Incident Optical Radiation Research Articles

Improved electrical characteristics of porous germanium photodiode obtained by phosphorus ion implantation

Germanium–based photodetectors are the standard technology used for radiometric measurement in the IR spectral range. Shallow PN+ junctions were obtained by using ion implantation in p–type germanium. N–type doping was achieved by phosphorus implantation at a dose of 1 × 1015 cm−2 doses and an energy of 150 keV followed by annealing at 600°C for 2 h. In order for the new detector to be used as a standard for radiometric measurement, a porous layer is introduced on the active surface. After metallisation by thermal evaporation of indium (In) on the front side and gold (Au) on the back side, the new porous germanium–based photodetector is essentially done. The electrical behaviour of this porous germanium photodiode is characterised by current–voltage measurements that are compared to the I–V characteristic of classical germanium photodiodes. The results of this study show a huge increase of the photocurrent, a significant increase of the shunt resistance and low series resistance. Effects of low shunt resistance are also discussed. AFM showed that the porous layer contributes to trap the incident optical radiation and to reduce the reflection coefficient fluctuations of the front face of the photodiode.

Strong self-focusing of axial symmetric laser beam due to quadratic nonlinearity

Strong self-focusing and defocusing of optical radiation with Gaussian spatial and temporal distribution due to cascading second harmonic generation without a significant change in the beam profile and the pulse shape for the fundamental wave under the condition of group velocity matching in the axial-symmetric case for a bulk medium is considered, and the maximum intensity of an optical beam increases in 70 times (or more) in comparison with the intensity of incident optical radiation. This result is obtained for pulse duration, which belongs to a time interval from microseconds to picoseconds, for which the dispersion of group velocity is negligible. Self-focusing of laser radiation allows the realization of the mode that is similar to Kerr-lens mode locking, but for laser systems generating the optical radiation with a duration from microseconds to picoseconds.

A Two-Dimensional (2D) Analytical Model for the Potential Distribution and Threshold Voltage of Short-Channel Ion-Implanted GaAs MESFETs under Dark and Illuminated Conditions

A two-dimensional (2D) analytical model for the potential distribution and threshold voltage of short-channel ion-implanted GaAs MESFETs operating in the sub-threshold regime has been presented. A double-integrable Gaussian-like function has been assumed as the doping distribution profile in the vertical direction of the channel. The Schottky gate has been assumed to be semi-transparent through which optical radiation is coupled into the device. The 2D potential distribution in the channel of the shortchannel device has been obtained by solving the 2D Poisson’s equation by using suitable boundary conditions. The effects of excess carrier generation due to the incident optical radiation in channel region have been included in the Poisson’s equation to study the optical effects on the device. The potential function has been utilized to model the threshold voltage of the device under dark and illuminated conditions. The proposed model has been verified by comparing the theoretically predicted results with simulated data obtained by using the commercially available ATLAS™ 2D device simulator.

A two-dimensional analytical model for the gate–source and gate–drain capacitances of ion-implanted short-channel GaAs metal-semiconductor-field effect transistor under dark and illuminated conditions

This paper presents an analytical model for the internal capacitances of short-channel ion-implanted GaAs MESFETs under dark and illuminated conditions. The device structure considered in this study is a normally-on self-aligned GaAs MESFET with indium tin oxide (ITO) as the Schottky metal for the gate electrode of the device. The gate area of the device is illuminated by an optical radiation of 0.87 μm from an external source which is coupled into the device through the semitransparent ITO metal at Schottky-gate to modulate the electrical and microwave characteristics of the device. The nonanalytic Gaussian doping profile commonly considered for the channel doping of an ion-implanted GaAs MESFET has been replaced by an analytic Gaussianlike function for the simplicity of the present model. The two-dimensional (2D) potential distribution obtained by solving the 2D Poisson’s equation by including the effect of photo-generated carriers due to the incident optical radiation has been utilized to model the depletion region width below the gate of the short-channel GaAs MESFETs. The depletion width model has then been used to model the internal gate-source and gate-drain capacitances of the device operating under both the linear and saturation regions. The effect of illumination on the Schottky junction at the gate of the MESFET has been modeled by considering an induced photo-voltage developed across the junction that is superimposed on the applied gate bias voltage. The proposed model has been verified by comparing the theoretically predicted results with simulated data obtained by using the commercially available ATLASTM 2D device simulator.

