WO2023115381A1 - Method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized lasers - Google Patents

Abstract

A method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized lasers, relating to the technical field of lasers. The method comprises the following steps: preparing a thin film sample from semiconductor nanocrystals, sample molecules being tightly and densely packed and fixed in position (S1); measuring the absorbance of the thin film sample by using an ultraviolet-visible absorption spectrometer (S2); generating a circularly polarized light beam by using a femtosecond laser and a circular polarizer, and focusing the circularly polarized light beam onto the thin film sample as pump light (S3); changing the intensity of the pump light, and measuring, by using another circularly polarized light beam as probe light, the intensity of a ground state bleaching signal of the thin film sample (S4); and determining whether the intensity of the ground state bleaching signal is equal to the absorbance, and if so, determining that the threshold light intensity of the pump light is a single-exciton gain threshold (S5).

Description

A Method for Single Exciton Gain in Semiconductor Nanocrystals Using Circularly Polarized Laser technical field

The invention relates to the field of laser technology, in particular to a method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light.

Background technique

Laser technology has a wide range of applications in communication, medical treatment, scientific research and other fields, and has great market demand. Laser threshold is one of the core issues in the field of laser research. How to reduce the laser threshold and find new materials with low threshold is the focus of laser technology.

At present, the research on the principle of laser has become mature, and the understanding of the laser threshold has gradually deepened. The basis of laser generation is the occurrence of amplified spontaneous emission. The electrons in the excited state jump back to the valence band, and generate photons through spontaneous emission, and these photons further induce the excited state electrons to generate stimulated emission, so as to achieve the purpose of light amplification. When stimulated emission produces more photons than stimulated absorption, the remaining photons will be emitted in the form of laser light. The incident light intensity just meets the laser emission condition is the threshold value.

In recent years, semiconductor nanocrystals have become a research hotspot of optical gain materials due to their excellent luminescent properties such as continuously tunable wavelength, narrow-band emission, and high photoluminescence quantum yield. However, in semiconductor nanocrystals, electrons in the excited state will combine with valence band holes to form excitons, and the interaction between excitons will make them combine to form multi-excitons. Due to the existence of Auger recombination, the multi-exciton Energy is lost in the form of heat, resulting in inefficient stimulated emission and difficult lowering of the threshold.

Contents of the invention

Aiming at the above-mentioned deficiencies in the prior art, the present invention provides a method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light.

The invention provides a method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light, comprising the following steps: using semiconductor nanocrystals to prepare thin film samples, so that the sample molecules are tightly packed and the position is fixed; using an ultraviolet-visible absorption spectrometer to measure The absorbance of the thin film sample; use a femtosecond laser and a circular polarizer to generate a beam of circularly polarized light, which is used as pump light to focus on the film sample; change the light intensity of the pump light, use another beam of circularly polarized light as the probe light, and detect The ground state bleaching signal intensity of the thin film sample; determine whether the ground state bleaching signal intensity is equal to the absorbance, and if so, determine that the critical light intensity of the pump light is the gain threshold of the single excitons.

Further, the semiconductor nanocrystal is a perovskite nanocrystal material, and the perovskite nanocrystal material includes CsPbBr ₃ , CsPbI ₃ , MAPbI ₃ or MAPbBr ₃ .

Further, the step of preparing the thin film sample includes: using a spin coater to spin coat the solution sample on a glass sheet, and after the solvent evaporates, the sample molecules are tightly packed and the position is fixed.

Further, measuring the absorbance of the film sample includes: measuring the absorbance of the film sample in the wavelength range of 400nm to 600nm, and the absorbance is measured according to the following formula:

In the formula, α ₀ represents the absorbance of the film sample in the wavelength range of 400nm–600nm; I ₀ represents the original intensity of white light; I represents the intensity of white light passing through the film sample without pump light excitation.

Further, the wavelength range of the pump light is 450nm-515nm.

Further, the wavelength range of the detection light is 450nm-550nm.

Further, the ground-state bleaching signal intensity is the change in the absorption of the probe light by the thin film sample under the conditions of pump light and no pump light excitation; the ground-state bleaching signal intensity is measured according to the following formula:

In the formula, Δα represents the ground state bleaching signal intensity of the film sample; I represents the intensity of the probe light passing through the film sample without pump light excitation; I ₁ represents the intensity of the probe light passing through the film sample under the excitation of pump light after the strength.

