- Nanowires/Nanostructures
- Quantum Cascade Laser Structures
- Superconductors, Superconductor/Colossal Magneto-Resistant Superlattices
- Photonic Crystals
- Magnetic Semiconductors, Other Magnetic Material
Recently (August 2001), IBM announced the first computer circuit composed of only a single carbon nanotube molecule. This shows one of many possible uses of nanotubes as well as the extreme advances in carbon nanotube fabrication in recent years. Although it is thought that this technology may become the basis of computer technology 20 to 30 years from now, there is a plethora of physics research that must be done first. Carbon nanotubes can be as small as 10 nm across and thus one expects quantum size effects to play an important role in any device based on these materials. It is also well known that small structural differences change a nanotubes properties from metallic to semiconducting to insulating. Films composed of aligned uniform nanotubes are starting to be manufactured, making optical techniques applicable to these materials.
I propose to make a number of measurements on nanotube films and other quantum structures (such as arrays of quantum dots). The initial main focus of the measurement will be broad spectral range ellipsometry measurements of nanotube films. This will allow me to identify the relevant structures in the electronic properties. Far infrared measurements will directly probe the free carriers in metallic and semiconducting samples and show vibrational modes in semiconducting and insulating samples which will indicate homogeneity and quality of the material. Semiconducting samples also are expected to have interesting features in the near infrared and visible ranges and can be studied as a function of doping. I propose to make these measurements as a function of temperature both in order to sharpen the spectral features and to study temperature related phase changes.
In addition to these initial measurements, a number of other measurements are possible. The micro Raman instrument in Prof. Pollak’s lab can be used to measure luminescence and Raman on a scale of 10s of microns and thus determine the local structure and uniformity of the samples. In addition, nano-surface photovoltage and NSOM spectroscopy could be used to probe single nanotube properties, either in films or in isolated nanotubes. Addionally the infrared ellipsometer and microscope at Brookhaven National Labs could be used to further study infrared properties if light intensity becomes a problem.
I am currently planning collaborative work with Maria Tamargo’s group at CCNY. They are working to produce wide bandgap II-VI quantum cascade lasers. Quantum cascade lasers should work with much higher efficiency than current infrared lasers, and thus could be a vast improvement for communications lasers (an enormous industry). I believe the proposed quantum cascade laser structures offer a very good opportunity for optical studies, both to characterize and optimize the devices and to study the device physics. To pursue this, I propose to do optical studies including ellipsometry (both in the visible and infrared regimes) and pump-probe infrared studies (possibly including pump-probe ellipsometry). Some of the infrared ellipsometry and the pump-probe infrared studies would be carried out at the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratories (BNL). I have spoken with the primary NSLS infrared representative, Larry Carr, who is interested in the work and expects we can get time to perform these experiments.
Ellipsometry is a powerful probe of both the electronic states and the physical structure of a sample. At Brooklyn College, there are two ellipsometers in Prof. Pollak’s group, which can be used for this project. Together these cover the entire range from the far-infrared to the ultraviolet (0.22 – 40 μm). The infrared ellipsometer at the NSLS takes advantage of the high infrared brilliance of the synchrotron light, allowing more precise and noise-free spectra than is possible with a conventional light source and can also be used for low temperature measurements.
These experiments will give direct information about the material quality and the quality of the interfaces in the structure. In addition, ellipsometry’s sensitivity to the electronic properties of the sample should reveal information about the device performance.
Pump-probe experiments at the infrared beamlines can be carried out using a high powered Ti-Sapphire laser tunable from 0.7 – 1μm (or 0.35 – 0.5μm using a frequency doubler). Basically this is a way to excite the samples optically and see what the electrons then do naturally (hopefully they will lase).
Recently, I have been involved in some of the first low temperature ellipsometric measurements on high-Tc superconductors in the near infrared, visible, and ultraviolet regimes. Although effects due to superconductivity are expected only in the far infrared due to the small size of any known superconducting gap, clear effects have been observed in the visible. Perhaps even more interesting, I have found signals which appear to be associated with lattice transformations and possibly charge ordering at high temperatures in the visible. This could point to high energy processes anticipated by many popular, non-phonon mediated high-Tc superconductor theories (or more theories with complex mechanisms involving phonons).
