Porous silicon-based three-dimensional optical waveguide preparation method based on femtosecond laser induced collapse densification mechanism

Abstract

The invention discloses a porous silicon-based three-dimensional optical waveguide preparation method based on a femtosecond laser local induced collapse densification mechanism, and belongs to the technical field of three-dimensional photon integration. By utilizing the hot working characteristic of high repetition frequency femtosecond laser, local collapse of a porous structure is induced through femtosecond laser direct writing in an oxidized porous silicon thin film material on a silicon substrate to form a densified track with an improved refractive index, and the size of a single track exceeds the Abbe diffraction limit; and arranging a plurality of sub-diffraction precision tracks in space according to a photonic lattice-like structure to form the porous silicon-based three-dimensional waveguide. According to the femtosecond laser direct writing glass waveguide, sub-diffraction order precision three-dimensional regulation and control of the waveguide section shape and the refractive index are achieved, the refractive index change is in the magnitude of 10 <-2 >, a more compact bending design superior to other reported femtosecond laser direct writing glass waveguides is provided, and by designing the waveguide structure, the bending performance of the femtosecond laser direct writing glass waveguide is improved. Flexible control of a mode field and low insertion loss when a standard optical fiber system is accessed are realized.

Description

Technical Field

[0001] The present invention belongs to the technical field of three-dimensional photonic integration, and in particular relates to a method for preparing a porous silicon-based three-dimensional optical waveguide based on a femtosecond laser-induced collapse and densification mechanism. Background Art

[0002] Traditional silicon-based photonic integration is limited by two-dimensional planar architectures, making it difficult to achieve three-dimensional high-density integration. It also faces problems such as complex processes, high costs, limited bandwidth, and difficulty integrating heterogeneous materials. Femtosecond laser direct writing technology provides a simple and efficient three-dimensional processing method. Although existing femtosecond laser direct writing technologies can produce three-dimensional waveguides in transparent materials such as glass and crystals, they suffer from problems such as low refractive index contrast, high bending loss, and inability to directly be compatible with silicon-based chips. The narrow bandgap and high refractive index characteristics of silicon materials themselves also limit the application of femtosecond laser direct writing technology in crystalline silicon. Silicon-based oxidized porous silicon films have potential in functional material integration due to their tunable refractive index and nanoporous properties, but existing laser direct writing methods make it difficult to achieve uniform refractive index modulation and low-loss waveguide preparation. Summary of the Invention

[0003] To address the above problems, the present invention proposes a method for fabricating porous silicon-based three-dimensional optical waveguides based on femtosecond laser localized induced collapse densification (FLICD). The technical solution is as follows:

[0004] In a first aspect, the present invention proposes a method for preparing a porous silicon-based three-dimensional optical waveguide based on a femtosecond laser-induced localized collapse densification mechanism, comprising the following steps:

[0005] (1) Preparing an oxidized porous silicon film on a silicon substrate;

[0006] (2) Using high-repetition-rate femtosecond laser direct writing technology, the local nanoporous structure is induced to collapse and densify inside the oxidized porous silicon film, forming a densification track with sub-diffraction precision. In this step, the femtosecond laser is focused into the interior of the oxidized porous silicon film, and local heat accumulation is triggered by nonlinear absorption, so that the nanoporous structure collapses in a controllable manner to form a densification track. The radial size of the track exceeds the diffraction limit due to the energy confinement caused by the ultrafast nonlinear effect, thereby achieving sub-diffraction precision control of the refractive index distribution.

[0007] A plurality of the densified tracks are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film serves as a cladding to prepare a porous silicon-based three-dimensional waveguide.

[0008] As a preferred feature of the present invention, the waveguide structure can be defined according to practical needs, such as fundamental mode shaped waveguides, LP11 mode control waveguides, directional couplers, and multi-channel waveguide arrays. The spatial arrangement of the traces can be divided for different waveguide cross-sectional shapes. This spatial arrangement can flexibly manipulate the refractive index and mode field distributions. When fabricated using femtosecond laser direct writing technology, densified traces are sequentially prepared from bottom to top within the oxidized porous silicon film.

