Surface Emitting Devices Based on a Semiconductor Coupled Multilayer Cavity for Novel Terahertz Light Sources (Special Section on Fabrication Technologies Supporting the Photonic/Nanostructure Devices)

Compact and room-temperature operable terahertz emitting devices have been proposed using a semiconductor coupled multilayer cavity that consists of two functional cavity layers and three distributed Bragg reflector (DBR) multilayers. Two cavity modes with an optical frequency difference in the terahertz region are realized since two cavities are coupled by the intermediate DBR multilayer. In the proposed device, one cavity is used as the active layer for two-color lasing in the near-infrared region by current injection and the other is used as the second-order nonlinear optical medium for difference-frequency generation of the two-color fundamental laser light. The control of the nonlinear polarization by face-to-face bonding of two epitaxial wafers with different orientations is quite effective to achieve bright terahertz emission from the coupled cavity. In this study, two-color emission by optical excitation was measured for the wafer-bonded GaAs/AlGaAs coupled multilayer cavity containing self-assembled InAs quantum dots (QDs). We found that optical loss at the bonding interface strongly affects the two-color emission characteristics when the bonding was performed in the middle of the intermediate DBR multilayer. The effect was almost eliminated when the bonding position was carefully chosen by considering electric field distributions of the two modes. We also fabricated the current-injection type devices using the wafer-bonded coupled multilayer cavities. An assemble of self-assembled QDs is considered to be desirable as the optical gain medium because of the discrete nature of the electronic states and the relatively wide gain spectrum due to the inhomogeneous size distribution. The gain was, however, insufficient for two-color lasing even when the nine QD layers were used. Substituting two types of InGaAs multiple quantum wells (MQWs) for the QDs, we were able to demonstrate two-color lasing of the device when the gain peaks of MQWs were tuned to the cavity modes by lowering the operating temperature

Current-injection two-color lasing in a wafer-bonded coupled multilayer cavity with InGaAs multiple quantum wells

Current-injection two-color lasing has been demonstrated using a GaAs/AlGaAs coupled multilayer cavity that is a good candidate for novel terahertz-emitting devices based on difference-frequency generation (DFG) inside the structure. The coupled cavity structure was fabricated by the direct wafer bonding of (001)- and (113)B-oriented epitaxial wafers for the efficient DFG of two modes in the (113)B side cavity, and two types of InGaAs multiple quantum wells (MQWs) were introduced only in the (001) side cavity as optical gain materials. The threshold behavior was clearly observed in the current–light output curve even at room temperature. Two-color lasing was successfully observed when the gain peaks of MQWs were considerably tuned to the cavity modes by the operating temperature

Ultrafast harmonic mode-locking of monolithic compound-cavity laser diodes incorporating photonic-bandgap reflectors

We present the first demonstration of reproducible harmonic mode-locked operation from a novel design of monolithic semiconductor laser comprising a compound cavity formed by a 1-D photonic-bandgap (PBG) mirror. Mode-locking (ML) is achieved at a harmonic of the fundamental round-trip frequency with pulse repetition rates from 131 GHz up to a record high frequency of 2.1 THz. The devices are fabricated from GaAs-Al-GaAs material emitting at a wavelength of 860 nm and incorporate two gain sections with an etched PBG reflector between them, and a saturable absorber section. Autocorrelation studies are reported which allow the device behavior for different ML frequencies, compound cavity ratios, and type and number of intra-cavity reflectors to be analyzed. The highly reflective PBG microstructures are shown to be essential for subharmonic-free ML operation of the high-frequency devices. We have also demonstrated that the single PBG reflector can be replaced by two separate features with lower optical loss. These lasers may find applications in terahertz; imaging, medicine, ultrafast optical links, and atmospheric sensing

Two-color surface-emitting lasers using a semiconductor coupled multilayer cavity

