Browsing by Subject "Optical gain"

Now showing 1 - 18 of 18

Open Access
Alloyed heterostructures of CdSexS1-x nanoplatelets with highly tunable optical gain performance
(American Chemical Society, 2017) Kelestemur Y.; Dede, D.; Gungor K.; Usanmaz, C. F.; Erdem, O.; Demir, Hilmi Volkan
Here, we designed and synthesized alloyed heterostructures of CdSexS1-x nanoplatelets (NPLs) using CdS coating in the lateral and vertical directions for the achievement of highly tunable optical gain performance. By using homogeneously alloyed CdSexS1-x core NPLs as a seed, we prepared CdSexS1-x/CdS core/crown NPLs, where CdS crown region is extended only in the lateral direction. With the sidewall passivation around inner CdSexS1-x cores, we achieved enhanced photoluminescence quantum yield (PL-QY) (reaching 60%), together with increased absorption cross-section and improved stability without changing the emission spectrum of CdSexS1-x alloyed core NPLs. In addition, we further extended the spectral tunability of these solution-processed NPLs with the synthesis of CdSexS1-x/CdS core/shell NPLs. Depending on the sulfur composition of the CdSexS1-x core and thickness of the CdS shell, CdSexS1-x/CdS core/shell NPLs possessed highly tunable emission characteristics within the spectral range of 560-650 nm. Finally, we studied the optical gain performances of different heterostructures of CdSexS1-x alloyed NPLs offering great advantages, including reduced reabsorption and spectrally tunable optical gain range. Despite their decreased PL-QY and reduced absorption cross-section upon increasing the sulfur composition, CdSexS1-x based NPLs exhibit highly tunable amplified spontaneous emission performance together with low gain thresholds down to ∼53 μJ/cm2.
Open Access
Amplified Spontaneous Emission and Lasing in Colloidal Nanoplatelets
(American Chemical Society, 2014-07) Guzelturk, B.; Kelestemur, Y.; Olutas, M.; Delikanli, S.; Demir, Hilmi Volkan
Colloidal nanoplatelets (NPLs) have recently emerged as favorable light-emitting materials, which also show great potential as optical gain media due to their remarkable optical properties. In this work, we systematically investigate the optical gain performance of CdSe core and CdSe/CdS core/crown NPLs having different CdS crown size with one- and two-photon absorption pumping. The core/crown NPLs exhibit enhanced gain performance as compared to the core-only NPLs due to increased absorption cross section and the efficient interexciton funneling, which is from the CdS crown to the CdSe core. One- and two-photon absorption pumped amplified spontaneous emission thresholds are found as low as 41 μJ/cm2 and 4.48 mJ/cm2 , respectively. These thresholds surpass the best reported optical gain performance of the state-of-the-art colloidal nanocrystals (i.e., quantum dots, nanorods,etc.) emitting in the same spectral range as the NPLs. Moreover, gain coefficient of the NPLs is measured as high as 650 cm 1 , which is 4-fold larger than the best reported gain coefficient of the colloidal quantum dots. Finally, we demonstrate a two-photon absorption pumped vertical cavity surface emitting laser of the NPLs with a lasing threshold as low as 2.49 mJ/cm2 . These excellent results are attributed to the superior properties of the NPLs as optical gain media.