[0009] As a preferred embodiment of the present invention, a porous silicon film is prepared by electrochemical etching and converted into an oxidized porous silicon film by thermal oxidation. The size (length, width and height) of the oxidized porous silicon film is designed according to the size of the target three-dimensional waveguide.

[0010] As a preferred embodiment of the present invention, in step (2), the repetition frequency of the femtosecond laser direct writing technology is greater than 500 kHz and the pulse width is less than 1 ps.

[0011] As a preference of the present invention, in step (2), the wavelength of the femtosecond laser direct writing technology is selected from 343 nm, 515 nm, 800 nm or 1030 nm.

[0012] As a preferred embodiment of the present invention, in step (2), the focusing of the femtosecond laser is achieved by an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50um-20mm / s.

[0013] As a preference of the present invention, the cross-sectional lateral dimension of the densified track with sub-diffraction-level precision is 200-500 nm (corresponding to the short axis of the elliptical cross section), and the longitudinal dimension is 800-1600 nm (corresponding to the long axis of the elliptical cross section).

[0014] As a preference of the present invention, the spacing between the densified tracks is 0.6-1 μm.

[0015] In the second aspect, the present invention proposes a porous silicon-based three-dimensional waveguide prepared by the above method, characterized in that the waveguide core layer is composed of a plurality of densely arranged densified tracks, and the effective refractive index difference between the waveguide core layer and the cladding layer is 10 -2 Magnitude.

[0016] As a preferred embodiment of the present invention, the working bandwidth of the waveguide covers the near-infrared band and can stably transmit high-power lasers exceeding 1.2W.

[0017] In a third aspect, the present invention provides an integrated photonic device, characterized in that it comprises a porous silicon-based three-dimensional waveguide prepared by the above method.

[0018] For example, an integrated photonic device is any of the following:

[0019] Mode converters that enable fundamental mode shape control;

[0020] Mode division multiplexer supporting high-order mode selection and control;

[0021] Directional couplers with customizable splitting ratios and adjustable coupling efficiency from 0-100% at a coupling spacing of 100 nanometers;

[0022] Multi-waveguide array with a channel spacing of 3μm and crosstalk suppression ≥15dB.

[0023] The beneficial effects of the present invention are:

[0024] (1) Three-dimensional programmable manufacturing capability with sub-diffraction precision: Through the high repetition rate femtosecond laser-induced local collapse and densification mechanism of porous silicon thin films, the diffraction limit of traditional laser processing has been broken through, and sub-micron trajectory control of 200-500nm in the horizontal direction and 800-1600nm in the vertical direction has been achieved. -2 The core-cladding refractive index difference of orders of magnitude can accurately arrange densified tracks in a photonic lattice structure in three-dimensional space, support flexible design of complex cross-sectional shapes such as circular, square, and hexagonal, and achieve high-precision control of the waveguide mode field distribution through the regulation of track spacing (0.6-1μm) and arrangement density, providing a manufacturing method with sub-diffraction level accuracy for photonic integration.

[0025] (2) The three-dimensional waveguide prepared by the present invention achieves an ultra-compact bending design while maintaining low transmission loss (≤2dB / cm@1550nm). The waveguide bending radius can be as low as 3mm (bending loss <1dB / cm@1550nm), which significantly improves the integration density of photonic devices and lays the foundation for large-scale photonic integrated circuits.

[0026] (3) This technology has both broadband compatibility, covering the entire near-infrared band, and high power carrying capacity, capable of transmitting lasers >1.2W. It supports advanced functions such as fundamental mode shape control, high-order mode selection, and mode division multiplexing. It can be successfully applied to passive devices such as mode converters and directional couplers, providing high-performance solutions for high-speed optical communications, high-power laser systems, and quantum optics. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] Figure 1 The principle of preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser-induced collapse densification (FLICD) mechanism: (a) Schematic diagram of the FLICD mechanism; (b) Cross-sectional scanning electron microscope image of the FLICD track; (c) Cross-sectional optical micrograph of the FLICD track; (d) Top-down optical micrograph of the FLICD track; (e) Schematic diagram of arranging multiple FLICD tracks to prepare three-dimensional waveguides.

