Thresholdless nanoscale coaxial lasers

Thresholdless coherence in a superradiant laser

Lasing threshold in the conventional lasers is the minimum input power required to initiate laser oscillation. It has been widely accepted that the conventional laser threshold occurring around a unity intracavity photon number can be eliminated in the input-output curve by making the so-called β parameter approach unity. The recent experiments, however, have revealed that even in this case the photon statistics still undergo a transition from coherent to thermal statistics when the intracavity mean photon number is decreased below unity. Since the coherent output is only available above the diminished threshold, the long-sought promise of thresholdless lasers to produce always coherent light has become questionable. Here, we present an always-coherent thresholdless laser based on superradiance by two-level atoms in a quantum superposition state with the same phase traversing a high-Q cavity. Superradiant lasing was observed without the conventional lasing threshold around the unity photon number and the photon statistics remained near coherent even below it. The coherence was improved by reducing the coupling constant as well as the excited-state amplitude in the superposition state. Our results pave a way toward always-coherent thresholdless lasers with more practical media such as quantum dots, nitrogen-vacancy centers and doped ions in crystals.

Plasmon near-field coupling and universal scaling behavior in shifted-core coaxial nano-cavity pair

We computationally and analytically investigate the plasmon near-field coupling phenomenon and the associated universal scaling behavior in a pair of coupled shifted-core coaxial nano-cavities. Each nano-cavity is composed of an InGaAsP gain medium sandwiched between a silver (Ag) core and an Ag shell. The evanescent coupling between the cavities lifts the degeneracy of the cut-off free transverse electromagnetic (TEM) like mode. The mode splitting of the supermodes is intensified by shifting the metal core position, which induces symmetry breaking. This coupling phenomenon is explained with spring-capacitor analogy and circuit analysis. The numerical simulation results reveal an exponential decay in the fractional plasmon wavelength relative to the ratio of gap distance and core shifting distance, which aligns with the plasmon ruler equation. In addition, by shifting the Ag cores in both cavities toward the center of the coupled structure, the electromagnetic field becomes strongly localized in nanoscale regions (hotspots) in the gain medium between the cavities, thus achieving extreme plasmonic nanofocusing. Utilizing this nanofocusing effect, we propose a refractive index sensor by placing a fluidic channel between the two cavities in close vicinity to the hotspots and reaching the highest sensitivity of ∼700nm/RIU.

Electrodynamic modeling of threshold-free lasing in photonic time crystals

An electrodynamic model is presented in this Letter to describe thresholdless lasers, utilizing the application of photonic time crystals (PTCs). By integrating the distinctive physical properties of PTCs and employing a comprehensive model based on a four-level system, the feasibility of achieving thresholdless laser operation is demonstrated. The proposed electrodynamic model comprehensively captures the intricate interplay between the electromagnetic field and the PTC medium. The model takes into account the ultrafast periodic variations in the refractive index of the PTCs, which arise from their time crystal-like behavior. Additionally, the dynamic response of the four-level system is considered, factoring in the processes of population inversion and relaxation. This Letter seeks to elucidate the underlying mechanisms that facilitate thresholdless laser operation in PTC-based systems. Through our electrodynamic modeling approach, we demonstrate that the ultrafast variations in the refractive index of PTCs give rise to a self-sustaining laser action, obviating the need for a lasing threshold. Moreover, we investigate the impact of various parameters, including pump power and modulation period, on the laser's performance and output characteristics. The developed electrodynamic model provides a comprehensive framework for comprehending and designing thresholdless lasers based on photonic time crystals. This research contributes to the advancement of thresholdless laser technology and opens up possibilities for applications in optical communications, sensing, and quantum photonics.

Frequency pushing enhanced by an exceptional point in an atom-cavity coupled system

We observed the frequency pushing of the cavity resonance as a result of the coupling of the cavity field with the ground state 138Ba in a high-Q cavity. A weak probe laser propagated along the axis of a Fabry-Pérot cavity while ground-state barium atoms traversed the cavity mode perpendicularly. By operating the atom-cavity composite in the vicinity of an exceptional point, we could observe a greatly enhanced frequency shift of the cavity transmission peak, which was pushed away from the atomic resonance, resulting in up to 41 ± 7 kHz frequency shift per atom from the empty cavity resonance. We analyzed our results by using the Maxwell-Schrödinger equation and obtained good agreement with the measurements.

Experimental demonstration of a nanobeam Fano laser

Microscopic single-mode lasers with low power consumption, large modulation bandwidth, and ultra-narrow linewidth are essential for numerous applications, such as on-chip photonic networks. A recently demonstrated microlaser using an optical Fano resonance between a discrete mode and a continuum of modes to form one of the mirrors, i.e., the so-called Fano laser, holds great promise for meeting these requirements. Here, we suggest and experimentally demonstrate what we believe is a new configuration of the Fano laser based on a nanobeam geometry. Compared to the conventional two-dimensional photonic crystal geometry, the nanobeam structure makes it easier to engineer the phase-matching condition that facilitates the realization of a bound-state-in-the-continuum (BIC). We investigate the laser threshold in two scenarios based on the new nanobeam geometry. In the first, classical case, the gain is spatially located in the part of the cavity that supports a continuum of modes. In the second case, instead, the gain is located in the region that supports a discrete mode. We find that the laser threshold for the second case can be significantly reduced compared to the conventional Fano laser. These results pave the way for the practical realization of high-performance microlasers.