Open Access
Colloidal optoelectronics of self-assembled quantum well superstructures
(2020-06) Erdem, Onur
Advances in the colloidal nanocrystal synthesis enabled creation of quasi twodimensional colloidal quantum wells (CQWs) in the last decade. These CQWs possess similar properties to those of epitaxially grown quantum wells while at the same time oﬀering the beneﬁts of low-cost synthesis and solubility in various solvents. Their atomically precise thickness and one-dimensional quantum conﬁnement grant them favorable properties such as narrow emission linewidth, reduced inhomogeneous broadening and giant oscillator strength. In addition, due to their quasi-two dimensional shape, they display intrinsic anisotropy. Because of this anisotropy, the particle interactions in closely packed ﬁlms depend greatly on the orientation of these CQWs. To fully utilize the interaction of CQWs with each other or with other particles in their proximity, we develop a selfassembly technique, which is used to deposit highly uniform thin CQW ﬁlms onto various solid substrates. This self-assembly technique allows us to deposit CQWs as a continuous monolayer while at the same time controlling their orientation throughout the substrate, thereby modifying their packing factor as well as nearﬁeld dipole-dipole interactions. This self-assembly technique is also employed to create large-area CQW ﬁlms of any desired thickness, simply by applying the same deposition technique on the same substrate as many times as desired. We use these self-assembled CQW ﬁlms to study the two main aspects of nanocrystal optoelectronics, namely, Förster resonance energy transfer (FRET) and optical gain, with CQWs. By using the orientation-controlled CQW monolayers, we show that the rate of FRET from colloidal quantum dots (QDs) to a monolayer of CQWs can be tuned via dipole-dipole interactions between QDs and CQWs. We use Förster’s theory of nonradiative energy transfer while taking into account the anisotropy of the excitonic CQW excitonic state as well as its delocalization throughout the CQW to account for our results. Next, we show that our multilayered CQW ﬁlms display optical gain in uncharacteriscally low thicknesses (as small as 40 nm) due to the tight packing and extremely uniform deposition of the CQWs. We furthermore study systematically the observed threshold of ampliﬁed spontaneous emission (ASE) in these CQW multilayers as a function of the ﬁlm thickness (i.e., the number of monolayers), and demonstrate that the gain threshold drops with increasing thickness, accompanied by the red-shift of the ASE peak. These trends can be explained by the varying degree of optical mode conﬁnement, which is a function of both the ﬁlm thickness as well as the wavelength of propagating mode. Our self-assembly technique allows to study and make use of the favorable properties of the CQWs including anisotropy and enhanced optical gain. Since this technique enables us to produce large-area ﬁlms displaying excellent homogeneity, it can be a benchmark building block for creating device-scale 2- or 3-dimensional superstructures from CQWs as well as from other types of colloidal nanocrystals to be utilized in both in- and out-of-plane optical applications.
Open Access
Colloidal quantum well light-emitting waveguides
(2023-07) Işık, Ahmet Tarık
Micro/nanoscale semiconductor light-emitting devices of colloidal nanocrystals offer low-cost solutions while delivering high performance in ambient lighting systems, displays, and photonic circuits. Colloidal quantum wells (CQWs) are excellent candidates as active materials for these optoelectronic devices owing to their superior properties including suppressed Auger recombination, large absorption cross-section, and narrow emission linewidth. In this thesis, as our first study, we proposed and demonstrated dual-color lasing using heterostructures of CQWs as the gain media in an all-solution-processed dual-color optical cavity for the first time. Here, a multilayered waveguide architecture consisting of green- and red-emitting CQWs, separated with a transparent low refractive index colloidal spacing layer of silica nanoparticles (NPs) suppressing otherwise detrimental nonradiative energy transfer between them, enabled amplified spontaneous emission (ASE) simultaneously in two colors at the threshold level of ~17 µJ/cm2 . We further adapted this multilayer waveguide configuration to a whispering-gallery-mode (WGM) cavity by fabricating a microdisk structure directly out of these layered CQWs-NPs-CQWs colloids. The resulting device showed dual-color multimode lasing both at 569 and 648 nm at the same time with the threshold of ~106 µJ/cm2 . Then, as the second study of this thesis, we developed a colloidal waveguide light-emitting diode (LED) structure of CQWs that changes the direction of light from the surface to the edge of the device by combining the active CQW region with a slit-shaped waveguide architecture that confines the light within the emissive layer and guides it through the lateral axis. Driving this LED waveguide of 900 µm in length by 150 µm in width at a current density level of 5.6 A/cm2 , we observed the output emission reached a luminance level of ~20,400 cd/m2 . These unique waveguiding architectures integrated into the light emitting devices of CQWs hold great promise for on-chip photonic applications including CQW dual-color excitation for biological imaging and CQW LED-based photonic integrated circuits.
