On-chip nonlinear photonics: hybrid integration of two-dimensional materials

At present, the performance of silicon-based electronic systems has approached its physical limit, and it is difficult to meet the requirements of information processing in the era of big data. Optical information processing has unmatched speed and energy consumption advantages. For example, all-optical information processing devices based on nonlinear optics can achieve ultra-high response speed in the femtosecond range, far exceeding that of electronic devices of the same class. Recently, the on-chip integrated nonlinear photonic devices have adopted the design idea of hybrid integration of two-dimensional new semiconductor materials and optical waveguides to achieve enhanced nonlinear effects, which is expected to further promote the development of photonic devices such as ultrafast optical switching, optical parametric amplification, all-optical logic computing, and quantum light sources.

Vincent Pelgrin and his collaborators from Aalto University in Finland recently published a review article in Light: Advanced Manufacturing titled Hybrid integration of 2D materials for on-chip nonlinear photonics.

This paper first summarizes the linear and nonlinear properties of different integrated optical systems and the nonlinear effects of different 2D materials, systematically summarizes the modeling and characterization of 2D material-on-chip photonic structure hybrid integrated systems, analyzes the hot applications of 2D material-integrated nonlinear optics in detail, and discusses the main challenges and future development prospects of the current hybrid integration of 2D materials and optical waveguides. The authors believe that with the exploration of various new 2D materials and the continuous improvement of optical waveguide integration technology, 2D material-optical waveguide integrated photonics will bring new breakthroughs in cutting-edge scientific research, industrial production and daily life.

Theoretical basis and preparation method of two-dimensional material-optical waveguide hybrid integrated system

Designing a hybrid two-dimensional material-waveguide integrated system is one of the key steps to effectively realize its optical function. In general, the eigenmode calculation is used to analyze its effective refractive index, transverse mode field distribution, and group velocity dispersion of the working wavelength to design the applicable waveguide and integration. Figure 1 simulates the TE mode field distribution of a planar buried strip silicon nitride (SiN) waveguide covering molybdenum disulfide (MoS2) at an operating wavelength of 1550 nm.

Figure 1: Mode field distribution of a hybrid integrated MoS?-SiN waveguide

According to the effective nonlinear properties of the waveguide (such as third-order nonlinear effects, the number of free carriers, and linear passive losses, etc.), the pulse propagation in the waveguide can be simulated in combination with the generalized nonlinear Schrödinger equation. Figure 2 simulates the propagation of nonlinear pulses in a SiN waveguide system, where the blue rectangular box region is the MoS2-SiN hybrid region, which greatly broadens the spectral width of the input pulse due to the enhanced nonlinear effect.

Figure 2: Transmission of nonlinear pulses in a hybrid integrated MoS?-SiN waveguide system

In addition, the authors summarize the main preparation methods (e.g., mechanical exfoliation, liquid phase exfoliation, and chemical vapor deposition) and transfer methods (e.g., wet transfer, dry transfer, or semi-dry transfer) of 2D materials to prepare integrated 2D material waveguide devices. In recent years, scientific researchers have also realized the direct growth of two-dimensional materials in waveguides, such as photonic crystal fibers.

Nonlinear optics of two-dimensional materials

The authors briefly review the nonlinear optical phenomena such as SHG, THG, Raman (CARS & SRS) and saturable absorption of typical 2D materials (such as graphene, black phosphorus, hexagonal boron nitride, transition metal chalcogenide, etc.) (Fig. 3b), explain the reasons for the significant differences in the conclusions of experimental studies on the nonlinear coefficients of 2D materials, and point out that the physical mechanism behind the nonlinear phenomena of 2D materials still needs to be improved.

Figure 3: Typical 2D materials and their nonlinear effects

Research progress of two-dimensional material-optical waveguide hybrid integrated system

The authors introduce in detail the latest research progress of two-dimensional material-optical waveguide hybrid integrated systems. Fig. 4a illustrates the enhancement of the four-wave mixing effect by using a graphene-silicon ring resonator system, Fig. 4b illustrates the tuning of a graphene-planar waveguide system with an applied electric field, and Fig. 4c and Fig. 4d illustrate the enhancement of second harmonics in transition metal dichalcogenides using planar silicon waveguides and optical fibers, respectively.

Fig. 4: Experimental progress on nonlinear effect enhancement in two-dimensional material-waveguide hybrid integrated systems

Figures 5a and 5b illustrate the enhanced Raman response of graphene using photonic crystal microcavities, while Figures 5c and 5d illustrate the saturable absorption of graphene by designing different waveguide structures.

Fig. 5: Research progress on Raman enhancement and saturable absorption experiments in two-dimensional material-waveguide hybrid integrated systems


The two-dimensional material-waveguide hybrid integration technology shows that it can bypass the physical limits of different integrated photonic systems, enrich and improve the performance of micro-nano photonic systems, and introduce new functions in physical, chemical and biological regulation. Finally, the authors analyze the use of novel characterization tools, novel two-dimensional materials with high nonlinear coefficients and their heterojunctions, complementary metal-oxide-semiconductor (CMOS)-compatible fabrication techniques, and new integrated platforms to achieve higher performance integrated optoelectronic devices, and look forward to their prospects in emerging directions such as broadband on-chip light sources, optical frequency comb generation, all-optical computing, optical parametric amplification, and optical quantum applications.

In summary, the authors believe that the research on hybrid integrated photonic technology for two-dimensional materials has just begun, and with the development of new structures and new two-dimensional layered materials, more high-efficiency on-chip optical functions are possible. At the same time, the authors point out some key scientific problems that need to be solved urgently in this field (such as the influence of propagation loss on the selection, reliability, and integration methods of 2D materials). (Source: Advanced Manufacturing WeChat public account)

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