum dots are doped, their biexciton radiative lifetime is 1 / 4. The single-exciton and biexciton Auger and radiative lifetimes of quantum dots with different diameters and doping levels have been obtained. According to the following relational formula, the single-exciton and biexciton lifetimes of quantum dots with different diameters and doping levels can finally be calculated:

[0107] (28)

[0108] In the formula represents the exciton lifetime, and represents the radiative lifetime. The values of the single-exciton lifetimes of quantum dots with different diameters and doping levels are as shown in (a) of Figure 13 , and the values of the biexciton lifetimes of quantum dots with different diameters and doping levels are as shown in (b) of Figure 13 .

[0109] After determining all the initial values and values in the total kinetic equation, we can solve it and obtain the different initial average number of excitons per quantum dot. The function of the number of photons in the cavity changing with time under the condition Taking the mercury selenide quantum dot 5 with a diameter of 5.8nm as an example, Calculated under the conditions like Figure 14 In order to calculate the pulse laser threshold , need to Analyze and define The delay between the peak time and the initial time is ,along with The increase of The law of change is that it increases first, If a certain critical value is reached decreases rapidly, and this critical value is the laser threshold The curve of the pulse laser threshold of mercury selenide quantum dot 5 changing with the quantum dot diameter is shown in Figure 8 As shown in (a).

[0110] Step 5: Calculate the continuous laser threshold of mercury selenide quantum dots 5 with different diameters.

[0111] For the calculation of the continuous laser threshold of quantum dots, it is necessary to establish the exciton distribution equation of each doping degree quantum dot branch under continuous light pumping before the laser is established in a steady state, with 1S e Examples of single-level doping branches:

[0112] (29)

[0113] (30)

[0114] (31)

[0115] In the equation, the 0 on the far right of the equation indicates a steady state. is the continuous optical pump intensity expressed as photon flux density, is the absorption cross-section of the quantum dot at the excitation wavelength. and the intrinsic absorption coefficient The following relationship exists:

[0116] (32)

[0117] For HgSe quantum dots 5, at an excitation wavelength of 400 nm, Take 4.63×105cm-1. Since the proportion of this branch in the quantum dot population is , so there is . Solving equations (29) to (31) can obtain the functions of , and relative to :

[0118] (33)

[0119] (34)

[0120] (35)

[0121] Having obtained the exciton distribution of each quantum dot branch with different doping levels at steady state, the gain coefficient of the quantum dot population at steady state can be calculated according to the following formula :

[0122] (36)

[0123] In the formula, is the refractive index of the gain medium. For this equation, there is a relationship between and . Substituting the exciton distribution of each quantum dot branch with different doping levels at steady state into equation (36) can establish the relationship between and . Substituting the initial exciton distribution corresponding to the pulsed laser threshold into equation (36) can calculate the laser threshold gain coefficient . If the gain coefficient of the quantum dot population at steady state under continuous optical pumping is exactly equal to the gain coefficient corresponding to the pulsed laser threshold, then the continuous optical pumping intensity satisfying this condition is the continuous laser threshold . The curve of the continuous laser threshold of mercury selenide quantum dot 5 varying with the quantum dot diameter is as shown in Figure 9 in (a).

