Single-exciton gain enables red, green, and blue colloidal quantum dot lasers

Compact lasers at visible wavelengths are used in many products, such as laser pointers, DVD players, and projection systems. Current semiconductor and solid-state laser technologies can produce multiple colors from ultraviolet through visible to infrared, but each model of laser typically covers only a very limited range of wavelengths related to the electronic properties of specific materials. Thus different semiconductor materials and technologies are required to cover the red, green, and blue (RGB) required by, e.g., display applications. Although red (III-phosphide) and blue (III-nitride) lasers are now fairly efficient devices, there are significant challenges to increasing the efficiency of green nitride lasers. Solid-state lasers that use nonlinear wavemixing techniques do allow discrete access to multiple wavelength points, but typically one (bulky) laser at a time.

Figure 1. Red, green, and blue (RGB) colloidal quantum dot (CQD) films excited in stripes by ultrashort optical pulses. (a) Photographs of emission stripes with pumping levels below and above amplified spontaneous emission (ASE) thresholds. (b) Coherent edge emission as a function of pump energy density for the RGB CQD films, with arrows indicating the ASE thresholds.

We have recently introduced a new nanotechnology material that makes full-visible-color lasers possible with a record-low energy pumping threshold.¹ The material is based on advances made by our collaborators at QD Vision Inc. (Lexington, MA) who have developed (Zn,Cd)Se-based (zinc and cadmium selenide) colloidal quantum dots (CQDs), which are synthesized in liquid solutions, as an exciting new source for high-efficiency luminescent material. We created 200 to 300nm thick, closely packed epitaxial-like films of these CQDs and showed that the CQD films are a physically innovative, practical way to achieve optical gain across the visible spectrum. Changing laser color requires, in principle, nothing more than controlling CQD size during the wet-chemistry synthesis. Our CQDs demonstrating laser action have CdSe alloy cores of nominal diameter 4.2, 3.2, and 2.5nm for RGB, respectively. A thin (∼1nm) layer of Zn_0.5Cd_0.5S (zinc cadmium sulfide) alloy forming an inorganic shell is one of the key factors in the successful material, as is the design of organic ligands, which provide a monolayer dielectric shell to promote, e.g., the CQD self-assembly in the solid film formation.

The use of CQDs for lasers has faced a problem known as biexciton gain, whose presence has been argued for a decade.² It arises because each ‘artificial atom’ (i.e., each individual CQD), has double degeneracy in the lowest energy level and therefore becomes transparent (the gain and the loss are balanced) if one exciton is generated, and reaches population inversion only if two excitons are generated. However, the nonradiative multiexciton Auger process is strongly enhanced in nanometer-sized particles and provides a competing pathway in attempts to build up a biexciton population inversion for optical gain. This has seemingly been a serious, fundamental limitation to pursuit of CQDs for lasers.³ Type-II CQDs, (in which, unlike type-I CQDs, excited electrons and holes are confined in separate regions) were introduced to enable single-exciton gain, for which the Auger process is physically absent.⁴ But in addition to presenting challenges in synthesis, the lower interband optical oscillator strength of these materials significantly reduces their optical gain, luminescence rate, and quantum efficiency, which is unfavorable for lasing applications.

Figure 2. Demonstration of optically pumped CQD vertical-cavity surface-emitting lasers (VCSELs) with ultrashort pulsed excitation. Photographs of an (a) red and (b) green CQD-VCSEL. (c) Spectra of a red CQD-VCSEL at different pumping levels.

In our engineered CQD solid films, we have enabled single-exciton gain without compromising the high performance of type-I CQDs. We have demonstrated amplified spontaneous emission (ASE)—i.e., cavityless laser action—with stripe-geometry pumped CQD films in RGB color (see Figure 1). The laser bright spots (visible at the right-hand edges of the stripes in Figure 1) occurred when the pumping level was above ASE thresholds of 90, 145, and 800μJcm⁻² for RGB, respectively, at least an order of magnitude improvement beyond other reports.^2,4 These films all demonstrate single-exciton gain, with an average number of excitons per CQD of 〈N〉∼0.80, 0.76, and 0.73 at the ASE thresholds for RGB, respectively.¹

The high performance of the CQD film enabled us to produce the first CQD vertical-cavity surface-emitting laser (VCSEL) under optical pumping (see video⁵), a high-water mark for any potential gain medium. We created each CQD-VCSEL by depositing CQDs between two distributed Bragg reflectors (reflectance >99%). Figure 2(a) and (b) shows the coherent CQD-VCSEL beams in the red and green colors, the first well-defined laser beams from a CQD system to our knowledge. Spectral analysis of the red CQD-VCSEL demonstrated a clear threshold behavior: see Figure 2(c). The laser onset occurred at a threshold of 60μJ cm⁻², corresponding to 〈N〉=0.53. The high quality factor, low-loss cavity enables optical gain close to the intrinsic limit of single-exciton gain, a fundamental feature in this new CQD system, enabling us to overcome the inhibiting role of Auger recombination.

In summary, we achieved single-exciton gain in our epitaxial-like CQD films, demonstrating ASE of all RGB colors and the first CQD-VCSELs at record-low thresholds in red and green. The results suggest a pathway toward full-color single-material lasers via nanomaterials. We now aim to transition the low-duty-cycle pulsed RGB laser demonstrations to the steady-state, or continuous-wave, regime. To this end, heat management and thermal stability of the epitaxial-like CQD films are among the current device engineering challenges.