Further, judging whether the intensity of the ground-state bleaching signal is greater than or equal to the absorbance includes: when α ₀ +Δα≤0, stimulated radiation compensates the absorption and produces optical gain, and determines that the critical light intensity of the pump light is a single exciton The gain threshold of the pump light, wherein the critical light intensity of the pump light is the light intensity of the pump light when I ₁ =I.

Compared with the prior art, the invention provides a method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light. By changing the polarization of the excitation light, the variation of the absorption and stimulated emission of the film sample after excitation with the incident power is detected. , compared with the absorbance, the threshold of optical gain is lower when excited by circularly polarized light, indicating that the gain is dominated by single excitons.

Description of drawings

Through the following description of the embodiments of the present invention with reference to the accompanying drawings, the above-mentioned and other objects, features and advantages of the present invention will be more clear, in the accompanying drawings:

Figure 1 schematically shows a flow chart of a method for realizing single-exciton gain in semiconductor nanocrystals using circularly polarized laser light;

Figure 2 schematically illustrates a simplified setup diagram of a method for achieving single-exciton gain in semiconductor nanocrystals;

Figure 3 schematically shows the principle of circularly polarized light generating single-exciton gain;

Figure 4 schematically shows the test results when the pump light and the probe light have the same/opposite circular polarization;

Fig. 5 schematically shows the gain threshold diagram under circularly polarized light/non-circularly polarized light excitation.

Detailed ways

In order to make the object, technical solution and advantages of the present invention clearer, the present invention will be described in further detail below in conjunction with specific embodiments and with reference to the accompanying drawings. Apparently, the described embodiments are some, but not all, embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by persons of ordinary skill in the art without making creative efforts belong to the protection scope of the present invention.

The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to be limiting of the invention. The terms "comprising", "comprising" and the like used herein indicate the presence of stated features, steps, operations and/or components, but do not exclude the presence or addition of one or more other features, steps, operations or components.

In the present invention, unless otherwise clearly specified and limited, the terms "installation", "connection", "connection", "fixation" and other terms should be understood in a broad sense, for example, it can be a fixed connection or a detachable connection, Or integrated; it can be mechanically connected, or electrically connected, or can communicate with each other; it can be directly connected, or indirectly connected through an intermediary, and it can be the internal communication of two components or the interaction relationship between two components. Those of ordinary skill in the art can understand the specific meanings of the above terms in the present invention according to specific situations.

All terms (including technical and scientific terms) used herein have the meaning commonly understood by one of ordinary skill in the art, unless otherwise defined. It should be noted that the terms used herein should be interpreted to have a meaning consistent with the context of this specification, and not be interpreted in an idealized or overly rigid manner.

Fig. 1 schematically shows a flowchart of a method for realizing single exciton gain in semiconductor nanocrystals by using circularly polarized laser light. As shown in FIG. 1 , the method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser provided by the embodiment of the present invention includes the following steps:

Step S1, using semiconductor nanocrystals to prepare a thin film sample, so that the sample molecules are tightly packed and the position is fixed;

Step S2, using an ultraviolet-visible absorption spectrometer to measure the absorbance of the film sample;

Step S3, using a femtosecond laser and a circular polarizer to generate a beam of circularly polarized light, which is used as pump light to focus on the film sample;

Step S4, changing the light intensity of the pump light, using another beam of circularly polarized light as the detection light to detect the ground state bleaching signal intensity of the film sample;

Step S5, judging whether the intensity of the ground state bleaching signal is equal to the absorbance, and if so, determining that the critical light intensity of the pump light is the gain threshold of the single excitons.

Experiments show that the threshold under circularly polarized light excitation is significantly lower than that dominated by biexcitons under non-circularly polarized light excitation, indicating that the gain is dominated by single excitons.

In the embodiment of the present invention, the semiconductor nanocrystal is a perovskite nanocrystal material, and the perovskite nanocrystal material includes CsPbBr ₃ , CsPbI ₃ , MAPbI ₃ or MAPbBr ₃ .

In the embodiment of the present invention, the step of preparing a thin film sample includes: using a spin coater to spin coat the solution sample on a glass plate, and after the solvent evaporates, the sample molecules are tightly packed and the position is fixed, so as to generate laser light emitted in a specific direction.

