dB Roll-off Frequency Research Articles

Visible to Short-Wave Infrared Photodetectors Based on ZrGeTe4 van der Waals Materials.

The self-terminated, layered structure of van der Waals materials introduces fundamental advantages for infrared (IR) optoelectronic devices. These are mainly associated with the potential for low noise while maintaining high internal quantum efficiency when reducing IR absorber thicknesses. In this study, we introduce a new van der Waals material candidate, zirconium germanium telluride (ZrGeTe4), to a growing family of promising IR van der Waals materials. We find the bulk form ZrGeTe4 has an indirect band edge around ∼0.5 eV, in close agreement with previous theoretical predictions. This material is found to be stable up to 140 °C and shows minimal compositional variation even after >30 days storage in humid air. We demonstrate simple proof-of-concept broad spectrum photodetectors with responsivities above 0.1 AW-1 across both the visible and short-wave infrared wavelengths. This corresponds to a specific detectivity of ∼109 cm Hz1/2 W-1 at λ = 1.4 μm at room temperature. These devices show a linear photoresponse vs illumination intensity relationship over ∼4 orders of magnitude, and fast rise/fall times of ∼50 ns, also verified by a 3 dB roll-off frequency of 5.9 MHz. As the first demonstration of photodetection using ZrGeTe4, these characteristics measured on a simple proof-of-concept device show the exciting potential of the ZrGeTe4 for room temperature IR optoelectronic applications.

Modeling and performance evaluation of in-line Fabry-Perot photothermal gas sensors with hollow-core optical fibers.

We study photothermal phase modulation in gas-filled hollow-core optical fibers with differential structural dimensions and attempt to develop highly sensitive practical gas sensors with an in-line Fabry-Perot interferometer for detection of the phase modulation. Analytical formulations based on a hollow-capillary model are developed to estimate the amplitude of photothermal phase modulation at low modulation frequencies as well as the -3 dB roll-off frequency, which provide a guide for the selection of hollow-core fibers and the pump modulation frequencies to maximize photothermal phase modulation. Numerical simulation with the capillary model and experiments with two types of hollow-core fibers support the analytical formulations. Further experiments with an Fabry-Perot interferometer made of 5.5-cm-long anti-resonant hollow-core fiber demonstrated ultra-sensitive gas detection with a noise-equivalent-absorption coefficient of 2.3×10-9 cm-1, unprecedented dynamic range of 4.3×106 and <2.5% instability over a period of 24 hours.

Three-Dimensional Integration of Black Phosphorus Photodetector with Silicon Photonics and Nanoplasmonics

We demonstrate the integration of a black phosphorus photodetector in a hybrid, three-dimensional architecture of silicon photonics and metallic nanoplasmonics structures. This integration approach combines the advantages of the low propagation loss of silicon waveguides, high-field confinement of a plasmonic nanogap, and the narrow bandgap of black phosphorus to achieve high responsivity for detection of telecom-band, near-infrared light. Benefiting from an ultrashort channel (∼60 nm) and near-field enhancement enabled by the nanogap structure, the photodetector shows an intrinsic responsivity as high as 10 A/W afforded by internal gain mechanisms, and a 3 dB roll-off frequency of 150 MHz. This device demonstrates a promising approach for on-chip integration of three distinctive photonic systems, which, as a generic platform, may lead to future nanophotonic applications for biosensing, nonlinear optics, and optical signal processing.

Demonstration of a GaAs-based 1550-nm continuous wave photomixer

An Er:GaAs-based 1550-nm CW photomixer is demonstrated. The related mechanism is extrinsic photoconductivity with optical absorption between the localized deep levels created by the Er and the extended states above the conduction band edge of GaAs. With the power boost made possible by a fiber-coupled erbium-doped-fiber amplifier, the Er:GaAs photomixers, operating at 1550 nm, radiate THz power levels easily measured by a Golay cell, and display a power spectrum having a −3 dB roll-off frequency of 307 GHz. This corresponds to a photocarrier lifetime of 520 fs, in good agreement with a previous measurement of the bandwidth of the same material in a photoconductive switch.

