A planar ion trap with an integrated waveguide was fabricated and characterized. The microdevice, consisting of a 1 mm-diameter one-hole ring trap and multi-mode optical waveguides, was made on a glass wafer using microfabrication techniques. The experimental results demonstrate that the microdevice can trap 1.5 μm- to 150 μm-diameter charged particles in air under an alternating electric field with the amplitude and frequency varying from 100 V to 750 V, and 100 Hz to 700 Hz, respectively. The on-chip waveguide is capable of detecting the presence of a particle in the trap, and the particle secular motion frequency was found to depend on the input alternating signal amplitude and frequency.
©2011 Optical Society of America
Manipulation of ions plays an important role in advancing many technologies, including ion-based quantum information processing (QIP), mass spectrometry (MS), and optical spectroscopy. A scalable quantum information processor, promising substantial advantages over conventional techniques, requires integration of all necessary functionalities for its implementation. However, the precise scaling rule and coordination of ion trapping and transport with multiple functionalities remain a difficult challenge. The development of an integrated system based on control, separation and detection of ions is even more elusive . In the field of life sciences, mass spectrometry is an indispensable analytical tool for chemical and biological research and discoveries. Specimens with diameters ranging from 10 nm to 20 μm, i.e. DNA, proteins, virus and cells, can be identified with high resolution and accuracy. To further extend the performance limits of MS, integration of advanced optical detection is considered a key approach . Similarly, optical spectroscopy of nanometer particles with important properties relies on ion trap capacity. Gas phase nanoparticles 3 nm to 15 nm in diameters are trapped, using a Trapped Reactive Atmospheric Particle Spectrometer (TRAPS) with a maximum trap capacity of 5 × 108 particles and a residence time of 12 sec, for X-ray and optical spectroscopy . Recently, a surface-electrode ion trap was developed with integrated fiber optics capable of efficiently collecting fluorescence light from the ion . It is obvious that ion manipulation finds use in many applications in miniaturized chemical detectors, vacuum technology, atomic clock and secure communications. Therefore, new approaches of manipulation of ions are needed for the next generation of advanced technologies [5–8].
In this paper, we report an on-chip planar ion trap with increased complexity and functionality by integration with waveguide optics. Our design is compatible with integrated microfabrication processing, and can be scaled to multiple macroscopic input and output optical ports.
2. Device design and fabrication
The device consists of a planar one-hole ion trap with an electrode on each side of the planar surface, and a waveguide on the trapping side of the planar surface. Figure 1(a) schematically shows the design of a typical microdevice. A Pyrex 7740 wafer with a thickness of 500 μm thickness was used as the substrate because it serves as a transparent electrical insulator for the top and bottom electrodes and allows for both micro-fabrication processing and mechanical through-wafer drilling. A single-hole metal electrode is formed on each side of the glass wafer with the top one for connection to an alternating electric field (AC) and the bottom one for connection to the ground. The two electrodes have the same dimensions and are accurately aligned in the planar coordinates, with a distance between the two electrodes defined by the wafer thickness. The hole-diameter is designed to be 1 mm, thus a 1 mm in diameter through-wafer hole needed to be realized using mechanical drilling. The local potential minimum of the AC field is centered along the z axis with a distance estimated as about 300 μm above the top surface of the planar trap [9, 10]. The ring geometry can be optimized to create the best trapping potential by varying the number and size of the ring . The optical waveguide is designed to have its length spanning from the edge of the electrode center hole to the edge of the chip, about 5 mm, with a cross-section area of 200 μm × 200 μm. The 200 μm waveguide height allows detection of secular motion of a particle suspended about 300 μm above the trap top surface. Photosensitive polymer SU-8 (Microchem) was selected for the waveguide material to fabricate the multimode waveguide because of its optical properties and compatibility with microfabrication processes . The waveguide’s sides and top surface are exposed to air while the bottom surface is in contact with the metal electrode on the glass. Since SU-8 has a refractive index of 1.589 at 633 nm wavelength, greater than those for air, Au, and Pyrex 7740, i.e. 1.0, 0.2, and 1.474, respectively, both scattered and emitted light can be collected and guided inside the SU-8 waveguide.
