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Core/shell CdSe/CdZnS nanoplatelets (NPLs) are a promising class of nanomaterials for lasing applications owing to their low thresholds and high stability of stimulated emission generation as compared with many other types of colloids. Moreover, they can be self-assembled into high-quality thin films by simple methods of deposition. However, the high throughput and reproducible methods for nanopatterning of such films for advanced light-emitting applications are still missing. In this work, we show direct laser writing on thin films assembled from CdSe/CdZnS NPLs using either spin-coating or the self-assembly method for the purpose of fabricating. Using theoretical calculations, we design the period of the structures to achieve high quality-factor optical modes in the emission band of NPLs. In these nanostructured NPL films, fabricated according to the design, we observe photoluminescence enhancement and directional outcoupling effects. The proposed approach holds great potential for LEDs with improved outcoupling and for distributed feedback lasers or lasers based on bound states in the continuum with directly written optical cavities. Published version This work was supported by NIRMA and the Ministry of Science and Higher Education of the Russian Federation (Project No. 075-15-2021-589).

Metasurfaces are subwavelength-thick constructs, consisting of discrete meta-atoms, providing discretized levels of phase accumulation that collectively approximate a designed optical functionality. The meta-atoms utilizing geometric phase with polarization-converting structures produced encouraging implementations of optical components including metalenses. However, to date, a pending and fundamental problem of this approach has been the low device efficiency that such resulting components suffer, an unwanted side effect of large lattice constants used for preventing inter-coupling of their meta-atoms. Although the use of near-field coupling for tuning electromagnetic resonances found its use in constructing efficient narrow-band designs, such structures fell short of providing high efficiency over a broad spectrum. Here, we propose and show that tightly packed fabric of identical dielectric nanopillar waveguides with continuously-tuned inter-coupling distances make excellent and complete achromatic metasurface elements. This architecture enables the scatterers to interact with the incoming wave extremely efficiently. As a proof-of-concept demonstration, we showed an achromatic cylindrical metalens, constructed from strongly coupled dielectric nanopillars of a single geometry as continuously-set phase elements in a 'meta-atomless' fashion, working in the entirety of 400-700 nm band. This metalens achieves over 85 percent focusing efficiency across this whole spectral range. To combat polarization sensitivity, we used hexagonally stacked nanopillars to build up a polarization-independent scatterer library. Finally, a circular metalens with polarization-independent operation and achromatic focusing was obtained. This is a paradigm shift in making an achromatic metasurface architecture by wovening identical nanopillars coupled into an irregular lattice constructed via careful tuning., Comment: 14 pages, 5 figures, 1 table

Applied physics letters 118(8), 089901 (2021). doi:10.1063/5.0045052, Published by American Inst. of Physics, Melville, NY

High-resolution compact-size focal plane arrays (FPAs) suffer the fundamental geometrical tradeoff between the optical resolution (pixel size miniaturization) and the optical crosstalk (spillover of neighboring pixel focusing). For FPAs, our previously reported metallic metasurfaces reached an unprecedented level of crosstalk suppression. However, practical utilization of these metallic microlens arrays has proved to be intrinsically limited due to the low device efficiency (of the order of 0.10) resulting from the fundamental absorption losses of metals and their cross-polarization scheme. Exceeding this limit, here we show highly efficient microlens designs enabled by dielectric metasurfaces for mid-wave infrared (MWIR) operation. These dielectric MWIR FPAs allow for a substantially high device efficiency over 0.80 without compromising the optical crosstalk performance. Systematically studying dielectric nanoantennas of silicon nanodisks that do not dictate the cross-polarization scheme using full-wave solutions, we found that the optical crosstalk is suppressed to low levels ≤≤ 3.0% while sustaining the high efficiency. A figure-of-merit (FoM) defined for the device performance as the focusing efficiency per optical crosstalk times the f-number achieves 84, which is superior to all other types of MWIR FPAs reported to date, all falling below a maximum FoM of 70. These findings indicate that the proposed approach can pave the way for the practical usage of metasurface microlens arrays in MWIR. Published version

Due to the limitation of the hole injection, p-type doping is essential to improve the performance of InGaN/GaN multiple quantum well light-emitting diodes (LEDs). In this work, we propose and show a depletion-region Mg-doping method. Here we systematically analyze the effectiveness of different Mg-doping profiles ranging from the electron blocking layer to the active region. Numerical computations show that the Mg-doping decreases the valence band barrier for holes and thus enhances the hole transportation. The proposed depletion-region Mg-doping approach also increases the barrier height for electrons, which leads to a reduced electron overflow, while increasing the hole concentration in the p-GaN layer. Experimentally measured external quantum efficiency indicates that Mg-doping position is vitally important. The doping in or adjacent to the quantum well degrades the LED performance due to Mg diffusion, increasing the corresponding nonradiative recombination, which is well supported by the measured carrier lifetimes. The experimental results are well numerically reproduced by modifying the nonradiative recombination lifetimes, which further validate the effectiveness of our approach. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

