Second-order nonlinear frequency conversion in InGaP-on-insulator waveguides

Nonlinear dielectric metasurfaces represent a rapidly advancing family of optical devices characterized by high nonlinear conversion efficiency with an ultrathin form factor. Here, we report on terahertz (THz) emission through difference-frequency generation (DFG) in an Aluminum Gallium Arsenide (AlGaAs) metasurface supporting magnetic toroidal dipolar resonances at the two pump wavelengths, which nonlinearly interact with each other. The simultaneous excitation of these high-Q resonances, together with the dispersive behavior of the nonlinear susceptibility, enables an exceptionally strong DFG response. Conversion efficiencies around 8 THz exceed those of an unstructured AlGaAs slab of the same thickness by over nine orders of magnitude. These results highlight the potential of DFG metasurfaces as compact and highly efficient platforms for THz generation.

Over the last decade, significant improvements in cost reduction as well as in conversion efficiency increase were achieved in large-scale industrial production, making PV cost competitive with other means of electricity generation. In many regions worldwide, PV achieves the lowest levelized cost of electricity. Several different factors made this tremendous achievement possible-namely economy of scale, a lean and efficient production process, and high conversion efficiencies. In this work, some of the key concepts and methods are described based on Hanwha Q CELLS’ experience. The methods and approaches for the fast transfer of cell technologies from laboratory to production and for accelerated progress in cell efficiency, quality, and reliability of the cell and module product are described. Over the last decade, the cell conversion efficiency increased by 0.5%abs per year. Currently, average cell conversion efficiencies exceeding 20% using boron-doped p-type multicrystalline (mc-Si) and 22%, using Czochralski-grown silicon (Cz-Si) substrates, are achieved on a multi-GW scale. Currently (as of Jan 2018) produced Q.ANTUM solar modules from these cells exhibit output powers of 325 Wp based on 120 half Cz cells and of 350 Wp based on 72 full multicrystalline cells. Besides just the performance, it is critical to investigate the reliability of solar modules based on p-type solar cells with dielectrically passivated rear. In terms of reliability, light-induced degradation (LID) is examined in detail, with conditions relevant for the activation of both the boron–oxygen (BO) defect, and “Light and Elevated Temperature Induced Degradation” (LeTID). While the formation of the BO defect has been considered the most prominent LID mechanism in boron-doped p-type Cz-Si, recently it was shown that LeTID is another important issue for passivated emitter and rear cells (PERC) on both mc-Si and Cz substrates. When not adequately suppressed, LeTID can occur on p-type PERC with degradation in output power of >6%; critically, it cannot be suppressed in a straightforward manner by conventional processing steps used to permanently deactivate the BO defect. In contrast to conventional PERC, Hanwha Q CELLS’ Q.ANTUM technology is shown to reliably suppress both LID due to BO defect formation and LeTID, for both p-type mc-Si and Cz-Si substrates.

The growing use of nonlinear imaging and spectroscopy - namely multiphoton imaging and coherent Raman spec-troscopy - has led to the development of a myriad of ultra-short, tunable, and energetic light sources, mostly based on second-order nonlinear frequency conversion in bulk materials, with outstanding achievements in terms of energy, peak power, and stability [1]. Although these sources fully satisfy the requirements for nonlinear imaging [2], they require high maintenance, limiting their use to a controlled environment. Fiber-based sources then appear as a solution to bring these experiments outside the lab, but are limited to biological tissue imaging be-cause of their limited pulse energy. To expand nonlinear imaging techniques to diluted media (e.g. gases), the approach of combining chirped pulse amplification with four-wave mixing in optical fibers to enhance pulse en-ergy has proven effective, as demonstrated by the recent realization of a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$\mu \mathrm{J}$</tex> -level fiber optical parametric oscillator (FOPCPO) pumped with highly chirped pulses [3]. Furthermore, this FOPCPO concept enables improved control of the generated bandwidth by adjusting the relative chirp of the pump and the resonant wave [4]. In this work, we investigated the dynamics of such a FOPCPO to verify its ability to meet the low-noise and high-stability require-ments of nonlinear imaging. In addition, we confirmed that parametric oscillators provide a more stable pulse train compared to parametric amplifiers [5].

