Practical Light Sources Research Articles

High power lateral coupled InAs/GaAs quantum dot distributed feedback lasers grown on Si(001) substrates

High-quality single-frequency semiconductor lasers play a key role in silicon optical integrated systems. Combining high density 8-stacked quantum dot (QD) material and low–loss laterally coupled gratings, we here demonstrate a high output power, low noise, and insensitivity to light feedback 1.3 µm InAs/GaAs QD distributed feedback (DFB) laser grown on Si(001) substrates. For a QD DFB laser of a 3 × 1500 µm2 cavity, it exhibits a high single-mode output light power of up to 25 mW at 20 °C and 1.8 mW at 70 °C, respectively and maintains a stable single–mode operation in the entirely measured temperature range with a maximum side mode suppression ratio (SMSR) of 56.5 dB. Furthermore, the laser has an average relative intensity noise value low to –155.9 dB/Hz and a Lorentzian linewidth narrow to 243 kHz. In addition, the laser shows an insensitivity to optical feedback with a feedback level of –24.9 dB. Lastly, a 7-channel QD DFB laser array emitting at wavelengths from 1274.5 nm to 1290.0 nm are also exhibited with all SMSRs of higher than 45 dB. The results achieved here enable a practical single-frequency Si-based light source for the development of high-performance silicon photonic chips.

Octave spanning and flat-top supercontinuum generation in standard foundry-compatible process-made Ge–Sb–Se chalcogenide glass waveguide

We reported near octave-spanning supercontinuum generation in Ge28Sb12Se60 chalcogenide glass rib waveguides. The waveguides were fabricated using foundry-compatible deep ultra-violet lithography, followed by plasma etching. We demonstrated an average propagation loss of ∼1.3 dB/cm at the 1550 nm telecommunication wavelength. With dispersion engineering, optimizing waveguide geometry, and pumping by a multi-wavelength femtosecond soliton fiber laser, we achieved a flat-top supercontinuum generation spanning from 1290 to 2400 nm and beyond . It has a 3 dB bandwidth of 436 nm and a 20 dB bandwidth of more than 900 nm. The implementation of such waveguides provides a practical broadband light source solution for on-chip spectroscopy and sensing systems.

Homodyne-detection-based discrete-variable quantum key distribution with a heralded single-photon source

Discrete-variable quantum key distribution (DV-QKD) has recently been implemented using a homodyne detection system, and a notable secret key rate can be achieved by employing an ideal single-photon source. However, most QKD implementations employ practical light sources, including a phase-randomized weak coherent source and a heralded single-photon source, which occasionally produce multiphotons and are vulnerable to photon-number-splitting (PNS) attacks. In this work, we propose a three-decoy-state method using a heralded single-photon source for homodyne-detection-based DV-QKD, thus making it immune to PNS attacks with current technology. Our simulation results demonstrate that our proposed protocol can achieve high-speed and secure key distribution over metropolitan distances. Our work paves a cost-effective path to realize DV-QKD and further incorporate it into classical telecommunication networks.

Frequency-tunable microwave quantum light source based on superconducting quantum circuits

A non-classical light source is essential for implementing a wide range of quantum information processing protocols, including quantum computing, networking, communication and metrology. In the microwave regime, propagating photonic qubits, which transfer quantum information between multiple superconducting quantum chips, serve as building blocks for large-scale quantum computers. In this context, spectral control of propagating single photons is crucial for interfacing different quantum nodes with varied frequencies and bandwidths. Here a deterministic microwave quantum light source was demonstrated based on superconducting quantum circuits that can generate propagating single photons, time-bin encoded photonic qubits and qudits. In particular, the frequency of the emitted photons can be tuned in situ as large as 200 MHz. Even though the internal quantum efficiency of the light source is sensitive to the working frequency, it is shown that the fidelity of the propagating photonic qubit can be well preserved with the time-bin encoding scheme. This work thus demonstrates a versatile approach to realizing a practical quantum light source for future distributed quantum computing.

Resonant inelastic tunneling using multiple metallic quantum wells.