Major fabrication steps for the microdevice are summarized schematically in Fig. 1(b). The fabrication started with patterning of a Pyrex 7740 wafer on both sides utilizing standard photolithography alignment techniques. This step defines the configuration of the trap electrodes using patterned photoresist. A deposition of a 50 nm Cr film on both sides of the wafer was performed using an evaporator, and was followed by a deposition of 500 nm thick Au on both sides of the wafer. The thin Cr film served as a seed layer for good adhesion of the Au film on the glass. After the two depositions of the metal layers, the photoresist was removed utilizing a lift-off process by immersing the wafer in acetone solution at room temperature. The wafer was washed in ethanol and de-ionized water (DI), and dried with N2. Thus the aligned metal electrodes were obtained on the top and bottom sides of the wafer. Next, a layer of SU-8 with a 200 μm thickness was spin-coated on the wafer top side. Following a double-step soft-baking process at 65 °C for 6 min. and 95 °C for 40 min., the film was exposed to UV radiation and was developed after a double-step post-exposure baking process at 65 °C for 5 min. and 95 °C for 15 min. A hard bake at 100 °C for 10 min. was performed after the developing process to finalize the waveguide fabrication. The center through-wafer hole for the trap, with a 1 mm diameter, was obtained by mechanical drilling, which completed the fabrication of the integrated device. Figure 1(c) is a picture of several fabricated microdevices with various waveguide configurations.
3. Experimental arrangement
Experiments were conducted at room temperature and ambient pressure to test the performance of the integrated devices. Figure 2(a) is an image of the experimental setup. To prevent any disturbance on the trapped particle by airflow, the microdevice was enclosed in a chamber during each experiment. The chamber was designed with access windows for optical fiber coupling to the on-chip waveguide and for particle loading. The device was first mounted on a printed circuit board (PCB) with electrical interconnections to an external AC power supply, as shown in Fig. 2(b). The chamber was made of clear acrylic, allowing in situ video monitoring of the trapped particle via two CCD cameras. A power supply (HP 8111A pulse/function generator, 20 MHz), an amplifier (Trek 50/750), and a multimeter (Agilent 34401A) were used to supply and monitor the AC potential with various amplitudes (100 V-750 V) and frequencies (100 Hz-700 Hz). A 650 nm laser diode was used to excite the particle from the bottom of the device via the through-hole or by coupling to one of the waveguides. Scattered light from the trapped particle was collected by the on-chip waveguide and detected using an external photodiode (Thorlabs, PDZ36A) couples via a multimode fiber (Thorlabs BFL 37-200). The time dependent photodiode signal was monitored using an oscilloscope (WON, PDS7102T), and were recorded and processed using a Fast Fourier Transform (FFT) to identify the oscillation frequency of the particle secular motion.
Figure 2(c) shows the design for a typical experiment. Diamond particles (C. R. Laurence Co. Inc.), 1.5 μm to 150 μm in diameter with a 3.5 g/cm3 density, were used in the experiments. We have also trapped glass microspheres  and paint powders , and found that diamond particles scattered light more efficiently. Electrostatic charge on the particles was created by rubbing the particles against a Teflon sheet using a brush. The charged particles were immediately dropped from the top access window of the chamber. One or several of these particles could be trapped at the same time depending on the operating conditions. Suspension of a single particle could be achieved by cautiously adjusting the frequency and/or amplitude of the potential, which resulted in ejection of particles while sometimes retaining a single particle in the trap. Experimental results concerning the trapping of a single particle are reported in the present work. Figure 2(d) is a typical image of a trapping experiment showing that a particle is suspended by the microdevice.
4. Results and discussions
The ion motion in the trap may be considered in two parts: the secular motion and the micro-motion. The secular motion is the motion of a particle trapped in a harmonic oscillator with a trap frequency lower than the frequency of the AC drive signal. The micromotion, on the other hand, has nearly the same frequency as the driving frequency but at smaller displacement compared to that of the secular motion. Detection of the secular motion of trapped particles is the interest of this work.
