On-chip Light Sources Research Articles

Spatially Modulating the Fluorescence Color of Mixed-Halide Perovskite Nanoplatelets through Direct Femtosecond Laser Writing.

Lead halide perovskites are widely applied in not only photovoltaics but also on-chip light sources and photon detection. To promote the incorporation of perovskite into integrated devices, microscale color patterning flexibility is a very important step. Here, we demonstrate spatially resolved modulation of the fluorescence of nanoplatelets (NPs) by femtosecond direct laser writing (fs-DLW). As the perovskite NP for the fs-DLW pattern is specially designed with a gradual bromide-iodide composition along the depth, the replacement of iodide ions by bromide ions can be activated under a controlled laser pulse and fluorescence is thus modulated from red to green. The effect of processing depth and NP thickness on fluorescence modulation is systemically investigated. The as-grown thick NP (thickness ≈ 500 nm) mainly exhibits a 690 nm emission from the bottom iodine-rich phase. After halide substitution induced by fs-DLW, a new fluorescence peak appears in the wavelength range of 540-600 nm; the peak position and intensity are controlled by the DLW conditions. The fluorescent color is spatially modulated from red to green, enabling microscale-resolved multicolor emission. Compared with other currently available techniques, microscale color patterning via fs-DLW is a straightforward mask-free one-step operation, yielding high spatial resolution and enabling three-dimensional patterning by the multiple-photon method. We demonstrate that arbitrary patterns can be drawn on a wide range of perovskite NPs, implying the potential applications in microencryption, sensors, multicolor displays, lasers, and light-emitting devices.

Microcavity Lasers on Silicon by Template-Assisted Selective Epitaxy of Microsubstrates

The vision of microchips that benefits from the high data rates of optical links has recently lead to novel concepts for the monolithic integration of active gain materials on silicon. In this letter, we use template-assisted selective epitaxy (TASE) for the monolithic integration of GaAs microsubstrates on silicon, which allows the subsequent integration of lattice-matched III-V laser cavities. We demonstrate the room-temperature operation of optically pumped microdisk-lasers with hexagonal top-facets and diameters as low as 1.2 μm. The devices exhibit uniform emission spectra, low lasing thresholds of P <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> = 13.6 ± 0.7 pJ/pulse, and a high-temperature stability (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> = 370 K). Scanning electron microscope and photoluminescence analysis of larger devices with a flat top-facet reveal a rotation of the crystal orientation and further reduce lasing thresholds. Our results show that the monolithic integration of III-V microsubstrates on silicon offers a versatile platform for on-chip light sources with excellent optical properties.

Silicon nitride nanobeam enhanced emission from all-inorganic perovskite nanocrystals.

Optically active perovskite nanocrystals have shown considerable promise for a myriad of applications, such as single photon source, light-emitting diodes and nanophotonics. Coupling those nanocrystals to photonic micro- and nanostructures will offer additional degrees of freedom to manipulate their optical properties. Herein, we demonstrate the coupling of perovskite nanocrystals to a mechanically robust, poly(methyl-methacrylate) (PMMA)-encapsulated silicon nitride nanobeam photonic crystal cavity at room temperature. As determined from the time-resolved photoluminescence decay measurements, we observed enhanced spontaneous emission from the perovskite nanocrystals by a factor of 1.4, consistent with finite difference time domain simulation. In addition, by varying the concentration of the perovskite nanocrystal in the PMMA layer, the effective index of the layer can be modified, allowing us to tune the cavity mode resonance. Our results show that solution-processable perovskite nanocrystals hold a promising prospect for applications such as on-chip light sources, optoelectronic devices and photonic integrated circuits.

Heteroepitaxial Integration of Mid-Infrared InAsSb Light Emitting Diodes on Silicon

Silicon photonics has emerged as the most promising technology for next-generation compact optoelectronic systems, but further development is still required to achieve efficient and reliable on-chip light sources. Direct epitaxial growth of antimonide-based compound semiconductor materials on silicon provides a pathway toward the monolithic integration of new, mid-infrared solid-state light sources and comprehensive photonic circuits on silicon platforms. Such devices have wide-ranging applications in environmental monitoring and medical diagnostics. This paper reports on the realization of a mid-infrared InAsSb light emitting diode directly integrated onto silicon using molecular beam epitaxy. The heteroepitaxial integration of the InAsSb p-i-n device onto silicon was achieved with the use of a novel, antiphase domain-free, GaSb-on-silicon buffer layer. The device exhibited efficient light emission at room temperature, peaking at around 4.5 μm, which corresponds well to the CO 2 atmospheric absorption band. An output power of 6 μW and an external quantum efficiency of 0.011% was measured at 300 K. These results demonstrate mid-infrared III-V light emitting diodes can be directly grown on silicon, which is an essential step towards the realization of the next generation, on-chip integrated light sources.

