Field emission properties of SiO2-wrapped CNT field emitter.

Carbon nanotubes (CNTs) exhibit unstable field emission (FE) behavior with low reliability due to uneven heights of as-grown CNTs. It has been reported that a mechanically polished SiO2-wrapped CNT field emitter gives consistent FE performance due to its uniform CNT heights. However, there are still a lack of studies on the comparison between the FE properties of freestanding and SiO2-wrapped CNTs. In this study, we have performed a comparative study on the FE properties of freestanding and SiO2-wrapped CNT field emitters. From the FE measurements, freestanding CNT field emitter requires lower applied voltage of 5.5 V µm-1 to achieve FE current density of 22 mA cm-2; whereas SiO2-wrapped field emitter requires 8.5 V µm-1 to achieve the same current density. This can be attributed to the lower CNT tip electric field of CNTs embedded in SiO2, as obtained from the electric field simulation. Nevertheless, SiO2-wrapped CNTs show higher consistency in FE current than freestanding CNTs. Under repeated FE measurement, SiO2-wrapped CNT field emitter achieves consistent FE behavior from the 1st voltage sweep, whereas freestanding field emitter only achieved consistent FE performance after 3rd voltage sweep. At the same time, SiO2-wrapped CNTs exhibit better emission stability than freestanding CNTs over 4000 s continuous emission.

Enhanced field emission properties of carbon nanotube bundles confined in SiO2 pits.

It has been widely reported that carbon nanotubes (CNTs) exhibit superior field emission (FE) properties due to their high aspect ratios and unique structural properties. Among the various types of CNTs, random growth CNTs exhibit promising FE properties due to their reduced inter-tube screening effect. However, growing random growth CNTs on individual catalyst islands often results in spread out CNT bundles, which reduces overall field enhancement. In this study, significant improvement in FE properties in CNT bundles is demonstrated by confining them in microfabricated SiO2 pits. Growing CNT bundles in narrow (0.5 µm diameter and 2 µm height) SiO2 pits achieves FE current density of 1-1.4 A cm-2, which is much higher than for freestanding CNT bundles (76.9 mA cm-2). From the Fowler Nordheim plots, confined CNT bundles show a higher field enhancement factor. This improvement can be attributed to the reduced bundle diameter by SiO2 pit confinement, which yields bundles with higher aspect ratios. Combining the obtained outcomes, it can be conclusively summarized that confining CNTs in SiO2 pits yields higher FE current density due to the higher field enhancement of confined CNTs.

Enhanced Carbon Nanotubes Growth Using Nickel/Ferrocene-Hybridized Catalyst.

Tall, crystalline carbon nanotubes (CNTs) are desired to successfully integrate them in various applications. As the crystallinity of CNTs improves with increasing growth temperatures, higher growth temperatures are required to obtain crystalline CNTs. However, in a typical chemical vapor deposition (CVD) process, CNT growth rate reduces when the growth temperature exceeds a specific level due to the degradation of the catalyst particles. In this study, we have demonstrated the improved catalytic activity of nickel/ferrocene-hybridized catalyst as compared to sole ferrocene catalyst. To demonstrate this, CNTs are grown on bare silicon (Si) as well as nickel (Ni) catalyst-deposited substrates using volatile catalyst source (ferrocene/xylene) CVD at the growth temperatures ranging from 790 to 880 °C. It was found that CNTs grown on bare Si substrate experience a reduction in height at growth temperature above 860 °C, whereas the CNTs grown on 10 nm Ni catalyst-deposited substrates experience continuous increase in height as the temperature increases from 790 to 880 °C. The enhancement in the height of CNTs by the addition of Ni catalyst is also demonstrated on 5, 20, and 30 nm Ni layers. The examination of CNTs using electron microscopy and Raman spectra shows that the additional Ni catalyst source improves the CNT growth rates and crystallinity, yielding taller CNTs with a high degree of structural crystallinity.

An optically tunable wideband optoelectronic oscillator based on a bandpass microwave photonic filter.