In vivo study of age-related changes in the optical properties of the skin

The optical properties of the skin (absorption coefficient, scattering coefficient, refractive index) may serve to characterize the skin and are important for correct light dosimetry in many optical diagnostic procedures and laser treatments especially photodynamic therapy and laser therapy. We determined in vivo the optical properties of tissues near the wrist, elbow and knee in subjects of different ages using diffuse reflectance spectroscopy, having in view the establishment of laser system types for the laser treatment of posttraumatic lesions in subjects of different ages. Diffuse reflection of light from biological tissue is due to the variation in refractive index of tissular and cellular components and the surrounding medium and depends on the wavelength of the incident optical radiation. The diffuse reflectance spectrum of the tissues tested showed two maxima localized at lamda(M1) is approximately 610 nm and lambda(M2) is approximately 675 nm. Laser systems which emit radiation at these wavelengths are not efficient for the treatment of joints, regardless of the subject's age. The deep tissues have a strong absorption in the range 630-700 nm, which indicates that for treating posttraumatic lesions we can use laser systems such as the He-Ne laser, the GaAlAs laser, and the InGaAlAs laser. Using Kramers-Kronig analysis of the diffuse reflectance spectra, the optical parameters n(omega) and k(omega) were determined. The age-dependent changes in these optical parameters of tissue must be taken into consideration and the use of laser treatments or optical diagnosis methods must be based on a knowledge of these properties and of the optical radiation parameters.

Evaluation of local photoconductivity of solar cells by microwave near-field microscopy technique

Local photoconductivity in surface layer of solar elements has been studied by the method of microwave near-field microscopy (MW NFM). Dependences of photoconductivity of a solar cell on the intensity and wavelength of the incident optical radiation are determined by measuring the coefficient of reflection of MW NFM resonator at the frequency 4.1 GHz.

Possibility of generating a sequence of attosecond subpulses upon second harmonic generation of high-intensity femtosecond pulses in an optically extended medium

Based on numerical simulation, we demonstrate the possibility of forming a sequence of equalintensity coupled subpulses upon second harmonic generation (SHG) of a high-intensity femtosecond pulse in an optically extended medium whose length is much smaller than the diffraction length of the incident optical radiation. The duration of individual subpulses can be ten (or more) times shorter than the duration of the initial femtosecond pulse. With respect to the spatial coordinate, the breakup of the initial Gaussian or hyper-Gaussian beam into a regular structure of subbeams, whose size can be from two to ten times smaller than the initial beam radius, also takes place. The SHG is shown to result in a limitation of the intensity of axially symmetric beams in a nonlinear focus. The subpulses being formed can propagate with velocities that differ substantially from the velocities of their propagation in the linear medium. The shape of the subpulses is close to a soliton shape, whereas the spatial distribution of the beam has a profile that can be approximated by a set of Gaussian and hyper-Gaussian distributions. The radiation at the double frequency leads to a phase modulation of the optical radiation at the fundamental frequency, which is necessary for the formation of subpulses and subbeams and is the cause of formation of these structures. The formation of subpulses occurs at the fundamental frequency at a small efficiency (≤8%) of energy conversion into the second harmonic.

Micro-Electro-Mechanical-Systems-Based Infrared Spectrometer Composed of Multi-Slit Grating and Bolometer Array

A micro-electro-mechanical-systems (MEMS)-based IR spectrometer has been designed and fabricated. The proposed micro-spectrometer consists of multi-slit grating and bolometric IR detector array. The grating is used for dispersion of an incident optical radiation into different wavelength components. The grating structure is patterned by reactive ion etching (RIE) of aluminum deposited on dielectric membrane, which is fabricated over silicon wafer. The IR detector part adopts a resistive bolometer that changes its resistance due to the temperature increment. The presence of IR at the specific wavelength can be detected by noticing the resistance change when temperature increases due to an incident IR ray. Vanadium oxide is adopted for resistive bolometer of IR spectrometer in this work because of its higher temperature coefficient of resistance (TCR) which is needed for the better sensitivity. Two parts that composes the MEMS-based IR spectrometer, aluminum multi-slit grating and thermal-type vanadium oxide IR detector, are processed independently and bonded as a final step.

