Red, Green and Blue CdSe/CdS Colloidal Quantum Dots Lasers
Following in the footsteps of fundamental studies of quantum confinement effect in low dimensional semiconductors, solution based colloidal II-VI CdSe/CdS core-shell quantum dots (CQDs) have become an appealing candidate for the next generation of luminescent materials. By changing the diameter, the bandgap of CQDs can be tuned to cover the whole visible spectrum using single family of materials.
Beyond LED applications which typical under modest electronic excitations, CQDs based lasers suffer from the non-radiative multiexcitonic Auger recombination process under high excitations required for optical amplification and lasing. This process is further enhanced by strong spatial confinement and electron/hole wavefunction overlapping inside the type-I CQDs. The fast Auger (i.e. on 100 ps time scale) competes with the radiative recombination and depletes the optical gain from CQDs materials. To mitigate the multiexciton susceptible Auger effect, compositional and structure engineering methods have been proposed to realize single-excitonic gain enabled CQD-VCSELs. The optical gain information is obtained by the ultrafast pump-probe spectroscopy.
In-plane feedback configuration has been used to develop CdSe/CdS based CQDs lasers. Second-order distributed feedback (DFB) grating is fabricated by standard electron beam lithography and ICP-RIE processes, embedded with dense CQD thin films, to demonstrate CQD-DFB lasers covering red, green and blue from one single family of materials (see below image).
Organic-Inorganic Halide Perovskites as Coherent Light Emitters
Turning from the II-VI based inorganic nanocrystals, a very different class of low-temperature solution-grown materials have virtually exploded to the scene of photovoltaics resaerch in the past couple of years. These are the hybrid organic-inorganic halide perovskites with the chemical form of ABX3 (A = CH3NH3+, CH(NH2)2+, Cs+; B = Pb2+; X = I–, Br–, Cl–). The uniform and continuous polycrystalline thin film (SEM shown below) is fabricated by spin-casting the perovskite precursors solution (i.e. CH3NH3I and PbI2 in DMSO), during which a second extraction solution (i.e. toluene) is dripped to induce solution supersaturation and perovskite crystallization. Different from quantum confinement effect in II-VI CQDs, the bandgap tuning in perovskites is realized by changing or mixing the halide elements in the chemical composition, with PL emission continuously covering the whole visible spectrum. The high absorption coefficients of perovskite materials (104 ~ 105 cm-1) above the bandgap benefit the photovoltaic and light emission applications. Given the fundamental property of continuous color tuning for light emission, the simplicity and low-cost synthesis of such solution-processed materials (perovskites, as well as II-VI CQDs) are attractive to develop solution-grown, high-throughput semiconductor lasers.
2. Perovskite Lasers Exploiting In-Plane Feedback
Following the initial ASE-based evidence for optical gain, perovskite laser application possibilities are exploited in the community. Here, part of my research work is to find the optimum laser cavity configuration to realize low-threshold, high-quality perovskite lasers. Based on the experience from the mature II-VI CQDs laser work, the in-plane feedback configuration is tested first due to its long optical path for amplification.
The figure below shows the perovskite DFB (PeDFB) laser realized on a second-order grating defined on quartz substrate. This geometry is useful as it enables in-plane feedback for the laser (from the 2nd order diffraction), and an output beam (from the 1st order diffraction) perpendicular to the grating plane. The quartz grating template is fabricated by electron beam lithography and ICP etching processes, followed by the perovskite thin film deposition onto the grating surface. The robust lasing output is achieved at near infrared, yet with a relatively high threshold.
Another cavity configuration which exploits the two-dimensional in-plane feedback is the photonic crystal (PhC). As optical resonators for microscale light emitters, photonic crystals are well-known optical nanostructures which have been deployed for high-end single crystal epitaxial III-V semiconductor diode lasers. The basic version is a 2D-PhC with perfect hexagonal lattice structure which provides in-plane distributed optical feedback enhancing the light-matter interaction. The lattice-periodic change in the refractive index creates a photonic band structure, which in a laser device design is exploited by matching the band edge (at Γ point in reciprocal space) of the photonic crystal to the gain spectrum of the active medium for maximal Bragg reflections. The perovskite laser based on the photonic crystal (PePhC) achieves nearly four-fold lasing threshold decrease compared with PeDFB devices.
Given the well-defined spatial output from the PePhC microlasers, a question arises of the scalability to compact multielement laser emitter array. Broadly, such pixelated array with individual addressability can be useful in applications such as high brightness displays, 3D modulated projections, etc. As a proof-of concept, below shows a 2D pixelated 4×4 PePhC microlaser array. By controlling the pump laser scanning system, different patterns (with coherent lasing output) of letters can be generated.
