Chih-Hsiung Chen

Theoretical model for intravalley and intervalley free-carrier absorption in semiconductor lasers: beyond the classical Drude model

Free-carrier absorption is calculated from the second-order perturbation theory of quantum mechanics by considering the interactions between carriers and polar optical phonons, deformation potential optical phonons, deformation potential acoustic phonons, piezoelectric acoustic phonons, and charged impurities in the intravalley transition and the intervalley transition. A formula is derived from our theoretical model for the coefficient of free-carrier absorption by incorporating the state-filling effect and the degenerate carrier distribution. Our results indicate that the classical Drude model is inadequate to describe many features of the free-carrier absorption. Alternatively, our theoretical model may provide an efficient method for investigating the effect of free-carrier absorption on the functionality or performance of the related optoelectronic device.

Theoretical modeling of nonequilibrium optical phonons and electron energy relaxation in GaN

The decay of the zone-center longitudinal optical (LO) phonon in GaN into a transverse optical (TO) phonon and a longitudinal acoustic (LA) phonon is theoretically investigated. Its decay into two LA phonons is forbidden. A theoretical model is presented to study the effect of nonequilibrium LO and TO phonons on the electron energy relaxation rate. The electron energy relaxation time is calculated, and its value is shown to strongly depend on the finite lifetimes of both LO and TO phonons. The individual contributions of A1 mode and E1 mode optical phonons in the overall electron energy relaxation processes are also discussed.

A small-signal analysis of the modulation response of high-speed quantum-well lasers: effects of spectral hole burning, carrier heating, and carrier diffusion-capture-escape

We present a small-signal analysis of the modulation response by simultaneously considering the effects of spectral hole burning, carrier heating, and carrier diffusion capture-escape. An explicit form of the small-signal modulation response is obtained and the nonlinear gain coefficients associated with each physical process are defined. Further simplifications in our results will give analytical forms for calculating the resonant frequency and damping rate of the modulation response. One of the simplified versions of our results is shown to agree with previous investigations. The effects of the carrier dephasing time, energy relaxation time, and diffusion-capture-escape times on the high-speed performance of QW lasers are theoretically investigated.

Theoretical modeling of the small-signal modulation response of carrier and lattice temperatures with the dynamics of nonequilibrium optical phonons in semiconductor lasers

A theoretical model is presented that is capable of simultaneously simulating the small-signal modulation response of the carrier density, photon density, electron temperature, hole temperature, populations of nonequilibrium longitudinal (LO) and transverse optical (TO) phonons at different wave vectors, and lattice temperature in semiconductor lasers. The phonon dynamics of nonequilibrium LO and TO phonons is calculated from first principles by considering the polar and deformation-potential interactions between carriers and optical phonons. Rate equations of the energy transfer among electrons, holes, photons, optical phonons, and acoustic phonons are given. The small-signal modulation responses of carrier and lattice temperatures are calculated. The different roles of carrier and lattice heating in semiconductor lasers are discussed.

Effects of electron–hole energy transfer on the nonlinear gain coefficients in the small-signal modulation response of semiconductor lasers

The effects of energy transfer between electrons and holes on the small-signal modulation response of semiconductor lasers are theoretically investigated. We calculate the electron energy relaxation time due to electron–hole scattering from the first principle. We show that its value is comparable to the electron energy relaxation time due to the electron–LO phonon scattering with the effect of nonequilibrium LO phonons. In such a case, the nonlinear gain coefficient due to carrier heating defined in the small-signal modulation response of semiconductor lasers is no longer a simple sum of the term due to electron heating and that due to hole heating.

Theoretical modeling of carrier and lattice heating effects for frequency chirping in semiconductor lasers

A theoretical model is presented that is capable of simultaneously simulating the frequency response of the photon density, carrier density, electron temperature, hole temperature, populations of nonequilibrium longitudinal optical (LO) and transverse optical (TO) phonons at different wave vectors, and lattice temperature under the modulation of small-signal current. Our results not only provide a more consistent theoretical model for frequency chirping but also illustrate the different roles of carrier and lattice heating in the frequency response of semiconductor lasers.

Effects of electron-hole energy transfer on the small-signal modulation response of semiconductor lasers

In this work, without assigning an arbitrary time constant and thus obscuring the role of electron-hole energy transfer, we calculate the electron energy relaxation rate due to electron-hole scattering from first-principles, and derive an explicit formula from this calculation. From our result, we define the electron energy relaxation time due to electron-hole scattering. We incorporate the effect of this electron-hole energy transfer into the rate equations of semiconductor lasers. We perform a small-signal analysis on these rate equations, and, without any approximation in our derivations, we obtain the modulation response function. Our result indicate that the nonlinear gain coefficient due to carrier heating defined in the small-signal modulation response of semiconductor lasers is no longer a simple sum of the term due to electron heating and that due to hole heating.

Second-order signal analysis on the direct-current amplitude-modulation of semiconductor lasers with effects of hot carriers and hot phonons

In this work, to characterise the effect of hot carriers, our theoretical model comprises rate equations of carrier-photon interaction including the energy-transfer processes among carriers, photons, and phonons in a semiconductor laser. We theoretically investigate the effect of hot carriers on the second-order harmonic distortion in semiconductor lasers. Compared to other theoretical models, our model comprises the degenerate carrier energy distribution and incorporates the effects of stimulated and nonstimulated (i.e., Auger) recombination heating. These are the major heating mechanisms in semiconductor lasers as well as the injection heating and free carrier absorption heating.

Effects of carrier heating on the frequency chirping of semiconductor lasers

Effects of carrier heating on the frequency chirping of semiconductor lasers are theoretically investigated. We show that the variation of carrier temperature, like the variation of carrier density, is also a major factor on the frequency chirping of semiconductor lasers.