On-chip spontaneous frequency conversion light source based on semiconductor nanowire-silicon nitride waveguide structure and preparation method thereof

Abstract

The invention discloses an on-chip spontaneous frequency conversion light source based on a semiconductor nanowire-silicon nitride waveguide structure, and belongs to the technical field of integrated optics and nonlinear optics. According to the light source, the high-nonlinearity characteristic of the semiconductor nanowire is utilized, and the semiconductor nanowire is embedded in the silicon nitride waveguide, so that the frequency up-conversion function which the silicon nitride waveguide does not have is realized. The light source structure comprises a silicon nitride waveguide serving as a light wave transmission channel, a semiconductor nanowire serving as a nonlinear medium core and a grating coupler integrated with the waveguide so as to realize efficient transmission of optical signals. The light source has the advantages of low power consumption, high integration level, easiness in butt joint with an existing optical fiber system and the like, and is suitable for an on-chip optoelectronic integrated system.

Description

1. Technical Field

[0001] The present invention relates to the technical fields of integrated optics, nonlinear optics, and photonic integrated circuits, and particularly relates to an on-chip spontaneous frequency conversion light source based on a composite structure of semiconductor nanowires and silicon nitride waveguides. 2. Background Art

[0002] With the rapid development of information technology, the requirements for the bandwidth and transmission rate of optical communication systems are increasing day by day. In the field of optical communication, traditional light sources have been difficult to meet the growing performance requirements due to their limitations in size, power consumption, and integration. Therefore, the development of new high-performance on-chip light sources has become the focus of photonics research.

[0003] Currently, on-chip photonic integrated circuits are mainly based on silicon-based waveguide structures. However, due to the relatively weak nonlinearity of silicon-based materials such as silicon nitride itself, it limits their performance in nonlinear optical applications. To overcome this limitation, it is urgent to achieve the integration of high-efficiency nonlinear materials and silicon nitride optical waveguides. The semiconductor nanowires grown by the top-down method are considered to be one of the ideal materials for realizing high-efficiency on-chip nonlinear optical functions due to their excellent nonlinear characteristics and sub-wavelength size characteristics. 3. Summary of the Invention

[0004] To solve the problems existing in the background art, the object of the present invention is to provide an on-chip spontaneous frequency conversion light source based on a semiconductor nanowire - silicon nitride waveguide structure in view of the deficiencies of the prior art.

[0005] The technical solution adopted by the present invention to solve its technical problems is as follows:

[0006] The present invention realizes efficient coupling with the external system by using a silicon nitride straight waveguide embedding semiconductor nanowires and cooperating with two grating couplers. The straight-line design of the silicon nitride straight waveguide, with grating couplers connected at both ends, ensures the efficient injection and extraction of optical signals. The semiconductor nanowires embedded in the straight waveguide provide the core of the nonlinear medium, realizing the frequency doubling conversion of optical signals. The settings of the oxide layer and the substrate layer provide the necessary support for the waveguide and the coupler. The continuous laser passes through the waveguide structure of the present invention, and through the nonlinear medium core composed of semiconductor nanowires and silicon nitride straight waveguides, frequency doubling conversion is realized.

[0007] The oxide layer adopted is silicon dioxide.

[0008] The substrate layer adopted is silicon.

[0009] The semiconductor nanowires adopted are CdS nanowires.

[0010] In a specific implementation, the semiconductor nanowire 5 is transferred onto the silicon dioxide SiO2 oxide layer 3, and then a silicon nitride thin film is deposited thereon to form a straight waveguide 2. The two ends of the straight waveguide are respectively connected to two grating couplers 1, ensuring efficient optical signal coupling with the external system.

[0011] The beneficial effects of the present invention are as follows:

[0012] The present invention has the advantages of low power consumption, high integration, and easy docking with existing optical fiber systems, and is suitable for on-chip optoelectronic integration systems. The present invention has achieved efficient second harmonic generation conversion from 1550 nm to 775 nm, and by simultaneously inputting 1550 nm and 1554 nm lasers, second harmonic generation outputs of 775 nm and 777 nm and sum frequency 776 nm multi-wavelength laser output are realized. 4. Description of the Drawings

[0013] Figure 1 Overall structure diagram of the on-chip spontaneous frequency conversion light source.

[0014] Figure 2 It is a schematic diagram of a silicon nitride waveguide embedding a semiconductor nanowire in the present invention.

[0015] Figure 3 It is an image of the structure of the present invention under an optical microscope.

[0016] Figure 4 It is a photo of the structure of the present invention under an optical microscope after laser injection.

[0017] Figure 5 It is a graph showing the change of the second harmonic intensity with the increase of the pump light intensity in the present invention.

[0018] Figure 6 It is a graph showing the relationship between the pump wavelength and the sum frequency intensity of the second harmonic. 5. Specific Embodiments

[0019] Taking the CdS nanowire as an example, the specific preparation process of the composite waveguide structure is as follows:

[0020] (1) Epitaxial growth of the nanowire: CdS nanowires 5 with a diameter of about 180 nm are prepared by chemical vapor deposition.

[0021] (2) Transfer of the nanowire: The CdS nanowire 5 is transferred onto a silicon substrate covered with 10 μm thick silicon dioxide using a mechanical transfer method.

[0022] (3) Fabrication of silicon nitride waveguides and grating couplers: First, a 300-nm-thick silicon nitride thin film is deposited by plasma-enhanced chemical vapor deposition (PECVD). Then, photoresist is spin-coated, and ZEP520A is uniformly spin-coated on the substrate transferred with nanowires using a spin coater, and the photoresist is cured by baking on a hot plate. The photoresist is patterned using electron beam lithography (EBL), and then developed by soaking in a developer and fixed in isopropyl alcohol for patterning. Finally, the pattern of the photoresist is transferred to the silicon nitride thin film through inductively coupled plasma etching (ICP). Finally, a de-glue step is performed, and the acetone is soaked to remove the excess photoresist, leaving the silicon nitride straight waveguide 3 and the grating coupler 1 that precisely embed the semiconductor nanowires.