Study and realization of a transfer detector based on porous silicon for radiometric measurements

A photodetector of high accuracy has been developed at the “Laboratoire de Métrologie des Rayonnements (L.M.R)—Tunisia” to serve as a transfer detector for optical power measurements in the visible spectral range, i.e. wavelength from 400 to 800 nm. The originality of the work, comparing with other ones, consists of realizing a pn + junction with a n + porous silicon layer on the front face and a zinc oxide (ZnO) film as a transparent electrode above the porous layer. This detector's structure permits to the porous silicon layer to trap the incident optical radiation and to reduce the reflection coefficient fluctuations of the front face to a value of about 7%. The uncertainties relating to the spatial response uniformity of the detector and especially in the central area with 5 mm diameter are equal to 0.5–0.7% for vertical and horizontal directions, respectively. The realized photodetector can roughly reach 10 mW of measured optical power. This value represents an earning of one decade comparing with commercial photodiodes that cannot measure optical power beyond 1 mW. The photodetector presents a linearity with a cumulated deviation of a few 10 −4 through more than 4 decades with an uncertainty of 1.5×10 −4 at 1 σ level. The photodetector was also indirectly calibrated with the cryogenic radiometer of the “Institut National de Métrologie (INM)–France” and provides a convenient standard with a relative standard uncertainty of a few 10 −3 at the 1 σ level. This uncertainty is equivalent to commercial transfer detectors used in metrology laboratories.

Near field properties in the vicinity of gold nanoparticles placed on various substrates for precise nanostructuring

We present theoretical results on the near field distribution in the vicinity of gold particles excited by laser radiation and placed on various substrate materials. The study is directed towards the precise nanostructuring of different material surfaces. The calculations are performed on the basis of the FDTD simulation technique. Metal (Au), semiconductor (Si) and dielectric (SiO2) substrate materials are investigated. Experimental results are shown to confirm the validity of the obtained FDTD simulation results. The results show that the field in the vicinity of the point of contact is enhanced, and the enhancement factor depends on the substrate material, particle size and the wavelength of the incident optical radiation. The characteristic size of the field enhanced area is found to be several times smaller than the gold particle size: for the case of the smallest particle diameter of 40 nm it is about 10 nm. The enhancement factor of the electric field on the substrate surface is highest when the substrate is gold metal and it decreases about two orders of magnitude when dielectric SiO2 substrate is used. The characteristic penetration depth of the enhanced field is within several tens of nanometres and the depth weakly depends on the substrate material and the laser excitation wavelength. The dependence of the gold particle size on the field enhancement factor is investigated as well. The present results can be used to predict and design precise nanostructuring on different materials with gold nanoparticles excited by a femtosecond laser pulse.

Correction of photoresponse nonuniformity for matrix detectors based on prior compensation for their nonlinear behavior

What we believe to be a novel procedure to correct the nonuniformity that is inherent in all matrix detectors has been developed and experimentally validated. This correction method, unlike other nonuniformity-correction algorithms, consists of two steps that separate two of the usual problems that affect characterization of matrix detectors, i.e., nonlinearity and the relative variation of the pixels' responsivity across the array. The correction of the nonlinear behavior remains valid for any illumination wavelength employed, as long as the nonlinearity is not due to power dependence of the internal quantum efficiency. This method of correction of nonuniformity permits the immediate calculation of the correction factor for any given power level and for any illuminant that has a known spectral content once the nonuniform behavior has been characterized for a sufficient number of wavelengths. This procedure has a significant advantage compared with other traditional calibration-based methods, which require that a full characterization be carried out for each spectral distribution pattern of the incident optical radiation. The experimental application of this novel method has achieved a 20-fold increase in the uniformity of a CCD array for response levels close to saturation.