The electric potential for a planar trap with a similar configuration was analyzed in our previous work . An AC electrical signal was applied with an instantaneous potential of the following form:10, 15, 16].
Particle trapping stability regions can be obtained from the trapping conditions. Figure 3 compares measured trapping regions corresponding to the AC amplitude Vac and frequency fac for three particle sizes, 15 μm, 60 μm and 150 μm. Summarizing all the experiments with provided combinations of amplitude and frequency of the operation AC signals, for each particle size, the area within the upper-bound and lower-bound lines corresponds to the stable trapping conditions of the particles with certain charge-to-mass ratios. Both the lower-bound and the upper-bound lines of the region are defined based on Eq. (2) according to the relationship of three parameters in the experiments: the applied AC voltage magnitude, Vac, the AC frequency, fac, and the particle charge-to-mass ratio, q0/m. The three shaded areas, obtained from present experimental data, cover the tested operation conditions for stable trapping of particles with three different sizes. The coverage of each area, i.e., the ranges of Vac, fac and q0/m, is limited due to the instrumentation used in the experiments.
From experimental trapping conditions of Vac and fac, the charge-to-mass ratio of the trapped particle, q0/m, can be evaluated using Eq. (2). The maximum charge-to-mass ratios of the trapped particles in present study, estimated based on the slopes of the region upper bounds, are approximately 8.5×10−4 C/kg and 1.5×10−3 C/kg for particles with diameters of 150 μm and 15 μm, respectively. The results also show that the trapping region for smaller particles is larger than that for bigger particles.
Equation (3) implies that the secular frequency of a trapped particle, ωz=2πfp, is linearly proportional to Vac. Its dependence on fac includes the drag effect with a drag parameter of γ, because the experiments are performed in ambient air, which can be estimated as :
Using the integrated waveguide, the particle secular motion can be detected by measurement of scattered laser light. A typical time dependent photodiode signal is shown in Fig. 4(a) using 15 μm-diameter diamond particles and a microdevice with a configuration shown in Fig. 2(d). The optical signal presents periodic characteristics with a frequency that can be revealed using fast Fourier transform (FFT) analysis, as shown in Fig. 4(b). Because the particle intersects the light path of the waveguide twice in one cycle of its secular motion, the particle secular frequency is half of the frequency of the optical time dependent output. As indicated in Fig. 4(b), the secular motion frequency is less than 29 Hz, much lower than the AC drive frequency of 120 Hz.
The measured dependences of the secular frequency on both the AC amplitude and the frequency are shown in Fig. 4(c) and Fig. 4(d), respectively. The results show that the frequency increases linearly with increasing AC amplitude, however, only a slight decrease in particle frequency is detected with increasing AC frequency under the tested frequency range. Curve fitting of the measured data for each dependence on the amplitude and frequency, as shown in Fig. 4(c) and Fig. 4(d), indicates that the charge-to-mass of the trapped particles is on the order of 10−4 C/kg, consistent with the calculated results shown in Fig. 3.
In summary, we have demonstrated a microfabricated device that can confine microparticles with 1.5 μm to 150 μm diameters using an AC drive voltage with amplitude and frequency values of 100 V to 750 V, and 100 Hz to 700 Hz, respectively. The on-chip waveguide is capable of detecting the oscillation of a suspended particle based on its secular motion. Furthermore, the characteristics of a particle, i.e. mass and charge, can be evaluated based on the experimental conditions and the measured data using this integrated microtrap.
The authors thank Mr. Dale Drew of the Department of Aerospace and Mechanical Engineering, University of Arizona, for assistance with the experimental apparatus. We acknowledge Dr. Hui Zhang and Prof. Jesse Zhu at the University of Western Ontario who provided samples of paint powders. WBW would like to acknowledge support from the DARPA Microsystems Technology Office. This work was supported by the DARPA YFA Program.
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