Ultrasmall InGa(As)P Dielectric and Plasmonic Nanolasers

Nanolasers have great potential for both on-chip light sources and optical barcoding particles. We demonstrate ultrasmall InGaP and InGaAsP disk lasers with diameters down to 360 nm (198 nm in height) in the red spectral range. Optically pumped, room-temperature, single-mode lasing was achieved from both disk-on-pillar and isolated particles. When isolated disks were placed on gold, plasmon polariton lasing was obtained with Purcell-enhanced stimulated emission. UV lithography and plasma ashing enabled wafer-scale fabrication of nanodisks with an intended random size variation. Silica-coated nanodisk particles generated stable subnanometer spectra from within biological cells across an 80 nm bandwidth from 635 to 715 nm.

Vertical Emitting Nanowire Vector Beam Lasers

Due to the peculiar structured light field with spatially variant polarizations on the same wavefront, vector beams (VBs) have sparked research enthusiasm in developing advanced super-resolution imaging and optical communications techniques. A compact VB nanolaser is intriguing for VB applications in miniaturized photonic integrated circuits. However, determined by the diffraction limit of light, it is a challenge to realize a VB nanolaser in the subwavelength scale because the VB lasing modes should have laterally structured distributions. Here, we demonstrate a VB nanolaser made from a 300 nm thick InGaAs/GaAs nanowire (NW). To select the high-order VB lasing mode, a standing NW as-grown from the selective-area-epitaxial (SAE) growth process is utilized, which has a bottom donut-shaped interface with the silicon oxide growth substrate. With this donut-shaped interface as one of the reflective mirrors of the nanolaser cavity, the VB lasing mode has the lowest threshold. Experimentally, a single-mode VB lasing mode with a donut-shaped amplitude and azimuthally cylindrical polarization distribution is obtained. Together with the high yield and uniformity of the SAE-grown NWs, our work provides a straightforward and scalable path toward cost-effective co-integration of VB nanolasers on potential photonic integrated circuits.

Ultra-small low-threshold mid-infrared plasmonic nanowire lasers based on n-doped GaN

An ultra-small mid-infrared plasmonic nanowire laser based on n-doped GaN metallic material is proposed and studied by the finite-difference time-domain method. In comparison with the noble metals, nGaN is found to possess superior permittivity characteristics in the mid-infrared range, beneficial for generating low-loss surface plasmon polaritons and achieving strong subwavelength optical confinement. The results show that at a wavelength of 4.2 µm, the penetration depth into the dielectric is substantially decreased from 1384 to 163 nm by replacing Au with nGaN, and the cutoff diameter of nGaN-based laser is as small as 265 nm, only 65% that of the Au-based one. To suppress the relatively large propagation loss induced by nGaN, an nGaN/Au-based laser structure is designed, whose threshold gain has been reduced by nearly half. This work may pave the way for the development of miniaturized low-consumption mid-infrared lasers.

Directly modulated parity-time symmetric single-mode Fabry-Perot laser

Effective manipulation of resonant mode, output power and modulation bandwidth of lasers are all of vital importance for practical application scenarios such as communication systems. We show that by breaking the parity-time (PT) symmetry, single mode operation lasing can be realized in an intrinsic multiple mode Fabry-Perot (FP) resonator. Two identical FP resonators are employed to establish a symmetric system and high output power can be achieved with lower fabrication difficulty and intracavity losses compared with ring resonators. The small-signal response and direct modulation of the PT-symmetric FP laser have also been demonstrated with electrical pumping. Our work opens new avenues for mode selection of high-performance FP lasers and provides a cost-effective candidate for practical applications such as communication systems.

Indefinite Graphene Nanocavities with Ultra-Compressed Mode Volumes

Explorations of indefinite nanocavities have attracted surging interest in the past few years as such cavities enable light confinement to exceptionally small dimensions, relying on the hyperbolic dispersion of their consisting medium. Here, we propose and study indefinite graphene nanocavities, which support ultra-compressed mode volumes with confinement factors up to 109. Moreover, the nanocavities we propose manifest anomalous scaling laws of resonances and can be effectively excited from the far field. The indefinite graphene cavities, based on low dimensional materials, present a novel rout to squeeze light down to the nanoscale, rendering a more versatile platform for investigations into ultra-strong light-matter interactions at mid-infrared to terahertz spectral ranges.

All-colloidal parity-time-symmetric microfiber lasers balanced between the gain of colloidal quantum wells and the loss of colloidal metal nanoparticles

Lasers based on semiconductor colloidal quantum wells (CQWs) have attracted wide attention, thanks to their facile solution-processability, low threshold and wide range spectral tunability. Colloidal microlasers based on whispering-gallery-mode (WGM) resonators have already been widely demonstrated. However, due to their microscale size typically supporting multiple modes, they suffer from multimode competition and higher threshold. The ability to control the multiplicity of modes oscillating within colloidal laser resonators and achieving single-mode lasers is of fundamental importance in many photonic applications. Here we show that as a unique, simple and versatile architecture of all-colloidal lasers intrinsically enabled by balanced gain/loss segments, the lasing threshold reduction and spectral purification can be readily achieved in a system of a WGM-supported microfiber cavity by harnessing the notions of parity-time symmetry (PT). In particular, we demonstrate a proof-of-concept PT-symmetric microfiber laser employing CQWs as the colloidal gain medium along with a carefully tuned nanocomposite of Ag nanoparticles (Ag NPs) incorporated into a PMMA matrix altogether and conveniently coated around a coreless microfiber as a rigorously tailored colloidal loss medium to balance the gain. The realization of gain/loss segments in our PT-symmetric all-colloidal arrangement is independent of selected pumping, reducing the complexity of the system and making compact device applications feasible, where control over the pumping is not possible. We observed a reduction in the number of modes, resulting in a reduced threshold and enhanced output power of the PT-symmetric laser. The PT-symmetric CQW-WGM microcavity architecture offers new opportunities towards simple implementation of high-performance optical resonators for colloidal lasers.

