Tzu-Chiang Chen

Electronic characteristics of doped InAs/GaAs quantum dot photodetector: temperature dependent dark current and noise density

The noise characteristics associated with dark current, photoconductive gain (PC), capture probability in doped InAs dots embedded in In0.1Ga0.9As/GaAs spacer layer have been proposed. The photoconductive and photovoltaic behaviors of the InAs/GaAs quantum dot infrared photodetector (QDIP) from the intersubband transition measurements are also clearly observed. Through noise measurement in dynamic signal analyzer (HP35670A) 1, the electronic bandpass filter frequencies are set up ranging from 3 to 10 KHz in a low noise current preamplifier (SR570) 2. The lock-in amplifier (SR830) 3 can be also used to measure and calibrate the noise density by means of the mean average deviation (MAD) contrast with noise spectra from HP35670A. The InAs/GaAs QDIP studied in this work belongs to n+-n-n+ structure with the top and free blocking barrier layers. It is observed that the owing blocking layer of QDIP not only suppress dark current successfully but also probably reduce the photocurrent 4-6. By systematically optoelectronic measurements and simulations, the modified model of noise current, photoconductive gain, and capture probability in the quantum devices have been proposed. It is shown that photoconductive gain is almost independent of bias under the lower bias, then increasing exponentially under higher bias and below the temperature of 80K. In contrast to quantum well infrared photodetector (QWIP), a higher photoconductive gain of the quantum dot infrared photodetector has been demonstrated and attributed to the longer lifetimes of excited carriers in quantum dots 7-10. At 80K, a photoconductive gain of tens of thousand is shown in the regions of higher biases. It is clear to note that the highest detectivity of the QDIP surprisingly approach to 3.0×1012 cmHz1/2/W at -0.6V under measured temperature 20 K. Under 80K, the average D* is obtained ~1010 cmHz1/2/W. To our knowledge, this is the one of highest D* data in the world.

A volatile-solvent gas fiber sensor based on polyaniline film coated on superstructure fiber Bragg gratings

A fiber sensor based on a polyaniline (PANI) film that is coated on the surface of an etched superstructure fiber grating to detect volatile solvent vapors is experimentally demonstrated. This sensing mechanism is based on the interaction of the testing gas with the polyaniline coating film, which changes the film index, resulting in a shift in the Bragg wavelength. The sensitivity of this sensor to ammonia (NH3) gas is about 0.073 pm ppm−1, which depends on the optical characteristics of the fiber grating, the diameter of the fiber cladding and the constituents of the sensing film. Methanol concentrations can also be measured using this sensing scheme. The sensitivity of this sensor must be improved to provide a simple, reliable, repeatable and non-destructive method for sensing various chemical gases.

Acousto-optic-induced cladding-mode reflection in a blazed-superstructure fiber grating

The coupling phenomenon between the cladding mode and the core mode in a blazed-superstructure fiber grating is experimentally demonstrated in this letter by applying both transverse and longitudinal acoustic waves. This is based on the difference between the cladding-mode wave vector and the core-mode wave vector, which is equivalent to the acoustic wave vector. By this means, a reflectivity-tunable multiwavelength switchable comb filter would be developed for the applications in fiber optics.

High-sensitivity pressure sensor based on a fiber Bragg grating

An all-fiber pressure sensor based on a fiber Bragg grating with the pressure sensitivity of 2.2x10-2 MPa-1 has been demonstrated. The physical configuration includes a FBG encapsulated in a polymer-half-filled metal cylinder with its end bonded to the central of a round plate attached to the surface of polymer, and the Young’s modulus of the polymer is four orders lower than FBG. This cylinder has two opening on opposite side of the wall at the polymer part. Under the pressure environment, the polymer can be pressurized along one radial direction only, and responds an axial force acting on the round plate, producing an axial strain on FBG. With a nice linearity, this sensor should be applied potentially for the measurement of mediums pressure, liquid level and depth underwater.