In the embodiment of the present invention, measuring the absorbance of the film sample includes: measuring the absorbance of the film sample in the wavelength range of 400nm to 600nm, and the absorbance is measured according to the following formula:

In the formula, α ₀ represents the absorbance of the film sample in the wavelength range of 400nm–600nm; I ₀ represents the original intensity of white light; I represents the intensity of white light passing through the film sample without pump light excitation.

In the embodiment of the present invention, the wavelength range of the pump light is 450nm-515nm.

In the embodiment of the present invention, the wavelength range of the detection light is 450nm˜550nm.

In the embodiment of the present invention, the ground-state bleaching signal intensity is the change in the absorption of the probe light by the film sample under the conditions of pump light and no pump light excitation; the ground-state bleaching signal intensity is measured according to the following formula:

In the formula, Δα represents the ground state bleaching signal intensity of the film sample; I represents the intensity of the probe light passing through the film sample without pump light excitation; I ₁ represents the intensity of the probe light passing through the film sample under the excitation of pump light after the strength.

In the embodiment of the present invention, judging whether the intensity of the ground-state bleaching signal is greater than or equal to the absorbance includes: when α ₀ +Δα≤0, the stimulated radiation compensates for the absorption and generates optical gain, and determining the critical light intensity of the pump light is The gain threshold of the single excitons, wherein the critical light intensity of the pump light is the light intensity of the pump light when I ₁ =I.

Through the embodiment of the present invention, the gain generated by the sample is measured by using absorption spectrum and transient absorption spectrum. First, measure the absorbance of the sample in the range of 400nm to 600nm with an ultraviolet-visible absorption spectrometer to obtain the ratio of the original intensity of white light to the intensity after passing through the sample. Then, the detection light intensity that passes through the sample when there is pump light is obtained through the experimental device of transient absorption spectroscopy. Compared with the case where biexcitons dominate under non-circularly polarized excitation, the gain threshold is significantly lower when excited with circularly polarized light, indicating that the gain is dominated by single excitons.

The present invention uses absorption spectrum and transient absorption spectrum to measure the gain produced by the sample. The specific device is as follows: the 1030nm femtosecond pulse generated by the femtosecond laser is divided into two beams after passing through the beam splitter, and one of them passes through the optical parametric amplifier. Continuously adjustable as pump light. The other beam passes through the BBO crystal (β-phase barium metaborate crystal, β-BaB ₂ O ₄ ) and the sapphire crystal, and then the wavelength range is expanded to 450nm-550nm as the probe light. By changing the time difference between the pump light and the probe light arriving at the sample, the changes that occur in the sample within a period of time after the pump light excites the sample can be obtained. After the pump light excites some electrons to the conduction band, the absorption of the probe light by the sample will decrease. At the same time, due to the presence of stimulated radiation, the intensity of the probe light passing through the sample will be greater than that transmitted through the sample without pump light excitation. The detection light intensity is called the ground state bleaching signal.

Figure 2 schematically shows a simplified setup diagram of a method for achieving single-exciton gain in semiconductor nanocrystals.

In order to measure the gain threshold situation under different polarization excitations, a simplified experimental setup for single exciton gain measurement as shown in Fig. 2 was adopted. After the pump light passes through the circular polarizer, a beam of left-handed circularly polarized light is generated, which is incident on the sample at a certain angle; then, another beam of left-handed circularly polarized light is focused on the sample by using a linear polarizer and a 1/4 wave plate. The transmitted light is collected after the sample to detect the ground state bleaching signal intensity of the sample. Combining this ground state bleaching signal intensity with the absorbance, the gain threshold can be calculated.

It should be noted that two achromatic prisms and an optical fiber collector are arranged behind the tested sample in FIG. 2 . It can be understood that achromatic prisms are mainly used for chromatic aberration correction and light focusing. The optical fiber collector is mainly used to collect the light required for the test, effectively improving the light collection efficiency.

Based on the above disclosure, the principle of achieving single-exciton gain with circularly polarized light will be described in detail below.

