Constant Background Illumination Research Articles

Space charge limitation on the response time of organic photodiodes

The dynamic response of an organic bulk heterojunction photodiode to small changes in applied bias or light intensity is investigated as function of the intensity of a constant background illumination by means of photoimpedance and transient photocurrent measurements. For bias voltages close to the open circuit voltage we find that the response timescale with the square root of the light intensity. The results can be quantitatively explained in terms of a space charge limitation on the photocurrent as predicted by Goodman and Rose (J. Appl. Phys. 42, 2823 (1971)). The relaxation time of the diode at open circuit corresponds to the lifetime of the slowest charge carrier in the diode. This relaxation time is determined by the dielectric constant and the smallest of the two carrier mobilities in the bulk heterojunction. This illustrates the importance of balanced carrier mobilities for obtaining diodes with fast response time at low bias for e.g. imaging arrays.

Analysis of the mechanisms of electron recombination in HgCdTe infrared photodiode

This paper presents an experimental study of minority carrier lifetime and recombination mechanisms in HgCdTe photodiode. The excitation light source is a wavelength-tunable pulsed infrared laser. A constant background illumination has been introduced to minimize the effect of the junction equivalent capacitor and resistance. The decay of the photo-generated voltage is recorded by a storage oscilloscope. By fitting the exponentially decay curve, the time constant has been obtained which is regarded as the photo-generated minority carrier lifetime of the HgCdTe photodiode. The experimental results show that the carrier lifetime is in the range of 18–407 ns at 77 K for the measured detectors of four Cd compositions. It was found that the Auger recombination process is more effective for low Cd composition while the radiative recombination process became more important for high composition materials. The Shockley–Read–Hall recombination processes could not be ignored for all Cd composition.

Photoconductivity anisotropy study in uniaxially aligned polymer based planar photodiodes

A study of photoconductivity in a planar photodiode based on the uniaxially aligned, semicrystalline polymer, poly(2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene) (PBTTT) is presented. The charge carrier photogeneration yield in bulk conjugated polymers is usually low but it could possibly be enhanced by applying a sufficiently large electric field, particularly along the direction of the polymer backbone. PBTTT is, in principle, an ideal system to study any anisotropy in carrier photogeneration because directional alignment of fully extended polymer chains can be achieved over macroscopic length scales. However, we have found that electron trapping in this polymer results in high photoconductive gain obscuring measurements of intrinsic photogeneration. We study the mechanism of photoconductive gain using steady-state and transient photocurrent measurement. Constant background illumination and the deposition of a surface layer of an electron acceptor material have been found to be effective in reducing the gain effects.

Experimental Evidence of Localized Oscillations in the Photosensitive Chlorine Dioxide-Iodine-Malonic Acid Reaction

The interaction between Hopf and Turing modes has been the subject of active research in recent years. We present here experimental evidence of the existence of mixed Turing-Hopf modes in a two-dimensional system. Using the photosensitive chlorine dioxide-iodine-malonic acid reaction (CDIMA) and external constant background illumination as a control parameter, standing spots oscillating in amplitude and with hexagonal ordering were observed. Numerical simulations in the Lengyel-Epstein model for the CDIMA reaction confirmed the results.

Illumination accelerates the decay of the O-intermediate of pharaonis phoborhodopsin (sensory rhodopsin II).

pharaonis phoborhodopsin (ppR, also called pharaonis sensory rhodopsin II [psRII]) is a member of the archaeal rhodopsin family and acts as a repellent phototaxis receptor of Natronobacterium pharaonis. Upon illumination, ppR is excited and undergoes a linear cyclic photoreaction, namely, a photocycle that constitutes photointermediates such as M- and O-intermediates (ppRM and ppRO, respectively). Under a constant background illumination (>600 nm) that irradiates ppRO, the decay rate of the flash-induced ppRO increased with an increase in the background light intensity, indicating the photoreactivity of ppRO. Azide did not influence the light-accelerated ppRO decay, but the time required for the cycle to be completed became shortened in an azide concentration-dependent manner because of acceleration of ppRM decay. Hence, the turnover rate of photocycling increased appreciably in the presence of both the background illumination and the azide. The observation reported previously (Schmies, G. et al. 2000, Biophys. J. 78:967-976) is discussed in connection with the present observations.

Algorithms for correlating severely obscured images in nonoverlapping zero-mean noise

In a previous report, we developed optimization algorithms showing how optical correlation filters operating with obscured inputs were affected by disjoint constant background illumination. We extend these studies by upgrading our algorithms to include the theoretical treatment of zero-mean disjoint noise, as well as constant background illumination. Computer simulations involving noise clutter but no background illumination are used to characterize the performance of our upgraded algorithms.

Multifocal pupillary light response fields in normal subjects and patients with visual field defects

The optimal conditions for recording focal pupillary light responses with a multifocal stimulation technique were determined, and the technique was applied to normal subjects and patients with visual field defects. Thirty-seven hexagonal stimuli were presented on a TV monitor with a visual field of 40° diameter under a constant background illumination. Using a slow (4.7 Hz) m-sequence, reliable focal responses were obtained in both normal subjects and patients. The pupillary field and visual field were well correlated in patients with retinal diseases, but the correlation was not strong in patients with optic-nerve diseases. Pupillary light responses were reduced in the blind hemifield in patients with post-geniculate lesions. These results indicate that the multifocal stimulation technique can be used clinically to obtain a pupillary field for objective visual field testing.

