Orhan Aytür

Fractional Fourier domains

It is customary to define the time-frequency plane such that time and frequency are mutually orthogonal coordinates. Representations of a signal in these domains are related by the Fourier transform. We consider a continuum of “fractional” domains making arbitrary angles with the time and frequency domains. Representations in these domains are related by the fractional Fourier transform. We derive transformation, commutation, and uncertainty relations among coordinate multiplication, differentiation, translation, and phase shift operators between domains making arbitrary angles with each other. These results have a simple geometric interpretation in time-frequency space. ijblicherweise wird die Zeit-Frequenz-Ebene so definiert, dal3 Zeit und Frequenz orthogonale Koordinaten darstellen. Signaldarstellungen in diesen Bereichen %ngen iiber die Fouriertransformation zusammen. Wir betrachten ein Kontinuum von “fraktionalen” Bereichen, die mit dem Zeitbereich und mit dem Frequenzbereich einen beliebigen Winkel einschlieljen. Signaldarstellungen in diesen Bereichen sind durch die “fraktionale Fouriertransformation” verkniipft. Wir zeigen Transformations-, Kommutations- und Unschsrfebeziehungen von Koordinatenmultiplikations-, Differentiations-, Verschiebungs- und Phasenverschiebungsoperatoren zwischen Bereichen, die beliebige Winkel einschlieljen. Diese Ergebnisse erlauben eine einfache geometrische Interpretation im Zeit-Frequenz-Raum.

Solar-blind AlGaN-based p-i-n photodiodes with low dark current and high detectivity

We report solar-blind Al/sub x/Ga/sub 1-x/N-based heterojunction p-i-n photodiodes with low dark current and high detectivity. After the p+ GaN cap layer was recess etched, measured dark current was below 3 fA for reverse bias values up to 6 V. The device responsivity increased with reverse bias and reached 0.11 A/W at 261 nm under 10-V reverse bias. The detectors exhibited a cutoff around 283 nm, and a visible rejection of four orders of magnitude at zero bias. Low dark current values led to a high differential resistance of 9.52/spl times/10/sup 15/ /spl Omega/. The thermally limited detectivity of the devices was calculated as 4.9/spl times/10/sup 14/ cm/spl middot/Hz/sup 1/2/W/sup -1/.

High-performance solar-blind photodetectors based on Al/sub x/Ga/sub 1-x/N heterostructures

Design, fabrication, and characterization of high-performance Al/sub x/Ga/sub 1-x/N-based photodetectors for solar-blind applications are reported. Al/sub x/Ga/sub 1-x/N heterostructures were designed for Schottky, p-i-n, and metal-semiconductor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I--V) characteristics led to a detectivity performance of 4.9/spl times/10/sup 14/ cmHz/sup 1/2/W/sup -1/. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2/spl times/10/sup 4/ was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.

Solar-blind AlGaN-based Schottky photodiodes with low noise and high detectivity

We report on the design, fabrication, and characterization of solar-blind Schottky photodiodes with low noise and high detectivity. The devices were fabricated on n−/n+ AlGaN/GaN heterostructures using a microwave compatible fabrication process. True solar-blind operation with a cutoff wavelength of ∼274 nm was achieved with AlxGa1−xN (x=0.38) absorption layer. The solar-blind detectors exhibited <1.8 nA/cm2 dark current density in the 0–25 V reverse bias regime, and a maximum quantum efficiency of 42% around 267 nm. The photovoltaic detectivity of the devices were in excess of 2.6×1012 cm Hz1/2/W, and the detector noise was 1/f limited with a noise power density less than 3×10−29 A2/Hz at 10 kHz.

High-speed visible-blind GaN-based indium–tin–oxide Schottky photodiodes

We have fabricated GaN-based high-speed ultraviolet Schottky photodiodes using indium–tin–oxide (ITO) Schottky contacts. Before device fabrication, the optical transparency of thin ITO films in the visible-blind spectrum was characterized via transmission and reflection measurements. The devices were fabricated on n−/n+ GaN epitaxial layers using a microwave compatible fabrication process. Our ITO Schottky photodiode samples exhibited a maximum quantum efficiency of 47% around 325 nm. Time-based pulse-response measurements were done at 359 nm. The fabricated devices exhibited a rise time of 13 ps and a pulse width of 60 ps.