In addition, I have recently been looking at the interplay of magnetism and superconductivity in superconductor/colossal magneto-resistant (CMR) superlattices. I have found that the proximity to the CMR material greatly reduces conductivity in the superconductor at all temperatures even though superconductivity persists. This elucidates some of the phenomena observed in the recently discovered Ru containing magnetic high-Tc superconductors, such as low optical conductivity in the far- and mid-infrared.
Photonic crystals are materials that are opaque to certain wavelengths of light while allowing the transmission of other wavelengths. The wavelength region where these materials are opaque is referred to as the photonic band gap. Photonic crystals offer promise for use in the next generation of communications and computing technology in which light is used instead of electrons for information transmission and processing. The unique optical properties of photonic crystals arise from the periodic variation in the refractive index of the material on a length scale near the wavelength of light.
Structures composed of photonic crystals may be useful in opto-electronics as electromagnetic micro-cavities for so-called "zero-threshold lasers" and single-mode light emitting diodes. Such structures would exhibit inhibited spontaneous emission, which could lower the power requirements and increase reliability, particularly of optical arrays. Alternately, they can show enhanced spontaneous emission which would allow faster modulation speeds for optical devices.
Various techniques are currently being tried to enhance the properties of photonic crystals, including inclusion of metallic, high refractive index, and luminescent nanoparticles. In this way, photonic crystals may be made to carry substantial electrical current but at the same time have transmission bands in the visible/infrared ranges. Some photonic crystals are also magnetic with important collective spin properties, while others may be superconductors at low temperature.
I propose to measure a variety of photonic crystals using several techniques. Again, the studies will begin with ellispsometric measurements across a broad energy range. This should reveal the basic energy structure and will allow these structures to be compared with conventional photonic materials. Since photonic crystals are usually less than 1 mm, this will require a sensitive setup and some patience. Key questions to answer are how the photonic band gap varies with thickness, overall particle size, and optical properties of the nanoparticles (or air) in the photonic crystal. In addition, I hope to be able to do some pioneering work on photonic crystals in magnetic systems, which are only beginning to be fabricated.
Naturally, photonic crystals have a periodicity of about 100-1000 nm and thus are good candidates for measurements based on microscopic measurements, such as micro- luminescence, micro-Raman, nano-surface photovoltage, and NSOM spectroscopy.
The discovery of ferromagnetism in III-Mn-V alloys is a major breakthrough that holds out possibilities of integrating giant spin-related effects into III-V-based electronics and optoelectronics. Other intriguing magnetic semiconductors are self-assembled magnetic semiconductor quantum dots (QDs) achieved by introducing Mn into the CdSe/ZnSe QD system, and semiconductor/magnetic superlattices. In these systems, not only the charge, but also the spin of the electron plays an important role in the properties of the material and any device constructed from the material. Recent experiments have demonstrated that spin-based nanostructures hold certain advantages, which make them attractive as candidates for the next generation of electronic devices.
I propose applying the optical techniques mentioned in the two previous sections to study ferromagnetic semiconductors and their multilayers. One technical problem to address is the large density of defects that form when Mn is introduced into the III-V. This type of disorder can be probed using a variety of optical tests and could lead to very productive collaborations with growers.
Other interesting magnetic materials include perovskite-type ferroelectric thin films and nano-crystalline materials, such as PbTiO3, BaTiO3, Pb(Zr,Ti)03, (Ba,Sr)TiO3, (Pb,Ba)TiO3, (Pb,La)TiO3. Properties of these materials are strongly dependent on microstructure and grain size. These materials possess large dielectric, pyroelectric, piezoelectric, and nonlinear optical coefficients that can be usefully exploited in opto-electronic and micro electronic technologies (e.g., memories, smart microsensors and actuators, multi-layer capacitors, and electro-optical modulators).
At a future date, a magnet with optical windows could be purchased to investigate (among other things) Bragg-confining systems based on diluted magnetic semiconductor multilayers. These systems offer the possibility of tuning (via an applied magnetic field) the relative band alignment between the constituent layer materials. In addition, the magnetic field induced metal-insulator transition in doped semiconductors could be investigated spectroscopically.