Net Quantum Efficiency Research Articles

Model of laser-induced temperature changes in solid-state optical refrigerators

We present an efficient and numerically stable method to calculate time-dependent, laser-induced temperature distributions in solids and provide a detailed description of the computational procedure and its implementation. This study combines the two-dimensional heat equation with laser-induced heat generation and temperature-dependent luminescence. The time-dependent optical response of a system is obtained numerically by the Crank–Nicolson method. This general model is applied to the specific case of optical refrigeration in ytterbium (Yb3+) doped fluorozirconate glass (ZBLAN). The laser-induced temperature change upon optical pumping and the respective transient luminescence response are calculated and compared to experimental data. The model successfully predicts the zero-crossing temperature, the net quantum efficiency, and the functional shape of the transients. We find that the laser-cooling transients have a fast and a slow component that are determined by the excited-state lifetime of the lumines...

Measurement of solid-state optical refrigeration by two-band differential luminescence thermometry

We present a non-contact optical technique for the measurement of laser-induced temperature changes in solids. Two-band differential luminescence thermometry (TBDLT) achieves a sensitivity of 7 mK and enables a precise measurement of the net quantum efficiency of optical refrigerator materials. The TBDLT detects internal temperature changes by decoupling surface and bulk heating effects via time-resolved luminescence spectroscopy. Several Yb 3+ -doped fluorozirconate (ZrF4‐BaF2‐LaF3‐AlF3‐NaF‐InF3, ZBLANI) glasses fabricated from precursors of varying purity and by different processes are analyzed in detail. A net quantum efficiency of 97.39±0.01% at 238 K (at a pump wavelength of 1020.5 nm) is found for a ZBLANI:1% Yb3+ lasercooling sample produced from metal fluoride precursors that were purified by chelate-assisted solvent extraction and dried in hydrofluoric gas. In comparison, a ZBLANI:1% Yb 3+ sample produced from commercialgrade metal fluoride precursors showed pronounced laser-induced heating that is indicative of a substantially higher impurity concentration. The TBDLT enables rapid and sensitive benchmarking of laser-cooling materials and provides critical feedback to the development and optimization of high-performance optical cryocooler materials. © 2010 Optical Society of America OCIS codes: 160.5690, 300.2530, 300.6430.

Pretreatment Dependence of Adsorption Properties of Merocyanine Dye at Rutile (110) and (100) TiO2 Surfaces Studied by C K-Edge NEXAFS

The adsorption properties of n-TiO2 rutile (100) and (110) surfaces which were prepared by simply annealing, photoetching, and HF-annealing methods were investigated by using the C K-edge NEXAFS. It was revealed that the interaction between the substrate and the dye molecules on the simply annealed surface was stronger than that on the HF-annealed surface, whereas the net quantum efficiency of the dye-sensitized photocurrent on HF-annealed surface was larger than that of the simply annealed surface for both surface faces. These results indicated that the energy difference between the LUMO of Mc[18,1] and the conduction band of TiO2 was large enough to give a nearly 100% quantum efficiency of electron transfer from photoexcited dye to TiO2 even in the case of the simply annealed surface. The enhancement of photocurrent of the HF-annealed surface comparing with that of the simply annealed surface was explained by the assumption that HF-annealing process can remove inactive layers (or surface defects) on TiO...

TERAHERTZ EMISSION FROM ELECTRICALLY PUMPED SILICON GERMANIUM INTERSUBBAND DEVICES

In this paper, we report on current pumped THz emitting devices based on intersubband transitions in SiGe quantum wells. The spectral lines occurred in a range from 5 to 12 THz depending on the quantum well width, Ge concentration in the well, and device temperature. A time-averaged power of 15 nW was extracted from a 16 period SiGe/Si superlattice with quantum wells 22 Å thick, at a device temperature of 30 K and a drive current of 550 mA. A net quantum efficiency of approximately 3 × 10-4 was calculated from the power and drive current, 30 times higher than reported for comparable quantum cascades utilizing heavy-hole to heavy-hole transitions and, taking into account the number of quantum well periods, approximately four times larger than for electroluminescence reported previously from a device utilizing light-hole to heavy-hole transitions.