Light radiation from surface plasmon polaritons in a structure of nanometal film on a subwavelength dielectric grating

A nanoscale light radiation source based on the diffraction radiation mechanism is proposed. In this mechanism, tunable and highly directional light emission from visible to ultraviolet frequency regime can be generated from surface plasmon polaritons (SPPs) excited by a moving electron beam above a nanometal film embedded on a subwavelength dielectric grating. The light emits in both the free-space and the dielectric substrate in specific directions. The radiation frequencies can be widely tuned by adjusting the beam energy or structure parameters. A remarkable enhancement (up to three orders of magnitude) of the radiation power density is found. The origin of the enhancement is the strong local field. Furthermore, an interesting physical phenomenon of radiation interference in the dielectric substrate is found. The physical origin of the interference is uncovered and discussed. With advances of miniature size, compatible on-chip integration, high power density, well directionality and wavelength tunability, our result has potential applications in electronic-photonic and nanophotonic systems as on-chip light sources, or in electron-emitter displays building into large-area array.

Electrically Driven Highly Tunable Cavity Plasmons

Electrically driven ultrafast plasmon sources with narrow line width and wide-range wavelength tunability are desirable choices for integrated nanophotonic circuits. These have a wide range of applications from the optical communication to the data processing. Here, we demonstrate a compact metal–insulator–metal tunneling junction as a plasmon source to meet these requirements simultaneously. It is consisted by a Ag nanowire (covered by a monolayer thiol molecular) cross-placed on a Au nanostripe. Cavity plasmons are excited by inelastic tunneling electrons as applying a bias voltage. The electroluminescence spectrum shows multiple peaks with line widths as narrow as tens of nanometers due to the excitation of third to fifth order cavity plasmon modes. The linearly tunable range of the third order cavity plasmon exceeds 200 nm by varying the diameter of the Ag nanowire ∼70 nm. Our work can be further developed for the multichannels and on-chip photonic light sources.

Perovskite Lasers: Highly Controllable Lasing Actions in Lead Halide Perovskite–Si3N4 Hybrid Micro‐Resonators (Laser Photonics Rev. 13(3)/2019)

In article number 1800189, Hongbing Fu, Shumin Xiao, Qinghai Song, and co-workers demonstrate high control over the lasing actions for perovskite nanosheets on a Si3N4 substrate. The cover highlights the laser actions of perovskite nanosheets covered on a Si3N4 disk in the central part. Combined with the mature CMOS technique, this demonstration shows the potential for perovskite nanosheets/Si3N4 disk configuration as on-chip light source. A technical path to guide the laser emission is shown, consisting of S3N4 waveguides and hybrid circle disks.

Efficient Background-Noise-Reduced Single-Mode Waveguides Platform for On-Chip Integrated Photonics

In this paper, we propose a modified platform for on-chip integrated photonics. The platform allows one to efficiently connect a tapered optical fiber or fiber lens to an on-chip waveguide with high alignment precision and to mechanically fix the fiber on the chip to prevent defocusing, lateral shift, and angular detuning during device operation. The metal screen of the connector-made of gold-prevents any penetration of the background radiation into the optical circuit, something which is not possible to achieve in all the cases using tapers or grating couplers. We performed impedance-matching analysis based on transmission line theory and numerically optimized the platform, allowing us to achieve a tapered fiber mode to on-chip waveguide mode transformation efficiency as high as 96% (-0.177 dB) at wavelengths of 785 and 1550 nm. The platform bandwidth (transformation efficiency ≥80%) is 151 nm. At the same time, transformation efficiency is stable (reduction less than 1%) in an offset range of ±2 nm-enough for modern on-chip and off-chip coherent light sources. The horizontal and vertical lateral alignment tolerances for the platform are 700 and 600 nm, respectively, at a transformation efficiency ≥80% while the longitudinal alignment tolerance is approximately 70 nm. Due to the relatively high lateral and longitudinal tolerances, it is possible to produce the platform at high efficiency using existing fabrication nanotechnologies. We believe that this platform accelerates the realization of fully on-chip Raman spectroscopes and may be of practical importance in other on-chip integrated photonic devices.

