Low power trapped cold atom platform based on surface plasmons

Abstract

The application discloses a low-power cold atom capturing platform based on surface plasmons, and a waveguide formed on the upper surface of a gold film can transmit surface plasmons; the evanescent field of red and blue detuned surface plasmons forms an optical dipole trap on the upper surface of the waveguide, which can stably trap cold atoms, and then the cold atoms are transmitted. The cold atom capturing platform is composed of a SiO2 base-gold film-SiO2 waveguide; a bidirectional red detuned TM mode and a unidirectional blue detuned optical TM mode are excited in the waveguide, and a stable optical dipole trap is formed on the upper surface of the waveguide for trapping cold atoms. Due to the enhancement effect of surface plasmons, a large atomic capturing trap can be obtained by using lower waveguide mode power. By adjusting the phase difference of the bidirectional red detuned mode, the dipole trap array can be moved to drive the cold atoms trapped therein to move, and the cold atoms can be guided on the waveguide surface.

Description

Technical Field

[0001] This invention relates to the field of integrated optics, and more particularly to a platform for low-power trapping of cold atoms based on surface plasmons. Background Technology

[0002] Integrated quantum-photonic chips, with their high scalability and strong light-matter interaction, have become a promising technology for quantum optics and quantum information processing research. In the past decade or so, quantum information processing has made significant progress by utilizing the quantum properties of atoms and photons; both atomic internal states and photonic states can provide carriers for the encoding, storage, and transmission of quantum information. Quantum information processing based on cold atoms and photons has been extensively studied. In recent years, photonic integrated chips have been widely applied in quantum information science, with the potential to extend photonic quantum information processors to tens of qubits. In contrast, previous research on cold atoms has largely relied on traditional optical devices, which are cumbersome, sensitive to environmental disturbances, and costly. Therefore, atomic chips have received considerable attention over the past few decades.

[0003] Due to the broad application prospects of photon-atomic interactions in photonic integrated circuits, researchers have recently focused on combining nanophotonic devices with neutral atoms to achieve efficient quantum devices. Methods for confining atoms to the surface of waveguide structures using near-field optical dipole traps have been developed, including nanofiber structures, two-dimensional array structures, photonic crystal nanocavities, and suspended ridge waveguides. However, these atom trapping methods are limited by large size, shallow potential wells, and high trapping power, making it difficult to generate efficient and stable atom trapping traps.

[0004] Atom trapping on waveguide surfaces involves inputting red-blue detuned light into the waveguide. The red-blue detuned light propagating within the waveguide generates an evanescent field on the waveguide surface. By utilizing the different evanescent field attenuation lengths of the red and blue detuned light, an optical dipole trap at a subwavelength height above the waveguide surface is obtained. The blue detuned light provides repulsion, while the red detuned light provides attraction. When the power ratio of these two lights is controlled, a stable trapping trap is generated on the waveguide surface through the combined effect of the optical dipole trap generated by the red-blue detuned light and the van der Waals potential of the waveguide surface. Currently, most researchers use this design to trap cold atoms. However, the potential trap depth of this scheme is limited by the waveguide structure. Once the waveguide structure is determined, the evanescent field distribution of the waveguide modes on the surface is fixed, thus limiting the effective potential trap depth.

[0005] In view of this, it is indeed necessary to provide a low-power platform for trapping cold atoms based on surface plasmons to solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a highly efficient and stable platform for low-power cold atom trapping based on surface plasmon resonances.

[0007] To achieve the above objectives, the present invention provides a low-power cold atom trapping platform based on surface plasmon resonance (SPR), comprising a waveguide, a gold film, and a substrate. The platform employs a combination of modes including unidirectional transmission of blue detuned TM0 and TM1 modes, and bidirectional transmission of red detuned TM0 mode. By using a preset mode power, the evanescent field of the combination mode can form an optical dipole trap on the upper surface of the waveguide to trap cold atoms.

[0008] As a further improvement of the present invention, the low-power cold atom trapping platform based on surface plasmons uses the evanescent field of surface plasmons to form an optical dipole trap.

[0009] As a further improvement of the present invention, the platform for low-power trapping cold atoms based on surface plasmon resonance adopts the bidirectional transmission red-detuned TMO mode, interferes with the waveguide to form a standing wave field, and generates a moving optical dipole trap by changing the phase difference of the bidirectional red-detuned wave, thereby driving the cold atoms trapped in the optical dipole trap to move.

