Nanocavity Laser Research Articles

Photonic crystal nanolasers in polydimethylsiloxane thin film for sensing quantities leading to strain

We propose and realize a 1D photonic crystal nanocavity laser embedded in a polydimethylsiloxane (PDMS) thin film. The nanolaser in PDMS exhibits a significant optical response to structural deformation. It can be attached to object surfaces or integrated into different configurations, enabling the detection of different quantities that induce strain in the film. In experiments, this nanolaser can detect temperature variations or micrometer-scale bending degrees by attaching it to a temperature-controllable or bendable plate, respectively. Moreover, we further utilize the film as a diaphragm of a chamber to demonstrate its potential as a highly sensitive pressure gauge and chemical sensor. By adjusting the thickness of the PDMS thin film and the position of the nanolaser, we experimentally achieved a minimum detectable gas pressure variation of 0.12 kPa and a sensing dynamic range of 46 dB. We also investigate the optical response of the nanolaser to the swelling of the PDMS thin film induced by different organic solvents in experiments. The experimental wavelength shift rates over time are proportional to different chemical vapors’ PDMS swelling ratios, which can be used to identify specific chemical vapors within the chamber that induce PDMS swelling. Based on the experimental results and the capability of reattaching to different objects or configurations, we believe that our PhC nanolaser demonstrated herein holds significant potential as a highly sensitive mechanical and chemical sensor.

Raman silicon nanocavity laser with efficient light emission from the edge of an adjacent waveguide.

A Raman nanocavity laser can emit light into free space and into a properly designed waveguide adjacent to the cavity by mode coupling. In common device designs, the emission from the edge of this waveguide is relatively weak. However, a Raman silicon nanocavity laser with strong emission from the waveguide edge would be advantageous for certain applications. Here we investigate the increase in the edge emission that can be achieved by adding photonic mirrors to the waveguides adjacent to the nanocavity. We experimentally compare devices with and without photonic mirrors: the edge emission for devices with mirrors is 4.3 times stronger on average. This increase is analyzed using coupled-mode theory. The results indicate that the control of the round-trip phase shift (between the nanocavity and the mirror) and an increase of the quality factors of the nanocavity are important for further enhancement.

PT symmetric single-mode line-defect photonic crystal lasers with asymmetric loss design.

The exploration of parity-time (PT) symmetry in micro-/nano-cavity lasers has recently gained immense research interest. The PT symmetric phase transition to single-mode lasing has been achieved by arranging the spatial distribution of optical gain and loss in single or coupled cavity systems. In terms of photonic crystal (PhC) lasers, a non-uniform pumping scheme is usually employed to enter the PT symmetry-breaking phase in a longitudinal PT symmetric system. Instead, we use a uniform pumping scheme to enable the PT symmetric transition to the desired single lasing mode in line-defect PhC cavities based on a simple design with asymmetric optical loss. The flexible control of gain-loss contrast is realized by removing a few rows of air holes in PhCs. We obtain single-mode lasing with a side mode suppression ratio (SMSR) of around 30 dB without affecting the threshold pump power and linewidth. The output power of the desired mode is six times higher than that in multimode lasing. This simple approach enables single-mode PhC lasers without sacrificing the output power, threshold pump power, and linewidth of a multimode cavity design.

Electrically‐Driven Photonic Crystal Lasers with Ultra‐low Threshold

AbstractLight sources with ultra‐low energy consumption and high performance are required to realize optical interconnects for on‐chip communication. Photonic crystal (PhC) nanocavity lasers are one of the most promising candidates for this role. In this work, a continuous‐wave PhC nanolaser with an ultra‐low threshold current of 10.2 µA emitting at 1540 nm and operated at room temperature is demonstrated. The lasers are InP‐based bonded on silicon (Si), and comprise a buried heterostructure active region and lateral p–i–n junction, feature CMOS‐compatible drive voltage, and exhibit low self‐heating. Carrier leakage is a fundamental limitation of the lateral pumping scheme that is identified as unwanted spontaneous emission from the InP p–i interface, limiting the injection efficiency to 3% which further decreases at higher current. The effect of fabrication disorder and p‐doping absorption on the Q‐factor is studied experimentally showing that p‐doping limits the Q‐factor to 8000, with a p‐doping absorption coefficient of 120 cm−1.

Quantum Fluctuations and Lineshape Anomaly in a High‐β Silver‐Coated InP‐Based Metallic Nanolaser

AbstractMetallic nanocavity lasers provide important technological advancement toward even smaller integrable light sources. They give access to widely unexplored lasing physics in which the distinction between different operational regimes, like those of thermal or coherent light emission, becomes increasingly challenging upon approaching a device with a near‐perfect spontaneous‐emission coupling factor . In fact, quantum‐optical studies have to be employed to reveal a transition to coherent emission in the intensity fluctuation behavior of nanolasers when the input–output characteristic appears thresholdless for nanolasers. Here, a new indicator for lasing operation in high‐ lasers is identified by showing that stimulated emission can give rise to a lineshape anomaly manifested as a transition from a Lorentzian to a Gaussian component in the emission linewidth that dominates the spectrum above the lasing threshold.