Tunnel nanojunctions based on inelastic electron tunneling (IET) have been heralded as a breakthrough for ultra-fast integrated light sources. However, the majority of electrons tend to tunnel through a junction elastically, resulting in weak photon-emission power and limited efficiency, which have hindered their practical applications to date. Resonant tunneling has been proposed as a way to alleviate this limitation, but photon-emissions under resonant tunneling conditions have remained unsatisfactory for practical IET-based light sources due to the inherent contradiction between high photon-emission efficiency and power. In this work, we introduce a novel approach that leverages much stronger resonant tunneling enhancement achieved by multiple metallic quantum wells, which has enabled the internal quantum efficiency to reach ∼1 and photon-emission power to reach ∼0.8 µW/µm2. Furthermore, this method is applicable with different electronic lifetimes ranging from 10 fs to 100 fs simultaneously, bringing practical implementation of IET-based sources one step closer to reality.

(Digital Presentation) Gesnoi Laser Technology for Photonic-Integrated Circuits

GeSn alloys have been regarded as a promising material for creating a complementary metal-oxide-semiconductor (CMOS)-compatible light source. Despite the remarkable progress in demonstrating GeSn lasers, an unavoidable intrinsic compressive strain introduced during epitaxial growth has prevented researchers from pushing the directness of GeSn gain media to the limit and realizing practical GeSn lasers. In this paper, we demonstrate a GeSn-based 1D photonic crystal nanobeam laser on a high-quality GeSn-on-insulator (GeSnOI) substrate which allows releasing the limiting compressive strain, thus improving the threshold and operating temperature. Pump-power-dependent photoluminescence measurements show a lasing threshold density of 18.2 kW cm−2 at 4 K for the released strain-free GeSn nanobeam, which is ~2 times lower than that of the unreleased GeSn nanobeam with compressive strain. The improved bandgap directness in the released GeSn nanobeam also allows achieving lasing action at higher operating temperatures up to 90 K compared to the unreleased laser device (<70 K). We also report a straightforward geometric strain-inversion technique that harnesses the harmful compressive strain to achieve ultrahigh tensile strain in GeSnOI nanowire, drastically improving the directness of the bandstructure. We achieve ~2.67% uniaxial tensile strain in ~120 nm wide nanowires, surpassing other values reported thus far. We also demonstrate unique superlattices comprising of indirect and direct bandgap GeSn are demonstrated in a single material only by applying a periodic tensile strain. Increased directness in tensile-strained GeSn significantly enhances the photoluminescence intensity by a factor of ~2.5. Our demonstration offers an avenue toward developing practical CMOS compatible light sources.

Near-infrared luminescence of erbium doped Ga2O3 films and devices based on silicon: Realization of energy transfer

1.54 μm photoluminescence and electroluminescence are achieved from erbium doped Ga2O3 films on silicon substrates and corresponding devices fabricated by sputtering, respectively. Efficient energy transfer between Ga2O3 and erbium ions is realized in optical pumping, with the energy transfer efficiency of 54.9%. The ∼1.54 μm electroluminescence of erbium ions here is proved to originate from indirect excitation, different from the conventional impact excitation by hot electrons. Herein, the light emitting devices present a turn-on voltage of ∼13 V and onset electric field of 1.4 MV/cm. This work demonstrates the potential of preparing practical electrical-driven silicon-based light sources from erbium doped Ga2O3 films.

1D photonic crystal direct bandgap GeSn-on-insulator laser

GeSn alloys have been regarded as a potential lasing material for a complementary metal–oxide–semiconductor-compatible light source. Despite their remarkable progress, all GeSn lasers reported to date have large device footprints and active areas, which prevent the realization of densely integrated on-chip lasers operating at low power consumption. Here, we present a 1D photonic crystal nanobeam with a very small device footprint of 7 μm2 and a compact active area of ∼1.2 μm2 on a high-quality GeSn-on-insulator substrate. We also report that the improved directness in our strain-free nanobeam lasers leads to a lower threshold density and a higher operating temperature compared to the compressive strained counterparts. The threshold density of the strain-free nanobeam laser is ∼18.2 kW cm−2 at 4 K, which is significantly lower than that of the unreleased nanobeam laser (∼38.4 kW cm−2 at 4 K). Lasing in the strain-free nanobeam device persists up to 90 K, whereas the unreleased nanobeam shows quenching of lasing at a temperature of 70 K. Our demonstration offers an avenue toward developing practical group-IV light sources with high-density integration and low power consumption.

