S. Rennon

1.55 μm single mode lasers with complex coupled distributed feedback gratings fabricated by focused ion beam implantation

Complex coupled GaInAsP/InP distributed feedback lasers were developed based on maskless focused ion beam lithography. By combining implantation enhanced wet chemical etching and implantation induced thermal quantum well intermixing a refractive index grating was defined self-aligned to a gain grating forming a complex coupled grating lateral to a ridge waveguide. The devices show single mode emission at wavelengths around 1.55 μm with linewidths 90%) over a large tuning range (88 nm) was achieved.

High-frequency properties of 1.55 μm laterally complex coupled distributed feedback lasers fabricated by focused-ion-beam lithography

Laterally complex coupled distributed feedback lasers have been fabricated by focused-ion-beam lithography on completely grown InGaAsP/InP laser structures emitting at 1.55 μm. The grating definition is based on implantation-enhanced quantum-well intermixing and wet chemical etching and allows the fabrication of complex coupled antiphase gratings without any further overgrowth step. Side-mode suppression ratios of 45 dB and bandwidths for direct modulation beyond 13 GHz were obtained for 300-μm-long devices. The lasers exhibit frequency-modulation response values lower than 200 MHz/mA and feature a low sensitivity to back-reflected light. Preliminary lifetime measurements over 7000 h continuous-wave operation at room temperature show no significant indication for long-time degradation.

Complex coupled distributed-feedback and Bragg-reflector lasers for monolithic device integration based on focused-ion-beam technology

Complex coupled distributed feedback (CC-DFB) and distributed Bragg-reflector (DBR) GaInAsP-InP lasers were fabricated by focused-ion-beam lithography. Due to the high single-mode yield and the simplified process technology, these devices are very suitable for monolithic device integration for wavelength-division-multiplexing (WDM) components. In a two-section device with a grating and an electrically separated waveguide gain section, the combination of a DFB laser with a passive waveguide, as well as the operation as a DBR laser with an unpumped grating section was investigated. The DFB and DBR lasers show very high single-mode stability in terms of wavelength detuning, bias current and temperature. For DBR lasers, a side-mode suppression ratio of well above 50 dB was achieved over a current range of more than 10 times the threshold current. Due to the good detuning properties. DFB Lasers were strongly detuned to the long wavelength side to suppress band edge absorption in passive waveguide sections. For a 3-mm-long waveguide the absorption was reduced by more than 20 dB. Although ion implantation is involved in the fabrication process of the lasers lifetime measurements over more than 10000 h of continuous-wave (CW) operation at room temperature show no significant device degradation.

Reactive ion etching of deeply etched DBR-structures with reduced air-gaps for highly reflective monolithically integrated laser mirrors

An electron cyclotron resonance reactive ion etching process is investigated for the fabrication of third order deeply etched distributed Bragg reflectors suitable for monolithically integrated laser mirrors. At a period of 550-nm, air-gaps as small as 150 nm down to a depth of 4.5 μm were realized in AlGaAs/GaAs heterostructures. The reflectivity of the deeply etched DBRs was tested by the device properties of microlasers with a cleaved facet on one side and a DBR on the other side. From the light output characteristic of the lasers a systematic improvement of the mirror reflectivity is observed by reducing the air-gap.

Selective growth of InP on focused-ion-beam-modified GaAs surface by hydride vapor phase epitaxy

The growth of InP islands on a planar focused-ion-beam (FIB)-modified (001) GaAs substrate was investigated in a hydride vapor phase epitaxy system. InP grew selectively on the FIB-implanted lines, forming continuous stripes, whereas isolated islands were observed outside the implanted area. The impact of the III/V ratio, crystallographic orientation of implanted lines, and implantation dose was explored. The choice of suitable growth conditions makes it possible to obtain continuous InP wires aligned in all possible directions. The results of this work could be used for the fabrication of future optoelectronic integrated circuits, which would include nanoscale structures, e.g., quantum-wire optical devices with GaAs electronic circuits.