We used photoluminescence spectroscopy to resolve two emission features in CdSe/CdMnS/CdS and CdSe/CdS core/multi-shell nanoplatelet heterostructures. The photoluminescence from the magnetic sample has a positive circular polarization with a maximum centered at the position of the lower energy feature. The higher energy feature has a corresponding signature in the absorption spectrum; this is not the case for the low-energy feature. We have also studied the temporal evolution of these features using a pulsed-excitation/time-resolved photoluminescence technique to investigate their corresponding recombination channels. A model was used to analyze the temporal dynamics of the photoluminescence which yielded two distinct timescales associated with these recombination channels. The above results indicate that the low-energy feature is associated with recombination of electrons with holes localized at the core/shell interfaces; the high-energy feature, on the other hand, is excitonic in nature with the holes confined within the CdSe cores. NRF (Natl Research Foundation, S’pore) ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

In this work, we propose a charge inverter that substantially increases the hole injection efficiency for InGaN/GaN light-emitting diodes(LEDs). The charge inverter consists of a metal/electrode, an insulator, and a semiconductor, making an Electrode-Insulator-Semiconductor (EIS) structure, which is formed by depositing an extremely thin SiO2insulator layer on the p+-GaN surface of a LED structure before growing the p-electrode. When the LED is forward-biased, a weak inversion layer can be obtained at the interface between the p+-GaN and SiO2insulator. The weak inversion region can shorten the carrier tunnel distance. Meanwhile, the smaller dielectric constant of the thin SiO2 layer increases the local electric field within the tunnel region, and this is effective in promoting the hole transport from the p-electrode into the p+-GaN layer. Due to the improved hole injection, the external quantum efficiency is increased by 20% at 20 mA for the 350 × 350 μm2LED chip. Thus, the proposed EIS holds great promise for high efficiency LEDs. Published version

The low p-type doping efficiency of the p-GaN layer has severely limited the performance of InGaN/GaN light-emitting diodes (LEDs) due to the ineffective hole injection into the InGaN/GaN multiple quantum well (MQW) active region. The essence of improving the hole injection efficiency is to increase the hole concentration in the p-GaN layer. Therefore, in this work, we have proposed a hole modulator and studied it both theoretically and experimentally. In the hole modulator, the holes in a remote p-type doped layer are depleted by the built-in electric field and stored in the p-GaN layer. By this means, the overall hole concentration in the p-GaN layer can be enhanced. Furthermore, the hole modulator is adopted in the InGaN/GaN LEDs, which reduces the effective valance band barrier height for the p-type electron blocking layer from ∼332 meV to ∼294 meV at 80 A/cm2 and demonstrates an improved optical performance, thanks to the increased hole concentration in the p-GaN layer and thus the improved hole injection into the MQWs. ASTAR (Agency for Sci., Tech. and Research, S’pore) Published version

We have studied the electroreflectance (ER) spectra of n- and p-type 4H–SiC polytype samples from 3 to 6.5 eV. The fundamental band gap and higher lying critical points are measured at room temperature. For this polytype, we observe band-gap narrowing in one of the structures with higher doping concentration.

In this letter, we present the microfabrication and application of arrays of silicon cantilever beams as microresonator sensors with nanoscale thickness to detect the mass of individual virus particles. The dimensions of the fabricated cantilever beams were in the range of 4–5 μm in length, 1–2 μm in width and 20–30 nm in thickness. The virus particles we used in the study were vaccinia virus, which is a member of the Poxviridae family and forms the basis of the smallpox vaccine. The frequency spectra of the cantilever beams, due to thermal and ambient noise, were measured using a laser Doppler vibrometer under ambient conditions. The change in resonant frequency as a function of the virus particle mass binding on the cantilever beam surface forms the basis of the detection scheme. We have demonstrated the detection of a single vaccinia virus particle with an average mass of 9.5 fg. These devices can be very useful as components of biosensors for the detection of airborne virus particles.

We introduce a proof-of-concept, optically controlled, optical switch based on the monolithic integration of a surface-illuminated photodetector and a waveguide electroabsorption modulator. We demonstrate unconstrained wavelength conversion over the entire center telecommunication wavelength band (C band) and optical switching up to 2.5 Gbit/s with extinction ratios exceeding 10 dB. Our approach offers both high-speed, low-power, switching operation and two-dimensional array scalability for the fabrication of chip-scale reconfigurable multichannel wavelength converters.

Semiconductor device-based sensing of chemical and biological entities has been demonstrated through the use of micro- and nanoscale field-effect devices and close variants. Although carbon nanotubes and silicon nanowires have been demonstrated as single molecule biosensors, the fabrication methods that have been used for creating these devices are typically not compatible with modern semiconductor manufacturing techniques and their large scale integration is problematic. These shortcomings are addressed by recent advancements in microelectronic fabrication techniques which resulted in the realization of nanowire-like structures. Here we report a method to fabricate silicon nanowires at precise locations using such techniques. Our method allows for the realization of truly integrated sensors capable of production of dense arrays. Sensitivity of these devices to changes in the ambient gas composition is also shown.