A new approach to quantum well infrared photodetectors: Staircase-like quantum well and barriers

We present a theoretical investigation of a novel staircase-like quantum well infrared photodetector (QWIP). The proposed structure makes use of quantum wells and barriers with increasing Al content both in the wells and in the barriers forming a staircase-like energy band diagram without applied bias. The detection wavelength is around λ = 12 μm at an applied electric field of F = 1.4 × 10 4 V/cm at room temperature. Device operation is based on inter-subband bound-to-bound transition. We have solved the energy band diagram of the structure self-consistently. We have also calculated the absorption coefficient, responsivity, total net quantum efficiency and dark current density at room temperature. The dark current density at the operating field was found to be around 10 −2 A/cm 2, which is lower than the conventional QWIPs in the literature.

The Emission of Terahertz Radiation from Doped Silicon Devices

ABSTRACTRecent interest in biological imaging, remote sensing, and biochemical spectroscopy has made nanoscale Terahertz (THz) emitters based on semiconductors become extremely attractive. THz lasers with good performance (emitting over 1 milliWatt CW at 10 K) have been fabricated from the group III-V compounds by molecular beam epitaxy, but such devices are complex with over 500 quantum wells and barriers and are incompatible with low cost Si processing. Similar devices from the SiGe system have had difficulties because the strain limits the total number of active layers, and have produced relatively poor performance (peak powers near 5 nW), even using strain symmetric active regions on virtual substrates of relaxed SiGe buffers. We report here on the characteristics of a new type of THz emitting device with higher powers (above 0.1 milliWatt) based on radiative impurity transitions in doped silicon, which can be simply fabricated without Ge alloying.The THz emitters were fabricated from both p-type and n-type doped silicon wafers with typical resistivities of 5 Ω-cm, using conventional photolithography and metal contact lift off. Samples were electrically pulsed and the electroluminescence was measured by Fourier Transform Infrared spectrometry. At 4.2 K, the emission spectra showed several peaks centered around 8.1 THz for the boron doped device, 7 to 13 THz for the gallium doped device, and 6.6 THz for the phosphorus doped device. The electroluminescence was attributed to radiative transitions from excited p-like to s-like hydrogenic dopant states, with emission energies in remarkable agreement with the known dopant absorption levels. Current-voltage measurement suggested an excitation mechanism based on impact ionization of neutral dopants by hot carriers. The net quantum efficiency (emitted photons per injected electron) was estimated to be up to 2 × 10-3. The temperature dependence will be reported with operation above 30 K.

Metal-semiconductor-metal traveling-wave photodetectors

We demonstrate a novel type of traveling wave photodetector: metal-semiconductor-metal traveling-wave photodetecor (MSM-TWPD). The demonstrated devices were fabricated using low-temperature grown GaAs (LTG-GaAs). In order to achieve high internal quantum efficiency, the narrow spacing between electrodes was fabricated by the self-aligned process without e-beam lithography. Electro-optical sampling measurement results at different optical pumping levels are reported. Ultrahigh bandwidth (0.8-ps, 570-GHz transform bandwidth) performance was observed even under high optical power illumination (/spl sim/1.8 mW) with 8.1% net quantum efficiency. Compared with a LTG-GaAs-based p-i-n TWPD and vertically illuminated MSM photodetector (PD), this novel TWPD has higher output saturation current with near terahertz electrical bandwidth, better quantum efficiency, and can be easily fabricated and integrated with other microwave devices. It thus promises the application in high-power distributed PD array or terahertz signal generation.

Advances in Detector Systems for Imaging Single Molecule Fluorescence

Advances in Detector Systems for Imaging Single Molecule Fluorescence

Device physics of quantum well infrared photodetectors

The author presents a detailed study of a wide variety of quantum well infrared photodetectors, which were chosen to have large differences in their optical and transport properties. Intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasicontinuum and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot-carrier mean free path, net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity.

Semiconductor superlattices and quantum wells for infrared optoelectronics

We present a theoretical investigation of a novel staircase-like quantum well infrared photodetector (QWIP). The proposed structure makes use of quantum wells and barriers with increasing Al content both in the wells and in the barriers forming a staircase-like energy band diagram without applied bias. The detection wavelength is around λ = 12 μm at an applied electric field of F = 1.4 × 104 V/cm at room temperature. Device operation is based on inter-subband bound-to-bound transition. We have solved the energy band diagram of the structure self-consistently. We have also calculated the absorption coefficient, responsivity, total net quantum efficiency and dark current density at room temperature. The dark current density at the operating field was found to be around 10−2 A/cm2, which is lower than the conventional QWIPs in the literature.

