Photon Gain Research Articles

Analytical and visual modeling of InGaN/GaN single quantum well laser based on rate equations

An analytical, visual and open source model based on solving the rate equations for InGaN/GaN single quantum well (QW) lasers has been carried out. In the numerical computations, the fourth-order Runge–Kutta method has been used for solving the differential rate equations. The rate equations which have been considered in this simulation include the two level rate equations for the well and separate confinement heterostructure (SCH) layers. We present a new and inexpensive modeling method with analytical, visual and open source capabilities to investigate and comprehend the QW laser characteristics such as time behavior of carriers in SCHs and QW, photon density, output power and gain, and also the output power versus current which presents the threshold current of the laser. The characteristics of the QW lasers, which include laser time response ( P– t), turn-on delay time of lasing and output power–current ( P– I) characteristic and related features such as threshold current and slope efficiency have been investigated. Our model accurately computes the P– t and P– I characteristics such as turn-on delay time, threshold current and slope efficiency, and also illustrates the effects of parameters such as the injection current and geometry.

Demonstration of a silicon photomultiplier with bulk integrated quenching resistors on epitaxial silicon

In this paper we present the experimental results of a silicon photomultiplier (SiPM) with bulk integrated quenching resistors on epitaxial silicon. Compared with existing SiPM with polysilicon quenching resistors on the surface or with MRS structure, it has potential advantages of high photon detection efficiency (PDE) while retaining a large micro-cell density and the fabrication process is also simplified. The SiPM with the micro-cell density up to 104/mm2 and the PDE up to 25.6% is demonstrated. The characteristics of dark count rate, single photon detection capability, gain, optical crosstalk and PDE have been investigated and discussed.

Theory of carrier and photon dynamics in quantum dot light emitters

AbstractWe present a microscopic theory describing the charge carrier and light emission dynamics in quantum dot (QD) light emitters. The theory covers non‐classical light emission (fluorescence and Raman emission) in the low carrier injection limit as well as laser emission and pulse amplification in the high carrier injection limit. The theoretical approach is based on QD Bloch equations including microscopically calculated Coulomb and electron–phonon scattering rates between bound QD, continuous wetting layer (WL) and bulk states. In the low carrier density limit, multi‐phonon relaxation is the dominant process, while at high charge carrier densities, Coulomb scattering dominates the dynamics. Using an equation of motion approach, we address (i) time‐resolved fluorescence and Raman emission, (ii) electrical injection and charge carrier transfer from bulk into WL and QD states, (iii) single photon emission and (iv) gain dynamics of QD amplifiers and lasing dynamics in QD vertical‐cavity surface‐emitting lasers (VCSELs) at high injection currents.

Impurity Study of Optical Properties in Fluorine-Doped Tin Oxide for Thin-Film Solar Cells

AbstractTin oxide (SnO2) is a durable, inexpensive transparent conducting oxide (TCO) material used for thin-film photovoltaic devices. However, the optical properties of conducting SnO2:F are generally not as good as in other conducting TCO materials such as ITO and ZnO:Al. Our previous analyses indicate that for thin-film solar cells, improving the optical properties of SnO2-coated glass could enhance photon collection and gain up to 10% additional photocurrent. Previously, we showed that some commercial SnO2 samples could have much higher optical absorption than others [2]. In this work, we continue our study on causes that could contribute to the high optical absorption of SnO2 films. The SnO2:F samples are fabricated by low-pressure metal-organic chemical vapor deposition or atmospheric-pressure chemical vapor deposition with tin precursors that includes different amounts of chlorine. Optical, electrical, and compositional analyses were performed. In addition to the free-carrier-introduced optical absorption, the non-active dopant also impacts the optical absorption. Among the SnO2 films fabricated with different precursors, the optical properties show a relationship based on the level of chlorine in the precursors and films. With a low-optical-absorption SnO2 layer, the solar cell could have better photon collection and a higher short-circuit current density.

Effect of two photon absorption on nonlinear pulse propagation in gain medium

The effects of two photon absorption (TPA) and gain dispersion on soliton propagation in amplified medium are investigated. For finite gain bandwidth, the effect of gain dispersion becomes significant along with TPA and is treated as perturbation in fundamental soliton propagation. Including these perturbing effects an analytical expression of integrated intensity is formulated applying a completely new methodology by adopting Rayleigh’s dissipation function in the framework of variational approach. With classical analogy, the Euler–Lagrange equation in non-conservative system is used to solve the problem analytically. In order to justify the analytical prediction a numerical verification is made by split-step beam propagation method following Ginzburg–Landau equation.