Cataloged from PDF version of article. We report enhanced sensitization of silicon through nonradiative energy transfer (NRET) of the excitons in an energy-gradient structure composed of a cascaded bilayer of green-and red-emitting CdTe quantum dots (QDs) on bulk silicon. Here NRET dynamics were systematically investigated comparatively for the cascaded energy-gradient and mono-dispersed QD structures at room temperature. We show experimentally that NRET from the QD layer into silicon is enhanced by 40% in the case of an energy-gradient cascaded structure as compared to the mono-dispersed structures, which is in agreement with the theoretical analysis based on the excited state population-depopulation dynamics of the QDs. (C) 2013 AIP Publishing LLC.

The effect of n-AlGaN versus p-AlGaN electron-blocking layers (EBLs) on the performance of InGaN/GaN light-emitting diodes is studied in this work. Experimental results suggest that the n-type EBL leads to higher optical output power and external quantum efficiency, compared to the devices with p-AlGaN EBL, which is commonly used today. Numerical simulations on the carrier distribution and energy band diagram reveal that the n-AlGaN EBL is more efficient in preventing electron overflow, while not blocking the hole injection into the active region, hence leading to higher radiative recombination rate within the multiple quantum wells active region. © 2013 AIP Publishing LLC.

InGaN/GaN light-emitting diodes (LEDs) with graded-thickness quantum barriers (GTQB) are designed and grown by metal-organic chemical-vapor deposition. The proposed GTQB structure, in which the barrier thickness decreases from the n-GaN to p-GaN side, was found to lead to an improved uniformity in the hole distribution and thus, radiative recombination rates across the active region. Consequently, the efficiency droop was reduced to 28.4% at a current density of 70 A/cm2, which is much smaller than that of the conventional equal-thickness quantum barriers (ETQB) LED, which is 48.3%. Moreover, the light output power was enhanced from 770 mW for the ETQB LEDs to 870 mW for the GTQB LEDs at 70 A/cm2. © 2013 AIP Publishing LLC. Published version

Efficient conversion from Mott-Wannier to Frenkel excitons is observed at room temperature. The time-resolved photoluminescence shows that the energy transfer rate and efficiency reach 0.262 ns(-1) and 80.9%, respectively. The energy transfer is enabled by strong dipole-dipole coupling in a hybrid inorganic/organic system of CdSe/ZnS core/shell heteronanocrystal and poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene] homopolymer composite, and the measured energy transfer efficiencies are consistent with the analytical model. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3529450]

We propose and demonstrate colloidal quantum dot hybridized, radial p-n junction based, nanopillar solar cells with photovoltaic performance enhanced by intimately integrating nanocrystals to serve as light harvesting agents around the light trapping pillars. By furnishing Si based nanopillar photovoltaic diodes with CdSe quantum dots, we experimentally showed up to sixfold enhancement in UV responsivity and similar to 13% enhancement in overall solar conversion efficiency. The maximum responsivity enhancement achieved by incorporation of nanocrystals in the nanopillar architecture is found to be spectrally more than four times larger than the responsivity enhancement obtained using planar architecture of the same device. (C) 2010 American Institute of Physics. [doi:10.1063/1.3485294]

By employing a spaced multilayer structure in a uniformly mixed P and N co-host, we developed a three-color white organic light emitting device with a stable Commission Internationale de L'Eclairage coordinates insensitive to the applied bias voltage. This insensitivity was achieved by suppressing the exciton quenching and shifting recombination zone effectively. Besides the achieved stable color coordinates of (0.411 ± 0.007, 0.382 ± 0.003), a high power efficiency of 30.7 lm/W and high color rendering index >80 were realized simultaneously. Published version

We imaged coplanar poled electro optic (EO) polymer films on transparent substrates with a multiple-photon microscope in reflection and correlated the second-harmonic light intensity with the results of Pockels coefficient (r33) measurements. This allowed us to make quantitative measurements of poled polymer films on non-transparent substrates like silicon, which are not accessible with traditional Pockels coefficient measurement techniques. Phase modulators consisting of silicon waveguide devices with EO polymer claddings with a known Pockels coefficient (from Vπ measurements) were used to validate the correlation between the second-harmonic signal and r33. This also allowed us to locally map the r33 coefficient in the poled area.

The authors present the design, growth, fabrication, experimental characterization, and theoretical analysis of blue quantum electroabsorption modulators that incorporate ∼5 nm thick In0.35 Ga0.65 NGaN quantum structures for operation between 420 and 430 nm. Growing on polar c plane on sapphire, they obtain quantum structures with zigzag potential profile due to alternating polarization fields and demonstrate that their optical absorption blueshifts with applied electric field, unlike the redshift of conventional quantum confined Stark effect. In InGaNGaN quantum structures, they report the largest absorption change of 6000 cm-1 for 6 V bias swing around 424 nm, holding promise for blue optical clock generation and injection directly into silicon chips. © 2007 American Institute of Physics.