Extraction of silver losses at cryogenic temperatures through the optical characterization of silver-coated plasmonic nanolasers

We report on the extraction of silver losses in the range 10 K-180 K by performing temperature-dependent micro-photoluminescence measurements in conjunction with numerical simulations on silver-coated nanolasers around near-infrared telecommunication wavelengths. By mapping changes in the quality factor of nanolasers into silver-loss variations, the imaginary part of silver permittivity is extracted at cryogenic temperatures. The latter is estimated to reach values an order of magnitude lower than room-temperature values. Temperature-dependent values for the thermo-optic coefficient of III-V semiconductors occupying the cavity are estimated as well. This data is missing from the literature and is crucial for precise device modeling. Our results can be useful for device designing, the theoretical validation of experimental observations as well as the evaluation of thermal effects in silver-coated nanophotonic structures.

Ultra-low threshold lasing through phase front engineering via a metallic circular aperture

Semiconductor lasers with extremely low threshold power require a combination of small volume active region with high-quality-factor cavities. For ridge lasers with highly reflective coatings, an ultra-low threshold demands significantly suppressing the diffraction loss at the facets of the laser. Here, we demonstrate that introducing a subwavelength aperture in the metallic highly reflective coating of a laser can correct the phase front, thereby counter-intuitively enhancing both its modal reflectivity and transmissivity at the same time. Theoretical and experimental results manifest a decreasing in the mirror loss by over 40% and an increasing in the transmissivity by 10⁴. Implementing this method on a small-cavity quantum cascade laser, room-temperature continuous-wave lasing operation at 4.5 μm wavelength with an electrical consumption power of only 143 mW is achieved. Our work suggests possibilities for future portable applications and can be implemented in a broad range of optoelectronic systems.

Low threshold lasing is widely required, especially for portable systems. Here the authors design a circular subwavelength metallic aperture in a QCL to shape its phase front and control diffraction losses, which in turn allows a lower threshold dissipation power, enabling the fabrication of shorter cavities.

Experimental demonstration of mode selection in bridge-coupled metallo-dielectric nanolasers

We experimentally demonstrate bridge-coupled metallo-dielectric nanolasers that can operate in the in-phase or out-of-phase locking modes at room temperature. By varying the length of the bridge, we show that the coupling coefficients can be realized in support of the stable operation of any of these two modes. Both coupled nanolaser designs have been fabricated and characterized for experimental validation. Their lasing behavior has been confirmed by the spectral evolution, light-in light-out characterizations, and emission linewidth narrowing. The operating mode is identified from the near-field and far-field emission pattern measurements. To the best of our knowledge, this is the first demonstration of mode selection in bridge-coupled metallo-dielectric nanolasers, which can serve as building blocks in nanolaser arrays for applications in imaging, virtual reality devices, and lidars.

Toward Microlasers with Artificial Structure Based on Single-Crystal Ultrathin Perovskite Films

A perovskite microlaser is potentially valuable for integrated photonics due to its excellent properties. The artificial microlasers were mostly made on polycrystalline films. Though a perovskite single crystal has significantly improved properties in comparison with its polycrystalline counterpart, an artificial microlaser based on single-crystal perovskite has been much less explored due to the difficulty in producing an ultrathin-single-crystal (UTSC) film. Here we show a device processing based on a perovskite UTSC film, confirming the high performance of the UTSC device with a quality factor of 1250. The single-crystal device shows 4.5 times the quality factor and 8 times the radiation intensity in comparison with its polycrystalline counterpart. The experiment first proved that hybrid perovskite microlasers with a subwavelength fine structure can be processed by focused ion beams (FIB). In addition, a wavelength-tunable distributed feedback (DFB) laser is demonstrated, with a tuning range of ∼4.6 nm. The research provides an easily applicable approach for perovskite photonic devices with excellent performance.

Strong Purcell effect in deep subwavelength coaxial cavity with GeSn active medium

We propose a deep subwavelength plasmonic cavity based on a metal-coated coaxial structure with Ge_0.9Sn_0.1 as the active medium. A fundamental surface plasmon polariton mode is strongly confined on the sidewall of the metal core, with the quality factor up to 5×10³ at 10 K. By reducing the cavity dimension to a few nanometers, this cavity mode shows a strong plasmon binding with the mode volume down to 8×10^-10 (λ/n)³, and significant size-dependent damping caused by the non-local optical response. The Purcell factor is achieved as high as 2×10⁹ at 10 K and 7×10⁸ at 300 K. This cavity design provides a systematic guideline of scaling down the cavity size and enhancing the Purcell factor. Our theoretical demonstration and understanding of the subwavelength plasmonic cavity represent a significant step toward the large-scale integration of on-chip lasers with a low threshold.

