Absorbance modulation enables lateral superresolution in optical lithography and transmission microscopy by generating a dynamic aperture within a photochromic absorbance-modulation layer (AML) coated on a substrate or a specimen. The absorbance-modulation is the property of photochromic molecules modulated between two states. The process is therefore solely controlled by far-field radiation at different wavelengths.
The applicability of this concept to reflection microscopy has not been addressed so far, although reflection imaging exhibits the important ability to image a wide range of samples, transparent or opaque, dielectric or metallic. We will present a simulation model for absorbance-modulation imaging (AMI) in confocal reflection microscopy and it is shown that imaging well beyond the diffraction limit is feasible. Our model includes the imaging properties of confocal microscopy, reflections at the boundaries, the photochromic process and diffraction due to propagation through a subwavelength aperture.
We derive an analytical design equation which estimates the dependence of the achievable resolution on relevant parameters, such as the AML properties and the applied light powers. This equation is very similar to the corresponding equation for STED (Stimulated emission depletion) microscopy and it is helpful for a fast design of the arrangement of optical setup and AML. As rapid scanning is relevant for a short imaging duration, we further derived an estimation for the pixel dwell time. We prove the validity of these equations by comparing the estimations with the complex numerical simulations. In addition, we show that a resolution enhancement down to 1/5 of the diffraction limit is possible.
The generation of a heterodyne carrier frequency via offset-lock in an optical phase-locked loop (OPLL) is a widespread technique in communication, spectroscopy and other fields. Commercial state-of-the-art laser-Doppler vibrometers (LDV) generate heterodyne frequency carrier by acoustooptic devices (Bragg cells) efficiently with the slow shear mode up to 409 MHz. Therefore, these LDVs are limited in measurement bandwidth and it is impossible to adjust the heterodyne carrier frequency to the optimal value in respect to the requested demodulation bandwidth. For RF-MEMS (radio-frequency microelectromechanical systems) testing, carrier frequencies in heterodyne LDVs have to exceed 1 GHz to enable the unambiguous reconstruction of the measured vibration, which is restricted by the conventional heterodyning techniques. Recently, we demonstrated a LDV microscope with the generation of a variable heterodyne carrier frequency up to 200 MHz by offset-lock in an OPLL with visible DBR semiconductor lasers. In this paper, we demonstrate the increase of heterodyne-frequency-carrier generation by conventional RF electronics up to 1.4 GHz and discuss the decisive OPLL parameters for the application of this technique to ultra-high-frequency laser-Doppler vibrometry. Our LDV microscope shows an (out-of-plane) vibration amplitude sensitivity of less than 1 pm/ √ Hz for vibration frequencies higher than 50 MHz, which enables the vibration measurement of most RF-MEMS. First measurements of resonances of a piezoelectric transducer are presented.
The analysis of materials and geometries in tensile tests and the extraction of mechanic parameters is an important field
in solid mechanics. Especially the measurement of thickness changes is important to obtain accurate strain information of
specimens under tensile loads. Current optical measurement methods comprising 3D digital image correlation enable
thickness-change measurement only with nm-resolution. We present a phase-shifting electronic speckle-pattern
interferometer in combination with speckle-correlation technique to measure the 3D deformation. The phase-shift for the
interferometer is introduced by fast wavelength tuning of a visible diode laser by injection current. In a post-processing
step, both measurements can be combined to reconstruct the 3D deformation. In this contribution, results of a 3Ddeformation
measurement for a polymer membrane are presented. These measurements show sufficient resolution for the
detection of 3D deformations of thin specimen in tensile test. In future work we address the thickness changes of thin
specimen under tensile loads.
The state-of-the-art technique for optical vibration analysis of macroscopic structures is laser-Doppler vibrometry in which a single-laser beam measures the motion in the beam direction. Thus, three laser beams are necessary to investigate three-dimensional (3-D) motions. The laser spots can be separated on macroscopic specimens with scattering surfaces to prevent optical crosstalk between the measurement beams, but such separation is impossible for a microscopic scatter point. We demonstrate a solution for this problem: an optical 3-D vibrometer microscope with a single-impinging laser beam, which collects scattered light from at least three directions. We prove that it is possible to realize a small laser focus of <3.5-μm diameter on a proper scatter point such as an etch hole of a microelectromechanical-systems device to obtain real-time, 3-D vibration measurements with megahertz vibration bandwidth and picometer amplitude resolution. A first measurement of operational-deflection shapes is presented.
Laser-Doppler vibrometry has become the state-of-the-art technique for broadband vibration analysis with picometer
resolution in microelectromechanical systems (MEMS). Displacement or velocity is detected only in direction of the
measurement beam and, thus, three impinging laser beams are necessary to investigate all components of a threedimensional
(3D) motion. This requirement is not problematic for 3D-vibration measurements on macroscopic objects
with scattering surfaces but for reflective microstructures. A general problem of measuring 3D vibrations with three laser
beams is optical crosstalk. This problem is especially critical for MEMS applications because the three beams have to be
positioned closely to achieve high lateral resolution. In this paper, we prove that it is possible to impinge the small laser focus of a single laser beam with 3.3 μm diameter on a proper edge, corner or etch hole of a MEMS device to obtain real-time, 3D-vibration measurements with picometer amplitude resolution without optical crosstalk. We present the first measurements of the 3D-vibrations in MEMS devices. We prove that our method can meet the requirement of the MEMS community for fast, full-3D, broad-bandwidth, vibration measurements with picometer amplitude resolution and micrometer spatial resolution.
The real-time measurement of three-dimensional vibrations is currently a major interest of academic research and
industrial device characterization. The most common and practical solution used so far consists of three single-point
laser-Doppler vibrometers which measure vibrations of a scattering surface from three directions. The resulting three
velocity vectors are transformed into a Cartesian coordinate system. This technique does also work for microstructures
but has some drawbacks: (1) The surface needs to scatter light, (2) the three laser beams can generate optical crosstalk if
at least two laser frequencies match within the demodulation bandwidth, and (3) the laser beams have to be separated on
the surface under test to minimize optical crosstalk such that reliable measurements are possible. We present a novel
optical approach, based on the direction-dependent Doppler effect, which overcomes all the drawbacks of the current
technology. We have realized a demonstrator with a measurement spot of < 3.5 μm diameter that does not suffer from
optical crosstalk because only one laser beam impinges the specimen surface while the light is collected from three