Papers by Venkatram Nalla
Solution-Grown CsPbBr<sub>3</sub>/Cs<sub>4</sub>PbBr<sub>6</sub>Perovskite Nanocomposites: Toward Temperature-Insensitive Optical Gain
Small, Jul 11, 2017
With regards to developing miniaturized coherent light sources, the temperature-insensitivity in ... more With regards to developing miniaturized coherent light sources, the temperature-insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature-insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II-VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands-free CsPbBr3 nanocrystals in a wider band gap Cs4 PbBr6 matrix by solution-phase synthesis. The unique electronic structures of CsPbBr3 nanocrystals enable temperature-insensitive gain spectrum while the lack of ligands and protection from Cs4 PbBr6 matrix ensure the thermal stability and high temperature operation. Specifically, a color drift-free stimulated emission irrespective of temperature change (20-150 °C) upon two-photon pumping is presented and the characteristic temperature is determined to be as high as ≈260 K. The superior gain properties of the CsPbBr3 /Cs4 PbBr6 perovskite nanocomposites are directly validated by a vertical cavity surface emitting laser operating at temperature as high as 100 °C. The results shed light on manipulating optical gain from the advantageous CsPbBr3 nanocrystals and represent a significant step toward the temperature-insensitive frequency-upconverted lasers.
Laser & Photonics Reviews, Aug 19, 2015
The newly engineered ternary CdZnS/ZnS colloidal quantum dots (CQDs) are found to exhibit remarka... more 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.
Asian Journal of Physics, Jun 30, 2021
Dissipation of electromagnetic energy through absorption is a fundamental process that underpins ... more Dissipation of electromagnetic energy through absorption is a fundamental process that underpins phenomena ranging from photovoltaics to photography, analytical spectroscopy, photosynthesis, and human vision. Absorption is also a dynamic process that depends on the duration of the optical illumination. Here we report on the resonant plasmonic absorption of a nanostructured metamaterial and the non-resonant absorption of an unstructured gold film at different optical pulse durations. By examining the absorption in travelling and standing waves, we observe a plasmonic relaxation time of 11 fs as the characteristic transition time. The metamaterial acts as a beam-splitter with low absorption for shorter pulses, while as a good absorber for longer pulses. The transient nature of the absorption puts a frequency limit of ~90 THz on the bandwidth of coherently-controlled, all-optical switching devices, which is still a thousand times faster than other leading switching technologies.
Nature Communications, Jan 12, 2018
Recently, coherent control of the optical response of thin films of matter in standing waves has ... more Recently, coherent control of the optical response of thin films of matter in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to demonstrations of all-optical image processing. Here we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, that controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fully fiberized network at wavelengths between 1530 nm and 1565 nm. The metadevice performance has been tested with optical signals equivalent to a bitrate of up to 40 Gbit/s and sub-milliwatt power levels. Since coherent absorption can operate at the single photon level and also with 100 THz bandwidth, w...
Angewandte Chemie, Oct 24, 2017
Multi-photon absorption (MPA) is among the most prominent nonlinear optical (NLO) effects and has... more Multi-photon absorption (MPA) is among the most prominent nonlinear optical (NLO) effects and has applications, for example in telecommunications, defense, photonics and biomedicines. Established MPA materials include dyes, quantum dots, organometallics and conjugated polymers, most often dispersed in solution. We demonstrate how metal-organic frameworks (MOFs), a novel NLO solid-state materials class, can be designed for exceptionally strong MPA behavior. MOFs consisting of zirconiumand hafnium-oxo-clusters and featuring a chromophore linker based on the tetraphenylethene (TPE) molecule exhibit record high twophoton absorption (2PA) cross section values, up to 3600 GM. The unique modular building-block principle of MOFs allows enhancing and optimizing their MPA properties in a theory guided approach by combining tailored charge polarization, conformational strain, threedimensional arrangement and alignment of the chromophore linkers in the crystal.
100 THz bandwidth optical modulators via coherent absorption of metamaterial
All-optical signal processing is one of the rising fields to eliminate the disadvantages of optic... more All-optical signal processing is one of the rising fields to eliminate the disadvantages of optical–electrical – optical conversion and continuing advances in terabits per second communications for high-performance computing. All-optical modulation via Coherent Perfect Absorption is the control of the phase or intensity of one light beam by another. A device based on coherent absorption has the advantage of being compact, fast, and intrinsically low power while demonstrating large modulations of light. Using femtosecond laser with variable pulse duration we probe the limits of switching that exploits coherent absorption in nanostructured gold and diamond metamaterials. Higher switching contrast ratios and modulation bandwidth exceeding 100THz has been observed.
, ICMAT& IUMRS-ICA. It incorporates referee's comments but changes resulting from the publishing ... more , ICMAT& IUMRS-ICA. It incorporates referee's comments but changes resulting from the publishing process, such as copyediting, structural formatting, may not be reflected in this document. The published version is available at: [].