[0028] Figure 2Mode control devices in Example 1: (a) Fundamental mode shape control waveguide. (b) LP11 mode control waveguide. Left: Designed waveguide cross section; Center: Measured waveguide cross section; Right: Measured near-field mode.

[0029] Figure 3 This is the directional coupler of Example 2: (a) Schematic diagram of the directional coupler structure; (b) The relationship between coupling efficiency and coupling spacing; (c) The relationship between coupling efficiency and coupling length.

[0030] Figure 4 Example 3: Eight-channel waveguide array: (a) Schematic diagram of the eight-channel waveguide array; (b) Optical micrograph of the waveguide array output; (c) Inter-channel crosstalk suppression results. Points of the same color represent the transmittance measured when inputting from the same channel. DETAILED DESCRIPTION

[0031] The present invention will be further described in detail below with reference to the accompanying drawings and examples, but the specific implementation manner of the present invention is not limited thereto.

[0032] The present invention proposes a method for preparing a porous silicon-based three-dimensional waveguide based on a femtosecond laser localized induced collapse densification mechanism, which is characterized by comprising the following steps:

[0033] (1) Preparing an oxidized porous silicon film on a silicon substrate;

[0034] (2) using high repetition rate femtosecond laser direct writing technology to induce local nanoporous structure collapse and densification inside the oxidized porous silicon film, forming a densification track with sub-diffraction level accuracy;

[0035] A plurality of the densified tracks are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film serves as a cladding to prepare a porous silicon-based three-dimensional waveguide.

[0036] Figure 1 The principle of preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser-induced collapse densification (FLICD) mechanism: (a) Schematic diagram of the FLICD mechanism; (b) Cross-sectional scanning electron microscope image of the FLICD track; (c) Cross-sectional optical micrograph of the FLICD track; (d) Top-down optical micrograph of the FLICD track; (e) Schematic diagram of arranging multiple FLICD tracks to prepare three-dimensional waveguides.

[0037] When using femtosecond laser direct writing technology, the wavelength is typically selected from 343nm, 515nm, 800nm, or 1030nm, with a repetition rate greater than 500kHz and a pulse width less than 1ps. The femtosecond laser is focused through an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50μm-20mm / s. The resulting densified tracks with sub-diffraction precision are spaced 0.6-1μm apart, with cross-sectional dimensions of 200-500nm horizontally and 800-1600nm vertically.

[0038] Example 1: Mode Control Device

[0039] In this embodiment, in order to achieve base mode shape control and LP11 mode selection and control, the femtosecond laser direct writing technical parameters are set as follows:

[0040] Laser parameters: central wavelength 1030nm, pulse width 214fs, repetition rate 1MHz, pulse energy 55nJ, linearly polarized light output;

[0041] Objective lens parameters: magnification 100×, numerical aperture NA = 0.8;

[0042] Scanning speed: 5mm / s;

[0043] Track spacing: 1 μm.

[0044] Preparation method of fundamental mode shape-controlled waveguide:

[0045] (1.1) Preparation of oxidized porous silicon film.

[0046] (1.2) In the oxidized porous silicon film, Figure 2 As shown in the waveguide cross-section design diagram in the first column of (a), multiple FLICD tracks are arranged into circular, square, and elliptical shapes to construct the waveguide core layer. The waveguide core layer matches the target waveguide structure, and the spacing between the tracks is 0.6μm.

[0047] (1.3) Using the above parameters, the local nanoporous structure is induced to collapse and densify from bottom to top inside the oxidized porous silicon film, forming a densification trajectory with sub-diffraction level accuracy, and obtaining Figure 2 The second column of (a) shows waveguides with circular, square, and elliptical cross-sections. The waveguide core consists of multiple FLICD tracks arranged in a two-dimensional photonic lattice array, and the cladding is untreated oxidized porous silicon.