Broadband PVDF Membrane Hydrophone for Comparisons of Hydrophone Calibration Methods up to 140 MHz

A PVDF membrane hydrophone has been constructed in particular for comparisons of broadband ultrasound hydrophone calibration methods and of the results obtained by different laboratories. Intercomparisons have to accompany the efforts currently undertaken to enhance the calibration frequency ranges and to implement the extension from the determination of amplitude-only to complex-valued calibration data. It can be expected that such hydrophone data will be used much more frequently in the future for exposure measurements on medical ultrasound equipment, in particular for the detection of nonlinearly distorted waveforms. The hydrophone design chosen has a foil thickness of 9 microm and an electrode diameter of 210 microm. A broadband differential preamplifier (-3 dB roll-off frequency: 95 MHz) is integrated to achieve a high signal-to-noise ratio over a broad frequency range (e.g., 26 dB-30 dB in the range 50 MHz to 140 MHz for measurements of nonlinearly distorted pulses). The hydrophone response was characterized by means of a primary interferometric calibration technique, by substitution calibration using time-delay spectrometry, and by complex broadband pulse calibration using nonlinear sound propagation. The results show a flat frequency response up to 40 MHz (maximum variations below +/-0.6 dB) and a thickness mode resonance at about 105 MHz. They indicate a useable bandwidth up to 140 MHz. The effective diameter as derived from directional response measurements is 240 microm at frequencies beyond 15 MHz.

Microwave mixing in microbridges made from YBa2Cu3O7−x thin films

We present a systematic study of the response of a YBa2Cu3O7−x thin-film microbridge to millimeter wave radiation. The dependencies of the microwave response spectrum on the bias voltage and operating temperature have been measured on samples made from YBa2Cu3O7−x films with different thicknesses. Once the bias voltage exceeds the value corresponding to the first maximum of differential resistance, the value of the −3 dB roll-off frequency of the microbridge response is drastically reduced. On the other hand, at temperatures close to Tc the roll-off frequency rises to about 20 GHz. For high resistivity YBa2Cu3O7−x films with relatively low upper critical magnetic fields the −3 dB roll-off frequency is about 7 GHz even at temperatures much smaller than Tc. To explain the results obtained we consider the presence of two additive mechanisms that contribute to the YBa2Cu3O7−x thin-film microbridge response to microwave radiation: vibration of magnetic vortices and heating of electrons and phonons. The competition between these two mechanisms defines the bias voltage and temperature dependencies of the −3 dB roll-off frequency as well as the recently reported dependence of the intermediate frequency bandwidth of YBa2Cu3O7−x hot-electron bolometer mixers on the local oscillator frequency. Numerical calculations based on this assumption are in a good qualitative agreement with the experimental results.

High speed characteristics of thin film write heads at deep submicron track width

This article presents a micromagnetic study of the dynamic magnetic switching process in the pole tip region of stitched-pole thin film write heads. The impacts of track width reduction and pole material saturation moment on the high frequency performance of the heads have been investigated. It is found that the magnetic switching process of the pole tip region at low recording frequencies can be characterized by three stages: the nucleation of a vortex (or vortices), followed by the motion of the vortex (or vortices), and finally the annihilation of the vortex (or vortices). At high frequencies, however, a residual vortex (or vortices) will always remain in the pole tips, resulting in lower head field amplitude. Frequency roll-off characteristics of the head field response have been calculated. The roll off is the result of residual vortex (or vortices), and the vortex moving speed limits the operational frequency range of the head. Reducing the track width lowers the head field 3 dB roll-off frequency. The simulation also finds that the use of 45/55 NiFe, which has significantly higher moment than permalloy, results in higher roll-off frequency as compared to permalloy.

Magnetic properties and high-frequency responses of high moment FeTaN/AlN laminates for high-data-rate magnetic recording

Soft magnetic FeTaN and novel FeTaN/AlN multilayer films with a saturation flux density of /spl sim/20 kG are obtained on both planar and sloping substrates. Flat frequency responses extend over 500 MHz for as-deposited 1000 /spl Aring/-thick FeTaN films. The 3 dB roll-off frequency for as-deposited 1 /spl mu/m-thick (FeTaN/ AlN)/sub 10/ laminates is only /spl sim/330 MHz. However, when the laminates are patterned into 20 /spl mu/m-wide strips along the hard axis, the frequency response improves and the 3 dB roll-off frequency is over 500 MHz, which offers the promise of data rates of over 1 Gb/s.

