Hybrid silicon nonlinear photonics [Invited] Download: 594次
1. INTRODUCTION
Silicon nonlinear photonics has been developed to meet the demand of on-chip optical processing and computation, owing to its potential low cost and CMOS compatibility [13" target="_self" style="display: inline;">–
However, the development of nonlinear photonics on silicon is still limited by the essential shortcomings of silicon. As a centrosymmetric material, silicon does not have the second-order nonlinear susceptibility, which is essential for efficient electro-optic modulation and second harmonic generation. The optical parametric nonlinear process observed commonly in silicon is the third-order process, including self-phase modulation, cross-phase modulation, and four-wave mixing [16–
What’s more, the band gap of silicon is 1.1 eV, which is smaller than the energy of two photons at the wavelength around 1550 nm, so silicon suffers from two-photon absorption (TPA) in the near-infrared. The absorption rate depends on the intensity of the incident field, which limits its application in situations requiring a large pump power (i.e., parametric amplification, Raman lasing, and Brillouin amplification) [12,13,2224" target="_self" style="display: inline;">–
To overcome these drawbacks and improve the performance of devices on the silicon platform, various materials with a better nonlinear property are introduced [79" target="_self" style="display: inline;">–
Two kinds of hybrid structures with distinct designs have been developed. The first one can be summarized as modifying the internal structure of silicon itself to obtain the desired nonlinearity. The other method introduces new materials with better nonlinear property to integrate with silicon. These new materials provide the desired nonlinearity while silicon confines the optical modes to nanoscale.
Here, we review the recent progress in hybrid silicon nonlinear photonics. First, we introduce the hybrid structures used to modify the nonlinear property of silicon. Two kinds of structures are discussed to break the essential symmetry and add the second-order nonlinear susceptibility on silicon. The free carrier density can also be controlled using p–i–n junctions. Next, we review the commonly used hybrid structures with new materials for enhancing the nonlinear photonic effects, including bulk materials and layered materials. Finally, we give a short summary.
2. MODIFICATION OF SILICON FOR NONLINEAR PHOTONICS
In this section, we focus on the first kind of hybrid structure, in which the structures can change the nonlinear optical property of silicon. The materials introduced only work as auxiliaries and do not contribute directly to the nonlinear process by themselves.
2.2 A. Enhancement of χ ( 2 ) Nonlinearity in Silicon
Because
Straining layers of
Fig. 1. (a) Diagram of the investigated silicon photonic crystal waveguide with straining layers on top [26]. (b) A top-view optical image of the strained silicon waveguides where a few waveguides are observed as yellow lines. A scanning electron microscopy image of the input facet of the waveguide is also shown. The waveguide is designed to realize second harmonic generation from mid-infrared to near-infrared [28]. (c) Three-dimensional sketch of the electric-field-induced second harmonic generation device with silicon ridge waveguide and spatially periodic patterning of the p–i–n junction. The electric field across the p–i–n junction induces the second-order nonlinear effect in a silicon waveguide. The periodic pattern is designed to alter the nonlinear susceptibility periodically for quasi-phase matching [30].
In addition to the mechanical method, the
Even though these two methods can induce
2.3 B. Manipulation of Free-Carriers in Silicon
Generally, the parametric nonlinear process requires a very high pump, while the essential and TPA-generated carriers induce parasitic nonlinear loss. In the resonant configuration, the threshold of the parametric oscillator and the Raman laser is proportional to the optical loss. The absorption of the free carriers pushes the pump threshold to values difficult to reach. Below the threshold, silicon is often used for quantum photon pair generation. The nonlinear loss significantly degrades the performance of the photon sources. Besides the loss effect, the lifetime of free carriers and the influence on the refractive index of silicon should also be considered.
An effective way to mitigate the influence of FCA is to control the density of free carriers in silicon. Fortunately, the reverse biased p–i–n diode can move the free carriers out of silicon. This method has been used to achieve net Raman gain in a silicon waveguide [31] and build low-threshold continuous wave Raman silicon laser [32]. For a silicon quantum photon source, increasing the coincidence-to-accidental and generation rates of quantum entangled photon pairs is also demonstrated in a silicon microcavity [33]. In addition, the reduction of the free carriers’ density enables the high-speed wavelength conversion [5] via four-wave mixing.