SIMULATIONS OF THE GENERATION OF TERAHERTZ RADIATION BY PHOTOMIXING IN LASER-ASSISTED FIELD EMISSION TO OPTIMIZE THE SIGNAL-TO-NOISE RATIO

Quantum simulations and experiments show that photomixing in laser-assisted field emission has promise as a new method for wide-band tunable sources of radiation at terahertz frequencies. The tunable bandwidth is only limited by the means for coupling power from the oscillations in the field emission current from photomixing, and not by the processes that generate this current. Photomixing is simulated as a stationary stochastic process in which the frequencies and phases of the incident optical radiation are random variables. The waveform of the current is determined by solving the Schrödinger equation at discrete time steps for which the effective potential barrier is a superposition of the incident radiation field and the static barrier. These samples satisfy the criteria of a Poisson process to allow for the discrete emission of electrons at a specified total current. The one-sided power spectral density for this current is calculated with the FFT to produce periodogram estimates. The simulations show that the signal-to-noise ratio may be increased by (1) raising the power flux density of each laser, (2) raising the DC static current, (3) reducing the linewidth of each laser, and (4) using a static current density of no more than 1010 A/m.

Properties of R.F. magnetron sputtered cadmium–tin–oxide and indium–tin–oxide thin films

The electrical, optical, structural, chemical, and etch properties of r.f. magnetron, sputter-deposited thin films of indium–tin–oxide (ITO) and cadmium–tin–oxide (CTO) are presented. Polycrystalline thin films of CTO and ITO with distinct diffraction peaks corresponding to the (220), (222), and (400) crystal planes and with an overall preferential 〈111〉 orientation were obtained at elevated deposition temperatures. The lattice parameter of these films was determined to be 0.948 and 1.026 nm for CTO and ITO, respectively. These thin films possess low resistivities in the 10 −4 Ω cm range, Hall mobilities up to 30 cm 2/V s, and carrier concentrations in the 10 20 cm −3 range. In excess of 85% of the perpendicularly incident optical radiation, in the 0.85 to 1.55 μm wavelength range, and up to 50% of the incident radiation at a wavelength of 365 nm was transmitted through the CTO and ITO conductors. A variety of wet and dry etching techniques were investigated in order to delineate features in the thin films. From investigations concerned with the Schottky and ohmic contact characteristics of CTO and ITO films deposited on undoped AlGaAs and InAlAs, ideality factors ranging from 1.1 to 1.2 and Schottky barrier heights ranging from 0.67 to 1.05 eV were obtained.

Optimization of front surface texturing processes for high-efficiency silicon solar cells

This paper presents the results of an experimental study regarding the increase in the efficiency of the silicon solar cells by texturing the front surface. Designing, patterning and surface etching processes led to refined structures with very low losses of the incident optical radiation. Photolithography has been used to generate patterns (disc hole) through the silicon dioxide layer grown at the beginning on silicon wafers. The holes (4 μm in diameter) have been uniformly distributed on the entire surface (2×2) cm2 and the distance between the hole centres was determined to be 20 μm. Semispherical walls have been defined in holes by isotropic etching up to join together of the wells.

Modeling of the nonlinear response of the intrinsic HgCdTe photoconductor by a two-level rate equation with a finite number of carriers available for photoexcitation.

A description of the nonlinear response of a HgCdTe photoconductor on the incident optical radiation is derived from a two-level rate equation for the photoexcited carriers. The model accounts for a limited number of electrons available for photoexcitation, and the possibility of photostimulated relaxation of an already excited photoelectron. The detector response as well as the level of incident optical power where saturation occurs is directly related to the physical constants of the detector material. The saturation of the detector is the same phenomenon as is known for the saturable absorbers-the detector material becomes transparent when the incident optical power exceeds the saturation power. Even though the description given here is applied to the HgCdTe detector, the general model can be applied to other detector materials as well. The parameters for the model are fitted to a thermoelectrically cooled HgCdTe detector and afterwards employed to predict the detector noise performance in a heterodyne setup. Unlike for liquid-nitrogen-cooled detectors, shot-noise-limited detection cannot be obtained with use of only thermoelectrical cooling (226 K) of the detector. However, the detector performance can be optimized to be able to perform Doppler measurements from aerosol backscatter by use of the model presented to optimize the applied optical power in the reference wave.