Fiber-pigtailing quantum-dot cavity-enhanced light emitting diodes

We report on a process for the fiber-coupling of electrically driven cavity-enhanced quantum dot light emitting devices. The developed technique allows for the direct and permanent coupling of p-i-n-doped quantum dot micropillar cavities to single-mode optical fibers. The coupling process, fully carried out at room temperature, involves a spatial scanning technique, where the fiber facet is positioned relative to a device with a diameter of 2 μm using the fiber-coupled electroluminescence of the cavity emission as a feedback parameter. Subsequent gluing and UV curing enable a rigid and permanent coupling between micropillar and fiber core. Comparing our experimental results with finite element method simulations indicates a cavity-to-fiber mode-coupling efficiency of ∼46%. Furthermore, we demonstrate pulsed current injection at a repetition rate exceeding 200 MHz as well as low-temperature operation down to 77 K of the fiber-coupled micropillar device. The technique presented in this work is an important step in the quest for efficient and practical quantum light sources for applications in quantum information.

Room‐Temperature Chiral Light‐Emitting Diode Based on Strained Monolayer Semiconductors (Adv. Mater. 36/2021)

Light-Emitting Diodes A room-temperature electrically tunable chiral light-emitting diode is realized by Jiang Pu, Taishi Takenobu, and co-workers in article number 2100601, using strained monolayer transition-metal dichalcogenides. The strain effects break the three-fold rotational symmetry, which results in different amounts of spin recombination at the K/K' valleys to generate chiral light emission driven by electric fields. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.

Room-Temperature Chiral Light-Emitting Diode Based on Strained Monolayer Semiconductors.

Room-temperature chiral light sources whose optical helicity can be electrically switched are one of the most important devices for future optical quantum information processing. The emerging valley degree of freedom in monolayer semiconductors allows generation of chiral luminescence via valley polarization. However, relevant valley-polarized light-emitting diodes (LEDs) have only been achieved at low temperatures (typically below 80 K). Here, a room-temperature chiral LED with strained transition metal dichalcogenide monolayers is realized. Spatially resolved polarization spectroscopy reveals that strain effects are crucial to yielding robust valley-polarized electroluminescence. The broken threefold rotational symmetry of strained monolayers induce inequivalent valley drifts at the K/K' valleys, resulting in different amounts of spin recombination driven by electric fields. Based on this scenario, ideally strained conditions are designed for LEDs on flexible substrates, in which the helicity of room-temperature valley-polarized electroluminescence is electrically tuned. The results provide a new pathway for practical chiral light sources based on monolayer semiconductors.

A practical-use hydrogen gas leak detector using CARS

The purpose of this research is to develop a non-contact, fast response, practical-use hydrogen gas leak detector using Coherent Anti-Stokes Raman Spectroscopy (CARS). To make it realize, the optimization of its practical light source was investigated minutely for this detector. The light source condition to generate the highest Anti-Stokes Raman light is obvious on the theory, while it is not matched with its practical condition under the physical constraints. In this study, the light source configuration was devised in which the laser beam (355 nm) is split into two optical paths and the Raman cell filled hydrogen gas is arranged on the one side. As a result, it was found that when its irradiation ratio (R = Stokes light/Pump light) was 0.140 ≤ R ≤ 0.173, the generation efficiency of the anti-Stokes light was high. The high generation efficiency of anti-Stokes light can be maintained even if the laser light intensity and hydrogen pressure to the Raman cell are changed. We discussed the difference between the obtained experimental results and the theoretical results. The detection limit of hydrogen gas concentration of our detector was set to 500 ppm. As a result, hydrogen gas up to 200 ppm ± 15% (lower detection limit: 157 ppm) could be measured. In addition, the measurement value of the hydrogen gas leaking from a nozzle was considered under the acceptance situation, too.

