Investigation of germanium quantum-well light sources.

In this paper, we report a broad investigation of the optical properties of germanium (Ge) quantum-well devices. Our simulations show a significant increase of carrier density in the Ge quantum wells. Photoluminescence (PL) measurements show the enhanced direct-bandgap radiative recombination rates due to the carrier density increase in the Ge quantum wells. Electroluminescence (EL) measurements show the temperature-dependent properties of our Ge quantum-well devices, which are in good agreement with our theoretical models. We also demonstrate the PL measurements of Ge quantum-well microdisks using tapered-fiber collection method and quantify the optical loss of the Ge quantum-well structure from the measured PL spectra for the first time.

Bias-dependence of luminescent coupling efficiency in multijunction solar cells.

In this work, we demonstrate an improved method to simulate the characteristics of multijunction solar cell by introducing a bias-dependent luminescent coupling efficiency. The standard two-diode equivalent-circuit model with constant luminescent coupling efficiency has limited accuracy because it does not include the recombination current from photogenerated carriers. Therefore, we propose an alternative analytical method with bias-dependent luminescent coupling efficiency to model multijunction cell behavior. We show that there is a noticeable difference in the J-V characteristics and cell performance generated by simulations with a constant vs. bias-dependent coupling efficiency. The results indicate that introducing a bias-dependent coupling efficiency produces more accurate modeling of multijunction cell behavior under real operating conditions.

Demonstration of a Ge/GeSn/Ge quantum-well microdisk resonator on silicon: enabling high-quality Ge(Sn) materials for micro- and nanophotonics.

We theoretically study and experimentally demonstrate a pseudomorphic Ge/Ge0.92Sn0.08/Ge quantum-well microdisk resonator on Ge/Si (001) as a route toward a compact GeSn-based laser on silicon. The structure theoretically exhibits many electronic and optical advantages in laser design, and microdisk resonators using these structures can be precisely fabricated away from highly defective regions in the Ge buffer using a novel etch-stop process. Photoluminescence measurements on 2.7 µm diameter microdisks reveal sharp whispering-gallery-mode resonances (Q > 340) with strong luminescence.

High-efficiency nanostructured window GaAs solar cells.

Nanostructures have been widely used in solar cells due to their extraordinary optical properties. In most nanostructured cells, high short circuit current has been obtained due to enhanced light absorption. However, most of them suffer from lowered open circuit voltage and fill factor. One of the main challenges is formation of good junction and electrical contact. In particular, nanostructures in GaAs only have shown unsatisfactory performances (below 5% in energy conversion efficiency) which cannot match their ideal material properties and the record photovoltaic performances in industry. Here we demonstrate a completely new design for nanostructured solar cells that combines nanostructured window layer, metal mesa bar contact with small area, high quality planar junction. In this way, we not only keep the advanced optical properties of nanostructures such as broadband and wide angle antireflection, but also minimize its negative impact on electrical properties. High light absorption, efficient carrier collection, leakage elimination, and good lateral conductance can be simultaneously obtained. A nanostructured window cell using GaAs junction and AlGaAs nanocone window demonstrates 17% energy conversion efficiency and 0.982 V high open circuit voltage.

Experimental demonstration of two methods for controlling the group delay in a system with photonic-crystal resonators coupled to a waveguide.

We measure the group delay in an on-chip photonic-crystal device with two resonators side coupled to a waveguide. We demonstrate that such a group delay can be controlled by tuning either the propagation phase of the waveguide or the frequency of the resonators.

Microlens performance limits in sub-2mum pixel CMOS image sensors.

CMOS image sensors with smaller pixels are expected to enable digital imaging systems with better resolution. When pixel size scales below 2 mum, however, diffraction affects the optical performance of the pixel and its microlens, in particular. We present a first-principles electromagnetic analysis of microlens behavior during the lateral scaling of CMOS image sensor pixels. We establish for a three-metal-layer pixel that diffraction prevents the microlens from acting as a focusing element when pixels become smaller than 1.4 microm. This severely degrades performance for on and off-axis pixels in red, green and blue color channels. We predict that one-metal-layer or backside-illuminated pixels are required to extend the functionality of microlenses beyond the 1.4 microm pixel node.

Improving characterization capabilities in new single-photon avalanche diode research.

Many novel and promising single-photon avalanche diodes (SPADs) emerged in recent years. However, some of them may demonstrate a very high dark count rate, even tens of megahertz, especially during the development phase or at room temperature, posing new challenges to device characterization. Gating operation with a width of 10 ns can be used to suppress the dark counts not coincident with the photon arriving time. However, as a side effect of the fast-gating operation, the gating response could be much higher than the avalanche signal and is usually removed by various circuit-based cancellation techniques. Here, we present an alternative method. A high-speed digital storage oscilloscope (DSO) is used to extract the weak avalanche signals from the large gating response background by waveform subtraction in software. Consequently, no complex circuit and precise tuning for each SPAD are needed. The avalanche detection threshold can be reduced to 5% of the full vertical scale of the DSO or 5 mV, whichever is greater. The timing resolution can be better than 2 ps for typical avalanche signals. Optical alignment and calibration are easy. The feasibility of on-wafer test with an RF probe station is discussed. All the advantages and features listed above make this method very useful in new SPAD research.

Optical confinement methods for continued scaling of CMOS image sensor pixels.

