Decay of dipole oscillation near planar interface

Details in the file link: Decay of dipole oscillation near planar interface

Mid-infrared Spectroscopy

As mentioned in a previous post on vibrational spectroscopy, one can use the infrared spectrum to determine the molecular structures, which may offer clues about the 'fingerprints' of the molecules. Compared to the weak vibrational overtone transitions which give rise to near infrared radiation, the mid-infrared gives much stronger signals that serves enough S/N ratio and works as the 'fingerprint' region of molecules. As an example, you can have a look at the absorption spectrum of Methane.

Absorption Spectrum of Methane

The biggest problem for laser spectroscopy in the mid-infrared is the lack of good laser sources in this spectrum range.

F.K. Tittel et al. has listed the available mid-infrared laser sources in the mid infrared in a book chapter, which is also given below:

Laser sources and their working range, courtesy[F.K. Tittel et al.]

Based on an excellent laser they developed in 2009, F. Adler et al. from JILA, Colorado has successfully developed a state-of-art mid-infrared laser spectroscopy. The work is excellent and the paper written is a classic to me, that's really worth to share with you in a summary of the paper in one short document: Google Drive--Mid-infrared Frequency comb Fourier Transform spectroscopy.

First Publication

My first publication related to my internship on Laser Speckle Contrast Analysis is now online, in which I perform the experiments and data analysis. 

Laser speckle analysis of flow in presence of static scatterers

I have introduced part of the project very briefly in a previous post on internship.

Mie Solution to sphere scattering

In the link below, you can find a short summary of J.A. Stratton's treatment on Sphere scattering in his classic book on 'Electromagnetic Theory'.

the notes--Mie solution of sphere scattering.

The very interesting part is the link between natural oscillation mode of the sphere, and the resonance condition upon plane wave incidence(which is the Mie's solution).
Hope you will enjoy it.

Extraordinary Optical Transmission through nano-hole array--- A revisit to the classical problem: dipole radiation near a lossy interface

Extraordinary Optical Transmission(EOT) phenomena of light through a nanohole array like Fig. 1 was rst observed by T. Ebbesen et al. in 1998. Since then, lots of researches try to explain the mechanism behind such phenomena, most of which attribute it to the Surface Plasmon Polariton(SPP).

Fig. 1, Nano-hole array

In a series of researches by H.T. Liu, P. Lalanne, et al[1, 4, 2], they are trying to build up a `microscopic' theory to explain the EOT phenomena, where each nano-hole is considered to be a elementary scatter, like the case in Fig. 2.

According to their analysis, not only SPP but also the so-called `qusi-Cylindrical wave' also plays an important role. Their theory is built on a fundamental problem:the radiation of dipole near an interface, which they named the `line source problem'. However, even the physical picture of the role of quasi-CW contribution has been widely cited, there are some obvious mathematical errors in their deviation of the line-source problem. In this document, I try to figure those 'messes' out and make a revisit to the classical problem: dipole radiation near a lossy interface based the previous work of W. Lucas, et al. and L. Novotny, et al. .

The document is provided in the link: Dipole radiation near interface. I hope the readers would find it useful. Request for the document can also be sent to my contact info. given in my CV.

Vibration Spectroscopy--Study Notes

Recently, I have been studying vibration spectroscopy, i.e. the vibration infrared and Raman spectroscopy, which indicates the vibration 'fingerprint'of molecules. This is a totally new topic to me, so I didn't know anything about it before, thus I started from zero. Now, I have finished the first step and understand the basics about it. My study notes to this topic can be found in the vibration spectroscopy notes.

Surface Plasmon Study notes

I am currently studying the physics of surface plasmons.
Starting with the wave equations and boundary conditions, I have discussed the very basics  in the Surface plasmon notes(constantly updating), which is mainly about the conditions for the existence of such surface waves at the interface of 2 different medium.

Interface between two medium

In order to fulfill the wave equation and boundary conditions, if there exists a TE polarized surface wave, it should have a real wavevector as follows:

,

 if there exists a TM polarized surface wave, it should have a real wavevector as follows:

.

For non-magnetic metals like gold/silver, their relative permeability u1=u2=1, and they normally have Re(epsilon)<<0, hence they are quite good materials for supporting surface plasmons in the visible range and normally the surface plasmons on the metal surface is TM polarized.