Direct bonded periodically poled MgO doped lithium niobate (PPMgLN) ridge waveguide is a new wavelength converter with high conversion efficiency. The optical field distribution of the ridge waveguide is simulated by employing finite-difference method (FDM), the mode overlap of propagated waves in the ridge waveguide is calculated and the relationship between the overlap coefficient and the waveguide structure sizes is also investigated. The overlap coefficient to difference frequency generation (DFG) process conversion efficiency calculation is firstly introduced.

The goal of integrated quantum photonics is to combine components for the generation, manipulation, and detection of nonclassical light in a phase-stable and efficient platform. Solid-state quantum emitters have recently reached outstanding performance as single-photon sources. In parallel, photonic integrated circuits have been advanced to the point that thousands of components can be controlled on a chip with high efficiency and phase stability. Consequently, researchers are now beginning to combine these leading quantum emitters and photonic integrated circuit platforms to realize the best properties of each technology. In this paper, we review recent advances in integrated quantum photonics based on such hybrid systems. Although hybrid integration solves many limitations of individual platforms, it also introduces new challenges that arise from interfacing different materials. We review various issues in solid-state quantum emitters and photonic integrated circuits, the hybrid integration techniques that bridge these two systems, and methods for chip-based manipulation of photons and emitters. Finally, we discuss the remaining challenges and future prospects of on-chip quantum photonics with integrated quantum emitters.

Solar Cell Capacitance Simulator in 1 Dimension (SCAPS-1D) is used to investigate the possibility of realizing ultrathin CdTe based solar cells with high and stable conversion efficiency. In the first step, we modified the conventional cell structure by substituting the CdS window layer with a CdS:O film having a wide band gap ranging from 2.42 to 3.17 eV. Thereafter, we simulated the quantum efficiency, as well as the parameters ofJ-Vcharacteristics, and showed how the thickness of CdS:O layer influences output parameters of Glass/SnO2/ZTO/CdS:O/CdTe1-xSx/CdTe/Ni reference cell. High conversion efficiency of 17.30% has been found using CdTe1-xSx(x=0.12) and CdTe layers of thickness 15 nm and 4 μm, respectively. Secondly, we introduced a BSR layer between the absorber layer and back metal contact, which led to Glass/SnO2/ZTO/CdS:O/CdTe1-xSx/CdTe/BSR/Ni configuration. We found that a few nanometers (about 5 nm) of CdTe1-xSxlayer is sufficient to obtain high conversion efficiency. For BSR layer, different materials with large band gap, such as ZnTe, Cu2Te, and p+-CdTe, have been used in order to reduce minority carrier recombination at the back contact. When ZnTe is used, high conversion efficiency of 21.65% and better stability are obtained, compared to other BSR.

Room-temperature, narrow-linewidth terahertz quantum cascade laser sources based on intracavity Cherenkov difference-frequency generation with emission at 3.5 THz, nearly 60µW power output, and conversion efficiency of approximately 0.55 mW/W2 are reported.

For fundamental tests of quantum physics as well as for quantum communications, non-classical states of light are an important tool. In this talk, we will present our work on developing nonlinear AlGaAs waveguides into a platform for quantum photonics in semiconductors. Most III-V semiconductors exhibit a large second-order optical nonlinearity, but phase-matching the nonlinear interaction is notoriously difficult. As a solution Bragg-reflection waveguides allow efficient creation of photon pairs through spontaneous parametric down-conversion. They have the potential to be homogeneously integrated with a pump laser and passive and active components on the chip.

Bandwidth enhancement and response flattening of wavelength conversion based on single-pass and double-pass cascaded second harmonic generation and difference frequency generation in step-segmented quasi-phase matched (QPM) gratings are investigated. For the same waveguide length, high conversion efficiency, flat response and broad signal bandwidth can be obtained simultaneously by optimizing the poling period of the step-segmented QPM structure. The conversion bandwidths in a 3-cm-long five-step-segmented waveguide are 134 nm for single-pass scheme and 132 nm for the double-pass one, respectively, which are both over the whole conventional band and long-wavelength band. The ripple on the matching response can keeps less than 0.2 dB as well.