Ultra-high on-chip optical gain in erbium-based hybrid slot waveguides

Efficient and reliable on-chip optical amplifiers and light sources would enable versatile integration of various active functionalities on the silicon platform. Although lasing on silicon has been demonstrated with semiconductors by using methods such as wafer bonding or molecular beam epitaxy, cost-effective mass production methods for CMOS-compatible active devices are still lacking. Here, we report ultra-high on-chip optical gain in erbium-based hybrid slot waveguides with a monolithic, CMOS-compatible and scalable atomic-layer deposition process. The unique layer-by-layer nature of atomic-layer deposition enables atomic scale engineering of the gain layer properties and straightforward integration with silicon integrated waveguides. We demonstrate up to 20.1 ± 7.31 dB/cm and at least 52.4 ± 13.8 dB/cm net modal and material gain per unit length, respectively, the highest performance achieved from erbium-based planar waveguides integrated on silicon. Our results show significant advances towards efficient on-chip amplification, opening a route to large-scale integration of various active functionalities on silicon.

Enhanced Second Harmonic Generation from Ferroelectric HfO2-Based Hybrid Metasurfaces.

Integrated nonlinear metasurfaces leading to high-efficiency optical second harmonic generation (SHG) are highly desirable for optical sensing, imaging, and quantum photonic systems. Compared to traditional metal-only metasurfaces, their hybrid counterparts, where a noncentrosymmetric nonlinear photonic material is incorporated in the near-field of a metasurface, can significantly boost SHG efficiency. However, it is difficult to integrate such devices on-chip due to material incompatibilities, thickness scaling challenges, and the narrow band gaps of nonlinear optical materials. Here, we demonstrate significantly enhanced SHG in on-chip integrated metasurfaces by using nanometer thin films of ferroelectric Y:HfO2. This material has the merit of CMOS compatibility, ultraviolet transparency up to 250 nm, and significant scalability down to sub-10 nm when deposited on silicon. We observe a 20-fold magnitude enhancement of the SHG intensity from the hybrid metasurface compared to a bare ferroelectric HfO2 thin film. Moreover, a 3-fold SHG enhancement is observed from the hybrid metasurface compared to a control structure using nonferroelectric HfO2, demonstrating a major contribution to the SHG signal from ferroelectric Y:HfO2. The effective second-order nonlinear optical coefficient χ(2) of Y:HfO2 is determined to be 6.0 ± 0.5 pm/V, which is comparable to other complex nonlinear photonic oxide materials. Our work provides a general pathway to build an efficient on-chip nanophotonic nonlinear light source for SHG using ferroelectric HfO2 thin films.

Shrinking synchrotron generation

Optics The generation of synchrotron radiation is typically achieved by accelerating charges in large magnetic fields. Synchrotron facilities are usually the realm of large international organizations. Henstridge et al. show that the interaction of a femtosecond light pulse moving in an arc on a specially designed metasurface can also generate synchrotron radiation. In this case, the synchrotron radiation at terahertz frequencies was produced by the nonlinear polarization induced by the light pulse. The results hold promise for the development of powerful on-chip light sources. Science , this issue p. [439][1] [1]: /lookup/doi/10.1126/science.aat5915

Room temperature continuous-wave nanolaser diode utilized by ultrahigh-Q few-cell photonic crystal nanocavities

Few-cell point-defect photonic crystal (PhC) nanocavities (such as LX and H1 type cavities), have several unique characteristics including an ultra-small mode volume (Vm), a small device footprint advantageous for dense integration, and a large mode spacing advantageous for high spontaneous-emission coupling coefficient (β), which are promising for energy-efficient densely-integratable on-chip laser light sources enhanced by the cavity QED effect. To achieve this goal, a high quality factor (Q) is essential, but conventional few-cell point-defect cavities do not have a sufficiently high Q. Here we adopt a series of modified designs of LX cavities with a buried heterostructure (BH) multi-quantum-well (MQW) active region that can achieve a high Q while maintaining their original advantages and fabricate current-injection laser devices. We have successfully observed continuous-wave (CW) lasing in InP-based L1, L2, L3 and L5 PhC nanocavities at 23°C with a DC current injection lower than 10 μA and a bias voltage lower than 0.9 V. The active volume is ultra-small while maintaining a sufficiently high confinement factor, which is as low as ~10-15 cm3 for a single-cell (L1) nanocavity. This is the first room-temperature current-injection CW lasing from any types of few-cell point-defect PhC nanocavities (LX or H1 types). Our report marks an important step towards realizing a nanolaser diode with a high cavity-QED effect, which is promising for use with on-chip densely integrated laser sources in photonic networks-on-chip combined with CMOS processors.

Core-shell p-i-n GaN nanowire LEDs by N-polar selective area growth.