Temperature dependent VCSEL optical characteristics based on graded Al x Ga 1-x As/GaAs distributed Bragg reflectors: reflectivity and beam profile analyses

The vertical cavity surface emitting laser (VCSEL) based on graded distributed Bragg reflectors (DBR) consisted of a top mirror of 20 pairs of AlxGa1-xAs/AlyGa1-yAs (x=0~0.9, y=0~0.12) quarter-wave stacks and a bottom mirror of 34 pairs of AlyGa1-yAs/AlxGa1-xAs quarter-wave stacks has been demonstrated. Using two proposed transfer matrix methods, the simulation of DBR reflectivity depending on temperature refractive index of AlxGa1-xAs and AlyGa1-yAs are discussed and investigated. The simulation results could be achieved to well predicted the DBR performance under operating temperature variances, i.e., the temperature on varying reflectivity and full width half maximum (FWHM), wavelength stop-band shifts of the laser reflector, where using the multi-layer films evolution software of essential Macleod and modified transfer matrix method, respectively. Under our simulation, assuming the physically VCSEL device feature such as the linear grading DBR structure sandwiched with a n-type GaAs substrate and air films, we have systematically studied the temperature effects on the key parameters of top and bottom DBR schemes. In contrast with the temperature dependent DBR on the 850nm-VCSEL characteristics simulated with the above two transfer methods, the temperature varying spectra of VCSEL are agreed with the our simulated results presented in this paper. Also the temperature dependent model of DBR based on refractive index of graded multi-AlxGa1-xAs/ AlyGa1-yAs has been proposed. So, a series of optoelectronic measurements experimentally confirm our results again. The maximum reflectivity of the top and bottom DBRs are 96.4 and 99.98%, respectively. The central wavelengths of the bandwidth spectra in the top and bottom DBR are same. i.e., 840nm. These results can be obtained the criteria for the high performance VCSEL design. The far-field patterns of transverse electromagnetic fields confined in <15μm active-layer aperture of selectively oxidized VCSEL have been observed.

A lateral pressure sensor using a fiber Bragg grating

We have designed an all optical high-sensitivity pressure sensor based on a fiber Bragg grating (FBG) encapsulated in a polymer-half-filled metal cylinder with the sensitivity of 1.87/spl times/10/sup -2/ Mpa/sup -1/. This sensor should be applied potentially for the measurement of mediums pressure, liquid level and depth underwater.

Investigation for optoelectronic characteristics and imaging performance of InAs quantum dot covered with In 0.1 Ga 0.9 As/GaAs multilayer based focal plane array

We report on a structure prepared by metalorganic chem- ical vapor deposition and molecular beam epitaxy techniques in- corporated with a standard process of compound semiconductor to achieve In0.1Ga0.9As/InAs/In0.1Ga0.9As quantum dot infrared photodetec- tor (QDIP)-based focal plane array. For investigating the mechanism of carrier transport and optoelectronic behavior, the photoresponse spec- tra and dark current were measured in agreement with the theoretical simulations. Furthermore, a model is proposed with the systematic anal- yses and explained for designing high-performance QDIPs based on the calculations of thermal activation energy and detectivity. For QDIP pho- toresponse measurements, the photovoltaic photoresponse is achieved, which can be attributed to the asymmetric morphology of quantum dot epitaxy. With increasing bias, the photoresponse spectra exhibit a red- shift due to band bending that generates a thinner triangle barrier which increases the escape possibility of the excited carriers situated at lower excited-state levels. The trapezoid-edged scheme enhances infrared cou- pling and increases the photoresponse intensity. A single-sided gradient AlxGa1-xAs (x = 0.25→0) barrier can suppress the dark-current under bias efficiently. The infrared imaging performance of InGaAs QDIP based on 320×256 FPA is also demonstrated in this paper. C 2011 Society of Photo-

Three-by-three port fused fiber grating coupler

We experimentally demonstrate that a 3×3 port fused fiber grating coupler can be implemented by combining the technique of fabricating fused-fiber couplers as well as that of writing fiber Bragg gratings for application in all-fiber optical filters. A Bragg grating with a reflectivity of 65% is written into the coupling region of a 3×3 fiber coupler with the coupling ratios of 60, 24, and 16%, respectively, for three different output ports. This device can be applied in optical multi/demultiplexing and add/drop filters or in special optical filters.