Embargo
Colloidal synthesis and optical properties of heterostructured quantum wells
(2024-08) Işık, Furkan
Colloidal quantum wells (CQWs) have emerged as auspicious gain materials for next-generation colloidal nanolasers owing to their exceptional optoelectronic properties including intrinsically suppressed Auger recombination, large absorption cross-section, low cost of production, and the ability to precisely tailor their attributes. However, the realization of their photonic devices faces fundamental challenges inherent to semiconductor nanocrystals in general, which can be tackled via the design and engineering of their advanced heterostructures. In this thesis, we proposed multiple design strategies to address scientific obstacles associated with using such CQWs as gain materials and developed a variety of their rational heterostructure designs by implementing advanced synthesis techniques, allowing us to systematically study the structure-property relationship. We investigated the optical gain performance of these CQW heterostructures through spectral and temporal spectroscopy techniques to elucidate the underlying mechanisms, which guided us to improve the associated structural aspects of CQWs. This approach culminated in the development of superior CQW heterostructures possessing low optical gain thresholds, giant material gain coefficients, and long gain lifetimes, addressing all main specifications quantifying the quality of a gain material. We also presented proof-of-concept device demonstrations showcasing the advancement in the gain aspect of these CQW heterostructures, such as high-performance amplified spontaneous emission in solution and whispering gallery mode lasing with ultra-low thresholds. The findings of this thesis indicate highly engineered CQW heterostructures offer excellent gain media.
Open Access
Coreless fiber‐based whispering‐gallery‐mode assisted lasing from colloidal quantum well solids
(Wiley-VCH Verlag, 2020-01) Sak, Mustafa; Taghipour, Nima; Delikanlı, Savaş; Shendre, S.; Tanrıöver, İbrahim; Gao, Y.; Yu, J.; Yanyan, Z.; Yoo, S.; Dang, C.; Demir, Hilmi Volkan; Foroutan, Sina
Whispering gallery mode (WGM) resonators are shown to hold great promise to achieve high‐performance lasing using colloidal semiconductor nanocrystals (NCs) in solution phase. However, the low packing density of such colloidal gain media in the solution phase results in increased lasing thresholds and poor lasing stability in these WGM lasers. To address these issues, here optical gain in colloidal quantum wells (CQWs) is proposed and shown in the form of high‐density close‐packed solid films constructed around a coreless fiber incorporating the resulting whispering gallery modes to induce gain and waveguiding modes of the fiber to funnel and collect light. In this work, a practical method is presented to produce the first CQW‐WGM laser using an optical fiber as the WGM cavity platform operating at low thresholds of ≈188 µJ cm−2 and ≈1.39 mJ cm−2 under one‐ and two‐photon absorption pumped, respectively, accompanied with a record low waveguide loss coefficient of ≈7 cm−1 and a high net modal gain coefficient of ≈485 cm−1. The spectral characteristics of the proposed CQW‐WGM resonator are supported with a numerical model of full electromagnetic solution. This unique CQW‐WGM cavity architecture offers new opportunities to achieve simple high‐performance optical resonators for colloidal lasers.
Open Access
Giant modal gain coefficients in colloidal II–VI nanoplatelets
(American Chemical Society, 2019) Güzeltürk, Burak; Pelton, M.; Olutaş, Murat; Demir, Hilmi Volkan
Modal gain coefficient is a key figure of merit for a laser material. Previously, net modal gain coefficients larger than a few thousand cm–1 were achieved in II–VI and III–V semiconductor gain media, but this required operation at cryogenic temperatures. In this work, using pump-fluence-dependent variable-stripe-length measurements, we show that colloidal CdSe nanoplatelets enable giant modal gain coefficients at room temperature up to 6600 cm–1 under pulsed optical excitation. Furthermore, we show that exceptional gain performance is common to the family of CdSe nanoplatelets, as shown by examining samples having different vertical thicknesses and lateral areas. Overall, colloidal II–VI nanoplatelets with superior optical gain properties are promising for a broad range of applications, including high-speed light amplification and loss compensation in plasmonic photonic circuits.
Open Access
High-efficiency optical gain in type-II semiconductor nanocrystals of alloyed colloidal quantum wells
(American Chemical Society, 2017) Guzelturk, B.; Kelestemur Y.; Olutas M.; Li, Q.; Lian, T.; Demir, Hilmi Volkan
Colloidal nanocrystals having controlled size, tailored shape, and tuned composition have been explored for optical gain and lasing. Among these, nanocrystals having Type-II electronic structure have been introduced toward low-threshold gain. However, to date, their performance has remained severely limited due to diminishing oscillator strength and modest absorption cross-section. Overcoming these problems, here we realize highly efficient optical gain in Type-II nanocrystals by using alloyed colloidal quantum wells. With composition-tuned core/alloyed-crown CdSe/CdSexTe1-x quantum wells, we achieved amplified spontaneous emission thresholds as low as 26 μJ/cm2, long optical gain lifetimes (τgain ≈ 400 ps), and high modal gain coefficients (gmodal ≈ 930 cm-1). We uncover that the optical gain in these Type-II quantum wells arises from the excitations localized to the alloyed-crown region that are electronically coupled to the charge-transfer state. These alloyed heteronanostructures exhibiting remarkable optical gain performance are expected to be highly appealing for future display and lighting technologies.