Nonlinear optical crystals lie at the core of ultrafast laser science and quantum communication technology. The emergence of 2Dmaterials provides a revolutionary potential for nonlinear optical crystals due to their exceptionally high nonlinear coefficients. However, uncontrolled stacking orders generally induce the destructive nonlinear response due to the optical phase deviation in different 2D layers. Therefore, conversion efficiency of 2D nonlinear crystals is typically limited to less than 0.01% (far below the practical criterion of >1%). Here, crystalline films of rhombohedral boron nitride (rBN) with parallel stacked layers are controllably synthesized. This success is realized by the utilization of vicinal FeNi (111) single crystal, where both the unidirectional arrangement of BN grains into a single-crystal monolayer and the continuous precipitation of (B,N) source for thick layers are guaranteed. The preserved in-plane inversion asymmetry in rBN films keeps the in-phase second-harmonic generation field in every layer and leads to a record-high conversion efficiency of 1% in the whole family of 2D materials within the coherence thickness of only 1.6µm. The work provides a route for designing ultrathin nonlinear optical crystals from 2D materials, and will promote the on-demand fabrication of integrated photonic and compact quantum optical devices.

We analyze the dynamics of sum-frequency generation (SFG) and difference-frequency generation (DFG) for broadband input fields. We demonstrate that amplitude modulation of the input fields significantly affects conversion efficiency of SFG and DFG, but frequency modulation does not affect conversion efficiency. Analytic results for SFG and DFG output-field intensities are thereby available for arbitrary fields. Self- and cross-phase modulation can affect the dynamics in the limit of very high field strengths, but these effects are of higher nonlinearity and therefore usually negligible. The quantum efficiency of DFG is shown to be generally higher than SFG, but the energy efficiency is much lower if the difference frequency is significantly less than the input frequencies. Optimal SFG quantum conversion efficiency for such systems approaches that of second-harmonic generation only if the quantum fluence (total number of photons) in the two input beams are equal. Optimal DFG quantum conversion efficiency occurs when the number of photons of frequency ${\mathrm{\ensuremath{\omega}}}_{2}$ is small yet sufficient for significant stimulation. A recently developed method for obtaining efficient SFG for multimode input fields is theoretically analyzed. The method involves using an arrangement with two or more nonlinear mixing crystals with a time-delay line situated between the crystals that delays one of the fundamental fields relative to the other. The efficiency in the second crystal depends on the cross-correlation function of the two fundamental fields upon leaving the first nonlinear crystal. The time-delay method is not effective for multimode DFG.

In this paper, a dielectric-filled circular waveguide TM01-TE11 mode converter is proposed, which has high conversion efficiency and a wide operating bandwidth. Filling the circular waveguide with dielectric material changes the local propagation characteristics, thus achieving a phase difference between the TE11 modes in the two halves of the circular waveguide during propagation. This, in turn, facilitates the completion of mode conversion with high efficiency. Compared with the conventional radial dielectric plate, this paper improves the method of filling the dielectric inside the circular waveguide by transforming it into a coaxial structure. This is followed by the incorporation of a radial dielectric plate, a modification that has been proven to enhance the conversion efficiency and extend the operational bandwidth. The mode converter operates at 9.7 GHz, and when the dielectric filler material is polytetrafluoroethylene (PTFE), both simulation and practical studies are carried out. The simulation results demonstrate that the maximum conversion efficiency of this mode converter is 99.2%, and the bandwidth with conversion efficiency greater than 90% is nearly 21.1%. The maximum conversion efficiency in the actual test is essentially consistent with the simulation results. The validity of the design scheme of this converter and the accuracy of the simulation study are demonstrated.

