Volume 7, Issue 11

One of the most exciting new developments in the plasmonic nanomaterials field is the discovery of their ability to mediate a number of photocatalytic reactions. Since the initial prediction of driving chemical reactions with plasmons in the 1980s, the field has rapidly expanded in recent years, demonstrating the ability of plasmons to drive chemical reactions, such as water splitting, ammonia generation, and CO 2 reduction, among many other examples. Unfortunately, the efficiencies of these processes are currently suboptimal for practical widespread applications. The limitations in recorded outputs can be linked to the current lack of a knowledge pertaining to mechanisms of the partitioning of plasmonic energy after photoexcitation. Providing a descriptive and quantitative mechanism of the processes involved in driving plasmon-induced photochemical reactions, starting at the initial plasmon excitation, followed by hot carrier generation, energy transfer, and thermal effects, is critical for the advancement of the field as a whole. Here, we provide a mechanistic perspective on plasmonic photocatalysis by reviewing select experimental approaches. We focus on spectroscopic and electrochemical techniques that provide molecular-scale information on the processes that occur in the coupled molecular-plasmonic system after photoexcitation. To conclude, we evaluate several promising techniques for future applications in elucidating the mechanism of plasmon-mediated photocatalysis.

Integration of superconducting nanowire single-photon detectors with nanophotonic waveguides is a key technological step that enables a broad range of classical and quantum technologies on chip-scale platforms. The excellent detection efficiency, timing and noise performance of these detectors have sparked growing interest over the last decade and have found use in diverse applications. Almost 10 years after the first waveguide-coupled superconducting detectors were proposed, here, we review the performance metrics of these devices, compare both superconducting and dielectric waveguide material systems and present prominent emerging applications.

The fascinating relationship between structure and property in nanowires has enabled a wealth of applications in photonics and electronics. The behavior of phonons in nanowires is also modified compared to their bulk counterparts. In this review, we provide an overview of the recent efforts to investigate the properties of acoustic phonons in nanowires using ultrafast optical methods. In particular, we focus on the calculation of the modified phonon dispersion in nanowires and how to address them optically. We then discuss experimental investigations in arrays of nanowires and a single nanowire. The analysis of phonon behavior reveals the possibility to perform advanced mechanical characterization and to vary the thermal properties of nanowires. The review concludes with a brief perspective on future research directions, from phonon-induced control over properties to three-dimensional (3D) acoustic nano-imaging.

Germanium (Ge) has played a key role in silicon photonics as an enabling material for datacom applications. Indeed, the unique properties of Ge have been leveraged to develop high performance integrated photodectors, which are now mature devices. Ge is also very useful for the achievement of compact modulators and monolithically integrated laser sources on silicon. Interestingly, research efforts in these domains also put forward the current revolution of mid-IR photonics. Ge and Ge-based alloys also present strong advantages for mid-infrared photonic platform such as the extension of the transparency window for these materials, which can operate at wavelengths beyond 8 μm. Different platforms have been proposed to take benefit from the broad transparency of Ge up to 15 μm, and the main passive building blocks are now being developed. In this review, we will present the most relevant Ge-based platforms reported so far that have led to the demonstration of several passive and active building blocks for mid-IR photonics. Seminal works on mid-IR optical sensing using integrated platforms will also be reviewed.

Terahertz quantum cascade laser (QCL) sources based on intra-cavity difference frequency generation are currently the only electrically pumped monolithic semiconductor light sources operating at room temperature in the 1–6-THz spectral range. Relying on the active regions with the giant second-order nonlinear susceptibility and the Cherenkov phase-matching scheme, these devices demonstrated drastic improvements in performance in the past several years and can now produce narrow-linewidth single-mode terahertz emission that is tunable from 1 to 6 THz with power output sufficient for imaging and spectroscopic applications. This paper reviews the progress of this technology. Recent efforts in wave function engineering using a new active region design based on a dual-upper-state concept led to a significant enhancement of the optical nonlinearity of the active region for efficient terahertz generation. The transfer of Cherenkov devices from their native semi-insulating InP substrates to high-resistivity silicon substrates resulted in a dramatic improvement in the outcoupling efficiency of terahertz radiation. Cherenkov terahertz QCL sources based on the dual-upper-state design have also been shown to exhibit ultra-broadband comb-like terahertz emission spectra with more than one octave of terahertz frequency span. The broadband terahertz QCL sources operating in continuous-wave mode produces the narrow inter-mode beat-note linewidth of 287 Hz, which indicates frequency comb operation of mid-infrared pumps and thus supports potential terahertz comb operation. Finally, we report the high-quality terahertz imaging obtained by a THz imaging system using terahertz QCL sources based on intra-cavity difference frequency generation.

A 2D dielectric grating produced by photolithography was employed as a microlens array (MLA), which was based on a smooth surface and a shape that may be defined roughly as a hemisphere of each lattice. Such a 2D MLA can focus an incident ultraviolet light beam into a matrix of light spots, which were estimated to be as small as 500 nm in diameter. Using a thin layer of photoresist (PR) to record the pattern of the focusing spots, we achieved an approximately inversed structure of periodically arranged holes in PR in submicron sizes. Filling these holes with gold using chemically synthesized colloidal gold nanoparticles produced a plasmonic grating consisting of gold nanoparticles larger than 580 nm in average diameter. Localized surface plasmon resonance in both first and second orders was observed, which was verified by the spectroscopic response and theoretical simulations. MLA can be thus repeatedly used as a master to produce plasmonic photonic structures with high reproducibility.

A number of light-absorbing devices based on plasmonic materials have been reported, and their device efficiencies (or absorption) are high enough to be used in real-life applications. Many light-absorbing applications such as thermophotovoltaics and energy-harvesting and energy-sensing devices usually require high-temperature durability; unfortunately, noble metals used for plasmonics are vulnerable to heat. As an alternative, refractory plasmonics has been introduced using refractory metals such as tungsten (3422°C) and transition metal nitrides such as titanium nitride (2930°C). However, some of these materials are not easy to handle for device fabrications owing to their ultra-high melting point. Here, we propose a light absorber based on chromium (Cr), which is heat tolerant due to its high melting temperature (1907°C) and is compatible with fabrication using conventional semiconductor manufacturing processes. The fabricated device has >95% average absorption of visible light (500–800 nm) independent of polarization states. To verify its tolerance of heat, the absorber was also characterized after annealing at 600°C. Because of its compactness, broadband operational wavelength, and heat tolerance, this Cr perfect absorber will have applications in high-temperature photonic devices such as solar thermophotovoltaics.

A saturable absorber is a building block for integrated ultrafast photonics and passive optical circuits. However, options currently available suffer from the bottlenecks of the necessity for fine control of the material preparation, large optical losses, and compatibility. This paper presents a complementary metal–oxide–semiconductor (CMOS)-compatible alternative based on an epsilon-near-zero (ENZ) medium, in which the real part of the dielectric constant vanishes. Excellent nonlinear optical modulations, including low linear optical losses, low bleaching threshold, moderate optical amplitude modulation, and high modulation speed of indium tin oxide (ITO) in its ENZ region are achieved. The use of ITO as an intracavity saturable absorber for optical switches of integrated waveguide chip lasers at 1.9 μm has been realized. A stable mode-locked waveguide laser with a repetition rate of 6.4 GHz and an average output power of 28.6 mW is achieved via carefully adjusting the intracavity three-surface interferometer (TSI). This work may pave the way for integrated photonics and electro-optics using a CMOS-compatible ENZ medium.