Highly efficient and stable electroluminescence from Er-doped Ga2O3 nanofilms fabricated by atomic layer deposition on silicon

Intense 1.53 μm electroluminescence (EL) is achieved from metal-oxide-semiconductor light-emitting devices based on Er-doped Ga2O3 (Ga2O3:Er) nanofilms fabricated by atomic layer deposition. Due to the wide bandgap and outstanding tolerance to electric field and electron injection of the amorphous Ga2O3 matrix, these silicon-based devices present a low turn-on voltage of ∼15 V, while the maximum injection current can reach 5 A/cm2. The optical power density of the EL emissions is improved to 23.73 mW/cm2, with the external quantum efficiency of 36.5% and power efficiency of 0.81%. The prototype devices show good stability and retain ∼90% initial EL intensity after operating consistently for 100 h. The EL originates from the impact excitation of doped Er3+ ions by hot electrons generated within dielectric layers. This work manifests the potential of fabricating practical Si-based light source from Ga2O3:Er nanofilms, enabling various optoelectronic applications.

All-fiber ultrafast laser generating gigahertz-rate pulses based on a hybrid plasmonic microfiber resonator

Ultrafast lasers generating high-repetition-rate ultrashort pulses through various mode-locking methods can benefit many important applications, including communications, materials processing, astronomical observation, etc. For decades, mode-locking based on dissipative four-wave-mixing (DFWM) has been fundamental in producing pulses with repetition rates on the order of gigahertz (GHz), where multiwavelength comb filters and long nonlinear components are elemental. Recently, this method has been improved using filter-driven DFWM, which exploits both the filtering and nonlinear features of silica microring resonators. However, the fabrication complexity and coupling loss between waveguides and fibers are problematic. We demonstrate a tens- to hundreds- of gigahertz-stable pulsed all-fiber laser based on a hybrid plasmonic microfiber knot resonator device. Unlike previously reported pulse generation mechanisms, the operation utilizes the nonlinear-polarization-rotation (NPR) effect introduced by the polarization-dependent feature of the device to increase intracavity power for boosting DFWM mode-locking, which we term NPR-stimulated DFWM. The easily fabricated versatile device acts as a polarizer, comb filter, and nonlinear component simultaneously, thereby introducing an application of microfiber resonator devices in ultrafast and nonlinear photonics. We believe that our work underpins a significant improvement in achieving practical low-cost ultrafast light sources.

Optimized designs for telecom-wavelength quantum light sources based on hybrid circular Bragg gratings.

We present a design study of quantum light sources based on hybrid circular Bragg gratings (CBGs) for emission wavelengths in the telecom O-band. The evaluated CBG designs show photon extraction efficiencies > 95% and Purcell factors close to 30. Using simulations based on the finite element method, and considering the influence of possible fabrication imperfections, we identify optimized high-performance CBG designs which are robust against structural aberrations. In particular, full 3D simulations reveal that the designs show robustness regarding lateral deviations of the emitter position in the device well within reported positioning accuracies of deterministic fabrication technologies. Furthermore, we investigate the coupling of the evaluated hybrid CBG designs to single-mode optical fibers, which is particularly interesting for the development of practical quantum light sources. We obtain coupling efficiencies of up to 77% for off-the-shelf fibers, and again proof robustness against fabrication imperfections. Our results show prospects for the fabrication of close-to-ideal fiber-coupled quantum light sources for long distance quantum communication.

Quantum random number generation with uncharacterized laser and sunlight

The entropy or randomness source is an essential ingredient in random number generation. Quantum random number generators generally require well modeled and calibrated light sources, such as a laser, to generate randomness. With uncharacterized light sources, such as sunlight or an uncharacterized laser, genuine randomness is practically hard to be quantified or extracted owing to its unknown or complicated structure. By exploiting a recently proposed source-independent randomness generation protocol, we theoretically modify it by considering practical issues and experimentally realize the modified scheme with an uncharacterized laser and a sunlight source. The extracted randomness is guaranteed to be secure independent of its source and the randomness generation speed reaches 1 Mbps, three orders of magnitude higher than the original realization. Our result signifies the power of quantum technology in randomness generation and paves the way to high-speed semi-self-testing quantum random number generators with practical light sources.

