Zurich Instruments Newsletter - Edition Q2/2011
- Interview Takeshi Fukuma, Kanazawa University: Subnanometers & Piconewtons
- Impressive: Upgrading our Customers’ Lock-in Amplifiers For Free
- Customer support: 11.02 software release
- Application: Label-free 3D molecular imaging with lock-in amplifiers
- Tips & Tricks: How to set the filters in a lock-in amplifier
- Company Agenda
Good ideas tend to spread around the world; so does good technology, inevitably showing up in more and more fields. Technology that was invented for one application will, if proven, soon find use in another. The iPhone, for example, absorbed a whole number of technologies from other fields. Apple did not invent most of the technologies, but their concept of combining and perfecting the right technologies is what yielded the iPhone’s exceptional usability.
This ZI Quarterly reports on today’s most advanced lock-in amplifier - actually a lab instrument with 2 PLLs and 4 universal PID controllers - that delivers unprecedented usability compared to other lock-ins. I am curious to see where this concept will be put to further use in the future.
Best wishes from Zurich,
CEO, Zurich Instruments
Interview Takeshi Fukuma, Kanazawa University
Subnanometers & Piconewtons
Your research subject reads "Molecular-scale analysis and measurements of biological phenomena by atomic force microscopy". Could you explain the scope of this work?
The distinctive features of our AFM include subnanometer-scale resolution and piconewton-order force sensitivity. So, we are particularly interested in the imaging of very small structures that have never been visualized before. For example, we have enabled to visualize lipid headgroups, β-sheets and α-helices of proteins with subnanometer resolution. We are also interested in the imaging of 3D force distribution at interfaces between a biological system and physiological solution. Our recent study suggests that the force distribution measured by AFM reflects not only distribution of water and ions but also distribution of mobile parts of biological molecules. We aim at establishing a method for visualizing such a nanobio interface.
How did you choose this challenging research as subject of specialization?
Frequency modulation AFM (FM-AFM) has traditionally been used in vacuum for atomic-scale studies on various materials. However, the use of FM-AFM in liquid was very limited due to the poor performance compared to that in vacuum. In 2004, I have presented a way to overcome this limitation and succeeded in atomic-resolution imaging by FM-AFM in liquid for the first time. After this instrumentation work, I was able to think of various new applications. Among them, I chose biological applications mainly due to the growing interests in nanobioscience, which was a global trend at that time.
How is your research going to impact the daily life of people? And how?
As is the same for all the nanoscale measurement technologies, AFM does not directly give a great impact on our daily life. However, it is a fundamental technology that helps to develop a wide range of materials and devices. For example, we have recently developed a method for visualizing surface potential distribution at a solid/liquid interface. This technique has attracted interests from various industrial companies developing contact lenses, batteries, cosmetics, etc. We have started to collaborate with these companies to help them in developing various products. In this way, we believe our research should indirectly but widely have an impact on our daily life.
Which parts of the typical AFM stop working in liquids and need to be customized?
For me, the most important part is the cantilever deflection sensor. The performance of a liquid-environment AFM is often limited by the noise from the cantilever deflection sensor as well as the low Q factor of the cantilever resonance. A typical AFM utilizes an optical beam deflection (OBD) sensor. We should note that the noise from an OBD sensor in liquid can be significantly larger than that in air.
What are the strategies to cope the low Q factor of cantilevers in liquids?
To be honest, I have never been able to overcome the theoretical limit determined by the Q factor. Instead, my strategy was to develop an instrument that can achieve the optimal performance limited only by the Q factor. First, I use a relatively stiff cantilever (spring constant > 10 N/m) to suppress the thermal vibration of a cantilever. Secondly, I use small oscillation amplitude (< 0.5 nm) to enhance the sensitivity to a short range interaction force. Finally, I use a low noise cantilever deflection sensor (floor noise < 20 fm/rtHz) to achieve the optimal performance limited only by the Q factor. Note that these conditions are suitable for true atomic resolution imaging but not necessarily suitable for other applications in liquid.
You spent a few years at Trinity College in Dublin. What do you think about Guinness?
Of course, I learned to drink Irish beer and enjoyed it very much. At the beginning of my stay, I was not able to drink a half pint. But at the time I left there, I found myself drinking four pints of Guinness in one night. So, my colleagues did very well in training me to drink a beer at a pub.
Upgrading our Customers’ Lock-in Amplifiers For Free
There was a time when lock-in amplifiers were bulky pieces of equipment, weighing 10 to 20 kilograms or more, and lots of buttons jammed on the front panel. In some models, manufacturers reduced the number of buttons by implementing hierarchical menus. The effect was that the user had to press buttons more often to change even the simplest setting. This is part of the past.
Zurich Instruments was the first to introduce fully host-based lock-in amplifiers, transferring the user-interface from the instrument to the computer. This architectural choice enabled us to invest in easy-to-use human interfaces and in the development of powerful data analysis tools. For instance, each Zurich Instruments lock-in amplifier is delivered with an oscilloscope, a frequency response sweeper and a spectroscope. Regular software updates bring new features that also support more applications.
Today, Zurich Instruments is upgrading all our lock-in amplifiers in the field with a high performance FFT spectrum analyzer. Convenient spectrum analysis is displayed in real-time on the computer screen. While the bandwidth of the displayed spectrum can be adjusted logarithmically by factors of 2 from 1.75 Hz up to 230 kHz, the FFT also has an extremely high resolution with up to 32’000 lines per spectrum. Such high line resolutions are made possible by the combination of the lock-in amplifier for data reduction with the subsequent FFT. This extremely useful tool will surely please users that need to characterize resonators, measure AM and FM modulated signals, or analyze frequency intermodulation. This functionality would cost thousands of dollars if purchased in an extra box.
