Effect of Hole Shift on Threshold Characteristics of GaSb-Based Double-Hole Photonic-Crystal Surface-Emitting Lasers.

Photonic-crystal (PC) surface-emitting lasers (SELs) with double-hole structure in the square-lattice unit cell were fabricated on GaSb-based type-I InGaAsSb/AlGaAsSb heterostructures. The relative shift of two holes was varied within one half of the lattice period. We measured the lasing wavelengths and threshold pumping densities of 16 PC-SELs and investigated their dependence on the double-hole shift. The experimental results were compared to the simulated wavelengths and threshold gains of four band-edge modes. The measured lasing wavelength did not exhibit switching of band-edge mode; however, the calculated lowest threshold mode switched as the double-hole shift exceeded one quarter of the lattice period. The identification of band-edge lasing mode revealed that modal gain discrimination was dominated over by its mode wavelength separation.

Effect of Etching Depth on Threshold Characteristics of GaSb-Based Middle Infrared Photonic-Crystal Surface-Emitting Lasers.

We study the effect of etching depth on the threshold characteristics of GaSb-based middle infrared (Mid-IR) photonic-crystal surface-emitting lasers (PCSELs) with different lattice periods. The below-threshold emission spectra are measured to identify the bandgap as well as band-edge modes. Moreover, the bandgap separation widens with increasing etching depth as a result of enhanced diffraction feedback coupling. However, the coupling is nearly independent of lattice period. The relationship between threshold gain and Bragg detuning is also experimentally determined for PCSELs and is similar to that calculated theoretically for one-dimensional distributed feedback lasers.

Fluorescent Fe Embedded Magnetic Nanodiamonds Made by Ion Implantation.

We demonstrate fluorescent Fe embedded magnetic nanodiamonds by ion implantation and two-step annealing. The diamond characteristics with a highly ordered core and a graphite surface layer are maintained after the implantation process. After the two-step annealing process, a bright red fluorescence associated with nitrogen-vacancy centers is observed. These new fluorescent magnetic nanodiamonds can be used as a dual-function in vivo tracer with both optical visibility and magnetic resonance imaging capabilities. They are potentially useful for the more advanced in vivo biological and medical applications.

PCSEL Performance of Type-I InGaAsSb Double-QWs Laser Structure Prepared by MBE.

This paper discusses the issue of controlling the epitaxial growth of mixed group V alloys to form a type-I InGaAsSb/AlGaAsSb double quantum wells (QWs) structure. We also discuss the run-to-run reproducibility of lattice-matched AlGaAsSb alloys and strained In0.35Ga0.65As0.095Sb0.905 in terms of growth parameters (V/III ratio, Sb2/As2 ratio). Molecular beam epitaxy (MBE) was used to grow two type-I InGaAsSb double-QWs laser structures differing only in the composition of the bottom cladding layer: Al0.85Ga0.15As0.072Sb0.928 (sample A) and Al0.5Ga0.5As0.043Sb0.957 (sample B). Both samples were respectively used in the fabrication of photonic crystal surface-emitting lasers (PCSELs). Sample A presented surface lasing action from circular as well as triangular photonic crystals. Sample B did not present surface lasing due to the deterioration of the active region during the growth of the upper cladding. Our findings underline the importance of temperature in the epitaxial formation of AlxGa1-xAsySb1-y in terms of lasing performance.

Fe Doped Magnetic Nanodiamonds Made by Ion Implantation as Contrast Agent for MRI.

We report in this paper a new MRI contrast agent based on magnetic nanodiamonds fabricated by Fe ion implantation. The Fe atoms that are implanted into the nanodiamonds are not in direct contact with the outside world, enabling this new contrast agent to be free from cell toxicity. The image enhancement was shown clearly through T2 weighted images. The concentration dependence of the T2 relaxation time gives a relaxivity value that is about seven times that of the regular non-magnetic nanodiamonds. Cell viability study has also been performed. It was shown that they were nearly free from cytotoxicity independent of the particle concentration used. The imaging capability demonstrated here adds a new dimension to the medical application of nanodiamonds. In the future one will be able to combine this capability of magnetic nanodiamonds with other functions through surface modifications to perform drug delivery, targeted therapy, localized thermal treatment and diagnostic imaging at the same time.

Wavelength tuning of surface plasmon coupled quantum well infrared photodetectors.

A novel method of detection wavelength tuning for surface plasmon coupled quantum well infrared photodetectors (QWIPs) was demonstrated. By changing of the thickness of the top contact layer, the detection wavelength can be adjusted. The displacement of the detection wavelength is related to the effective dielectric constant of the dielectric layers in the device structure. The peak wavelength moves toward longer wavelength as the contact layer thickness decreases. With a proper match of the 2D metal hole array and the QW absorption region, the responsivity can be kept within a reasonable range for samples with different top contact layer thicknesses.

GaSb-based mid infrared photonic crystal surface emitting lasers.

We demonstrated for the first time above room temperature (RT) GaSb-based mid-infrared photonic crystal surface emitting lasers (PCSELs). The lasers, under optical pumping, emitted at λ(lasing)~2.3µm, had a temperature insensitive line width of 0.3nm, and a threshold power density (P(th)) ~0.3KW/cm2 at RT. Type-I InGaAsSb quantum wells were used as the active region, and the photonic crystal, a square lattice, was fabricated on the surface to provide optical feedback for laser operation and light coupling for surface emission. The PCSELs were operated at temperatures up to 350K with a small wavelength shift rate of 0.21 nm/K. The PCSELs with different air hole depth were studied. The effect of the etched depth on the laser performance was also investigated using numerical simulation based on the coupled-wave theory. Both the laser wavelength and the threshold power decrease as the depth of the PC becomes larger. The calculated results agree well with the experimental findings.