3. Perovskite Lasers in Vertical Cavity Configuration
Because of the polycrystalline (grain size 30 ~ 300 nm) nature of solution processed perovskite thin films, even if in-plane feedback should benefit from relative long optical path (amplification), light scattering from their polycrystalline microstructure (Rayleigh-Mie range) can readily introduce unwanted or confounding optical losses and/or interference. Improvement of perovskite synthesis can reach smaller grain size films while a trade-off seems to exist as the higher density of grain surfaces and boundaries can induce more non-radiative recombination. To further lower the lasing threshold, vertical cavity configuration is pursued after PeDFB and PePhC devices.
As with established epitaxially grown (and ubiquitous) single crystal inorganic III-V compound semiconductor vertical cavity surface emitting lasers (VCSEL), one common denominator is a planar Fabry-Perot microcavity with high reflectivity (> 99%) multilayer stacks of distributed Bragg reflectors (DBR). The DBR stacks are usually part of the epitaxial growth process. Such in-situ complete VCSEL fabrication flow is not (yet) available for the solution processed perovskites, and thus as a first pass approach the prototype perovskite vertical cavity laser devices (PeVCSEL) are made by integrating perovskite thin films with discrete III-V wide bandgap GaN-based DBRs. The particular DBR exploits recently developed innovations for manipulating index of refraction of epitaxial GaN layers by their nano-porositication (further details in last section about light engine for atomic clock).
Under this monolithic integrated PeVCSEL configuration, the lasing threshold gets a ten-fold decrease from the PePhC devices, which has a contribution from the high-quality (Q ~ 1100) cavity. In addition to achieving laser action with high temporal and spatial coherence, these structures have enabled the study of optical gain dynamics and phase transition ( i.e. β factor) from spontaneous emission to laser regime, while addressing the critical issue of device degradation (device lifetime ~ 4 hours under continuous pulse pumping in the ambient). Taking advantages of the high quality PeVCSEL, the quasi-steady state operation at room temperature has been realized under longer pulse excitation (5 ns, comparable with PL lifetime of perovskites), which is critical for eventually continuous-wave perovskite lasers.
In aim to both improve the device robustness and explore possibility of green wavelength (interested in semiconductor lasers), green perovskite CH(NH2)2PbBr3 thin films are used as the optical gain medium. To overcome the disadvantage in nanoporous-GaN DBR based PeVCSEL where perovskite film is “sandwiched” between two mirrors (namely the air gaps between DBRs still made air-sensitive perovskite accessible by the ambient), dielectric DBR consists of ten pairs alternating HfO2/SiO2 layers is fabricated by sputtering process. In this configuration, the perovskite film is hermetic sealed after top DBR directly sputtered onto perovskite film surface. The high reflectivity (~ 99.6%) of the dielectric DBRs improve the cavity Q factor to ~ 1400, and the lasing threshold from the green PeVCSEL is one order magnitude lower than green II-VI CQDs lasers under similar cavity configuration and optical pumping condition. This dielectric sputtering fabrication flow can be applied in different types of substrates (e.g. flexible polymers). The realization of the flexible green PeVCSEL devices help us to envision further practical applications of perovskite based laser devices, such as large-area laser emitter arrays on curved surfaces (e.g. wallpaper).
Research work on the optical gain origin of perovskite materials, thermal management optimization, continuous-wave operation perovskite lasers are underway, hopefully with more exciting results in the near future.
See this review article for more information about my work on “Coherent Light Emitters from Solution Chemistry: Inorganic II-VI Nanocrystals and Organometallic Perovskites“.
Other useful reviews about perovskite light emission applications can be found in “Perovskite Photonic Sources” and “Perovskite Materials for Light Emitting Diodes and Lasers” literature.
Ultracompact UV Semiconductor Laser Engine for Atomic Clocks
Developing compact (In, Al)GaN multiple quantum wells (MQWs) based electrical injected junction vertical cavity surface emitting lasers (VCSELs) at 369.1 nm with ultranarrow linewidth for ytterbium atomic clock applications (collaboration with Prof. Jung Han from Yale University). At present the pumping of a Yb-ion transition in an atomic clock is enabled with a frequency-doubled, external cavity laser that cannot be easily miniaturized. The wavelength of commercial solid-state diode laser is unfortunately limited to above 370 nm. Thus the development of direct 369.1 nm emission electrical injected UV-VCSEL enables the possiblity to realize miniature Yb-ions optically driven atomic clocks.
A key piece of components is the nanoporous (NP) GaN based DBR (see below SEM image), where specific GaN layers (with high doping level) is electrochemically nanoporousified to obtain lower index of refraction, while other GaN layers (with low doping level) remain intact (high refractive index) during electrochemical etching process, leading to a high reflectivity DBR nanostructure. Moreover, the NP GaN based DBR still holds large portion of electrical conductivity when compared with structure before electrochemical etching, making it a both electrically and optically active highly reflective mirror for VCSEL applications. This project is still ongoing and more results are coming.