Two-color surface-emitting lasers were demonstrated, employing a GaAs/AlGaAs coupled multilayer cavity composed of two cavity layers and three distributed Bragg reflector (DBR) multilayers. InGaAs multiple quantum wells (MQWs) with two different well widths were introduced only in the upper cavity, sandwiched between p-type and n-type DBRs. This current-injection type device exhibited two-color lasing in the near-infrared region under room temperature pulsed conditions. This two-color lasing was obtained when the lower cavity had an optimal thickness relative to the upper cavity thickness and the MQW emission properties

Terahertz oscillation and stimulated emission from planar microcavities

In the past decades, the miniaturization in optics led to new devices with structural sizes in the range of the light wavelength, where the photonic modes are con- fined and the number of states is limited. In the smallest microcavities, i.e. micrometer sized optical resonators, the propagation of only one mode is permitted that is simultaneously amplified internally. This particularly strong enhancement of the electric field is directly related to the quality factor of the cavity. By introducing an optical dipole into a high-Q microcavity, the spontaneous emission is amplified at the cavity mode frequency enabling stimulated emission in an inverted system. Although some of theses cavity e®ects can only be understood by quantum elec- trodynamic theory, most mechanisms are accessible by classical and semi-classical approaches. In this thesis, one-dimensional planar microcavities with quality factors up to 4500 have been fabricated by physical vapor deposition of dielectric thin films and organic active materials. A new cavity design based on anisotropic dielectric mirrors grown by oblique angle deposition microcavities with two energetically shifted orthogonally polarized modes is presented. The application of these anisotropic structures for terahertz di®erence signal generation is demonstrated in spectrally and time resolved transmission experiments, where optical beats with repetition rates in the terahertz range are observed. Optically pumped organic vertical cavity surface emitting lasers (VCSELs) have been realized by applying an organic solid state laser compound and high reflectance distributed Bragg reflectors. These lasers combine a very low laser threshold with small beam divergence and good stability. A transfer of the anisotropic design towards an organic VCSEL results in the generation of two perpendicularly polarized laser modes with a splitting adjustable by the fabrication conditions. The observation of an oscillation of two laser modes in a photomixing experiment proves a phase coupling mechanism. This demonstrates the potential of the anisotropic cavity design for a passive or active component in a terahertz radiation source or frequency generator. Furthermore, microcavities with two and three coupled resonators are investigated. By the application of time-resolved transmission experiments, spatial oscil- lations of the internal electric field - photonic Bloch oscillations - are successfully demonstrated. In combination with the anisotropic microcavities, this is a second concept for the modulation of transmitted light with terahertz frequencies. All experiments are accompanied by numerical or analytical models. Transmission experiments of continuously incident light and single laser pulses are compared with transfer matrix simulations and Fourier transform based approaches. For the modeling of emission experiments, a plane wave expansion method is successfully used. For the analysis of the organic VCSEL dynamics, we apply a set of rate equations that explains the gain switching process

Vertical-external-cavity surface-emitting lasers and quantum dot lasers

The use of cavity to manipulate photon emission of quantum dots (QDs) has been opening unprecedented opportunities for realizing quantum functional nanophotonic devices and also quantum information devices. In particular, in the field of semiconductor lasers, QDs were introduced as a superior alternative to quantum wells to suppress the temperature dependence of the threshold current in vertical-external-cavity surface-emitting lasers (VECSELs). In this work, a review of properties and development of semiconductor VECSEL devices and QD laser devices is given. Based on the features of VECSEL devices, the main emphasis is put on the recent development of technological approach on semiconductor QD VECSELs. Then, from the viewpoint of both single QD nanolaser and cavity quantum electrodynamics (QED), a single-QD-cavity system resulting from the strong coupling of QD cavity is presented. A difference of this review from the other existing works on semiconductor VECSEL devices is that we will cover both the fundamental aspects and technological approaches of QD VECSEL devices. And lastly, the presented review here has provided a deep insight into useful guideline for the development of QD VECSEL technology and future quantum functional nanophotonic devices and monolithic photonic integrated circuits (MPhICs).Comment: 21 pages, 4 figures. arXiv admin note: text overlap with arXiv:0904.369

