GaAsP Photodiode Research Articles

Simultaneous measurement of DC two-photon absorption signal offset and amplitude of the intensity autocorrelation in the focusing of femtosecond pulses.

In this work, the DC two-photon absorption signal offset (${{\rm DC}_{\rm TPA}}$DCTPA) and the amplitude of the autocorrelation (${{\rm A}_{\rm AC}}$AAC) are measured simultaneously around the focal point of an apochromatic microscope objective using the z-scan autocorrelation technique. The ${{\rm A}_{\rm AC}}$AAC is obtained from the nonlinear sensor response given by the two-photon-absorption, generated in a GaAsP photodiode, for femtosecond laser pulses. We verify that the change in the ${{\rm DC}_{\rm TPA}}$DCTPA signal along $z$z is coincident with the amplitude of the intensity autocorrelation, and that the highest amplitude of the AC is reached at the same position as the highest amplitude of the ${{\rm DC}_{\rm TPA}}$DCTPA signal. The ${{\rm DC}_{\rm TPA}}$DCTPA signal is typically used as a reference for the alignment in a collinear intensity autocorrelator, and we show that it can also be used as a practical procedure to estimate the depth of focus. The ${{\rm DC}_{\rm TPA}}$DCTPA signal measurement allows us to locate the optimum spatial-temporal coupling given by the highest amplitude of the intensity autocorrelation. Additionally, we find a variation in the pulse duration within the same region due to the radially varying group delay dispersion.

Determination of the contributions of two simultaneous absorption orders using 2-beam action spectroscopy.

The concept of a 2-beam action (2-BA) spectroscopy was recently introduced as a method for determining the order of effective nonlinear absorption in multiphoton photoresists. Here we demonstrate that the 2-BA approach can be extended to any measureable observable generated by linear and/or nonlinear absorption. As an example, 2-beam constant-amplitude photocurrent spectroscopy is used to study absorption of a tightly focused, mode-locked or continuous-wave, 800 nm laser by a GaAsP photodiode. The effective order of the absorption process can be measured at any desired value of the photocurrent or photovoltage. A self-consistent framework is presented for using non-integral 2-BA exponents to determine the relative contributions of two absorption mechanisms of different order. The dependence of the ratio of the quadratic and linear contributions on the average excitation power is used to verify that these are the dominant orders of absorption in the photodiode with 800 nm excitation.

Conversion efficiency of Si-InGaAs and GaAsP-Si-Ge lateral beam splitting photovoltaic devices

This paper deals with the development and characterization of two photovoltaic systems based on the lateral beam splitting principle, with components able to convert different spectral bands of the solar radiation. One is based on a Si cell and a InGaAs photodiode coupled through a long-pass dichroic filter with 950 nm cut-off. A second system is working with a Si photodiode coupled with Ge and a GaAsP photodiodes and two long-pass dichroic filters of 600 nm and 950 nm cut-off in cascade. The systems have been tested under illumination with a Sun simulator and under direct solar radiation in various two-terminal configurations. The performance of the two systems have been compared in same illumination conditions. Both systems, as compared with the Si component alone, proved to significantly increase the photovoltaic performance, achieving at best a conversion efficiency of 20%, so almost doubling its efficiency in best cases.

Optimizing ultrashort laser pulse compression by two photon absorption

Demonstrated is an approach for relative optimization of ultrashort pulses using two-photon generated photocurrent in a GaAsP photodiode. Two-photon absorption is a nonlinear process, allowing for highly sensitive tuning of ultrashort laser systems.

Two-photon microscopy using an Yb^3+-doped fiber laser with variable pulse widths

Most of the two-photon fluorescence microscopes are based on femtosecond Ti:Sapphire laser sources near the 800 nm wavelength. Here, we introduce a new confocal two-photon microscope system using a mode-locked Yb(3+)-doped fiber laser. The mode-locked fiber laser produces 13 ps pulses with large positive chirping at a repetition rate of 36 MHz with an average power of 80 mW. By using an external grating pair pulse compressor, the pulse width and the frequency chirping of the laser output are controlled for optimum two-photon excitation. For a given objective lens, the optimum condition was obtained by monitoring the two-photon-induced-photocurrent in a GaAsP photodiode at the sample position. The performance of this pulse width optimized two-photon microscope system was demonstrated by imaging Vybrant DiI-stained dorsal root ganglion cells in 2 and 3 dimensions.

Transient Absorption for Characterization of Quantum Dot Intermediate Band Solar Cells

ABSTRACTWe use transient absorption methods to characterize the sequential two-photon absorption in a quantum-dot super-lattice based intermediate band solar cell (QD-IBSC). Using collinear, orthogonally polarized beams generated from an Optical Parametric Oscillator (OPO) at varying time delay, tuned stepwise from 1050nm to 1250nm, we use the solar cell photocurrent as a direct measure of the transient absorption by measuring the differential photo-current as a function of time delay between two energetically degenerate, 100fs pulses. For comparison, we measure the pulse autocorrelation in the same geometry using a GaAsP photodiode, where all observed photocurrent is derived from instantaneous two-photon absorption. Our measurements show that at high intensity, the measurement is dominated by instantaneous two photon absorption, with a simultaneous sequential two-photon photocurrent which persists beyond the pulse overlap. Our measurements demonstrate the method can reveal carrier dynamics in a working QD-IBSC, and their dependence on energy. The method could potentially give details of the band structure formed in the QD-IBSC. Such knowledge may benefit device development and future designs of IBSCs based on QD superlattices or alternative intermediate band materials or device structures.