Even though FCA plays a negative role due to the absorption of free carriers, one should be aware that the real part of the refractive index also depends on the free carriers’ density. This provides an additional method to build modulators by modulating the density of the free carriers in silicon. High-speed electro-optic modulators up to
3. BULK-MATERIAL-ASSISTED SILICON NONLINEAR PHOTONICS
In the above section, we mainly focus on hybrid structures used to remold the essential properties of silicon. However, the hybrid structures require careful design and fabrication, and TPA and FCA still remain as the limitations. A more radical method is to introduce new materials to replace parts of the nonlinear function of silicon. By exploiting new materials with the desired nonlinear properties and making hybrid silicon photonic devices, the advantages of both materials can be used. Generally, there are three guidelines to select new materials: (1) the material should possess better nonlinear properties to compensate the shortcomings of silicon, such as second-order nonlinearity, larger third-order nonlinearity, and lower TPA rates; (2) the combination of the new material and silicon should not destroy the confinement property provided by silicon; and (3) the material should be CMOS-compatible. In the following section, we divide the materials into bulk materials and layered materials. These two types of materials for a silicon hybrid platform have different functions and require different structure designs, based on their shapes, integration methods, and optical properties.
Bulk materials are often used for the cladding of silicon waveguides. In this case, the materials not only contribute to the mode distributions, but also offer nonlinearities. It should be noted that the refractive index of most materials is lower than that of silicon, so the mode confinement offered by silicon may be destroyed while the nonlinear interaction strength scales as the inverse of the effective mode area
Since the hybrid structures are complex and the refractive indexes of the new materials are different from that of silicon, the geometries should be carefully designed to simultaneously fulfill the phase-matching condition, the dispersion properties, and large mode overlap, as well as the tight mode confinement [40].
3.4 A. Utilization of Bulk Materials with Large χ ( 2 ) Nonlinearity
Due to the symmetric property of the lattice, the second-order nonlinear susceptibility of bulk silicon is approximately zero. However,
3.5 B. Utilization of Bulk Materials with Large χ ( 3 ) Nonlinearity
In traditional nonlinear silicon photonics, third-order nonlinearity has been used to realize super-continuum generation, four-wave mixing, and photon pair generation. However, the competition between Kerr, and TPA and FCA effects has been a crucial topic for a long time. Even though silicon has a relative high
The complex number
Fig. 2. (a) Schematic of a nanoslot waveguide covered by a nonlinear optical organic material. (b) Experimental setup of the all-optical demultiplexing by four-wave mixing. Inset: 1, diagram of the data signal; 2, diagram of the 42.7 GHz pump; 3, the spectrum at the output of the DUT (green) and after bandpass-filtering (blue); 4, diagram of the demultiplexed signal [7].
Similar to organic materials, silicon–chalcogenide (SC) is another promising hybrid silicon structure with a high FOM. It is well known that chalcogenide has a higher Kerr nonlinearity and a lower TPA rate than silicon [59]. With careful optimization for the structural design, the SC structure can significantly increase the FOM factor by five times [60]. Despite the organic and inorganic materials, nanocomposites can also be used to exploit new hybrid approaches (Si–Si nc) [61]. Si nanocrystal has
For most hybrid approaches described above, silicon waveguides still play an important role in optical confinement, while the cladding materials provide the desired nonlinearity. The commonly used silicon waveguides are single stripe waveguides and nanoslot waveguides. No matter which hybrid structure one chooses, the common principle is to reduce the energy density in silicon to mitigate the detrimental effects of TPA. Another essential problem for efficient nonlinear interactions is the phase-matching condition. The geometric design of the hybrid structure should simultaneously meet the demands of high FOM and phase matching.
3.6 C. Utilization of Optical Nanowires with Nonlinearity
Replacing the nonlinear parts of silicon devices with better nonlinear components is also an alternative approach, provided that the coupling efficiency between different components is sufficiently high. In this approach, the introduced structures are separated and have different functions [69]. Very recently, Chen
Fig. 3. (a) Schematic of a freestanding nanowire evanescently coupled with integrated silicon waveguide. (b) SEM image of the MZI consisting of a U-shaped 300 nm wide silicon waveguide and a 950 nm diameter CdS free-standing nanowire. The inset shows a close-up view of the right-hand coupling region. (c) Optical micrograph of the integrated nanowire–silicon resonators under a 976 nm wavelength excitation from a tapered fiber probe [70].
4. LAYERED-MATERIAL-ASSISTED SILICON NONLINEAR PHOTONICS
In recent years, layered material has attracted much attention for its unique and excellent optical and electronic properties. It has great potential for building light sources, modulators, switches, and photon detectors. As a representative, graphene has been studied a lot in the last decade and used for various applications in electro-optic modulation, all-optical modulation, and parametric nonlinear photonics [75], relying on photon absorption and large
Table 1. Reported Layered Materials for Nonlinear Silicon Photonics
|
4.3 A. Utilization of Layered Materials with Parametric Nonlinear Processes
Some layered materials, such as
Fig. 4. (a) Schematic design of the hybrid integration of onto a silicon waveguide for second harmonic generation (left). Emission spectrum when excited from grating and free space (right) [80]. (b) Scanning electron micrograph of the fabricated silicon photonic crystal cavity with monolayer on top, indicated by the orange outline. Visible stripes of holes inside the monolayer region are due to the ripped monolayer during exfoliation (left). The spectrum of the second harmonic waves (right) [84]. (c) Scanning electron micrograph of the tuned photonic crystal cavity (left). Steady-state input/output optical bistability for the quasi-TE cavity mode with laser-cavity (right) [88].
