Jichul Shin

Lasing characteristics of low threshold microcavity lasers using half‐wave spacer layers and lateral index confinement

Data are presented characterizing threshold and transverse mode behavior of microcavity lasers, which use a half‐wavelength cavity spacer layer surrounding a single quantum well active region. Selective conversion of AlAs into AlxOy is used to define lateral device dimensions of 2, 5, and 8 μm. Initial results demonstrate a continuous‐wave room‐temperature lasing threshold current of 97 μA for a 2 μm device and 220 μA for an 8 μm device. We show that lasing operation is influenced by the AlxOy located only 200 A from the quantum well.

Characterization of a Direct-Current Glow Discharge Plasma Actuator in Low-Pressure Supersonic Flow

DOI: 10.2514/1.27197 An experimental study of a direct-current, nonequilibrium glow plasma discharge in the presence of a Mach 2.85 supersonic flow is presented. The discharge is generated with pinlike electrodes flush-mounted on a plane surface with sustaining currents between 25 to 300 mA. In the presence of a supersonic flow, two distinct discharge modes (diffuse and constricted) are observed depending on the flow and discharge operating conditions. The effect of the dischargeonthe flow(“plasmaactuation”)ischaracterizedbytheappearanceofaweakshockwaveinthevicinityof the discharge. The shock is observed at low powers (10 W) for the diffuse discharge mode but is absent for the higher power (100 W) constricted mode. High-speed laser schlieren imaging suggests that plasma actuation is rapid as it occurs on a time scale that is less than 220 s. Rotational (gas) and vibrational temperature within the dischargeareestimatedbyemissionspectralline fitsofN2 andN 2 rovibronicbandsnear365–395nm.Theelectronic temperatures are estimated by using the Boltzmann plot method for Fe(I) atomic lines. Rotational temperatures are found to be high (1500 K) in the absence of a flow but drop sharply (500 K) in the presence of a supersonic flow for both the diffuse and constricted discharge modes. The vibrational and electronic temperatures are measured to be about 3000 K and 1.25 eV, respectively, and these temperatures are the same with and without flow. The gas temperature spatial profiles above the cathode surface are similar for the diffuse and constricted modes indicating that dilatational effects due to gas heating are similar. However, complete absence of flow actuation as indicated visually by the shock indicates that electrostatic forces may also play an important role in high-speed plasma-flow actuation phenomena. Analytical estimates using cathode sheath theory indicate thation pressure within sheath can besignificant,resulting ingascompressionwithin sheathandacorrespondingexpansionaboveit. Theexpansion,in turn, may fully negate the dilatational effect in the constricted case resulting in an apparent absence of forcing in the constricted case.

Resonant cavity light emitting diode with an AlxOy/GaAs reflector

A resonant cavity light emitting diode is fabricated which makes use of a Ag mirror/contact and a buried AlxOy/GaAs Bragg reflector. The buried AlxOy is formed by the selective conversion of AlAs. The differential light output efficiency is measured and found to be ∼5.5% when the AlxOy/GaAs reflector is present versus ∼2.5% for a control device without the AlxOy/GaAs mirror. The measured efficiencies are compared with calculations.

Dynamics of pulse phenomena in helium dielectric-barrier atmospheric-pressure glow discharges

A study of pulse phenomena in conventional parallel-plate dielectric-barrier controlled atmospheric-pressure glow (DB-APG) discharges in helium is reported. Stable DB-APG discharges are found to occur at arbitrarily low frequencies as long as the gas voltage exceeds the Paschen breakdown voltage, i.e., no lower limit of ∼1 kHz exists for DB-APG operation. The interpulse preionization phenomenon is found to be an artifact of typical ∼10 kHz operation of DB-APG discharges and does not play a role in the formation of a stable pulse. Multiple pulses result from repeated temporally separated breakdown events in the discharge. A relatively simple zero-dimensional model that treats only the Paschen breakdown mechanism in the discharge and charge trapping phenomena at the dielectric surfaces can be used to simulate all important qualitative features of DB-APG phenomena. Finally, we show that control of the pulse intensity, number of pulses in a pulse train, and the time interval between pulse trains can be achiev...

Improved mode stability in low threshold single quantum well native‐oxide defined vertical‐cavity lasers

Single quantum well active region vertical‐cavity surface‐emitting lasers (VCSELs) fabricated using a ‘‘native‐oxide’’ technique are compared with three quantum well active region VCSELs. The single quantum well active region exhibits improved transverse mode stability under a variety of operating conditions. The suggested reason is due to the reduced gain of the single quantum well, which results in greater lasing mode selectivity. For the single quantum well active region, a continuous wave threshold current of 178 μA at room temperature is measured for a 5 μm diam VCSEL and a pulsed threshold of 160 μA.

