Low Threshold Quantum Dot Lasers - The Journal of Physical

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Low Threshold Quantum Dot Lasers Veena Hariharan Iyer, Rekha Mahadevu, and Anshu Pandey J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00430 • Publication Date (Web): 15 Mar 2016 Downloaded from http://pubs.acs.org on March 16, 2016

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The Journal of Physical Chemistry Letters

Low Threshold Quantum Dot Lasers Veena Hariharan Iyer, Rekha Mahadevu and Anshu Pandey* Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore 560012. AUTHOR INFORMATION Corresponding Author * [email protected]

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ABSTRACT. Semiconductor quantum dots have replaced conventional inorganic phosphors in numerous applications. Despite their overall successes as emitters, their impact as laser materials has been severely limited. Eliciting stimulated emission from quantum dots requires excitation by intense short pulses of light typically generated using other lasers. In this letter, we develop a new class of quantum dots that exhibit gain under conditions of extremely low levels of continuous wave illumination. We observe thresholds as low as 74 mW/cm2 in lasers made from these materials. Due to their strong optical absorption as well as low lasing threshold, these materials could possibly convert light from diffuse, polychromatic sources into a laser beam.

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Lasers are employed in almost every area of science and technology. Solid state laser materials such as titanium doped sapphire (Ti:Sapphire), neodymium doped garnet (Nd:YAG), as well as semiconductor laser diodes are among the most popular sources of laser radiation. Despite the ever increasing role of lasers in modern technology, the efficiencies of solid state lasers continue to suffer from major bottlenecks1,2. A prominent deficiency of solid state laser materials is their poor absorption of light from most pump sources. While the development of ~800 nm laser diodes has greatly benefitted Nd:YAG lasers, other materials such as Ti:Sapphire still require optical pumping through more cumbersome techniques. Semiconductor quantum dots (QDs) have large, broadband absorption cross sections, and also exhibit large spectral tunability of their emission bands3. While these properties are lacking in conventional solid state phosphors, QDs suffer from yet other problems that have prevented their applications in real-world lasers. Most serious is the rather severe restriction on the pump source. QDs typically require pumping by short pulsed lasers in order to exhibit amplified spontaneous emission (ASE) or gain. In a typical laboratory setting, this implies that semiconductor laser diodes are used to pump an Nd:YAG or Nd:Yittrium Lithium Fluoride laser. The output of such a laser is then frequency doubled and used to pump an amplified, short-pulsed Ti:Sapphire system. The output from such a laser is yet again frequency doubled and then used to pump QDs4,5,6. The development of QDs capable of lasing under continuous wave pump excitation is thus imperative for real-world applications. In this regard, the recent demonstration of continuous wave lasing in quantum wells is particularly noteworthy7. While quantum wells outperform existing colloidal materials, the observed threshold fluences still necessitate the use of high power laser diodes to pump these materials.


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Here we report the preparation of colloidally grown QDs that can lase at extremely low fluences, of the order of tens of milliwatts per square centimetre. The extraordinary performance of these materials arises in part from their controlled spontaneous lifetimes. Briefly, it has been conjectured8 that like three level systems, the gain properties of QDs can be improved by developing materials with suitable spontaneous emission lifetimes. Spontaneous emission is the major loss mechanism in a three level laser system. Enhancing spontaneous lifetimes ensures that energy delivered by the pump is retained for a longer time, and losses are minimized. At the same time, the stimulated emission cross section is itself a function of the spontaneous lifetime, and falls as the lifetime increases. For very long lifetimes, the laser material is thus unable to overcome cavity losses, and the onset of lasing cannot occur for even the highest fluences. It is therefore necessary to develop QDs with precisely tuned excitonic lifetimes in order to achieve gain. We synthesized ZnTe based trilayer QDs9 (see figure 1a, b and also figure S1). The ZnTe core is synthesized using a standard method. Subsequently, a ZnSe shell is over grown by successive additions of Zinc and Selenium precursors. Figure 1c shows the x-ray diffraction (XRD) patterns of ZnTe and ZnTe/ZnSe QDs. The diffraction pattern of these QDs is observed to be a straightforward sum of the diffraction patterns of the two semiconductor layers. This implies an abrupt semiconductor-semiconductor interface, as reported by us previously for other ZnTe based QDs10. An analysis of the XRD line widths confirms that the size of the core is unchanged during shell growth indicating negligible alloying at the interface (see Figure S2). In order to further ascertain that the presence of Selenium in the QD core had no bearing on photophysics, we intentionally prepared QDs where the core was an alloy with composition ZnTeSe (Figure 1d). Analysis of XRD patterns within the framework of Vegard’s law indicates a composition of


