Toshihiko Baba

Photonic crystals and microdisk cavities based on GaInAsP-InP system

This paper presents a preliminary guide to realize microcavity semiconductor lasers exhibiting spontaneous emission control effects. It includes: 1) theoretical consideration on the effects; 2) processing techniques for semiconductor microcavities; and 3) some demonstrations of photonic crystal and microdisk cavity. It was shown that, even with a spectral broadening of electron transition, thresholdless lasing operation and alternation of spontaneous emission rate are expected in a cavity satisfying the single mode condition that only one mode is allowed in the transition spectrum. An ideal three-dimensional (3-D) photonic crystal has the potentiality for realizing this condition. In two-dimensional (2-D) crystals and microdisk cavities, thresholdless operation is also expected, but the alternation of spontaneous emission rate may be negligible due to the insufficient optical confinement. In the experiment, some processing techniques for GaInAsP-InP system were investigated and methane-based reactive ion beam etching was selected because of the smooth sidewalls and adaptability to arbitrary structures. A GaInAsP-InP 2-D photonic crystal constructed by submicron columns was fabricated using this method. Owing to the slow surface recombination of this material, a polarized photoluminescence and peculiar transmission spectra were observed at room temperature (RT), which can be explained by a photonic band calculation. However, some technical improvement is necessary for clear demonstration of photonic bandgap, which is minimally required for device applications. In contrast to this, a GaInAsP-InP microdisk cavity of 2 /spl mu/m in diameter, which corresponds to the cavity volume 2.5 times the single-mode condition, has achieved RT lasing with threshold current as low as 0.2 mA. Further reduction of diameter and realization of continuous-wave (CW) operation will provide a significant regime for the observation of spontaneous emission control effects.

Room temperature continuous wave operation and controlled spontaneous emission in ultrasmall photonic crystal nanolaser

Photonic crystal slab enables us to form an ultrasmall laser cavity with a modal volume close to the diffraction limit of light. However, the thermal resistance of such nanolasers, as high as 10(6) K/W, has prevented continuous-wave operation at room temperature. The present paper reports on the first successful continuous-wave operation at room temperature for the smallest nanolaser reported to date, achieved through fabrication of a laser with a low threshold of 1.2 muW. Near-thresholdless lasing and spontaneous emission enhancement due to the Purcell effect are also demonstrated in a moderately low Q nanolaser, both of which are well explained by a detailed rate equation analysis.

Spontaneous emission factor of a microcavity DBR surface-emitting laser

The spontaneous emission factor (SEF) of a microcavity distributed Bragg reflector (DBR) surface-emitting laser has been obtained theoretically to investigate the possibility of the thresholdless lasing operation. Formulas expressing the spontaneous emission in a three-dimensional microcavity were obtained. By introducing the distribution of mode density in wavevector space, it is shown that the radiation pattern of spontaneous emission is deeply modified by the microcavity and is different from that in free space. Based on this result, the SEF and the emission lifetime are calculated as a function of emission spectral width and the size of the active region. It is found that the SEF exceeds 0.1, even though the spectral width is as large as 30 nm when the transverse size is smaller than 0.5 mu m and the DBR reflectivity is larger than 90%. >

Ultrasmall and ultralow threshold GaInAsP-InP microdisk injection lasers: design, fabrication, lasing characteristics, and spontaneous emission factor

We have calculated lasing characteristics of current injection microdisk lasers of several microns in diameter, taking account of the scattering loss at center posts and the carrier diffusion effect. We found that the optimum width of the disk wing exposed to the air is 0.6-0.7 /spl mu/m and the minimum threshold current is nearly 10 /spl mu/A for the disk diameter of 2 /spl mu/m. The internal differential quantum efficiency can be 95% if the transparent carrier density is reduced to 7.5/spl times/10/sup 17/ cm/sup -3/ and the diffusion constant is increased to 8 cm/sup 2//s. In the experiment, we have obtained the room temperature continuous-wave operation of a GaInAsP-InP device of 3 /spl mu/m in diameter, for the first time, with a record low threshold of 150 /spl mu/A. This achievement was mainly owing to the reduction of the scattering loss at the disk edge, and hence the reduction of the threshold current density. The spontaneous emission factor was estimated to be 6/spl times/10/sup -3/. This value was much reduced by the large detuning of the lasing wavelength against the spontaneous emission peak. A larger value over 0.1, which is expected for such a small device, will be obtained by the wavelength tuning and the narrowing of the spontaneous emission spectrum.