[0048] The prepared waveguide was tested under the condition of transmitting 1550nm light, and the results were Figure 2 From the measured diagram of the near-field mode corresponding to the third column of (a), it can be seen that the waveguide transmits with the corresponding mode field shape.

[0049] Preparation method of LP11 mode control waveguide:

[0050] (2.1) Preparation of oxidized porous silicon film.

[0051] (2.2) In the oxidized porous silicon film, Figure 2 As shown in the waveguide cross-section design diagram in (b), multiple FLICD tracks are arranged into a spatially coupled structure to construct a waveguide core layer. The waveguide core layer matches the target waveguide structure, and the spacing between the tracks is 0.6μm.

[0052] (2.3) Using the above parameters, the local nanoporous structure is induced to collapse and densify from bottom to top inside the oxidized porous silicon film, and the Figure 2 (b) The second column of the two-lobed centrosymmetric fan-shaped waveguide has a core layer composed of multiple FLICD tracks arranged in a two-dimensional photonic lattice array, and the cladding is untreated oxidized porous silicon.

[0053] The prepared waveguide is used to excite the 980nm light LP11 mode, and the rotation control of the LP11 mode is achieved by rotating the coupling structure (0° / 45° / 90°) along the central axis. Figure 2 From the measured diagram of the near-field mode corresponding to the third column in (b), it can be seen that the waveguide transmits with the corresponding mode field shape.

[0054] It can be seen from the experimental results that the present invention successfully achieves fundamental mode and high-order mode control, demonstrates flexible mode field regulation capabilities, and is suitable for complex photonic integrated chips.

[0055] Example 2: Directional Coupler

[0056] In order to achieve controllable coupling efficiency and splitting ratio design, the femtosecond laser direct writing technical parameters are set as follows:

[0057] Laser parameters: central wavelength 1030nm, pulse width 214fs, repetition rate 1MHz, pulse energy 55nJ, linearly polarized light output;

[0058] Objective lens parameters: magnification 100×, numerical aperture NA = 0.8;

[0059] Scanning speed: 5mm / s;

[0060] Track spacing: 1 μm.

[0061] Preparation method of directional coupler waveguide:

[0062] (3.1) Preparation of oxidized porous silicon film.

[0063] (3.2) The structure of the coupler is as follows Figure 3As shown in (a), a dual-waveguide symmetrical coupling structure is adopted in the oxidized porous silicon film according to the target waveguide cross-section design diagram, and the waveguides constituting the two arms are both designed with a circular cross-section and a diameter of 9 μm.

[0064] (3.3) Using the above parameters, the localized nanoporous structure was sequentially induced to collapse and densify within the porous silicon oxide film from bottom to top. The coupling length was controlled to be constant at 1 mm, resulting in a set of directional couplers with coupling gaps ranging from 0.5 to 5 μm in 0.5 μm increments. Furthermore, a set of directional couplers with coupling gaps ranging from 0.5 to 3 mm in 0.5 mm increments were also fabricated, controlling the coupling gap to be constant at 500 nm. The waveguide core of the directional coupler consisted of multiple FLICD tracks arranged in a two-dimensional photonic lattice-like array, and the cladding consisted of untreated porous silicon oxide.

[0065] The relationship between the test coupling efficiency and the coupling gap and coupling length is as follows: Figure 3 As shown in (b) and (c), efficient energy transfer is achieved within the coupling gap range of ≤1μm, and the modulation range is 0-1, demonstrating the ability to design and prepare precise splitting ratio directional couplers and verifying the feasibility of submicron modulation gap.

[0066] Example 3: Multi-waveguide array

[0067] In this embodiment, in order to achieve fan-in and fan-out of a low-crosstalk multi-waveguide array, the femtosecond laser direct writing technical parameters are set as follows:

[0068] Laser parameters: central wavelength 1030nm, pulse width 214fs, repetition rate 1MHz, pulse energy 55nJ, linearly polarized light output;

[0069] Objective lens parameters: magnification 100×, numerical aperture NA = 0.8;

[0070] Scanning speed: 5mm / s;

[0071] Track spacing: 1 μm.