A second Zurich Instruments product upgrade applies to users that need to measure at low frequencies with large bandwidths. The new feature is called Sinc filtering, and permits to completely remove the omega and 2 omega components in the measurement result. These omega components are the result of the demodulation and can disturb the measurement result considerably at low frequencies. The Sinc filter completely removes the unwanted components with a notch filter, with an attenuation that is better than 80 dB. Zurich Instruments supports Sinc filtering up to 10 kHz frequency range, which is a factor of at least 10 better than competing instruments.
Free of charge, Zurich Instruments is upgrading all our customers’ HF2LI lock-in amplifiers with the new software release 11.02 (additional release details are provided below in 11.02 software release).
11.02 software release
In addition to the FFT spectrum analyzer and Sinc filtering, the new 11.02 software release provides several other improvements. For instance the frequency response sweeper has several new features, the graphical user interface supports the new HF2LI-PID option, and the transfer speed of the USB interface has been increased by 50%. Furthermore, square-wave signal generation has been added, and programmers will enjoy a series of new functions, as well as official support for MATLAB and Python 2.6.
All users of the HF2IS Impedance Spectroscope, HF2LI Lock-in Amplifier, and HF2PLL Phase-locked Loop are warmly recommended to switch to the new 11.02 software release. For support and access credentials to the download area from Zurich Instruments, do not hesitate to contact email@example.com.
Label-free 3D molecular imaging with lock-in amplifiers
Recent measurement techniques based on Raman spectroscopy enable 3D imaging of unlabelled molecular species. These new techniques are likely to become very important in the field of biochemical microscopy, as complements to fluorescence imaging techniques and MRI which currently are widely used to capture images of organs. A high performance lock-in amplifier is a key element in these new measurement techniques.
Raman spectroscopy can be used to identify molecules based on the vibrational frequencies of chemical bonds excited by a laser. Among others, there are two very promising technologies which have recently been developed, “Coherent Anti-Stokes Raman Scattering” (CARS) and “Stimulated Raman Scattering” (SRS). Both are non-linear Raman techniques in which vibrational states of molecules are excited by synchronized laser pulses of different wavelengths. The resulting optical signal which is emitted from the tissue can be used to identify molecular bonds and thereby reconstruct images of living tissue. The lasers can be focused on one point, much like in two-photon microscopy, which allows for 2D and also 3D imaging. For monitoring fast phenomena, so-called video-rate CARS/SRS is used. This technique allows for the monitoring of transients, as for example in the diffusion of an injected drug in tissue.
High Performance Lock-In Amplifier
The lock-in amplifier plays an important role for both CARS and SRS. Both of these imaging techniques suffer from a low signal-to-noise ratio, as lasers have a large amount of noise, especially at low frequencies. In order to optimize the signal-to-noise ratio, laser modulation techniques and subsequent narrow-band filtering using a lock-in amplifier are applied. The requirements for fast imaging and efficient noise suppression make the high performance HF2LI lock-in amplifier a good solution. The HF2LI results in sharp CARS and SRS imaging for several reasons: sophisticated digital filtering (with steep roll-off filters) leads to a very high dynamic reserve of 120 dB (100 times better than the 80 dB of older, analog lock-in amplifiers) and efficient noise suppression. Furthermore, the HF2LI minimum time-constant of 780 ns allows for video-rate CARS and SRS with sharper images. Low jitter in the HF2LI output signals further improves image contrast.
CARS image of carbon nanotubes between titanium electrodes (using HF2LI). Courtesy of Desiré Whitmore, University of California, Irvine, USA
Tips & Tricks
How to set the filters in a lock-in amplifier
Phase sensitive detection, both in lock-ins and impedance spectroscopy, is a fundamental technique to extract weak signals from large background noise, and the filter setting is one of the most important parameters. Generally, one uses a large bandwidth (small time-constant) to observe fast transient signals. But, larger bandwidth also means more noise: the amount of noise power in the demodulated signal increases proportionally to the filter bandwidth. Using a small bandwidth (big time-constant) reduces the amount of noise in the output, but only slow or static signals can be observed.
The empirical approach to set the correct filter bandwidths is to measure several traces of the same physical quantity with different filter settings: the filters in each measurement should be 3-10 times larger than in the previous one. While doing this, the user should make sure that the sampling rate is about 10 times the filter bandwidth. The correct filter setting is achieved when the signal of interest is not smoothed by the reduced filter bandwidth, and increasing the bandwidth does not reveal any additional information about the signal dynamics but only lets through a larger amount of noise.
The comparison between traces with different filter settings can be done very easily with the HF2, which can demodulate the input signal with different filter bandwidths simultaneously. Moreover, the Spectroscope tab in the HF2 graphical interface allows one to visualize and compare the demodulated signals on the same plot. (To achieve a large SNR while detecting fast transients is particularly tricky because one has to use other methods to lower the noise level than simple bandwidth narrowing of lock-in amplification, for example: better shielding of the cables to reject interferences, amplifiers with lower noise, etc.)
In the next newsletter (Edition Q3/2011), we will address the related question of how to set the sampling rate in a lock-in amplifier. Stay tuned.
- SFP 2011, Congrès Générale, Société Française de Physique, Bordeaux, July 4-8
- NC-AFM 2011, Non-Contact Atomic Force Microscopy, Lindau, September 19-22