Rapid, in-line, non-interferometric auto- and cross-correlator for microscopy

We demonstrate a simple, in-line scheme for determining the duration of ultrafast pulses in the focal region of a high-numerical-aperture microscope objective. Photocurrent generated in a GaAsP photodiode by two-photon absorption of orthogonally-polarized laser beams that meet at a slight angle is used to autocorrelate lasers non-interferometrically. Cross-correlation between two lasers is also demonstrated. This setup, which can be built readily by a microscope user who is not an optics expert, allows for the rapid characterization of pulses that can be hundreds of fs long while making it possible for all of the laser intensity to be employed for nonlinear optical microscopy after the pulse duration has been measured.

Characterization of GaAsP trap detector for radiometric measurements in ultraviolet wavelength region

A trap detector was constructed of three Schottky-type 10×10mm2 GaAsP photodiodes. The spectral reflectance of the trap detector was calculated from the measured spectral reflectances of a single GaAsP photodiode in the wavelength range between 240 and 600nm, and compared to the measured spectral reflectance of the trap detector at three laser wavelengths. The absolute spectral responsivity of the trap detector was measured. The internal quantum efficiencies (IQE) of the trap detector and a single photodiode were calculated in the wavelength region between 250 and 400nm from the spectral reflectances and responsivities. The comparison revealed reduction of the apparent IQE of the trap detector as compared to the single photodiode at the level of 10%. The spatial uniformity of the responsivity of the trap detector was measured, and the corresponding uncertainty component at 325nm was calculated to be 4×10−4. The effect of moderate ultraviolet exposure at the level of 50mJ∕cm2 on the stability of the responsivity of GaAsP photodiode was studied and found to be below 2×10−3 at all used wavelengths.

Difference in the photocurrent of semiconductor photodiodes depending on the polarity of current measurement through a contribution from the photoemission current

When using semiconductor photodiodes in the VUV, photoelectrons emitted from the photodiode surface form an additional current circuit when electrons reach a vacuum chamber wall surrounding the photodiode. In the case where the rear electrode of the photodiode is grounded, and hence the sensing terminal of an electrometer is connected to the front electrode, the electrometer measures the superimposed current, that is, the photocurrent internally generated in the p–n junction plus the photoemission current. Experiments done for diffusion-type silicon photodiodes and Schottky-type GaAsP photodiodes in the wavelength range from 50 nm to 200 nm show that the photoemission contribution has peaks of about 10% and 25%, respectively, of the internal photocurrent at around 100 nm for both without an extraction voltage. Therefore, for precise measurements of semiconductor photodiode photocurrents, the sensing terminal of an electrometer should be connected to the rear electrode of a photodiode to avoid any contribution from the photoemission current.

Adaptive pulse compression by two-photon absorption in semiconductors

We investigate the adaptive optimization of broadband laser pulses, using a closed-loop learning algorithm in which the merit function is derived from two-photon absorption in semiconductors. Photoluminescence experiments with CdS thin films and photocurrent measurements of a GaAsP photodiode have been performed. The experimental data demonstrate that reliable and accurate pulse compression to the bandwidth limit can be achieved, unperturbed by nontrivial phase effects. Therefore two-photon absorption proves to be an easy-to-implement alternative to second-harmonic generation for the compression of broadband laser pulses.

Spectral and intensity dependence of spatially resolved two-photon conductivity defects on a GaAsP photodiode

Femtosecond laser excitation through a near-field scanning optical microscope is used to study spatially resolved defects in the two-photon conductivity of a GaAs0.6P0.4 diffusion type photodiode. Two types of defects are observed when the photodiode is excited with femtosecond pulses below the bulk band gap energy. Photocurrent enhancement defects show a higher photocurrent than the surrounding areas and are driven by both one-and two-photon processes. These defects are often correlated with pits in the photodiode surface. Photocurrent depression defects have the normal two-photon power dependence and are not associated with surface pits. Based on the low measured coverage of both defect sites, the performance of GaAsP in auto- and cross correlators will be unaffected in most situations. A deviation in alloy stoichiometry, in which localized areas are rich in arsenic while poor in phosphorus, provides the best explanation for the origin of the enhancement defects.

Measurement of pulse asymmetry by three-photon-absorption autocorrelation in a GaAsP photodiode

Three-photon absorption in a GaAsP photodiode is used for what is believed to be the first time for nonlinear optical autocorrelation at wavelengths from 1.4 to 1.6microm . Theoretical intensity dependence and contrast ratios are achieved, and the ratio of the three- to the two-photon-absorption coefficient is measured. Finally, it is demonstrated that pulse asymmetry can be measured with no direction-of-time ambiguity by use of unbalanced three-photon-absorption autocorrelation.