Figure 3 schematically shows the principle of circularly polarized light generating single-exciton gain. As shown in Figure 3, considering the doubly degenerate valence and conduction bands, the electron spin can take ±1/2. When excited with non-polarized light, the nanocrystals in the ground state will absorb, reducing the intensity of the transmitted light, while the nanocrystals in the single-exciton state will not only absorb part of the photons, but also generate part of the photons due to stimulated radiation, Since the number of photons absorbed and generated are equal, the light intensity does not change when the excitation light passes through the nanocrystal, and the nanocrystal is transparent to the excitation light. However, when the nanocrystal is in the biexciton state, the conduction band population has been filled and the population inversion is realized, so the nanocrystal will not absorb the excitation light, but will emit additional photons due to the stimulated radiation, resulting in gain. In the case of non-polarized light excitation, when the number of biexciton nanocrystals generated after excitation is greater than the number of nanocrystals in the ground state, photons can be emitted. In the case of circularly polarized light excitation, only electrons satisfying the transition selection rule can be excited to the conduction band. Therefore, only when the nanocrystal is in the ground state, it will absorb the excitation light. And when the nanocrystal is in the single exciton state, the gain can already be generated. Therefore, in the case of excitation by a particular circularly polarized light, the condition for gain to occur is that the number of nanocrystals in the single exciton state is greater than the number of nanocrystals in the ground state. Compared with the case of non-polarized light excitation, when using circularly polarized light excitation, the threshold condition to be satisfied is lower, and the gain is mainly dominated by single excitons.

The following experiment proves that the single-exciton gain effect can be achieved by using circularly polarized light excitation.

Fig. 4 schematically shows the test results when the pump light and the probe light have the same/opposite handedness circular polarization. Fig. 5 schematically shows the gain threshold diagram under circularly polarized light/non-circularly polarized light excitation.

Embodiment 1 is a spin lifetime test. In the case of left-handed circularly polarized light excitation, left-handed and right-handed circularly polarized light are respectively used to detect the ground state bleaching signal of the sample, and the experimental results shown in FIG. 4 can be obtained. After the sample is excited by left-handed circularly polarized light, the signal detected by using left-handed circularly polarized light rises rapidly to the maximum value, while the signal detected by right-handed circularly polarized light rises slowly, and the two coincide after 7 ps, which indicates that the pumping The light generates electrons with an angular momentum of -1/2. Due to the transition selection rule, only the left-handed circularly polarized light can detect the corresponding ground-state bleaching signal. As the electron spin relaxes to 1/2, the right-handed circular polarization The light gradually detects the signal, therefore, the polarization degree generated by the excitation light can be obtained as 75% through the amplitude difference of the two signals. This shows that within 7ps, the polarization of electrons can be achieved by using circularly polarized light excitation, reducing the degeneracy of energy levels, and only generating single excitons, which reflects the feasibility of single exciton gain.

Since the incident light polarization can only be maintained at 7 ps, the gain data at 1.5 ps after excitation is mainly detected. Adjust the incident light power, and record the change curve of the ground state bleaching signal with the power. When α ₀ +Δα≤0, there is gain. In order to compare with the case of biexciton gain, Fig. 5 shows the graph of the ground state bleaching signal versus power under circularly polarized light/non-circularly polarized light excitation. It can be seen that when non-circularly polarized light is used for excitation, the gain is given by When biexcitons dominate, the threshold is 140 μJ/cm ² , while when excited with circularly polarized light, the threshold is 90 μJ/cm ² , which is significantly smaller than the case of non-circularly polarized excitation. Thus, it is experimentally demonstrated that single-exciton gain can be achieved using circularly polarized light.

To sum up, the embodiment of the present invention provides a method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light. By changing the polarization of the excitation light, the absorption and stimulated emission of the thin film sample after excitation vary with the incident power. Compared with the absorbance, it can be concluded that the threshold of optical gain is lower when excited by circularly polarized light, indicating that the gain is dominated by single excitons.

It should also be noted that, unless specifically described or the steps must occur sequentially, the order of the above steps is not limited to the one listed above, and can be changed or rearranged according to the desired design. Moreover, the above-mentioned embodiments can be mixed and matched with each other or with other embodiments based on design and reliability considerations, that is, technical features in different embodiments can be freely combined to form more embodiments.