Characteristics of High Speed Electro-thermal Jet Activated by Pulsed DC Discharge

Abstract Experimental study of synthetic jet produced by pulsed direct current (DC) discharge is presented. High velocity jet is activated electro-thermally by high frequency pulsed DC discharge in small cavity. A cavity of 2.38 mm diameter cylinder bounded by circular electrode is made in a ceramic plate and a small orifice of 1.78 mm diameter is drilled in the middle of cavity. High frequency pulsed DC discharge instantaneously heats air in the cavity and produces high velocity jet at the exit of the orifice. Schlieren imaging at high framing rate of 100 kHz reveals the presence of supersonic precursor shock followed by the jet emerging from the orifice. The jet velocity reaches as high as about 300 m/s. Jet with smaller cavity volume produces lesser effect and jet velocity reaches maximum at certain cavity volume with given discharge current and orifice size. As duty time of pulse increases from 5 to 20 μs at fixed frequency of 5 kHz, the jet velocity also increases and becomes nearly constant with further increase in duty time. At fixed duty time of 20 μs, higher frequency pulsing of 10 kHz produces degradation of the jet as the discharge pulse continues. The jet developed in this study is demonstrated to be strong enough to penetrate deep into supersonic boundary layer and to produce a bow shock when the jet is issued into Mach 3 supersonic flow.

Run-to-run variations, asymmetric pulses, and long time-scale transient phenomena in dielectric-barrier atmospheric pressure glow discharges

The dielectric-barrier (DB) discharge is an important approach to generate uniform non-equilibrium atmospheric-pressure glow discharges. We report run-to-run variations, asymmetric pulse formation and long time-scale transient phenomena in these discharges. For similar DB discharge geometric and operating conditions, we observe significant run-to-run variations as manifested in the different voltage–current waveforms at the start of each new run. These run-to-run variations are also accompanied by asymmetric pulses at the start of each run. The variations are observed to drift to a repeatable true steady-state condition on time scales of order tens of minutes to hours. Asymmetric pulse waveforms drift to a symmetric pulse waveform at the true steady state. We explore reasons for these phenomena and rule out thermal drift during a discharge run and gas-phase impurity buildup as potential causes. The most plausible explanation appears to be variations in the surface characteristics of the DBs between two consecutive runs owing to varying inter-run environmental exposure and the conditioning of the dielectric surface during a run owing to plasma–surface interactions. We speculate that the dielectric surface state affects the secondary electron emission coefficient of the surface which in turn is manifested in the discharge properties. A zero-dimensional model of the discharge is used to explore the effect of secondary electron emission.

Threshold characteristics of planar and index‐guided microcavity lasers

Lasing threshold modes are studied experimentally for microcavity lasers fabricated to operate either as planar cavities, in which the mode is established by the emitter distribution, or as index‐guided modes in the planar cavity with weak three‐dimensional confinement. For no lateral index guiding, the lasing mode area is suggested to depend on the gain saturation. For lateral index confinement to a small area, spectral and radiation pattern measurements show that the spontaneous emission coupling is increased, and reduces the lasing threshold even in the presence of an increased loss.

Microdischarge-assisted ignition of dielectric-barrier high-pressure glow discharges

The ignition characteristics of dielectric-barrier high-pressure glow (DB-HPG) discharges in the presence of dc microdischarges are studied for pure helium and pure nitrogen working gases. In the presence of an array of microdischarges integrated with one of the DB-HPG electrodes, a substantial reduction of DB-HPG ignition voltage is observed for both working gases. The discharge structure within the DB-HPG volume gap is localized near the microdischarge holes when the DB-HPG first turns on in the presence of microdischarges, but subsequently expands to cover the entire volume at higher voltages for the pressures studied. The helium discharge is generally more uniform than the nitrogen gas. Our studies indicate that the localized discharge has a glow-like character rather than a Townsend- or streamer-like one.

Schlieren Imaging of Flow Actuation Produced by Direct-Current Surface Glow Discharge in Supersonic Flows

Schlieren imaging of flow field structure produced by direct-current surface glow discharges in diffuse mode is studied experimentally in the presence of Mach 2.85 supersonic flow. An array of four discharge pairs with a total discharge power of about 150 W is established just upstream of a 30deg wedge. Schlieren imaging reveals that for cathodes located upstream of the anodes, the discharge produces significant interaction with the flow, by changing the attached oblique shock structure at the foot of the wedge, while this interaction effect is completely absent in the cathode downstream case. We conclude that gas-heating-related dilatational effect alone cannot explain the observed phenomena.