Mehmet Unlu

Significant performance enhancement in photoconductive terahertz optoelectronics by incorporating plasmonic contact electrodes

Even though the terahertz spectrum is well suited for chemical identification, material characterization, biological sensing and medical imaging, practical development of these applications has been hindered by attributes of existing terahertz optoelectronics. Here we demonstrate that the use of plasmonic contact electrodes can significantly mitigate the low-quantum efficiency performance of photoconductive terahertz optoelectronics. The use of plasmonic contact electrodes offers nanoscale carrier transport path lengths for the majority of photocarriers, increasing the number of collected photocarriers in a subpicosecond timescale and, thus, enhancing the optical-to-terahertz conversion efficiency of photoconductive terahertz emitters and the detection sensitivity of photoconductive terahertz detectors. We experimentally demonstrate 50 times higher terahertz radiation powers from a plasmonic photoconductive emitter in comparison with a similar photoconductive emitter with non-plasmonic contact electrodes, as well as 30 times higher terahertz detection sensitivities from a plasmonic photoconductive detector in comparison with a similar photoconductive detector with non-plasmonic contact electrodes.

Frequency Tunable Microstrip Patch Antenna Using RF MEMS Technology

A novel reconfigurable microstrip patch antenna is presented that is monolithically integrated with RF microelectromechanical systems (MEMS) capacitors for tuning the resonant frequency. Reconfigurability of the operating frequency of the microstrip patch antenna is achieved by loading it with a coplanar waveguide (CPW) stub on which variable MEMS capacitors are placed periodically. MEMS capacitors are implemented with surface micromachining technology, where a 1-mum thick aluminum structural layer is placed on a glass substrate with a capacitive gap of 1.5 mum. MEMS capacitors are electrostatically actuated with a low tuning voltage in the range of 0-11.9 V. The antenna resonant frequency can continuously be shifted from 16.05 GHz down to 15.75 GHz as the actuation voltage is increased from 0 to 11.9 V. These measurement results are in good agreement with the simulation results obtained with Ansoft HFSS. The radiation pattern is not affected from the bias voltage. This is the first monolithic frequency tunable microstrip patch antenna where a CPW stub loaded with MEMS capacitors is used as a variable load operating at low dc voltages

Switchable Scattering Meta-Surfaces for Broadband Terahertz Modulation

Active tuning and switching of electromagnetic properties of materials is of great importance for controlling their interaction with electromagnetic waves. In spite of their great promise, previously demonstrated reconfigurable metamaterials are limited in their operation bandwidth due to their resonant nature. Here, we demonstrate a new class of meta-surfaces that exhibit electrically-induced switching in their scattering parameters at room temperature and over a broad range of frequencies. Structural configuration of the subwavelength meta-molecules determines their electromagnetic response to an incident electromagnetic radiation. By reconfiguration of the meta-molecule structure, the strength of the induced electric field and magnetic field in the opposite direction to the incident fields are varied and the scattering parameters of the meta-surface are altered, consequently. We demonstrate a custom-designed meta-surface with switchable scattering parameters at a broad range of terahertz frequencies, enabling terahertz intensity modulation with record high modulation depths and modulation bandwidths through a fully integrated, voltage-controlled device platform at room temperature.

A 15–40-GHz Frequency Reconfigurable RF MEMS Phase Shifter

This paper presents a novel frequency reconfigurable phase shifter using the RF microelectromechanical systems (MEMS) technology. The phase shifter is based on the triple-stub circuit topology composed of three stubs that are connected by two transmission lines that are all implemented as distributed MEMS transmission lines. The insertion phase of the circuit is controlled by changing the electrical lengths of the stubs and the connecting transmission lines, while having ideally zero reflection coefficient at all times. The phase shifter has theoretically no specific limits on the frequency; in other words, it can be reconfigured to work at any frequency between 15-40 GHz, with adjustable phase steps, while providing a constant time delay within a 2%-3% instantaneous bandwidth around any selected center frequency in the above-mentioned band. The phase shifter is fabricated monolithically using a surface micromachining process on a quartz substrate and occupies 10.8 mm × 5.9 mm. Measurement results show that the phase shifter has average phase error of 1.6 °, 3.7 °, and 4.7 °, average insertion loss of 3.1, 5, and 8.2 dB and average return loss of 19.3, 15.8, and 13.7 dB at 15, 30, and 40 GHz, respectively. To the best of our knowledge, this paper demonstrates the first phase shifter in the literature that can work at any given frequency in a targeted band with adjustable phase steps.