Quantum efficiency of cesium iodide photocathodes in the 120–220 nm spectral range traceable to a primary detector standard

Differently prepared CsI samples have been investigated in the 120–220 nm spectral range for their quantum efficiency, spatial uniformity and the effect of radiation aging. The experiments were performed at the PTB radiometry laboratory at the Berlin synchrotron radiation facility BESSY. A calibrated GaAsP Schottky photodiode was used as transfer detector standard to establish traceability to the primary detector standard, because this type of photodiode – unlike silicon p-on-n photodiodes – proved to be of sufficiently stable response when exposed to vacuum ultraviolet radiation. The paper reviews the experimental procedures that were employed to characterize and calibrate the GaAsP photodiode and reports the results that were obtained on the investigated CsI photocathodes.

Spatially resolved femtosecond time correlation measurements on a GaAsP photodiode

Femtosecond photoconductive near-field microscopy on a GaAsP diffusion type photodiode is used to investigate the response of the pure semiconductor material and impurity defect sites. At an excitation energy of 780 nm, below the bulk band gap, impurity sites ≈1 μm in diameter are observed. These sites exhibit a linear dependence of photoconductivity on light intensity instead of the usual two-photon dependence. The linear response on the defect sites gives rise to a first-order interferometric autocorrelation, while the typical GaAsP semiconductor material in other regions yields a second-order interferometric autocorrelation, which can be used for determinations of temporal shape, phase, and linear chirp of pulses. The results indicate that high concentrations of impurity sites can limit the application of a GaAsP photodiode as a femtosecond auto- or cross-correlator.

Using GaAsP photodiodes to characterize ultrashort pulses under high numerical aperture focusing in microscopy

SummaryWe demonstrate for the first time that the two‐photon response of GaAsP photodiodes can be used to conveniently characterize the spatial and temporal profiles of ultrashort pulses at the focus of a high numerical aperture system.

Autocorrelation measurement of 6-fs pulses based on the two-photon-induced photocurrent in a GaAsP photodiode

We experimentally characterize the two-photon response of a GaAsP photodiode by use of a femtosecond Ti:sapphire laser tuned below the diode bandgap. The photodiode is shown to be highly suitable for real-time second-order autocorrelation measurements of pulses as short as 6fs in duration and with energies as small as a few picojoules.

Multiple- and single-shot autocorrelator based on two-photon conductivity in semiconductors

Two-photon-induced conductivity has been observed in Si and GaAsP photodiodes and in a CdS photo-conductive cell at room temperature and applied to the ultrashort optical pulse measurement with a simplified Michelson-type arrangement. The efficiency of the two-photon conductivity is found to be 3 x 10(-14)I A/W, where I is the intensity of the incident pulse in watts per squared centimeter. A single-shot pulse-width measurement has also been performed by using a two-dimensional silicon CCD array. Use of these inexpensive photodetectors with nonlinear characteristics has proved to be convenient for intensity correlators because of their instantaneous response to temporal coincidence of optical pulses and ease of optical arrangement.

Spectral quantum efficiencies of semiconductor photodiodes in the far ultraviolet region

The spectral quantum efficiencies and reflectances of p-i-n-type Si photodiodes and a Schottky-type GaAsP photodiode were measured in the wavelength region between 105 to 300 nm. These results are compared with an optical model using complex refractive indices. The spectral relative external quantum efficiencies are well interpreted within the model by assuming that the production energy of an electron-hole pair, ε, is wavelength independent. It is suggested that surface recombination of minority carriers occurs in some devices. The internal quantum efficiency for the GaAsP photodiode is shown to be constant in the photon energy range below about 5.4 eV. The recombination loss and the ε value are separately estimated.

Design of an ultraviolet radiometer 2 Detector optical characteristics

In Part 1 of this paper, GaAsP Schottky barrier photodiodes were shown to be superior to silicon-based devices in terms of high current linearity and noise performance. In this paper, GaAsP devices are also shown to have significant advantages over silicon in terms of UV stability and radiometric performance when used with optical filters. Metal-dielectric interference filters (MDIFs) and glass absorption filters (GAFs) also have acceptable UV stability. A seven-channel UV radiometer for use in a space environment simulation facility and which uses GaAsP photodiodes in combination with MDIFs and GAFs is described.

A real-time microcomputer-assisted array spectroradiometer for measurement and photobiological weighting of irradiance

We have modified a real-time array spectrometer to a spectroradiometer that allows on-line measurement of threshold exposure times for the UV-erythema and direct pigmentation, as well as of colour, by only one portable instrument. The spectral procedure used involves three stages of measuring and data processing; the rapid pick-up of the spectral power distribution of the incident radiation within the range from 200 to 800 nm, with a spectral resolution of 1.5 nm and shortest measuring time of 1.5 ms; the absolute detection of irradiance of an integral part of the spectrum by three GaAsP photodiodes near the entrance slit, with arched Teflon diffusers in front of it; and the photobiological weighting of measured spectral irradiances by the built-in microcomputer.