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ZnTe0.87Se0.1311. While it is possible to prepare QDs with alloyed12,13,14 as well as unalloyed cores, there are no qualitative differences in optical properties of either system. We have employed both types of cores in this work with equal success. We further over coated these structures with an alloy layer (CdZnS) in order to achieve greater electron-hole separation. In the past, the usage of a thiol has been shown to give rise to alloy QDs in the case of the CdSe/CdS system15. While our synthesis does not employ thiols as the only sulphur source, we find that the presence of dodecanethiol does lead to the formation of a CdZnS alloy(see Figure S3). Figure 1e shows the XRD patterns observed for a CdZnS over layer on ZnSe. The pattern indicates a lattice spacing intermediate to CdS and ZnS, consistent with a binary semiconductor alloy. In terms of Vegard’s law, the pattern corresponds to a Zn0.41Cd0.59S alloy11.

Figure 1a.Schematic of the core/ multishell QD material. Band offsets dictate that holes are localized to the core, while electrons are transferred to the shell. b. TEM image of the nanocrystals. Scale bar: 50 nm. c. XRD patterns observed during ZnSe shell growth. We observe that the pattern for the ZnTe/ZnSe material is a straightforward sum of the patterns of individual patterns, suggesting epitaxial growth with minimal interfacial alloying. Standard patterns are


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shown both above and below. Substrate artefacts at 33 and 38 degrees were removed. d. XRD patterns of a ZnTeSe core exhibits peak positions intermediate to ZnTe and ZnSe, consistent with a ZnTe0.87Se0.13 alloy. e. XRD patterns of the CdZnS over layer on ZnSe. As dictated by band offsets, in ZnTe/ZnSe/CdZnS QDs, holes relax to the ZnTe core, while electrons become localized to the outer CdZnS shell after complete excitonic cooling16. As a consequence, these QDs exhibit optical band gaps that are much narrower than the band gaps of their constituent semiconductors. Figure 2a shows absorption spectra observed during various stages of growth. The original ZnTe cores as well as the ZnTe/ZnSe core/shell structures have fairly wide band gaps, 3.7 and 2.5eV respectively in figure 2a. The overgrowth of CdZnS causes the band edge to red-shift significantly, and the absorption onset now occurs in the near-infrared. The band edge position varies from 2 to 1.5 eV, depending on the shell thickness. The band edge photoluminescence (PL) emission from these materials also shows a corresponding red shift (figure 2b). We analyzed the emission characteristics of a film of ZnTe/ZnSe/CdZnS QDs. This particular sample exhibits a lifetime of 0.8 µs (Figure 2c) that is expected to be suitable for low threshold continuous wave excitation of stimulated emission. Films were prepared by drop casting cleaned QDs onto an indented glass substrate. Films with 0.1 OD at 500 nm were produced in this manner. These were illuminated using a 405 nm continuous wave laser focused into a stripe with dimensions 300 µm by 2 mm using a cylindrical lens. The excitation beam was chopped at 137 Hz. Sample emission was collected using the same lens and focussed onto a silicon photodiode. The sample emission at 137 Hz was isolated using lockin detection. Figure S4 shows the least counts of this setup. Figure 2d shows the results of this measurement on a linear plot. Sample emission is associated with a very distinct threshold


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at 11mW/cm2, and falls sharply for smaller fluences. This threshold corresponds to a slope change of 362% in the emission-excitation curve. The existence of a distinct, finite threshold is consistent with the interpretation of the continuous wave pumping of optical amplification in these substrates. Additionally, the emission from these materials also exhibits unusual dependencies on the proximity to reflectors, as well as the excitation pump irradiance. These observations that consistently indicate low threshold stimulated emission are shown in figures S5-S10.

