to advance
measurement science at the atomic and nanometer scale,
focusing on precision optical metrology, quantum devices,
nanoscale plasmas, and nanooptical systems.
INTENDED OUTCOME AND BACKGROUND
This strategic element focuses on developing
and exploiting precision metrology
at the interface between atomic and
nanoscale systems. Systems under study
include quantum dots and wires, ultracold
atomic quantum gases, metallic
nanoparticles, and those with nanoscale
features induced on surfaces by highly
charged ions. Such systems arise in
advanced 193 nm and 157 nm lithography,
plasma etching of semiconductor
wafers, nanolasers, detectors, biomarkers
and sensors, nanomaterials, quantum
devices and quantum information, and atomic clocks.
Our research combines theory and
experiment. Theory is used to extend
the fundamental understanding of systems
at the atomic/nanoscale interface
as necessary to interpret experiment, to
explore new applications in nanoscale
and quantum technologies, and to
motivate new and enhanced precision
metrology. We are developing the
theoretical understanding needed to
create nanooptics structures that will be
needed in emerging quantum and nanoscale technologies.
Experiment is used to develop new precision
measurement tools for this
regime, to collect precise data essential
for the applications mentioned, and to
further the understanding of these systems.
We have developed precision
metrology to make accurate displacement
measurements to subatomic
dimensions by use of frequency combs
locked to a cesium atomic clock. We
have made the precise measurements of
the refractive index of water needed by
the semiconductor industry to develop
immersion lithography for sub-100 nm
optical lithography. And we are now
expanding our expertise by beginning to
probe the nanooptics and nanomechanical
properties of nanoscale and quantum-coherent systems.
Accomplishments
Designing the Nanoworld: Nanostructures, Nanodevices, and Nanooptics
Developing and exploiting precision metrology for quantum and nanotechnology requires nanoscale modeling of ultrasmall structures, devices, their dynamical operation, and their response to probes.
Atomic-scale simulations of the electronic and optical properties of complex nanosystems at the nano/molecular interface are being carried out. These systems include nanocrystals, self-assembled dots, nanodot arrays and solids, and bio/nanohybrids. These simulations provide benchmarks for precise experimental tests of the atomic-scale sensitivity of nanosystems. The work is providing the foundation needed to build design tools for engineering nanolasers, detectors, biomarkers and sensors, quantum devices, and nanomaterials. For example, recent work has demonstrated the critical importance of strain, even in the smallest nanocrystals, for understanding the optical properties of these systems.
Nanoscale simulations of optical fields near nanosystems are also being carried out. Results are being used to design nanoprobes and nanocavities, for use in precision nanooptics metrology. Results are also being used to design and model the nanooptics highway, that is, a collection of nanoparticles used to generate, transport, and collect photons on the nanoscale, well below the diffraction limit that governs the classical transport of photons. Nanooptics highways will be critical for the quantum transport of excitations in quantum devices and in the metrology of these devices. Theory is being developed to quantify and optimize this quantum transport.
Selectable Resistance Magnetic Tunnel Junctions by Highly Charged Ion Modification
Within a broad program to explore methods to fabricate structures with novel electronic properties at the nanometer scale, we are using highly charged ions (HCIs) to produce ensembles of nano-features within magnetic tunnel junctions (MTJs). MTJs are widely recognized as probable long-term solutions to the magnetic recording industry’s need for ultrasensitive magnetic sensors for use as new hard drive “read” heads.
The leading technical challenge is producing MTJs whose resistance-area (RA) product (two dimensional resistivity) falls in a range that allows for both high signal-to-noise and large bandwidths. Contemporary approaches have focused on producing MTJ layer structures with uniform RA products, whereas our approach is to produce a layer structure that is a superposition of high and low RA product regions, whose average RA product is determined by the relative density of each region.