Numerical simulation of temperature-dependence on distributed Bragg reflector (DBR) and performance analyses for proton-implant/oxide confined VCSEL: comparison with transmission matrix, matrix calculating methods, and Macleod model

This paper mainly focuses on the simulation for temperature-dependent Distributed Bragg Reflector (DBR) of 850nm vertical cavity surface emitting laser (VCSEL) with Transmission Matrix (TMM), Matrix Calculating Methods (MCM) and Macleod Model and performance for comparison with proton-implant/oxide confined process on VCSEL. Using well-developed temperature-dependent DBR-reflectivity solver with Mathcad simulator, we have successfully compared the Macleod Model simulator with theoretical self-developed solution based on the Transmission Matrix (TMM), Matrix Calculating Methods (MCM) and find very good agreement with previous results while accounting for influences of conjugated part of refractive index and graded Al compositions of DBR materials. Moreover, optoelectronic performance of Proton-Implant/Oxide Confined 850nm VCSEL have been demonstrated on this paper using temperature-dependent power output, voltage/injection current, transverse operating wavelengths, optical spectral characteristics, slope efficiency and transverse optical modes with an approximated Marcatilis method extracted and measurement from systematically measuring experiments. Through adequate and precise LD device design and processes, we have proposed the high performance single-mode proton implanted in contrast to the oxide confined 850 nm VCSEL. Under nominal temperature-variety and keeping operating temperature of 30°C,the threshold voltage, injecting current, peak-wavelength and differential resistance of the proton implanted VCSEL with the optical aperture in the dimension of 10 &mgr;m are 1.8 V, 3.2 mA, 851 nm and 36.8 ohm, respectively.

Calculations of bandstructures on the lens and pyramid-shaped InAs quantum dot for confirming the photoluminescence and photoresponse

Electronic and optical properties of ideal and real quantum dots (QDs) are extensively studied and derived for the recent decade. Strain caused by the differences of the lattice constants of dot and wetting, barrier materials are decisive for both the self-assembly mechanisms and the electro-optical properties. The research is mainly investigated for realizing the strain effects on the optical properties of InAs/GaAs self-assembled QDs embedded in GaAs barrier layer incorporated with the three-dimensional (3D) Schördinger equation and solved by using finite element method (FEM). From 3D QD geometrical profiles establishing by the spatially geometric equations, the confined electron and hole bandstructures on altering sized lens and pyramidal shape-like QDs with numerical calculations and strained heterostructure of the finite element approximations have been proposed. Applying the fast FEM models, it is demonstrated that the correspondence of ground, excited eigenstates, the probability of density function (|Ψ|2) of the confined levels from the single InAs QD to a matrix of nine QDs to obtain the transition energy and coordinated absorption wavelength to be predicted and summarized clearly. Through calculating energy levels within the conduction and valence band edge confinement on the InAs/GaAs heterostructure with FEM to contradistinguish with corrected to optical transitions and linear absorption spectra can be achieved for verifying to being the specific wavelengths from photoluminescence (PL) and photoresponse (PR) measurements for quantum dot infrared photodetector. By fitting the energy differences among the subbands, the geometrical shape and size of QD can be predicted. Inducting the tendency from single QD to the matrix of 9 QDs, the step-wise bands have been obtained being some regularity clearly. And from the transmission electron microscope (TEM) measurement, the dominant sizes of QDs in the really grown wafer remain the consistent with the numerical analyses applied in 3D QD profile that is interpreted using spatially geometric equations.