Gregory C. Dyer

Transmission line theory of collective plasma excitations in periodic two-dimensional electron systems: Finite plasmonic crystals and Tamm states

We present a comprehensive theory of the one-dimensional plasmonic crystal formed in the grating gated two-dimensional electron gas (2DEG) in semiconductor heterostructures. To describe collective plasma excitations in the 2DEG, we develop a generalized transmission line theoretical formalism consistent with the plasma hydrodynamic model. We then apply this formalism to analyze the plasmonic spectra of 2DEG systems with step-like periodic changes of electron density and/or gate screening. We show that in a periodically modulated 2DEG, a plasmonic crystal is formed and derive closed-form analytical expressions describing its energy band spectrum for both infinite and finite size crystals. Our results demonstrate a non-monotonic dependence of the plasmonic band gap width on the electron density modulation. At so-called transparency points where the plasmon propagates through the periodic 2DEG in a resonant manner, the plasmonic band gaps vanish. In semi-infinite plasmonic crystals, we demonstrate the formation of plasmonic Tamm states and analytically derive their energy dispersion and spatial localization. Finally, we present detailed numerical analysis of the plasmonic band structure of a finite four-period plasmonic crystal terminated either by an Ohmic contact or by an infinite barrier on each side. We trace the evolution of the plasmonic band spectrum, including the Tamm states, with changing electron density modulation and analyze the boundary conditions necessary for formation of the Tamm states. We also analyze interaction between the Tamm states formed at the opposite edges of the short length plasmonic crystal. The validity of our theoretical approach was confirmed in experimental studies of plasmonic crystals in short modulated plasmonic cavities which demonstrated excellent quantitative agreement between theory and experiment.

Far infrared edge photoresponse and persistent edge transport in an inverted InAs/GaSb heterostructure

Direct current (DC) transport and far infrared photoresponse were studied an InAs/GaSb double quantum well with an inverted band structure. The DC transport depends systematically upon the DC bias configuration and operating temperature. Surprisingly, it reveals robust edge conduction despite prevalent bulk transport in our device of macroscopic size. Under 180 GHz far infrared illumination at oblique incidence, we measured a strong photovoltaic response. We conclude that quantum spin Hall edge transport produces the observed transverse photovoltages. Overall, our experimental results support a hypothesis that the photoresponse arises from direct coupling of the incident radiation field to edge states.

Transmission and Reflection of Terahertz Plasmons in Two-Dimensional Plasmonic Devices

Plasmons in two-dimensional semiconductor devices will be reflected by discontinuities, notably, junctions between gated and non-gated electron channels. The transmitted and reflected plasmons can form spatially- and frequency-varying signals, and their understanding is important for the design of terahertz detectors, oscillators, and plasmonic crystals. Using mode decomposition, we studied terahertz plasmons incident on a junction between a gated and a nongated channel. The plasmon reflection and transmission coefficients were found numerically and analytically and studied between 0.3 and 1 THz for a range of electron densities. At higher frequencies, we could describe the plasmons by a simplified model of channels in homogeneous dielectrics, for which the analytical approximations were accurate. At low frequencies, however, the full geometry and mode spectrum had to be taken into account. The results agreed with simulations by the finite-element method. Mode decomposition thus proved to be a powerful method for plasmonic devices, combining the rigor of complete solutions of Maxwells equations with the convenience of analytical expressions.

Rectified diode response of a multimode quantum cascade laser integrated terahertz transceiver

We characterized the DC transport response of a diode embedded in a THz quantum cascade laser as the laser current was changed. The overall response is described by parallel contributions from the rectification of the laser field due to the non-linearity of the diode I-V and from thermally activated transport. Sudden jumps in the diode response when the laser changes from single mode to multi-mode operation, with no corresponding jumps in output power, suggest that the coupling between the diode and laser field depends on the spatial distribution of internal fields. The results demonstrate conclusively that the internal laser field couples directly to the integrated diode.