Continuous-wave upconversion lasing with a sub-10 W cm-2 threshold enabled by atomic disorder in the host matrix

Microscale lasers efficiently deliver coherent photons into small volumes for intracellular biosensors and all-photonic microprocessors. Such technologies have given rise to a compelling pursuit of ever-smaller and ever-more-efficient microlasers. Upconversion microlasers have great potential owing to their large anti-Stokes shifts but have lagged behind other microlasers due to their high pump power requirement for population inversion of multiphoton-excited states. Here, we demonstrate continuous-wave upconversion lasing at an ultralow lasing threshold (4.7 W cm⁻²) by adopting monolithic whispering-gallery-mode microspheres synthesized by laser-induced liquefaction of upconversion nanoparticles and subsequent rapid quenching (“liquid-quenching”). Liquid-quenching completely integrates upconversion nanoparticles to provide high pump-to-gain interaction with low intracavity losses for efficient lasing. Atomic-scale disorder in the liquid-quenched host matrix suppresses phonon-assisted energy back transfer to achieve efficient population inversion. Narrow laser lines were spectrally tuned by up to 3.56 nm by injection pump power and operation temperature adjustments. Our low-threshold, wavelength-tunable, and continuous-wave upconversion microlaser with a narrow linewidth represents the anti-Stokes-shift microlaser that is competitive against state-of-the-art Stokes-shift microlasers, which paves the way for high-resolution atomic spectroscopy, biomedical quantitative phase imaging, and high-speed optical communication via wavelength-division-multiplexing.

Upconversion microlasers present a lot of advantages but also require high pumping powers. Here the authors present a high-performing microlaser based on anti-Stokes-shift in upconversion nanoparticles synthesized using a technique of liquid quenching.

Ultralow-threshold laser using super-bound states in the continuum

Wavelength-scale lasers provide promising applications through low power consumption requiring for optical cavities with increased quality factors. Cavity radiative losses can be suppressed strongly in the regime of optical bound states in the continuum; however, a finite size of the resonator limits the performance of bound states in the continuum as cavity modes for active nanophotonic devices. Here, we employ the concept of a supercavity mode created by merging symmetry-protected and accidental bound states in the continuum in the momentum space, and realize an efficient laser based on a finite-size cavity with a small footprint. We trace the evolution of lasing properties before and after the merging point by varying the lattice spacing, and we reveal this laser demonstrates the significantly reduced threshold, substantially increased quality factor, and shrunken far-field images. Our results provide a route for nanolasers with reduced out-of-plane losses in finite-size active nanodevices and improved lasing characteristics.

Plasmon-exciton coupling dynamics and plasmonic lasing in a core-shell nanocavity

Plasmonic nanolasers based on the spatial localization of surface plasmons (SPs) have attracted considerable interest in nanophotonics, particularly in the desired application of optoelectronic and photonic integration, even breaking the diffraction limit. Effectively confining the mode field is still a basic, critical and challenging approach to improve optical gain and reduce loss for achieving high performance of a nanolaser. Here, we designed and fabricated a semiconductor/metal (ZnO/Al) core-shell nanocavity without an insulator spacer by simple magnetron sputtering. Both theoretical and experimental investigations presented plasmonic lasing behavior and SP-exciton coupling dynamics. The simulation demonstrated the three-dimensional optical confinement of the light field in the core-shell nanocavity, while the experiments revealed a lower threshold of the optimized ZnO/Al core-shell nanolaser than the same-sized ZnO photonic nanolaser. More importantly, the blue shift of the lasing mode demonstrated the SP-exciton coupling in the ZnO/Al core-shell nanolaser, which was also confirmed by low-temperature photoluminescence (PL) spectra. The analysis of the Purcell factor and PL decay time revealed that SP-exciton coupling accelerated the exciton recombination rate and enhanced the conversion of spontaneous radiation into stimulated radiation. The results indicate an approach to design a real nanolaser for promising applications.

Quasinormal modes expansions for nanoresonators made of absorbing dielectric materials: study of the role of static modes

The interaction of light with photonic resonators is determined by the eigenmodes of the system. Modal theories based on quasinormal modes provide a natural tool to calculate and understand light scattering by nanoresonators. We show that, in the case of resonators made of absorbing dielectric materials, eigenmodes with zero eigenfrequency (static modes) play a key role in the modal formalism. The excitation of static modes builds a non-resonant contribution to the modal expansion of the scattered field. This non-resonant term plays a crucial physical role since it largely contributes to the off-resonance signal to which resonances are added in amplitude, possibly leading to interference phenomena and Fano resonances. By considering light scattering by a silicon nanosphere, we quantify the impact of static modes. This study shows that the importance of static modes is not just formal. Static modes are of prime importance in an expansion truncated to only a few modes.

Scaling of metal-clad InP nanodisk lasers: optical performance and thermal effects

A key component for optical on-chip communication is an efficient light source. However, to enable low energy per bit communication and local integration with Si CMOS, devices need to be further scaled down. In this work, we fabricate micro- and nanolasers of different shapes in InP by direct wafer bonding on Si. Metal-clad cavities have been proposed as means to scale dimensions beyond the diffraction limit of light by exploiting hybrid photonic-plasmonic modes. Here, we explore the size scalability of whispering-gallery mode light sources by cladding the sidewalls of the device with Au. We demonstrate room temperature lasing upon optical excitation for Au-clad devices with InP diameters down to 300 nm, while the purely photonic counterparts show lasing only down to 500 nm. Numerical thermal simulations support the experimental findings and confirm an improved heat-sinking capability of the Au-clad devices, suggesting a reduction in device temperature of 450 - 500 K for the metal-clad InP nanodisk laser, compared to the one without Au. This would provide substantial performance benefits even in the absence of a plasmonic mode. These results give an insight into the benefits of metal-clad designs to downscale integrated lasers on Si.

Extreme field confinement in zigzag plasmonic crystals

We propose extreme field confinement in a zigzag plasmonic crystal that can produce a wide plasmonic bandgap near the visible frequency range. By applying a periodic zigzag structure to a metal-insulator-metal plasmonic waveguide, the lowest three plasmonic crystal bands are flattened, creating a high-quality broadband plasmonic mirror over a wavelength range of 526-909 nm. Utilizing zigzag plasmonic crystals in a three-dimensional tapered metal-insulator-metal plasmonic cavity, extreme field confinement with a modal volume of less than 0.00005 λ ³ can be achieved even at resonances over a wide frequency range. In addition, by selecting the number of zigzag periods in the plasmonic crystal, critical coupling between the cavity and the waveguide can be achieved, thereby maximizing the field intensity with an enhancement factor of 10⁵ or more. We believe that zigzag plasmonic crystals will provide a powerful platform for implementing broadband on-chip plasmonic devices.