Figure 2a. Optical absorption of ZnTe (green), ZnTe/ZnSe (red), ZnTe/ZnSe/CdZnS QDs (blue and black). b. PL emission spectra observed from ZnTe/ZnSe/CdZnS (red, orange and green solid) and ZnSeTe/ZnSe/CdZnS QDs (black, dashed). c. ZnTe/ZnSe/CdZnS QDs with a lifetime of 814 ns in dilute solution. d. Consistent with ASE, films show a sharp threshold above which the samples become highly emissive. This corresponds to a threshold of 11mW/cm2. We prepared a multimode laser by drop casting a ZnSeTe/ZnSe/CdZnS QD film with lifetime 1.3 µs onto a partially reflecting (90%) mirror. A dichroic mirror reflective over 700-930 nm was placed at the other end (See Figure S11 for the mirror characterization). This combination of


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mirrors has a finesse of 52, and expected mode widths as narrow as 0.57 nm in absence of any other inhomogenieties. The QD film however scatters light, reducing the effective finesse to 3. A continuous wave pump beam at 405 nm was introduced from the side of the 800 nm mirror. The output is collected using a lens from the partially silvered end mirror. A long pass filter (495 nm) is used to separate the pump beam from the laser emission. The emission is spectrally resolved using an uncooled charge coupled device (CCD). Figure 3a shows the spectrum of laser described above. The emission lineshape is consistent with a multimode laser and also indicates the broad gain bandwidth afforded by the QD material. At increasing pump irradiances, additional modes upto 400 meV away from the central mode become observable. This is expected in general from any multimode device and has been observed in the case of multimode lasers prepared from other types of nanostructures e.g. ZnO nanowires.17-19 The number of modes in the laser can be described using a multimode laser theory for inhomogenous media.20 The scaling exponent (0.33) predicted for a system with competing modes is in good agreement with our experimentally observed behavior. This is presented in figure S12. Individual modes realized within our laser are however as narrow as 8 meV (Figure 3a inset). In contrast, to the observations made using a resonator configuration, the emission of a QD film in absence of a resonator is significantly broader (Figure 3a, dashed curve). It is also noteworthy that the emission maximum does not correspond to the most intense modes observed in the presence of a resonator. Selective transmission of an optical element can lead to the appearance of a line like pattern even in spontaneous emission. It is therefore noteworthy that the resonator employed here is most transmissive above 1.8 eV (See Figure S11). From Figure 3a, it is also apparent that no intense lines or emission is observed above this energy. This is despite the observation of


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emission from a normal sample film at these energies. By virtue of the dichroic back mirror, the resonator is least lossy at 1.55 eV. The sample emits at these wavelengths, causing a buildup of photons around this energy region within the resonator. These photons in turn cause stimulated emission from the sample, causing the energy to be funneled into these modes, while sample emission is suppressed at all other wavelengths. From figure 3a, it is also clear that there is an abrupt appearance of the emission from the device. For example, the mode observed at 1.53 eV exhibits a pump threshold of 74 mW/cm2 that corresponds to the excitation of less than one exciton per QD (Figure 3b). The mode shows a nonlinear initial increase in intensity followed by a subsequent linear increase at higher pump powers, after the onset of stable laser operation. This behavior is a hallmark of multimode laser operation, and has been predicted theoretically as well as observed in experiments involving nanostructure lasers. Consistent with the operation of a multimode laser with an inhomogenously broadened gain medium, other modes become feasible at higher powers. For example, figure 3c shows the emergence of two new modes at 102 mW/cm2 and 707 mW/cm2. A comparative study of other modes is described in figure S13. While reference 8 predicts the attainment of thresholds lower than 1 W/cm2 for materials with comparable parameters, the attainment of even lower thresholds indicates operation possibly under a sub - single exciton gain regime21. The low operation threshold suggests several potential applications for these materials. It is also apparent from Figure 3a that the spectral positions of various modes evolve with power. The spectral positions are observed to have a logarithmic dependence on the pump irradiance. The data shown in figure 3d can be fit to slopes of 2 meV and 3.5 meV respectively on a logarithmic scale.


