We have investigated the intersubband relaxation times in asymmetrical step quantum wells with a high width ratio and observed population inversion between the state in the step and the state in the well. Based on the observed population inversion a novel intersubband laser structure at 10 micrometer wavelength is proposed.
The frequency dependence of the various parameters which determines the feasibility of intersubband lasers based on quantum wells are discussed and the more desirable frequency range in terms of each parameter is described. These parameters include the optical gain, the confinement factor, the relaxation rate, and the optical losses. The advantages and disadvantages of employing both electrical and optical pumping for creating population inversion for intersubband lasers are also discussed. Then, the potential problems concerning the realization of some of the proposed laser structures based on intersubband transitions in quantum wells are discussed and the more promising schemes are determined. Based on the recently published experimental results on resonant tunneling processes in coupled quantum wells as well as the free carrier absorption in doped layers, it is concluded that the realization of proposed electrically pumped lasers at far-infrared frequencies is extremely difficult at the present time. This is due to the high free carrier absorption in the doped layers and the inability to invert the population of the upper and lower subbands (laser states) of the quantum well in the active layer. Optically pumped structures do not necessary rely on the resonant tunneling process for creating population inversion and also doped injector/collector layers may be avoided in these structures and hence they could be more promising for the realization of intersubband lasers at far-infrared frequencies.
A well established selective etching technology for GaAs IMPATT diodes was employed to fabricate GaAs TUNNETT diodes on diamond heat sinks. The improved heat dissipation led to significantly higher dc to RF conversion efficiencies and doubled the available RF output power of diodes on diamond heat sinks compared to diodes from the same MBE-grown material on integral heat sinks. An RF output power of more than 80 mW with a corresponding dc to RF conversion efficiency of 5.25% was measured at 106.9 GHz. To the authors' knowledge, this is the first successful demonstration of GaAs TUNNETT diodes on diamond heat sinks, and the dc to RF conversion efficiencies and RF power levels are the highest reported to date from any single device made of group III-V materials (GaAs, InP, etc.) at this frequency. A free-running TUNNETT diode oscillator at 107.7 GHz showed an excellent phase noise of less than -94 dBc/Hz, measured at a frequency off-carrier of 500 kHz and an RF power of 40 mW. Phase-locking of TUNNETT diode oscillators has been demonstrated in a setup with a EIP 578 frequency counter and external down-conversion.
An optical GaAs/AlGaAs heterojunction bipolar transistor technology has been developed using transparent indium-tin-oxide emitter contacts for unobstructed through-the-top optical access. Optical tuning and injection locking has been observed in 6 GHz microwave oscillators made with this device. Optical tuning ranges up to 25 MHz and locking ranges up to 2.5 MHz have been demonstrated with the optical injection of RF power at 30 dB below the oscillator power level.
This paper describes a coherent detection receiver (CDR) with adaptive image rejection capability which is also compatible with existing monolithic integrated optics and MMIC technology. It details the component selection and circuit layout tradeoffs that were considered for the design and fabrication of the CDR in monolithic IC format. An optical `half receiver' IC with waveguides, phase shifter, 3 dB coupler, and balanced mixer diodes has been constructed and tested. Measurements and results for this chip are given and discussed. An initial design and simulated results for the microwave (intermediate frequency) IC are also given and discussed.
The existence of negative resistance in double barrier resonant tunneling structures has led to the proposal of various applications for these devices. For many of these applications, stability is an important consideration. This paper will discuss the effect that various device parameters have on stability and on the capability of high frequency device operation. It is concluded that the circuit and device conditions required for stable operation greatly reduce the amount of power that can be produced by these devices at microwave and millimeter-wave frequencies.