Thermal Simulation and Experimental Analysis of Optically Pumped InP-on-Si Micro- and Nanocavity Lasers.

There is a general trend of downscaling laser cavities, but with high integration and energy densities of nanocavity lasers, significant thermal issues affect their operation. The complexity of geometrical parameters and the various materials involved hinder the extraction of clear design guidelines and operation strategies. Here, we present a systematic thermal analysis of InP-on-Si micro- and nanocavity lasers based on steady-state and transient thermal simulations and experimental analysis. In particular, we investigate the use of metal cavities for improving the thermal properties of InP-on-Si micro- and nanocavity lasers. Heating of lasers is studied by using Raman thermometry and the results agree well with simulation results, both revealing a temperature reduction of hundreds of kelvins for the metal-clad cavity. Transient simulations are carried out to improve our understanding of the dynamic temperature variation under pulsed and continuous wave pumping conditions. The results show that the presence of a metal cladding not only increases the overall efficiency in heat dissipation but also causes a much faster temperature response. Together with optical experimental results under pulsed pumping, we conclude that a pulse width of 10 ns and a repetition rate of 100 kHz is the optimal pumping condition for a 2 μm wide square cavity.

Sub-100-nW-threshold Raman silicon laser designed by a machine-learning method that optimizes the product of the cavity Q-factors.

Raman silicon lasers based on photonic crystal nanocavities with a threshold of several hundred microwatts for continuous-wave lasing have been realized. In particular, the threshold depends on the degree of confinement of the excitation light and the Raman scattering light in the two nanocavity modes. Here, we report lower threshold values for Raman silicon nanocavity lasers achieved by increasing the quality (Q) factors of the two cavity modes. By using an optimization method based on machine learning, we first increase the product of the two theoretical Q values by a factor of 17.0 compared to the conventional cavity. The experimental evaluation demonstrates that, on average, the actually achieved product is more than 2.5 times larger than that of the conventional cavity. The input-output characteristic of a Raman laser with a threshold of 90 nW is presented and the lowest threshold obtained in our experiments is 40 nW.

Detection of negatively ionized air by using a Raman silicon nanocavity laser.

The performance of a Raman silicon laser based on a high quality-factor nanocavity depends on the degree of free-carrier absorption, and this characteristic may be useful for certain applications. Here we demonstrate that laser oscillation in a Raman silicon nanocavity laser stops abruptly after an exposure to a weak flux of negatively ionized air for a few seconds. Spectral measurements reveal that the laser interruption is mainly caused by the transfer of extra electrons from the negatively ionized air molecules to the silicon nanocavity. These electrons affect the efficiency of the Raman laser by free carrier absorption. We find that the laser output gradually recovers as the extra electrons escape from the nanocavity and confirm that such a detection of ionized air is repeatable. These results show that a Raman silicon nanocavity laser can be used for the detection of ionized air with a high spatial resolution.

Low- and high-β lasers in the class-A limit: photon statistics, linewidth, and the laser-phase transition analogy

Nanocavity lasers are commonly characterized by the spontaneous coupling coefficient β that represents the fraction of photons emitted into the lasing mode. While β is conventionally discussed in semiconductor lasers where the photon lifetime is much shorter than the carrier lifetime (class-B lasers), little is known about β in atomic lasers where the photon lifetime is much longer than the other lifetimes and only the photon degree of freedom exists (class-A lasers). We investigate the impact of the spontaneous coupling coefficient β on lasing properties in the class-A limit by extending the well-known Scully–Lamb master equation. We demonstrate that in the class-A limit all the photon statistics are uniquely characterized by β and that the laser phase transition-like analogy becomes transparent. In fact, β perfectly represents the “system size” in phase transition. Finally, we investigate the laser-phase transition analogy from the standpoint of a quantum dissipative system. Calculating a Liouvillian gap, we clarify the relation between β and the continuous phase symmetry breaking.

Plasmonic Quantum Dot Nanocavity Laser: Hybrid Modes

The hybrid modes in the plasmonic quantum dot (QD) laser are modeled using the Marctili method. The model is then used to study the mode characteristics. The modes are going to cutoff point at zero propagation constant, while it goes to surface plasmon polaritons (SPPs) mode at higher photon energy. This behavior was different from that of waveguide modes shown in the dielectric waveguide. At plasmon resonance, hybrid mode is exactly one mode: surface plasmon polariton mode (perfect electric conductor).