Rigorous characterization method for photon-number statistics.

Characterization of photon statistics of a light source is one of the most basic tools in quantum optics. Existing methods rely on an implicit and unverifiable assumption that the source never emits too many photons to stay within the measuring range of the detectors. As a result, they fail to fulfill the demand arising from emerging applications of quantum information such as quantum cryptography. Here, we propose a characterization method using a conventional Hanbury-Brown-Twiss setup to produce rigorous bounds on emission probabilities of low photon numbers from an unknown source. As an application, we show that our characterization method can be used for a practical light source in a quantum key distribution protocol to forsake the commonly used a priori assumption without significant change in efficiency. Our versatile and flexible formula for rigorous bounds will make an essential contribution to the optics toolbox in the era of quantum information.

1.5 μm quantum-dot diode lasers directly grown on CMOS-standard (001) silicon

Electrically pumped on-chip C-band lasers provide additional flexibility for silicon photonics in the design of optoelectronic circuits. III–V quantum dots, benefiting from their superior optical properties and enhanced tolerance to defects, have become the active medium of choice for practical light sources monolithically grown on Si. To fully explore the potentials of integrated lasers for silicon photonics in telecommunications and datacenters, we report the realization of 1.5 μm room-temperature electrically pumped III–V quantum dot lasers epitaxially grown on complementary metal-oxide-semiconductor (CMOS)-standard (001) Si substrates without offcut. A threshold current density of 1.6 kA/cm2, a total output power exceeding 110 mW, and operation up to 80 °C under pulsed current injection have been achieved. These results arose from applying our well-developed InAs/InAlGaAs/InP QDs on low-defect-density InP-on-Si templates utilizing nano-patterned V-grooved (001) Si and InGaAs/InP dislocation filters. This demonstration marks a major advancement for future monolithic photonic integration on a large-area and cost-effective Si platform.

Andreev Reflection in a Superconducting Light-Emitting Diode.

We experimentally demonstrate Cooper-pair injection into a superconducting light-emitting diode by observing Andreev reflection at the superconductor-semiconductor interface, overcoming the contradicting requirements of an electrically transparent interface and radiative recombination efficiency. The device exhibits electroluminescence enhancement at the quasi-Fermi energy at temperatures below Tc. The theoretically predicted conductance and electroluminescence spectra based on Cooper-pair injection into the semiconductor correspond well to our experimental results. Our findings pave the way for practical superconductor-semiconductor quantum light sources.

(Invited) Strain-Engineered Low-Threshold Group IV Lasers for Photonic-Integrated Circuits

Photonic-integrated circuits (PICs) are a key enabler for the ultimate miniaturization of next-generation optical computers that can fit in the palm of your hand. However, the realization of fully functional PICs is currently limited by the absence of an efficient light source on silicon (Si), thus making a practical Si-compatible light source the Holy Grail. Especially, it has been thus far considered close to impossible to create a practical on-chip laser using Si-compatible group IV semiconductor materials (e.g. germanium (Ge)) owing to their indirect bandgap nature. Strain-engineered Ge material has recently garnered much attention for the dramatic reduction of the lasing threshold of group IV lasers because large tensile strain fundamentally alters the Ge’s bandstructure. Here, we present our recent research works on highly strained group IV materials on a Si chip for the realization of low-threshold on-chip lasers. In the first part, we will introduce our innovative strain engineering platforms that can create direct bandgap Ge nanowires by inducing >5% elastic tensile strain. In the second part, we will present the first experimental observation of low-threshold optically pumped lasing in highly strained Ge nanowires. The strain engineering of direct bandgap germanium-tin will also be described for the ultimate reduction of the lasing threshold.