Michael Clement Wanke

Frequency doubling and tripling of terahertz radiation in a GaAs/AlAs superlattice due to frequency modulation of Bloch oscillations

We report on frequency doubling and tripling of THz radiation in a voltage-biased GaAs/AlAs superlattice. By use of a corner cube antenna system, radiation from the Santa Barbara free-electron laser (frequency 0.7 THz) was guided into a superlattice mesa element and the second and third harmonic were coupled out of the mesa. Without bias only radiation of the third harmonic was generated, while the biased superlattice emitted radiation of both the second and third harmonic. We attribute the harmonic generation to frequency modulation of damped Bloch oscillations of the miniband electrons in the superlattice.

Terahertz frequency response of an In0.53Ga0.47As/AlAs resonant‐tunneling diode

We have measured the room‐temperature, broad‐band, terahertz response of a high‐speed In0.53Ga0.47As/AlAs resonant‐tunneling diode from 120 GHz to 3.9 THz using the free‐electron lasers at UCSB. The ‘‘rectified’’ response is measured with a conventional probe station by using the tungsten probe tip as a whisker antenna. Normalizing the rectified response in the resonant‐tunneling regime with the off‐resonant response we remove the extrinsic frequency dependence controlled by the antenna and the RC time constant and measure an intrinsic relaxation time.

Third harmonic generation by Bloch-oscillating electrons in a quasioptical array

We compute the third harmonic field generated by Bloch-oscillating electrons in a quasioptical array of superlattices under THz irradiation. The third harmonic power transmitted oscillates with the internal electric field, with nodes associated with Bessel functions in eEd/ℏω. The nonlinear response of the array causes the output power to be a multivalued function of the incident laser power. The output can be optimized by adjusting the frequency of the incident pulse to match one of the Fabry-Perot resonances in the substrate. Within the transmission-line model of the array, the maximum conversion efficiency is 0.1%.

Atmospheric Propagation of THz Radiation

In this investigation, we conduct a literature study of the best experimental and theoretical data available for thin and thick atmospheres on THz radiation propagation from 0.1 to 10 THz. We determined that for thick atmospheres no data exists beyond 450 GHz. For thin atmospheres data exists from 0.35 to 1.2 THz. We were successful in using FASE code with the HITRAN database to simulate the THz transmission spectrum for Mauna Kea from 0.1 to 2 THz. Lastly, we successfully measured the THz transmission spectra of laboratory atmospheres at relative humidities of 18 and 27%. In general, we found that an increase in the water content of the atmosphere led to a decrease in the THz transmission. We identified two potential windows in an Albuquerque atmosphere for THz propagation which were the regions from 1.2 to 1.4 THz and 1.4 to 1.6 THz.

Fabrication of quantum dots in undoped Si/Si0.8Ge0.2 heterostructures using a single metal-gate layer

Enhancement-mode Si/SiGe electron quantum dots have been pursued extensively by many groups for their potential in quantum computing. Most of the reported dot designs utilize multiple metal-gate layers and use Si/SiGe heterostructures with Ge concentration close to 30%. Here, we report the fabrication and low-temperature characterization of quantum dots in the Si/Si0.8Ge0.2 heterostructures using only one metal-gate layer. We find that the threshold voltage of a channel narrower than 1 μm increases as the width decreases. The higher threshold can be attributed to the combination of quantum confinement and disorder. We also find that the lower Ge ratio used here leads to a narrower operational gate bias range. The higher threshold combined with the limited gate bias range constrains the device design of lithographic quantum dots. We incorporate such considerations in our device design and demonstrate a quantum dot that can be tuned from a single dot to a double dot. The device uses only a single metal-gate...

Third Harmonic Generation in a Gaas/Algaas Superlattice in the Bloch Oscillator Regime

A GaAs/AlGaAs superlattice driven at 600 GHz exhibits strong, power dependent third harmonic generation. Conversion efficiency up to 0.1% was observed. The power dependence agrees semi-quantitatively with a simple model describing Bloch oscillations driven by a strong THz electric field.

Terahertz harmonic generation from Bloch-oscillating superlattices in quasi-optical arrays

We explored harmonic generation by Bloch oscillation in miniband superlattices driven by intense THz radiation from the UCSB free electron lasers, as a function of both THz intensity and applied DC bias. To accomplish this we integrated micrometers size superlattice mesas in a quasi-optical array which amplified a plane wave incident normal to the array and coupled it into the growth direction of the superlattice. We were able to successfully measure both second and third harmonic generation quantitatively. The harmonics are compared to a quasi-classical picture of Bloch oscillation.