One-dimensional carbon nanostructures for terahertz electron-beam radiation

One-dimensional carbon nanostructures such as nanotubes and nanoribbons can feature near-ballistic electronic transport over micron-scale distances even at room temperature. As a result, these materials provide a uniquely suited solid-state platform for radiation mechanisms that so far have been the exclusive domain of electron beams in vacuum. Here we consider the generation of terahertz light based on two such mechanisms, namely, the emission of cyclotronlike radiation in a sinusoidally corrugated nanowire (where periodic angular motion is produced by the mechanical corrugation rather than an externally applied magnetic field), and the Smith-Purcell effect in a rectilinear nanowire over a dielectric grating. In both cases, the radiation properties of the individual charge carriers are investigated via full-wave electrodynamic simulations, including dephasing effects caused by carrier collisions. The overall light output is then computed with a standard model of charge transport for two particularly suitable types of carbon nanostructures, i.e., zigzag graphene nanoribbons and armchair single-wall nanotubes. Relatively sharp emission peaks at geometrically tunable terahertz frequencies are obtained in each case. The corresponding output powers are experimentally accessible even with individual nanowires, and can be scaled to technologically significant levels using array configurations. These radiation mechanisms therefore represent a promising paradigm for light emission in condensed matter, which may find important applications in nanoelectronics and terahertz photonics.DMR-1308659/National Science Foundationhttp://ultra.bu.edu/papers/Tantiwanichapan-2016-PRB-CNT-THz.pd

Aperiodic Multilayer Graphene Based Tunable and Switchable Thermal Emitter at Mid-infrared Frequencies

Over the past few decades, there have been tremendous innovations in electronics and photonics. The development of these ultra-fast growing technologies mostly relies on fundamental understanding of novel materials with unique properties as well as new designs of device architectures with more diverse and better functionalities. In this regard, the promising approach for next-generation nanoscale electronics and photonics is to exploit the extraordinary characteristics of novel nanomaterials. There has been an explosion of interest in graphene for photonic applications as it provides a degree of freedom to manipulate electromagnetic waves. In this thesis, to tailor the broadband blackbody radiation, new aperiodic multilayer structures composed of multiple layers of graphene and hexagonal boron nitride (hBN) are proposed as selective, tunable and switchable thermal emitters. To obtain the layer thicknesses of these aperiodic multilayer structures for maximum emittance/absorptance, a hybrid optimization algorithm coupled to a transfer matrix code is employed. The device simulation indicates that perfect absorption efficiency of unity can be achieved at very narrow frequency bands in the infrared under normal incidence. It has been shown that the chemical potential in graphene enables a promising way to design electrically controllable absorption/emission, resulting in selective, tunable and switchable thermal emitters at infrared frequencies. By simulating different aperiodic thermal emitters with different numbers of graphene layers, the effect of the number of graphene layers on selectivity, tunability, and switchability of thermal emittance is investigated. This study may contribute towards the realization of wavelength selective detectors with switchable intensity for sensing applications

Electronic Band Structure of Quantum Cascade Laser

Multiple quantum well structure is the subject of theoretical and experimental research over the last two decades due to the possibility of making novel electronic and optoelectronic devices. The phenomenon of resonant tunneling makes it a prime candidate for all tunneling-based quantum devices with one-dimensional confinement. THz laser design using multilayered low-dimensional semiconductor structure is one such example, where miniband formation and its energy difference with lowest quantum state play crucial factor in governing device performance. Quantum cascade laser (QCL) is one of such candidate, which speaks in favor of research using multiple-quantum-well (MQW) structure. In the proposed chapter, transmission coefficient of multiple quantum-well structure is numerically computed using propagation matrix technique, and its density of states is calculated in the presence and absence of electric field applied along the direction of quantum confinement. Absorption coefficient is also calculated for its possible application as optical emitter/detector. Based on the electronic and photonic properties investigated, electronic band structure of the quantum cascade laser (formed using the MQW structure) is computed. Formation of miniband is tailored with variation of external bias is shown