Light Engineering in Nanometer Space

Significant advances have been made in photonic integrated circuit technology, similar to the development of electronic integrated circuits. However, the miniaturization of cavity resonators, which are the essential components of photonic circuits, still requires considerable improvement. Over the past decades, various optical cavities have been utilized to implement next-generation light sources in photonic circuits with low energy, high data traffic, and integrable physical sizes. Nevertheless, it has been difficult to reduce the size of most commercialized cavities beyond the diffraction limit while maintaining high performance. Herein, recent advancements in subwavelength metallic cavities that can improve performance, even with the use of lossy plasmonic modes, are reviewed. The discussion is divided in three parts according to light engineering methods: subwavelength metal-clad cavities engineered using intermediate dielectric cladding; implementation of plasmonic cavities and waveguides using plasmonic crystals; and development of deep-subwavelength plasmonic waveguides and cavities using geometric engineering. A direction for further developments in photonic integrated circuit technology is also discussed, along with its practical application.

Plasmonic Nanolasers in On-Chip Light Sources: Prospects and Challenges

The plasmonic nanolaser is a class of lasers with the physical dimensions free from the optical diffraction limit. In the past decade, progress in performance, applications, and mechanisms of plasmonic nanolasers has increased dramatically. We review this advance and offer our prospectives on the remaining challenges ahead, concentrating on the integration with nanochips. In particular, we focus on the qualifications for electrical pumping, energy consumption, and ultrafast modulation. At last, we evaluate the strategies for on-chip source construction design and further threshold reduction to achieve a long-term room-temperature electrically pumped plasmonic nanolaser, the ultimate goal toward practical applications.

Optical confinement in the nanocoax: coupling to the fundamental TEM-like mode

The nanoscale coaxial cable (nanocoax) has demonstrated optical confinement in the visible and the near infrared. We report on a novel nanofabrication process which yields optically addressable, sub-µm diameter, and high aspect ratio metal-insulator-metal nanocoaxes made by atomic layer deposition of Pt and Al₂O₃. We observe sub-diffraction-limited optical transmission via the fundamental, TEM-like mode by excitation with a radially polarized optical vortex beam. Our experimental results are based on interrogation with a polarimetric imager. Finite element method numerical simulations support these results, and their uniaxial symmetry was exploited to model taper geometries with both an electrically large volume, (15λ)³, and a nanoscopic exit aperture, (λ/200)².

Real-time dynamic wavelength tuning and intensity modulation of metal-clad nanolasers

To realize ubiquitously used photonic integrated circuits, on-chip nanoscale sources are essential components. Subwavelength nanolasers, especially those based on a metal-clad design, already possess many desirable attributes for an on-chip source such as low thresholds, room-temperature operation and ultra-small footprints accompanied by electromagnetic isolation at pitch sizes down to ∼50 nm. Another valuable characteristic for a source would be control over its emission wavelength and intensity in real-time. Most efforts on tuning/modulation thus far report static changes based on irreversible techniques not suited for high-speed operation. In this study, we demonstrate in-situ dynamical tuning of the emission wavelength of a metallo-dielectric nanolaser at room temperature by applying an external DC electric field. Using an AC electric field, we show that it is also possible to modulate the output intensity of the nanolaser at high speeds. The nanolaser's emission wavelength in the telecom band can be altered by as much as 8.35 nm with a tuning sensitivity of ∼1.01 nm/V. Additionally, the output intensity can be attenuated by up to 89%, a contrast sufficient for digital data communication purposes. Finally, we achieve an intensity modulation speed up to 400 MHz, limited only by the photodetector bandwidth used in this study, which underlines the capability of high-speed operation via this method. This is the first demonstration of a telecom band nanolaser source with dynamic spectral tuning and intensity modulation based on an external E-field to the best of our knowledge.

Tailoring the lineshapes of coupled plasmonic systems based on a theory derived from first principles

Coupled photonic systems exhibit intriguing optical responses attracting intensive attention, but available theoretical tools either cannot reveal the underlying physics or are empirical in nature. Here, we derive a rigorous theoretical framework from first principles (i.e., Maxwell's equations), with all parameters directly computable via wave function integrations, to study coupled photonic systems containing multiple resonators. Benchmark calculations against Mie theory reveal the physical meanings of the parameters defined in our theory and their mutual relations. After testing our theory numerically and experimentally on a realistic plasmonic system, we show how to utilize it to freely tailor the lineshape of a coupled system, involving two plasmonic resonators exhibiting arbitrary radiative losses, particularly how to create a completely "dark" mode with vanishing radiative loss (e.g., a bound state in continuum). All theoretical predictions are quantitatively verified by our experiments at near-infrared frequencies. Our results not only help understand the profound physics in such coupled photonic systems, but also offer a powerful tool for fast designing functional devices to meet diversified application requests.