In the description of the present invention, it should be understood that the orientation or positional relationship indicated by the terms "upper", "lower", "front", "rear", "left", "right" etc. are based on those shown in the accompanying drawings. Orientation or positional relationship is only for the convenience of describing the present invention and simplifying the description, and does not indicate or imply that the referred device or element must have a specific orientation, be constructed and operated in a specific orientation, and thus should not be construed as a limitation of the present invention.

Throughout the drawings, the same elements are indicated by the same or similar reference numerals. Conventional structures or constructions will be omitted when it may obscure the understanding of the present invention. And the shape, size, and positional relationship of each component in the figure do not reflect the actual size, proportion and actual positional relationship. In addition, in the description of the present invention, "plurality" means at least two, such as two, three, etc., unless otherwise specifically defined.

The specific embodiments described above have further described the purpose, technical solutions and beneficial effects of the present invention in detail. It should be understood that the above descriptions are only specific embodiments of the present invention and are not intended to limit the present invention. Any modifications, equivalent replacements, improvements, etc. made within the spirit and principles of the present invention shall be included within the protection scope of the present invention.

Claims (8)

1. A method utilizing circularly polarized lasers to realize single-exciton gain in semiconductor nanocrystals, characterized in that it comprises the following steps:

   Using semiconductor nanocrystals to prepare thin film samples, so that the sample molecules are tightly packed and the position is fixed;

   Measure the absorbance of the thin film sample using an ultraviolet-visible absorption spectrometer;

   Using a femtosecond laser and a circular polarizer to generate a beam of circularly polarized light, which is used as pump light to focus on the thin film sample;

   changing the light intensity of the pump light, using another beam of circularly polarized light as the detection light to detect the ground state bleaching signal intensity of the thin film sample;

   Judging whether the intensity of the ground-state bleaching signal is equal to the absorbance, and if so, determining that the critical light intensity of the pump light is the gain threshold of the single excitons.
2. The method for utilizing circularly polarized laser light to realize monoexciton gain in semiconductor nanocrystals according to claim 1, wherein the semiconductor nanocrystals are perovskite nanocrystal materials, and the perovskite nanocrystal materials include CsPbBr ₃ , _CsPbI3 , _MAPbI3 or _MAPbBr3 .
3. The method of utilizing circularly polarized laser light to realize single exciton gain in semiconductor nanocrystals according to claim 1, wherein the step of preparing a thin film sample comprises:

   The solution sample was spin-coated onto a glass slide using a spin coater, and the sample molecules were tightly packed and the position was fixed after the solvent evaporated.
4. The method for realizing single-exciton gain in semiconductor nanocrystals by utilizing circularly polarized laser light according to claim 1, wherein said measuring the absorbance of said film sample comprises:

   Measure the absorbance of the film sample in the wavelength range of 400nm to 600nm, and the absorbance is measured according to the following formula:

   

   In the formula, α ₀ represents the absorbance of the film sample in the wavelength range of 400nm–600nm; I ₀ represents the original intensity of white light; I represents the intensity of white light passing through the film sample without pump light excitation.
5. The method for realizing single-exciton gain in semiconductor nanocrystals by using circularly polarized laser light according to claim 1, wherein the wavelength range of the pump light is 450nm-515nm.
6. The method for realizing single exciton gain in semiconductor nanocrystals by using circularly polarized laser light according to claim 1, wherein the wavelength range of the probe light is 450nm-550nm.
7. The method for realizing single-exciton gain in semiconductor nanocrystals by utilizing circularly polarized laser light according to claim 4, wherein the ground state bleaching signal intensity is the same as that of the thin film under the conditions of pump light and no pump light excitation. The amount of change in the sample's absorption of the probe light;

   The ground state bleaching signal intensity is measured according to the following formula:

   

   In the formula, Δα represents the ground-state bleaching signal intensity of the film sample; I represents the intensity of white light passing through the film sample without pump light excitation; I ₁ represents the intensity of the probe light passing through the film sample under the excitation of pump light Strength of.
8. The method for realizing monoexciton gain in semiconductor nanocrystals by using circularly polarized laser light according to claim 7, wherein said judging whether the ground state bleaching signal intensity is greater than or equal to said absorbance comprises:

   When α ₀ +Δα≤0, the stimulated radiation compensates the absorption and produces optical gain, and the critical light intensity of the pump light is determined to be the gain threshold of the single excitons, wherein the pump light The critical light intensity is the light intensity of the pump light when I ₁ =I.

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