L. T. Florez

Vertical-cavity surface-emitting lasers: Design, growth, fabrication, characterization

The authors have designed, fabricated, and tested vertical-cavity surface-emitting lasers (VCSEL) with diameters ranging from 0.5 mu m to>50 mu m. Design issues, molecular beam epitaxial growth, fabrication, and lasing characteristics are discussed. The topics considered in fabrication of VCSELs are microlaser geometries; ion implementation and masks; ion beam etching packaging and arrays, and ultrasmall devices. >

Application of reflectance difference spectroscopy to molecular‐beam epitaxy growth of GaAs and AlAs

We perform an accuracy analysis of several possible reflectance–difference (RD) configurations that are compatible with standard molecular‐beam epitaxy (MBE) growth chambers, and describe in detail an optical‐bridge system that can determine relative changes in RD signals as small as 5×10−5 under standard growth conditions. Using this system, we determine the RD response of GaAs for changes in surface conditions at different wavelengths and correlate these to simultaneously measured reflection high‐energy electron diffraction (RHEED) intensities. Maximum anisotropies are found at 2.0–2.5 and 3.5 eV for Ga on GaAs and Al on AlAs, respectively, providing a way of spectrally distinguishing Ga–Ga and Al–Al dimers for surface‐chemical investigations, and suggesting that these photon energies are also optimal for modifying growth by light. At photon energies well removed from these anisotropy maxima, RD signals follow changes in surface structure, as RHEED. Our RD‐RHEED correlations provide insight concerning c...

Dynamic, polarization, and transverse mode characteristics of vertical cavity surface emitting lasers

The dynamic, polarization, and transverse mode characteristics of strained InGaAs-GaAs quantum well vertical cavity surface emitting lasers (VCSELs) emitting at 0.98 mu m are investigated. The dynamic behavior of VCSELs with high and low operating voltages and series resistances is compared. A large wavelength chirp in the lasing spectrum was observed for the lasers with high voltage/resistance, even under low-duty-cycle pulse operation. This is thought to be due to resistive heating close to the laser junction. It is observed that the transverse mode structure of VCSELs and their dependence on laser dimensions and drive current are highly analogous to those of edge emitting lasers, whereas the polarization characteristics of the two types of lasers are significantly different. >

Van der Waals bonding of GaAs epitaxial liftoff films onto arbitrary substrates

Epitaxial liftoff is an alternative to lattice‐mismatched heteroepitaxial growth. Multilayer AlxGa1−xAs epitaxial films are separated from their growth substrates by undercutting an AlAs release layer in HF acid (selectivity ≳108 for x≤0.4). The resulting AlxGa1−xAs films tend to bond by natural intermolecular surface forces to any smooth substrate (Van der Waals bonding). We have demonstrated GaAs thin‐film bonding by surface tension forces onto Si, glass, sapphire, LiNbO3, InP, and diamond substrates, as well as self‐bonding onto GaAs substrates. In transmission electron microscopy the substrate and thin‐film atomic lattices can be simultaneously imaged, showing only a thin (20–100 A) amorphous layer in between.

Multiple wavelength tunable surface-emitting laser arrays

Techniques to achieve wavelength multiplexing and tuning capabilities in vertical-cavity surface-emitting lasers (VCSELs) are described, and experimental results are given. The authors obtained 140 unique, uniformly separated, single-mode wavelength emissions from a 7*20 VCSEL array. Large total wavelength span ( approximately 430 AA) and small wavelength separation ( approximately 3 AA) are obtained simultaneously with uncompromised laser performance. All 140 lasers have nearly the same threshold currents, voltages, and resistances. Wavelength tuning is obtained by using a three-mirror coupled-cavity configuration. The three-mirror laser is a two-terminal device and requires only one top contact. Discrete tuning with a range as large as 61 AA is achieved with a small change in drive current of only 10.5 mA. >

Surface-Emitting Microlasers for Photonic Switching and Interchip Connections

Vertical-cavity electrically pumped surface-emitting microlasers are formed on GaAs substrates at densities greater than two million per square centimeter. Two wafers were grown with ln0.2Ga0.8As active material composing three 80 ? thick quantum wells in one and a single quantum well (SQW) 100 ? thick in the other. Lasing was seen in devices as small as 1 .5 ?m diameter with <0.05 ?m3 active material. SQW microlasers 5 x 5 ?m square had room-temperature cw current thresholds as low as 1.5 mA with 983 nm output wavelength. 10 x 10 ?m square SQW microlasers were modulated by a pseudorandom bit generator at 1 Gb/s with less than 10-10 bit error rate. Pulsed output >170 mW was obtained from a 100?m square device. The laser output passes through the nominally transparent substrate and out its back side, a configuration well suited for micro-optic integration and photonic switching and interchip connections.

Transverse mode characteristics of vertical cavity surface-emitting lasers

Transverse mode characteristics of vertical cavity surface‐emitting (VC‐SE) lasers are described. The mode structure is investigated as a function of the transverse dimension for proton‐implanted gain‐guided VC‐SE lasers. A comparison is made to an air‐post index‐guided structure. The lasing modes and the evolution of the modes with increasing drive current for the VC‐SE lasers are observed to be highly analogous to those of the edge‐emitting lasers. Broad‐area gain‐guided lasers lase in the fundamental TEM00 mode near threshold. At higher currents, high‐order modes are successively excited. A 5 μm square proton‐implanted gain‐guided VC‐SE laser emits a single mode. On the other hand, an air‐post index‐guided SE laser, due to the large index difference between the laser and the cladding, emits multiple transverse modes. Moreover, we show that the gain‐guided VC‐SE lasers exhibit better device characteristics than the air‐post index‐guided lasers.

Two‐dimensional phase‐locked arrays of vertical‐cavity semiconductor lasers by mirror reflectivity modulation

Coupling of two‐dimensional (2D) vertical‐cavity surface‐emitting lasers (VCSELs) to give a coherent supermode is described. The top metal layer of a stained‐layer InGaAs quantum well VCSEL structure was patterned laterally by depositing various metals with different optical reflectivities. This lateral reflectivity patterning defined a 2D laser array sharing the same ‘‘supercavity’’. It is shown that these 2D arrays oscillate in a stable single, coherent 2D supermode. This was achieved with a simple planar process and without significant deterioration of threshold current and efficiency relative to an equivalent broad‐area VCSEL.

Large two‐dimensional arrays of phase‐locked vertical cavity surface emitting lasers

The phase‐locking of two‐dimensional (2D) arrays incorporating a large number of electrically pumped, vertical cavity surface emitting lasers (VCSELs) is described. The InGaAs/GaAs quantum well VCSELs are phase locked by patterning the reflectivity of the laser back mirrors. Structures with both weak and strong modulation of the mirror reflectivity have been studied. The strongly modulated (weakly coupled) structures exhibit superior coherence and beam patterns, with up to 27×27 lasers oscillating in virtually a single (highest order) supermode. The weakly modulated (strongly coupled) arrays also emit highly coherent beams, but with lower threshold currents and higher differential efficiencies. Weakly modulated arrays of 20×20 elements (120×120 μm2) exhibited threshold currents of <1 mA per laser and ≳300 mW pulsed output powers. These coherent 2D arrays should be useful for optical signal processing and high optical power applications.

Reflectance‐difference spectroscopy system for real‐time measurements of crystal growth

We describe a reflectance‐difference spectroscopy system for real‐time in situ optical studies of crystal growth. Either changes in surface chemistry or surface structure can be monitored depending on wavelength.