A monolithic phased array using 3-bit DMTL RF MEMS phase shifters

This paper presents a phased array system designed at 15 GHz employing 3-bit distributed MEMS transmission line (DMTL) type phase shifters which are monolithically integrated with the feed network of the system and the radiating elements on the same substrate. The phase shifter can give 0deg-360deg phase shift with 45deg steps at 15 GHz which is used to obtain various combinations of progressive phase shift in the excitation of radiating elements. The phased array is composed of four linearly placed microstrip patch antennas. In order to monolithically integrate the patch antennas and phase shifters, tapered lines with low return loss from microstrip to coplanar waveguide (CPW) have been designed. The design of the phased array system and its components is given. Since the DC biasing schema of a MEMS system is also an important issue in terms of the RF losses, the paper also addresses the effect of the bias lines on the loss characteristics of the phase shifters. Moreover, the process steps, which are used in the fabrication of the phased array, are also summarized

A Reconfigurable RF MEMS Triple Stub Impedance Matching Network

This paper presents a reconfigurable triple stub impedance matching network using RF MEMS technology centered at 10GHz. The device is capable of covering impedances on the whole Smith Chart. The device structure consists of three variable length stubs which are designed as distributed MEMS transmission lines and two lambdag/8 length CPW transmission lines connecting the stubs. The variable length stubs are implemented with 12 MEMS switches over CPW lines and CPW lines connecting the switches. lambdag/8 spacing between the stubs is selected to obtain a uniform distribution of the impedance points on the Smith Chart. Initial measurement results of the fabricated structure show a good agreement with the simulation results

Miniature multi-contact MEMS switch for broadband terahertz modulation

A miniature MEMS switch is designed, fabricated, and incorporated in a reconfigurable metallic mesh filter for broadband terahertz modulation. The mechanical, electrical, and geometrical properties of the MEMS switch are set to enable broadband terahertz modulation with relatively low modulation voltage, high modulation speed, and high device reliability. The implemented miniature MEMS switch exhibits an actuation voltage of 30 V, a fundamental mechanical resonance frequency of 272 kHz, and an actuation time of 1.23 μs, enabling terahertz modulation with a record high modulation depth of more than 70% over a terahertz band of 0.1-1.5 THz, with a modulation voltage of 30 V and modulation speeds exceeding 20 kHz.

Design, Fabrication, and Experimental Characterization of Plasmonic Photoconductive Terahertz Emitters

In this video article we present a detailed demonstration of a highly efficient method for generating terahertz waves. Our technique is based on photoconduction, which has been one of the most commonly used techniques for terahertz generation (1-8). Terahertz generation in a photoconductive emitter is achieved by pumping an ultrafast photoconductor with a pulsed or heterodyned laser illumination. The induced photocurrent, which follows the envelope of the pump laser, is routed to a terahertz radiating antenna connected to the photoconductor contact electrodes to generate terahertz radiation. Although the quantum efficiency of a photoconductive emitter can theoretically reach 100%, the relatively long transport path lengths of photo-generated carriers to the contact electrodes of conventional photoconductors have severely limited their quantum efficiency. Additionally, the carrier screening effect and thermal breakdown strictly limit the maximum output power of conventional photoconductive terahertz sources. To address the quantum efficiency limitations of conventional photoconductive terahertz emitters, we have developed a new photoconductive emitter concept which incorporates a plasmonic contact electrode configuration to offer high quantum-efficiency and ultrafast operation simultaneously. By using nano-scale plasmonic contact electrodes, we significantly reduce the average photo-generated carrier transport path to photoconductor contact electrodes compared to conventional photoconductors (9). Our method also allows increasing photoconductor active area without a considerable increase in the capacitive loading to the antenna, boosting the maximum terahertz radiation power by preventing the carrier screening effect and thermal breakdown at high optical pump powers. By incorporating plasmonic contact electrodes, we demonstrate enhancing the optical-to-terahertz power conversion efficiency of a conventional photoconductive terahertz emitter by a factor of 50 (10).

Reconfigurable reflectarray using RF MEMS technology

This paper presents the design of a reconfigurable microstrip slot-coupled patch reflectarray using RF MEMS switches. The reflectarray is designed on two back-to-back bonded glass substrates front side of which contains the microstrip patch antenna elements and the backside contains the phase shifting elements. The phase shifting elements consist of microstrip lines the lengths of which are adjusted with MEMS switches resulting with adjustable phase characteristics of each antenna element. A transmission line based model is used to model the unit element of the array which is used to obtain a phase design curve and the design is verified with HFSS. The 4 × 1 array is then designed at 26 GHz to verify the phase design curve and according to the HFSS simulations, the main beam of the array is rotated by 14° by reconfiguring the length of the microstrip transmission lines with RF MEMS switches.

Development of a Fiber Laser with Independently Adjustable Properties for Optical Resolution Photoacoustic Microscopy.

Photoacoustic imaging is based on the detection of generated acoustic waves through thermal expansion of tissue illuminated by short laser pulses. Fiber lasers as an excitation source for photoacoustic imaging have recently been preferred for their high repetition frequencies. Here, we report a unique fiber laser developed specifically for multiwavelength photoacoustic microscopy system. The laser is custom-made for maximum flexibility in adjustment of its parameters; pulse duration (5–10 ns), pulse energy (up to 10 μJ) and repetition frequency (up to 1 MHz) independently from each other and covers a broad spectral region from 450 to 1100 nm and also can emit wavelengths of 532, 355, and 266 nm. The laser system consists of a master oscillator power amplifier, seeding two stages; supercontinuum and harmonic generation units. The laser is outstanding since the oscillator, amplifier and supercontinuum generation parts are all-fiber integrated with custom-developed electronics and software. To demonstrate the feasibility of the system, the images of several elements of standardized resolution test chart are acquired at multiple wavelengths. The lateral resolution of optical resolution photoacoustic microscopy system is determined as 2.68 μm. The developed system may pave the way for spectroscopic photoacoustic microscopy applications via widely tunable fiber laser technologies.