Multi-wavelength microresonator based on notched-elliptical polymer microdisks with unidirectional emission

A three-dimensional notched-elliptical microdisk with a wavelength-size notch on the boundary is proposed as a multi-wavelength and unidirectional emission lasing source. The device contains multiple properly designed two-dimensional whispering gallery mode-based polymer notched microdisks with different dimensions for use as a multi-wavelength source. It can have a relatively high optical quality factor of 4000, unidirectional emission with low far-field divergence ∼4°, and the efficiency of emission is as high as 84.2%. The effect of the notch size on the far-field divergence is analyzed, and the multi-wavelength lasing performance is characterized, demonstrating that the resonator is robust and reliable. This work paves a unique but generic way for the design of compact multi-wavelength microlasers.

Room-Temperature Lasing from Mie-Resonant Nonplasmonic Nanoparticles

Subwavelength particles supporting Mie resonances underpin a strategy in nanophotonics for efficient control and manipulation of light by employing both an electric and a magnetic optically induced multipolar resonant response. Here, we demonstrate that monolithic dielectric nanoparticles made of CsPbBr₃ halide perovskites can exhibit both efficient Mie-resonant lasing and structural coloring in the visible and near-IR frequency ranges. We employ a simple chemical synthesis with nearly epitaxial quality for fabricating subwavelength cubes with high optical gain and demonstrate single-mode lasing governed by the Mie resonances from nanocubes as small as 310 nm by the side length. These active nanoantennas represent the most compact room-temperature nonplasmonic nanolasers demonstrated until now.

Realizing spin Hamiltonians in nanoscale active photonic lattices

Spin models arise in the microscopic description of magnetic materials and have been recently used to map certain classes of optimization problems involving large degrees of freedom. In this regard, various optical implementations of such Hamiltonians have been demonstrated to quickly converge to the global minimum in the energy landscape. Yet, so far, an integrated nanophotonic platform capable of emulating complex magnetic materials is still missing. Here, we show that the cooperative interplay among vectorial electromagnetic modes in coupled metallic nanolasers can be utilized to implement certain types of spin Hamiltonians. Depending on the topology/geometry of the arrays, these structures can be governed by a classical XY Hamiltonian that exhibits ferromagnetic and antiferromagnetic couplings, as well as geometrical frustration. Our results pave the way towards a scalable nanophotonic platform to study spin exchange interactions and could address a variety of optimization problems.

Ten years of spasers and plasmonic nanolasers

Ten years ago, three teams experimentally demonstrated the first spasers, or plasmonic nanolasers, after the spaser concept was first proposed theoretically in 2003. An overview of the significant progress achieved over the last 10 years is presented here, together with the original context of and motivations for this research. After a general introduction, we first summarize the fundamental properties of spasers and discuss the major motivations that led to the first demonstrations of spasers and nanolasers. This is followed by an overview of crucial technological progress, including lasing threshold reduction, dynamic modulation, room-temperature operation, electrical injection, the control and improvement of spasers, the array operation of spasers, and selected applications of single-particle spasers. Research prospects are presented in relation to several directions of development, including further miniaturization, the relationship with Bose-Einstein condensation, novel spaser-based interconnects, and other features of spasers and plasmonic lasers that have yet to be realized or challenges that are still to be overcome.

Beam steering of a single nanoantenna

Nanoantennas play an important role as mediators to efficiently convert free-space light into localized optical energy and vice versa. However, effective control of the beam direction of a single nanoantenna remains a great challenge. In this paper, we propose an approach to steer the beam direction of a single nanoantenna by adjusting two antenna modes with opposite phase symmetry. Our theoretical study confirmed that the combination of even- and odd-symmetric modes with a phase difference of π/2 enables effective beam steering of a single nanoantenna whose steering angle is controlled by adjusting the amplitude ratio of the two antenna modes. To implement our theory in real devices, we introduced asymmetric trapezoidal nano-slot antennas with different side air-gaps of 10 and 50 nm. The trapezoidal nanoantennas can simultaneously excite the dipole and quadrupole modes in a single nanoantenna and enables effective beam steering with an angle of greater than 35° near the resonance of the quadrupole mode. In addition, the steering angle can also be controlled by adjusting the degree of asymmetry of the trapezoidal slot structure. We believe that our beam steering method for a single nanoantenna will find many potential applications in fields such as imaging, sensing, optical communication, and quantum optics.

Engineering III-V Semiconductor Nanowires for Device Applications

III-V semiconductor nanowires offer potential new device applications because of the unique properties associated with their 1D geometry and the ability to create quantum wells and other heterostructures with a radial and an axial geometry. Here, an overview of challenges in the bottom-up approaches for nanowire synthesis using catalyst and catalyst-free methods and the growth of axial and radial heterostructures is given. The work on nanowire devices such as lasers, light emitting nanowires, and solar cells and an overview of the top-down approaches for water splitting technologies is reviewed. The authors conclude with an analysis of the research field and the future research directions.

Observation of PT-symmetry in a fiber ring laser

A wavelength-tunable single-mode laser with a sub-kilohertz linewidth based on parity-time (PT)-symmetry is proposed and experimentally demonstrated. The proposed PT-symmetric laser is implemented based on a hybrid use of an optical fiber loop and a thermally tunable integrated microdisk resonator (MDR). The MDR, implemented based on the silicon-on-insulator, operates with the optical fiber loop to form two mutually coupled cavities with an identical geometry. By controlling two light waves passing through the two cavities, with one having a gain coefficient and the other a loss coefficient but with an identical magnitude, a PT-symmetric laser is implemented. Thanks to an ultranarrow passband of the cavity due to PT-symmetry, single-longitudinal mode lasing is achieved. The tuning of the wavelength is implemented by thermally tuning the MDR. The proposed PT-symmetric laser is demonstrated experimentally. Single-longitudinal mode lasing at a wavelength of around 1555 nm with a sub-kilohertz linewidth of 433 Hz is implemented. The lasing wavelength is continuously tunable from 1555.135 to 1555.887 nm with a tuning slope of 75.24 pm/°C.

