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Hilmi Volkan Demir

Publications and source records attributed to Hilmi Volkan Demir.

27 records · Page 2Linked to original sources
arXiv2019

Control of LED Emission with Functional Dielectric Metasurfaces

The improvement of light-emitting diodes (LEDs) is one of the major goals of optoelectronics and photonics research. While emission rate enhancement is certainly one of the targets, in this regard, for LED integration to complex photonic devices, one would require to have, additionally, precise control of the wavefront of the emitted light. Metasurfaces are spatial arrangements of engineered scatters that may enable this light manipulation capability with unprecedented resolution. Most of these devices, however, are only able to function properly under irradiation of light with a large spatial coherence, typically normally incident lasers. LEDs, on the other hand, have angularly broad, Lambertian-like emission patterns characterized by a low spatial coherence, which makes the integration of metasurface devices on LED architectures extremely challenging. A novel concept for metasurface integration on LED is proposed, using a cavity to increase the LED spatial coherence through an angular collimation. Due to the resonant character of the cavity, extending the spatial coherence of the emitted light does not come at the price of any reduction in the total emitted power. The experimental demonstration of the proposed concept is implemented on a GaP LED architecture including a hybrid metallic-Bragg cavity. By integrating a silicon metasurface on top we demonstrate two different functionalities of these compact devices: directional LED emission at a desired angle and LED emission of a vortex beam with an orbital angular momentum. The presented concept is general, being applicable to other incoherent light sources and enabling metasurfaces designed for plane waves to work with incoherent light emitters.

physics.optics↗
arXiv2019

Sub-single exciton optical gain threshold in colloidal semiconductor quantum wells with gradient alloy shelling

Colloidal semiconductor quantum wells have emerged as a promising material platform for use in solution-processable light-generation including colloidal lasers. However, application relying on their optical gain suffer from a fundamental complication due to multi-excitonic nature of light amplification in common II-VI semiconductor nanocrystals. This undesirably increases the optical gain threshold and shortens the net gain lifetime because of fast nonradiative Auger decay. Here, we demonstrate sub-single exciton level of optical gain threshold in specially engineered CdSe/CdS@CdZnS core/crown@gradient alloyed shell colloidal quantum wells. This sub-single exciton ensemble-averaged gain threshold of Ng = 0.80 (per particle) resulting from impeded Auger recombination along with a large absorption cross-section of quantum wells enables us to observe the amplified spontaneous emission starting at a low pump fluence of 800 nJ cm-2, at least three-folds better than the previously best reported values among all colloidal semiconductor nanocrystals. Moreover, long optical gain lifetimes of 800 ps accompanied with modal gain coefficients of 2,000 cm-1 are achieved. Finally, using these gradient shelled quantum wells, we show a vertical cavity surface-emitting colloidal laser operating at an ultralow lasing threshold of 7.5 micro-joule cm-2. These results represent a significant step towards the realization of solution-processable electrically-driven colloidal lasers.

physics.app-ph↗
arXiv2019

Sustained biexciton emission in colloidal quantum wells assisted by dopant-host interaction

Biexcitons have been considered as one of the fundamental building blocks for quantum technology because of its overwhelming advantages in generating entangled photon pairs. Although many-body complexes have been demonstrated recently in mono-layer transition metal dichalcogenides (TMDs), the low emission efficiency and scale up capability hinder their applications. Colloidal nanomaterials, with high quantum efficiency and ease of synthesis/processing, are regarded to be an appealing complement to TMDs for biexciton sources. However, a progress towards biexciton emission in colloidal nanomaterials has been challenging largely by small binding energy and ultrafast non-radiative multiexciton recombination. Here, we demonstrate room-temperature biexciton emission in Cu-doped CdSe colloidal quantum wells (CQWs) under continuous-wave excitation with intensity as low as ~10 W/cm2. The characteristics of radiative biexciton states are investigated by their super linear emission with respect to excitation power, thermal stability and transient photophysics. The interaction between the quantum confined host carriers and the dopant ions increases biexciton binding energy by two folds compared to the undoped CQWs. Such strong binding energy together with suppressed Auger recombination and efficient, spectrally narrow photoluminescence in a quasi-2D semiconductor enables sustained biexciton emission at room temperature, providing a potential solution for efficient, scalable and stand-alone quantum devices.

cond-mat.mes-hall↗
arXiv2018

Highly efficient visible colloidal lead-halide perovskite nanocrystal light-emitting diodes

Lead-halide perovskites have been attracting attention for potential use in solid-state lighting. Following the footsteps of solar cells, the field of perovskite light-emitting diodes (PeLEDs) has been growing rapidly. Their application prospects in lighting, however, remain still uncertain due to a variety of shortcomings in device performance including their limited levels of luminous efficiency achievable thus far. Here we show high-efficiency PeLEDs based on colloidal perovskite nanocrystals (PeNCs) synthesized at room temperature possessing dominant first-order excitonic radiation (enabling a photoluminescence quantum yield of 71% in solid film), unlike in the case of bulk perovskites with slow electron-hole bimolecular radiative recombination (a second-order process). In these PeLEDs, by reaching charge balance in the recombination zone, we find that the Auger nonradiative recombination, with its significant role in emission quenching, is effectively suppressed in low driving current density range. In consequence, these devices reach a record high maximum external quantum efficiency of 12.9% reported to date and an unprecedentedly high power efficiency of 30.3 lm W-1 at luminance levels above 1000 cd m-2 as required for various applications. These findings suggest that, with feasible levels of device performance, the PeNCs hold great promise for their use in LED lighting and displays.