[0072] Preparation method of multi-waveguide array:

[0073] (4.1) Preparation of oxidized porous silicon film.

[0074] (4.2) The structure of the eight-channel waveguide array adapter is as follows Figure 4 As shown in (a), an eight-channel parallel array design is adopted in the oxidized porous silicon film according to the target waveguide cross-section design diagram. The waveguide of each channel adopts a circular cross-section design with a diameter of 12μm. The center spacing between adjacent waveguides transitions from 150μm (section I) to 15μm (section III), and an S-shaped bend transition is adopted (section II).

[0075] (4.3) Using the above parameters, the local nanoporous structure is induced to collapse and densify from bottom to top inside the oxidized porous silicon film to form a trajectory, as shown in the following example: Figure 4 In the waveguide structure shown in (b), the waveguide core layer is composed of multiple FLICD tracks arranged in a two-dimensional photonic lattice array, and the cladding is untreated oxidized porous silicon.

[0076] Test the insertion loss and crosstalk suppression performance of each channel at 1550nm, such as Figure 4 As shown in (c), the insertion loss of all channels is stabilized at 2.45±0.14dB, with crosstalk suppression >15dB and >25dB for edge channels. The waveguide fabricated in this invention can achieve a bending radius as low as 3mm (bending loss <1dB / cm@1550nm), outperforming conventional femtosecond laser-written glass waveguides.

[0077] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be considered as equivalent replacement methods and are included in the scope of protection of the present invention.

Claims

1. A method for preparing porous silicon-based three-dimensional optical waveguides based on the femtosecond laser localized induced collapse densification mechanism, characterized in that: The following steps are involved: (1) Preparing an oxidized porous silicon film on a silicon substrate; (2) using high repetition rate femtosecond laser direct writing technology to induce local nanoporous structure collapse and densification inside the oxidized porous silicon film, forming a densification track with sub-diffraction level accuracy; A plurality of the densified tracks are arranged in a preset three-dimensional structure to form a waveguide core layer, and the untreated area in the porous silicon film serves as a cladding to prepare a porous silicon-based three-dimensional waveguide.

2. The method for preparing a porous silicon-based three-dimensional waveguide based on a femtosecond laser localized induced collapse densification mechanism according to claim 1, characterized in that: In step (2), the repetition frequency of the femtosecond laser direct writing technology is greater than 500 kHz and the pulse width is less than 1 ps.

3. The method for preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser localized induced collapse densification mechanism according to claim 1, characterized in that: In step (2), the wavelength of the femtosecond laser direct writing technology is selected from 343nm, 515nm, 800nm or 1030nm.

4. The method for preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser localized induced collapse densification mechanism according to claim 1, characterized in that: In step (2), the focusing of the femtosecond laser is achieved by an objective lens with a numerical aperture NA ≥ 0.80, and the scanning speed is 50 μm-20 mm / s.

5. The method for preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser localized induced collapse densification mechanism according to claim 1, characterized in that: The cross-sectional dimensions of the densified track with sub-diffraction precision are 200-500 nm in lateral dimension and 800-1600 nm in longitudinal dimension.

6. The method for preparing porous silicon-based three-dimensional waveguides based on the femtosecond laser localized induced collapse densification mechanism according to claim 1, characterized in that: The spacing between the densified tracks is 0.6–1 μm.

7. A porous silicon-based three-dimensional waveguide prepared according to any one of claims 1 to 6, characterized in that: The waveguide core is composed of several densely arranged densified tracks, and the refractive index difference between the waveguide core and the cladding is 10 -2 Magnitude.

8. The porous silicon-based three-dimensional waveguide according to claim 8, characterized in that: The operating bandwidth of the waveguide covers the O / C / L bands and can stably transmit high-power lasers exceeding 1.2W.

9. An integrated photonic device, characterized in that: A porous silicon-based three-dimensional waveguide prepared according to any one of claims 1 to 6.

Images