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Owen Martinez
Owen Martinez

Small Photonic Chip Offers A Big Improvement In Precision Optics

In addition to expanding the number of potential applications, optics and sensors can also take laser performance to a new level. For example, integrated devices are already critical to optical-coherence tomography, a noninvasive procedure for taking 2-D and 3-D images of retinal tissue. To determine the full potential of integrated laser-based systems, we first examined the precision optics and photonic-sensor sectors, looking at core technologies, recent growth, and go-forward adoptions. We found that both markets are now thriving and that the uptick in integrated laser devices could increase their value even further.

Small Photonic Chip offers a big improvement in Precision Optics

Some laser manufacturers and end customers are already pursuing such deals to facilitate the creation of devices that integrate precision-optics, sensors, and lasers. For instance, a major supplier of lithography systems recently acquired a precision-optics company to gain additional capabilities for extreme-ultraviolet and deep-ultraviolet products. Another leading industrial-applications company acquired minority stakes in some laser-technology firms to boost its capabilities in materials-processing applications. It also acquired a company that manufactures many of the photonics components and products used in sensors for autonomous driving, smartphones, and digital data transmission.

Optics and photonics technologies are ubiquitous: they are responsible for the displays on smart phones and computing devices, optical fiber that carries the information in the internet, advanced precision manufacturing, enhanced defense capabilities, and a plethora of medical diagnostics tools. The opportunities arising from optics and photonics offer the potential for even greater societal impact in the next few decades, including solar power generation and new efficient lighting that could transform the nation's energy landscape and new optical capabilities that will be essential to support the continued exponential growth of the Internet.

Engineers envisioned all of these advances continuing and converging to the point where photonics would merge with electronics and eventually replace it. Photonics would move bits not just across countries but inside data centers, even inside computers themselves. Fiber optics would move data from chip to chip, they thought. And even those chips would be photonic: Many expected that someday blazingly fast logic chips would operate using photons rather than electrons.

These small quantum chips have again been designed for boson sampling, which could be interesting in its own right for studying the interactions between squeezed states. More importantly, said Vernon, the X-series offers a crucial building block for creating a universal quantum computer that will outperform classical computers.

Stand-alone SNSPDs are ideal for all fiber and free-space applications, but they would lead to coupling losses whenever they need to be interfaced with on-chip devices. Furthermore, to achieve high detection efficiency, these structures must be relatively long and dense meanders are needed (typically covering >200 μm2 [82]), compromising the speed of the detector and making the devices more prone to fabrication imperfections [97], [98], [99]. Although for applications in quantum information Science and for the detection of faint light, integration of single-photon detectors with on-chip photonic structures represents, in general, a challenging task, thanks to their relatively simple structure, SNSPDs are less demanding. A new approach where detectors are integrated with the experiment directly on the respective chip has been proposed in 2009 by Hu et al. [100]. In this new configuration, a single nanowire segment is placed on top of a photonic waveguide such that photons are absorbed along their direction of propagation, as depicted in Figure 1B. This approach allows for drastically enhancing the interaction length over which the propagating field can be absorbed by the nanowire. Wires of significantly shorter overall length than those of meander-type detectors can then be used to efficiently absorb photons travelling on a chip. Such short wires possess a smaller kinetic inductance and therefore feature smaller recovery times, resulting in higher photon counting rates. In 2011, Sprengers et al. [101] presented the first experimental realization of a superconducting nanowire single-photon detector consisting of an NbN nanowire fabricated on top of a GaAs waveguide. The on-chip detection efficiency (OCDE) was determined as 19.7%, and a timing jitter of 60 ps was measured. Subsequently, Pernice et al. [102] demonstrated NbN superconducting nanowire single-photon detectors embedded with silicon (Si) waveguides featuring 91% OCDE, sub-nanosecond decay time and 18-ps timing jitter in the NIR range. The combination of SNSPDs and nanophotonic represents an added value for performances tunability. The absorption of the detector can be in fact engineered by controlling the evanescent coupling trough waveguide and detector geometry [103], and state-of-the-art nanophotonic structures can be exploited to improve or add new functionalities to the detector, such as high efficiency and spectral resolution by embedding them in microring resonators [104], wavelengths demultiplexer [105] or photonic crystal cavities [106]. In addition, travelling wave geometry evanescent coupling could play an important role in the mitigation of artifacts due to the position dependency of the detection efficiency. It has been in fact observed [102] from mode simulation that, for evanescent coupling, the optical field is concentrated in the side of the nanowire, resulting in a higher absorption efficiency than in the nanowire center. This could have important consequences in the detection mechanism and in the detector performances of the geometrical jitter, for instance, as discussed in the next section.

