3. Material platforms for nonlinear integrated photonics

Numerous materials for integrated nonlinear optics have been investigated: among them, Si and related materials, such as SiN, amorphous silicon (a-Si), and SiC; glasses, such as silica, high-index glass, and chalcogenide glasses; III–V semiconductors, in particular AlGaAs; ferroelectrics, in particular lithium niobate (LN). Other recently investigated materials are tantalum pentoxide (Ta₂O₅) and vanadium dioxide (VO₂) [^[44]44 and references therein]. Correspondingly, several material platforms have been developed to achieve the goal of a dense components’ integration. Each platform has its advantages and disadvantages and offers variable nonlinear efficiencies and integration densities depending on the values of the nonlinear coefficients and refractive index contrast. An additional limiting factor is related to losses that are determined by both the quality of the grown material and the maturity of the fabrication processes, which affect, for instance, the roughness and absorption of the surface passivation layer. Although in the longer term the growth and fabrication quality for all these materials will eventually level up, there still remains one important limiting factor that clearly separates them regarding high power densities, namely two photon absorption (TPA). TPA is determined by the band gap of the material and by the working wavelength, which is often in the telecom range due to the historically developed infrastructure of sources, detectors, etc. In this regard, small-band-gap semiconductors, such as Si ^{[45][46][47][48][49]}[45-49] and GaAs [50, ^[50][51]51], are fundamentally limited and there seems to be no way around this problem other than to change the working wavelength and all the surrounding infra-structure.

Silicon photonics research and development has much progressed, and both component performance and integration complexity have made significant steps forward in the past decade ^{[45][46][47][48][49]}[45-49]. Nowadays, silicon-based platforms, in particular silicon-on-insulator (SOI), are among the most mature for PIC realization. In the SOI platform, the functional optical elements are situated in the thin top silicon layer, and the insulator is typically made of SiO₂. The SOI platform has become the foundation of silicon photonics for several reasons, including strong optical confinement of silicon due to the significant refractive index difference between silicon and SiO₂, which enables very compact optical devices with low optical propagation loss, high process yield, low cost, etc.

Among the numerous nonlinear waveguide platforms that have been explored, the group of materials capable of combining both passive waveguides, for light steering and nonlinear manipulation, and active functionalities (laser sources, modulators, and detectors) monolithically on the same chip is that of III–V semiconductors ^[50][51][50, 51]. This has been a strong driving force, stimulating development of nonlinear optical devices based on III–V semiconductors and addressing challenges associated with loss mitigation and nonlinearity enhancement in these platforms. The main III–V integrated nonlinear photonic platforms considered to date are GaAs ^[50][51][50, 51] and its AlGaAs derivative [52], InP and InGaAsP, III-nitrides AlN [53], and GaN [54], as well as GaP [55] and its ternary derivative InGaP. We note that AlN [53] shows optical properties similar to SiN (refractive index, transparency, and n₂) but also possesses second-order nonlinearity.

Considering AlGaAs, the structures used in NLIP can be divided into three-layer, two-layer, and multi-layer platforms depending on the number of epitaxially grown layers with different material compositions. Figure 5 shows examples of these three AlGaAs platforms; the waveguides sketched in Figure 5h,i are often employed for phase matching of the χ⁽²⁾ processes.

Figure 5. Schematics of AlGaAs platforms and waveguide geometries. (a–c) 3-layer platform with strip-loaded, nanowire, and half-core waveguides, respectively. (d–f) 2-layer platforms with suspended nanorib, suspended nanowire, and AlGaAs-OI waveguides, respectively. (g–i) Multi-layer platform with multi-quantum-well waveguide, modulated-χ⁽²⁾ waveguide, and Bragg-reflector waveguide, respectively. Reproduced from [52] under Creative Commons License.

In recent years, lithium-niobate-on-insulator (LNOI) wafer fabrication process has been rapidly advancing ^[56][57][58][56-58]. This technology is revolutionizing the lithium niobate industry, enabling higher performance, lower costs, and entirely new devices and applications. Availability of LNOI wafers has sparked significant interest in the platform for integrated optical applications, as LNOI offers the attractive material properties of lithium niobate while also permitting the same strong optical confinement and high optical element integration density that has driven the success of more mature silicon photonics platforms. Many of the key building blocks for highly integrated PICs have been demonstrated in this platform, including low-loss optical waveguides, electro-optical interfaces for ultra-fast modulation, and nonlinear optical elements and resonators. However, further work needs to be completed to make LNOI an attractive and competitive integrated optical platform: (i) optical interfacing to LNOI waveguides has to be improved, reducing fiber-to-chip coupling losses; (ii) optical gain media need to be demonstrated in this platform, either via bonding previously-doped lithium niobate in the LNOI wafer fabrication process or by doping the LNOI wafer after fabrication using ion implantation techniques; (iii) development of photodetectors on LNOI waveguides requires further investigation ^[56][57][58][56-58].