Our strategy is to irradiate high quality oxides with very dilute doses of HCIs, thereby introducing local regions of reduced or ablated oxide at each ion’s impact site. The HCI impact sites result in a very low RA product compared to the high RA product of the regions that were not struck by ions. The RA product of the HCI modified oxide is selected by the dose of HCIs. For example, 100 HCIs per square micrometer have been found to reduce the RA product by a factor of 100.
Laser Cooling of Nanomechanical Systems
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Figure 7. (a) Monolithic titanium cylinder housing sample and collection optics; (b) Sample package containing wafer with quantum dots; (c) Electrodes fabricated to generate surface acoustic wave. |
Using conventional cryogenic techniques, it is possible to cool nanoscale mechanical resonators to the point that their mechanical modes are near the quantum ground state. To actually reach the ground state, and to be able to measure and control the motion, we use a quantum dot embedded in the mechanical resonator to couple its mechanical energy to optical radiation. The mechanical motion of the resonator modifies the energy spectrum of the quantum dot in a manner analogous to that in which the optical spectrum of a trapped ion is modified by its motion about its equilibrium position. By detuning an incident laser beam to the red-shifted motional sideband, trapped ions can be laser cooled to the quantum ground state of the trap. A similar result is predicted for the system comprised of the nanomechanical resonator with an embedded quantum dot. In both cases, the final temperature is determined by spectroscopic measurement of the fluorescence.
We are implementing this cooling process. The first step is to study the optomechanical coupling of a high-frequency acoustic wave to a quantum dot. We have fabricated a device, as shown in Fig. 7, which creates a surface acoustic wave on a sample containing many quantum dots; one individual dot will be isolated spectroscopically. We have constructed a microscope to capture the dot fluorescence with an optical fiber. The entire microscope is cooled to below 4 K in high vacuum in a closed-cycle cryostat.
The spectroscopy is demanding; the signal is weak (a single quantum emitter) and the resolution required is more than an order of magnitude higher than that provided by grating spectrometers conventionally used in quantum dot spectroscopy. We have built a Fabry-Perot cavity with a resolution of 50 MHz and a throughput of better than 90 %. The cavity transmission is collected by a single-photon counting module. We use the cavity in tandem with a grating monochromator to unambiguously see the emission of a single quantum dot.
A Few Quantum Dot, Ultralow-Threshold Laser
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Figure 8. SEM image of 1.8 μm diameter microdisk cavity. Insert: Top-view simulation showing whispering gallery cavity modes. |
Stimulated emission and lasing are by nature cooperative phenomena and typically involves large number of emitters. A single-state laser not only represents an interesting limit to the number of contributing states, it also offers a way to probe the system’s characteristics as it moves from a continuous distribution of states, to a discrete distribution, to finally a single state. There are also technological motivations to this endeavor, as more efficient lasers requiring smaller numbers of emitters will have an impact in a variety of technologies.
A single-state laser has been recently observed in atomic systems, but requires the preparation of an ensemble of atoms in the same state. This has not been observed in solids. We have shown lasing in solids in which only a few states contribute to the lasing action. We use semiconductor quantum dots (QDs) placed in a high quality factor (Q) optical microcavity. The cavity is a microdisk structure with a diameter of 1.8 μm, and supports whispering-gallery modes in the vicinity of disk perimeter. The Q exceeds 15,000. The microdisk and simulation of the cavity mode is shown in Fig. 8.
We observe discrete QD states in the photoluminescence and we control the position of the QD states with respect to the cavity mode by adjusting the sample temperature. Even when the cavity mode is positioned where no apparent state is aligned we observe lasing. Our model indicates this is due to the coupling of tails of the emitter with the very high Q cavity. However, when the cavity mode is aligned with an emitter state, the lasing threshold drops by a factor of about 3–4. Because of off-resonance coupling of the cavity mode and emitter tails, even when a single discrete QD state is aligned with the cavity mode and lases we do not have a purely single state laser. However, this does represent the first time the alignment of such a single QD state produces such a dramatic reduction in laser threshold.
First strategic focus |
Second strategic focus |
Third strategic focus
"Technical Activities 2005-2007" - Table of Contents |