Photonic crystal nanocavities are often used to enhance the interaction between the layered materials and optical modes in silicon. Compared with a traditional waveguide, it can provide much stronger nonlinear interaction. It can confine the optical mode to an ultrasmall volume to the order of
In terms of third-order nonlinearity, graphene, black phosphorus,
4.4 B. Utilization of Layered Materials with Light Absorption
TPA and FCA in silicon material have a negative impact on a parametric process, including Kerr, Raman, and Brillouin processes. On the contrary, the nonparametric nonlinear absorption and free carrier effect in silicon can be appropriately controlled to make all-optical modulators. Since the absorption rate and the free carrier induced refractive index change are relevant to the light intensity, the transmission of one mode can be controlled or modified by another light mode [89].
As a zero-band gap material with unique optoelectronic properties, graphene has found applications in photodetectors, solar cells, modulators, and absorbers [90–
Fig. 5. (a) Schematic picture of an in-plane all-optical modulation in graphene-on-silicon suspended membrane waveguides (left). Pump output power at 100 kHz at different input powers (right) [76,77]. (b) Three-dimensional schematic illustration of a graphene-silicon hybrid nanophotonic wire. The probe light is coupled into and out of the silicon-on-insulator (SOI) nanowire by using grating couplers with adiabatic tapers. The pump light is emitted through a fiber on top of the SOI-nanowire (up). Dynamic responses of the output power for TE- and TM-polarization modes of hybrid nanophotonic wires with a locally modulated optical pump (down) [78].
The photo-absorptive property of graphene provides the ability to build photon detectors. Because the layer is very thin, the absorption of a few layers of graphene is weak. For a single layer, the absorption is only about 2%. When using a silicon waveguide and cavity, the light absorption in graphene can be enhanced significantly. More than 90% of the light can be absorbed by graphene in a silicon nanocavity, which is promising for detectors with high responsivity. In addition, the absorption and heating of the graphene can increase the nonlinear thermal response of silicon. The thermal nonlinear optical bistability has been observed in a graphene silicon waveguide Fabry–Perot resonator [79].
4.5 C. Utilization of Layered Materials with Light Emission
Silicon is an indirect band gap material, so it is hard to integrate a light source on traditional silicon photonics. Even though Raman laser and frequency combs have been demonstrated on a silicon chip, these protocols have a strict requirement of high pump power and high
Fig. 6. (a) Optical image of bulk (greenish region) and monolayer (contoured region) on PMMA. (b) Scanning electron micrograph of an undercut silicon nanobeam cavity. The dimensions of the nanobeam cavity are 7.2 μm long, 0.365 μm wide, and 0.22 μm thick. The tightly confined mode in the nanocavity ensures the strong coupling between the layered materials and photons. (c) Left: PL spectra of the nanobeam laser with increasing pump power levels at room temperature, which corresponds to an estimated spectral resolution of 0.41 nm. Right: The log–log plot of light in versus light out for two cavity modes and for a background spontaneous emission shows a clear transition from the spontaneous emission to eventual lasing [83].
5. SUMMARY
In this review, we have summarized the recent progress in hybrid nonlinear silicon photonics. The hybrid platform takes advantage of silicon and other materials. On one hand, the CMOS-compatible silicon fabrication technology lays the foundation for making high-performance devices. The high optical confinement offered by silicon waveguides and cavities as well as its large nonlinear susceptibility greatly enhances the nonlinear interaction. On the other hand, hybrid structures compensate for the shortcomings of silicon and develop diverse applications. The cladding materials with a second-order nonlinear effect make second harmonic generation and electro-optic modulation possible on a silicon chip. Higher FOM ensures a more efficient and higher data rate wavelength conversion. The integration of silicon with layered materials is naturally compatible and has been demonstrated to realize on-chip room temperature lasing, frequency conversion, all-optical modulation, and photon detection. We believe the combination of silicon and other nonlinear photonic materials can improve the performance and expand the application of on-chip nonlinear silicon photonics.
[3] C. K. J. Leuthold, W. Freude. Nonlinear silicon photonics. Nat. Photonics, 2010, 4: 535-544.
[20] T. Baba. Slow light in photonic crystals. Nat. Photonics, 2008, 2: 465-473.
[74] R. Yan, D. Gargas, P. Yang. Nanowire photonics. Nat. Photonics, 2009, 3: 569-576.
[84]
[87]
Article Outline
Ming Li, Lin Zhang, Li-Min Tong, Dao-Xin Dai. Hybrid silicon nonlinear photonics [Invited][J]. Photonics Research, 2018, 6(5): 05000B13.