Superconducting nanowire single photon detectors provide high detection efficiency from UV to mid-infrared wavelengths [19], [112], [132]. As discussed in the Introduction, a cloud of quasiparticles is generated upon photon absorption in the nanowire, which in turn switches to a resistive state triggering a detection event. The quasiparticle concentration during the initial stages of the detection process depends on the ratio between the incident photon energy and the superconducting energy gap [35]. The higher the photon energy (i.e. shorter wavelengths), the more quasiparticles are generated, and it will be more likely to operate the detector in the deterministic regime. The detection efficiency will further depend on the relation between quasiparticle cloud size and nanowire geometry as current crowding and quasiparticle diffusion affect the performance [19]. Considering only the superconducting material specific properties, there is no lower limit for the shortest detectable wavelength. On the other hand, the longest detectable wavelength is determined by the superconducting gap and by the nanowire width. NbN and WSi nanowires have shown saturated spectral response up to 5.5 μm [112], evidently limited by the capability to fabricate (narrow) nanowires with higher uniformity. For waveguide-integrated SNSPDs, the spectral range of the detector is determined not only by material properties and geometry of the superconducting strip but also by the waveguiding material. Several material systems that are well suited for fabricating photonic integrated circuits have limited transparency windows. Silicon, for example, is a very popular choice for many integrated optics applications due to its high refractive index, which guarantees strong mode confinement inside a waveguide. However, silicon is only transparent for wavelength in the NIR spectral region and can therefore not take advantage of the full spectral bandwidth of waveguide-integrated SNSPDs. Alternative photonic integrated circuit material systems such as aluminum nitride (AlN) [133], silicon nitride (SiN) [111] and the more exotic polycrystalline diamond [115], however, feature extremely large optical transparency windows and have been demonstrated to be suitable for realizing high-performance SNSPDs, as discussed in Section 3.1. Among these materials, diamond is particularly attractive because it provides optical transparency from the UV to the mid-infrared. Beyond the material platform, an additional bandwidth limitation of waveguide-integrated SNSPDs arises from the waveguide geometry required for low-loss propagation of transverse electric and magnetic field modes (e.g. the waveguide cross-sections determine the maximum wavelength for which a propagating mode is still supported). Lastly, the mechanism for coupling light into an optical waveguide on a chip may underlie further bandwidth constraints, as it is, for example, the case when using optical grating couplers [134].

Silicon has a long tradition in integrated electronics and represents today one of the most used technological platform for integrated photonics. Through the advent of SOI, high-density devices with foundry-ready CMOS technology can be realized nowadays. Silicon has a very high refractive index (n=3.45 at 1550 nm) allowing tight light confinement and very low bending losses, which results in the possibility of integrating dense optical structures [2]. One of the main limitations in the application of silicon photonics is that, due to its small band gap (1.1 eV), it is not possible to guide visible light in silicon. This represents a major drawback especially considering that for most applications in biology and the life Sciences, investigation in the visible spectra is essential. Efficient single-photon generation and detection on a photonic chip are two main challenges for the integrated photonics community. Being able to efficiently generate and detect single-photon is a key aim for the photonic community but would be of limited use without an established photon manipulation technology. In this context, a further limitation of Si is related to thermal instability processes due to two-photon absorption (TPA) and free carrier absorption, which limits fabrication devices with active functionalities [175], [176], [177]. In addition, because of its centrosymmetric structure, Si does not provide the electro-optic effect, which would enable ultrafast on-chip modulation of light and phase shifts. 041b061a72


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