In order to utilize solar power for the production of solar electricity and solar fuels on a global scale, it is necessary to develop solar photon conversion systems that have an appropriate combination of very high efficiency (delivered watts/m2) and low capital cost ($/m2). One potential promising approach to attain high conversion efficiencies well above the Shockley-Queisser limit of 33% is to utilize the unique properties of semiconductor nanostructures (viz., quantum dots/rods/films) and singlet fission chromophores to control the relaxation dynamics of photogenerated carriers and photochemical excited states, respectively. The former process creates enhanced photocurrent through efficient photogenerated electron-hole pair multiplication (electron-hole pairs exist as excitons in size-quantized nanostructures), or enhanced photopotential through hot carrier transport, transfer, and extraction processes. The latter process of singlet fission generates two triplets from an excited singlet state. These fundamental dynamics in various bulk and nanoscale semiconductors and in molecular chromophores have been studied using transient absorption, photoluminescence, photocurrent and THz spectroscopy with fs to ns time resolution. The predictions that the generation of more than one electron-hole pair or exciton per absorbed photon would be an efficient process in nanostructures, and via singlet fission, have been confirmed in different classes of materials, molecules, and their architectures in photon conversion devices. Selected aspects of this work will be summarized and recent advances will be discussed, including the very remarkable and beneficial theoretical effects of combining MEG or singlet fission with solar concentration and controlling the thicknesses of the photoactive layers in a two-photon photoelectrosynthesis cell structure for solar fuels.

Dye-sensitized solar cells (DSSCs) have aroused intense interest and been regarded as one of the most prospective solar cells, due to low-cost, flexibility, simple device fabrication and high conversion efficiency under weak light, in comparison to the conventional photovoltaic devices. Very recently, G2E in Swiss and G24i in UK including Korean and Japan companies have demonstrated commercial and prototyped components based on DSSC technology with liquid electrolytes. However, the unit costs, long-term device stability and power conversion efficiency must be further improved for real-life applications. For this purpose, considerable efforts have been devoted to the search for low cost Pt-free catalysts that exhibit high electrochemical activity and fast electron transfer kinetics, while a platinum (Pt) catalyst is still the most efficient and widely used counter electrode (CE) in DSSCs, but it has some problems that make it unsatisfactory, such as its high manufacturing cost, due to scarcity, and poor long-term stability for the iodide/triiodide redox couple in a DSSC. As a result, significant efforts have been devoted to finding possible alternatives to Pt, including carbon blacks, carbon nanotubes, functionalized graphene, and heteroatom-doped GnPs as efficient metal-free electrocatalysts. One of the most promising candidates as CE materials is non-metallic heteroatom-doped graphene nanoplatelets (GnPs). Heteroatom-doped GnPs gave high power conversion efficiency (PCE) of DSSCs employing cobalt electrolytes. However, they are not sufficiently active in iodidde electrolytes, which are more common and desirable electrolytes. Thus, the better elcetrocatalytical ability and electrochemical stability of carbon-basedmaterials toward the redox couples still need to be significantly improved for the practical applicationof DSSCs. Very recently, a series of edge-fuctionalized heteroatom-doped and metalliod-doped graphene nanoplatelets (GnPs) have been prepared through a simple mechanochemical ball-milling process. In this presentation, comparative study of edge-functionalized heteroatom-doped and metalloid-doped graphene nanoplatelets as metal-free counter electrodes for highly efficient dye-sensitized solar cells employong bothe redox shuttles will be described to understand and establish the structure-property relationship between sort of fuctionalization and cell performance with regard to the present GnPs through the same fabrication process under the same experimental condition.

Detecting the electric-field waveform of an optical pulse from the terahertz to the visible spectral domain provides a complete characterization of the average field waveform and holds great potential for quantum optics, time-domain (including frequency-comb) spectroscopy, high-harmonic generation, and attosecond science, to name a few. The field-resolved measurements can be performed using electro-optic sampling, where a laser pulse is characterized through an interaction with another pulse of a much shorter duration. The measured pulse train must consist of identical pulses, including their equal carrier-envelope phase (CEP). Due to the limited coverage of broadband laser gain media, creating CEP-stable pulse trains in the mid-infrared typically requires nonlinear frequency conversion, such as difference frequency generation, optical parametric amplification, or optical rectification. These techniques operate in a single-pass geometry, often limiting efficiency. In this work, we demonstrate field-resolved analysis of the pulses generated in a resonant system, an optical parametric oscillator (OPO). Due to the inherent feedback, this device exhibits a relatively high conversion efficiency at a given level of input power. By electro-optic sampling, we prove that a subharmonic OPO pumped with CEP-stable few-cycle fiber-laser pulses generates a CEP-stable mid-infrared output. The full amplitude and phase information renders dispersion control straightforward. We also confirm the existence of an exotic “flipping” state of the OPO directly in the time domain, where the electric field of consecutive pulses has the opposite sign.