Ari D. Feldman

Very high laser-damage threshold of polymer-derived Si(B)CN-carbon nanotube composite coatings.

We study the laser irradiance behavior and resulting structural evolution of polymer-derived silicon-boron-carbonitride (Si(B)CN) functionalized multiwall carbon nanotube (MWCNT) composite spray coatings on copper substrate. We report a damage threshold value of 15 kWcm(-2) and an optical absorbance of 0.97 after irradiation. This is an order of magnitude improvement over MWCNT (1.4 kWcm(-2), 0.76), SWCNT (0.8 kWcm(-2), 0.65) and carbon paint (0.1 kWcm(-2), 0.87) coatings previously tested at 10.6 μm (2.5 kW CO2 laser) exposure. Electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy suggests partial oxidation of Si(B)CN forming a stable protective SiO2 phase upon irradiation.

Atomic clock with 1×10(-18) room-temperature blackbody Stark uncertainty.

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

Evaluating the thermal damage resistance of graphene/carbon nanotube hybrid composite coatings.

We study laser irradiation behavior of multiwalled carbon nanotubes (MWCNT) and chemically modified graphene (rGO)-composite spray coatings for use as a thermal absorber material for high-power laser calorimeters. Spray coatings on aluminum test coupon were exposed to increasing laser irradiance for extended exposure times to quantify their damage threshold and optical absorbance. The coatings, prepared at varying mass % of MWCNTs in rGO, demonstrated significantly higher damage threshold values at 2.5 kW laser power at 10.6 μm wavelength than carbon paint or MWCNTs alone. Electron microscopy and Raman spectroscopy of irradiated specimens show that the coating prepared at 50% CNT loading endure at least 2 kW.cm−2 for 10 seconds without significant damage. The improved damage resistance is attributed to the unique structure of the composite in which the MWCNTs act as an efficient absorber of laser light while the much larger rGO sheets surrounding them, dissipate the heat over a wider area.

Electronic Enhancement of the Exciton Coherence Time in Charged Quantum Dots

Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information and photonics research. Nature sets the ultimate limit, however, given by the strength of the systems coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the optical coherence time of the charged exciton transition in quantum dots embedded in a photonic waveguide. By manipulating the electronic wave functions through an applied lateral electric field, we increase the coherence time from ∼1.4 to ∼2.7  ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to ∼6  nm, which leads to a 30% weaker transition dipole moment. The ability to electronically control the coherence time opens new avenues for quantum communication and novel coupling schemes between distant qubits.Minimizing decoherence due to coupling of a quantum system to its fluctuating environment is at the forefront of quantum information science and photonics research. Nature sets the ultimate limit, however, given by the strength of the system’s coupling to the electromagnetic field. Here, we establish the ability to electronically control this coupling and enhance the coherence time of a quantum dot excitonic state. Coherence control is demonstrated on the positively charged exciton transition (an electron Coulomb-bound with two holes) in quantum dots embedded in a photonic waveguide by manipulating the electron and hole wavefunctions through an applied lateral electric field. With increasing field up to 15 kV cm−1, the coherence time increases by a factor of two from ∼ 1.4 ns to ∼ 2.7 ns. Numerical calculations reveal that longer coherence arises from the separation of charge carriers by up to ∼ 6 nm, which leads to a 30% weaker transition dipole moment. The ability to electrostatically control the coherence time and transition dipole moment opens new avenues for quantum communication and novel coupling schemes between distant qubits. ∗ kevin.silverman@nist.gov 1 ar X iv :1 51 0. 05 58 6v 1 [ co nd -m at .m es -h al l] 1 9 O ct 2 01 5 In the solid state, three-dimensional quantum confinement of charge carriers in a semiconductor quantum dot (QD) decouples them from their surroundings, resulting in robust optical coherence of the exciton (Coulomb-bound electron-hole pairs) and trion (excitons bound to an additional electron or hole) states.[1] Excitonic coherence in QDs is a fundamental property of light-matter interaction and plays an important role in opto-electronics. From a quantum information perspective, the coherence time (T2) is a key parameter for quantum phenomena including the duration of Rabi oscillations,[2] fidelity of spin-photon entanglement,[3] cavity-emitter coupling,[4] and photon indistinguishability.[5, 6] The leading source of exciton and trion decoherence at elevated temperatures is coupling of charge carriers to phonons, which destroys coherence on a picosecond timescale. At cryogenic temperatures where electron-phonon scattering is absent, a one-nanosecond T2 time has been measured corresponding to a sub-μeV homogeneous linewidth γ (inversely proportional to T2, see Fig. 1(a)), which is limited primarily by the excited-state recombination lifetime.[7–9] Fast and deterministic control of T2 would be an enabling technology for future semiconductor QD photonic devices. Simply increasing T2 would extend the time available for coherent rotations of electronic states about the Bloch sphere. With careful control one can optimize the trade off between fast rotation times to suit the properties of the source and longevity of coherence for robust operations. The coherence time also sets the extent of the single-photon wavepacket, which can be leveraged to improve the purity and indistinguishability of single photons generated from unique QDs for linear optical quantum computing applications.[10] With control of the coherence time faster than 1/T2, more advanced operations such as dynamic tunability of the Rabi frequency and coherent storage of QD qubits can be envisioned. Despite the above-mentioned utility, control of excitonic coherence in QDs has not been intensely explored. This is especially true when it comes to increasing the coherence time. A large body of work is devoted to changing the spontaneous emission lifetime (T1) of QDs by embedding them in nanostructures with a modified vacuum density of optical states. Large enhancements of the emission rate can be achieved via the Purcell effect,[11–13] and if T1 becomes smaller than T2/2, the coherence time will decrease by further reducing T1. The radiative lifetime can also be increased in these structures, but this has no effect on the coherence time as it is already limited by pure dephasing in these cases. Alternatively, one can control the spontaneous emission rate by manipulating the electron

