Siddharth Dhomkar

Long-term data storage in diamond

Optical control of trapped charge in diamond makes it possible to store and retrieve arbitrary data sets in three dimensions. The negatively charged nitrogen vacancy (NV−) center in diamond is the focus of widespread attention for applications ranging from quantum information processing to nanoscale metrology. Although most work so far has focused on the NV− optical and spin properties, control of the charge state promises complementary opportunities. One intriguing possibility is the long-term storage of information, a notion we hereby introduce using NV-rich, type 1b diamond. As a proof of principle, we use multicolor optical microscopy to read, write, and reset arbitrary data sets with two-dimensional (2D) binary bit density comparable to present digital-video-disk (DVD) technology. Leveraging on the singular dynamics of NV− ionization, we encode information on different planes of the diamond crystal with no cross-talk, hence extending the storage capacity to three dimensions. Furthermore, we correlate the center’s charge state and the nuclear spin polarization of the nitrogen host and show that the latter is robust to a cycle of NV− ionization and recharge. In combination with super-resolution microscopy techniques, these observations provide a route toward subdiffraction NV charge control, a regime where the storage capacity could exceed present technologies.

Vertical correlation and miniband formation in submonolayer Zn(Cd)Te/ZnCdSe type-II quantum dots for intermediate band solar cell application

High resolution x-ray diffraction based reciprocal space mapping is employed to investigate vertical correlation in submonolayer Zn(Cd)Te/ZnCdSe type-II quantum dots (QDs). The average lateral deviation from one dot to another is found to decrease from 13%–17% to 8%–11% with an increase in QD size. Narrower photoluminescence with a better yield is obtained for the sample with improved vertical correlation, indicating smaller QD size distribution along with partial suppression of non-radiative recombination paths. Observed reduction in radiative lifetimes and supportive calculations demonstrate enhanced hole-hole wavefunction overlap pointing towards possibility of miniband formation, an advantageous feature for an intermediate band solar cell.

Determination of excitonic size with sub-nanometer precision via excitonic Aharonov-Bohm effect in type-II quantum dots

A spectral analysis of the Aharonov-Bohm (AB) oscillation in magneto-photoluminescence intensity was performed for stacked type-II ZnTe/ZnSe quantum dots (QDs). Very narrow AB oscillations (∼0.3 T) allowed for probing of both the lateral size distribution in the stack ensemble of QDs and the size of type-II excitons as determined by the electronic orbit with sub-nanometer precision. Two sets of stacks with excitonic size of 18.2 and 17.5 nm are determined to be present in the sample.

Radiative transitions in stacked type-II ZnMgTe quantum dots embedded in ZnSe

Sub-monolayer quantities of Mg are introduced in multilayer stacked ZnMgTe quantum dots (QDs) embedded in ZnSe barriers in order to reduce the hole confinement energy by controlling the bandgaps and band-offsets of ZnTe/ZnSe system having type-II band alignment. The photoluminescence (PL) emission from such ZnMgTe/ZnSe QD structure is found to be a broad band centered at 2.35 eV. The higher energy side of the PL band shows a larger blue-shift with increasing excitation intensity and a faster life-time decay due to a greater contribution of the emission from the smaller size dots and the isoelectronic bound excitons. It is found that the characteristic decay time of the PL evolves along the band with a value of 129 ns at 2.18 eV to 19 ns at 2.53 eV. The temperature dependent PL emission is controlled by two thermally activated processes: ionization of electrons away from QD state to the barrier (EA1 ∼ 3 meV) by breaking the type-II excitons and thermal escape of the holes from the ground state to the barri...

Optimization of growth conditions of type-II Zn(Cd)Te/ZnCdSe submonolayer quantum dot superlattices for intermediate band solar cells

Intermediate band solar cells (IBSCs) have been predicted to be significantly more efficient than the conventional solar cells, but have not been realized to their full potential due to the difficulties related to the fabrication of practical devices. The authors report here on growth and characterization of Zn(Cd)Te/ZnCdSe submonolayer quantum dot (QD) superlattices (SLs), grown by migration enhanced epitaxy. These QDs do not exhibit formation of wetting layers, which is one of the culprits for the unsatisfactory performance of IBSCs. The ZnCdSe host bandgap is ∼2.1 eV when lattice matched to InP, while the Zn(Cd)Te-ZnCdSe valence band offset is ∼0.8 eV. These parameters make this material system an excellent candidate for a practical IBSC. The detailed structural analysis demonstrates that the process of desorption of Cd and the preferential incorporation of Zn facilitates the formation of unintentional strained ZnSe-rich layer at the QD-spacer interface. The growth conditions have been then optimized s...

