In photonics we study the interaction of light with matter. More specifically, the field studies the propagation and the generation of light in different media such as dielectrics, air and metals.
Photonics has many applications such as sensing (gas sensing, biosensing), telecommunication, lighting, photovoltaics, CD/DVD drives and so on. For most photonic applications, the wavelengths of interest are between the visible and the infrared, as shown in Figure 1.2.
A recent trend in photonics is the drive towards miniaturization of com- ponents and integrating many of them on a single chip. These so-called (nano)photonic integrated circuits have a better performance, are cheaper, are more robust, and consume less power than bulk photonics, than low-contrast integrated photonics and than electronics.
(a) Layers of the SOI stack. (b) Standard etch depths of 70 and 220 nm.
Figure 1.3: The layers of a standard SOI stack. The thickness of the bottom
Silicon layer depends on the way the SOI stack was fabricated and whether or not the final wafer is thinned. The top layer is patterned to create nanophotonic structures. Typically, etch depths of 70 and 220 nm are used.
in wavelengths that are useful for telecommunication (1310 nm and 1550 nm), and thanks to the high index contrast, we can produce very small devices. To make nanophotonic chips, typically one starts from a Silicon On Insulator (SOI) wafer, see Figure 1.3(a). Using different resists and etching processes, the wafer is then paterned (see Figure 1.3(b).
1.2.1 Fabrication of nanophotonic chips
There are essentially two ways to define optical features on-chip. Both methods are based on a resist that covers the chip. Part of the resist is then removed, and in a next step, the unprotected parts of the chip can be etched, or other mate- rials can be deposited on top of it. The first method for modifying the resist is by using electron beam lithography. In electron beam lithography (often abbre- viated as e-beam lithography), a beam of electrons is incident on the resist in order to remove parts of it. For example: photonic wire waveguides are fabri- cated in [15,16] and photonic crystal cavities are fabricated in [17,18]. Even though e-beam steering can allow accurate dimensional control, it is slow and unsuitable for mass production due to the small writing area. Figure 1.4 shows some examples of nanophotonic components.
The second way to define optical features is by using a resist that is sensitive to light (a photoresist). Photoresists are used a lot in electronic chip fabrication, and moreover silicon is a good material to guide light, so we can reuse standard Complementary Metal Oxide Semiconductor (CMOS) technology to manufac- ture photonic chips. In this technology, the SOI wafer is patterned using deep UV lithography. Recent advances in the lithography processes made it possi-
(a) Crossing (b) Taper
(c) SEM image of a crossing (d) SEM image of multiple tapers
Figure 1.4: Some examples of nanophotonic subcomponents created by opti-
cal lithography. These components are building blocks for integrated optical circuits. Because a nanophotonic circuit is planar, crossings (left) are sometimes needed. Tapers (right) are used to spread light from a narrow waveguide to a broad one. On the bottom, Scan- ning Electron Microscope (SEM) pictures of the fabricated devices are shown.
ble to accurately pattern optical structures with a dimensional control of 1-5 nm [19], which enables mass-fabrication of nanophotonic devices and circuits. A detailed step-by-step description of the process we use at imec can be found in [19].
One big challenge when it comes to guiding light on a chip, is to control the light in a very accurate manner. The features that exhibit guiding properties are only sub-micron scale (e.g., 450 nm thick and 220 nm high for a rectangular waveguide), and the phase of the light is very sensitive to slight variations in these dimensions. Surface roughness causes scattering and back reflections, which lead to more losses and performance degradation. We will discuss the impact of this precision on the performance of our devices in chapter 3 and 6.
1.2.2 Nonlinearities
Nonlinear processes cause a change in the refractive index n of the material. De- pending on the used materials and the type of nonlinearity, the strengths of the effects can vary over different orders of magnitude. Furthermore, the timescale at which the different nonlinear effects occur varies from the microsecond (µs)
scale to the femtosecond ( f s) scale.