Nanoscale patterning by focused ion beam enhanced etching for optoelectronic device fabrication

Abstract A technique for highly resolved maskless patterning is obtained by combining focused ion beam lithography with wet chemical etching. When exposing InP to a focused Ga + -ion beam it acts like a photoresist with hydrofluoric acid (HF) as the appropriate developer. This technology allows the fabrication of filter gratings for single mode emitting lasers. Distributed feedback (DFB) and distributed Bragg-reflector (DBR) lasers with a high single mode stability and side mode suppression ratios (SMSR) above 50 dB were realized. The devices show no degradation after more than 10 000 h of cw operation.

Grazing-incidence diffraction strain analysis of a laterally-modulated multiquantum well system produced by focused-ion-beam implantation

Focused Ga+ ion beam implantation was used to define a laterally periodic modulation of the electronic band gap in a GaAs/Ga0.97In0.03As/Al0.2Ga0.8As/GaAs [001] multiquantum well structure. The samples were investigated as-implanted and after a rapid thermal annealing (60 s at 650 and 800 °C) by means of x-ray grazing-incidence diffraction. The method provides a separate inspection of the induced strain and the damage profiles as a function of depth below the sample surface. For samples with an ion dose of 5×1013 cm−2, we found a nearly uniform lateral strain amplitude of about 2×10−3 up to the maximum information depth of about 500 nm. It was accompanied by the appearance of structural defects. Rapid thermal annealing at 650 °C has reduced the strain amplitude by a factor of five as well as the density of volume defects. The maximum strain amplitude were found in a depth of about 100 nm. After rapid thermal annealing at 800 °C, the strain has disappeared.

Edge-emitting microlasers with one active layer of quantum dots

High performance edge-emitting microlasers with deeply etched distributed Bragg reflectors (DBRs) were fabricated on an AlGaAs-GaAs laser structure with a single GaInAs quantum dot (QD) active layer. Mirror reflectivities well above 90% were achieved by third-order narrow air-gap Bragg reflectors with /spl lambda//4 air-gaps. DBR lasers with 160-/spl mu/m-long cavities and cleaved mirrors on one side show differential efficiencies of 0.87 W/A and output powers of more than 50 mW at 980-nm emission wavelength in continuous wave (CW) operation at room temperature. With deeply etched DBRs on both sides of the cavity CW operating microlasers with cavity lengths down to 16 /spl mu/m could be realized with a minimum threshold current of 1.2 mA for a 30-/spl mu/m cavity length. All lasers are emitting at the QD ground state at room temperature. Twenty-/spl mu/m-long devices show CW threshold currents of about 3 mA, output powers above 1 mW, and single mode emission with >25 dB sidemode suppression ratios. First, high-frequency measurements mere performed proving that these QD microlasers are well suited for large-scale integrated high-speed optical data processing with modulation frequencies well above 10 GHz.

Photonic crystals for optoelectronic devices

We present results on the fabrication and characterization of two and one dimensional photonic crystals for optoelectronic device applications. By using high resolution electron beam lithography 2D and 1D photonic crystals structures are defined on GaAlAs/GaAs and InP/InGaAsP waveguide layers. A crucial step of the patterning is the dry etching, in which structures on a (sub) 100 nm scale with aspect ratios (width to height) of ten or more have to be obtained. We have realized straight waveguides, waveguides with sharp bends, waveguides with build - in cavities as well as lasers with 1D and 2 D photonic crystal mirrors. By using a build in quantum dot layer, optical modes in the passive structures can be investigated. In the InGaAsP as well as in the InGaAs material system ridge waveguide lasers with photonic crystal mirrors have been realized. For the InGaAs system 1D Bragg reflectors with reflectivities above 95 percent have been obtained. These mirrors are essential for mircolasers with active resonator length down to 12 micrometers . These are the shortest edge emitting lasers realized to date.