GaN nanowire LEDs with radial p-i-n junctions were grown by molecular beam epitaxy using N-polar selective area growth on Si(111) substrates. The N-polar selective area growth process facilitated the growth of isolated and high-aspect-ratio n-type NW cores that were not subject to self-shadowing effects during the subsequent growth of a conformal low-temperature Mg:GaN shell. LED devices were fabricated from single-NW and multiple-NW arrays in their as-grown configuration by contacting the n-type core through an underlying conductive GaN layer and the p-type NW shell via a metallization layer. The NW LEDs exhibited rectifying I-V characteristics with a sharp turn-on voltage near the GaN bandgap and low reverse bias leakage current. Under forward bias, the NW LEDs produced electroluminescence with a peak emission wavelength near 380 nm and exhibited a small spectral blueshift with increasing current injection, both of which are consistent with electron recombination in the p-type shell layer through donor-acceptor-pair recombination. These core-shell NW devices demonstrate N-polar selective area growth as an effective technique for producing on-chip nanoscale light sources.

Tuning Lasing Emission toward Long Wavelengths in GaAs-(In,Al)GaAs Core–Multishell Nanowires

Semiconductor nanowire (NW) lasers are attractive as integrated on-chip coherent light sources with strong potential for applications in optical communication and sensing. Realizing lasers from individual bulk-type NWs with emission tunable from the near-infrared to the telecommunications spectral region is, however, challenging and requires low-dimensional active gain regions with an adjustable band gap and quantum confinement. Here, we demonstrate lasing from GaAs-(InGaAs/AlGaAs) core-shell NWs with multiple InGaAs quantum wells (QW) and lasing wavelengths tunable from ∼0.8 to ∼1.1 μm. Our investigation emphasizes particularly the critical interplay between QW design, growth kinetics, and the control of InGaAs composition in the active region needed for effective tuning of the lasing wavelength. A low shell growth temperature and GaAs interlayers at the QW/barrier interfaces enable In molar fractions up to ∼25% without plastic strain relaxation or alloy intermixing in the QWs. Correlated scanning transmission electron microscopy, atom probe tomography, and confocal PL spectroscopy analyses illustrate the high sensitivity of the optically pumped lasing characteristics on microscopic properties, providing useful guidelines for other III-V-based NW laser systems.

Coupling Single Photons from Discrete Quantum Emitters in WSe2 to Lithographically Defined Plasmonic Slot Waveguides

We report the observation of the generation and routing of single plasmons generated by localized excitons in a WSe2 monolayer flake exfoliated onto lithographically defined Au-plasmonic waveguides. Statistical analysis of the position of different quantum emitters shows that they are (3.3 ± 0.7) times more likely to form close to the edges of the plasmonic waveguides. By characterizing individual emitters, we confirm their single-photon character via the observation of antibunching in the signal ( g(2)(0) = 0.42) and demonstrate that specific emitters couple to modes of the proximal plasmonic waveguide. Time-resolved measurements performed on emitters close to and far away from the plasmonic nanostructures indicate that Purcell factors up to 15 ± 3 occur, depending on the precise location of the quantum emitter relative to the tightly confined plasmonic mode. Measurement of the point spread function of five quantum emitters relative to the waveguide with <50 nm precision is compared with numerical simulations to demonstrate the potential for greater increases in the coupling efficiency for ideally positioned emitters. The integration of such strain-induced quantum emitters with deterministic plasmonic routing is a step toward deep-subwavelength on-chip single quantum light sources.

Photoluminescence evolution in WS2 via optical irradiation and substrate interactions

Transition metal dichalcogenides, such as monolayer MoS2 and WS2, can be easily transferred onto or integrated with various substrates, and are therefore promising on-chip light sources. Although are direct-bandgap materials with more efficient optical gain compared to indirect band-gap materials such as silicon, they are more sensitive to the surrounding environment and substrate materials than bulk direct-bandgap semiconductors. Herein, the photoluminescence intensity of WS2 transferred onto silicon nitride is demonstrated to be tunable and substantially enhanced by violet laser excitation, owing to irradiation-induced chemical doping with atmospheric moisture and oxygen, which lead to changes in the densities of neutral excitons and trions; this process is reversible in vacuum conditions. However, with continuous excitation, WS2 with trapped molecules is more easily damaged by laser irradiation than bulk semiconductors. Upon coating with Al2O3, the stability of WS2 under laser irradiation is enhanced. We show that the isolation of WS2 from air can extend the lifetime of transition metal dichalcogenides on chip-communication devices. The results of this study indicate that further optimization is required to realize practical on-chip applications of these monolayer materials as light sources.