[0124] In this embodiment, zinc blende phase mercury selenide quantum dot 5 with a diameter of 4.8 nm to 6.6 nm is used as the gain medium. As shown in Figure 5 in (a) and Figure 6 in (a), due to the weakened quantum confinement effect, when the diameter of mercury selenide quantum dot 5 is large enough, its first electron energy level 1S e will shift to near the ambient Fermi level, which will cause the 1S e energy level to be occupied by doped electrons in the unexcited state. As the quantum dot diameter further increases, the 1S e energy level will further shift down below the ambient Fermi level, which will cause the 1S eThe probability of the energy level being occupied by doped electrons increases. As shown in Figure 6 (a) and Figure 7 (a), when the diameter of the mercury selenide quantum dot 5 is greater than 4.8 nm, it can ensure that the probability of the 1S e energy level being occupied by electrons is large enough, that is, greater than 0.78, so that the gain threshold of the mercury selenide quantum dot 5 is lower than . As the diameter of the quantum dot further increases, the gain threshold will continue to decrease. As shown in Figure 6 (a) and Figure 8 (a), if the diameter of the mercury selenide quantum dot 5 is greater than 6.6 nm, the probability of electrons occupying the second electron energy level 1P e will be too large, greater than 0.32. The doped electrons on the 1P e energy level are not helpful for the bleaching of the ground state absorption, but will provide an additional path for Auger recombination. The too-fast Auger recombination rate is not conducive to the establishment of the laser and will lead to an increase in the laser threshold. As shown in Figure 8 (a), using the zinc blende phase mercury selenide quantum dot 5 with a diameter of 4.8 nm - 6.6 nm as the gain medium of the laser, a pulsed laser threshold less than 1 in terms of the number of excitons per quantum dot can be achieved for pulsed laser emission. The emission wavelength of the exciter covers the range of 1500 nm - 2100 nm. Among them, using the mercury selenide quantum dot 5 with a diameter of 5.8 nm as the gain medium, the laser achieves pulsed laser emission.

[0125] This embodiment also provides a method for preparing a low-threshold quantum dot laser for preparing the above-mentioned low-threshold quantum dot laser. The preparation method includes:

[0126] (1) Using the thermal injection method, zinc blende phase mercury selenide quantum dots 5 with a diameter range of 4.8 nm - 6.6 nm are prepared by controlling the reaction temperature and growth time;

[0127] (2) Provide a quartz substrate 1, and use reactive ion etching to form a silica periodic grating 2 on the quartz substrate 1;

[0128] (3) Disperse the mercury selenide quantum dots 5 in a toluene solution with a concentration of 20 mg / ml to obtain a toluene dispersion of zinc blende phase mercury selenide quantum dots 5, and then spin-coat the toluene dispersion of the zinc blende phase mercury selenide quantum dots 5 on the silica periodic grating 2 at a speed of 2500 revolutions per minute to form a quantum dot layer 3;

[0129] (4) On the quantum dot layer 3 obtained in step (3), spin-coat the 500F spin-on glass produced by Filmtronics at a speed of 3000 revolutions per minute to obtain a silica protective layer 4.

[0130] Example 2: Based on Example 1, the quantum dots in this example are mercury sulfide quantum dots 6, and their specific structure is as follows Figure 2 As shown in (b), the mercury sulfide quantum dots 6 are in the sphalerite phase, with a diameter of 4 nm to 5.2 nm.

[0131] The calculation model based on which the diameter range of the mercury sulfide quantum dots 6 is selected is as follows:

[0132] Steps 1 to 3 are the same as in Example 1. Formulas (1) to (13) are used to obtain: Figure 5 The positions of the energy levels of the mercury sulfide quantum dot 6 shown in (b) relative to the vacuum energy level as the diameter changes, as shown in Figure 6 (b) shows the 1S of HgS quantum dot 6 e Energy level, 1P e Energy level, 1D e The curve of the electron occupancy rate on the energy level changing with the diameter of the quantum dot, such as Figure 7 (b) shows a curve of the gain threshold of the mercury sulfide quantum dot 6 as a function of the quantum dot diameter. It should also be noted that the mercury sulfide quantum dot 6 , The values are 0.65eV and 9.00eV respectively; the energy difference between the valence band top of mercury sulfide material and the ambient Fermi level is 1.15eV; the , Take 0.036 respectively m 0, 0.31 m 0.

[0133] Step 4 is the same as Example 1, but the quantum dot diameters adapted by formulas (23) and (24) are different: for mercury sulfide quantum dots 6 with a diameter less than or equal to 4.6 nm, formula (23) and formula (14) to formula (22) are combined to form a set of total dynamic equations describing the quantum dot-light field coupling system; for mercury sulfide quantum dots 6 with a diameter greater than 4.6 nm, formula (24) and formula (20) to formula (22) are combined to form another set of total dynamic equations describing the quantum dot-light field coupling system.

[0134] Finally, according to the calculation results, Figure 8 (b) is a graph showing the pulse laser threshold of the mercury sulfide quantum dot 6 as a function of the quantum dot diameter.

