Resonance excitation method based on double high-Q mode microcavity

Abstract

The application relates to a resonance excitation method based on a double-high-Q-value mode microcavity, and belongs to the technical field of optoelectronic devices. The resonance excitation method reduces the quality factor of a polarization mode providing a window for resonance excitation light, widens the spectral line width, reduces the difficulty of producing overlap, and improves the tolerance of a resonant cavity.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of optoelectronic devices, and particularly relates to a resonance excitation method based on a double-high-Q-value mode microcavity. BACKGROUND

[0002] For many years, an ideal single-photon source has been the goal pursued by people. A quantum dot system can well meet the requirement of exciting only one photon at a time due to unique physical characteristics. However, since a quantum dot is not an ideal two-level system, the homogeneity of photons is difficult to guarantee. Resonance excitation as a means of generating excitons can avoid unnecessary relaxation time and guarantee the homogeneity of the emitted photons. However, this also brings certain problems. Since the pump light and the emitted single photon have the same wavelength during resonance excitation, how to effectively filter the excitation light becomes a problem to be solved. So far, the most effective method is polarization filtering. A linearly polarized pump laser is used to excite the quantum dot, and an orthogonal polarizer is used at the single-photon collection end to eliminate the excitation light. However, in the double-degenerate transition of a single-exciton quantum dot, the optical selection rule stipulates that the resonance fluorescence photon is randomly right-handed or left-handed circularly polarized. While the pump light is filtered, the single photon with the same resonance characteristics is also filtered, reducing the system efficiency of the quantum dot single-photon source by at least two times. A possible solution is to excite the quantum dot into the waveguide mode of the microcylinder from the side and collect the single photon from the top. However, the spatially orthogonal excitation and collection method has only tried this method in a low-Purcell-effect and non-polarized quantum dot device, and has not realized a high-performance single-photon source without background light.

[0003] Wang H, He Y, Chung T H, et al. Toward optimal single-photon sources from polarized microcavities [J]. Nature Photonics, 2019, 13(11): 770-775.]. The asymmetric elliptical microcylinder cavity has two orthogonal polarization microcavity modes, one is the mode polarized along the long axis of the ellipse and the other is the mode polarized along the short axis of the ellipse, hereinafter referred to as V mode and H mode. The quantum dot can be coupled to a certain polarization mode, and the single photon in this polarization state is excited by the polarization selective Purcell effect. Specifically, the quantum dot coupled to the H mode is excited by the resonant excitation light polarized in the V direction, and the quantum dot is excited to emit single photons polarized in the H direction with a high probability through the polarization selective Purcell effect. Finally, the excitation light polarized in the V direction is filtered by a polarizer to achieve a certain indistinguishability and emission efficiency. However, there is a problem. Generally, the H mode and the V mode of the microcavity are not at the same wavelength. How to make the V polarized excitation light with the same wavelength as the H mode coupled into the microcavity? At this time, the two polarization modes of the microcavity need to have a reasonable overlap. This reasonable overlap not only needs to ensure that the resonant excitation light polarized in the V direction with the same wavelength as the H mode is coupled into the microcavity to provide energy for the quantum dot to generate excitons, but also needs to control the degree of overlap to ensure the purity of the polarization selective Purcell effect and avoid excessive interference of the V mode on the H mode. It should be emphasized here that when the size of the microcavity is relatively large, the quality factors of the V and H modes are similar. When the quality factor of the microcavity is low, the line widths of the two modes are wide, and it is relatively easy to control the ellipticity of the microcavity to adjust the distance between the two polarization modes. When the quality factor of the microcavity is high, it is difficult to control the reasonable overlap due to the narrowing of the cavity mode line width, and the non-symmetry of the microcavity needs to be precisely controlled. Therefore, the quality factor of the microcavity is restricted, which affects the improvement of the Purcell coefficient. This causes a contradiction, which limits the further improvement of the spontaneous emission rate of the quantum dot and the application of this resonant excitation method in the strong coupling system. SUMMARY

[0004] (I) Technical problems to be solved

[0005] The technical problem to be solved by the present application is to eliminate part of the contradiction in the resonant excitation of the existing single photon source, and to provide an improved resonant excitation method.

