We have recently developed an ultrafast terahertz-pulse-coupled scanning tunneling microscope (THz-STM) that can
image nanoscale dynamics with simultaneous 0.5 ps temporal resolution and 2 nm spatial resolution under ambient
conditions. Broadband THz pulses that are focused onto the metallic tip of an STM induce sub-picosecond voltage
transients across the STM junction, producing a rectified current signal due to the nonlinear tunnel junction currentvoltage
(I-V) relationship. We use the Simmons model to simulate a tunnel junction I-V curve whereby a THz pulse
induces an ultrafast voltage transient, generating milliamp-level rectified currents over sub-picosecond timescales. The
nature of the ultrafast field emission tunneling regime achieved in the THz-STM is discussed.
We detail a new ultrafast scanning tunneling microscopy technique called THz-STM that uses terahertz (THz) pulses coupled to the tip of a scanning tunneling microscope (STM) to directly modulate the STM bias voltage over subpicosecond time scales . In doing so, THz-STM achieves ultrafast time resolution via a mode complementary to normal STM operation, thus providing a general ultrafast probe for stroboscopic pump-probe measurements. We use THz-STM to image ultrafast carrier trapping into a single InAs nanodot and demonstrate simultaneous nanometer (2 nm) spatial resolution and subpicosecond (500 fs) temporal resolution in ambient conditions. Extending THz-STM to vacuum and low temperature operation has the potential to enable studies of a wide variety of subpicosecond dynamics on materials with atomic resolution.
In this study, we investigated the effect of substrate temperature on the change in structural and morphological properties
of thin film Gallium Arsenide (GaAs) deposited by pulsed laser deposition (PLD) on Silicon (Si) substrate. The growths
were conducted at different substrate temperatures (25º C - 600º C). X-ray Diffraction (XRD), Atomic Force Microscopy
(AFM) and Scanning Electron Microscopy (SEM) were used to study the crystal structure and surface quality of the
films. It was observed that the films were increasingly more crystalline in the (111) orientation and also larger in crystal
grain size with increase in substrate temperature 285º C and above. The deposited GaAs films on Si were smooth, dense
and free of voids, pinholes and cracks for a wide range of temperature.
Standoff identification of explosive residues may offer early warnings to many hazards plaguing present and future
military operations. The greatest challenge is posed by the need for molecular recognition of trace explosive compounds
on real-world surfaces. Most techniques that offer eye-safe, long-range detection fail when unknown surfaces with no
prior knowledge of the surface spectral properties are interrogated. Inhomogeneity in the surface concentration and
optical absorption from background molecules can introduce significant reproducibility challenges for reliable detection
when surface residue concentrations are below tens of micrograms per square centimeter. Here we present a coupled
standoff technique that allows identification of explosive residues concentrations in the sub microgram per square
centimeter range on real-world surfaces. Our technique is a variation of standoff photoacoustic spectroscopy merged
with ultraviolet chemical photodecomposition for selective identification of explosives. We demonstrate the detection of
standard military grade explosives including RDX, PETN, and TNT along with a couple of common compounds such as
diesel and sugar. We obtain identification at several hundred nanograms per centimeter square at a distance of four
Several growths of Si nanodots on Si and GaAs substrates were conducted by pulsed laser deposition (PLD) using a KrF
laser of 248nm, 15ns, 12Hz and a Ti-sapphire laser of 800nm, 130fs, 1kHz at 1x10<sup>-5</sup>mbar vacuum. The laser fluencies on
a Si target were varied from 3 to 32J/cm<sup>2</sup> for the nanosecond (ns) PLD growths and 1-2.75J/cm<sup>2</sup> for the femtosecond (fs)
PLD. Wide range of nanodots from 20nm to a few micron size droplets were observed from both the ns and fs PLD.
Auger electron spectroscopy of the nanodots was conducted and which indicated that the nanodots were without
A technique using a mask consisting of an array of small holes was used to obtain high density nanodots with uniform
size. The array of 100nm diameter holes was created by E-beam lithography. With this technique we have achieved
100nm Si dots with 300nm spacing between them, with few defects. We have observed that laser fluences closer to the
ablation threshold work better for deposition using the EBL mask. In summary, we have demonstrated the growth of
100nm Si nanodots in an array with very few defects using the EBL masking technique.
A microfluidic flow cytometric technique capable of obtaining information on nanometer-sized organelles in single cells in a label-free, noninvasive optical manner was developed. Experimental two-dimensional (2D) light scattering patterns from malignant lymphoid cells (Jurkat cell line) and normal hematopoietic stem cells (cord blood CD34+ cells) were compared with those obtained from finite-difference time-domain simulations. In the simulations, we assumed that the mitochondria were randomly distributed throughout a Jurkat cell, and aggregated in a CD34+ cell. Comparison of the experimental and simulated light scattering patterns led us to conclude that distinction from these two types of cells may be due to different mitochondrial distributions. This observation was confirmed by conventional confocal fluorescence microscopy. A method for potential cell discrimination was developed based on analysis of the 2D light scattering patterns. Potential clinical applications using mitochondria as intrinsic biological markers in single cells were discussed in terms of normal cells (CD34+ cell and lymphocytes) versus malignant cells (THP-1 and Jurkat cell lines).