[0135] Step 5 is the same as in Example 1, and finally the following is obtained: Figure 9 (b) shows a curve of the continuous laser threshold of the mercury sulfide quantum dot 6 as a function of the quantum dot diameter. It should also be noted that the mercury sulfide quantum dot 6 at an excitation wavelength of 400 nm Take 2.70×105 cm -1 。

[0136] In this embodiment, zinc blende phase mercury sulfide quantum dots 6 with a diameter of 4 nm to 5.2 nm are used as the gain medium. As shown in (b) of Figure 5 and (b) of Figure 6 , due to the weakened quantum confinement effect, when the diameter of the mercury sulfide quantum dots 6 is large enough, its first electron energy level 1S e will shift to the vicinity of the ambient Fermi level, which will cause the 1S e energy level to be occupied by doped electrons in the unexcited state. As the diameter of the quantum dots further increases, the 1S e energy level will further shift downward below the ambient Fermi level, which will cause the probability of the 1S e energy level being occupied by doped electrons to increase. As shown in (b) of Figure 6 and (b) of Figure 7 , choosing the diameter of the mercury sulfide quantum dots 6 to be greater than 4 nm can ensure that the probability of the 1S e energy level being occupied by electrons is large enough, that is, greater than 0.90, so that the gain threshold of the mercury sulfide quantum dots 6 is lower than . As the diameter of the quantum dots further increases, the gain threshold will continue to decrease. As shown in (b) of Figure 6 and (b) of Figure 8 , if the diameter of the mercury sulfide quantum dots 6 is greater than 5.2 nm, the probability of electrons occupying the second electron energy level 1P e will be too large, greater than 0.26. The doped electrons on the 1P e energy level are not helpful for the bleaching of the ground state absorption, but will provide an additional path for Auger recombination. The too-fast Auger recombination rate is not conducive to the establishment of the laser and will cause an increase in the laser threshold. As shown in (b) of Figure 8 , using zinc blende phase mercury sulfide quantum dots 6 with a diameter of 4 nm to 5.2 nm as the gain medium of the laser can achieve pulsed laser emission with a pulsed laser threshold less than 1 expressed by the number of excitons per quantum dot on average. The emission wavelength of the exciter covers the range of 1100 nm to 1250 nm. Among them, using mercury sulfide quantum dots 6 with a diameter of 4.6 nm as the gain medium, the laser realizes pulsed laser emission.

[0137] This embodiment also provides a method for preparing a low-threshold quantum dot laser for preparing the above-mentioned low-threshold quantum dot laser. The difference between the preparation method and that of Embodiment 1 is only that different quantum dots are prepared in the first step:

[0138] (1) Using the thermal injection method, zinc blende phase mercury sulfide quantum dots 6 with a diameter range of 4 nm to 5.2 nm were prepared by controlling the reaction temperature and growth time.

[0139] As Figure 9 shown, in lasers using mercury selenide quantum dots 5 or mercury sulfide quantum dots 6 as the gain medium, the continuous laser threshold expressed in terms of the excitation light intensity is greater than 30 KW cm -2 and 300 KW cm -2 respectively, exceeding the thermal limit that general quantum dots can withstand, and it is difficult to obtain sustainable continuous laser emission. This is mainly because the Auger recombination rate of bare core quantum dots is still too fast. Therefore, a core / alloy / shell structure needs to be adopted to construct a gentle interface potential barrier and passivate the surface of the quantum dots to suppress Auger recombination. A larger volume also helps the quantum dots to absorb pump photons more fully.

[0140] Example 3: Based on Example 1, the quantum dots described in this example are quantum dots 7 with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide. Its specific structure is as Figure 2 shown in (c). The quantum dots 7 with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide are composed of a zinc blende phase mercury selenide core 8, a mercury cadmium selenide alloy layer 9 coated on the surface of the mercury selenide core 8, and a cadmium selenide shell 10 coated on the surface of the mercury cadmium selenide alloy layer 9. The cadmium element in the mercury cadmium selenide alloy layer 9 gradually increases from the inside to the outside. The effective exciton diameter of the quantum dots 7 with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide is 4.8 nm to 6.4 nm.

