Hung-I Lin

TEMPERATURE DEPENDENCE OF PHOTOLUMINESCENCE SPECTRA IN INAS/GAAS QUANTUM DOT SUPERLATTICES WITH LARGE THICKNESSES

In this report, we investigate the thermal relaxation of the photoluminescence (PL) in InAs/GaAs quantum dot superlattices with large thicknesses that have two to more than three times the critical thickness for spontaneous island formation. It is found that the linewidth first decreases and then increases with increasing temperature. In addition to thermionic emission, we suggest that carrier repopulation among quantum dots plays an important role in the PL quenching. The temperature dependence of PL peak energy following a Varshni relation was attributed to the dilation of lattice and electron-lattice interaction. The emission intensity quenches rapidly when the temperature rises to around 60 K, indicating the existence of defect-related centers in the vicinity of InAs/GaAs interfaces. In addition, we performed the measurement of the activation energy of PL quenching at different emission energy. We found that the loss mechanism of PL quenching based on the activation of electron-hole pairs from quantum...

AlGaAs BELOW HALF BANDGAP: THE SILICON OF NONLINEAR OPTICAL MATERIALS

Very few nonlinear optical materials are actually useful for high throughput all-optical devices. However, AlGaAs does satisfy all of the nonlinear optical figures of merit when used with photons of energy less than one half the semiconductor bandgap. Here we review our measurements of the pertinent nonlinear coefficients in waveguides and various device applications to all-optical switching in the communications band around 1550 nm.

Study of surface Fermi level and surface state distribution in InAlAs surface-intrinsic- n+ structure by photoreflectance

The built‐in electric field and surface Fermi level in the InAlAs surface‐intrinsic‐n+ structures were studied by room‐temperature photoreflectance. The samples were grown by molecular beam epitaxy with an undoped layer thickness of 1000 A. The undoped layer was subsequently etched to 800, 600, 400, and 200 A. Different chemical solutions were used in the etching process and the built‐in electric field is found independent of the etching process. While the surface Fermi level, in general, varies with the undoped layer thickness, there exists, for each Al concentration, a certain range of thicknesses within which the surface Fermi level is weakly pinned. From the dependence of electric field and surface Fermi level on the undoped layer thickness, we conclude that the surface states distribute over two separate regions within the energy band gap and the densities of surface states are as low as 1.02±0.05×1011 cm−2 for the distribution near the conduction band and 2.91±0.05×1011 cm−2 for the distribution nea...

Built-in electric field and surface Fermi level in InP surface-intrinsic n+ structures by modulation spectroscopy

The techniques of the photoreflectance and electroreflectance (ER) were used to study the built-in electric fields and the surface Fermi levels of InP surface-intrinsic-n+ (SIN+) structures. The substrates of SIN+ structures are either Fe-doped semi-insulated InP or Sn-doped N+ InP with the same doping concentrations as its buffer layer. The built-in electric field and the Fermi level were calculated from the Franz–Keldysh oscillations of the photoreflectance spectra. Our studies found that for the samples with the same doping concentration in the buffer layer and substrate, the built-in electric field increases as their top layer thickness decreases. The surface Fermi level, on the other hand, remains approximately constant. For samples with a semi-insulated substrate, the photoreflectance spectra indicate the simultaneous existence of two built-in electric fields, one in the top layer and the other at the interface region between the buffer layer and substrate. ER spectra were measured with the applicat...

Dependence of electron effective mass on alloy composition of InAlGaAs lattice matched to InP studied by optically detected cyclotron resonance

The electron effective mass of InAlGaAs lattice matched to InP has been determined as a function of Al content. The electron effective mass is obtained from far‐infrared optically detected cyclotron resonance (ODCR). In ODCR, the carriers are provided by optical pumping, and hence no doping is necessary. Unlike previous reports, we are able to detect the cyclotron resonance signal of a thin intrinsic epilayer at low temperature. Thus corrections of nonparabolicity are not required. In addition, from photoluminescence measurement, we determine the band‐gap energy. Both the effective mass and band‐gap energy show a nonlinear variation with Al composition.

Dissolvable and Recyclable Random Lasers

The dissolvable and recyclable random laser can be dissolved in water, accompanying the decay of emission intensity and the increment in lasing threshold. It can be reused after deionized treatment, exhibiting reproducibility with recycling processes.

Two-photon transitions between bound-to-continuum states in AlGaAs/GaAs multiple quantum well

We have experimentally observed room-temperature exciton resonances resulting from interband two-photon transitions between bound-to-continuum states. The excitons exhibit reduced binding energy and broadened resonances compared to that of excitons resulting from two-photon transitions between bound states. This trend is consistent with our theoretical prediction.

Determination of interband transition dipole moment of InAs/InGaAs quantum dots from modal absorption spectra

Interband-transition dipole moment of InAs/InGaAs quantum dots is determined for the first time by the segmented-modal-absorption method. The extracted dipole moment near 1.3_¿m wavelength is 32±2 Debye consistent with those reported in the literature.

A White Random Laser

Random laser with intrinsically uncomplicated fabrication processes, high spectral radiance, angle-free emission, and conformal onto freeform surfaces is in principle ideal for a variety of applications, ranging from lighting to identification systems. In this work, a white random laser (White-RL) with high-purity and high-stability is designed, fabricated, and demonstrated via the cost-effective materials (e.g., organic laser dyes) and simple methods (e.g., all-solution process and self-assembled structures). Notably, the wavelength, linewidth, and intensity of White-RL are nearly isotropic, nevertheless hard to be achieved in any conventional laser systems. Dynamically fine-tuning colour over a broad visible range is also feasible by on-chip integration of three free-standing monochromatic laser films with selective pumping scheme and appropriate colour balance. With these schematics, White-RL shows great potential and high application values in high-brightness illumination, full-field imaging, full-colour displays, visible-colour communications, and medical biosensing.

Transient and Flexible Hyperbolic Metamaterials on Freeform Surfaces

Transient technology is deemed as a paramount breakthrough for its particular functionality that can be implemented at a specific time and then totally dissolved. Hyperbolic metamaterials (HMMs) with high wave-vector modes for negative refraction or with high photonic density of states to robustly enhance the quantum transformation efficiency represent one of the emerging key elements for generating not-yet realized optoelectronics devices. However, HMMs has not been explored for implementing in transient technology. Here we show the first attempt to integrate transient technology with HMMs, i.e., transient HMMs, composed of multilayers of water-soluble and bio-compatible polymer and metal. We demonstrate that our newly designed transient HMMs can also possess high-k modes and high photonic density of states, which enables to dramatically enhance the light emitter covered on top of HMMs. We show that these transient HMMs devices loss their functionalities after immersing into deionized water within 5 min. Moreover, when the transient HMMs are integrated with a flexible substrate, the device exhibits an excellent mechanical stability for more than 3000 bending cycles. We anticipate that the transient HMMs developed here can serve as a versatile platform to advance transient technology for a wide range of application, including solid state lighting, optical communication, and wearable optoelectronic devices, etc.