Development of a Bolometer Detector System for the NIST High Accuracy Infrared Spectrophotometer.

A bolometer detector system was developed for the high accuracy infrared spectrophotometer at the National Institute of Standards and Technology to provide maximum sensitivity, spatial uniformity, and linearity of response covering the entire infrared spectral range. The spatial response variation was measured to be within 0.1 %. The linearity of the detector output was measured over three decades of input power. After applying a simple correction procedure, the detector output was found to deviate less than 0.2 % from linear behavior over this range. The noise equivalent power (NEP) of the bolometer system was 6 × 10−12 at the frequency of 80 Hz. The detector output 3 dB roll-off frequency was 200 Hz. The detector output was stable to within ± 0.05 % over a 15 min period. These results demonstrate that the bolometer detector system will serve as an excellent detector for the high accuracy infrared spectrophotometer.

Dynamic range performance of a high speed, highsaturation InGaAs/InP pin waveguide photodetector

An InGaAs/InP pin waveguide photodetector with a 3 dB roll-off frequency of 39 GHz using a simple design is reported. The two-tone spurious-free dynamic range of an external modulation fibre optic link employing the waveguide photodetector is measured and is in excess of 110 dB/Hz2/3 for a photocurrent of 5 mA.

Amplitude-, phase-, and frequency modulation of far-infrared radiation by optical excitation of silicon

The 118.6 μm radiation of an H 2O-laser is modulated in amplitude, phase, and frequency by optical generation of free carriers in a silicon wafer. Two compact, diode-pumped CW Nd:YAG-lasers (λ = 1.06 μm, P = 40 mW each) are used for optical excitation. They are frequency tunable and can be electrically modulated in output power. 70% maximal effective modulation of the FIR intensity and 74° maximal phase modulation are achieved. Modulation frequencies for amplitude and phase up to 80 kHz have been realized, the degree of modulation decreasing with increasing modulation frequency. FIR frequency modulation is accomplished by generating an optically induced “travelling” grating using two Nd:YAG-lasers. Frequency shifts up to 15 MHz were achieved. With 26 mW effective excitation power the maximal efficiency was 1.5 × 10 −3 with a 3 dB roll-off frequency of 35 kHz for the Si wafer used.

Far-infrared frequency modulation by optically induced gratings in silicon

Frequency modulation of 119 μm (2.5 THz) H2O laser radiation by optically induced gratings in Si is reported. The gratings were excited using two diode-pumped, frequency-tunable Nd:YAG lasers. Frequency shifts up to 15 MHz were generated. With 26 mW effective optical exitation power a maximal efficiency of 1.5×10−3 was achieved, with a 3 dB roll-off frequency of 35 kHz for the Si wafer used in the experiment. Results of numerical model calculations are presented, which show that efficiencies in the 10% region should be achievable with moderate optical excitation power. It is pointed out that the principle of optically induced gratings offers a promising tool to transfer frequency modulation from the near-infrared to the far-infrared spectral range.

A quantitative heterodyne experiment with extrinsic silicon at 10.6 μm

We report the results of a series of experiments on the use of an extrinsic silicon photoconductor for heterodyne detection at 10.6 μm. An arsenic-doped sample was for these measurements, having a donor density of 3 × 10 16 cm −3. The measurements were made at approximately 5 K, with a CO 2 laser providing the local oscillator power, and a calibrated blackbody at 900 K providing the signal. The signal-to-noise ratio was measured as a function of detector bias voltage for local oscillator powers ranging from 30 to 280 mW. The measured signal-to-noise ratio was found to be within 4 dB of the theoretically expected value. The 3 dB roll-off frequency was measured from the signal roll-off and from the laser-induced generation-recombination noise roll-off. Both measurements agreed closely. The detector response time determined from this measurement varied between 4 and 10 nsec, depending on the laser power and bias voltage.