Wideband and low dispersion slow light by chirped photonic crystal coupled waveguide.

Previously, we discussed an optical delay device consisting of a directional coupler of two different photonic crystal (PC) waveguides. It generates wideband and low dispersion slow light. However, it is easily degraded by a large reflection loss for a small imperfection of the coupling condition. In this paper, we propose and theoretically discuss a PC coupled waveguide, which allows more robust slow light with lower loss. For this device, unique photonic bands with a zero or negative group velocity at the inflection point can be designed by the structural tuning. Finite difference time domain simulation demonstrates the stopping and/or back and forth motion of an ultrashort optical pulse in the device combined with the chirped structure. For a signal bandwidth of 40 GHz, the average group index of the slow light will be 450, which gives a 1 ns delay for a device length of 670 microm. The theoretical total insertion loss at the device and input/output structures is as low as 0.11 dB.

Slow light with low dispersion and nonlinear enhancement in a lattice-shifted photonic crystal waveguide

We discovered that a silicon-on-insulator photonic crystal waveguide whose lattice is shifted along the waveguide generates wideband, low-dispersion, slow light with excellent reproducibility. We observed delayed transmission of picosecond optical pulses, as well as two-photon absorption and self-phase modulation enhanced by a high internal light intensity in the slow-light regime.

Dispersion-controlled optical group delay device by chirped photonic crystal waveguides

Previously, we proposed and demonstrated chirped photonic crystal waveguides, in which some structural parameters are gradually changed so that the photonic band characteristic is smoothly shifted. In this letter, we discuss an optical delay line composed of two index-chirped waveguides in a directional coupler structure. A large delay is realized by a low average group velocity of <c∕350 near the band-edge with almost perfect dispersion compensation even for a short optical pulse. Finite difference time domain simulation demonstrates that such a device is possible in a practical design.

Analysis of finite 2D photonic crystals of columns and lightwave devices using the scattering matrix method

The scattering matrix method was applied to the analysis of finite two-dimensional photonic crystals and lightwave devices. Results indicated that 1) the light transmission at the photonic band gap (PBG) is suppressed to less than -30 dB in the densely packed and honeycomb crystals, both of which are composed of only four rows of unit cells of semiconductor columns and 2) this PBG effect is weakened to half when the nonuniformity from 10 to 30% is brought to the diameter of columns. Also, the light propagation in defect waveguides with abrupt bends, a branch and a directional coupler was demonstrated by this method. It was found that the coupling loss at the input end of the waveguide is drastically changed by the shape of the input end. The reflection loss at 600 bends was estimated to be less than 1 dB, and the excess loss at an abrupt Y-branch was estimated to be 0-4.6 dB, depending on the frequency of the input wave. The demultiplexing and power dividing functions were expected in a directional coupler with a submicron coupling length, which is considered to be due to antiguide characteristics of the waveguides.

Efficiency enhancement in a light-emitting diode with a two-dimensional surface grating photonic crystal

We demonstrate a light-emitting diode exhibiting 1.7–2.7-fold enhancement in light extraction efficiency compared with that for a planer device. It has a two-dimensional surface grating photonic crystal, which diffracts internal light with a large solid angle into the escape light cone. Due to its shallow grating separated from the active layer and micron order lattice constant, the fabrication process is simple, applicable to arbitrary semiconductor devices, and free from process-induced nonradiative recombinations.

Low-group-velocity and low-dispersion slow light in photonic crystal waveguides.

Photonic crystal slab line defect waveguides with slightly small innermost holes are theoretically expected to show light transmission with low-group-velocity and low-dispersion (LVLD) characteristics owing to a linear and almost flat photonic band. In this study, the LVLD characteristics of such waveguides were experimentally confirmed by using modulation phase shift measurement and transmission of ultrashort optical pulses. These results will be applicable to buffering and nonlinearity enhancement of optical signals.