Photoexcited escape probability, optical gain, and noise in quantum well infrared photodetectors

We present a detailed and thorough study of a wide variety of quantum well infrared photodetectors (QWIPs), which were chosen to have large differences in their optical and transport properties. Both n- and p-doped QWIPs, as well as intersubband transitions based on photoexcitation from bound-to-bound, bound-to-quasicontinuum, and bound-to-continuum quantum well states were investigated. The measurements and theoretical analysis included optical absorption, responsivity, dark current, current noise, optical gain, hot carrier mean free path, net quantum efficiency, quantum well escape probability, quantum well escape time, as well as detectivity. These results allow a better understanding of the optical and transport physics and thus a better optimization of the QWIP performance.

Oxidative photofragmentation of .alpha.,.beta.-amino alcohols via single electron transfer: cooperative reactivity of donor and acceptor ion radicals in photogenerated contact radical ion pairs

The studies presented in this paper show that α,β-amino alcohols undergo a very clean C−C bond cleavage upon SET (single electron transfer) oxidation by photoexcited electron acceptors in a process which generally culminates in two-electron reduction of the acceptors. For a number of different α,β-amino alcohols, the oxidative fragmentation occurs in a high chemical yield (>90%), yet with low to medium quantum efficiencies (0.0001−0.1) which vary strongly depending on the properties of electron donor (D), acceptor (A), and solvent. The net quantum efficiency reflects the competition between back electron transfer and the chemical redox process. Detailed mechanistic studies were carried out to investigate the visible light induced oxidative fragmentation of α,β-amino alcohols in the presence of electron acceptors including thioindigo (TI), 9,10Dicyanoanthracene (DCA), 2,6,9,10-tetracyanoanthracene (TCA), and 1,4-dicyanonaphthalene (DCN)

Energy transfer and quantum yield in Photosystem II

The photoreduction and dark reoxidation of Q α and Q β, the primary electron acceptors of Photosystems (PS) IIα and IIβ, respectively, in the presence of 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU) were studied in tobacco chloroplasts by means of fluorescence and absorbance measurements. The magnitude of a correction for an absorbance change by the oxidizing side of PS II needed in our previous study of the quantum yield of Q reduction (Biochim. Biophys. Acta 635 (1981), 111–120) has been determined. The absorbance change occurs in PS IIα mainly. The maximum fluorescence yield was found to be the same as in the mutant Su/su, which has a 3-fold higher reaction center concentration and a lower PS IIα to PS IIβ ratio. The kinetics of the light-induced fluorescence increase were measured after various pretreatments and the corresponding kinetics of the integrated fluorescence deficit were analyzed into their α and β components. From the results the contribution to the minimum fluorescence level, the degree of energy transfer between units, and the quantum efficiency of Q reduction were calculated for both types of PS II. This led to the following conclusions. The absence of energy between PS IIβ antennae is confirmed. Fluorescence quenching in PS IIα was adequately described by the matrix model, except for a decrease in the energy transfer between units during photoreduction of Q α, possibly due to the formation of ‘islets’ of closed centers. PS II reaction centers in which Q is reduced do not significantly quench fluorescence. The ratio of variable to maximum fluorescence, 0.77 in PS IIα and 0.92 in PS IIβ, multiplied by the fraction of Q remaining in the reduced state after one saturating flash, 0.88 in PS IIα and greater than 0.95 in PS IIβ, leads to a net quantum efficiency of Q reduction in the presence of DCMU and NH 2OH of 0.68 in PS IIα and about 0.90 in PS IIβ. These values are in good agreement with the measured overall quantum efficiency of Q reduction.

Schottky Barrier Photodiodes with Antireflection Coating

Schottky barrier diodes can be used for fast and efficient photodetectors if the incident light is coupled into the depletion layer of the diode and if electron-hole pairs are created by the internal photoelectric effect in the depletion layer. Fast response of the diode is achieved by designing a Schottky barrier with a small RC product. High quantum efficiency is obtained by coupling the light through a thin metal layer into the depletion region of the diode and by using an antireflection coating on the metal layer for matching the incident light beam. Schottky barrier photodiodes have been made with thin semitransparent gold layers on n-type epitaxial silicon and with zinc sulfide as an antireflection coating. A net quantum efficiency of 70 percent has been achieved at the He-Ne laser wavelength of 6328 Å. The pulse response of packaged diodes with 0.5-nanosecond wide pulses shows a symmetrical pulse shape with only small distortion due to carrier diffusion and reactance in the completed package. The diode structure is suitable for detector arrays. It is also useful for optical time domain reflectometry. The technique of coupling light through metal layers can be extended to other optical devices which require efficient transfer of radiation into a semiconductor through conducting electrodes.