Characterization of the three-well active region of a quantum cascade laser using contactless electroreflectance

Quantum cascade (QC) lasers with emission at wavelengths below 4 μm are difficult to achieve from conventional III-V materials systems lattice matched to GaAs and InP due to the limited conduction band offset (CBO) of those materials that results from the presence of intervalley scattering. The II-VI materials ZnCdSe/ZnCdMgSe, with a CBO as high as 1.12 eV and no intervalley scattering, are promising candidates to achieve this goal. Using molecular beam epitaxy (MBE), the authors grew a QC laser structure with a three-well active region design made of ZnCdSe and ZnCdMgSe multilayers closely lattice matched to InP. A test structure, which contains only the active region of the QC laser separated by quaternary barrier layers, was also grown. The test structure was characterized by contactless electroreflectance (CER). Photoluminescence measurements and a model based on the transfer matrix method were used to identify the CER transitions. The energy levels obtained for the test structure were then used to pr...

Determination of shape anisotropy in embedded low contrast submonolayer quantum dot structures

We describe a procedure for the morphological characterization of hard-to-image submonolayer quantum dot structures. This procedure employs high resolution x-ray diffraction based reciprocal space mapping, accompanied by rigorous diffraction modeling for precise determination of the morphology of submonolayer quantum dots. Our modelling results and experimental data clearly show that the investigated quantum dots are anisotropically elongated along the [110] orientation. Complementary polarization dependent photoluminescence measurements, combined with our previously reported magneto-photoluminescence data, confirm this conclusion. Our formalism enables direct extraction of structural information of complex embedded three-dimensional structures, which, due to their low electron density contrast with respect to the surrounding host matrix, cannot be readily investigated by traditional electron diffraction techniques.

Determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer quantum dots via spectral analysis of optical signature of the Aharanov-Bohm excitons

For submonolayer quantum dot (QD) based photonic devices, size and density of QDs are critical parameters, the probing of which requires indirect methods. We report the determination of lateral size distribution of type-II ZnTe/ZnSe stacked submonolayer QDs, based on spectral analysis of the optical signature of Aharanov-Bohm (AB) excitons, complemented by photoluminescence studies, secondary-ion mass spectroscopy, and numerical calculations. Numerical calculations are employed to determine the AB transition magnetic field as a function of the type-II QD radius. The study of four samples grown with different tellurium fluxes shows that the lateral size of QDs increases by just 50%, even though tellurium concentration increases 25-fold. Detailed spectral analysis of the emission of the AB exciton shows that the QD radii take on only certain values due to vertical correlation and the stacked nature of the QDs.

Orientation-independent room temperature optical 13C hyperpolarization in powdered diamond

Shining light on diamond particles makes them MRI-“bright,” opening avenues for room temperature hyperpolarized liquids. Dynamic nuclear polarization via contact with electronic spins has emerged as an attractive route to enhance the sensitivity of nuclear magnetic resonance beyond the traditional limits imposed by magnetic field strength and temperature. Among the various alternative implementations, the use of nitrogen vacancy (NV) centers in diamond—a paramagnetic point defect whose spin can be optically polarized at room temperature—has attracted widespread attention, but applications have been hampered by the need to align the NV axis with the external magnetic field. We overcome this hurdle through the combined use of continuous optical illumination and a microwave sweep over a broad frequency range. As a proof of principle, we demonstrate our approach using powdered diamond with which we attain bulk 13C spin polarization in excess of 0.25% under ambient conditions. Remarkably, our technique acts efficiently on diamond crystals of all orientations and polarizes nuclear spins with a sign that depends exclusively on the direction of the microwave sweep. Our work paves the way toward the use of hyperpolarized diamond particles as imaging contrast agents for biosensing and, ultimately, for the hyperpolarization of nuclear spins in arbitrary liquids brought in contact with their surface.

Charge dynamics in near-surface, variable-density ensembles of nitrogen-vacancy centers in diamond

Although the spin properties of superficial shallow nitrogen-vacancy (NV) centers have been the subject of extensive scrutiny, considerably less attention has been devoted to studying the dynamics of NV charge conversion near the diamond surface. Using multicolor confocal microscopy, here we show that near-surface point defects arising from high-density ion implantation dramatically increase the ionization and recombination rates of shallow NVs compared to those in bulk diamond. Further, we find that these rates grow linearly, not quadratically, with laser intensity, indicative of single-photon processes enabled by NV state mixing with other defect states. Accompanying these findings, we observe NV ionization and recombination in the dark, likely the result of charge transfer to neighboring traps. Despite the altered charge dynamics, we show that one can imprint rewritable, long-lasting patterns of charged-initialized, near-surface NVs over large areas, an ability that could be exploited for electrochemical biosensing or to optically store digital data sets with subdiffraction resolution.