Current trends in PIC material technologies, however, indicate that there is no waveguide material technology that nowadays can address the needs for all the potential applications of PICs. Integration of different material technologies and functionalities (e.g., nonlinearity) can occur through two different routes: (i) hybrid integration and (ii) heterogeneous integration. The former is a process that connects two or more PICs or photonic device chips, usually from different material technologies into one single package. This process is, in general, performed at the packaging stage after fabrication of the PIC and photonic device chips. The latter, instead, is a process that combines two or more material technologies into a single PIC chip. This process is generally performed at the early-to-mid stages of fabrication of the PIC chip, as in the case of unpatterned III–V thin-films integrated onto pre-processed silicon photonic wafers ^[59][60][61][59-61].

In order to address the shortcomings of the SOI platform, several novel waveguide platforms have been developing based on heterogeneous integration of other material systems on silicon substrates, with the common requirement of remaining compatible with the complementary-metal-oxide-semiconductor (CMOS) technology [61]. As a recent example, photonic components in SiC (specifically: waveguides, 1D and 2D photonic crystal cavities, microdisk, and microring resonators), based on thin layers of SiC on insulator (SiCOI), have been implemented [62, ^[62][63]63]. High nonlinearity and low loss were demonstrated in waveguides and ring resonators fabricated in amorphous SiC (a-SiC) grown directly on silica, using plasma-enhanced chemical vapor deposition, with loss as low as 3 dB/cm, providing a very scalable material growth. The intrinsic quality factor of the microring resonator was around 160,000 [64]. Much higher quality factors have been achieved by using the 4H polytype of silicon carbide (4H-SiC). For instance, a microring fabricated in high-purity semi-insulating 4H-SiC exhibited a Q = 1.1 × 10⁶, corresponding to a waveguide loss of 0.38 dB/cm [64].

4. Nonlinear photonics devices

Here, let us focus on three main areas of applications: all-optical computing, all-optical processing and nonlinear photonics sources [65].

In all-optical computing, the key strength provided by optical technologies is the parallelism of information transfer and processing onto multiple wavelength channels. Parallel access to information points is permitted, due to the capability to use light waves of distinct wavelengths within the same device. In addition, being photon the information carrier, there is no propagation delay in different parts of the optical system. However, the implementation of logic operations using the photon signal is a very challenging frontier research, because of the fundamental requirement of a very efficient light-control of the light. Ultrafast all-optical switch is a fundamental component for all-optical computing. It can be defined as a structure with a pump light controlling the ON/OFF transition of the signal light. Optical devices performing digital functions are expected to fulfill the following criteria: 1) ultra-compactness; 2) low power consumption (∼fJ/bit); 3) high-speed operation (scalable beyond 100 Gb/s); 4) parallel operation on multiple wavelength channels, reducing the need for large fan-outs and redundant parallel processing structures; 5) preservation of information (e.g., phase) carried in the optical domain, usually lost in optical–electronic conversion; 6) the ability to transparently process a data channel regardless of its data rate or its modulation format; 7) scalability, and 8) cascading. Unfortunately, as underlined by Grinblat et al. in 2020, at that date no known structure behaved as an ideal switch [66].

A method widely used in all-optical switches is based on the third-order nonlinear optical effects, i.e., on the effect of intensity-dependent refractive index. The advantage of this method is its strong universality; in principle, complex all-optical devices could be realized based on it. The main obstacle, limiting its applications, is the intrinsic material bottleneck limitation, due to the contradiction between the huge third-order nonlinear coefficient and the ultrafast response time (i.e., the larger the third-order nonlinear coefficient, the slower the response time).

An important enabling technology to implement all-optical switching operation is provided by high Q/ V_m resonators, Q being the quality factor and V_m the mode volume of the cavity. The field intensity enhancement inside the cavity makes it possible to utilize optical nonlinearities at a low input power (a few hundred μW to mW level), achieving picosecond–femtosecond response time and lower energy consumption [67, ^[67][68]68]. Microring resonators are among the most widely used configurations for realization of all-optical switches due to their simple design and easy experimental realization ^{[69][70][71][72]}[69-72]. The switching principle, based on nonlinear effects, relies on a shift in resonant wavelength: the refractive index variation in the nonlinear material changes the resonant frequency of the microring and/or the coupling between the waveguide and the microring. As a consequence, switch control of the signal light output can be enabled. As an example, Figure 6 presents a schematic illustration of an all-optical switch based on a microring resonator [71].