MOCVD Growth of InGaAs/GaAs/AlGaAs Laser Structures with Quantum Wells on Ge/Si Substrates

The paper presents the results of the application of MOCVD growth technique for formation of the GaAs/AlAs laser structures with InGaAs quantum wells on Si substrates with a relaxed Ge buffer. The fabricated laser diodes were of micro-striped type designed for the operation under the electrical pumping. Influence of the Si substrate offcut from the [001] direction, thickness of a Ge buffer and insertion of the AlAs/GaAs superlattice between Ge and GaAs on the structural and optical properties of fabricated samples was studied. The measured threshold current densities at room temperatures were 5.5 kA/cm2 and 20 kA/cm2 for lasers operating at 0.99 μm and 1.11 μm respectively. In order to obtain the stimulated emission at wavelengths longer than 1.1 μm, the InGaAs quantum well laser structures with high In content and GaAsP strain-compensating layers were grown both on Ge/Si and GaAs substrates. Structures grown on GaAs exhibited stimulated emission under optical pumping at the wavelengths of up to 1.24 μm at 300 K while those grown on Ge/Si substrates emitted at shorter wavelengths of up to 1.1 μm and only at 77 K. The main reasons for such performance worsening and also some approaches to overcome them are discussed. The obtained results have shown that monolithic integration of direct-gap A3B5 compounds on Si using MOCVD technology is rather promising approach for obtaining the Si-compatible on-chip effective light source.

Super-Radiant Emission from Quantum Dots in a Nanophotonic Waveguide

Future scalable photonic quantum information processing relies on the ability of integrating multiple interacting quantum emitters into a single chip. Quantum dots provide ideal on-chip quantum light sources. However, achieving quantum interaction between multiple quantum dots on-a-chip is a challenging task due to the randomness in their frequency and position, requiring local tuning technique and long-range quantum interaction. Here, we demonstrate quantum interactions between separated two quantum dots on a nanophotonic waveguide. We achieve a photon-mediated long-range interaction by integrating the quantum dots to the same optical mode of a nanophotonic waveguide and overcome spectral mismatch by incorporating on-chip thermal tuners. We observe their quantum interactions of the form of super-radiant emission, where the two dots collectively emit faster than each dot individually. Creating super-radiant emission from integrated quantum emitters could enable compact chip-integrated photonic structures that exhibit long-range quantum interactions. Therefore, these results represent a major step toward establishing photonic quantum information processors composed of multiple interacting quantum emitters on a semiconductor chip.

Quantum photonics model for nonclassical light generation using integrated nanoplasmonic cavity-emitter systems

The implementation of non-classical light sources is becoming increasingly important for various quantum applications. A particularly interesting approach is to integrate such functionalities on a single chip as this could pave the way towards fully scalable quantum photonic devices. Several approaches using dielectric systems have been investigated in the past. However, it is still not understood how on-chip nanoplasmonic antennas, interacting with a single quantum emitter, affect the quantum statistics of photons reflected or transmitted in the guided mode of a waveguide. Here we investigate a quantum photonic platform consisting of an evanescently coupled nanoplasmonic cavity-emitter system and discuss the requirements for non-classical light generation. We develop an analytical model that incorporates quenching due to the nanoplasmonic cavity to predict the quantum statistics of the transmitted and reflected guided waveguide light under weak coherent pumping. The analytical predictions match numerical simulations based on a master equation approach. It is moreover shown that for resonant excitation the degree of anti-bunching in transmission is maximized for an optimal cavity modal volume $V_{c}$ and cavity-emitter distance $s$. In reflection, perfectly anti-bunched light can only be obtained for specific $(V_{c},s)$ combinations. Finally, our model also applies to dielectric cavities and as such can guide future efforts in the design and development of on-chip non-classical light sources using dielectric and nanoplasmonic cavity-emitter systems.

Direct modulation of a single InP/InAs nanowire light-emitting diode

Telecom-band single nanowire (NW) light-emitting diodes (LEDs) on silicon platforms are promising for use as future on-chip light sources. To realize such an LED, we grew p-i-n junction InP/InAs NWs by the vapor-liquid-solid method and used them to fabricate single NW-LED composites on Si wafers. Using a micro-photoluminescence system and a current injection probe setup, we were able to estimate the optical and electrical properties. In addition, we demonstrated the dynamic signal modulation capabilities of our device through a digital eye diagram with a pseudorandom binary sequence signal. This telecom-band single NW-LED employing Gbps signal modulation has the potential for use as an on-chip light source.