Morphological and Electrical Characterization of MWCNT Papers and Pellets.

Six types of commercially available multiwall carbon nanotube soot were obtained and prepared into buckypapers by pellet pressing and by filtration into a paper. These samples were evaluated with respect to thickness, compressibility and electrical conductivity. DC conductivity results by two-point and four-point (van der Pauw) measurement methods as a function of preparation parameters are presented. Topology was investigated qualitatively by way of scanning electron microscopy and helium ion microscopy and from this, some generalizations about the nanotube structural properties and manufacturing technique with respect to conductivity are given.

Novel Free-Carrier Pump-Probe Analysis of Carrier Transport in Semiconductors

We have developed a pump-probe configuration to measure the carrier lifetime using the transient free-carrier density. The free-carrier absorption varies as λ² Δn/μ, where λ is 10.6 µm in this paper. We measure the transient photoconductive decay that is proportional to Δn * μ. The data product gives Δβ * Δσ∼ λ²Δn(t)². The mobility variation is nullified by multiplying the data from the two parallel measurements. From the product data, both Δn(t) and μ(Δn) can be determined. A large increase in Δα and decrease in μ are observed and caused by space-charge effects in regions of high injection. These data show the unexpected and remarkable result that the lifetime is relatively constant up to an injection level of about three times the doping level. However, the mobility decreases by about a factor of six over the same injection range.

Transient mobility in silicon as seen by a combination of free-carrier absorption and resonance-coupled photoconductive decay

The combination of the resonance-coupled photoconductive decay (RCPCD) apparatus and a pump-probe free carrier absorption experiment results in a method of viewing transient mobility. RCPCD uses an Nd:YAG laser operating at 1064 nm to pump the p-type silicon wafer, and a microwave coil antenna detects the transient excess-carrier concentration. The pump-probe experiment uses the same pump laser and a 10.6 μm CO2 laser with HgCdTe photodetector to measure the transient change in absorption. The change in conductivity detected by RCPCD is directly proportional to the excess-carrier concentration (Δn) and mobility (μ), whereas the pump-probe experiment has an inversely proportional relationship. By mathematically combining these signals at equivalent optical fluxes, a quantity proportional to the mobility emerges. The mobility is shown to vary both temporally and with respect to injection, countering the assumption that mobility is constant for photoconductive decay measurements. Theory and results are discu...

Atomic Clock with1×10−18Room-Temperature Blackbody Stark Uncertainty

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

An atomic clock with

The Stark shift due to blackbody radiation (BBR) is the key factor limiting the performance of many atomic frequency standards, with the BBR environment inside the clock apparatus being difficult to characterize at a high level of precision. Here we demonstrate an in-vacuum radiation shield that furnishes a uniform, well-characterized BBR environment for the atoms in an ytterbium optical lattice clock. Operated at room temperature, this shield enables specification of the BBR environment to a corresponding fractional clock uncertainty contribution of 5.5×10(-19). Combined with uncertainty in the atomic response, the total uncertainty of the BBR Stark shift is now 1×10(-18). Further operation of the shield at elevated temperatures enables a direct measure of the BBR shift temperature dependence and demonstrates consistency between our evaluated BBR environment and the expected atomic response.

1\times 10^{-18}

The space charge limited current (SCLC) effect was analyzed on undoped crystalline silicon wafers at high injection levels. Space charge limited currents develop when the electric field from the injected carriers exceeds that of the background doping. This becomes a relevant phenomenon when applied to materials that use undoped wafers such as back-contact solar cells. It may also be applicable to silicon and non-silicon concentrator cells. The SCLC effect may be significant in the epitaxial thin film materials used in space photovoltaics. Our study uses Resonant Coupled Photoconductive Decay (RCPCD) to analyze and view SCLC via photoconductive carrier lifetime measurements. An undoped Si sample was subjected to the high injection of excess carries by using a pulsed laser source. The excess carrier densities were well above background doping, creating an excess carrier density of approximately 3 × 1017 cm−3. This is well above the background carrier density of 1.73 × 1013 cm−3 (measured by Capacitance-Voltage techniques). The SCLC is detected by observing irregularities, such as positive slope of the photoconductive decay curve in the initial, very high injection portion of the process. A theory of SCLC will be developed and data will be presented.