Figure 6. Schematic illustration of an all-optical switch made of an add-drop microring resonator. The dotted line represents the transmittance spectrum of a cold cavity. The solid line is the transmittance when inputs are applied. A resonant shift occurs due to the optical Kerr effect. Two different wavelengths λ₁, and λ₂ are used for the operation. (a) λ₁ will drop (high) when only λ₁ is inputted (high). (b) λ₂ will not drop (low) when only λ₂ is inputted (high). (c) λ₂ will drop (high) when both λ₁ and λ₂ are inputted (high). As a result, λ₂ can be switched off and on by turning λ₁ signal on and off. Reprinted with permission from [71] © The Optical Society.

The basic building block of an optical flip-flop device is the optical bistable switch. In this device, the output of the system takes on one of two possible states, depending on the states of the inputs. Optical bistable operation permits optical read–write memory operation, opening the possibility of an integrated optical logic circuit on a single chip. A typical way of forming a bistable optical device is to place a saturable absorber inside a resonator. As the input intensity is increased, the field inside the cavity also increases, lowering the absorption of material and thus increasing the field intensity further. If the intensity of the incident field is subsequently lowered, the field inside the cavity tends to remain large because the absorption of the material system has again been reduced. However, even if single bistable switches have already been demonstrated on different platforms, it is worth noting that the next big challenge is the realization of a complex system, where a number of bistable switches are connected in tandem and in parallel ^{[73][74][75][76][77][78][79][80]}[73-80].

Electronic transistors, as the basic unit of logic circuits, have succeeded in supporting large-scale integrated circuits for computers. Any complex logic circuit can express a combination of three basic logic gates, AND, OR, and NOT, and these basic gates can be constructed with a universal transistor. Although electronic logic gates have enabled creation of integrated circuits with high density and functionality, optical logic gates cannot reach the far requirements of large-scale optical computing circuits even today [81]. On the other hand, optical transistors, as the core hardware of optical gates, so far have not been effectively exploited. Moreover, the universal optical transistor does not seem to exist or be practical for optical gates. Many different approaches to optical transistors have been proposed. Much early work was based on optical bistability [82] using nonlinear optical phenomena, mostly in resonators, whereas others worked on the optically controlled switching of light-exploiting single molecules [83] or quantum dots [84].

Today, in optical networks, data are encoded on photons for transmission, while information processing is often carried out through optical–electrical–optical (OEO) conversions. In the last 20 years, with the aim to assist/replace some of the electronic modules used in network routers, optical signal processing systems (OSPs) have been investigated. OSP refers to a broad range of techniques with the aim to process and manipulate the signal, i.e., amplitude, phase, wavelength, and polarization of optical waves, directly in the optical domain. Optical signal processing techniques employ a wide range of devices and various nonlinearities to achieve multiple network functionalities. Widely used functionalities demonstrated in nonlinear PICs include wavelength ^{[85][86][87][88]}[85-88] and format conversions [89], routing [90], phase-sensitive amplification [91], optical multiplexing and demultiplexing ^{[92][93][94][95][96]}[92-96], optical memory^[97] [97], all-optical tunable delay [98], and signal regeneration [99]. Thus far, most of the existing OSP research has relied on third-order nonlinearities, such as four-wave mixing (FWM), self-phase modulation (SPM), and cross-phase modulation (XPM) ^{[100][101][102][103][104][105]}[100-105].

As an example, C-band wavelength conversion in Si photonic wire waveguides with submicron cross-section was demonstrated by means of nondegenerate FWM (see fig. 7) [105]). The nonresonant character of the FWM allowed to demonstrate frequency tuning of the idler from ∼ 20 GHz to > 100 GHz, thus covering several C-band DWDM channels.

Figure 7 Output spectra from a single-mode silicon photonic-wire waveguide with a cross- section of 220 nm × 445 nm and length of L = 4.2 mm fabricated on a SOI. Pump (λp ~1435 nm) and signal laser sources were multiplexed and launched into the waveguide using a tapered fiber in copropagating configuration; several signal wavelengths (λs =1545.5, 1548.5, 1550.5, 1552.5, and 1555.5 nm) were used. On each side of the signal wavelength employed, newly-generated satellite peaks are clearly seen. Reprinted with permission from [105] © The Optical Society