[0023] Design of silicon nitride straight waveguide 2: The thickness of the silicon nitride thin film is 300 nm, and the width of the silicon nitride waveguide is 1.0 μm.

[0024] Design of grating coupler 1: Grating structure parameters: period T1 = 1.026 μm, duty cycle ff1 = 31.1%, coupling angle θ1 = 15°, and the grating coupling efficiency is 32%.

[0025] Example 1: Using the device structure as Figure 1 shown, when a continuous laser with a wavelength of 1550 nm is input as the pump light, it is injected into the silicon nitride straight waveguide 2 through the grating coupler 1, and frequency doubling conversion is achieved through the semiconductor nanowire 5, and finally, a laser with a wavelength of 775 nm is output from the other side of the straight waveguide 2. The optical microscope image of this process is as Figure 4 shown. When the output power of the laser is set to 10 mW, the transmission loss of the silicon nitride waveguide is about 10 dB / cm, and the power drops to 8.5 mW at the grating coupler. The pump light power collected by the power meter is 115 μW, and the SHG power is 90 pW. As can be seen from Figure 5 , as the pump light intensity increases, the intensity of the second harmonic also increases, indicating a good dependence of the nonlinear conversion efficiency of the light source on the pump light intensity, and demonstrating the performance changes of the present invention under different pump conditions.

[0026] Example 2: Using lasers with wavelengths of 1550 nm and 1554 nm as pump light sources simultaneously, they are injected into the silicon nitride straight waveguide 2 through the grating coupler 1, and frequency doubling conversion is achieved through the semiconductor nanowire 5, and lasers with wavelengths of 775 nm, 777 nm, and their sum frequency 776 nm are output. As can be seen from Figure 6 , the relationship between the pump wavelength and the sum frequency (SFG) intensity of the second harmonic (SHG) is marked with the peaks of SHG1, SHG2, and their SFG, reflecting the nonlinear optical effects under different pump wavelengths. These peaks reveal the effectiveness of the structure of the present invention in multi-wavelength laser output.

[0027] The overall size of the light source device fabricated according to the above embodiments of the present invention has a length of less than 550 μm and a width of less than 50 μm.

Claims

1. An on-chip spontaneous frequency conversion light source based on a semiconductor nanowire-silicon nitride waveguide structure, characterized in that: It includes a semiconductor nanowire 5 and an on-chip integrated waveguide. The on-chip integrated waveguide includes two grating couplers 1, an oxide layer 3, a substrate layer 4, and a straight waveguide 2. Among them, the straight waveguide 2 is disposed on the oxide layer 3 and above the substrate layer 4. The semiconductor nanowire 5 is embedded in the straight waveguide 2. The grating couplers 1 are disposed on the oxide layer 3 and at both ends of the straight waveguide 2 for realizing efficient injection and extraction of optical signals. The semiconductor nanowire 5 is provided in the middle of the straight waveguide 2 to form a nonlinear medium core for realizing frequency doubling conversion of optical signals.

2. The on-chip spontaneous frequency conversion light source according to claim 1, characterized in that: The straight waveguide 2 is designed to be linear and is respectively connected to the two grating couplers 1 at both ends for realizing efficient injection and extraction of optical signals.

3. The on-chip spontaneous frequency conversion light source according to claim 1, wherein: By adjusting the parameters of the grating coupler 1, the regulation of the optical intensity at the output end is realized.

4. The on-chip spontaneous frequency conversion light source according to claim 1, characterized in that: The oxide layer 3 uses silicon dioxide.

5. The on-chip spontaneous frequency conversion light source according to claim 1, wherein: The substrate layer 4 uses silicon.

6. The on-chip spontaneous frequency conversion light source according to claim 1, characterized in that: The straight waveguide 2 uses a silicon nitride waveguide and is designed as a strip waveguide.

7. The on-chip spontaneous frequency conversion light source according to claim 1, wherein: The semiconductor nanowire 5 uses a CdS nanowire or semiconductor nanowires such as ZnO, CdSe, and CdTe.

8. The on-chip spontaneous frequency conversion light source according to claim 1, wherein: The pump light source is a continuous laser, which is injected into the silicon nitride straight waveguide 2 through the grating coupler 1 and undergoes frequency doubling conversion through the semiconductor nanowire 5.

9. A method for fabricating an on-chip spontaneous frequency conversion light source based on a semiconductor nanowire-silicon nitride waveguide structure, characterized in that It includes the following steps: Step A: Prepare the semiconductor nanowire. Step B: Transfer the semiconductor nanowire to the oxide layer on the silicon substrate. Step C: Deposit a silicon nitride thin film on the oxide layer to form a waveguide. Step D: Form the grating coupler. Step E: Embed the semiconductor nanowire in the waveguide.

10. The method according to claim 9, wherein: The coupler is designed to optimize the optical signal injection and extraction efficiency with the on-chip system.

11. The method according to claim 9, characterized in that: The thickness of the silicon nitride thin film and the waveguide width are adjusted according to the transmission characteristics of the optical signal to realize the best optical signal transmission efficiency.

12. A preparation method of an on-chip spontaneous frequency conversion light source based on a semiconductor nanowire-silicon nitride waveguide structure, characterized in that: It includes transferring the semiconductor nanowire 5 to the oxide layer 3, then depositing a silicon nitride thin film, preparing and forming a waveguide structure, and preparing a composite structure spontaneous frequency conversion light source. The light source is designed to be compatible with the on-chip system.

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