Advanced Materials, Feb 27, 2018
We present the first experimental demonstration of coherent light-by-light modulation at fewoptic... more We present the first experimental demonstration of coherent light-by-light modulation at fewoptical-cycle (6 fs) pulse durations, enabled by a nanostructured polycrystalline diamond broadband metasurface absorber only 170 nm thick. Photonic metamaterials -artificial media, structurally engineered at the sub-wavelength scale to provide passive and active functionalities not available in natural materials, enjoy considerable attention, not least as planar media with strong, controllable absorption for applications in light energy harvesting, photo-catalysis, photovoltaics, photo-detection and displays. Recently, 'coherent control' of absorption has emerged as a new mechanism for controlling light-with-light, with potential applications ranging from signal processing in coherent information networks to excitation-selective spectroscopy. The concept is truly scalable, having been demonstrated at wavelengths from visible to microwave, and can function at very low intensities (down to the single photon level ) and in the quantum regime. It allows for the implementation of various all-optical analogue logical functions in 'four-port devices' (with two inputs and two outputs), which in the simplest case may comprise a thin (substantially sub-wavelength thickness) layer of absorbing material illuminated from both sides by two mutually coherent
Nonlinear optical and exciton properties of CuInS2 nanocrystals
Journal of Physical Chemistry C, Jul 23, 2009
Asian Journal of Physics, 2021
Dissipation of electromagnetic energy through absorption is a fundamental process that underpins ... more Dissipation of electromagnetic energy through absorption is a fundamental process that underpins phenomena ranging from photovoltaics to photography, analytical spectroscopy, photosynthesis, and human vision. Absorption is also a dynamic process that depends on the duration of the optical illumination. Here we report on the resonant plasmonic absorption of a nanostructured metamaterial and the non-resonant absorption of an unstructured gold film at different optical pulse durations. By examining the absorption in travelling and standing waves, we observe a plasmonic relaxation time of 11 fs as the characteristic transition time. The metamaterial acts as a beam-splitter with low absorption for shorter pulses, while as a good absorber for longer pulses. The transient nature of the absorption puts a frequency limit of ~90 THz on the bandwidth of coherently-controlled, all-optical switching devices, which is still a thousand times faster than other leading switching technologies.
The development of coherent optical networks and processing are catalysing increasing attention a... more The development of coherent optical networks and processing are catalysing increasing attention as solutions to accelerate the data transfer speed and data processing. Conventional technology in coherent optical networks can perform at the maximum speed of 100 Gb/s. Here we evaluate the effect of plasmonic finite response time on the coherent perfect absorption process for a plasmonic metamaterial absorber, to achieve 100 Tb/s. All-optical modulation means control of the phase or intensity of one light beam by another. In the coherent perfect absorption scenario, the interference of two counter-propagating coherent beams on a highly absorbing material of sub- wavelength thickness can either lead to nearly total transmission or to nearly total absorption of the incident light, depending on their mutual intensity and phase. We study the coherent modulation of the total energy as a function of the pulse duration, from few hundreds fs down to 6 fs. Our measurements allow us to assess th...
Optical nonlinearity in plasmonic metal-cored fibres
Nature communications, Jan 12, 2018
Recently, coherent control of the optical response of thin films in standing waves has attracted ... more Recently, coherent control of the optical response of thin films in standing waves has attracted considerable attention, ranging from applications in excitation-selective spectroscopy and nonlinear optics to all-optical image processing. Here, we show that integration of metamaterial and optical fibre technologies allows the use of coherently controlled absorption in a fully fiberized and packaged switching metadevice. With this metadevice, which controls light with light in a nanoscale plasmonic metamaterial film on an optical fibre tip, we provide proof-of-principle demonstrations of logical functions XOR, NOT and AND that are performed within a coherent fibre network at wavelengths between 1530 and 1565 nm. The metadevice has been tested at up to 40 gigabits per second and sub-milliwatt power levels. Since coherent absorption can operate at the single-photon level and with 100 THz bandwidth, we argue that the demonstrated all-optical switch concept has potential applications in c...
Advanced materials (Deerfield Beach, Fla.), Jan 27, 2018
Diamond is introduced as a material platform for visible/near-infrared photonic metamaterials, wi... more Diamond is introduced as a material platform for visible/near-infrared photonic metamaterials, with a nanostructured polycrystalline diamond metasurface only 170 nm thick providing an experimental demonstration of coherent light-by-light modulation at few-optical-cycle (6 fs) pulse durations. "Coherent control" of absorption in planar (subwavelength-thickness) materials has emerged recently as a mechanism for high-contrast all-optical gating, with a speed of response that is limited only by the spectral width of the absorption line. It is shown here that a free-standing diamond membrane structured by focused ion beam milling can provide strong, spectrally near-flat absorption over a visible to near-infrared wavelength range that is wide enough (wider than is characteristically achievable in plasmonic metal metasurfaces) to facilitate coherent modulation of ultrashort optical pulses comprising only a few oscillations of electromagnetic field.