The pixels that make up CMOS image sensors have steadily decreased in size over the last decade. This scaling has two effects: first, the amount of light incident on each pixel decreases, making optical efficiency, i.e., the collection of each photon, more important. Second, diffraction comes into play when pixel size approaches the wavelength of visible light, resulting in increased spatial optical crosstalk. To address these two effects, we investigate and compare three methods for guiding incident light from the microlens down to the photodiode. Two of these techniques rely on total internal reflection (TIR) at the boundary between dielectric media of different refractive indices, while the third uses reflection at a metal-dielectric interface to confine the light. Simulations are performed using a finite-difference time-domain (FDTD) method on a realistic 1.75-mum pixel model for on-axis as well as angled incidence. We evaluate the optical efficiency and spatial crosstalk performance of these methods compared to a reference pixel and find significant (10%) improvement for the TIR designs with properly chosen parameters and nearly full spatial crosstalk elimination using metal to confine the light. We also show that these improvements are comparable to those achieved by thinning the image sensor stack.

Silicon single-photon avalanche diodes with nano-structured light trapping.

Silicon single-photon avalanche detectors are becoming increasingly significant in research and in practical applications due to their high signal-to-noise ratio, complementary metal oxide semiconductor compatibility, room temperature operation, and cost-effectiveness. However, there is a trade-off in current silicon single-photon avalanche detectors, especially in the near infrared regime. Thick-junction devices have decent photon detection efficiency but poor timing jitter, while thin-junction devices have good timing jitter but poor efficiency. Here, we demonstrate a light-trapping, thin-junction Si single-photon avalanche diode that breaks this trade-off, by diffracting the incident photons into the horizontal waveguide mode, thus significantly increasing the absorption length. The photon detection efficiency has a 2.5-fold improvement in the near infrared regime, while the timing jitter remains 25 ps. The result provides a practical and complementary metal oxide semiconductor compatible method to improve the performance of single-photon avalanche detectors, image sensor arrays, and silicon photomultipliers over a broad spectral range.The performance of silicon single-photon avalanche detectors is currently limited by the trade-off between photon detection efficiency and timing jitter. Here, the authors demonstrate how a CMOS-compatible, nanostructured, thin junction structure can make use of tailored light trapping to break this trade-off.

Solar water splitting by photovoltaic-electrolysis with a solar-to-hydrogen efficiency over 30.

Hydrogen production via electrochemical water splitting is a promising approach for storing solar energy. For this technology to be economically competitive, it is critical to develop water splitting systems with high solar-to-hydrogen (STH) efficiencies. Here we report a photovoltaic-electrolysis system with the highest STH efficiency for any water splitting technology to date, to the best of our knowledge. Our system consists of two polymer electrolyte membrane electrolysers in series with one InGaP/GaAs/GaInNAsSb triple-junction solar cell, which produces a large-enough voltage to drive both electrolysers with no additional energy input. The solar concentration is adjusted such that the maximum power point of the photovoltaic is well matched to the operating capacity of the electrolysers to optimize the system efficiency. The system achieves a 48-h average STH efficiency of 30%. These results demonstrate the potential of photovoltaic-electrolysis systems for cost-effective solar energy storage.

On-chip plasmonic waveguide optical waveplate.

Polarization manipulation is essential in almost every photonic system ranging from telecommunications to bio-sensing to quantum information. This is traditionally achieved using bulk waveplates. With the developing trend of photonic systems towards integration and miniaturization, the need for an on-chip waveguide type waveplate becomes extremely urgent. However, this is very challenging using conventional dielectric waveguides, which usually require complex 3D geometries to alter the waveguide symmetry and are also difficult to create an arbitrary optical axis. Recently, a waveguide waveplate was realized using femtosecond laser writing, but the device length is in millimeter range. Here, for the first time we propose and experimentally demonstrate an ultracompact, on-chip waveplate using an asymmetric hybrid plasmonic waveguide to create an arbitrary optical axis. The device is only in several microns length and produced in a flexible integratable IC compatible format, thus opening up the potential for integration into a broad range of systems.

Antenna electrodes for controlling electroluminescence.

Optical antennas can control the emission from quantum emitters by modifying the local density of optical states via the Purcell effect. A variety of nanometallic antennas have been implemented to enhance and control key photoluminescence properties, such as the decay rate, directionality and polarization. However, their implementation in active devices has been hampered by the need to precisely place emitters near an antenna and to efficiently excite them electrically. Here we illustrate a design methodology for antenna electrodes that for the first time facilitates simultaneous operation as electrodes for current injection and as antennas capable of optically manipulating the electroluminescence. We show that by confining the electrically excited carriers to the vicinity of antenna electrodes and maximizing the optical coupling of the emission to a single, well-defined antenna mode, their electroluminescence can be effectively controlled. This work spurs the development of densely integrated, electrically driven light sources with tailored emission properties.

Enhanced second-harmonic generation in AlGaAs/AlxOy tightly confining waveguides and resonant cavities.

We demonstrate second-harmonic generation (SHG) from sub-micrometer-sized AlGaAs/AlxOy artificially birefringent waveguides. The normalized conversion efficiency is the highest ever reported. We further enhanced the SHG using a waveguide-embedded cavity formed by dichroic mirrors. Resonant enhancements as high as approximately 10x were observed. Such devices could be potentially used as highly efficient, ultracompact frequency converters in integrated photonic circuits.

Dichroic mirror embedded in a submicrometer waveguide for enhanced resonant nonlinear optical devices.

We report the design, fabrication and characterization of novel dichroic mirrors embedded in a tightly confining AlGaAs/Al(x)O(y) waveguide. Reflection at the first-harmonic wavelength as high as 93% is achieved, while high transmission is maintained at the second-harmonic wavelength. The measured cavity spectrum is in excellent agreement with finite-difference time-domain simulations. Such a mirror is essential for achieving resonant enhancement of second-harmonic generation.