InGaAs-based high-performance p-i-n photodiodes

We have designed, fabricated, and characterized high-speed and high-efficiency InGaAs-based p-i-n photodetectors with a resonant cavity enhanced structure. The devices were fabricated by a microwave-compatible process. By using a postprocess recess etch, we tuned the resonance wavelength from 1605 to 1558 nm while keeping the peak efficiencies above 60%. The maximum quantum efficiency was 66% at 1572 nm which was in good agreement with our theoretical calculations. The photodiode had a linear response up to 6-mW optical power, where we obtained 5-mA photocurrent at 3-V reverse bias. The photodetector had a temporal response of 16 ps at 7-V bias. After system response deconvolution, the 3-dB bandwidth of the device was 31 GHz, which corresponds to a bandwidth-efficiency product of 20 GHz.

Simultaneous phase matching of optical parametric oscillation and second-harmonic generation in aperiodically poled lithium niobate

We report a simple ad hoc method for designing an aperiodic grating structure to quasi-phase match two arbitrary second-order nonlinear processes simultaneously within the same electric-field-poled crystal. This method also allows the relative strength of the two processes to be adjusted freely, thereby enabling maximization of the overall conversion efficiency. We also report an experiment that is based on an aperiodically poled lithium niobate crystal that was designed by use of our method. In this crystal, parametric oscillation and second-harmonic generation are simultaneously phase matched for upconversion of a femtosecond Ti:sapphire laser to 570 nm. This self-doubling optical parametric oscillator provides an experimental verification of our design method.

High-speed solar-blind photodetectors with indium-tin-oxide Schottky contacts

We report AlGaN/GaN-based high-speed solar-blind photodetectors with indium-tin-oxide Schottky contacts. Current–voltage, spectral responsivity, and high-frequency response characterizations were performed on the fabricated Schottky photodiodes. Low dark currents of <1 pA at 20 V reverse bias and breakdown voltages larger than 40 V were obtained. A maximum responsivity of 44 mA/W at 263 nm was measured, corresponding to an external quantum efficiency of 21%. True solar-blind detection was ensured with a cutoff wavelength of 274 nm. Time-based high-frequency measurements at 267 nm resulted in pulse responses with rise times and pulse-widths as short as 13 and 190 ps, respectively. The corresponding 3-dB bandwidth was calculated as 1.10 GHz.

Non-orthogonal domains in phase space of quantum optics and their relation to fractional Fourier transforms

Abstract It is customary to define a phase space such that position and momentum are mutually orthogonal coordinates. Associated with these coordinates, or domains, are the position and momentum operators. Representations of the state vector in these coordinates are related by the Fourier transformation. We consider a continuum of “fractional” domains making arbitrary angles with the position and momentum domains. Representations in these domains are related by the fractional Fourier transformation. We derive transformation, commutation, and uncertainty relations between coordinate multiplication, differentiation, translation, and phase shift operators making arbitrary angles with each other. These results have a simple geometric interpretation in phase space and applications in quantum optics.

High-speed InSb photodetectors on GaAs for mid-IR applications

We report p-i-n type InSb-based high-speed photodetectors grown on GaAs substrate. Electrical and optical properties of photodetectors with active areas ranging from 7.06/spl times/10/sup -6/ cm/sup 2/ to 2.25/spl times/10/sup -4/ cm/sup 2/ measured at 77 K and room temperature. Detectors had high zero-bias differential resistances, and the differential resistance area product was 4.5 /spl Omega/ cm/sup 2/. At 77 K, spectral measurements yielded high responsivity between 3 and 5 /spl mu/m with the cutoff wavelength of 5.33 /spl mu/m. The maximum responsivity for 80-/spl mu/m diameter detectors was 1.00/spl times/10/sup 5/ V/W at 4.35 /spl mu/m while the detectivity was 3.41/spl times/10/sup 9/ cm Hz/sup 1/2//W. High-speed measurements were done at room temperature. An optical parametric oscillator was used to generate picosecond full-width at half-maximum pulses at 2.5 /spl mu/m with the pump at 780 nm. 30-/spl mu/m diameter photodetectors yielded 3-dB bandwidth of 8.5 GHz at 2.5 V bias.