physics.app-ph↗
arXiv2017

Temperature-dependent Optoelectronic Properties of Quasi-2D Colloidal Cadmium Selenide Nanoplatelets

Colloidal Cadmium Selenide (CdSe) nanoplatelets (NPLs) are a recently developed class of efficient luminescent nanomaterial suitable for optoelectronic device applications. A change in temperature greatly affects their electronic bandstructure and luminescence properties. It is important to understand how-and-why the characteristics of NPLs are influenced, particularly at elevated temperature, where both reversible and irreversible quenching processes come into picture. Here we present a study on the effect of elevated temperature on the characteristics of colloidal CdSe NPLs. We used an effective-mass envelope function theory based 8-band k$\cdot$p model and density-matrix theory considering exciton-phonon interaction. We observed the photoluminescence (PL) spectra at various temperatures for their photon emission energy, PL linewidth and intensity by considering the exciton-phonon interaction with both acoustic and optical phonons using Bose-Einstein statistical factors. With rise in temperature we observed a fall in the transition energy (emission redshift), matrix element, Fermi factor and quasi Fermi separation, with reduction in intraband state gaps and increased interband coupling. Also, there was a fall in the PL intensity, along with spectral broadening due to an intraband scattering effect. The predicted transition energy values and simulated PL spectra at varying temperatures exhibit appreciable consistency with experimental results. Our findings have important implications for application of NPLs in optoelectronic devices, such as NPL lasers and LEDs, operating much above room temperature.

cond-mat.mes-hall↗
arXiv2017

Simple and Complex Metafluids and Metastructures with Sharp Spectral Features in a Broad Extinction Spectrum: Particle-Particle Interactions and Testing the Limits of the Beer-Lambert Law

Metallic nanocrystals (NCs) are useful instruments for light manipulation around the visible spectrum. As their plasmonic resonances depend heavily on the NC geometry, modern fabrication techniques afford a great degree of control over their optical responses. We take advantage of this fact to create optical filters in the visible-near IR. Our systems show an extinction spectrum that covers a wide range of wavelengths (UV to mid-IR), while featuring a narrow transparency band around a wavelength of choice. We achieve this by carefully selecting the geometries of a collection of NCs with narrow resonances that cover densely the spectrum from UV to mid-IR except for the frequencies targeted for transmission. This fundamental design can be executed in different kinds of systems, including a solution of colloidal metal NCs (metafluids), a structured planar metasurface or a combination of both. Along with the theory, we report experimental results, showing metasurface realizations of the system, and we discuss the strengths and weaknesses of these different approaches, paying particular attention to particle-particle interaction and to what extent it hinders the intended objective by shifting and modifying the profile of the planned resonances through the hybridization of their plasmonic modes. We have found that the Beer-Lambert law is very robust overall and is violated only upon aggregation or in configurations with nearly-touching NCs. This striking property favors the creation of metafluids with a narrow transparency window, which are investigated here.

cond-mat.mes-hall↗
arXiv2016

Copper-Doped Colloidal Semiconductor Quantum Wells for Luminescent Solar Concentrators

Doping of bulk semiconductors has revealed widespread success in optoelectronic applications. In the past few decades, substantial effort has been engaged for doping at the nanoscale. Recently, doped zero-dimensional quantum dots made of semiconductor nanocrystals have been demonstrated to be promising materials for luminescent solar concentrators (LSCs) as they can be engineered for providing Stokes-shifted tunable emission in the solar spectrum. However, existing doped colloidal quantum dots suffer from moderately low quantum efficiency and intrinsically small absorption cross-section, which together fundamentally limit their effective usage. Here, we show the first account of copper doping into two-dimensional atomically flat CdSe colloidal quantum wells. Copper doping via post-synthesis partial cation exchange into 3-5 monolayers of CdSe and CdSe/CdS core-shell colloidal quantum wells (CQWs) enables Stokes-shifted, tunable dopant induced highly efficient photoluminescence emission having high quantum efficiencies (>43%), accompanied by an order of magnitude higher absorption cross-section than that of the doped quantum dots. Based on these exceptional properties, we demonstrated a flexible prototype luminescent solar concentrators using these newly synthesized doped colloidal quantum wells. These findings may open up new directions for deployment of CQWs in LSCs for advanced solar light harvesting technologies.

cond-mat.mes-hall↗
arXiv2014

Plasmonic Metastructures and Nanocomposites with a Narrow Transparency Window in a Broad Extinction Spectrum

We propose and describe plasmonic nanomaterials with unique optical properties. These nanostructured materials strongly attenuate light in a broad wavelength interval ranged from 400 nm to 5 um but exhibit a narrow transparency window centered at a given wavelength. The main elements are nanorods and nano-crosses of variable sizes. The nanomaterial can be designed as a solution, nanocomposite film, metastructure or aerosol. The principle of the formation of the transparency window in the abroad extinction spectrum is based on the narrow lines of longitudinal plasmons of single nanorods and nanorod complexes. To realize the spectrum with a transmission window, we design a nanocomposite material as a mixture of nanorods of different sizes. Simultaneously, we exclude nanorods of certain length from the nanorod ensemble. The width of the plasmonic transparency window is determined by the intrinsic and radiative broadenings of the nanocrystal plasmons. We also describe the effect of narrowing of the transparency window with increasing the concentration of nanocrystals. Two well-established technologies can be used to fabricate such nano- and metamaterials, the colloidal synthesis and lithography. Nanocomposites proposed here can be used as optical materials and coatings for shielding of electromagnetic radiation in a wide spectral interval with a simultaneous possibility of communication using a narrow transparency window.

cond-mat.mes-hall↗