Fluorescence depletion mechanisms in super-resolving STED microscopy

We prove that stimulated emission is by far the dominant quenching mechanism for providing super-resolution in fluorescence microscopy with a red-shifted depletion beam. Our evidences are based on simultaneously measuring fluorescence quenching and photon gain in the quenching beam. Measurements were performed for several fluorescent dyes including fluorescent proteins over a wide spectral range of their emission spectra. We found that, for each fluorophore, the wavelength dependence of both signals closely follows that of the stimulated emission cross-section.

Characterisation of Geiger-mode avalanche photodiodes for medical imaging applications

Recently developed multipixel Geiger-mode avalanche photodiodes (G-APDs) are very promising candidates for the detection of light in medical imaging instruments (e.g. positron emission tomography) as well as in high-energy physics experiments and astrophysical applications. G-APDs are especially well suited for morpho-functional imaging (multimodality PET/CT, SPECT/CT, PET/MRI, SPECT/MRI). G-APDs have many advantages compared to conventional photosensors such as photomultiplier tubes because of their compact size, low-power consumption, high quantum efficiency and insensitivity to magnetic fields. Compared to avalanche photodiodes and PIN diodes, they are advantageous because of their high gain, reduced sensitivity to pick up and the so-called nuclear counter effect and lower noise. We present measurements of the basic G-APD characteristics: photon detection efficiency, gain, inter-cell crosstalk, dynamic range, recovery time and dark count rate.

Simulations of a photopumped X-ray laser using the H-like Cl−Li-like Se scheme

Calculations of the modal photon densities and gain in a photopumped Cl XVII-Se XXXII X-ray laser are presented. In this paper we undertake a realistic simulation of the generation of both Cl and Se plasmas, using a high-power optical laser, which includes radiation from both Ly- α fine-structure components of H-like Cl pumping the 2p 3/2–5d 5/2 transition in Li-like Se. The calculations are performed in two dimensions in a realistic geometry taking into account plasma gradients. This gives information about the spatial extent and time evolution of X-ray lasing gain on the 5–4 transitions (39.5 A ̊ ) in Li-like Se. We find that gain (about 200 cm −1 ) is expected only when the optical laser includes a pre-pulse. Calculations show that the absorption of pumping radiation in the pumped plasma can reduce the gain by 20%. Time-dependent calculations have shown that the gain is reduced by 30% in comparison to the steady-state calculations. The effect of the spectral profile and self-radiation of 5d 5/2–2p 3/2 transition in Li-like Se reduces the gain by about 2%.

Simulations of Al XIII–Fe XXIV X-ray laser photopumping scheme

Calculations of the modal photon densities and gain in a photopumped H-like Al–Li-like Fe X-ray laser scheme are presented. Previous simulations of this scheme using a zero-dimensional model took only the Al Ly- α 2 pumping radiation into account. Here we undertake a more realistic simulation of the generation of both Al and Fe plasmas using a high-power optical laser, which includes both Ly- α 2 and Ly- α 1 radiation from the H-like Al ions pumping 2s 1/2–5p 1/2 and 2s 1/2–5p 3/2 transitions in Li-like Fe. In this paper, we present two-dimensional calculations in a realistic geometry taking plasma gradients into account. Information about the spatial extent and time evolution of gain on the 5–4 transitions in Li-like Fe is presented.

Further simulations of the gain in a K XIX/Cl XVII resonantly photopumped X-ray Laser

Calculations of the modal photon densities and gain in a resonantly photopumped K XIX/Cl XVII X-ray laser are presented. Previous simulations of this scheme in a novel geometry only took into account the pumping effects of the H-like Ly- α emission from the laser-produced K plasmas on the populations of a similarly produced plasma of H-like Cl ions. Here we extend this work to include the effects of the resonance line emission of He-like K. Furthermore, calculations of the gain are extended to two dimensions, giving information on the spatial extent of the lasing region. Importantly, we find that whilst the emission from the He-like K indeed modifies the computed characteristics of the laser to some extent, the reabsorption of certain spectral regions of this line emission by ground state He-like K ions modifies the spectrum as seen by the H-like Cl ions, resulting in sufficient detuning so as to prevent such emission from destroying the gain on the 4–3 X-ray lasing transitions (for example, the 4f 7/2–3d 5/2 line at 64.8 Å).