[0141] The calculation method for selecting the effective exciton diameter range of the quantum dots 7 with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide is as follows:

[0142] The calculation of the laser threshold of core / alloy / shell structure quantum dots still uses the same model as that of bare core quantum dots. It should be particularly noted that the energy level position and doping degree of core / alloy / shell structure quantum dots with a specific effective exciton diameter are the same as those of the corresponding bare core quantum dots with the same diameter. The core / alloy / shell structure only affects the Auger lifetime and absorption cross-section of the quantum dots. For the calculation of the total absorption cross-section of core / alloy / shell structure quantum dots, the volume of the core material is calculated using the effective exciton diameter, the volume of the shell material is calculated using the difference between the total diameter and the effective exciton diameter of the quantum dots, and then based on the respective inherent absorption coefficients of the core and shell materials combined with Equation (32) to calculate the total absorption cross-section. The inherent absorption coefficient of the cadmium selenide shell 10 at an excitation wavelength of 400 nm is 0.80×10 5 cm -1。The absorption cross-sectional areas of quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with different effective exciton diameters at an excitation wavelength of 400 nm are as Figure 15 shown. Additionally, the suppression of Auger recombination in the core / alloy / shell structure mainly stems from the spatial separation of electron-hole pairs and a smooth confinement potential. The effective exciton diameter has little effect on the Auger lifetime. Therefore, assuming that the biexciton lifetime of a core / alloy / shell structured quantum dot with a total diameter of 12 nm is 1.5 ns in the undoped case and ignoring its variation with the effective exciton diameter, the single-exciton and biexciton lifetimes of core / alloy / shell structured quantum dots with different doping levels can be calculated based on the statistical scaling of the carrier number. The specific values are shown in the following table:

[0143] Table 1. Single-exciton lifetimes of core / alloy / shell structured quantum dots with different doping levels

[0144]

[0145] The table gives the single-exciton (X), 1S e level single-doped quantum dot single-exciton (X-), and 1S e level double-doped and 1P e level with 0 - 6 additional electrons doped quantum dot single-exciton (X2-,0-~X2-,6-) lifetimes ( ), radiative lifetimes ( ), and Auger lifetimes ( ).

[0146] Table 2. Biexciton lifetimes of core / alloy / shell structured quantum dots with different doping levels

[0147]

[0148] The table gives the biexciton (XX), 1S e level single-doped quantum dot biexciton (XX-), and 1S e level double-doped and 1P e level with 0 - 6 additional electrons doped quantum dot biexciton (XX2-,0-~XX2-,6-) lifetimes ( ), radiative lifetimes ( ), and Auger lifetimes ( ).

[0149] Substitute the single-exciton and biexciton lifetimes of the quantum dots with the above different doping levels of the core / alloy / shell structure, as well as the absorption cross-sectional areas of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide having different effective exciton diameters, into Steps 4 to 5 of Example 1, and a curve showing the variation of the pulsed laser threshold of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with respect to the effective exciton diameter of the quantum dots with the core / alloy / shell structure can be obtained, as shown in Figure 10 as shown in (a) of Figure 11 . A curve showing the variation of the continuous laser threshold of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with respect to the effective exciton diameter of the quantum dots with the core / alloy / shell structure is as shown in as shown in (a) of

[0150] In this example, the total diameter of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide is 12 nm. The effective exciton diameter of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide is equal to the diameter of the mercury selenide quantum dots 5 having the same band-edge emission wavelength as the quantum dots with the core / alloy / shell structure. The cadmium element proportion in the cadmium mercury selenide alloy layer 9 gradually increases linearly from 0% to 100% from the inside to the outside. Correspondingly, the mercury element decreases linearly from 100% to 0%. In the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, the bandgap width of the cadmium selenide shell 10 is much larger than that of the mercury selenide core 8. Therefore, the influence of the shell of the quantum dots with the core / alloy / shell structure on the effective exciton diameter can be ignored, and the effective exciton diameter is jointly determined by the core diameter, the alloy layer composition, and the thickness. The effective exciton diameter of the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide is equal to the sum of the diameter of the mercury selenide core 8 and the thickness of the cadmium mercury selenide alloy layer 9. And the diameter of the mercury selenide core 8 cannot be less than 2 nm to prevent excessive defect states, and the thickness of the cadmium mercury selenide alloy layer 9 cannot be less than 2 nm to prevent the formation of a non-smooth interface potential barrier.