[0006] (II) Technical solutions

[0007] In order to solve the above technical problems, the application provides a resonance excitation method based on a double-high-Q-value mode microcavity, comprising the following steps: constructing two modes with a preset Q value in a resonant cavity forbidden band containing a quantum dot; splitting the microcavity mode by appropriately changing the symmetry of the resonant cavity, so that the H mode split from the short-wave basic mode and the V mode split from the long-wave basic mode are overlapped; coupling the quantum dot into the asymmetric resonant cavity; exciting the quantum dot coupled with the H mode by using the V-polarized resonance excitation light; promoting the quantum dot to excite the H-polarized single photon with a high probability by the polarization-selected Purcell effect; and finally filtering the V-polarized excitation light by using a polarizer to achieve a certain indistinguishability and emission efficiency.

[0008] Preferably, the change of the symmetry of the resonant cavity is achieved by changing the symmetry of the XY section of the resonant cavity to split the microcavity mode into two orthogonal polarization modes.

[0009] Preferably, the change of the symmetry of the resonant cavity is achieved by changing the XY section of the resonant cavity to split the microcavity mode into two orthogonal polarization modes.

[0010] Preferably, the construction of the two modes with a preset Q value is achieved by introducing a thickness gradient layer in the microcolumn cavity to affect the construction of the microcavity mode and promote the generation of two modes with a quality factor higher than the preset value.

[0011] Preferably, the H mode and the V mode are the microcavity modes polarized along the long axis of the XY elliptical section and the microcavity modes polarized along the short axis of the XY elliptical section, respectively.

[0012] Preferably, the overlap of the two polarization modes can provide a window for the V-polarized resonance excitation light to be coupled into the resonant cavity.

[0013] Preferably, the resonance excitation method is achieved based on the polarization-selected Purcell effect, and the Purcell factor represents the influence of the resonant cavity on the spontaneous emission rate of the quantum dot, and the formula is as follows:

[0014]

[0015] Wherein, Q is the quality factor of the microcavity, V is the mode volume of the microcavity, n, λ c , ω c , ω, δω are the effective refractive index, the resonant wavelength of the resonant cavity, the resonant frequency of the resonant cavity, and the center frequency and full width at half maximum of the exciton transition, respectively.

[0016] The application also provides an application of the method in a columnar resonant cavity single photon source.

[0017] The application also provides an application of the method in a strong coupling scenario.

[0018] The application further provides an application of the method in the field of quantum information processing technology.

[0019] (III) Beneficial Effects

[0020] 1. The resonance excitation method can be applied to a strong coupling scenario and has extremely important application value in the field of quantum information processing.

[0021] 2. The resonance excitation method reduces the quality factor of the polarization mode providing a window for resonance excitation light, broadens the spectral linewidth, reduces the difficulty of producing overlap, and improves the tolerance of resonant cavity manufacturing.

[0022] 3. The resonance excitation method can make the mode wavelengths of two modes completely overlap by fine adjustment, and use the difference in the quality factors of the two modes to generate single photons with fixed polarization. In a weak coupling system, the improved method can make quantum dots coupled with microcavity modes with a higher Purcell factor, eliminate the limitation of the Purcell factor, and realize higher intensity excitation of the light source.

[0023] 4. The resonance excitation method benefits from the high quality factor of the microcavity and can even realize high-efficiency resonance excitation in a strong coupling system.

[0024] 5. The resonance excitation method is suitable for all quantum dot-resonant cavity single photon sources with two basic modes and polarization degeneracy. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The figure is a cross-sectional and cross-sectional schematic diagram of the micro-column resonant cavity single photon source of the application.

[0026] Figure 2 The figure is an optical bandgap and a mode spectrum diagram corresponding thereto under different ellipticity factors of the resonance excitation method of embodiment 1 of the application applied to a column resonant cavity single photon source.

