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Journal of Nanophotonics

Sub-100-nm-wide slit for detecting ground state atoms with near-field photoionization

Tomohiro Sato

Published Sep 1, 2007• 3 citations
Publisher

Abstract

We fabricated a 50-nm-wide slit on a glass substrate using the lift-off process with a bilayer resist including Cr. The nanometric slit is designed for detecting a small number of neutral atoms in the ground state with high spatial accuracy by way of two-step photoionization with two-color near-field lights. Conducting the finite difference time domain simulations, we drew the spatial profiles of near-field lights generated in the vicinity of the slit for both cases of normal illumination and total-internal reflection (TIR) illumination with a linearly polarized light beam. In the slit-parallel-incident TIR configuration with s-polarization, we obtained a good throughput of 1.1 x 10-2 and the highest extinction ratio of 2.2 x 10-7 for a wavelength of 780 nm. In this case, the ionization efficiency of low-energy 87Rb atoms in the 5S1/2 hyperfine state is estimated to exceed 1%.

DOI
10.1117/1.2794356

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arXiv:1810.10385•Oct 24, 2018•0 citations
We fabricated a 50-nm-wide slit on a glass substrate using the lift-off process with a bilayer resist including Cr. The nanometric slit is designed for detecting a small number of neutral atoms in the ground state with high spatial accuracy by way of two-step photoionization with two-color near-field lights. Conducting the finite difference time domain simulations, we drew the spatial profiles of near-field lights generated in the vicinity of the slit for both cases of normal illumination and total-internal reflection (TIR) illumination with a linearly polarized light beam. In the slit-parallel-incident TIR configuration with s-polarization, we obtained a good throughput of 1.1 x 10-2 and the highest extinction ratio of 2.2 x 10-7 for a wavelength of 780 nm. In this case, the ionization efficiency of low-energy 87Rb atoms in the 5S1/2 hyperfine state is estimated to exceed 1%.
We develop a nanoslit to detect a small number of neutral atoms in the ground state. The detection scheme of using two-step photoionization is very sensitive and the use of nanometric near-field lights leads to an ultrahigh spatial resolution. An edge-sharpened nanoslit is fabricated with FIB milling. Considering the case of detecting Rb atoms, the polarization-dependent spatial distribution of near-field light generated at the nanoslit by total-internal reflection of a 476.5-nm Ar-ion laser beam is obtained with a scanning near-field optical microscope. Based on the experimental results, the ionization efficiency is estimated for both s-polarization and p-polarization. We also discuss the discrepancy between the experimental value and the numerical one obtained from the finite difference time domain simulations in the p-polarization case.
We describe a slit-type detector for ground-state atoms with nanometer optical near fields. The detector with a slit width of 70 nm and a slit length of 100 µm is fabricated by processing a silicon-on-insulator wafer with photolithography and anisotropic etching. Measurement with a fiber probe of the intensity distribution on the slit reveals that the spatial resolution is 96 nm. Rubidium atoms can be selectively detected by means of two-step photoionization. The detection efficiency is estimated at 25% for Rb atoms with a mean velocity of 10 m/s. The atom detector can be used for atom deflection experiments with a fiber probe.
Summary form only given. We have proposed deflection by an optical near field on a fiber probe to control atoms with high spatial accuracy. In this case, the deflection angle is estimated to be 0.1 degree for a Rb atom with an incident velocity of 10 m/s. It leads to the deviation of 10 /spl mu/m from the incident axis at 1 cm. in the downstream. To detect the deflected atoms with an accuracy of 1%, we need the spatial resolution of 100 nm. However, a commercial detector such as MCP has a low resolution of 50 /spl mu/m at most and the highest resolution that has been reported is 1 /spl mu/m to our knowledge. In addition, these are applied to metastable atoms and have less detection efficiency for the ground state atoms we manipulate. For drastic improvement, we present here a slit-type atom detector with a nanometric lateral resolution. Since the number of atoms deflected by the optical near field is very small, it is important to detect atoms with high efficiency. For this purpose, we use photoionization with two-color optical near fields.
We describe atom deflection with a slit-type deflector as an elemental technique of precise atom control using near-field light. In the demonstration experiments, cold Rb atoms deflected by repulsive near-field light induced in a 200?nm-wide slit are detected by means of two-step photoionization. Then, we consider a near-field-light lens of atomic de Broglie waves by developing the atom deflector. The feasibility of nanofocusing is shown by numerical analyses with Fourier atom optics. Finally, we describe atom funnels with evanescent light to generate a cold atomic beam required for atom manipulation using nanometric near-field light. The flux intensity of cold Rb atoms outputted from a 240??m outlet is estimated to be 7.5 ? 107?atom?cm?2?s?1 at a blue detuning of 1.2?GHz. In order to increase the flux intensity to 1012?atom?cm?2?s?1, we develop a Si-made funnel with a micron-sized outlet.
Ultracold atom-traps on a chip enhances the practical application of atom traps in quantum information processing, sensing, and metrology. Plasmon mediated near-field optical potentials are promising for trapping atoms. The combination of plasmonic nanostructures and ultracold atoms has the potential to create a two dimensional array of neutral atoms with lattice spacing smaller than that of lattices created from interfering light fields -- the optical lattices. We report the design, fabrication and characterization of a nano-scale array of near-field optical traps for neutral atoms using plasmonic nanostructures. The building block of the array is a metallic nano-disc fabricated on the surface of an ITO-coated glass substrate. We numerically simulate the electromagnetic field-distribution using Finite Difference Time Domain method around the nanodisc, and calculate the intensity, optical potential and the dipole force for 87^{87}87Rb atoms. The optical near-field generated from the fabricated nanostructures is experimentally characterized by using Near-field Scanning Optical Microscopy. We find that the optical potential and dipole force has all the desired characteristics to trap cold atoms when a blue-detuned light-field is used to excite the nanostructures. This trap can be used for effective trapping and manipulation of isolated atoms and also for creating a lattice of neutral atoms having sub-optical wavelength lattice spacing. Near-field measurements are affected by the influence of tip on the sub-wavelength structure. We present a deconvolution method to extract the actual near-field profile from the measured data.