Open Access
Highly stable, near-unity efficiency atomically flat semiconductor nanocrystals of CdSe/ZnS hetero-nanoplatelets enabled by ZnS-Shell hot-injection growth
(WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim, 2019) Yemliha, Yemliha; Quliyeva, Ulviyya; Güngör, Kıvanç; Erdem, Onur; Kelestemur, Yusuf; Mutlugün, Evren; Kovalenko, M.; Demir, Hilmi Volkan
Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c‐ALD) provides the ability to produce their core/shell heterostructures. However, as c‐ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near‐unity efficiency CdSe/ZnS NPLs are shown using hot‐injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI‐shell hetero‐NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c‐ALD shell‐coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI‐shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm−2. Despite being annealed at 500 K, these ZnS‐HI‐shell NPLs possess low gain thresholds as small as 25 µJ cm−2. These findings indicate that the proposed 573 K HI‐shell‐grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.
Open Access
Liquid interface self-assembly of colloidal nanoplatelets for optoelectronics
(Springer Singapore, 2022-10-28) Erdem, Onur; Demir, Hilmi Volkan
In this chapter, we discuss how liquid interface self-assembly can contribute to the utilization of colloidal semiconductor nanoplatelets in optoelectronics. Self-assembled nanoplatelet mono- or multilayers can be used as two-dimensional optically active waveguides, gain media of ultra-thin lasers, or energy transfer-based photosensitizers.
Open Access
Liquid-liquid diffusion ‐ assisted crystallization: a fast and versatile approach toward high quality mixed quantum dot ‐ salt crystals
(Wiley-VCH Verlag, 2015) Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.; Rengers, C.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.
Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligand-exchanged QDs into salt matrices. Furthermore, in a modified two-step approach, the seed-mediated LLDC provides the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt films as an excellent color-conversion layer is demonstrated. These findings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.
Open Access
Low-threshold optical gain and lasing of colloidal nanoplatelets
(IEEE, 2014-10) Keleştemur, Yusuf; Güzeltürk, Burak; Olutaş, Murat; Delikanlı, Savaş; Demir, Hilmi Volkan
Semiconductor nanocrystals, which are also known as colloidal quantum dots (CQDs), are highly attractive materials for high performance optoelectronic device applications such as lasers. With their size, shape and composition tunable electronic structure and optical properties, CQDs are highly desired for achieving full-color, temperature-insensitive, low-threshold and solution-processed lasers [1, 2]. However, due to their small size, they suffer from the nonradiative multiexciton Auger Recombination (AR), where energy of a bound electron-hole pair is transferred to a third particle of either an electron or a hole instead of radiative recombination. Therefore, CQDs having suppressed AR are strongly required for achieving high quality CQD-based lasers. To address this issue, CQDs having different size, shape and electronic structure have been synthesized and studied extensively [3-5]. Generally, suppression of AR and lower optical gain thresholds are achieved via reducing the wavefunction overlap of the electron and hole in a CQD. However, the separation of the electron and hole wavefunctions will dramatically decrease the oscillator strength and optical gain coefficient, which is highly critical for achieving high performance lasers. Therefore, colloidal materials with suppressed AR and high gain coefficients are highly welcomed. Here, we study optical gain performance of colloidal quantum wells [6] of CdSe-core and CdSe/CdS core/crown nanoplatelets (NPLs) that demonstrate remarkable optical properties with ultra-low threshold one- and two-photon optical pumping. As a result of their giant oscillator strength, superior optical gain and lasing performance are achieved from these colloidal NPLs with greatly enhanced gain coefficient [7]. © 2014 IEEE.