[0027] Figure 3 The figure is a resonant cavity mode distribution diagram when two polarization modes are the same in the resonance excitation method of embodiment 2 of the application applied to a column resonant cavity single photon source. DETAILED DESCRIPTION

[0028] In order to make the purpose, content, and advantages of the application more clear, the specific embodiments of the application are further described in detail below in combination with the drawings and examples.

[0029] The present application aims at eliminating part of the contradiction in the existing single photon source resonance excitation, and proposes an improved resonance excitation method based on double high-Q value mode microcavities: two modes with high quality factors (Q values) are constructed in the forbidden band of the resonant cavity containing quantum dots, the symmetry of the resonant cavity is appropriately changed to make the microcavity mode split, and the H mode split from the short-wave basic mode and the V mode split from the long-wave basic mode overlap. The quantum dots are coupled to the asymmetric resonant cavity, the quantum dots coupled to the H mode are excited by the V-polarized resonance excitation light, the Purcell effect of polarization selection is used to promote the quantum dots to excite H-polarized single photons with high probability, and finally the V-polarized excitation light is filtered by a polarizer to realize almost perfect indistinguishability and emission efficiency.

[0030] Since the two polarization modes are split in different basic modes, the quality factors are not related, so the quality factor of the polarization mode providing a window for the resonance excitation light is greatly reduced, the spectral linewidth is broadened, and the difficulty of overlap is reduced. Even by fine adjustment, the mode wavelengths of the two modes are completely overlapped, and the polarization single photons are generated by using the quality factor difference of the two modes. At the same time, the quality factor of the polarization mode coupled to the quantum dots is no longer limited, so that this resonance method can be applied to the strong coupling scene.

[0031] In the above method, the change (destruction) of the symmetry of the resonant cavity is achieved by changing the symmetry of the XY cross section of the resonant cavity, and the microcavity mode is split into two orthogonal polarization modes, for example, the XY cross section is changed from a circle to an ellipse [Gayral B, Gerard J M, Legrand B, et al. Optical study of GaAs / AlAs pillar microcavities with elliptical cross section [J]. Applied Physics Letters, 1998, 72(12): 1421-1423.].

[0032] In the above method, the construction of the two high-Q value modes is achieved by introducing a thickness gradient layer in the microcylinder cavity to affect the composition of the microcavity mode and promote it to generate two modes with high quality factors.

[0033] In the above method, the H mode and the V mode are in turn the microcavity mode polarized along the long axis of the XY elliptical cross section and the microcavity mode polarized along the short axis of the XY elliptical cross section.

[0034] The purpose of the overlap of the two polarization modes in the above method is to provide a window for the coupling of the V-polarized resonant excitation light into the resonant cavity. Since the quantum dots are buried in the middle layer of the entire resonant cavity, without the corresponding resonant cavity mode providing a window, the excitation light is difficult to couple into the resonant cavity, and the quantum dots are effectively excited.

[0035] The resonant excitation method in the above method is based on the polarization-selective Purcell effect [Moreau E, Robert I, Gerard J, et al. Single-mode solid-state single photon source based on isolated quantum dots in pillar microcavities [J]. Applied Physics Letters, 2001, 79 (18): 2865-2867.], which is an improvement on the previously proposed resonant excitation method [Wang H, He Y, Chung T H, et al. Towards optimal single-photon sources from polarized microcavities [J]. Nature Photonics, 2019, 13 (11): 770-775.]. The quantum dots are coupled to an asymmetric geometric birefringent cavity, the quantum dots coupled to the H mode are excited by V-polarized resonant excitation light, the polarization-selective Purcell effect is used to promote the quantum dots to emit H-polarized single photons with a high probability, and finally a polarizer is used to filter the V-polarized excitation light, achieving almost perfect indistinguishability and emission efficiency. The Purcell factor represents the influence of the resonant cavity on the spontaneous emission rate of the quantum dots, and its formula is as follows:

[0036] Where Q is the quality factor of the microcavity, V is the mode volume of the microcavity, n, λ c 、ω c , ω, δω are the effective refractive index, the resonant wavelength of the resonant cavity (microcavity mode wavelength), the resonant frequency of the resonant cavity, and the center frequency and full width at half maximum of the exciton transition, respectively.