Ultrafast Plasmonic Optical Switching Structures and Devices

[This submission is based on the contacts with Prof. Pavesi by code JMDC(100%.2019.FPHY.832).] Plasmonic structures possess rich physics related to the sensitivity of plasmon resonance to the change in the environmental dielectric constant, the enhanced light scattering and optical extinction, and the local field enhancement enabled strong light-matter interactions, which have been applied in refractive-index sensors, optical feedback in various micro- or nano-cavity lasers, surface enhanced Raman scattering spectroscopy, and high-sensitivity molecular detection. However, ultrafast optical response is another important aspect of plasmons, which can be utilized to achieve switching of optical signals in different spectral bands. These optical switching designs are very important for applications in optical logic circuits and optical communication system. In this review, we summarize a series of reports on ultrafast plasmonic optical switches, where we focus our discussions on the structural and device designs, instead of on their physics. By categorizing the designs of optical switches into different groups by their featured performances, we intend to propose the development trend and the commonly interested mechanisms of such ultrafast optical switches. We hope this review will supply helpful concepts and technical information for further development and new applications of ultrafast optical switching devices.

Detrimental Fluctuation of Frequency Spacing Between the Two High-Quality Resonant Modes in a Raman Silicon Nanocavity Laser

To understand how we can maximize the fabrication yield, we study the fluctuations of the cavity parameters of an ultralow-threshold Raman Si laser that employs two high-quality resonant modes of a photonic crystal heterostructure nanocavity. We measure the quality (Q) factors, the resonant wavelengths (λ), and the frequency spacing between the resonant modes (Δf) for 40 nanocavity-based Raman Si lasers with the same design. The Q, λ, and Δf fluctuate due to random variations of the cavity geometry and we find that 12 out of 40 devices provide laser emission. While the fluctuations of Q and λ in these 12 cavities are basically the same as those in the other 28 cavities, the detuning between Δf and the Raman shift of Si for the 12 cavities is less than 0.024 THz, which is strictly smaller than the detuning for the other 28 cavities. A large fluctuation of Δf is, thus, detrimental for the yield of Raman Si lasers. Numerical simulations reveal that a decrease in the random variations of the air hole positions and radii is important for high yields.

Implementing a Raman silicon nanocavity laser for integrated optical circuits by using a (100) SOI wafer with a 45-degree-rotated top silicon layer

Raman silicon (Si) lasers based on high-quality-factor photonic crystal nanocavities are very compact and can operate at excitation powers of less than one microwatt. For the best performance, the nanocavity of the Raman Si laser has to be fabricated along the [100] crystal direction of the Si-on-insulator (SOI) wafer to enhance the Raman gain. On the other hand, Si photonic devices are usually fabricated along the [110] direction because crystalline Si can be easily cleaved along [110]. This rotation by 45 degrees of the nanocavity with respect to the cleavable direction can be problematic for various applications. Here we report a Raman Si nanocavity laser fabricated on a modified (100) SOI wafer in which the crystal orientation of the top Si layer is rotated in plane by 45 degrees relative to the crystal direction of the support substrate. We observe room temperature continuous wave laser oscillation with a sub-microwatt threshold and a maximum energy efficiency of 5.6%. It is found that the photonic-slab warpage induced by compressive stress is reduced in this 45°-rotated SOI wafer.

Whispering-gallery mode hexagonal micro-/nanocavity lasers [Invited

Whispering-gallery-mode (WGM) hexagonal optical micro-/nanocavities can be utilized as high-quality (Q) resonators for realizing compact-size low-threshold lasers. In this paper, the progress in WGM hexagonal micro-/nanocavity lasers is reviewed comprehensively. High-Q WGMs in hexagonal cavities are divided into two kinds of resonances propagating along hexagonal and triangular periodic orbits, with distinct mode characteristics according to theoretical analyses and numerical simulations; however, WGMs in a wavelength-scale nanocavity cannot be well described by the ray model. Hexagonal micro-/nanocavity lasers can be constructed by both bottom-up and top-down processes, leading to a diversity of these lasers. The ZnO- or nitride-based semiconductor material generally has a wurtzite crystal structure and typically presents a natural hexagonal cross section. Bottom-up growth guarantees smooth surface faceting and hence reduces the scattering loss effectively. Laser emissions have been successfully demonstrated in hexagonal micro-/nanocavities synthesized with various materials and structures. Furthermore, slight deformation can be easily introduced and precisely controlled in top-down fabrication, which allows lasing-mode manipulation. WGM lasing with excellent single-transverse-mode property was realized in waveguide-coupled ideal and deformed hexagonal microcavity lasers.

Room temperature nanocavity laser with interlayer excitons in 2D heterostructures.