Open Access
Optical gain and lasing of colloidal semiconductor quantum wells intimately integrated into optical cavities
(2019-07) Sak, Mustafa
Colloidal semiconductor quantum wells, also known as nanoplatelets (NPLs), attract an increasingly greater deal of interest as a promising material platform for light-generating applications. The superior optical properties of NPLs including their ultra-large absorption cross-sections, purely homogeneous broadening, and suppressed Auger recombination make them highly attractive for solution-processable color convertors, LEDs and lasers. In this thesis, we studied the optical gain properties and performance levels of tailored heterostructures of such NPLs intimately integrated into various optical cavities. To do so, we systematically measured their amplified spontaneous emission under one- and twophoton absorption excitations. Also, with these hetero-NPLs as the gain media, we have proposed and demonstrated a new class of practical whispering gallery mode (WGM) NPL-fiber architecture with high stability and low lasing thresholds enabled by record low waveguide loss coefficients in its class. Moreover, we have developed a single-mode vertical-cavity surface-emitting laser (VCSEL) of these hetero-NPLs closely integrated into the wedge cavity of a pair of distributed Bragg reflectors, leading to a record low lasing threshold in its class. The findings obtained in these WGM NPL-laser and NPLVCSEL structures indicate that these NPLs are excellent for high-performance colloidal lasing.
Open Access
Optical gain in ultrathin self‐assembled bi‐layers of colloidal quantum wells enabled by the mode confinement in their high‐index dielectric waveguides
(Wiley-VCH Verlag, 2020) Foroutan-Barenji, Sina; Erdem, Onur; Gheshlaghi, Negar; Altıntaş, Yemliha; Demir, Hilmi Volkan
This study demonstrates an ultra‐thin colloidal gain medium consisting of bi‐layers of colloidal quantum wells (CQWs) with a total film thickness of 14 nm integrated with high‐index dielectrics. To achieve optical gain from such an ultra‐thin nanocrystal film, hybrid waveguide structures partly composed of self‐assembled layers of CQWs and partly high‐index dielectric material are developed and shown: in asymmetric waveguide architecture employing one thin film of dielectric underneath CQWs and in the case of quasi‐symmetric waveguide with a pair of dielectric films sandwiching CQWs. Numerical modeling indicates that the modal confinement factor of ultra‐thin CQW films is enhanced in the presence of the adjacent dielectric layers significantly. The active slabs of these CQW monolayers in the proposed waveguide structure are constructed with great care to obtain near‐unity surface coverage, which increases the density of active particles, and to reduce the surface roughness to sub‐nm scale, which decreases the scattering losses. The excitation and propagation of amplified spontaneous emission (ASE) along these active waveguides are experimentally demonstrated and numerically analyzed. The findings of this work offer possibilities for the realization of ultra‐thin electrically driven colloidal laser devices, providing critical advantages including single‐mode lasing and high electrical conduction.
Open Access
Optical microfluidic waveguides and solution lasers of colloidal semiconductor quantum wells
(2020-07) Maskoun, Joudi
Microﬂuidics has become an important technology platform oﬀering many applications including point-of-care systems, lab-on-a-chip (LOC) devices, and drug delivery and separation. For this technology to reach its full potential, many improvements and components are being heavily researched and utilized to help broaden the range of its applications. One such important application is the implementation of lasers in microﬂuidic networks. Microﬂuidic lasers are being employed as sensors and light sources for use in chemical and biological reaction promoting and ﬂow cytometry. Microﬂuidic ampliﬁed spontaneous emission (ASE) and lasing using ﬂuorescent dyes embedded in liquid-liquid waveguides has been previously reported. The performance of these devices may be signiﬁcantly improved using colloidal semiconductor quantum wells, also known as nanoplatelets (NPLs), which possess optical properties desirable for lasing. In this work, diﬀerent than previous works, optical microﬂuidic waveguides and solution lasers of NPLs are proposed and demonstrated. To this end, a Fabry-P´erot cavity is created in a microﬂuidic channel encapsulated with polydimethylsiloxane (PDMS) to achieve in-solution lasing with NPLs. The microﬂuidic devices are fabricated using soft lithography and implemented as a platform for observing optical gain from NPLs. Because of its many advantages over other materials for microﬂuidic devices, such as its ease of fabrication, solvent compatibility, transparency and availability, PDMS is chosen as the base material for our microﬂuidic device. Combined with the desirable optical properties of the NPLs, PDMS can provide easy integration of laser media into ﬂexible microﬂuidic networks. Using capillary as well as pressure-driven ﬂows, record low optical gain thresholds were achieved. Using capillary forces, single-mode lasing was demonstrated on an on-chip Fabry-P´erot cavity from red-emitting NPLs. The use of pressure-driven ﬂow allowed for the observation of gain from a liquid-liquid waveguide. These microfabricated NPL solution lasers have the potential to provide compact and inexpensive coherent light sources for applications in microﬂuidics and integrated optics.
