Kian-Giap Gan

A racetrack mode-locked silicon evanescent laser

By utilizing a racetrack resonator topography, an on-chip mode locked silicon evanescent laser (ML-SEL) is realized that is independent of facet polishing. This enables integration with other devices on silicon and precise control of the ML-SELs repetition rate through lithographic definition of the cavity length. Both passive and hybrid mode-locking have been achieved with transform limited, 7 ps pulses emitted at a repetition rate of 30 GHz. Jitter and locking range are measured under hybrid mode locking with a minimum absolute jitter and maximum locking range of 364 fs, and 50 MHz, respectively.

Vertical-cavity surface-emitting laser active regions for enhanced performance with optical pumping

We have developed an improved active region design for optically pumped vertical-cavity surface-emitting lasers. The design makes use of carrier-blocking layers to segment the absorber and promote uniform carrier populations in the quantum wells with pump efficiencies near 75%. A model to calculate the carrier distribution in the active region and a design methodology are presented along with a metric to describe the carrier uniformity in the quantum wells. Experimental verification of the performance improvements shows an over 50% reduction in device thresholds and an increase of 20/spl deg/C in maximum operating temperatures.

Ultrahigh-power-bandwidth product and nonlinear photoconductance performances of low-temperature-grown GaAs-based metal-semiconductor-metal traveling-wave photodetectors

Maximum-output-power and bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors (PDs). In this paper, we report record high-peak output voltage (/spl sim/ 30 V) together with ultrahigh-speed performance (1.8 ps, 190 GHz) observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak output power and ultrahigh-electrical bandwidth performances were achieved due to superior MSM microwave guiding structure, short carrier trapping time, and the capability to take high bias voltage (/spl sim/ 30 V) with a LTG-GaAs layer. Under such a high bias voltage, a significant nonlinear photocurrent increase with the bias voltage was observed. The nonlinear photoconductance and ultrahigh-output power-bandwidth performances opens a new way in the application of high-performance optoelectronic mixers and photomixer devices.

High-speed and high-power performances of LTG-GaAs based metal-semiconductor-metal traveling-wave-photodetectors in 1.3-/spl mu/m wavelength regime

In this letter, we demonstrated ultrahigh bandwidth and high output power performances of low-temperature-grown (LTG) GaAs-based metal-semiconductor-metal traveling wave photodetectors (MSM TWPDs) in the long wavelength regime (/spl sim/1300 nm). Ultrahigh bandwidth (1.3-ps pulsewidth with 234 GHz transformed 3-dB electrical bandwidth) was achieved with long-absorption-length (70-/spl mu/m) devices due to the improved microwave property in the MSM TWPDs and their high velocity-mismatch bandwidth. Under high optical power illumination, these 70-/spl mu/m-long MSM TWPD devices also exhibited superior output power-bandwidth-product performance due to their large absorption volumes. To the best of our knowledge, the demonstrated peak-output-voltage-bandwidth product (3.55 V, 160 GHz, 568 GHz-V) is the highest among the reported photodetectors for Icing optical communication wavelength (1.2 /spl mu/m-1.6 /spl mu/m) applications.

Ultrahigh power-bandwidth-product performance of low-temperature-grown-GaAs based metal-semiconductor-metal traveling-wave photodetectors

High-output-power and high-bandwidth performances are usually two tradeoff parameters in the design of high-speed photodetectors. In this letter, we report high peak-output-voltage (∼20 V) and peak-output-current (∼400 mA, 50 Ω load) together with ultrahigh-speed performances (1.5 ps, 220 GHz), observed in low-temperature-grown-GaAs (LTG-GaAs) based metal-semiconductor-metal (MSM) traveling-wave photodetectors (TWPDs) at a wavelength of 800 nm. Ultrahigh-peak-output-power and ultrahigh-electrical-bandwidth performances were achieved due to the superior MSM microwave guiding structure and a short carrier trapping time in the LTG-GaAs layer, which reduced the space-charge screening effect and increased the photoabsorption volume without sacrificing electrical bandwidth significantly. We also observed different bias-dependent nonlinear behaviors in MSM TWPDs under high and low illuminated optical power excitations, which are possibly dominated by the space-charge screening and the lifetime increasing effects, respectively.

Ultrafast valence intersubband hole relaxation in InGaN multiple-quantum-well laser diodes

The ultrafast carrier dynamics in InGaN multiple-quantum-well (MQW) laser diodes were investigated using a time-resolved bias-lead monitoring technique. From the optical selection rules of TE and TM polarized light, one can selectively excite and probe different valence-subband-to-conduction-subband transitions in the MQW structure with different polarized pump and probe light. The subband structure of the MQW structure of the laser diode was calculated and is verified by electroluminescence measurement. Using this technique, ultrafast valence intersubband hole relaxation processes (τ<0.35 ps) were found to dominate the observed carrier dynamics.

Measurement of gain, group index, group velocity dispersion, and linewidth enhancement factor of an InGaN multiple quantum-well laser diode

Gain, group index, group velocity dispersion (GVD), temperature variation of refractive index, and linewidth enhancement factor of an In/sub 0.15/Ga/sub 0.85/N/In/sub 0.02/Ga/sub 0.98/N multiple quantum-well blue laser diode was measured using the Fourier transform method as a function of wavelength from 400 to 410 nm. At the lasing wavelength (403.5 nm), the group index is 3.4, the GVD (dn/sub g//d/spl lambda/) is -37 /spl mu/m/sup -1/, the temperature variation of refractive index dn/dT is 1.3/spl times/10/sup -4/ K/sup -1/, and the linewidth enhancement factor is 5.6.

1.3 μm wavelength vertical cavity surface emitting laser fabricated by orientation-mismatched wafer bonding: A prospect for polarization control

We propose and demonstrate a long-wavelength vertical cavity surface emitting laser (VCSEL) which consists of a (311)B InP-based active region and (100) GaAs-based distributed Bragg reflectors (DBRs), with an aim to control the in-plane polarization of output power. Crystal growth on (311)B InP substrates was performed under low-migration conditions to achieve good crystalline quality. The VCSEL was fabricated by wafer bonding, which enables us to combine different materials regardless of their lattice and orientation mismatch without degrading their quality. The VCSEL was polarized with a power extinction ratio of 31 dB.

Traveling-wave photodetectors with high power-bandwidth and gain-bandwidth product performance

Traveling-wave photodetectors (TWPDs) are an attractive way to simultaneously maximize external quantum efficiency, electrical bandwidth, and maximum unsaturated output power. We review recent advances in TWPDs. Record high-peak output voltage together with ultrahigh-speed performance has been observed in low-temperature-grown GaAs (LTG-GaAs)-based metal-semiconductor-metal TWPDs at the wavelengths of 800 and 1300 nm. An approach to simultaneously obtain high bandwidth and high external efficiency is a traveling-wave amplifier-photodetector (TAP detector) that combines gain and absorption in either a sequential or simultaneous traveling-wave structure.

Electron relaxation and transport dynamics in low-temperature-grown GaAs under 1 eV optical excitation

Electron relaxation and transport dynamics in low-temperature-grown GaAs under 1 eV optical excitation was investigated by femtosecond transient transmission measurement and electro-optical sampling measurement in bulk samples and fabricated devices. An increase in the electron lifetime can be observed when the electron density is higher than 3×1017 cm−3. This effect is attributed to prolonged electron relaxation due to intervalley scattering of highly excited electrons and associated hot phonon effects. Our conclusion is further supported by bias-dependent studies where intervalley scattering was achieved using high electric fields.