Feasibility of surface plasmon lasing in HgTe quantum wells with population inversion

Surface plasmon lasing in semiconductor gain media at far-infrared frequencies requires simultaneously long non-radiative recombination times and large plasmon propagation length. In this paper, we show that these conditions are realized in mercury-telluride quantum wells (HgTe QWs) near the topological transition. We derive the conditions of surface plasmon amplification in HgTe QWs with interband population inversion. To this end, we calculate the spatially-dispersive high-frequency conductivity of pumped HgTe QWs taking into account their realistic band structure, and compare the interband gain with Drude absorption and collisionless Landau damping. An extra necessary condition of plasmon lasing is revealed, namely, the non-equilibrium carrier density should be high enough to make the plasmon spectrum overlap with the frequency domain of interband excitations. The latter condition limits the processes of both stimulated and spontaneous plasmon emission at low temperatures, and should have a strong impact on the recombination kinetics of HgTe QWs at low temperatures.

Mid-Infrared Lasing in Lead Sulfide Subwavelength Wires on Silicon

Vapor-liquid-solid (VLS) growth of nanoscale or subwavelength scale semiconductor wires (nanowires) has been proven to be an important and effective approach to producing high-quality, substrate insensitive photonic materials with a flexible and ever-expanding coverage of wavelengths for lasing and other photonic applications. However, the materials and lasing demonstrations have so far been limited to mostly ultraviolet to visible wavelengths, with a few exceptions in the short-wavelength infrared range. A further extension to longer wavelengths (such as mid-infrared, MIR) using narrower band gap semiconductors encounters severe challenges: the ever decreasing radiative efficiency due to the Auger and other nonradiative channels with wavelengths demands extremely high material quality and significantly narrows the material choices. This situation is very unsatisfactory, given many important applications that demand materials and lasers of subwavelength scales for MIR wavelengths in an integrated platform, especially on silicon. Here we report our results on lasing demonstration in MIR (3-4 μm) based on a unique combination of high-quality material growth on a silicon substrate and the choice of an intrinsically strong MIR material in lead sulfide (PbS). Lasing is demonstrated from single wires both on the original silicon substrate and on the sapphire substrates after transferring, with sizes of lasing wires down to below half of the normalized volume (volume of wires divided by the wavelength cubed) and operating temperature up to 180 K. Such subwavelength wire lasers could be important for a wide range of MIR applications on silicon-based integrated photonic platforms, such as chemical and environmental sensing, free-space communications, and many others.

Stochastic polarization switching induced by optical injection in bimodal quantum-dot micropillar lasers

Mutual coupling and injection locking of semiconductor lasers is of great interest in non-linear dynamics and its applications for instance in secure data communication and photonic reservoir computing. Despite its importance, it has hardly been studied in microlasers operating at μW light levels. In this context, vertically emitting quantum dot micropillar lasers are of high interest. Usually, their light emission is bimodal, and the gain competition of the associated linearly polarized fundamental emission modes results in complex switching dynamics. We report on selective optical injection into either one of the two fundamental mode components of a bimodal micropillar laser. Both modes can lock to the master laser and influence the non-injected mode by reducing the available gain. We demonstrate that the switching dynamics can be tailored externally via optical injection in very good agreement with our theory based on semi-classical rate equations.

Plasmonic Nanolasers Enhanced by Hybrid Graphene-Insulator-Metal Structures

Graphene is a two-dimensional (2D) structure that creates a linear relationship between energy and momentum that not only forms massless Dirac fermions with extremely high group velocity but also exhibits a broadband transmission from 300 to 2500 nm that can be applied to many optoelectronic applications, such as solar cells, light-emitting devices, touchscreens, ultrafast photodetectors, and lasers. Although the plasmonic resonance of graphene occurs in the terahertz band, graphene can be combined with a noble metal to provide a versatile platform for supporting surface plasmon waves. In this study, we propose a hybrid graphene-insulator-metal (GIM) structure that can modulate the surface plasmon polariton (SPP) dispersion characteristics and thus influence the performance of plasmonic nanolasers. Compared with values obtained when graphene is not used on an Al template, the propagation length of SPP waves can be increased 2-fold, and the threshold of nanolasers is reduced by 50% when graphene is incorporated on the template. The GIM structure can be further applied in the future to realize electrical control or electrical injection of plasmonic devices through graphene.

Lasing action in low-resistance nanolasers based on tunnel junctions

We experimentally demonstrate the lasing action of a new nanolaser design with a tunnel junction. By using a heavily doped tunnel junction for hole injection, we can replace the p-type contact material of a conventional nanolaser diode with a low-resistance n-type contact layer. This leads to a significant reduction of the device resistance and lowers the threshold voltage from 5 V to around 0.95 V at 77 K. The lasing behavior is verified by the light output versus the injection current (L-I) characterization and second-order coherence function measurements. Because of less Joule heating during current injection, the nanolaser can be operated at temperatures as high as 180 K under CW pumping. The incorporation of heavily doped tunnel junctions may pave the way for other nanoscale cavity design for improved heat management.

Measuring the frequency response of optically pumped metal-clad nanolasers

We report on our initial attempt to characterize the intrinsic frequency response of metal-clad nanolasers. The probed nanolaser is optically biased and modulated, allowing the emitted signal to be detected using a high-speed photodiode at each modulation frequency. Based on this technique, the prospect of high-speed operation of nanolasers is evaluated by measuring the D-factor, which is the ratio of the resonance frequency to the square root of its output power(f_R/Pout1/2). Our measurements show that for nanolasers, this factor is an order of magnitude greater than that of other state-of-the-art directly modulated semiconductor lasers. The theoretical analysis, based on the rate equation model and finite element method simulations of the cavity is in full agreement with the measurement results.