Lead Affects Learning by Hermissenda crassicornis.

Figure 1. (A) Temporal transfer functions measured by Batra and Barlow (dashed line), Brodie et al. (solid line), and in our preliminary experiments with filled circles) and without (unfilled circles) background illumination. (B) TTFfrom three single ommatidia in as many eyes. Filled circles denote responses recorded with constant background illumination and unfilled circles those recorded without background. Error bars denote standard deviation.

Determination of the mobility and transport properties of photocarriers in Bi12GeO20 by the time-of-flight technique

A direct measurement of a key material parameter in photorefractivity, the charge carrier mobility, has been achieved only recently by means of a holographic time-of-flight technique. In this paper we report, as far as we know, the first successful direct determination of electron mobility using the classical time-of-flight method. This consists in measuring the velocity of a sheet of excess carriers, created by a short-duration excitation pulse of strongly absorbed photons close to the surface. These carriers drift through the sample under the action of an applied electric field. However, the technique could not be used in its original configuration. A constant background illumination was required in order to saturate traps and to prevent space charge build up. Optimal illumination conditions (wavelength below 550 nm, suitable integrated number of photons) were found under which a quasifree, nondispersive, charge transport was observed. The mobility is limited by interaction with a shallow trap, the population of which can be modulated by the additional monochromatic illumination. An unexpected high value was found as compared to results published earlier. This value is probably very close to the microscopic collision-limited mobility. This explains the relatively small spread of the results obtained with different nominally undoped Bi12GeO20 (BGO) samples. The values lie in the range 0.2–1.0 cm2 V−1 s−1. They are in excellent agreement with that measured elsewhere in BSO using the holographic time-of-flight technique.

Time-of-flight experiments on 2D electron gases

Transport of electrons in GaAs/AlGaAs modulation-doped heterostructures is studied by means of the Time-of-Flight technique. By applying a constant background illumination, we are able to distinguish between lateral transport processes and effects of perpendicular transport across the GaAs/AlGaAs interface. It is shown that the initial part of the photocurrent signal is due to lateral transport only. This part of the signal is further investigated as a function of sample length and applied electric field.

Effects of tank colour on growth and smoltification of Atlantic salmon ( Salmo salar L.)

Atlantic salmon parr, Salmo salar, were reared in dark green or light grey tanks under a dual photoperiod, consisting of a constant background illumination (LD 24:0) from incandescent bulbs and additional light simulating a natural photoperiod (LDN). Additional light was either yellow light from incandescent bulbs or artificial daylight from fluorescent tubes. There were no significant differences in growth between groups in green or grey tanks, or between groups receiving additional light of different spectral composition. There were strong indications of smoltification in grey tanks, whereas in the green tanks the fish were not considered to be smolts.

Application of spatial filtering in thermal lensing detection

In thermal lens spectroscopy, the application of a circular stop can result in reducing the constant background illumination of a detector, while only slightly affecting the thermal lens signal. Improvement of the signal to background ratio in the filtered detection of thermal lensing is estimated and its relation to S/N in thermal lens spectrometer is discussed. An experiment with nearly threefold increase of useful signal to constant background ratio is reported.

The operation of connexions between photoreceptors and large second-order neurones in dragonfly ocelli

Photoreceptors and large second-order neurones (L-neurones) of dragonfly ocelli have been penetrated simultaneously with microelectrodes to study the operation of the synapse between them. The responses of L-neurones to changes in illumination are of opposite polarity and are more phasic than those of photoreceptors. 1. When pulses of light are superimposed on a constant background illumination, the phasic nature of L-neurone responses is often enhanced. Sometimes, during a light stimulus, an L-neurone is depolarised relative to its resting potential. With rapidly repeated short pulses of light, responses of L-neurones decrement markedly, although the full response of photoreceptors is maintained. 2. Passive membrane properties of L-neurones cannot contribute significantly to the cutback in the hyperpolarising response of these neurones to light on. 3. When depolarising or hyperpolarising currents are injected into a photoreceptor, responses in an L-neurone it synapses with are of opposite polarity and markedly phasic. The voltage gain of the connexion between one photoreceptor and an L-neurone can be as great as nine. 4. No evidence for feedback connexions from L-neurones to photoreceptors has been found. 5. Conductance measurements on some L-neurones indicate that synaptically-induced currents may make a small contribution to the cutback in the hyperpolarising response to light on. 6. Some L-neurones make rapidly decrementing inhibitory connexions with other L-neurones. 7. A variety of neurones which respond to changes in illumination by alterations in spike rate have been found. Some of these connect with L-neurones. 8. Possible mechanisms for the cutback in the hyperpolarising response of an L-neurone at light on include intrinsic properties of the pre- or postsynaptic terminals, and excitatory synapses made by small second-order neurones on L-neurones.