[0010] As a further improvement of the present invention, the waveguide is located above the gold film.

[0011] As a further improvement of the present invention, both the waveguide and the substrate are made of silicon dioxide.

[0012] As a further improvement of the present invention, the width of the waveguide is 700–900 nm, and the height of the waveguide is 90 nm–130 nm. As a further improvement of the present invention, the thickness of the gold film is 40–60 nm.

[0013] The beneficial effects of this invention are as follows: compared with existing methods for preparing cold atom traps, trapping and transporting cold atoms in integrated optics, it has the following advantages:

[0014] (1) The low-power cold atom trapping platform based on surface plasmons of the present invention traps cold atoms by placing a waveguide on a gold film and generating surface plasmons at the interface between the gold film and the waveguide, thereby enhancing the evanescent field on the upper surface of the waveguide and generating an efficient atom trapping trap to trap cold atoms, thereby reducing the waveguide mode optical power required to trap cold atoms.

[0015] (2) The present invention obtains an optical dipole trap through surface plasmon enhancement. The bidirectional transmission of red detuned surface plasmon interference generates a standing wave field, forming a trap array. By changing the phase difference of the red detuned wave, the trap array can be moved, thereby driving the cold atoms in it to move and realizing the transmission of cold atoms along the waveguide surface.

[0016] (3) The low-power cold atom trapping platform based on surface plasmons of the present invention has waveguide dimensions in the subwavelength range, and features integrability and ease of fabrication. Attached Figure Description

[0017] Figure 1 This is a schematic diagram of the structure of the low-power cold atom trapping platform based on surface plasmons of the present invention.

[0018] Figure 2 The diagram shows a cross-sectional view of the waveguide structure and the optical field distribution of the propagation mode within the waveguide, as described in this invention. Figure 2 (a) is a schematic diagram of the waveguide cross-section. Figure 2 (b) is the optical field distribution diagram of the propagation mode in the waveguide.

[0019] Figure 3 This is a potential energy distribution diagram of the atomic trapping trap of the present invention.

[0020] Figure 4 The graph shows the variation of the effective potential depth of the atomic trapping trap of the present invention with the red-blue laser power ratio under different waveguide widths.

[0021] Figure 5 This is a graph showing the variation of the effective potential depth of the atomic trapping trap of the present invention with the red-blue laser power ratio at different waveguide heights.

[0022] Figure 6 This is a graph showing the effective potential well depth of the atomic trapping trap of the present invention as a function of Pr / Pb for different Pb values.

[0023] Figure 7 This is a graph showing the effective potential well depth of the atomic trapping trap of the present invention as a function of Pr / Pb for different Pb1 values. Detailed Implementation

[0024] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0025] It should also be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and / or processing steps closely related to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0026] According to a specific embodiment of the present invention, such as Figure 1 The waveguide 20 in the platform 100 for low-power cold atom trapping based on surface plasmons is made of silicon dioxide, the gold film 10 is made of Au, and the substrate 30 is a silicon dioxide substrate 30.

[0027] In some embodiments, the waveguide 20 has a width of 700-900 nm, preferably 800 nm, and a height of 90 nm-130 nm, preferably 110 nm.

[0028] In some embodiments, the thickness of the gold film 10 is 40–60 nm, preferably 50 nm.

[0029] In some embodiments, the incident light of waveguide 20 uses bidirectional red detuned light with a wavelength of 850 nm and a TM0 mode. The two unidirectional blue detuned light beams used have wavelengths of 760 nm and 765 nm, respectively, and operate in TM0 and TM1 modes.

[0030] The platform 100 for low-power cold atom trapping based on surface plasmons of the present invention employs a combination of modes including unidirectional transmission of blue detuned TM0 and TM1 modes, and bidirectional transmission of red detuned TM0 mode. By using a preset mode power, the evanescent field of the combination mode can form an optical dipole trap on the upper surface of the waveguide 20 to trap cold atoms.

[0031] Furthermore, an optical dipole trap is formed using the evanescent field of surface plasmons.

