Biomolecules can exhibit low-lying vibrational modes in the THz region which are detectable in transmission
given a strong molecular dipole moment and optical depth, and a spectrometer of adequate sensitivity. The nucleic
acids are particularly interesting because of applications such as label-free gene assay, bio-agent detection, etc. However
for nucleic acids, sample preparation and THz coupling are of paramount importance because of the strong absorption by
liquid water and the small concentration of molecules present in physiological solutions. Concentration methods
become necessary to make the THz vibrational modes detectable, either by concentrating the nucleic-acid sample itself
in a small volume but large area, or by concentrating the THz radiation down to the volume of the sample. This paper
summarizes one type of the first method: nanofluidic channel arrays for biological nucleic acids; and two types of the
second method: (1) a circular-waveguide pinhole, and (2) a
circular-waveguide, conical-horn coupling structure, both for
DNA crystals. The first method has been demonstrated on a very short artificial nucleic acid [small-interfering (si) RNA
(17-to-25 bp)] and a much longer, biological molecule [Lambda-phage DNA (48.5 kbp)]. The second method has been
demonstrated on small (~100 micron) single crystals of DNA grown by the sitting-drop method.
A novel approach for the generation of THz radiation that utilizes "interband" transitions and tunneling processes
occurring simultaneously within double-barrier (DB) GaSb/InAs/GaSb broken-gap (BG) resonant-tunneling-diodes
(RTDs) is discussed. This paper focuses on the architectural and cavity designs for realizing TE polarized emission from
single DB-BG-RTD devices and quantum-dot pillar arrays. Design techniques useful for mitigating CB drive current (&
the associated thermal heating) while at the same time optimizing output power and power efficiency are discussed.
This paper will illustrate the potential of InAs/GaSb broken-gap structures for providing a solution to the well-known
and long-standing terahertz (THz) frequency gap in source technology. In a double-barrier GaSb/InAs/GaSb
heterostructure, the ultrafast heavy-hole interband tunneling can be utilized to achieve electron depopulation of a quasibound,
heavy-hole level located in the valence-band of the right GaSb barrier region. A population inversion is then
created using electron injection into the conduction-band resonant state of the double-barrier structure. Degrading
nonradiative processes such as acoustic phonon, optical phonon and Auger recombination are suppressed in this spatially
separated two-level energy system. Hence, heavy-hole interband tunneling prevails over nonradative transition rates and
establishes a population inversion at relatively high operating temperatures. Detailed simulations predict a significant
optical (total) gain of ~0.001 which is comparable with GaAs/AlGaAs quantum well laser in spite of the small overlap of
conduction band and heavy-hole wavefunctions. The TE emission allows for implementation of vertical surface
emission in large area, for single or arrayed devices. Also, lateral quantum confinement of arrayed systems can be used
to reduce the conduction band current density (and undesired thermal heating) and increase the quantum efficiency.
While the unique spectral information associated with chemical and biological molecules within the terahertz frequency
regime (~ 3.0-3.0 millimeters) motivates its use for practical sensing applications, limiting factors at the macroscale
(weak spectral absorption, broad line widths and masking geometrical effects introduced by the samples) provides
motivation for man-engineered sensing materials that allow for the transduction of the spectral information about target
molecules from the nanoscale. This brief letter will overview work being performed by our research group to define
molecular-level functionality that will be useful for realizing "THz/IR-sensitive" materials. Here the goal is to define
switchable molecular components that when incorporated into larger DNA-based nanoscaffolds lead to THz and/or IR
regime electronic and/or photonic material properties that are dictated in a predictable manner by novel functionality
paradigms. In particular, theoretical modeling and design studies are being performed to engineer organic and biological
switches that can be incorporated into DNA-based architectures that enable the precise extraction of nanoscale
information (e.g., composition, dynamics, conformation) through electronic/photonic transformations to the macroscale.
Hence, these studies seek to define new spectral-based sensing modalities useful for characterizing bio-molecules
A novel optically-triggered (OT) interband resonant-tunneling-diode (I-RTD) device (based on AlGaSb/InAs/AlGaSb
heterostructures) concept for generating terahertz (THz) frequency oscillations has been previously presented that shows
promise for achieving enhanced output power levels under pulsed operation. The main concept is to utilize novel
nanoscale mechanisms to achieve an externally driven relaxation oscillation that consists of two phases. Namely, the
first phase is a valence band (VB) well hole-charging transient produced by a natural Zener (interband) tunneling
process and the second is a discharging transient induced by optical annihilation of the VB well hole-charge by
externally-injected photon flux. While the initial simulation results for a practical diode-laser implementation clearly
show the superiority of this new oscillator concept (i.e., excellent output power capability, ~10mW, over broad portions
of the THz regime, ~300-600GHz), the specific optical-triggering conditions required by the AlGaSb/InAs based
material systems (i.e., photonic-energy ~4.7 μm, intensity level ~3.5x107 W/cm2 and a pulse repetition frequency (PRF)
equal to the THz oscillation period) are technically too demanding to meet for continuous-wave (CW) mode operation.
Hence, this paper will report on variations and extensions of the original OT-I-RTD oscillator concept. Specifically,
modifications to the device structure will be considered to allow for OT operation at 1.55 μm where the optical
technology is more robust. Here the specific focus will be in the introduction of In1-xGaxAs /GaSbyAs1-y hetero-systems
and the application of band-engineering to assess the potential of a 1.55 μm based OT-I-RTD oscillator design.
The double-barrier AlGaSb/InAs/AlGaSb heterostructure with staggered bandgap alignment can admit significant interband tunneling current in addition to the conduction band electron transport. The resulting positive hole-charge accumulation in the right valence-band (VB) well will electrostatically modify the spatial potential profile across the device structure, thereby effectively altering the conduction of conduction-band electron transport. A sequentially triggered optical discharging process can be used to annihilate, or substantially reduce, the trapped holes that are generated from the interband tunneling process. Hence, an artificially induced electro-optic interaction can be used to return the device to its initial state and to produce a two-cycle oscillation process - i.e., one with a interband-induced charging transient followed by a optically-induced discharging transient to the initial state. These charging-discharging cycles obtained from this hybrid type of interband resonant-tunneling-diode (I-RTD) device constitute steady-state oscillatory behavior at very high frequency and produce alternating-current (ac) power as long as very short (i.e., sub-picosecond) and intense far-infrared laser pulses are presented to the diode. Initial studies of non-optimized structures and designs predict impressive figures of merit for oscillation frequencies (e.g., ~ 300-600 GHz) and substantial output powers (e.g., ~ 10 mW) for very modest device areas (i.e., 100 μm2). This paper will present physics-based I-RTD diode simulation results to precisely describe transport dynamics and transient electric current for both charging (initiated by Zener tunneling) and discharging (artificially induced by photons flux) processes. A basic electro-optical design concept and modeling approach for the analysis and synthesis of non-linear hybrid I-RTD circuits will also be presented. The main objectives of this paper are: (1) to perform a detailed assessment of the ac output power and efficiency of an optically-triggered (OT) I-RTD hybrid oscillator in the frequency range approximately 300 to 600 GHz, and (2) to prescribe the general requirements for realizing a diode-laser pair upon a single solid-state platform in the future. Therefore, guidelines for a practical engineering implementation and performance estimates for an OT-I-RTD hybrid oscillator design will be presented.