Yuichi Kawamura

Ultrafast 1.55 μm all‐optical switching using low‐temperature‐grown multiple quantum wells

Low‐temperature grown surface‐reflection all‐optical switching has been demonstrated with ultrafast photoresponse (1.5 ps), low switching energy (2 pJ), high‐contrast (13 dB), polarization independence, and wide operation wavelength range in the 1.55 μm band using low‐temperature‐grown Be‐doped strained InGaAs/InAlAs multiple quantum wells. The combination of low‐temperature growth and Be‐doping contributes to the ultrafast photoresponse. Additionally, the introduction of compressive strain and a mirror with 1% reflectivity greatly enhances optical nonlinearities.

Ultrafast 1.55‐μm photoresponses in low‐temperature‐grown InGaAs/InAlAs quantum wells

Doping with Be was found to be very effective for shortening of carrier lifetime in InGaAs/InAlAs multiple quantum wells (MQWs) grown at low temperature by molecular beam epitaxy. The MQW materials have carrier lifetimes controllable from a few tens of picoseconds to 1 ps in the 1.55‐μm wavelength region, coupled with a large optical nonlinearity due to an excitonic feature, implying applicability to ultrafast optical devices in the fiber‐optic communication. The carrier lifetime was measured by a time‐resolved pump‐probe method using an optical source based on a 1.535‐μm semiconductor laser. We also investigated the resistivity, carrier density, and Hall mobility in the MQWs.

InGaP/InGaAlP double‐heterostructure and multiquantum‐well laser diodes grown by molecular‐beam epitaxy

Room‐temperature continuous‐wave (cw) operation is achieved in the MBE (molecular‐beam epitaxy)‐grown InGaP/InGaAlP double‐heterostructure (DH) visible laser diodes with a threshold current of 110 mA. The lasing wavelength and threshold current density under pulsed operation are 666 nm and as low as 3.9 kA/cm2, respectively. This result is achieved by the introduction of H2 into the growth chamber during growth, the continuous growth from one layer to the next layer, and the introduction of a GaAs buffer layer. InGaP/InGaAlP quantum well structures are also grown. From photoluminescence measurements, the conduction‐band discontinuity ΔEc is estimated to be 0.43 of the band‐gap difference ΔEg. Furthermore, the multiquantum‐well (MQW) structure is found to be stable under thermal treatment at temperatures as high as 750 °C. Room‐temperature pulsed operation of InGaP/InGaAlP MQW laser diodes is achieved for the first time. The lasing wavelength is 658 nm with a threshold current density of 7.6 kA/cm2. cw ope...

High-speed InGaAlAs/InAlAs multiple quantum well optical modulators

High-speed modulation over 22 GHz for waveguided InGaAlAs/InAlAs multiple quantum well (MQW) optical modulators is described. A large on/off ratio of over 25 dB is demonstrated with a low-drive voltage (6 V) operating in the 1.55- mu m wavelength region. The design and characteristics of MQW p-i-n modulators are discussed. The causes of large-insertion loss and the required drive voltage bandwidth figure of merit for the MQW modulator are discussed. The frequency response measurements show that the response speed is limited by the RC time constant of the device. This suggests that the speed can be further enhanced by decreasing the size and capacitance of the device. >

Molecular beam epitaxial growth of InGaAlP visible laser diodes operating at 0.66–0.68 μm at room temperatures

The room temperature pulsed operation of In0.49Ga0.31Al0.20P/In0.49Ga0.51−x AlxP/In0.49Ga0.31Al0.20P(x=0.00–0.03) double heterostructure (DH) laser diodes have been achieved for the first time. The lasing wavelength was 0.66–0.68 μm with a threshold current density of 2.6–3.6×104A/cm2 at 26 °C. These results were achieved by growing DH wafers by molecular beam epitaxy (MBE). Key points in the successful MBE growth of these DH wafers were, first, the realization of low resistance p‐type and n‐type InGaAlP layers by reducing contamination in the growth chamber. This was done by installing a substrate loading room with an interlock valve and a substrate transfer mechanism. The second was the realization of an abrupt p‐n junction by the use of Si instead of Sn as an n‐type dopant.

Monolithic integration of a DFB laser and an MQW optical modulator in the 1.5 µm wavelength range

A monolithically integrated device with an improved structure consisting of an InGaAsP/InP DFB laser and an In-GaAs/InAlAs MQW optical modulator was fabricated by the LPE (liquid phase epitaxy)/MBE (molecular beam epitaxy) hybrid growth technique. The DFB laser in this device was operated at 1.556 μm under CW condition at room temperature. A narrow coupling region between laser and modulator results in a depth of modulation as high as 55 percent at a modulator reverse bias voltage of -5 V. High-speed modulation with a response time of 300 ps was also achieved in this monolithic device.

Impact ionization rates in an InGaAs/InAlAs superlattice

An In0.53Ga0.47As/In0.52Al0.48As superlattice avalanche photodiode is fabricated by molecular beam epitaxy, and ionization rates are measured. The electron ionization rate is enhanced by a factor of 20 over hole ionization rate. This is the first time that such a remarkable increase has been observed for a superlattice. The increase is attributed to the large conduction‐band offset.

Molecular beam epitaxial growth of InGaAlP on (100) GaAs

InGaAlP epitaxial layers that are lattice matched to (100) GaAs substrates have been successfully grown for the first time by molecular beam epitaxy. The surface of the grown crystal is mirror smooth, and the full width at half‐maximum of the double crystal x‐ray diffraction pattern is less than 100 sec. The energy gap at the Γ point increases from 1.9 eV (InGaP) to 2.5 eV (InAlP) with increasing AlP mole fraction. The optical waveguide effect was also observed in InGaP/InGaAlP double heterostructure wafers.

InGaAsP/InAlAs superlattice avalanche photodiode

The fabrication of an InGaAsP-InAlAs superlattice avalanche photodiode using a gas source molecular beam epitaxy is discussed. A quaternal alloy of InGaAsP was used for the well layers in order to suppress the dark current due to the tunneling effect. With this structure, the valance band discontinuity almost vanishes and a gain bandwidth of 110 GHz was obtained. >

Refractive indices of In0.49Ga0.51−xAlxP lattice matched to GaAs

The refractive indices of In0.49Ga0.51P, In0.49Al0.51P, and In0.49Ga0.29Al0.22P, lattice matched to GaAs grown by molecular‐beam epitaxy, are determined from double‐beam reflectance measurements for photon energies ranging from 0.6 to 1.3 eV. Variation of the In0.49Ga0.51−xAlxP, refractive index with Al composition x and photon energy is calculated according to the single‐effective‐oscillator model. These analytical results are then compared with experimental data.