[0032] Furthermore, by employing the bidirectional red-detuned TM0 mode, the waveguide 20 is interfered with to form a standing wave field. By changing the phase difference of the bidirectional red-detuned wave, a moving optical dipole trap is generated, which drives the cold atoms trapped in the optical dipole trap to move.

[0033] In some embodiments, atoms in the evanescent field are subjected to the dipole effect, which confines them to the deepest part of the potential well. The optical dipole potential traps neutral atoms; for 87Rb atoms, the optical dipole potential can be expressed as:

[0034]

[0035] The natural linewidths of the D1 and D2 transitions are γ1 = 2π × 5.75 MHz and γ2 = 2π × 6.07 MHz, respectively. I(x,z) represents the evanescent field intensity.

[0036] When an atom approaches the surface of waveguide 20, it is attracted by the van der Waals force, which can be expressed as:

[0037]

[0038] In the calculation of the optical trap potential on the surface of waveguide 20, the presence of 87Rb ground-state atoms in the two-color evanescent field on waveguide 20 is considered. The blue and red detuning fields at 760 nm, 765 nm, and 850 nm will generate optical dipole potentials, and the total potential energy includes both the optical dipole potential and the van der Waals potential, i.e.:

[0039] U = U ob +U or +U vdW (3)

[0040] Wherein, Uob is the repulsive potential generated by the blue detuned evanescent field, and Uor is the attractive potential generated by the red detuned evanescent field.

[0041] 1. Waveguide 20 design

[0042] Figure 2 (a) is a schematic cross-section of waveguide 20, with the gold film 10 having a thickness of 50 nm. The potential well performance of the waveguide 20 surface is mainly related to the width w and thickness h of the waveguide 20. Different waveguide 20 modes exist, such as... Figure 2 As shown in (b), the electric field distributions of the fundamental mode TM0 and the higher-order mode TM1 are displayed when the incident light wavelengths are 760 nm and 765 nm, respectively. Furthermore, this invention does not support the TE mode, but only considers the TM mode.

[0043] Figure 3 The potential energy distribution of the atom trapping well, calculated using MATLAB, was generated at approximately 125 nm on the surface of waveguide 20 using 850 nm red detuned light with a power of 0.345 mW and 760 nm blue detuned light with a power of 1.5 mW. The well depth reached 0.1393 mK. In the figure, white "☆" indicates the minimum potential value, and white "x" indicates the saddle point of the well. The effective well depth U is the difference between the saddle point depth and the minimum potential value. The figure clearly shows that this invention can form a stable potential well on the surface of waveguide 20, and the effective well depth can stably trap 87Rb cold atoms in experiments.

[0044] Figure 4 The graph shows the variation of the effective potential depth of the atomic trapping trap with the red-blue detuned power ratio under different waveguide 20 widths. To select a suitable waveguide 20 size, this invention simulates the variation of the effective potential depth of the trapping trap with the waveguide 20 width. Figure 4The graph shows the effective well depth as a function of Pr / Pb for different waveguide 20 widths when Pb = 1.5 mW, Pb1 = 0.3 mW, and h = 110 nm. Different colored curves represent different widths. It can be observed that the peak value of the effective well depth for different widths appears near Pr / Pb = 0.23, and the peak value increases as the waveguide 20 width decreases. To find the relationship between U and w, a more intuitive graph showing the effective well depth as a function of waveguide 20 width was calculated and plotted, as shown below. Figure 4 The illustration shows the effective well depth of waveguide 20 with a width of 750 nm to 950 nm when h = 110 nm, Pb = 1.5 mW, and Pr / Pb = 0.22. It is clearly observed that U decreases as the width of waveguide 20w increases. When the width w is less than 750 nm or greater than 950 nm, the mode distribution of the captured light in waveguide 20 is significantly worse. Considering that the mode transmittance of the input light decreases with increasing width, the waveguide 20 width chosen in this invention is ultimately 800 nm.

[0045] Figure 5 The relationship between the effective potential well depth and Pr / Pb was studied under different waveguide 20 heights. Different colored curves in the figure correspond to different waveguide 20 heights. It can be seen from the figure that changing the waveguide 20 height shifts the peak value of the effective potential well. Based on the mode transmittance under different waveguide 20 heights, the waveguide 20 height selected in this invention is 110 nm.