Let's consider the permittivity of metal can be approximated as the simplified Drude model, where dissipation term is not considered:

then the wavevector-frequency relationship can be expressed as:

where k~=k/kp, w~=w/wp, kp=wp/c, where wp is the plasma frequency.

A typical configuration for SPR sensing is given in the following image:

Kretschmann confi guration

To excite the surface plasmons, we need to match the wavevectors  components which is parallel to the interface.

 In the above configuration, we are only able to excite surface plasmons on the gold-sensing area interface. When the permittivity of the sensing area changes, the k-w relation for the surface plasmon also changes, as a result, we will excite surface plasmons at different wavevector and frequency. The figure below serves as an example:

Surface plasmon excitation condition when the permittivity of the

sensing area varies

In a recent publication, Z.F. Yu, S.H. Fan [2011] argue from a theoretical point of view that "The extraordinary spectral sensitivity of surface plasmon resonance sensors is commonly attributed to the modal overlap or unique dispersion of surface plasmons.In contrast to this belief, we show that such high sensitivity is due to the multimode nature of the sensing scheme."

The main results of their publication is given as follows:
the spectral sensitivity S can be written as

Some Research news media

SPIE seems to adapt to the new media age quite well.
SPIE Newsroom: There is an app. on android for this.
SPIE.TV: Lots of interesting talks with slides of SPIE conferences, I watched one talk by L.V. Wang, which is quite well developed: ideas clear and inspiring. 

Sciencedaily is quite a good news sources. 
Physics normally reports some excellent publication on APS, is a good sources for physics research.
Medgadget provides nice news on Medical technology news.

Compressed Sensing (study)

From now on, I will start a self book study session on Compressed Sensing, which I consider to be an important math tool for future research.
I will start with the "Introduction to Compressed Sensing" by Mark A. Davenport et al.
The study notes will be constantly updated in the file "CS_notes", it is mainly a summary of my study progress of "Introduction to Compressed Sensing" with some additional background material for me to understand, thus most of the material in the notes belongs to Mark A. Davenport et al., it is important to note that.

The Compressed Sensing has two equally important part:
(1). Sensing Matrix M, this is vital for practical measurement design, and the construction of the sensing matrix M requires knowledge of the sparsity of the data we want to measure.
(2). Signal Recovery via l1-minimization.

Up to June 13th, I have finished the first part--properties and construction of sensing matrix. But it is really difficult to understand the theory, compared to the Shannon sampling theory. I think partly due to the writing and partly due to the early stage of Compressed Sensing itself.  Hope the later study can be easier.

Agora20120509--waves in complex media

Agora is a paper discussion session of our group.
This week, I will chair the discussion for the review paper of A. Mosk '

Controlling waves in space and time for imaging and focusing in complex media'. 

Based on the wave equation given above and Maxwell equations, if the material is non-absorbing and reciprocal, then the wave exhibit time reversal symmetry, due to this symmetry, a wave that propagating outwards can thus be back-propagated to the exact starting. 

It is due to the development of Spatial Light Modulator(SLM) that now one can control the field at optical frequency at a much simple and fast way, even for strongly scattering medium.  

Our Agora session of this month ended on May 23th, 2012. A summary of the discussion have been given in Agora_may2012. If people are interested in, you can have a look. 

Internship finished--Laser Speckle Contrast Analysis for Hemodynamics measurement

The internship is with a company on cardiology. The purpose is to build up a Laser speckle contrast imaging setup and algorithm in order to measure important parameters for blood flow, for instance, the heart rate and velocity.

An example of Laser Speckle pattern

A laser speckle is the coherence interference pattern of surface or material due to the local refractive index variation. Speckle sometimes is considered as noise for coherent optical imaging method like OCT. But actually, it contains much information about the material itself and the illuminating light. For example, the laser speckle contrast is modified by the periodic pulsed blood flow,  one can easily extract the heart rate info from the speckles.

Laser spot on a fingertip

Master Thesis--Extended Nijboer Zernike theory for imaging simulation of optical systems

I have successfully passed the master thesis on the topic 'Extended Nijboer Zernike theory'. A link to the abstract of this work can be found in the group's blog--Lei Wei Master thesis