# Presentation: 572385

## NIRT:  Semiconductor nanostructures and photonic crystal microcavities for quantum information processing at terahertz frequenciesM. S. Sherwin and P. Petroff, UC Santa Barbara, Craig Pryor, University of Iowa, and J. Vuckovic, Stanford University, CCF-0507295

- We have fabricated, characterized, and calculated
- the quantum states of self-assembled cylindrical
- “quantum posts,” a new type of semiconductor
- nanostructure. Quantum posts have precisely-
- controllable heights which can be more than an
- order of magnitude greater than the heights of
- conventional self-assembled InGaAs quantum dots.
- The photoluminescence spectrum of a single
- 23 nm high quantum post is shown as a function
- of electric field applied along the growth direction
- (axis of the post). The red dashed line follows one
- of the peaks, exhibiting a very steep, linear red-shift
- with increasing electric field.   This red-shift occurs
- as the electron and hole are attracted to opposite
- ends of the quantum post.  The slope of the line
- increases with quantum post length, and can be used
- to measure the effective length of a post.

- J. He *et al.*, Nano Lett. **7**, 802 (2007)
- H. Krenner Physica E, in press

**Quantum posts:**
- Transmission electron
- micrographs of  23 nm
- high (top left) and 40 nm
- (top right) quantum
- posts.
- Bottom:
- interband
- photoluminescence
- spectrum vs. electric field
- (note electric field
- increases moving
- right to left)

**Notes:**
Semiconductor devices are shrinking to ever smaller length scales.  At length scales measured in nanometers, quantum physics tells us that the states which can be occupied by electrons are determined by an equation very similar to the equations for sound waves.  In an organ pipe, the frequency of the sound waves that can be emitted depends on the length of the pipe--the very long pipes make low bass notes, short pipes high treble notes.  The quantum posts that we have manufactured and are characterizing can be thought of as tiny organ pipes for electrons.  The emission from these organ pipes is in the form of light rather than sound.  Like organ pipes, quantum posts can be made in different lengths which are highly controllable.   The longest posts we can make, when occupied by a single electron, are expected to absorb and emit light at frequencies near 1012 cycles/s (1 terahertz), about 500 times smaller than  the frequency of visible light and 500 times higher than the frequency of a microwave oven.   This frequency range is technologically underserved, and reliable nanostructures that resonate at terahertz frequencies open doors to science and technology ranging from storing bits of information in a quantum information processor to creating a source of terahertz radiation for use in security-related applications.
Quantum posts can also emit radiation in the near-infrared frequency range, close to visible wavelengths.  The figure above shows that the near IR emission frequency can be strongly tuned by varying the electric field across the post.  The longer are the posts, the steeper is the tuning curve.  This strong tuning occurs because negative electrons and positive holes are pulled to opposite ends of the quantum post.  Thus, in addition to their potential for terahertz technology, quantum posts are extremely promising as quantum light sources at near-IR frequencies.  The relatively large volume of quantum posts, as compared to usual semiconductor nanostructures called quantum dots, is expected to enhance the ability to couple light emitted from quantum posts to resonators for light.  Such coupling is critical for some methods of quantum information processing.

## NIRT:  Semiconductor nanostructures and photonic crystal microcavities for quantum information processing at terahertz frequenciesM. S. Sherwin and P. Petroff, UC Santa Barbara, Craig Pryor, University of Iowa, and J. Vuckovic, Stanford University, CCF-0507295

- The development and control of quantum posts opens new doors for  quantum information processing and terahertz science and technology. Making robust, three-D quantum confined structures with transitions at terahertz frequencies is highly desirable for emitters, detectors, and other applications. An electron trapped in a 40 nm long quantum post is expected to have a 1st-excited to ground state transition near 1.7 THz (6.5 meV--see figure at right) with long lifetimes. With their relatively large volumes, quantum posts embedded in suitably designed microcavities tuned to a near-ir or terahertz resonance offer enhanced likelihood of strongly coupling the cavity mode with the quantum states of the quantum post, a necessary step for many applications in quantum information processing.

**Educational Impact:  **One elementary school science teacher, five undergraduates, 10 graduate students, and two post-doctoral researchers have been involved with activities sponsored by this grant.
- The teacher, two undergrads and one grad. students have developed two hands-on educational modules which are leant to elementary school teachers and cover *all of the California state standards for 4th-graders in the area of circuits and electricity and magnetism.*

**NIRT:  Semiconductor nanostructures and photonic crystal microcavities for quantum information processing at terahertz frequencies**M. S. Sherwin and P. Petroff, UC Santa Barbara, Craig Pryor, University of Iowa, and J. Vuckovic, Stanford University, CCF-0507295

- E (meV)

- Nanostructures for THz:
- Wave functions and
-  lowest two electronic
- energy levels computed
- for a 40 nm high quantum
- post using 8-band k.p
-  theory (C. Pryor).

**Notes:**
The science of this project is highly interdisciplinary in nature, involving a experimental and theoretical physicists, materials scientists, and electrical engineers.  Students involved in this project receive unusually broad training which prepares them extremely well for our nation’s science and engineering workforce.

In addition to technical training, this award supports a very special educational outreach program.  California, like many states, has adopted educational standards on which students are tested at various grade levels.  For example, the standards for fourth grade science include an understanding of the difference between parallel and series circuits, as well as some understanding of the relationship between electricity and magnetism.  The vast majority of elementary school teachers are not prepared to teach these concepts and there is little or no support available to them from school systems pinched by dwindling budgets.  A fourth-grade science specialist, two undergraduates (one hispanic, who will translate materials into Spanish) and a graduate student have worked together to design and build hands-on materials called “QUESTboards” that give students hands-on experience with series and parallel circuits, and the relationship between magnetism and electricity.  The teachers love these because they help their students understand and meet the standards (and, we all hope, raise test scores), the students love them because they’re fun to play with, and we love them because we get welcomed to bring a little bit of physics into elementary school classrooms.