A position-sensitive photon event counting detector applied tofluorescence imaging of dyes in sol-gel matrices

A photon event counting imaging detector, originallydeveloped for astronomical applications, has been adapted for use on alow light level fluorescencemicroscope. It is based on a 40 mm diameter, three-microchannel plate imageintensifier with a photon gain of 107 when operating in the photon eventcounting mode. The intensifier's output screen is lens coupled to a fullframe progressive scan CCD camera which transfers the photonevent data into a PC via a framegrabber.The image is built up from the photon events in the frames usingsoftware-based event processing. The high sensitivity of the photon countingapproach means that low excitation light levels and/or a low probeconcentration can be used. Moreover, the inherent linearity betweenincident intensity and thenumber of detected events provides intrinsic photometricaccuracy independent of camera gain and offset.To demonstrate the viability of photon event counting imaging appliedto fluorescence studies, we have imaged4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran-(DCM-) and rhodamine 6G-doped thin films produced bythe sol-gel technique using tetraethylorthosilicate (TEOS) as aprecursor.We show that fluorescence imaging is a feasible tool for finding cracks,streaks or other defects and for assessing the uniform thickness of thefinished films. In addition, we image the fluorescence in responseto an excitation beam illuminating rhodamine 6G-doped bulk monolithsol-gel samples. The results are analysed using the Lambert-Beer law.

Static and dynamic characteristics analysis of all-optical wavelength conversion using directionally coupled semiconductor optical amplifiers

We analyze the wavelength-converting operation of directionally coupled semiconductor optical amplifiers (DCSOA's) with respect to their static and dynamic characteristics. A complete dynamic DCSOA model based on the modified time-dependent transfer-matrix method is developed. This model accurately considers longitudinal variations of carrier and photon densities, gain, refractive index, coupling coefficients, and lateral optical fields. The extinction ratio, modulation bandwidth, and frequency chirping of DCSOA-based wavelength converters are investigated. For most characteristics, the DCSOA scheme has advantages over cross-gain modulation. Furthermore, by optimizing the injection current into the DCSOA, even better performance can be attained.

Calculations of the modal photon densities and gain in a K/Cl resonantly photopumped X-ray laser

A calculation of the modal photon densities and gain in a resonantly photopumped X-ray laser is presented. The one-dimensional Lagrangian hydrodynamics code MED103 is used to simulate the laser irradiation of planar Potassium and Chlorine targets. A line transfer algorithm is used to calculate the modal photon density of the Hydrogen-like Ly- α emission from the Potassium plasma taking into account the gradients in density, temperature and velocity. The relative position of the chlorine and potassium targets is chosen to optimise the pumping whilst maintaining a realistic experimental geometry. The steady-state zero-dimensional code Galaxy is then used to calculate the gain at the peak of the laser pulses on the 4f 7/2→3d 5/2 X-ray lasing transition (64.8 Å) in hydrogenic chlorine as well as other possible lasing transitions. A comparison is also made with a back-to-back target geometry which has been used in previous experiments.

Light amplification without population inversion: Time-dependent study of (1+1)-photon emission from autoionizing states.

In a time-dependent dressed-state calculation we have studied the time development of (1+1)-photon amplification without population inversion from an autoionizing (AI) state which is coupled to another AI state embedded into the same continuum. We have shown that the (1+1)-photon gain evolves with time over the single-photon gain and it can be controlled by changing the relative strength of the coupling of the resonant intermediate state with the ground state and with the two AI states. We have also shown that three-peaked gain on the upper probe field evolves with time and we have demonstrated that the modification of the second peak and the appearance of the third peak are solely due to the presence of the second AI state.

Fast efficient Ca atomic resonance filter at 423 nm

An optically pumped active Ca atomic resonance filter is demonstrated, applicable to background-limited optical communications through scatter channels. In pump saturation, the filter should detect 50% of the incident 423-nm signal power with an internal photon gain of 6 and a response time of 10 micros, 2 orders of magnitude faster than a passive Ca filter. Response time of 100 micros has been demonstrated, limited by available pump power. The filter maintains the wide field of view and reduced solar background associated with atomic absorption at the Ca Fraunhofer line while permitting higher data rate communications.