Current optical networks are mostly based on time-division-multiplexing (TDM) and wavelength-division-multiplexing (WDM). In the former, multiple relatively low-bit-rate streams of data with the same carrier frequency are interleaved to create a single high-bit-rate stream, while the latter involves simultaneous propagation of multiple data signals, each at a different wavelength in a single optical line. Wavelength-division-multiplexing (WDM) techniques offer very effective utilization of the fiber bandwidth directly in the wavelength domain. A first issue of current optical networks concerns wavelength-blocking. In order to overcome it, a fundamental piece is represented by wavelength conversion devices. They can be obtained using different nonlinear effects, such as FWM or XPM [100-105]. Another issue is to increase transmission bandwidth, and an option is to combine TDM and WDM. In this process, demultiplexing an ultra-high-data-rate time-multiplexed signal to speeds receivable through electronics is achieved by wavelength conversion. Specifically, information multiplexed in the time domain through optical time demultiplexing (OTDM) can be converted into parallel lower-data-rate wavelength or spatial channels. This process has been achieved utilizing FWM, whereby a relatively low repetition rate pump switches out temporally multiplexed channels by converting them to new wavelengths (the idler in the FWM process) ^{[92][93][94][95][96]}[92-96]. There are, of course, other routes that can be followed to increase transmission bandwidth and/or to develop advanced processing chips. The goal of increasing bandwidth density of on-chip interconnects without increasing the number of waveguides, waveguide crossings, and chip footprint, for instance, may be reached by exploiting mode-division-multiplexing (MDM) in conjunction with WDM. Polarization-division-multiplexing (PDM) and orthogonal frequency-division-multiplexing (OFDM) are two other effective methods to increase the spectral efficiency of a communication system [96].

When signal modulation rate increases, signal degradation in the optical channel caused by dispersion, nonlinearity, and noise becomes a critical issue. Conventionally, signal regeneration in an optical system is performed through optical–electrical–optical (OEO) conversion, in which a weak and distorted signal is detected, restored in electronics, and retransmitted onto an optical fiber. Regenerators are designed to increase system performance, reduce data degradation, and enhance signal-to-noise ratio (SNR) for higher link capacity. In general, regenerators perform three signal processing functions: (1) reamplifying, (2) reshaping, and (3) retiming the signal. When the data rate is becoming higher and higher (towards 100 Gb/s), optoelectronic regeneration schemes will be very hard to implement or even impossible. Thus, all-optical regeneration, either 2R (re-amplification, re-shaping) or 3R (re-amplification, re-shaping, re-timing), has become a key technology to improve signal quality [99].

Finally, nonlinear optical sources provide an outstanding example of new possibilities offered by integrated nonlinear photonic chips, such as generation of new classes, named supercontinuum generation [106], and microcombs [107], which are capable of generating coherent, ultra-broadband light sources that cannot be produced in linear photonic systems.

Supercontinuum generation is a device functionality that has important applications in many areas of photonic integrated circuits, particularly in WDM applications. As an example, it could be beneficial to use a single broadband laser source, select by filtering specific wavelength channels, and then modulate these channels instead of using a separate laser for each wavelength. An experimental demonstration of a tunable continuum source in silicon photonic wires (SPWs) with a power-dependent broad spectrum was reported in [108]. As can be seen in Figure 14, a spectral broadening of more than 350 nm was observed upon propagation of ultrashort 1.3 μm wavelength optical pulses in a 4.7 mm long single-mode waveguide.

Figure 8. Supercontinuum generation in a 4.7-mm-long silicon-photonic-wire waveguide for several input central wavelengths at P 0 ≈ 1W. The inset shows that the spectral broadening increases as λ₀ approaches the ZGVD wavelength of 1290 nm. Reprinted with permission from [108] © The Optical Society

Optical frequency combs (OFCs) are often referred to as optical rulers: their spectra consist of a precise sequence of discrete and equally spaced spectral lines that represent precise marks in frequency. This discrete ensemble of equally spaced laser frequencies that distinguish OFCs from other light sources is the spectral counterpart of the regular train of short pulses emitted by mode-locked lasers. The OFCs solve the challenge of directly measuring optical frequencies and are now exploited as the most accurate time references available, ready to replace the current standard for time. Laser frequency combs can provide integrated sources of mutually coherent laser lines for terabit-per-second transceivers, parallel coherent light detection, or photonics-assisted signal processing. Mode-locked lasers were initially used for comb generation. Thereafter, comb emission was demonstrated in continuous-wave (cw) laser-pumped resonators through cascaded third-order parametric processes [109]. Because of the relatively low strength of third-order nonlinearity, generation of Kerr combs requires small interaction volumes and high-Q resonators. For these reasons, small resonators are particularly suited to reach broadband comb generation with quite moderate pump power. Advancements in the fabrication technology of optical micro-cavities may enable realizing ultra-fast and stable optical clocks and pulsed sources with extremely high repetition-rates in the form of compact and integrated devices. In this framework, demonstration of planar high-Q resonators, compatible with silicon technology ^[110][111][110, 111], has revealed a unique opportunity for these devices to provide entirely new capabilities for photonic-integrated technologies. It can also be recalled that electro-optic (EO) modulation, in materials with second-order nonlinearity, provides an interesting alternative to Kerr (third-order nonlinearity) resonators for the generation of OFCs; the electrical controllability of EO combs guarantees a greater versatility and also an excellent comb stability and phase coherence [112].

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