[0037] Assuming that the charged quantum dots are resonantly coupled to the H mode and are detuned from the V mode, the Purcell factors of the two modes are both related to P, where P is the spatial overlap of the quantum dots and the resonant cavity. Therefore, the ratio of the spontaneous emission rates of the emitted H and V polarized light of the quantum dots can be used to evaluate the polarization characteristics of the emitted photons.

[0038] The resonant excitation method described in the present application is further described below by way of examples and in conjunction with the accompanying drawings.

[0039] Example 1

[0040] The embodiment is applied to an InGaAsP / (InP-slit) micro-pillar resonant cavity single photon source with an elliptical cross section containing InAs / InP quantum dots, and the specific structure is as shown in Figure 1 .

[0041] The resonant excitation method can be applied to an InGaAsP / (InP-slit) micro-pillar resonant cavity single photon source with an elliptical cross section containing InAs / InP quantum dots, and the resonant cavity structure in the single photon source is as shown in Figure 1 , including an InP substrate, an InP center layer, an InAs / InP quantum dot photon emission source, an InGaAsP layer, and a conventional DBR with a constant thickness composed of an (InP-slit) layer and a DBR with a gradually changing thickness. The long axis length of the InGaAsP layer is D / α (α≤1), and the short axis length is Dα; the long axis length of the InP layer is d / β (β≤1), and the short axis length is dβ, and the long axis and the short axis of the InGaAsP and the InP are located on the same straight line. Generally, we let α=β, and define e=α -1 -1 as an elliptical factor. The thickness of the InP layer in the conventional DBR is t1=λ B / 4, where λ B is the Bragg wavelength, which is about 1.55 μm; the thickness of the InGaAsP layer is t2=λ B / (4n2), where n2 is the refractive index of InGaAsP. The thicknesses of the InP-slit layer and the InGaAsP layer in the DBR with a gradually changing thickness are linearly reduced to the center of the resonant cavity according to t 1i =t1[1-ρ(2i-1)] and t 2i =t2(1-2ρi), respectively, where i is the layer number of the gradually changing DBR, and ρ is the gradual change rate, that is, the proportion of the thickness reduction of each layer. The thickness of the InP center layer is t0=t1(1-2ρN), where N is the number of pairs of the single-sided gradually changing DBR (the number of the gradually changing DBRs on the upper part and the lower part is the same). The InGaAsP / (InP-slit) on the top and the bottom of the conventional DBR is 4 pairs and 6.5 pairs, respectively, the single-sided gradually changing DBR is N=3, and the height of the micro-pillar resonant cavity is about 7 μm.

[0042] When the symmetry of the microcavity is not broken, the microcavity can exist in two basic modes at the same time through reasonable parameter adjustment, and in order to distinguish, they are re-marked as MODE A and MODE O. As shown in the spectrum of Figure 2 , when the microcavity becomes elliptical, these modes will be divided into four modes. Here, we re-label these basic modes as MODE A and O in the H mode, and MODE A and O in the V mode, which are called MODE O V , O Hand MODE A V A H By reasonably adjusting the effective radius D of InGaAsP and the effective radius d of InP, MODE A H The wavelength was fixed at 1.55 μm, while the Q-factor was optimized to maintain it at a high level. Figure 2 As can be seen from this, under different e values, MODEA H It remains in the 1.55μm band with a very narrow linewidth. As e increases, mode splitting intensifies, leading to mode... V Blue shift and gradually approach MODEA H At e = 0.2, the wavelength shifts to the vicinity of 1.55 μm, resulting in overlap. Due to MODE O V Splitting into another fundamental mode, the Q factor will not be related to the MODEA. H Because of this correlation, its Q factor is low, resulting in a wider spectral linewidth. That is, regardless of how narrow the mode's linewidth is, the ellipticity factor can be controlled to allow the mode splitting into another fundamental mode to function correctly. V To create a suitable overlap.

