Gregory M. Peake

Low-dislocation-density GaN from a single growth on a textured substrate

The density of threading dislocations (TD) in GaN grown directly on flat sapphire substrates is typically greater than 10{sup 9}/cm{sup 2}. Such high dislocation densities degrade both the electronic and photonic properties of the material. The density of dislocations can be decreased by orders of magnitude using cantilever epitaxy (CE), which employs prepatterned sapphire substrates to provide reduced-dimension mesa regions for nucleation and etched trenches between them for suspended lateral growth of GaN or AlGaN. The substrate is prepatterned with narrow lines and etched to a depth that permits coalescence of laterally growing III-N nucleated on the mesa surfaces before vertical growth fills the etched trench. Low dislocation densities typical of epitaxial lateral overgrowth (ELO) are obtained in the cantilever regions and the TD density is also reduced up to 1 micrometer from the edge of the support regions.

Resonantly Enhanced Second-Harmonic Generation Using III–V Semiconductor All-Dielectric Metasurfaces

Nonlinear optical phenomena in nanostructured materials have been challenging our perceptions of nonlinear optical processes that have been explored since the invention of lasers. For example, the ability to control optical field confinement, enhancement, and scattering almost independently allows nonlinear frequency conversion efficiencies to be enhanced by many orders of magnitude compared to bulk materials. Also, the subwavelength length scale renders phase matching issues irrelevant. Compared with plasmonic nanostructures, dielectric resonator metamaterials show great promise for enhanced nonlinear optical processes due to their larger mode volumes. Here, we present, for the first time, resonantly enhanced second-harmonic generation (SHG) using gallium arsenide (GaAs) based dielectric metasurfaces. Using arrays of cylindrical resonators we observe SHG enhancement factors as large as 10(4) relative to unpatterned GaAs. At the magnetic dipole resonance, we measure an absolute nonlinear conversion efficiency of ∼2 × 10(-5) with ∼3.4 GW/cm(2) pump intensity. The polarization properties of the SHG reveal that both bulk and surface nonlinearities play important roles in the observed nonlinear process.

The MAC - a miniature atomic clock

The authors are developing a chip-scale atomic clock (CSAC), more than two orders of magnitude smaller and lower power than any existing technology. As an intermediate milestone, en route to the ultimate CSAC objectives, we have developed a miniature atomic clock (MAC), combining the low-power CSAC physics package with a low-parts count, low-power digital control and microwave system. The MAC is a complete packaged atomic clock, with overall size of 10 cm/sup 3/, power consumption >200 mW, and short-term stability /spl sigma//sub y/(/spl tau/)/spl sim/4/spl times/10/sup -10//spl tau//sup - 1/2 /. The MAC provides a valuable testbed for the further development and refinement of the CSAC physics package as well as for the development of the CSAC control electronics prior to undertaking the costly and time-consuming size-reduction effort which will be necessary to meet the ultimate CSAC objectives. The MAC itself may find applications in commercial and military timing systems which require the relatively small size and power consumption of the MAC now, rather than wait for the evolution of the 1 cm/sup 3/, 30 mW CSAC.

VCSELs for atomic clocks

The spectroscopic technique of coherent population trapping (CPT) enables an all-optical interrogation of the groundstate hyperfine splitting of cesium (or rubidium), compared to the optical-microwave double resonance technique conventionally employed in atomic frequency standards. All-optical interrogation enables the reduction of the size and power consumption of an atomic clock by two orders of magnitude, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources due to their low power consumption and circular output beam. Several research teams are currently using VCSELs for DARPAs chip-scale atomic clock (CSAC) program with the goal of producing an atomic clock having a volume < 1 cm^3, a power consumption < 30 mW, and an instability (Allan deviation) < 1x10^-11 during a 1-hour averaging interval. This paper describes the VCSEL requirements for CPT-based atomic clocks, which include single mode operation, single polarization operation, modulation bandwidth > 4 GHz, low power consumption (for the CSAC), narrow linewidth, and low relative intensity noise (RIN). A significant manufacturing challenge is to reproducibly obtain the required wavelength at the specified VCSEL operating temperature and drive current. Data are presented that show the advantage of operating at the D1 (rather than D2) resonance of the alkali atoms. Measurements of VCSEL linewidth will be discussed in particular, since atomic clock performance is especially sensitive to this parameter.