A quantum amplifier model for predicting temporal variations in the output power from a semiconductor laser on a picosecond time scale

A new model for simulating temporal fluctuations in the power emitted by a semiconductor laser is described. Light in the cavity is assumed to circulate in the form of traveling photon packets, in which the photon number fluctuates due to the processes of spontaneous emission, stimulated emission, absorption, scattering, and reflection. The dipole dephasing time T plays a critical role in modeling the interaction of the photon packets and gain medium. The Monte Carlo method is used to simulate the temporal behavior of a continuously pumped Fabry-Perot laser. The laser output power is found to exhibit periodic fluctuations at the cavity transit time frequency (longitudinal mode beat frequency). The amplitude of these fluctuations, as well as the relaxation oscillation, which occurs at a much lower frequency, is strongly influenced by the magnitude of T. The results of these simulations are related to the temporal behavior expected from a conventional FP laser.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimum facet reflectivity for high speed lasers

The issue of optimum facet reflectivity for large bandwidth semiconductor lasers is addressed. The laser facet reflectivity, is chosen for maximising the relaxation oscillation frequency, by optimisation of the photon lifetime and maximum obtainable intracavity photon density. The optimum mirror reflectivity also depends on the relative importance of photon density-induced facet damage, parasitic leakage, heating, and gain saturation. In practice, we find both theoretically and experimentally, that higher mirror reflectivities can result in higher modulation bandwidths. The issue of optimum mirror reflectivity is also of special interest for surface emitting lasers where the cavity length is very short but the mirror reflectivity is very high.

Multichordal charge exchange recombination spectroscopy on Doublet III (abstract)

Single shot, multipoint ion temperature and plasma rotation profiles have been routinely obtained on the Doublet III tokamak for 32 consecutive time slices with 20-ms resolution. A six-chord tangentially viewing spectroscopic diagnostic has been built to look at radiation emitted by fully stripped low-Z impurity ions (He, C, O) that have undergone charge exchange recombination with hydrogen atoms from a 3-MW heating beam. The main components of the instrument are a single monochromator for wavelength dispersion, a single image intensifier tube for photon gain, and a pair of 1024-element linear photodiode arrays for detection. A special arrangement of fiber optics allows simultaneous data acquisition from all chords without the use of scanning mirrors or other moving parts. Ion temperature profiles taken under a variety of plasma conditions will be presented.

A Diode Intensifier Tube Designed for Scintillation Detection

An input and output faceplates diode intensifier tube having a large ratio (.8) useful-external diameters has been designed with a view to reach very good performance in scintillation detection. The photoelectrons emitted by the bialkali photocathode are accelerated up to about 10 kV and collected on a fast blue phosphor screen (P 47). The design is described and the main technological problems which has to be solved are reported, in particular the one allowing to overcome wall charge effects. The experimental results on image distortions, electron and photon gains, single electron resolution, dark noise, as well as on the phosphor screen characteristics are given. An assembly of this tube working as a high photon gain (?30 at 10 kV) light pre-amplifier, optically coupled to a fast photomultiplier could allow to obtain significant improvements of the FWHM energy resolution in scintillation detection by using a rather large NaI(T1) scintillator block (e.g. 9.4 % at 122 KeV instead of 10.5 % obtained with the fast PMT used alone).

Nonequilibrium kinetics of coupled photons and electrons in two-level systems of the laser type

Rate equations for photons and for electrons in systems of two groups of energy levels (energy separation kTηG) are set up allowing for absorption, stimulated and spontaneous emission, nonradiative transitions, photon loss or dissipation as well as photon gain from the ambient, and an externally applied pumping rate. The rate equations are discussed qualitatively, and the stability of the solutions is obtained by the use of phase portraits. The quantitative discussion includes steady-state photon distributions. Earlier work showing that these distributions can obey the Bose–Einstein law with a nonzero chemical potential kTF is taken into the laser regime, which was not covered by recent relevant work. The second-order phase transition that occurs at threshold is established explicitly, and it is shown that the transition requires the neglect of certain generation terms in the rate equations. Some time-dependent solutions of the rate equations are also discussed, using the slaving principle of adiabatic elimination for the electrons. The application of this work to semiconductors is discussed.