Position and mode dependent coupling of terahertz quantum cascade laser fields to an integrated diode

A Schottky diode integrated into a terahertz quantum cascade laser waveguide couples directly to the internal laser fields. In a multimode laser, the diode response is correlated with both the instantaneous power and the coupling strength to the diode of each lasing mode. Measurements of the rectified response of diodes integrated in two quantum cascade laser cavities at different locations indicate that the relative diode position strongly influences the laser-diode coupling. ∗ Now at Soraa, Freemont, California, 94555 USA † mcwanke@sandia.gov 1 ar X iv :1 60 5. 03 11 8v 1 [ co nd -m at .m es -h al l] 1 0 M ay 2 01 6 Quantum cascade lasers (QCLs) may be considered one of the most remarkable achievements in quantum engineering due to both the intensity and the broad tailorability of their emission. Since the operating range of these unipolar, intersubband lasers was extended to the terahertz (THz) band of the spectrum, a variety of applications requiring a compact high-power (>mW) source between 1-5 THz have become accessible. Of particular interest is the use of a THz QCL as a local oscillator (LO) for heterodyne mixing. THz QCLs provide ample power for mixing; however, it is non-trivial to efficiently couple the THz LO power from a QCL to a mixer such as a planar Schottky diode. One possible solution is to directly integrate a Schottky diode mixer into the core of a THz QCL to create a THz transceiver. We previously observed the direct coupling of the internal QCL fields to an integrated diode, however, several questions concerning the precise nature of this coupling remain open. For practical applications, the response of a Schottky diode mixer should be linear in both the LO and signal field amplitudes. However, prior measurements suggested that both the mode structure and the instantaneous power of the laser may affect the laserdiode coupling and lead to a non-linear response to the QCL (LO) power. In this letter we examine how the rectified response of Schottky diodes embedded into the core of THz QCLs depends upon diode position and QCL bias current. To determine the effect of diode position upon the diode’s coupling with the laser fields, we compare the rectified response of diodes with different relative positions in the laser waveguide to the emission spectra of two otherwise identical 2.8 THz QCL transceivers. The studied THz QCLs have a Schottky diode embedded into the core of the 3 mm long by 170 μm wide waveguide, as illustrated in Fig. 1. Both transceivers were cleaved from the same row of the processed die, and thus have identical cavity lengths. Sample A has the diode located by design at the center of the QCL waveguide relative to the laser facets, 1.5 mm from both facets. Sample B has the diode shifted +4 μm from that of the diode in Sample A. Given the slight uncertainty of the cleave planes relative to the diode position, the exact locations of the diodes in Samples A and B may differ from design. But the relative positions of the two diodes are fixed by the device layouts. Rectified and intermediate frequency (IF) signals result from the coupling of THz laser fields to a Schottky diode. If only nearest-neighbor modes in a Fabry-Perot laser (FP) cavity separated by the angular frequency ωFP are considered, the rectified and IF signals,

THz Transceiver Characterization: LDRD Project 139363 Final Report

LDRD Project 139363 supported experiments to quantify the performance characteristics of monolithically integrated Schottky diode + quantum cascade laser (QCL) heterodyne mixers at terahertz (THz) frequencies. These integrated mixers are the first all-semiconductor THz devices to successfully incorporate a rectifying diode directly into the optical waveguide of a QCL, obviating the conventional optical coupling between a THz local oscillator and rectifier in a heterodyne mixer system. This integrated mixer was shown to function as a true heterodyne receiver of an externally received THz signal, a breakthrough which may lead to more widespread acceptance of this new THz technology paradigm. In addition, questions about QCL mode shifting in response to temperature, bias, and external feedback, and to what extent internal frequency locking can improve stability have been answered under this project.

Terahertz-based target typing.

The purpose of this work was to create a THz component set and understanding to aid in the rapid analysis of transient events. This includes the development of fast, tunable, THz detectors, along with filter components for use with standard detectors and accompanying models to simulate detonation signatures. The signature effort was crucial in order to know the spectral range to target for detection. Our approach for frequency agile detection was to utilize plasmons in the channel of a specially designed field-effect transistor called the grating-gate detector. Grating-gate detectors exhibit narrow-linewidth, broad spectral tunability through application of a gate bias, and no angular dependence in their photoresponse. As such, if suitable sensitivity can be attained, they are viable candidates for Terahertz multi-spectral focal plane arrays.