[0043] Example 2

[0044] This embodiment uses the micropillar resonant cavity structure described in Embodiment 1.

[0045] When e = 0.20, D = 0.9 μm, and d = 0.222 μm, MODE O V and A H The mode wavelengths are all around 1.55 μm. (Example of mode distribution) Figure 3 As shown. At this time, the Q factor of the H mode is as high as 1.38 × 10. 5 Q / √V is approximately 4.58 × 10 5 MODE O M The quality factor is 732.8, and Q / √V is approximately 603. Theoretically, [Song H, Takemoto K, Miyazawa T, et al. High quality-factor Si / SiO2-InP hybrid micropillar cavities with submicrometer diameter for 1.55-μm telecommunication band[J]. Optics Express, 2015, 23(12): 16264-16272], Q / √V > 10. 4 This is sufficient to enable strong coupling between quantum dots and microcavity modes. Therefore, it possesses a high Q factor (Q ~ 10). 5 V<1(λ / n) 3 AH The modes can achieve high efficiency coherent control in the 1.55 μm band. Meanwhile, the huge difference in quality factors of the two modes makes the mode O V The quantum dot can be provided with energy without interfering with the coupling between the quantum dot and the H mode.

[0046] The Purcell factor formula is:

[0047]

[0048] Since both polarization modes are around 1.55 μm, the quantum dot is adjusted by temperature or other means to couple with the two modes. At this time, the detuning term of the Purcell factors of the two modes, i.e. the ratio of the two mode F p s, i.e. the ratio of the two mode F

[0049] The above is only the preferred embodiment of the present application, it should be noted that for those skilled in the art, without departing from the technical principles of the present application, can make a number of improvements and modifications, these improvements and modifications should also be considered as the protection scope of the present application.

Claims

1. A method for resonant excitation based on a double high-Q mode microcavity, characterized in that, The method comprises the following steps: constructing two modes with a preset Q value in a resonant cavity band containing quantum dots, splitting the microcavity modes by appropriately changing the symmetry of the resonant cavity, causing the H mode split from the short-wave basic mode and the V mode split from the long-wave basic mode to overlap, coupling the quantum dots into the asymmetric resonant cavity, exciting the quantum dots coupled with the H mode by using resonant excitation light with V polarization, promoting the quantum dots to emit single photons with H polarization with a high probability by the polarization-selected Purcell effect, and finally filtering the excitation light with V polarization by using a polarizer to achieve indistinguishability and emission efficiency. The resonant excitation method is realized based on the polarization-selected Purcell effect, and a Purcell factor represents the influence of the resonant cavity on the spontaneous radiation rate of the quantum dots, and the formula is as follows: wherein Q is the quality factor of the microcavity, V is the mode volume of the microcavity, n, λ c , ω c , ω, δω are the effective refractive index, the resonance wavelength of the resonant cavity, the resonance frequency of the resonant cavity, the center frequency of the exciton transition and its full width at half maximum, respectively.

2. The method of claim 1, wherein, The change of the symmetry of the resonant cavity is realized by changing the XY section of the resonant cavity into an ellipse, and the microcavity mode is split into two orthogonal polarization modes.

3. The method of claim 1, wherein, The XY section of the resonant cavity is changed into an ellipse when the symmetry of the resonant cavity is changed.

4. The method of claim 1, wherein, The construction of the two modes with a preset Q value is realized by introducing a thickness gradient layer into the microcavity to affect the construction of the microcavity mode and promote the generation of two modes with a quality factor higher than the preset value.

5. The method of claim 1, wherein, The H mode and the V mode are microcavity modes polarized along the long axis of the XY ellipse section and the short axis of the XY ellipse section, respectively.

6. The method of claim 1, wherein, The overlap of the two polarization modes can provide a window for coupling the resonant excitation light with V polarization into the resonant cavity.

7. Application of the method in any one of claims 1 to 6 to a columnar resonant cavity single-photon source.

8. Application of the method in any one of claims 1 to 6 to a strong coupling scenario.

9. Application of the method in any one of claims 1 to 6 to the field of quantum information processing technology.

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