[0151] In this example, by coating a cadmium mercury selenide alloy layer 9 with a gradually increasing cadmium element content from the inside to the outside on the surface of the zinc blende phase mercury selenide core 8, a smooth interface potential barrier is constructed and the surface of the quantum dots is passivated, thereby suppressing Auger recombination and being able to further reduce the laser threshold to obtain sustainable continuous laser emission. Using the quantum dots 7 with the core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with an effective exciton diameter of 4.8 nm to 6.4 nm as the gain medium can make the first electron energy level 1S e have a relatively large electron occupancy probability, while the electron occupancy probability on the second electron energy level 1P e will not be too large. As shown in Figure 10 as shown in (a) of As low as 0.52 or below. Among them, using quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with an effective exciton diameter of 5.8 nm as the gain medium, the laser achieved pulsed laser emission. As shown in (a) of Figure 11 , using quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with an effective exciton diameter of 4.8 nm - 6.4 nm as the gain medium, the laser also achieved a continuous laser threshold expressed by the excitation light intensity less than 1 KW cm -2 continuous laser emission. The emission wavelength of the laser covered 1500 nm - 2000 nm. Among them, using quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with an effective exciton diameter of 5.8 nm as the gain medium, the laser achieved continuous laser emission.

[0152] This embodiment provides a method for preparing a quantum dot laser with a low threshold for preparing the above-mentioned quantum dot laser with a low threshold. The difference between the preparation method and that of Embodiment 1 is only that different quantum dots are prepared in the first step:

[0153] (1) The mercury selenide core 8 is prepared by the thermal injection method, and the diameter of the mercury selenide core 8 is controlled by controlling the reaction temperature and growth time; the cadmium mercury selenide alloy layer 9 is prepared by the successive ionic layer adsorption reaction method, and the element ratio and thickness of the cadmium mercury selenide alloy layer 9 are controlled by controlling the injection rates and injection amounts of mercury ions, cadmium ions, and selenium ions; the cadmium selenide shell 10 is prepared by the successive ionic layer adsorption reaction method; the effective exciton diameter of the quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide is controlled by controlling the diameter of the mercury selenide core 8 and the thickness of the cadmium mercury selenide alloy layer 9, and finally, quantum dots 7 with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide with a total diameter of 12 nm and an effective exciton diameter range of 4.8 nm - 6.4 nm are obtained.

[0154] Embodiment 4: Based on Embodiment 3, the quantum dots in this embodiment are quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide, and its specific structure is as shown in (d) of Figure 2 . The quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide are composed of a zinc blende phase mercury sulfide core 12, a cadmium mercury sulfide alloy layer 13 coated on the surface of the mercury sulfide core 12, and a cadmium sulfide shell 14 coated on the surface of the cadmium mercury sulfide alloy layer 13; the cadmium element in the cadmium mercury sulfide alloy layer 13 gradually increases from the inside to the outside, and the effective exciton diameter of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide is 4 nm - 5 nm.

[0155] The calculation method for selecting the effective exciton diameter of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide is the same as that in Example 3. It should be noted that the intrinsic absorption coefficient of the cadmium sulfide shell 14 at an excitation wavelength of 400 nm is 0.68×10 5 cm -1 . The absorption cross-sectional area value of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide at an excitation wavelength of 400 nm as shown in Figure 15 can be obtained; the curve of the pulsed laser threshold of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide varying with the effective exciton diameter of the core / alloy / shell structure quantum dots as shown in (b) in Figure 10 ; the graph of the continuous laser threshold of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide varying with the effective exciton diameter of the core / alloy / shell structure quantum dots as shown in (b) in Figure 11 .