Atomically thin layered two-dimensional (2D) materials have provided a rich library for both fundamental research and device applications. Bandgap engineering and controlled material response can be achieved from artificial heterostructures. Recently, excitonic lasers have been reported using transition metal dichalcogenides; however, the emission is still the intrinsic energy bandgap of the monolayers. Here, we report a room temperature interlayer exciton laser with MoS2/WSe2 heterostructures. The onset of lasing was identified by the distinct kink in the "L-L" curve and the noticeable spectral linewidth collapse. Different from visible emission of intralayer excitons in monolayer components, our laser works in the infrared range, which is fully compatible with the well-established technologies in silicon photonics. Long lifetime of interlayer excitons relaxes the requirement of the cavity quality factor by orders of magnitude. Room temperature interlayer exciton lasers might open new perspectives for developing coherent light sources with tailored optical properties on silicon photonics platforms.

Three-dimensional photonic crystal simultaneously integrating a nanocavity laser and waveguides

Three-dimensional (3D) integration of photonic devices is an ultimate route toward highly integrated photonic circuits. 3D photonic crystals (PCs) are a promising platform for 3D integration because of their complete photonic bandgaps (cPBGs) which enables full control of light. However, simultaneous integration of active and passive devices into a 3D PC has been hindered due to fabrication difficulties. Here, we simultaneously integrate a nanocavity laser and waveguides into a 3D PC with a cPBG at near-IR wavelengths using micromanipulation technology. The proposed plate-insertion stacking method allowed fabrication of 3D PCs with a large number of layers, enabling integration of the active and passive circuit components. Laser emission from the photo-excited nanocavity laser was observed from the output port of the waveguides, demonstrating successful guiding of the light from the nanocavity laser in the 3D PC. This work paves the way for 3D photonic circuits using 3D PCs with cPBGs.

Lasing Dynamics of Optically-Pumped Ultralow-Threshold Raman Silicon Nanocavity Lasers

A Raman laser based on a high-$Q$ nanocavity in silicon has potential as a light source in photonic integrated circuits, but it is unclear how the significant enhancement of nonlinear optical effects by the cavity affects the lasing dynamics. Spectrally resolved time-domain measurements reveal that free carriers generated by two-photon absorption induce various dynamical effects during the initial lasing process, even at very low threshold power. Surprisingly, the Raman laser signal exhibits large oscillations at high excitation power. This insight will be useful for incorporating such Raman nanocavity lasers into device designs.

Light Emission Properties of 2D Transition Metal Dichalcogenides: Fundamentals and Applications

Abstract2D transition metal dichalcogenides (2D TMDs) with energy bandgaps covering the visible and near infrared spectral region have enormous advantages in various photonic and optoelectronic applications, such as light emitting devices, transistors, photovoltaic devices, photodetectors, and nanocavity lasers. Understanding the mechanisms and controlling the optical properties of 2D TMDs is of crucial importance for further applications in integrated photonics. This review focuses on the light emission properties of 2D TMDs and their heterojunctions (HJs), including the fundamentals, tunability, and device applications. The band structures and excitonic properties related to light emission in 2D TMDs are discussed. The approaches to control, tune, and enhance light emission are also discussed. Subsequently, the light emission phenomena in 2D TMD HJs and their mechanisms associated with interlayer exciton, charge transfer, and energy transfer processes are described. Finally, the future potentials and challenges in TMD photonics are presented.

Rate equation description of quantum noise in nanolasers with few emitters

Rate equations for micro- and nanocavity lasers are formulated which take account of the finite number of emitters, Purcell effects as well as stochastic effects of spontaneous emission quantum noise. Analytical results are derived for the intensity noise and intensity correlation properties, g(2), using a Langevin approach and are compared with simulations using a stochastic approach avoiding the mean-field approximation of the rate equations. Good agreement between the two approaches is found even for large values of the spontaneous emission beta-factor, i.e., for threshold-less lasers, as long as more than about ten emitters contribute to lasing. A large value of the beta-factor improves the noise properties.

High-Q photonic crystal cavities in all-semiconductor photonic crystal heterostructures

Photonic crystal cavities enable the realization of high Q-factor and low mode-volume resonators, with typical architectures consisting of a thin suspended periodically-patterned layer to maximize confinement of light by strong index guiding. We investigate a heterostructure-based approach comprising a high refractive index core and lower refractive index cladding layers. Whilst confinement typically decreases with decreasing index contrast between the core and cladding layers, we show that, counter-intuitively, due to the confinement provided by the photonic band structure in the cladding layers, it becomes possible to achieve Q-factors $>10^4$ with only a small refractive index contrast. This opens up new opportunities for implementing high Q-factor cavities in conventional semiconductor heterostructures, with direct applications to the design of electrically-pumped nano-cavity lasers using conventional fabrication approaches.