Photoelectric polarization-sensitive broadband photoresponse from interface junction states in graphene

Graphene has established itself as a promising optoelectronic material. Many details of the photoresponse (PR) mechanisms in graphene in the THz-to-visible range have been revealed, however, new intricacies continue to emerge. Interface junctions, formed at the boundaries between parts of graphene with different number of layers or different stacking orders, and making connection between electrical contacts, provide another peculiar setup to establish PR. Here, we experimentally demonstrate an enhanced polarization sensitive photoelectric PR in graphene sheets containing interface junctions as compared to homogenous graphene sheets in the visible, infrared, and THz spectral regions. Our numerical simulations show that highly localized electronic states are created at the interface junctions, and these states exhibit a unique energy spectrum and enhanced probabilities for optical transitions. The interaction of electrons from interface junction states with electromagnetic fields generates a polarization-sensitive PR that is maximal for the polarization direction perpendicular to the junction interface.

Dispersion studies in THz plasmonic devices with cavities

Analytical and numerical studies of the dispersion properties of grating gated THz plasmonic structures show that the plasmonic crystal dispersion relation can be represented in terms of effective index of the dielectric medium around the 2DEG for the plasmons. Forbidden energy band gaps are observed at Brillion zone boundaries of the plasmonic crystal. FDTD calculations predict the existence of the plasmonic modes with symmetrical, antisymmetrical and asymmetrical charge distributions. Breaking the translational symmetry of the crystal lattice by changing the electron concentration of the two dimensional electron gas (2DEG) under a single gate line in every 9th gate induces a cavity state. The induced cavity state supports a weekly-coupled cavity mode.

Interferometric measurement of far infrared plasmons via resonant homodyne mixing

We present an electrically tunable terahertz two dimensional plasmonic interferometer with an integrated detection element that down converts the terahertz fields to a DC signal. The integrated detector utilizes a resonant plasmonic homodyne mixing mechanism that measures the component of the plasma waves in-phase with an excitation field functioning as the local oscillator. Plasmonic interferometers with two independently tuned paths are studied. These devices demonstrate a means for developing a spectrometer-on-a-chip where the tuning of electrical length plays a role analogous to that of physical path length in macroscopic spectroscopic tools such as Fourier transform interferometers.

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,

Resonant bolometric subterahertz detection in a 2D plasmonic cavity.

The two-dimensional plasma resonance excited in the channel of a field effect transistor has recently been utilized as the frequency-selective absorber in a monolithic far infrared plasmonic cavity detector. In this article we discuss the relevant parameters pertaining to engineering the plasmonic cavity and an integrated detection element as constituent elements of a resonant far infrared detector. The spectra of low-order plasmon modes in 18 μm and 34 μm long two-dimensional plasmonic cavities with 4 μm period grating gates have been measured. When the length of the plasma cavity is significantly larger than the gate length or period, the cavity length rather than grating period defines the plasmon wavevector. Electronic noise sources are considered; random telegraph noise is suggested as a dominant noise source when the device is operated as a highly resistive bolometric detector.

Coherent phenomena in terahertz 2D plasmonic structures: strong coupling, plasmonic crystals, and induced transparency by coupling of localized modes

The device applications of plasmonic systems such as graphene and two dimensional electron gases (2DEGs) in III-V heterostructures include terahertz detectors, mixers, oscillators and modulators. These two dimensional (2D) plasmonic systems are not only well-suited for device integration, but also enable the broad tunability of underdamped plasma excitations via an applied electric field. We present demonstrations of the coherent coupling of multiple voltage tuned GaAs/AlGaAs 2D plasmonic resonators under terahertz irradiation. By utilizing a plasmonic homodyne mixing mechanism to downconvert the near field of plasma waves to a DC signal, we directly detect the spectrum of coupled plasmonic micro-resonator structures at cryogenic temperatures. The 2DEG in the studied devices can be interpreted as a plasmonic waveguide where multiple gate terminals control the 2DEG kinetic inductance. When the gate tuning of the 2DEG is spatially periodic, a one-dimensional finite plasmonic crystal forms. This results in a subwavelength structure, much like a metamaterial element, that nonetheless Bragg scatters plasma waves from a repeated crystal unit cell. A 50% in situ tuning of the plasmonic crystal band edges is observed. By introducing gate-controlled defects or simply terminating the lattice, localized states arise in the plasmonic crystal. Inherent asymmetries at the finite crystal boundaries produce an induced transparency-like phenomenon due to the coupling of defect modes and crystal surface states known as Tamm states. The demonstrated active control of coupled plasmonic resonators opens previously unexplored avenues for sensitive direct and heterodyne THz detection, planar metamaterials, and slow-light devices.