Danielle R. Chamberlin

Real-time imaging using a 2.8 THz quantum cascade laser and uncooled infrared microbolometer camera

Real-time imaging in the terahertz (THz) spectral range was achieved using a milliwatt-scale, 2.8 THz quantum cascade laser and an uncooled, 160 x 120 pixel microbolometer camera modified with Picarin optics. Noise equivalent temperature difference of the camera in the 1-5 THz frequency range was estimated to be at least 3 K, confirming the need for external THz illumination when imaging in this frequency regime. Despite the appearance of fringe patterns produced by multiple diffraction effects, single-frame and extended video imaging of obscured objects show high-contrast differentiation between metallic and plastic materials, supporting the viability of this imaging approach for use in future security screening applications.

Using a ferrocenylsilane-based block copolymer as a template to produce nanotextured Ag surfaces: uniformly enhanced surface enhanced Raman scattering active substrates

We report a method of fabricating nanotextured Ag surfaces using a template of self-assembled inorganic-containing block copolymer, polystyrene-b-polyferrocenylsilane. The Ag surfaces with periodically ordered nanoscale features created by the self-organized block copolymer are capable of producing enhanced Raman signals. Using benzenethiol as a probe molecule, an enhancement factor of up to 106 has been observed. More importantly, the enhancement is very uniform; less than 10% Raman signal variation has been obtained. Furthermore, since the size and spacing of the nanostructures can be adjusted by tailoring the polymer chain length, the electromagnetic field can potentially be tuned to achieve even higher surface-enhanced Raman scattering (SERS) activity. This inorganic-containing block copolymer template approach not only provides a simple and straightforward method to fabricate SERS active substrates but also offers a means to experimentally examine the SERS mechanism.

New p-Ge THz laser spectrometer for the study of solutions: THz absorption spectroscopy of water

We present the development of a high power, tunable far-infrared p-germanium laser spectrometer for the study of dissolved biomolecules in the THz range. As a first application we report on the measurement of the absorption coefficient α for liquid water in the frequency range from 81to96cm−1. Using the p-Ge laser spectrometer we were able to penetrate through layers of up to 100μm thickness. We discuss the advantages and the limitations of this THz spectrometer. We present an analysis of the experimental data based on a χ2 test to provide an objective procedure to minimize the influence of systematic effects, for example of interference due to multiple reflections within the sample chamber. The measured absorption coefficient α lies between (410±6) and (490±6)cm−1 at 81 and 96cm−1, respectively.

Narrow linewidth intervalence-band emission from germanium terahertz lasers

We demonstrate narrow linewidth emission from p-Ge terahertz lasers at low magnetic fields where intervalence-band emission dominates. The volume of these laser crystals is 32 mm3, which is one order of magnitude smaller than the lasers for which the emission spectrum has been measured previously. The frequencies emitted are shown to be selected by the optical cavity and do not shift with the magnetic field, as has been observed for larger laser crystals.

Failure mode analysis of oxide VCSELs in high humidity and high temperature

High-speed fiber-optic transceiver modules using parallel optics require that oxide-confined vertical-cavity surface-emitting lasers (VCSELs) be moisture resistant in a non-hermetic package. We have found that the conventional storage 85/85 (85/spl deg/C/85% relative humidity) test does not adequately characterize oxide VCSELs moisture resistance. We have identified three failure modes in the oxide VCSELs under operating conditions in high humidity. In this paper, we discuss the failure mechanisms including dislocation growth, semiconductor cracks, and aperture surface degradation, all associated with operation under high relative humidity. Understanding of these failure modes has led to more appropriate qualification standards and environmentally robust oxide VCSELs.

Imaging at 3.4 THz with a quantum-cascade laser

We have assembled a single-frequency imaging system at 3.4 THz with a quantum-cascade laser. Images of electronic and biological applications are demonstrated. We operate the laser with a peak output power of 2.5 mW at a 7% duty cycle and a 22 K operating temperature. The minimum spot size is 340 microm. The signal is detected with a single-element deuterated triglycine sulfate detector, and images are captured by scanning of the sample.

Low-resistance tunnel junctions on GaAs substrates using GaInNAs

Using molecular-beam epitaxy, we have fabricated p-n tunnel junctions lattice matched to GaAs substrates that consist of highly C-doped Ga0.93In0.07As on the p side, and highly Si-doped Ga0.93In0.07N0.02As0.98 on the n side. The introduction of N on the n side of the tunnel junction: (1) lowers the conduction band edge while leaving the valence band edge unchanged, (2) allows the introduction of In to further lower the band gap while remaining lattice matched to GaAs, and (3) increases the maximum Si donor activation that can be achieved. All three of these effects increase the tunneling probability for carriers across the junction and decrease the specific resistance. We estimate the active C acceptors on the p side to be 1.5×1020 cm−3 and the active Si donors on the n side to be 1.8×1019 cm−3. Because of the mutual passivation effect between N and Si atoms, the Si doping level was increased to 1.0×1020 cm−3 to achieve this active net donor concentration. The specific resistance of test tunnel junctions ...

Reliability and failure mechanisms of oxide VCSELs in non-hermetic enviroments

High speed fiber optic transceiver modules using parallel optics require that oxide-confined vertical-cavity surface-emitting lasers (VCSELs) be moisture resistant in non-hermetic environments. Conventional storage 85/85 (85°C and 85% relative humidity) testing without a bias does not adequately characterize oxide VCSEL’s moisture resistance. Oxide VCSELs do not fail or degrade significantly under such conditions. With a bias, however, we have found that moisture can cause failure modes not seen in dry reliability testing. Without proper device design and fabrication, these failure modes lead to high failure rates in oxide VCSELs. In this paper, we first discuss the failure mechanisms we have identified, including dense dislocation network growth, semiconductor cracking and aperture surface degradation, all in high humidity and high temperature under operating conditions. We then report the results of environmental reliability tests on Agilent’s oxide VCSELs developed for the parallel optics modules. The results from a large number of wafers produced over an extended period of time have shown consistent, robust environmental reliability.

Dual-wavelength THz imaging with quantum cascade lasers

We have assembled an imaging system using quantum cascade lasers at frequencies of both 3.4 and 2.3 THz. Images at the two frequencies and the resulting absorption coefficients are compared. We demonstrate imaging in both reflection and transmission. The lasers are operated in a closed-cycle refrigerator and we use a peak output power of >2.5 mW at 7-10% duty cycle and 22-40 K operating temperature. The focal spot size is approximately 300 microns for both lasers and is not diffraction-limited. The signal is detected with a single-element DTGS detector, and images are captured by scanning the sample. Applications enabled by longer wavelengths are demonstrated, as well as the determination of chemical information through imaging at two wavelengths.

Optical properties of InAlAs/GaAsSb heterostructures grown by metalorganic vapor phase epitaxy

The optical properties of GaAsSb/InAlAs heterostructures grown on InP have been measured with cathodoluminescence, photoluminescence, and Fourier-transform infrared spectroscopy (FTIR) at <10 K. A heterostructure with heavily carbon-doped GaAsSb shows little shift of emission wavelength with incident intensity. A heterostructure with undoped GaAsSb has emission that is redshifted with respect to the doped structure and has a blueshift of 6 meV per decade of increasing beam intensity. Low-temperature FTIR shows that the lower energy level transition is redshifted in the undoped sample relative to the doped sample. These observations are consistent with the presence of donor-acceptor (D–A) pair recombination in the undoped structures, which is overwhelmed by free electron-bound acceptor recombination in the doped structure. We conjecture that the anomalously large difference between emission and absorption and blueshift in D–A transition observed result from spontaneous ordering in the GaAsSb.