Time-resolved infrared transmittance and reflectance of a propagating melt in GaAs

The time-resolved infrared transmittance and reflectance of a melt induced by a 10 nsec optical-laser pulse has been observed in an undoped crystalline GaAs wafer. Picosecond pulsed, 2.86 GHz repetition rate, infrared radiation from a free-electron laser was used to study the formation and propagation of the melt in real time. The back reflectance (probed from the side opposite to the incident optical radiation) displays interference oscillations as the melt propagates in the sample. The measurements are in agreement with model calculations which describe the melt with the Drude free-carrier model.

Realization of an all-optical triode and diode with a two-level-atom–loaded diffraction grating

A finite-difference time-domain full-wave vector Maxwell equation solver is coupled with a two-level-atom model to simulate the scattering of an ultrafast pulsed Gaussian beam from a finite-length, metallic lamellar grating loaded with two-level atoms. The atomic medium is taken to be resonant at or near the frequency of the incident optical radiation. The highly resonant material and grating behaviors are then combined to realize an all-optical triode at low powers and an all-optical diode at high powers. Simulation results demonstrate the operating characteristics of these triode and diode configurations.

The incorporation of microscopic material models into the FDTD approach for ultrafast optical pulse simulations

We are developing full-wave vector Maxwell equation solvers for use in studying the physics and engineering of linear and nonlinear integrated photonics systems. Particular emphasis has been given to the interaction of ultrafast optical pulses with nonresonant and resonant optical materials and structures. Results are reviewed that simulate the interaction of ultrafast optical pulses with structures (e.g., gratings of finite length) filled with materials exhibiting resonant loss or gain. In particular, we consider structures loaded with atomic media resonant at or near the frequency of the incident optical radiation. Interest in these problems follows from our desire to design micron-sized linear and nonlinear guided-wave couplers, modulators, and switches. These resonant problems pose interesting FDTD modeling issues because of the many time and length scales involved. To understand the physics underlying the small-distance scale and short-time scale interactions, particularly in the resonance regime of the materials and the associated device structures, a first principles approach is desirable. Thus, the results presented are based upon a quantum mechanical two-level atom model for the materials. The resulting Maxwell-Bloch model requires a careful marriage between microscopic (quantum mechanical) material models of the resonant material systems and the multidimensional, macroscopic Maxwell's equations solver. The FDTD numerical issues are discussed. Examples are given to illustrate the design and control of these resonant large-scale optical structures. An optical triode is designed and characterized with the FDTD Maxwell-Bloch simulator.

A new type of gas sensor based on the semiconductor-cleft-semiconductor structure

The capacitive characteristics of a semiconductor-cleft-semiconductor structure created by means of cleavage are studied. The advantage of this method is the possibility of using various materials in different phase states as the dielectric in such a structure. The capacity of this structure depends on the applied voltage, the intensity of the incident optical radiation and the density of the charge on the surface of the semiconductors. Relaxation oscillations can form in the structure when the cleft is filled with an antiferroelectric.

Grating coupling to surface plasma waves II Interactions between first- and second-order coupling

For grating periods larger than the excitation wavelength, multiple-grating orders couple incident optical radiation to the surface plasma waves (SPW’s) characteristic of the metal–air interface. For a grating period that is an integral multiple of the wave vector of these surface modes, two resonances become degenerate in coupling angle. There are also permitted diffraction orders at this coupling angle. The vicinity of this multiple-mode coupling resonance, where several free-space electromagnetic modes, as well as two surface modes, are coupled by different orders of a grating, is known as a minigap region. Not surprisingly, the response surface displays complex dependences on frequency, angle, and grating profile. A detailed experimental and theoretical study is presented of the optical response at 633 nm in the (+1, −2) minigap region for Ag films deposited on photolithographically defined 870-nm-period gratings. Measurements of both the 0-order reflectance and the −1-order diffraction are presented for a wide progression of grating depths. The SPW resonances depend on the grating depth, and this variation is used to tune through the minigap region for a fixed wavelength and period. Similar measurements are presented for a single grating as a function of wavelength through the minigap region. In both measurements the 0-order response shows only a single broad minimum as the resonances approach degeneracy, while the −1-order diffraction shows clearly defined momentum gaps. A simple theoretical model based on the Rayleigh hypothesis is presented that gives a good qualitative picture of the response. The response surfaces are sensitive to the grating profile, and detailed modeling requires inclusion of higher-order grating components.