Lasing in Ni Nanodisk Arrays

We report on lasing at visible wavelengths in arrays of ferromagnetic Ni nanodisks overlaid with an organic gain medium. We demonstrate that by placing an organic gain material within the mode volume of the plasmonic nanoparticles both the radiative and, in particular, the high ohmic losses of Ni nanodisk resonances can be compensated. Under increasing pump fluence, the systems exhibit a transition from lattice-modified spontaneous emission to lasing, the latter being characterized by highly directional and sub-nanometer line width emission. By breaking the symmetry of the array, we observe tunable multimode lasing at two wavelengths corresponding to the particle periodicity along the two principal directions of the lattice. Our results are relevant for loss-compensated magnetoplasmonic devices and topological photonics.

Intensity noise and bandwidth analysis of nanolasers via optical injection

A measurement method that can be used to extract the relative intensity noise of a nanolaser is introduced and analyzed. The method is based on optical injection of emission from a nanolaser, serving as a master oscillator, transferring its intensity fluctuations to a low-noise semiconductor laser serving as a slave oscillator. Using the stochastic rate equation formalism, we demonstrate that the total relative intensity noise of the system is a weighted superposition of the relative intensity noise of individual lasers. We further discuss the analytical relations that can be used to extract the relative intensity noise spectrum of a nanolaser. Finally, we use mutual correlation as a mathematical tool to quantify the degree of resemblance between the injected and extracted intensity fluctuations, theoretically confirming that the spectra are at least 97% correlated within the 3-dB bandwidth when an injection strength is chosen properly.

Exceptional singular resonance in gain mediated metamaterials

We study the scattering of optical field by a hybridized metamaterial with properly imprinted gain. We predict that an occasionally real-eigen valued singularity in the interaction matrix of the coupled dark-bright meta-molecule would produce a high-Q resonance. This effect is demonstrated in full-wave three-dimensional finite element optical simulation. Field is efficiently amplified at this resonance. Further investigation shows that the resonance is associated with an exceptional point. The difference of this exceptional singularity from other high-Q resonances such as the spectral singularities in the scattering or transfer matrixes of parity-time symmetric systems and the bound states in the continuum is discussed. The non-Hermitian nature of the exceptional singularity promises some nonlinear applications.

Accumulation and directionality of large spontaneous emission enabled by epsilon-near-zero film

We demonstrate the spectral accumulation of large spontaneous emission (SE) for nanocavities with different sizes in the coupled Ag nanorod and epsilon-near-zero (ENZ) film system. This effect originates from the slowing down of the spectral shift of resonant nanocavities at the wavelength where the permittivity of the substrate vanishes, i.e., the resonance "pinning" near the ENZ frequency. In addition, most far field radiation of the emitter is concentrated in the forward field with small solid angle due to the impedance mismatch between the ENZ film and the free space. This kind of size-relaxed nanocavity for directional SE has potential applications in the bright single photon sources, plasmon-based nanolasers, and on-chip nanodevices.

Quantum-dot micropillar lasers subject to coherent time-delayed optical feedback from a short external cavity

We investigate the mode-switching dynamics of an electrically driven bimodal quantum-dot micropillar laser when subject to delayed coherent optical feedback from a short external cavity. We experimentally characterize how the external cavity length, being on the same order than the microlaser's coherence length, influences the spectral and dynamical properties of the micropillar laser. Moreover, we determine the relaxation oscillation frequency of the micropillar by superimposing optical pulse injection to a dc current. It is found that the optical pulse can be used to disturb the feedback-coupled laser within one roundtrip time in such a way that it reaches the same output power as if no feedback was present. Our results do not only expand the understanding of microlasers when subject to optical feedback from short external cavities, but pave the way towards tailoring the properties of this key nanophotonic system for studies in the quantum regime of self-feedback and its implementation to integrated photonic circuits.

Applications of nanolasers

Nanolasers generate coherent light at the nanoscale. In the past decade, they have attracted intense interest, because they are more compact, faster and more power-efficient than conventional lasers. Thanks to these capabilities, nanolasers are now an emergent tool for a variety of practical applications. In this Review, we explain the intrinsic merits of nanolasers and assess recent progress on their applications, particularly for optical interconnects, near-field spectroscopy and sensing, optical probing for biological systems and far-field beam synthesis through near-field eigenmode engineering. We highlight the scientific and engineering challenges that remain for forging nanolasers into powerful tools for nanoscience and nanotechnology.

On-Chip Monolithically Fabricated Plasmonic-Waveguide Nanolaser

Plasmonic-waveguide lasers, which exhibit subdiffraction limit lasing and light propagation, are promising for the next-generation of nanophotonic devices in computation, communication, and biosensing. Plasmonic lasers supporting waveguide modes are often based on nanowires grown with bottom-up techniques that need to be transferred and aligned for use in optical circuits. Here, we demonstrate a monolithically fabricated ZnO/Al plasmonic-waveguide nanolaser compatible with the fabrication requirements of on-chip circuits. The nanolaser is designed with a plasmonic metal layer on the top of the laser cavity only, providing highly efficient energy transfer between photons, excitons, and plasmons, and achieving lasing in the ultraviolet region up to 330 K with a low threshold intensity (0.20 mJ/cm² at room temperature). This work demonstrates the realization of a plasmonic-waveguide nanolaser without the need for transfer and positioning steps, which is the key for on-chip integration of nanophotonic devices.