Low temperature and high magnetic field spectroscopic ellipsometry system.

We report on the design and implementation of a spectral ellipsometer at near-infrared wavelength (700-1000 nm) for samples placed in high magnetic fields (up to 14 T) at low temperatures (~4.2 K). The main optical components are integrated in a probe, which can be inserted into a conventional long-neck He dewar and has a very long free-space optical path (~1.8 m×2). A polarizer-sample-(quarter-wave plate)-rotating analyzer configuration was employed. Two dielectric mirrors, one before and one after the sample in the optical path, helped to reflect the light back to the analyzer and a two-axis piezo-driven goniometer under the sample holder was used to control the direction of the reflected light. Functional test results performed on an intrinsic GaAs wafer and analysis on the random error of the system are shown. We obtained both amplitude and phase ellipsometric spectra simultaneously and observed helicity transformation at energies near the GaAs exciton transitions in the phase spectra. Significant shifts of them induced by magnetic fields were observed and fitted with a simple model. This system will allow us to study the collective magneto-optical response of materials and spatial dispersive exciton-polariton related problems in high external magnetic fields at low temperatures.

High-quality planar light emitting diode formed by induced two-dimensional electron and hole gases.

A high-quality planar two-dimensional p-i-n light emitting diode in an entirely undoped GaAs/AlGaAs quantum well has been fabricated by using conventional lithography process. With twin gate design, two-dimensional electron and hold gases can be placed closely on demand. The electroluminescence of the device exhibit high stability and clear transition peaks so it is promising for applications on electrically-driven single photon sources.

Investigation of quantum dot passively mode-locked lasers with excited-state transition.

Monolithic passively mode-locked quantum dot lasers with excited-state transition were investigated in a broad operating range without ground-state lasing. Optical and electrical characteristics of these mode locked lasers were studied in detail at different levels of injection current and absorber bias. Very different behaviors in the evolution of the hysteresis, the optical spectra and the evolution of repetition frequency were observed between our lasers and conventional quantum dot lasers with ground-state transition. Possible mechanisms behind these observed phenomena were proposed and discussed. A minimum pulse width of 3.3 ps and an externally compressed pulse width of 0.78 ps were obtained.

Voltage-tunable dual-band quantum dot infrared photodetectors for temperature sensing.

We report voltage-tunable 3-5 µm & 8-12 µm dual-band detection in the InAs/Al0.3Ga0.7As/In0.15Ga0.85As confinement-enhanced dots-in-a-well quantum dot infrared photodetectors. The capability in temperature sensing is also demonstrated. Distinct response peaks at 5.0 µm and 8.6 µm were observed in the photocurrent spectra with working temperature up to 140K. The two peaks correspond to the transition paths from the quantum dot ground state to the quantum well state and the quantum dot excited state, respectively. At 77K, the response ratio of the 8.6 µm peak over the 5.0 µm peak changes from 0.29 at -3V to 5.8 at + 4.8V. Excellent selectivity between the two peaks with bias voltage makes the device attractive for third-generation imaging systems with pixel-level multicolor functionality.

Effects of crossed states on photoluminescence excitation spectroscopy of InAs quantum dots.

In this report, the influence of the intrinsic transitions between bound-to-delocalized states (crossed states or quasicontinuous density of electron-hole states) on photoluminescence excitation (PLE) spectra of InAs quantum dots (QDs) was investigated. The InAs QDs were different in size, shape, and number of bound states. Results from the PLE spectroscopy at low temperature and under a high magnetic field (up to 14 T) were compared. Our findings show that the profile of the PLE resonances associated with the bound transitions disintegrated and broadened. This was attributed to the coupling of the localized QD excited states to the crossed states and scattering of longitudinal acoustical (LA) phonons. The degree of spectral linewidth broadening was larger for the excited state in smaller QDs because of the higher crossed joint density of states and scattering rate.

Impacts of coulomb interactions on the magnetic responses of excitonic complexes in single semiconductor nanostructures.

We report on the diamagnetic responses of different exciton complexes in single InAs/GaAs self-assembled quantum dots (QDs) and quantum rings (QRs). For QDs, the imbalanced magnetic responses of inter-particle Coulomb interactions play a crucial role in the diamagnetic shifts of excitons (X), biexcitons (XX), and positive trions (X-). For negative trions (X-) in QDs, anomalous magnetic responses are observed, which cannot be described by the conventional quadratic energy shift with the magnetic field. The anomalous behavior is attributed to the apparent change in the electron wave function extent after photon emission due to the strong Coulomb attraction by the hole in its initial state. In QRs, the diamagnetic responses of X and XX also show different behaviors. Unlike QDs, the diamagnetic shift of XX in QRs is considerably larger than that of X. The inherent structural asymmetry combined with the inter-particle Coulomb interactions makes the wave function distribution of XX very different from that of X in QRs. Our results suggest that the phase coherence of XX in QRs may survive from the wave function localization due to the structural asymmetry or imperfections.

Time-domain and spectral-domain investigation of inflection-point slow-light modes in photonic crystal coupled waveguides.

We report on spectral-domain and time-domain measurements and numerical calculations of group velocities in a photonic crystal coupled waveguide, where the unique guided mode band structure has a flat band region within the photonic band gap allowing for slow light observation. The spectral dependence of group velocity, which is measured by interference method, indicates the existence of slow light modes around the inflection point of the unique flat band, rather than at the band edge. Time-domain observation of optical pulses propagating along two-dimension slab photonic crystal coupled waveguides is also demonstrated by using a high speed oscilloscope. By adjusting the wavelength of the input pulses toward the flat band of the coupled defect modes, an increasing duration time between reference and output pulses are clearly observed. An extremely small group velocity of 0.017c is thus obtained. Calculated group velocities show good agreement with our measured results.