Seongsin M. Kim

Biomedical terahertz imaging with a quantum cascade laser

We present biomedical imaging using a single frequency terahertz imaging system based on a low threshold quantum cascade laser emitting at 3.7THz (λ=81μm). With a peak output power of 4mW, coherent terahertz radiation and detection provide a relatively large dynamic range and high spatial resolution. We study image contrast based on water/fat content ratios in different tissues. Terahertz transmission imaging demonstrates a distinct anatomy in a rat brain slice. We also demonstrate malignant tissue contrast in an image of a mouse liver with developed tumors, indicating potential use of terahertz imaging for probing cancerous tissues.

High-frequency modulation characteristics of 1.3-/spl mu/m InGaAs quantum dot lasers

In this letter, we report results of small-signal modulation characteristics of self-assembled 1.3-/spl mu/m InGaAs-GaAs quantum dot (QD) lasers at room temperature. The narrow ridge-waveguide lasers were fabricated with multistack InGaAs self-assembled QDs in active region. A high characteristic temperature of T/sub o/=210 K with threshold current density of 200A/cm/sup 2/ was obtained. Small-signal modulation bandwidth of f/sub -3 dB/=12 GHz was measured at 300 K with differential gain of dg/dn/spl cong/2.4/spl times/10/sup -14/ cm/sup 2/ from detailed characteristics. We observed that a limitation of modulation bandwidth in high current injection appeared with gain saturation. This property can direct future high-speed QD laser design.

Long-wavelength GaInNAs(Sb) lasers on GaAs

The boom in fiber-optic communications has caused a high demand for GaAs-based lasers in the 1.3-1.6-/spl mu/m range. This has led to the introduction of small amounts of nitrogen into InGaAs to reduce the bandgap sufficiently, resulting in a new material that is lattice matched to GaAs. More recently, the addition of Sb has allowed further reduction of the bandgap, leading to the first demonstration of 1.5-/spl mu/m GaAs-based lasers by the authors. Additional work has focused on the use of GaAs, GaNAs, and now GaNAsSb barriers as cladding for GaInNAsSb quantum wells. We present the results of photoluminescence, as well as in-plane lasers studies, made with these combinations of materials. With GaNAs or GaNAsSb barriers, the blue shift due to post-growth annealing is suppressed, and longer wavelength laser emission is achieved. Long wavelength luminescence out to 1.6 /spl mu/m from GaInNAsSb quantum wells, with GaNAsSb barriers, was observed. In-plane lasers from these samples yielded lasers operating out to 1.49 /spl mu/m, a minimum threshold current density of 500 A/cm/sup 2/ per quantum well, a maximum differential quantum efficiency of 75%, and pulsed power up to 350 mW at room temperature.

Multiple-quantum-well GaInNAs-GaNAs ridge-waveguide laser diodes operating out to 1.4 /spl mu/m

In this letter, results from a ridge waveguide laser diode (LD) structure, with three GaInNAs quantum wells (QWs) and GaNAs barriers, are presented. The sample was grown by solid source molecular beam epitaxy with an RF plasma nitrogen source. These devices differ from previously reported GaInNAs QWs LDs that used GaAs as the barrier material. The introduction of nitrogen into the barriers reduces the spectral blue shift caused by post-growth annealing. Long wavelength emission out to 1.405 /spl mu/m was observed. The devices exhibited threshold current densities as low as 1.5 kA/cm/sup 2/, high differential efficiency of 0.67 W/A, and a maximum output power of 350 mW.

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.

Design and analysis of perfect terahertz metamaterial absorber by a novel dynamic circuit model

Metamaterial terahertz absorbers composed of a frequency selective layer followed by a spacer and a metallic backplane have recently attracted great attention as a device to detect terahertz radiation. In this work, we present a quasistatic dynamic circuit model that can decently describe operational principle of metamaterial terahertz absorbers based on interference theory of reflected waves. The model comprises two series LC resonance components, one for resonance in frequency selective surface (FSS) and another for resonance inside the spacer. Absorption frequency is dominantly determined by the LC of FSS while the spacer LC changes slightly the magnitude and frequency of absorption. This model fits perfectly for both simulated and experimental data. By using this model, we study our designed absorber and we analyze the effect of changing in spacer thickness and metal conductivity on absorption spectrum.

Photoluminescence study of InAsSb/InAsSbP heterostructures grown by low‐pressure metalorganic chemical vapor deposition

Photoluminescence has been measured for double‐ and separate‐confinement InAsSb/InAsSbP heterostructures grown by low‐pressure metalorganic vapor deposition. A measurement of the integrated luminescence intensity at the temperature range of 77–300 K shows that over a wide range of excitation level (1–5×102 W/cm2) the radiative transitions are the dominant mechanism below T∼170 K. Auger recombination coefficient C=C0 exp(−Ea/kT) with C0≊5×10−27 cm6/s and Ea≊40 meV has been estimated.

InSb and InSb:N multiple quantum dots

The authors report the growth and structural properties of InSb and InSb:N quantum dots on InAs and GaAs substrates. Strain induced self-assembled quantum dots are grown using solid-source molecular beam epitaxy. For improved growth control, they developed a growth technique similar to atomic-layer epitaxial methods. InSb and InSb:N multiple quantum dots formed on both InAs and GaAs. They explain the formation of multiple quantum dots by the anisotropic distribution of strain energy within the quantum dot, the long adatom lifetime during atomic-layer epitaxy, and the low bond energy of InSb.

Multicolor InGaAs quantum-dot infrared photodetectors

In this letter, we report two-color InGaAs quantum-dot infrared photodetectors. The InGaAs self-assembled quantum dots were grown on InGaP matrix via metal-organic chemical vapor deposition and fabricated as intersubband photoconductive structure. Bias-controlled two-color photoconductive responses were obtained at two different infrared window, mid-infrared of /spl lambda/=5.5 /spl mu/m and far-infrared of /spl lambda/=9.2 /spl mu/m at 77 K. Our devices were operated with low dark currents and high optical gains, and resulted in peak detectivity D/sup */ of 4.7/spl times/10/sup 9/ cm Hz/sup 1/2//W at /spl lambda/=5.5 /spl mu/m with bias of -2.0 V, and 7.2/spl times/10/sup 8/ cm Hz/sup 1/2//W at /spl lambda/=9.2 /spl mu/m with bias of -0.8 V.

Failure mode analysis of oxide VCSELs in high humidity and high temperature

High-speed fiber-optic transceiver modules using parallel optics require that oxide-confined vertical-cavity surface-emitting lasers (VCSELs) be moisture resistant in a non-hermetic package. We have found that the conventional storage 85/85 (85/spl deg/C/85% relative humidity) test does not adequately characterize oxide VCSELs moisture resistance. We have identified three failure modes in the oxide VCSELs under operating conditions in high humidity. In this paper, we discuss the failure mechanisms including dislocation growth, semiconductor cracks, and aperture surface degradation, all associated with operation under high relative humidity. Understanding of these failure modes has led to more appropriate qualification standards and environmentally robust oxide VCSELs.