The fastest nonlinearity that we will encounter in this dissertation is the Kerr effect. The Kerr effect causes the refractive index to change in response to an applied electric field. This can be either an externally applied field or the opti- cal field itself. In the latter case, n = n0+ n2I , where I is the intensity of the light
and n2is the Kerr constant. As n2is usually very small, this effect is only relevant
for high intensities. For silicon, n2is on the order of 10−13cm2/W for telecom
wavelengths [20] (which is still a factor 200 higher than in silicon oxide). In res- onant structures, the Kerr effect can cause a bistability of the output, and very interesting nonlinear behavior arises when coupling several of these cavities.
One of the other important nonlinearities that has to be taken into account is the temperature effect. Due to temperature changes caused by high opti- cal powers or resistive heating, the refractive index can vary according to n =
n0+d nd T∆T . For silicon, d n
d T ' 1.86 · 10−4K−1[21,22]. Sometimes heaters are po-
sitioned on top of the optical structures in order to control the refractive index, for example in nanophotonic beam steering [23,24].
In nanophotonic resonators such as photonic crystal cavities and ring res- onators, the intensity inside the resonating structure can become very high, meaning that nonlinear effects will play a more important role. In addition these resonant devices are very sensitive to phase changes caused by refractive index changes. We can estimate the wavelength shift∆λ using the following equation:
∆λ
λ =
∆n
n , (1.1)
where∆n is the change in refractive index. In the case of a thermal effect, for
λ ' 1550 nm and n ' 3 and a temperature increase of ten degrees, this results in
a significant shift of about 1 nm.
1.2.3 Building blocks for optical neural networks
As we explained previously, nanophotonics could be used as a platform to cre- ate an artificial neural network. Artificial neural networks, which we will discuss in more detail in chapter 2, consist of a large number of nonlinear elements that are connected to each other and together perform computation. There are several potential building blocks to consider when designing such a nanopho- tonic neural network. Semiconductor Optical Amplifiers (SOAs) have been ex- tensively investigated in the doctoral thesis of K.T. Vandoorne [8], and ring res- onators are being investigated by T. Van Vaerenbergh, see for example [25,26]. Another interesting class of components are photonic crystal cavities, which are the emphasis of this doctoral thesis. The main differences between photonic crystal cavities and the other devices are:
• Photonic crystal cavities are passive devices (as opposed to SOAs, which consume approximately 1 mW per SOA). If the insertion loss (IL) of a cav- ity is sufficiently low, we do not need much regeneration of the signal in the network, leading to low-power reservoirs.
• Cavities store energy in a cavity mode. This leads to a considerable build- up of energy, which causes nonlinear effects such as temperature effects, the plasma dispersion effect due to free carriers, and Kerr-nonlinearities to become present. This is an advantage when a reservoir task needs non- linearity. Furthermore, the cavity has a time constant, which is a memory mechanism that is similar to that in leaky hyperbolic tangent reservoirs. By playing with the dimensions of the device, we can modify this time constant.
• The resonance mechanism for a ring resonator and a photonic crystal cavity are very similar. However, a photonic crystal cavity is inherently bidirectional. This can be advantageous because this adds additional feedback paths into the system.
• Photonic crystal cavities can have two possible architectures. Either the cavity is put within the guiding structure (inline cavity), which means that the transmission only equals unity when the device is at resonance. In the other case, the cavity is next to the waveguide (side cavity), and the trans- mission equals zero at resonance. This makes them behave fundamen- tally different, and gives additional degrees of freedom when designing a reservoir.
• The cavity can be made to be very compact, which means we can put a large number of cavities on one chip. This is especially true for 1D wire cavities (which are discussed in section 3.3), which can be fabricated with a very small footprint of about 10-20µm using SOI technology. Ring res- onators on the other hand, with bend radii of 5µm and larger, have a footprint of at least 100µm2, and SOAs, with a length of at least 500µm, are relatively large compared to the other two devices.