Direct Modulation Characteristics of Composite Resonator Vertical-Cavity Lasers

We report the small-signal modulation characteristics of a monolithic dual resonator vertical cavity surface emitting laser. The modulation response is described by a system of rate equations with two independent carrier populations and a single longitudinal optical mode. The independent optical overlaps and differential gains of the two active regions can each be adjusted to maximize the output response. We show that under certain conditions, the composite resonator may achieve a higher bandwidth than a single cavity laser with the same photon density. We find the relaxation oscillation frequency to depend mainly on the total photon density and not the individual currents in the two cavities. With appropriate current injection, the composite resonator laser achieves a maximum -3-dB bandwidth of 12.5 GHz and a maximum modulation current efficiency factor of approximately 5GHz/ma1/2

The Miniature Atomic Clock - Pre-Production Results

The authors have developed a miniature atomic clock (MAC) for applications requiring atomic timing accuracy in portable battery-powered applications. Recently, we have completed a pre-production build of 10 devices in order to evaluate unit-to-unit performance variations and to gain statistical confidence in the performance specifications, environmental sensitivity, and manufacturability.

VCSELs for atomic sensors

A new generation of small low-power atomic sensors, including clocks, magnetometers, and gyroscopes, is being developed based on recently available MEMS and VCSEL technologies. These sensors rely on spectroscopic interrogation of alkali atoms, typically rubidium or cesium, contained in small vapor cells. The relevant spectroscopic wavelengths (in vacuum) are 894.6 nm (D1) and 852.3 nm (D2) for cesium, and 795.0 nm (D1) and 780.2 nm (D2) for rubidium. The D1 wavelengths are either preferred or required, depending on the application, and vertical-cavity surface-emitting lasers (VCSELs) are preferred optical sources because of their low power consumption and circular output beam. This paper describes the required VCSEL characteristics for atomic clocks and magnetometers. The fundamental VCSEL requirement is single-frequency output with tunability to the particular spectroscopic line of interest. Single-polarization and single-transverse-mode operation are implicit requirements. VCSEL amplitude noise and frequency noise are also important because they contribute significantly to the sensor signal-to-noise ratio. Additional desired VCSEL attributes are low cost, low power consumption, and several years of continuous operating lifetime. This paper also describes the 894-nm VCSELs that we have developed for cesium-based atomic sensors. In particular, we discuss VCSEL noise measurements and accelerated lifetime testing. Finally, we report the performance of prototype atomic clocks employing VCSELs.

Single transverse mode operation of electrically pumped vertical-external-cavity surface-emitting lasers with micromirrors

We report an electrically pumped vertical-external-cavity surface-emitting laser (VECSEL) that is designed for wafer-scale fabrication. Single-mode continuous-wave operation is demonstrated at a wavelength of 970 nm. The device structure incorporates a curved micromirror output coupler that is produced using a micromolding process. In addition to outlining the VECSEL fabrication process, we quantify its spatial and spectral modal characteristics.

Large S-section-ring-cavity diode lasers: directional switching, electrical diagnostics, and mode beating spectra

Semiconductor lasers with monolithically integrated ring cavities exceeding 1 cm in perimeter were fabricated and characterized optically and electrically. Directional switching was observed, influenced by S-section seeding of unidirectional operation. The lasing threshold was identified by differential current-voltage measurements, which are also shown to be useful in monitoring directional switching. Mode beating was observed at three radio-frequency bands: 7.6, 15.2, and 22.9 GHz.

Frequency beating between monolithically integrated semiconductor ring lasers

Optoelectronic integrated circuits incorporating a pair of optically independent large-cavity semiconductor ring lasers (SRLs), directional couplers, waveguides, Y-junction mixer, and photodetectors are demonstrated. Counterclockwise and clockwise output beams from the two SRLs are collected separately and mixed prior to detection. Frequency beating between modes of two SRLs is measured. The beat frequency is fine-tuned by an integrated Joule heater, designed for thermal control of the lasing wavelength. No signs of frequency lock-in in the vicinity of zero detuning are observed, which makes this structure a promising candidate for applications in ring laser gyros and optical rotation sensors.