Quantum Dots - Laser Application
II-VI semiconductor colloidal Quantum Dots (QDs) are highly fluorescent nanocrystals which are prepared through organometallic synthesis in solution phase. They have been widely used in practice as fluorescent labels in many bio-imaging applications. Quasi-zero dimensional Wannier exciton states arise in QDs as a result of the strong quantum confinement which makes QDs be very interesting and unique: Just change the size of QDs (no need to change material), emission colors will change. Bigger QDs will emit redder light and smaller QDs will emit bluer. Extensive research has been carried out to exploit their associated electronic/optical properties.
Strong quantum confinement in 3 dimensions enhances density of states at band edge leading to low lasing threshold. Controllable emission wavelength of QDs allows making laser at any colors, which are not available for semiconductor laser at this moment. Difference of energy in electron states in QDs is large enough to inhibit depopulation of lowest states, which results in temperature insensitive lasing threshold.
Full Color Amplified Spontaneous Emission (ASE) Results:
a, Experimental configuration with a cylindrical lens focusing ultrashort (100 fs) pulse laser excitation at a fixed wavelength of λ = 400 nm as a stripe on a CQD film (with a slit for precise stripe length control). b, Plan view photographs of emission from excited stripes of RGB CQD films; stripe length and width are 1 mm and 10.5 µm, respectively. The RGB bright spots at the film edges seen above threshold levels are clear visual evidence of ASE. c, Edge emission as a function of pump energy density for RGB CQD films, with arrows indicating ASE energy density thresholds of 90 (red), 145 (green) and 800 µJ cm−2 (blue), respectively. d,e, Spectral analysis of edge emission, together with absorption peaks of the lowest exciton state for red and green CQD films. The lowest exciton absorption peaks were isolated from full spectrum absorbance results. The full-width at half-maximum (FWHM) of the spectral peaks narrows from 28 nm to 7 nm for these red and green CQD films.
Very low ASE threshold enable by single exciton gain in type I colloidal quantum dots alows us to make the first practical colloidal-quantum-dot vertical-cavity surface-emitting laser (CQD-VCSEL).
a, Schematic of a vertically pump CQD-VCSEL with a long pass filter to remove any residual pump excitation beam. CQD gain medium is placed inside a wedge cavity for a variable cavity length. The wedge angle is 1.2 × 10−3 rad, and two DBRs have reflectivity higher than 99%. b,c, Photographic images of red and green CQD-VCSELs showing spatially well-defined output beams, which are collinear with the pump beam. d, Spectra from a red CQD-VCSEL structure below and above threshold. Inset: single-mode lasing for a green CQD-VCSEL from a shorter cavity. From the linewidth of laser emission, the quality factor of the cavity was estimated to be ~1,300. e, Emergence of laser modes from spontaneous emission in a CQD-VCSEL when increasing pump power.
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