An optoelectronic oscillator (OEO) with wideband frequency tunability and stable output based on a bandpass microwave photonic filter (MPF) has been proposed and experimentally demonstrated. Realized by cascading a finite impulse response (FIR) filter and an infinite impulse response (IIR) filter together, the tunable bandpass MPF successfully replaces the narrowband electrical bandpass filter in a conventional single-loop OEO and serves as the oscillating frequency selector. The FIR filter is based on a tunable multi-wavelength laser and dispersion compensation fiber (DCF) while the IIR filter is simply based on an optical loop. Utilizing a long length of DCF as the dispersion medium for the FIR filter also provides a long delay line for the OEO feedback cavity and as a result, optical tuning over a wide frequency range can be achieved without sacrificing the quality of the generated signal. By tuning the wavelength spacing of the multi-wavelength laser, the oscillation frequency can be tuned from 6.88 GHz to 12.79 GHz with an average step-size of 0.128 GHz. The maximum frequency drift of the generated 10 GHz signal is observed to be 1.923 kHz over 1 hour and its phase noise reaches the -112 dBc/Hz limit of our measuring equipment at 10 kHz offset frequency.

Cavity-length optimization for high energy pulse generation in a long cavity passively mode-locked all-fiber ring laser.

In order to achieve higher pulse energy in a passively mode-locked fiber ring laser, a long cavity length is commonly implemented. However, a long cavity operating in the anomalous dispersion regime also leads to pulse broadening, which reduces the average pulse power. In this paper, the trade-off between cavity length and average pulse power is investigated with the aim of optimizing the cavity length to achieve maximum pulse energy. Numerical simulation results, presented here, indicate that there exists an optimum cavity length for which the pulse energy is maximum and the optimum length shifts as the pump power changes. The simulation results for a pump power of 500 mW are verified by measurements carried out on a long cavity nonlinear polarization rotation mode-locked all-fiber ring laser operating in the anomalous dispersion regime. With a repetition rate of 266 kHz for the dissipative solitons, we achieve a pulse energy of 139.1 nJ for a cavity length of 700 m. Higher pulse energy can be expected by using a pump laser diode with higher pump power.

Noise conversion from pump to the passively mode-locked fiber lasers at 1.5 µm.

We characterize the noise conversion from the pump relative intensity noise (RIN) to the RIN and phase noise of passively mode-locked lasers at 1.5 µm. Two mode locking mechanisms, nonlinear polarization rotation (NPR) and semiconductor saturable absorber mirror (SESAM), are compared for noise conversion for the first time. It is found that the RIN and the phase noise of both types of lasers are dominated by the noise converted from the pump RIN and thus, can be predicted with the measured pump RIN and noise conversion ratios. The SESAM laser is found to show an excess noise conversion from the laser RIN to the laser phase noise due to the slow saturable absorber effect.

Bidirectional passively mode-locked soliton fiber laser with a four-port circulator.

We present an all-fiber bidirectional passively mode-locked soliton laser with what we believe is a novel cavity configuration. Using a four-port circulator, we incorporate two different semiconductor saturable absorber mirrors (SESAMs) into the laser cavity, which enables bidirectional mode locking. The laser allows the generation of two independent countercirculating mode-locked pulse trains, each with an individual fundamental repetition rate that can be adjusted by varying the SESAM pigtail length. Two countercirculating pulse trains with repetition rates of 21.3 and 15.2 MHz are obtained simultaneously. By controlling the intracavity loss imposed on these two pulse trains, either one of the two pulse trains can be switched on or off. The bidirectional operation with other repetition rates is also demonstrated.

Photonic measurement of microwave frequency based on phase modulation.

A photonic approach for microwave frequency measurement is proposed. In this approach, an optical carrier is modulated by an unknown microwave signal through a phase modulator. The modulated optical signal is then split into two parts; one part passes through a spool of polarization maintaining fiber (PMF) and the other one, through a dispersion compensation fiber (DCF), to introduce different microwave power penalties. After the microwave powers of the two parts are measured by two photodetectors, a fixed frequency-to-power mapping is established by obtaining an amplitude comparison function (ACF). A proof-of-concept experiment demonstrates frequency measurement over a range of 10.5 GHz, with measurement error less than +/-0.07 GHz.

Simultaneous dual free spectral range microwave photonic filter using a high-birefringence chirped grating in a Sagnac loop.

We propose a continuously tunable, dual free spectral range (FSR) photonic microwave notch filter configuration using a high-birefringence linearly chirped fiber Bragg grating (Hi-Bi LCFBG) that is connected in a Sagnac loop using a Hi-Bi coupler. The configuration employs double sideband modulation and can generate two FSRs simultaneously. The larger FSR corresponds to the differential time delay of the Hi-Bi LCFBG and the Hi-Bi pigtails of the coupler; the smaller FSR corresponds to the time delay between the arms of the Sagnac loop. Measured results demonstrate dual FSR, a large notch rejection, and that the FSR is easily tunable by tuning the LCFBG. We also present the filter transfer function for the design. Experimental results agree well with the theoretical analysis.