[0156] In this example, by coating a cadmium mercury sulfide alloy layer 13 with a gradually increasing cadmium element content from the inside to the outside on the surface of the zinc blende phase mercury sulfide core 12, a gentle interface barrier is constructed and the surface of the quantum dots is passivated, thereby suppressing Auger recombination and being able to further reduce the laser threshold to obtain sustainable continuous laser emission. Using the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide with an effective exciton diameter of 4 nm to 5 nm as the gain medium can make the first electron energy level 1S e have a large electron occupancy probability, while the electron occupancy probability on the second electron energy level 1P e is not too large. As shown in (b) in Figure 10 , the pulsed laser threshold of the laser expressed by the number of excitons per quantum dot is as low as 0.35 and below. Among them, using the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide with an effective exciton diameter of 4.6 nm as the gain medium, the laser achieved pulsed laser emission. As shown in (b) in Figure 11 , using the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide with an effective exciton diameter of 4 nm to 5 nm as the gain medium, the laser also achieved a continuous laser threshold expressed by the excitation light intensity less than 1 KW cm -2 continuous laser emission. The emission wavelength of the laser covered the range of 1100 nm to 1230 nm. Among them, using the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide with an effective exciton diameter of 4.8 nm as the gain medium, the laser achieved Continuous laser emission.

[0157] This embodiment provides a method for preparing a quantum dot laser with a low threshold, which is used to prepare the above-mentioned quantum dot laser with a low threshold. The difference between this preparation method and that of Embodiment 3 is only that different quantum dots are prepared in the first step:

[0158] (1) The mercury sulfide core 12 is prepared by the thermal injection method, and the diameter of the mercury sulfide core 12 is controlled by controlling the reaction temperature and growth time; the mercury cadmium sulfide alloy layer 13 is prepared by the continuous ion layer adsorption reaction method, and the element ratio and thickness of the mercury cadmium sulfide alloy layer 13 are controlled by controlling the injection rates and injection amounts of mercury ions, cadmium ions and sulfur ions; the cadmium sulfide shell 14 is prepared by the continuous ion layer adsorption reaction method; the effective exciton diameter of the quantum dots 11 with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide is controlled by controlling the diameter of the mercury sulfide core 12 and the thickness of the mercury cadmium sulfide alloy layer 13, and finally, quantum dots 11 with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide with a total diameter of 12 nm and an effective exciton diameter range of 4 nm to 5 nm are obtained.

[0159] Based on the quantum confinement effect, the above-mentioned embodiment controls the electronic energy levels 1S e and 1P e by adjusting the quantum dot diameter, and further controls the relative positions of the 1S e energy level and the 1P e energy level with respect to the environmental Fermi level, and then controls the electron doping probabilities on the 1S e energy level and the 1P e energy level. The electron doping on the 1S e energy level and the 1P e energy level obtained by this method is stable in the atmospheric environment, and the obtained quantum dot laser has the characteristics of low gain threshold, low laser threshold, sustainable continuous laser output, designable emission wavelength and environmental stability.

[0160] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, rather than to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those of ordinary skill in the art should understand that they can still modify the technical solutions described in the foregoing embodiments, or perform equivalent replacements for some of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A quantum dot laser with a low threshold, characterized in that, It includes a quartz substrate, a silica periodic grating layer, a quantum dot layer, and a silica protective layer stacked in sequence. The quantum dot layer is composed of closely packed quantum dots doped with electrons. The quantum dots are mercury selenide quantum dots, mercury sulfide quantum dots, quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, or quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide. When the quantum dots are mercury selenide quantum dots, the mercury selenide quantum dots are in the zinc blende phase and have a diameter of 4.8 nm to 6.6 nm. When the quantum dots are mercury sulfide quantum dots, the mercury sulfide quantum dots are in the zinc blende phase and have a diameter of 4 nm to 5.2 nm. When the quantum dots are quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, the quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide are composed of a zinc blende phase mercury selenide core, a cadmium mercury selenide alloy layer coated on the surface of the mercury selenide core, and a cadmium selenide shell coated on the surface of the cadmium mercury selenide alloy layer. The cadmium element in the cadmium mercury selenide alloy layer gradually increases from the inside to the outside. The effective exciton diameter of the quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide is 4.8 nm to 6.4 nm. When the quantum dots are quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide, the quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide are composed of a zinc blende phase mercury sulfide core, a cadmium mercury sulfide alloy layer coated on the surface of the mercury sulfide core, and a cadmium sulfide shell coated on the surface of the cadmium mercury sulfide alloy layer. The cadmium element in the cadmium mercury sulfide alloy layer gradually increases from the inside to the outside. The effective exciton diameter of the quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide is 4 nm to 5 nm.