Angewandte Chemie International Edition, 2017
Multi‐photon absorption (MPA) is among the most prominent nonlinear optical (NLO) effects and has... more Multi‐photon absorption (MPA) is among the most prominent nonlinear optical (NLO) effects and has applications, for example in telecommunications, defense, photonics, and bio‐medicines. Established MPA materials include dyes, quantum dots, organometallics and conjugated polymers, most often dispersed in solution. We demonstrate how metal–organic frameworks (MOFs), a novel NLO solid‐state materials class, can be designed for exceptionally strong MPA behavior. MOFs consisting of zirconium‐ and hafnium‐oxo‐clusters and featuring a chromophore linker based on the tetraphenylethene (TPE) molecule exhibit record high two‐photon absorption (2PA) cross‐section values, up to 3600 GM. The unique modular building‐block principle of MOFs allows enhancing and optimizing their MPA properties in a theory‐guided approach by combining tailored charge polarization, conformational strain, three‐dimensional arrangement, and alignment of the chromophore linkers in the crystal.
Advanced Functional Materials, 2017
Recently, all-inorganic cesium lead halide perovskite nanocrystals (IPNCs) (CsPbX 3 , X = Cl, Br,... more Recently, all-inorganic cesium lead halide perovskite nanocrystals (IPNCs) (CsPbX 3 , X = Cl, Br, I) were discovered to possess superior optical gain properties appealing for solutionprocessed cost-effective lasers. Yet, the potential of such materials has not been exploited for practical laser devices, rendering the prospect as laser media elusive. Herein, we realized for the first time the challenging but practically desirable vertical cavity surface emitting lasers (VCSELs) based on the CsPbX 3 IPNCs, featuring low threshold (9 μJ/cm 2 ), unidirectional output (beam divergence of ~3.6º) and favorable stability. The lasing wavelength can be tuned across red, green and blue region maintaining comparable thresholds, which is promising in developing single source-pumped full-color visible lasers. It is fully demonstrated that the characteristics of the VCSELs can be versatilely engineered by independent adjustment of the cavity and solution processable nanocrystals. Our results represent a significant leap towards practical laser sources leveraging on the advantageous CsPbX 3 IPNCs.
Chemistry of Materials, 2017
The development of alternative nonlinear optical metamaterials has attracted much attention recen... more The development of alternative nonlinear optical metamaterials has attracted much attention recently due to technological demands. Upconversion emission via a simultaneous two-photon absorption process is a nonlinear process that is widely studied in synthetically challenging organic compounds. Hereby, we report 9 metal organic frameworks constructed with various combinations of the following ligands: trans,trans-9,10-bis(4pyridylethenyl) anthracene, trans,trans-9,10-bis(4-pyridylethynyl) anthracene, 1,4-bis[2-(4′-pyridyl)ethenyl]benzene, 4,4′-stilbene dicarboxylate, 4,4′-biphenyl dicarboxylate, 4,4′-benzene dicarboxylate, and benzene-1,3,5-tricarboxylate. Altering the auxiliary carboxylate ligands not only changes the structure but also varies the two-photon excited fluorescence. The two-photon excited emission is enhanced when longer spacer ligands are used and when they are packed in more expanded structures in hms topology. Unusually, the emission becomes stronger when a pair of pyridyl type ligands are perfectly aligned in parallel which could be due to reduction in nonradiative decay caused by molecular rotation. The comparison of two-photon absorption cross sections with their action cross section counterpart revealed a dissimilar trend. High level of absorption in MOFs does not necessitate the formation of a highly excited emissive state. To the best of our knowledge, this is the first example of a systematic structural-property relationship study on the two-photon excited fluorescence in metal organic frameworks.
Solution-Grown CsPbBr3 /Cs4 PbBr6 Perovskite Nanocomposites: Toward Temperature-Insensitive Optical Gain
Small (Weinheim an der Bergstrasse, Germany), Jan 11, 2017
With regards to developing miniaturized coherent light sources, the temperature-insensitivity in ... more With regards to developing miniaturized coherent light sources, the temperature-insensitivity in gain spectrum and threshold is highly desirable. Quantum dots (QDs) are predicted to possess a temperature-insensitive threshold by virtue of the separated electronic states; however, it is never observed in colloidal QDs due to the poor thermal stability. Besides, for the classical II-VI QDs, the gain profile generally redshifts with increasing temperature, plaguing the device chromaticity. Herein, this paper addresses the above two issues simultaneously by embedding ligands-free CsPbBr3 nanocrystals in a wider band gap Cs4 PbBr6 matrix by solution-phase synthesis. The unique electronic structures of CsPbBr3 nanocrystals enable temperature-insensitive gain spectrum while the lack of ligands and protection from Cs4 PbBr6 matrix ensure the thermal stability and high temperature operation. Specifically, a color drift-free stimulated emission irrespective of temperature change (20-150 °C) u...
Nonlinear optical studies of Phthalocyanines in solutions and thin films of PMMA studied using cw and nanosecond pulse excitation
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Papers by Venkatram Nalla