[0046] 2. Simulation results of trapping potential wells

[0047] Figure 6 The effective well depth as a function of Pr / Pb was calculated for different Pb values ​​when Pb1 = 0.3 mW. The graph shows that U initially increases and then decreases with increasing Pr / Pb. This is because the optical well is formed by the combined action of the optical dipole force generated by the red-blue detuned light and the van der Waals force on the surface of waveguide 20. When Pr / Pb is very small, the proportion of blue detuned light is large, providing a strong repulsive potential, resulting in a shallow well depth. Conversely, when Pr / Pb is large, the proportion of red detuned light is large, providing a strong attractive potential. However, an excessively large attractive potential also leads to a low potential barrier and a shallow well depth. Therefore, both excessively large and small Pr / Pb values ​​result in a very small effective well depth or even a highly unstable well. Only by controlling the repulsive force generated by the blue detuned evanescent field and the attractive force generated by the red detuned evanescent field—that is, finding an optimal Pr / Pb—can an optimal well be formed on the surface of waveguide 20. The figure also shows that as Pb increases, the Pr / Pb corresponding to the peak value shifts to the right, and the peak value also increases.

[0048] Figure 7This diagram illustrates the relationship between the effective well depth U of the optical trap and Pr / Pb when Pb = 1.5 mW. Different colored curves correspond to different Pb1 values. From a single curve, it is clear that the effective well depth increases with increasing Pb1. Therefore, Pb1 has a significant impact on the effective well depth U. To better understand this relationship... Figure 7 The illustration shows the relationship between U and Pb1 when Pr / Pb = 0.225. The graph shows that when Pb1 is very small (i.e., the blue detuned light power in TM1 mode is very small), the corresponding effective potential well depth is also very small. However, when Pb1 > 0.25 mW, further increases in Pb1 result in a very slow increase in U, which tends to saturate. This is because the blue detuned light in TM1 mode fills the excessively small repulsive potential on both sides of the potential well to repair the potential barrier. When Pb1 is too small, it cannot completely repair the barrier; when Pb1 is too large, the barrier is completely repaired, and the excess Pb1 has almost no further effect. Therefore, this invention uses Pb1 = 0.3 mW.

[0049] In summary, this invention utilizes the enhancement effect of surface plasmon resonance (SPR) to design a low-power cold atom trapping platform 100 based on SPR, which consists of a silicon dioxide waveguide 20 placed above a gold film 10. Free electrons in the gold film 10 interact with photons in the waveguide 20 to form a mixed excited state; that is, free electrons undergo collective oscillation under irradiation with light waves of the same resonance frequency, forming SPR at the interface between the gold film 10 and the waveguide 20. The SPR forms an enhanced optical field on the surface of the waveguide 20. By inputting low-power red-blue detuned light into the waveguide 20, a large cold atom trap can be formed on the surface of the waveguide 20.

[0050] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A platform for low-power trapping of cold atoms based on surface plasmon resonances, characterized in that, The platform includes a waveguide, a gold film, and a substrate. The low-power cold atom trapping platform based on surface plasmon resonance employs a combination of modes, including unidirectional transmission of blue detuned TM0 and TM1 modes and bidirectional transmission of red detuned TM0 mode. By using a preset mode power, the evanescent field of the combination mode can form an optical dipole trap on the upper surface of the waveguide to trap cold atoms. The platform for low-power trapping cold atoms based on surface plasmons adopts the bidirectional transmission red-detuned TMO mode, interferes with the waveguide to form a standing wave field, and generates a moving optical dipole trap by changing the phase difference of the bidirectional red-detuned wave, thereby driving the cold atoms trapped in the optical dipole trap to move. The waveguide is located above the gold film.

2. The platform for low-power cold atom trapping based on surface plasmons according to claim 1, characterized in that, The platform for low-power cold atom trapping based on surface plasmons uses the evanescent field of surface plasmons to form an optical dipole trap.

3. The platform for low-power cold atom trapping based on surface plasmons according to claim 1, characterized in that, Both the waveguide and the substrate are made of silicon dioxide.

4. The platform for low-power cold atom trapping based on surface plasmons according to claim 1, characterized in that, The width of the waveguide is 700~900nm, and the height of the waveguide is 90nm~130nm.

5. The platform for low-power cold atom trapping based on surface plasmons according to claim 1, characterized in that, The thickness of the gold film is 40~60nm.

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