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Figure 3. a. Continuous wave QD laser spectrum (solid lines). The pump irradiances in W/cm2 are indicated by the numbers. The dashed curve is the spectrum of a QD film, without a resonator. Inset: Modes as narrow as 8 meV are observed. b. Laser emission exhibits a distinctly nonlinear initial rise (solid, blue curve) followed by a subsequent linear evolution (dashed). The data shown here are for the mode at 1.53eV. Inset: The same data on a linear-linear plot. c. Pump dependence of modes at 1.42 and 1.32 eV. d. The peak positions of various modes have a logarithmic dependence on fluence, however the magnitude of the shift varies from line to line.

In order to fully realize practical applications of QD lasers, it is also important to demonstrate the existence of a well-defined laser beam. We show in figure 4a, a photograph of the observation of spatially well-defined beam from a QD laser when it is operated above threshold. In this figure, a 400 nm continuous wave diode pump delivers 90 mW into the QD laser. A set of lenses are employed to collimate the output; a long pass filter (450 nm) is used to separate out the laser emission from the pump light. The beam is subsequently projected on a screen. Figures S14-S18 show the behavior of incoherent emitters as well the pump dependence


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of the beam. The spatial coherence of a typical QD laser is shown in Figure 4b. Unlike light derived from an incoherence source (Light Emitting Diode (LED) with a parabolic collimator, blue diamonds in figure 4b), the QD laser emission shows no measurable divergence upto a distance of 40 cm. This data has been taken by progressively moving a photodiode detector with an 13 mm2 area away from the light source. Despite collimation, the light from a non-point-like incoherent source diverges, causing a decrease in the signal received by the detector. Instead, the intensity of emission from the QD laser source is unaltered, indicating a high degree of spatial coherence.

Figure 4. a. Photograph of the operation of a continuous wave QD laser. From left to right, 405 nm CW pump source, focusing lens, QDs in resonator, two collimating lenses (not visible), longpass filter, Laser spot on screen. The figure width approximately corresponds to 30 cm. b. The laser beam is found to have a high degree of spatial coherence. Here a detector with no collection elements is moved away from the QD laser source (red circles). The distance is measured from the collimating optic closest to the detector. No drop in signal (associated with beam divergence) is noticed for over 40 cm. For comparison, a LED source with a collimating paraboloid shows a rapidly decaying signal intensity that falls off roughly as the square of distance. The observation of low threshold lasing in QDs represents a significant new development. Combined with their broad, strong absorption bands and tunable emission, it makes QDs the most efficient and flexible among gain media. While optical amplification has been reported


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recently in a variety of other unconventional materials including organo-lead halide perovskites22,23 as well as 2-D semiconductors4,24, the threshold fluences observed in this work are the lowest in any known material system. Colloidal QDs are thought of as promising materials for various opto-electronic devices such as solar cells25,26, field-effect transistors27,28, light-emitting diodes29 and electroluminescent displays30,31. The observation of continuous wave lasing in these materials represents the realization of a long pursued goal as well as the beginning of a novel application area for these materials.

ASSOCIATED CONTENT Supporting Information. A description on the synthesis of ZnTe/ZnSe/CdZnS and ZnTeSe/ZnSe/CdZnS, characterization of CdZnS layer, fabrication of QD lasers, power dependence of QD emission spectrum, determination of absorption cross-sections, quantum yields and lifetime, evolution of line widths of the XRD pattern with shell thickness, size histogram of QDs, transmission spectra of dichroic and partially reflecting silver mirror, laser stability, power dependent emission spectra and additional sample characterization data. AUTHOR INFORMATION Notes The authors declare no competing financial interests. ACKNOWLEDGMENT AP thanks DST and IISc for generous funding. RM acknowledges CSIR for a fellowship. REFERENCES


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Low Threshold Quantum Dot Lasers - The Journal of Physical

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