2. The quantum dot laser with a low threshold according to claim 1, characterized in that, The product of the grating period and the effective refractive index of the silica periodic grating is equal to the wavelength of the band-edge emission peak of the quantum dots in the quantum dot layer.

3. The quantum dot laser with a low threshold according to claim 1, characterized in that, The low-threshold quantum dot laser further includes a pump source for exciting the quantum dot layer.

4. A method for preparing a quantum dot laser with a low threshold, which is used to prepare the quantum dot laser with a low threshold described in any one of claims 1 to 3, characterized in that, It includes the following steps: S1. Prepare quantum dots. S2. Based on the quartz substrate, use reactive ion etching to form a silica periodic grating layer on the quartz substrate. S3. Disperse the quantum dots obtained in step S1 in a toluene solution to obtain a toluene dispersion of the quantum dots, and then spin-coat the toluene dispersion of the quantum dots on the silica periodic grating layer obtained in step S2 to obtain a quantum dot layer. S4. On the quantum dot layer obtained in step S3, obtain a silica protective layer by spin-coating spin-on glass.

5. The preparation method of the quantum dot laser with a low threshold according to claim 4, wherein The quantum dots in step S1 are mercury selenide quantum dots, mercury sulfide quantum dots, quantum dots with a core / alloy / shell structure of mercury selenide / cadmium mercury selenide / cadmium selenide, or quantum dots with a core / alloy / shell structure of mercury sulfide / cadmium mercury sulfide / cadmium sulfide. When the quantum dots are mercury selenide quantum dots, they are prepared by the thermal injection method, and the diameter of the quantum dots is controlled by controlling the reaction temperature and growth time. When the quantum dots are mercury sulfide quantum dots, they are prepared by the thermal injection method, and the diameter of the quantum dots is controlled by controlling the reaction temperature and growth time. When the quantum dots are quantum dots with a core / alloy / shell structure of mercury selenide / mercury cadmium selenide / cadmium selenide, it consists of a mercury selenide core, a mercury cadmium selenide alloy layer, and a cadmium selenide shell. The mercury selenide core is prepared by the thermal injection method, and the diameter of the mercury selenide core is controlled by controlling the reaction temperature and growth time; the mercury cadmium selenide alloy layer and the cadmium selenide shell are prepared by the successive ionic layer adsorption reaction method, and at the same time, by controlling the injection rates and injection amounts of mercury ions, cadmium ions, and selenium ions, the elemental ratio and thickness of the mercury cadmium selenide alloy layer are controlled; the diameter of the effective exciton is controlled by controlling the diameter of the mercury selenide core and the thickness of the mercury cadmium selenide alloy layer; When the quantum dots are quantum dots with a core / alloy / shell structure of mercury sulfide / mercury cadmium sulfide / cadmium sulfide, it consists of a mercury sulfide core, a mercury cadmium sulfide alloy layer, and a cadmium sulfide shell. The mercury sulfide core is prepared by the thermal injection method, and the diameter of the mercury sulfide core is controlled by controlling the reaction temperature and growth time; the mercury cadmium sulfide alloy layer and the cadmium sulfide shell are prepared by the successive ionic layer adsorption reaction method, and at the same time, by controlling the injection rates and injection amounts of mercury ions, cadmium ions, and sulfur ions, the elemental ratio and thickness of the mercury cadmium sulfide alloy layer are changed; the diameter of the effective exciton is controlled by controlling the diameter of the mercury sulfide core and the thickness of the mercury cadmium sulfide alloy layer.

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