Open Access
Thickness-tunable self-assembled colloidal nanoplatelet films enable ultrathin optical gain media
(American Chemical Society, 2020) Erdem, Onur; Foroutan, Sina; Gheshlaghi, Negar; Güzeltürk, B.; Altıntaş, Yemliha; Demir, Hilmi Volkan
We propose and demonstrate construction of highly uniform, multilayered superstructures of CdSe/CdZnS core/shell colloidal nanoplatelets (NPLs) using liquid interface self-assembly. These NPLs are sequentially deposited onto a solid substrate into slabs having monolayer-precise thickness across tens of cm2 areas. Because of near-unity surface coverage and excellent uniformity, amplified spontaneous emission (ASE) is observed from an uncharacteristically thin film having 6 NPL layers, corresponding to a mere 42 nm thickness. Furthermore, systematic studies on optical gain of these NPL superstructures having thicknesses ranging from 6 to 15 layers revealed the gradual reduction in gain threshold with increasing number of layers, along with a continuous spectral shift of the ASE peak (∼18 nm). These observations can be explained by the change in the optical mode confinement factor with the NPL waveguide thickness and propagation wavelength. This bottom-up construction technique for thickness-tunable, three-dimensional NPL superstructures can be used for large-area device fabrication.
Open Access
Ultralow-threshold up-converted lasing in oligofluorenes with tailored strong nonlinear absorption
(Royal Society of Chemistry, 2015) Guzelturk, B.; Kanibolotsky, A.L.; Orofino-Pena, C.; Laurand, N.; Dawson, M.D.; Skabara P.J.; Demir, Hilmi Volkan
Nonlinear optical response in organic semiconductors has been an attractive property for many practical applications. For frequency up-converted lasers, to date, conjugated polymers, fluorescent dyes and small organic molecules have been proposed but their performances have been severely limited due to the difficulty in simultaneously achieving strong nonlinear optical response and high performance optical gain. In this work, we show that structurally designed truxene-based star-shaped oligofluorenes exhibit strong structure-property relationships enabling enhanced nonlinear optical response with favorable optical gain performance. As the number of fluorene repeat units in each arm is increased from 3 to 6, these molecules demonstrate a two-photon absorption cross-section as high as 2200 GM, which is comparable to that of linear conjugated polymers. Tailored truxene oligomers with six fluorene units in each arm (T6) show two-photon absorption pumped amplified spontaneous emission with a threshold as low as 2.43 mJ cm-2, which is better than that of the lowest reported threshold in organic semiconductors. Furthermore, we show a frequency up-converted laser using the newly designed and synthesized star-shaped oligomer T6 with a threshold as low as 3.1 mJ cm-2, which is more than an order of magnitude lower than that of any conjugated polymer. Thus, these oligomers with enhanced nonlinear optical properties are highly attractive for bio-integrated applications such as photodynamic therapy and in vivo bio-sensing. © The Royal Society of Chemistry 2015.
Open Access
Unraveling the ultralow threshold stimulated emission from CdZnS/ZnS quantum dot and enabling high ‐ Q microlasers
(Wiley-VCH Verlag, 2015) Wang Y.; Fong, K. E.; Yang, S.; Ta, V.; Gao, Y.; Wang, Z.; Nalla, V.; Demir, Hilmi Volkan; Sun, H.
The newly engineered ternary CdZnS/ZnS colloidal quantum dots (CQDs) are found to exhibit remarkably high photoluminescence quantum yield and excellent optical gain properties. However, the underlying mechanisms, which could offer the guidelines for devising CQDs for optimized photonic devices, remain undisclosed. In this work, through comprehensive steady-state and time-resolved spectroscopy studies on a series of CdZnS-based CQDs, we unambiguously clarify that CdZnS-based CQDs are inherently superior optical gain media in the blue spectral range due to the slow Auger process and that the ultralow threshold stimulated emission is enabled by surface/interface engineering. Furthermore, external cavity-free high-Q quasitoroid microlasers were produced from self-assembly of CdZnS/ZnS CQDs by facile inkjet printing technique. Detailed spectroscopy analysis confirms the whispering gallery mode lasing mechanism of the quasitoroid microlasers. This tempting microlaser fabrication method should be applicable to other solution-processed gain materials, which could trigger broad research interests. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA.