I am a Chinese Peasant Worker

Ok,this time it is our work :)

http://www.photonics.com/content/spectra/2008/June/research/91966.aspx

Easy-to-Tune All-Fiber Bandpass Filter

Fiber bend sets upper and lower cutoff of low-loss filter.

by Breck Hitz

Tunable bandpass filters are crucial components in numerous photonic applications, including telecommunications. All-fiber filters eliminate the need to couple into bulky and lossy external components, and many approaches to all-fiber tunable filters have been explored by researchers worldwide. Now scientists at Tianjin University and at FiberHome Telecommunication Technologies Co. Ltd., in Wuhan, both in China, have demonstrated an all-fiber tunable bandpass filter that avoids the complex tuning mechanisms — e.g., acousto-optic, electro-optic, temperature or pressure — of previous all-fiber filters.
The bandwidth and center frequency of the new filter are set merely by changing the bend radii of a pair of fibers. The bend in one fiber sets the long-wavelength cutoff, and the bend in the other sets the short-wavelength cutoff (Figure 1).

It is a huge disaster. Many building was destroyed by the earthquake. And many people gone. What a terrible nature. God bless us.

Oh, it summer again. The time is flying. And now the campus is full of strong flower perfume.

Just For Fun

http://www.markwang.com/chinamap/

College of Optical Sciences - The University of Arizona

The Institute of Optics- University of Rochester

College of Optics & Photonics - the University of Central Florida

What do the terms "optics" and "photonics" mean to you? Leading researchers have had their say - and now it’s your turn.

It started off as an innocent enough question. Claire Bedrock, the publisher of Journal of Optics A: Pure and Applied Physics (and also, incidentally, optics.org), asked her editorial board whether the name of the journal should be changed to something shorter and, er, snappier.

The responses - from some of the world’s most respected optical scientists - revealed fundamental differences in perception between the terms "optics" and "photonics". Or, as Massimo Santarsiero of the University of Roma Tre put it: "This just confirms that not all of us agree on the meaning of the words optics and photonics, and surely misunderstandings of this kind would be unavoidable in the whole community of opticists (... sorry!)"

Nikolay Zheludev of the University of Southampton in the UK delved into the etymology of the words to try to tease out their true meaning:

• The Greek term όptικά (optics) referred specifically to matters and instruments of vision. Strictly speaking optics is the science of vision.

• Photonics derives from the Greek photon (fοtον), which means light. Photonics is the science of light.

"In my view, this implies that optics is a much narrower term than photonics," he concluded. "In fact, photonics embraces optics."

But Lifeng Li of China’s Tsinghua University along with several other respondents, had a very different view: "To me the term optics in the modern and broad sense means the science and technology that deals with light, and photonics is a subdiscipline of optics."

Meanwhile, Mark Stockman at Georgia State University argues that names such as optics and photonics are just tags, and their meaning does not always correspond to their linguistic roots. "Optics is conventionally used for the general science of light," he said. "Photonics was originally introduced as a counterpart of electronics - using photons instead of electrons to process and transfer signals and information - but now the meaning of photonics is gradually becoming wider."

Naturally, we’d like to know your views. Can "photonics" be used interchangeably with "optics", or do they have quite distinct meanings? Please let us know what you think by using the commenting tool at the bottom of this article.

About the author

Susan Curtis is editor of optics.org.

It is really cool~~

A photonic crystal that can confine and guide light in all three dimensions offers new possibilities for novel laser designs.

Vertical fluorescence, SEM and infrared micrograph images of the structures, showing how the photonic crystal developed by Braun's team can guide light along a straight line, or make it bend through 90°. Image (f) shows the light coming from the top of structure (a).

US scientists have created a three dimensional pathway for light inside a 100 nm x 100 nm photonic crystal made using self-assembly techniques. Once inside the crystal, light can be directed through tiny waveguide-like structures to travel along a straight line, follow a curve and even bend around very narrow corners – without sustaining large losses (Nature Photonics doi:10.1038/nphoton.2007.252).

detail links

http://optics.org/cws/article/research/32276

It is really shorter and shorter, amazing.

detail links:

http://optics.org/cws/article/research/32296

Femtosecond pulse stabilization captures attosecond electron transport in real time.

Attosecond electron transport captured by femtosecond pulse stabilization
Image: Menlo Systems

An international team at the Max Planck Institute of Quantum Optics in Germany has made the first attosecond measurement in condensed matter, which could lead to coherent control in solids on the atomic scale. It is a step towards electronics in which the speed is limited only by the time required for an electron to travel between neighboring atoms.

In the proof-of-principle experiment, pulses of extreme ultraviolet light of 300 attoseconds duration fell upon the surface of a tungsten crystal, along with an infrared pulse comprising a few well-controlled oscillation cycles of its electric field.

The ultraviolet photons excited both loosely-bound electrons and electrons bound tightly to the cores of the atoms forming the crystal lattice. The difference in arrival time of these two types of electron on the crystal surface was measured at 110 attoseconds, as resolved by the rapidly oscillating phase-stabilized infrared pulses.

The generation of the attosecond ultraviolet pulses and the detection of the released electrons on the surface both employed phase-stabilization technology developed by Menlo Systems. This technology can stabilize the phase of ultrashort-pulse high-power lasers to about 100 mrad over several hours.

US researchers have made a nanolaser that could enable data to be stored with a density of 10 Tbit per square inch.

The nanolaser can focus light with a power of over 200 nW into a spot just 35 nm across. The result suggests that data storage beyond 10 Tbit per square inch could be possible at last. "Furthermore, our nanolaser technology could be scaled down to a spot size as small as 10 nm," team leader Sakhrat Khizroev of the University of California at Riverside told nanotechweb.org. "This experiment could have a great impact on the magnetic data storage industry and especially enable so-called heat-assisted magnetic recording (HAMR) – one of the most promising data storage technologies of the future."

Graduate student Rabee Ikkawi tuning a near-field optical system to test the nanolaser technology. Another member of the group at UCR, post-doctoral student Alexander Krichevsky, was instrumental in developing a unique near-field optical module to test both continuous and patterned magnetic media that could be implemented with heat-assisted magnetic recording (HAMR). Credit: Sakhrat Khizroev.

The data storage industry is now looking for alternatives to its conventional recording technology, known as longitudinal magnetic recording, for the first time since it was introduced over five decades ago. This is because the technology will one day reach the fundamental, superparamagnetic limit – beyond which the magnetic bits in conventional recording media become so small that the stored information is unstable. ......

It is so cool, more details is here

http://optics.org/cws/article/research/32159/1/071207

Yes,I has been this city. So I take care about the news about Jena.

Education, research and industry are the three backbones that underpin the optical activities in Jena, Germany. Jacqueline Hewett speaks to key players in the region to get a flavour of how these factors have been exploited to create today's thriving optical hub.

It is hard to describe the city of Jena in Germany as anything other than an optical stronghold. Today, it is home to a cluster of optical firms ranging from established multinationals such as Carl Zeiss and Jenoptik, right through to many innovative small- to medium-sized companies, two universities and world-renowned research institutes.

Jenoptik produces single diode lasers as part of its business model is to create, control and capture light. Credit: Jenoptik.

Jena's optical history dates back to 1846 when Carl Zeiss himself established his first workshop in the city – a move that effectively laid the foundation for today's thriving industry.

........

Details links

http://optics.org/cws/article/industry/30916

The photonic crystal fiber produces ultrafast pulses at low powers and across a broad spectral range, as well as breaking new conceptual ground in photonics.

The profile of transmitted light through the Kagome-lattice PCF. The fiber can accommodate guidance broadband guidance in the core with a continuum of cladding modes without interaction.
Image credit: Fetah Benabid

A new design of hollow-core photonic crystal fiber (HC PCF) has been developed by an international team led by Fetah Benabid of Bath University in the UK. One immediate result has been a method to produce attosecond laser pulses more efficiently than previous techniques.

The fiber developed by the team has a core based on a Kagome lattice, a geometric structure formed by interlaced triangles. This gives the fiber the counter-intuitive ability to carry core-guided modes and cladding continuum modes at the same time, unlike conventional PCFs where light is transmission is confined to the core. The result is transmission bandwidths that are claimed to be several times wider than conventional photonic crystal fibers.

"This is a new theory of photonic guidance in HC PCFs," Benabid explained to optics.org. "The ability of a fiber to have a core-guided mode and a cladding continuum that shares a strong longitudinal phase match is novel. It's the first experimentally observed instance of the so-called Von Neumann-Wigan quasi-bound state in a continuum, which was first predicted in the 1930s."

The team's discovery could have wide-ranging implications. "This new photonic guidance theory could create new designs for next-generation photonic materials such as broadband HC PCFs and photonic crystals," said Benabid. "The similarity between this guidance and the Von Neumann-Wigner states would also constitute a new conceptual bridge between photonics and quantum mechanics."

“This constitutes a new conceptual bridge between photonics and quantum mechanics.”

The fiber's unique properties have led directly to a second breakthrough, the efficient generation of a broad spectrum of ultrafast pulses from a hydrogen-filled PCF through stimulated Raman scattering.

The conventional technique to create attosecond pulses is high-harmonic generation (HHG), which produces central wavelengths in the XUV or soft X-ray region through the firing of a very intense laser pump pulse into a gas. Benabid's fiber was able to produce ultrashort pulses more simply using through stimulated Raman scattering.

The diffracted spectrum generated in a Kagome-lattice hollow-core PC filled with hydrogen. The spectrum spans from the UV to mid-IR.
Image credit: Fetah Benabid

"In our fiber the pump excites the hydrogen to create a very broad and coherent comb-like spectrum spanning from the UV to the mid-IR," said Benabid. "This type of excitation is also new. Creating a coherent spectrum usually requires a special type of excitation, necessitating extremely short pump pulses or finely tuned multi-pump excitation - and in both cases a huge amount of peak power."

Benabid's fiber is claimed to require a pump pulse with power levels six orders of magnitude lower and five orders of magnitude longer than those previously needed for HHG.

The University of Bath team: (l-r) Francois Couny, Fetah Benabid and Phil Light
Image credit: Fetah Benabid

"Our new techniques could enable low-cost compact systems for sub-femtosecond optical pulse generation at wavelengths spanning the UV-IR," commented Benabid. "This would have huge impact in areas such as laser science, materials science and biological research."

About the author

Tim Hayes is a reporter for optics.org and Optics & Laser Europe.

The European Commission report includes data on the 2006 salaries of thousands of scientists working in the public and private sectors across 38 countries.

The result is that the chinese scientists are the cheapest among them. Oh,my god....

Detail is here :

http://www.nature.com/news/2007/071128/pdf/450597a.pdf

Inspired by the way a carnivorous plant captures insects, US scientists have designed a new material that could facilitate high-speed adaptive optics.

The researchers based their material on the way a Venus flytrap works.

“We got the inspiration to build this material from way a Venus flytrap works,” Alfred Crosby, an assistant professor at the University of Massachusetts at Amherst, explained to optics.org. “This plant can change the shape of its lobes from concave to convex at very high speeds – around 100 ms.”

The new material consists of a number of lens-shaped patterns fabricated on a silicon substrate. Each individual element looks like a hemispherical shell, which can be in either a concave or convex orientation. Like the flytrap, the lenses in the material can switch between convex and concave orientations at speeds of around 30 ms.

Such high-speed transitions are made possible by a phenomenon called "snap buckling instability". When the pressure applied on a curved surface exceeds a certain threshold, it changes the orientation from a convex to a concave shape, or vice versa.

“It’s like taking a tennis ball and cutting it into two pieces,” says Crosby. “If you place one half between two vertical stands, and start pushing the bulge, the ball changes shape and begins to deform. Then at one critical point, it stops deforming and changes to a convex shape.”

The curved surfaces in the new material react in the same way, but in this case the stimulus can come in the form of pressure, heat or an electrical current. In their first prototype, the team used a fluid-derived swelling to demonstrate the rapid transition.

When the individual lenses change shape, the surface as a whole undergoes a transition that modifies its reflectivity or its focal length. This means that the material could be used in outdoor signs where the reflectivity of the surface keeps changing, and also as an adaptive lens with a variable focal length.

The micrographs show the surface of the new material when all the lenses are in the convex (top) and concave (bottom) positions.

According to Crosby, each lens in the surface can be fabricated in various shapes and sizes. In this work, the team built hemispherical lenses with diameters ranging from 50 µm to 500 µm, with an inter-lens spacing of 10–50 µm. Crosby and his team found that as the lenses got smaller, the transition speeds went up.

Hongrui Jiang of the University of Wisconsin, who last year produced a liquid lens that mimics the human eye, commented: “This work takes a biomimetic approach to realizing snapping polymer membranes triggered by environmental parameters. It could potentially be applied in arrays of on/off operating devices, such as optical switches and as actuators that control other components.” Jiang added that the fabrication process must now be improved to produce lenses with highly uniform shapes and smoother textures.

The Massachusetts team is now working on a nanoscale version of the material, which could be used in adhesives and paints. “We are also looking to trigger the transitions with voltage changes instead of fluid-derived swelling,” said Crosby. “Such a feature would be most useful for optical devices.”

About the Venus flytrap

The Venus flytrap is a carnivorous plant that is found in Eastern United States. Writing about it in his diaries, Charles Darwin called it “one of the most amazing in the world”, because of the incredible speed at which it traps insects.

From the outside, it has green coloured leaves that terminate with lobes that look like two palms attached at the knuckles. In this sense, they can be thought of as concave. When an insect or fly sits on its inner surface, the lobes change to a convex shape, instantly sealing the insect inside. The reaction time is estimated to be about 100 ms – one of the fastest movements in the plant kingdom.

The researchers reported their work in Advanced Materials.

About the author

A L Narayan is a freelance science writer based in Singapore.

Congratulations to the Chinese National Space Agency (CNSA) and the researchers for Chang'e-1

  by Baz Luhrman

Everybody's Free To Wear Sunscreen
---------------------------------------------------------------------------
Ladies and Gentlemen of the class of '97... wear sunscreen.

If I could offer you only one tip for the future, sunscreen would be IT.

The long term benefits of sunscreen have been proved by scientists whereas the rest of my advice has no basis more reliable than my own meandering experience.

I will dispense this advice now.

Enjoy the power and beauty of your youth. Never mind. You will not understand the power and beauty of your youth until they have faded. But trust me, in 20 years you'll look back at photos of yourself and recall in a way you can't grasp now how much possibility lay before you and how fabulous you really looked.

You are NOT as fat as you imagine.

Don't worry about the future; or worry, but know that worrying is as effective as trying to solve an algebra equation by chewing bubblegum. The real troubles in your life are apt to be things that never crossed your worried mind; the kind that blindside you at 4pm on some idle Tuesday.

Do one thing every day that scares you.

Sing.

Don't be reckless with other people's hearts, don't put up with people who are reckless with yours.

Floss.

Don't waste your time on jealousy; sometimes you're ahead, sometimes you're behind. The race is long, and in the end, it's only with yourself.

Remember compliments you receive, forget the insults; if you succeed in doing this, tell me how.

Keep your old love letters, throw away your old bank statements.

Stretch.

Don't feel guilty if you don't know what you want to do with your life. The most interesting people I know didn't know at 22 what they wanted to do with their lives, some of the most interesting 40 year olds I know still don't.

Get plenty of calcium.

Be kind to your knees, you'll miss them when they're gone.

Maybe you'll marry, maybe you won't, maybe you'll have children, maybe you won't, maybe you'll divorce at 40, maybe you'll dance the funky chicken on your 75th wedding anniversary. Whatever you do, don't congratulate yourself too much or berate yourself, either. Your choices are half chance, so are everybody else's. Enjoy your body, use it every way you can. Don't be afraid of it, or what other people think of it, it's the greatest instrument you'll ever own.

Dance. Even if you have nowhere to do it but in your own living room.

Read the directions, even if you don't follow them.

Do NOT read beauty magazines, they will only make you feel ugly.

Get to know your parents, you never know when they'll be gone for good.

Be nice to your siblings; they are your best link to your past and the people most likely to stick with you in the future.

Understand that friends come and go, but for the precious few you should hold on. Work hard to bridge the gaps in geography in lifestyle because the older you get, the more you need the people you knew when you were young.

Live in New York City once, but leave before it makes you hard; live in Northern California once, but leave before it makes you soft.

Travel.

Accept certain inalienable truths, prices will rise, politicians will philander, you too will get old, and when you do you'll fantasize that when you were young prices were reasonable, politicians were noble and children respected their elders.

Respect your elders.

Don't expect anyone else to support you. Maybe you have a trust fund, maybe you'll have a wealthy spouse; but you never know when either one might run out.

Don't mess too much with your hair, or by the time you're 40, it will look 85.

Be careful whose advice you buy, but, be patient with those who supply it. Advice is a form of nostalgia, dispensing it is a way of fishing the past from the disposal, wiping it off, painting over the ugly parts and recycling it for more than it's worth.

But trust me on the sunscreen.

US researchers believe that tiny optical versions of capacitors, resistors and inductors can be made by adjusting the size, shape and composition of sub-wavelength nanoparticles.

Snapshot in time: full wave simulations of the electric (top) and the magnetic field distribution in a finite length (850 nm) straight nanowire. (Image credit: University of Pennsylvania)

Nader Engheta and colleagues at the University of Pennsylvania are laying down plans for optical nanocircuits that could replace conventional electronics. Apart from being much smaller, optical nanocircuits have the potential to offer more bandwidth and increase data handling capacity thanks to their higher operating frequency.

As Engheta explained, developers will have to get to grips with a different way of thinking to make the most of the technology. Specifically, this means considering a material's local permittivity rather than its conductivity.

"At lower frequencies, the substrate of a classic circuit board is very poorly conducting and elements are connected with each other only through the shorting metallic wires attached to their terminals," he told nanotechweb.org. "When you move into the infrared and visible spectrum, the displacement current that enables optical nanocircuits may also flow in the surrounding background material and connect elements in an unwanted manner."

Engheta's proposal for optical shorting wires involves the use of a special class of nanowaveguides formed by a core with a relatively large permittivity surrounded by a thin concentric shell of low permittivity. The team has considered a cylindrical waveguide just over 83 nm in diameter with a core measuring 50 nm across, and the simulation results are encouraging.

"We looked at the distribution of optical displacement current density on a transverse cross section of a straight (850 nm) optical shorting wire and the model shows that the current is confined within the core," said Engheta. "What's more, the longitudinal component of the optical electric field in the core is very small, which means that the potential drop between the two ends of our interconnecting structure is low – another key requirement."

Critics will point out that the work to date is purely computational, but the group is taking steps towards realizing a prototype circuit. Engheta is keeping the design under wraps, but he did reveal that the team has started working with the university's nanofabrication group and plans to begin its proof-of-concept experiments early next year.

The researchers presented their work in Optics Express.

About the author

James Tyrrell is editor of nanotechweb.org.

Dynamically confining light in volumes smaller than the wavelength is an important step towards the complete control of light.

Schematic diagram of the experiment, showing the nanocavity, the mirror, and the light pulses.
Image credit: Nature Materials

A team from Kyoto University has demonstrated a method for dynamically changing the lifetime of a photonic crystal resonator, which in effect can trap a pulse of light inside the system. Susumu Noda and his colleagues showed that the Q factor of their system, a measure of the number of time periods of light after which most of the light has left the system, could be increased by a factor of four from 3,000 to 12,000 within just a few picoseconds (Nature Materials, 6, 862).

"This is a completely new approach in this field, a new concept for dynamically controlling the Q factor that has not been used before," Noda explained to optics.org.

Nanocavities with high Q factors have been demonstrated previously, with Noda's team recently reporting one with a world-beating Q factor of two million, but controlling the Q factor dynamically is the key to developing useful applications for the technology.

"Slowing and/or stopping light could lead to quantum information processing applications, where nanocavities would be integrated on a chip and the transfer, storage and exchange of photons would be possible through integrated waveguides," Noda said. "The important issue is how to deliberately control the storage and release of photons from such a high-Q nanocavity."

The team fabricated its resonator in a two-dimensional photonic crystal. The structure consists of a waveguide with a mirror located at one end and a microresonator coupled to the waveguide.

Scanning electron micrograph of the fabricated sample, and schematic diagram of the measurement system.
Image credit: Nature Materials

The microresonator was formed by three missing airholes in the photonic crystal lattice, making a nanocavity in the crystal located five hole-rows away from the waveguide. The mirror was created by a line defect at one end of the waveguide, where the slightly modified lattice constant acted as a perfect reflector.

When a probe pulse of light was fed in through the waveguide, some light waves were reflected back from the cavity and interfered constructively with those reflected back from the mirror, yielding a large coupling of the system to the external light.

The team's breakthrough was to exploit the change in refractive index that can be induced in a silicon-based photonic crystal when it is irradiated with another pulse of light. This change is caused by the non-linear response of the crystal and can be preserved over several nanoseconds, long after the pulse that caused it has faded away.

This change in refractive index affects the interference between the waves reflected directly from the nanocavity and those returning from the mirror, with the result that it can be made either constructive or destructive. When destructive, the interference dramatically reduces the coupling to the external light, and therefore traps the light inside the system for a significantly longer period of time.

"When the cavity Q factor increases, the photon's lifetime is increased and the operational speed becomes slow," Noda explained. "So the Q factor needs to be made small when we introduce the photons, and then increase rapidly before the photons leak out of the nanocavity. And, if necessary, decreased again so that they can be deliberately released quickly when desired. Until now there has been no way to achieve such dynamic control of the nanocavity Q. This work demonstrates a way to do so for the first time."

About the author

Tim Hayes is a reporter for optics.org and Optics & Laser Europe.

Taday is a holiday for person who are still single.

An optical modulator based on negative-index metamaterials could be faster and more compact than existing telecoms devices.

One drawback of so-called negative-index metamaterials (NIMs) is that they tend to absorb much of the infrared light passing through them. Now a team of US researchers has turned that shortcoming to their advantage, using it to design a new type of optical switch – or modulator – that promises to be faster and more compact than existing devices (App. Phys. Lett. 91 173175).

The researchers, from Hewlett-Packard Laboratories in Palo Alto, and University of California, Berkeley, claim this is the first time that optical modulation has been seen in a NIM at near-infrared wavelengths.

The team based their modulator on a "fishnet" NIM, because this structure is known to have both a negative electrical permittivity and a negative magnetic permeability for infrared light at certain wavelengths. As well as being responsible for the negative refraction of light, this "double resonance" also affects how much light is transmitted by the NIM.

To make their modulator, the team sandwiched an 80 nm layer of silicon between two 25 nm layers of silver. The silver layers were perforated with tiny rectangular holes using nanoimprint lithography and electron-beam lithography to form an square array of criss-crossed wires. The wires were separated by 320 nm in both directions; the wire widths were 220 nm in one direction and 110 nm in the perpendicular direction.

"These are basically resonant structures," explains Hewlett-Packard's Shih-Yuan Wang, who led the work. The magnetic resonance is related to the sandwich structure, while the electric resonance is caused by the crosshatching of wires.

The device was then studied by shining two laser beams on it. One beam was a 1700 nm wavelength infrared laser that was shone through the device to see how much light it would transmit. The second beam was a 532 nm wavelength visible laser that was used to control the transmission of the infrared light.

The visible laser was able to control infrared transmission by creating electron—hole pairs in the silicon layer, the presence of which causes a slight shift in the wavelength at which resonances occur. The result is a 50% increase in the amount of infrared light transmitted by the device when the visible laser is on.

A 50% change is already enough to make a useable modulator, says Wang, but he believes that a contrast of 90% is possible with further refinement of the structure. What's more, by repeating the experiment using very short laser pulses, the team found that the material could be switched in as little as 58 ps – a figure defined by the time it takes for the electrons and holes to respond to the laser light.

This means that the modulator could be switched on and off very fast, several tens of gigahertz in the current device, and Wang believes that 100 GHz could be possible with device optimization. This is much faster than any commercial modulator available today.

The modulator operates in the near infrared part of the spectrum, making it a good candidate for use in optical communications devices. "The closest competitor is the electro-absorption modulator," says Wang. "But with that you are limited to the bandgap of the semiconductor you are using. With our structure we can design to any wavelength if we change the dimensions of the criss-cross pattern."

There's plenty of work to do, however. The modulator is optically pumped in its existing form; a useable device would require electrical activation.

About the author

Pauline Rigby is a science journalist based in the UK.

Journal of Optics A: Pure and Applied Optics presents a special issue dedicated to research papers presented at the NANOMETA-2007 conference, a new European Topical Meeting which took place January 2007, in Seefeld, Austria.

Recent advances in nanofabrication now allow sub-wavelength photonic structures to be manufactured which are of similar complexity to those used in the microwave community for many decades. The idea of the NANOMETA meeting was to bring together the extensive experience of microwave electrical engineers in structured materials and the emerging community of photonics researchers interested in the physics of light coupled to nanostructures.

The overlap between these two groups of scientists became apparent a few years ago with the possibility to revolutionize electrodynamics by developing media that displayed negative refraction, for which the term “metamaterials” was coined. Today this term covers materials with all sorts of unusual electromagnetic functionalities that may be achieved by sub-wavelength structuring.

The special issue covers the diverse topics discussed at the NANOMETA meeting, and includes negative index materials, 2D and 3D photonic bandgap structures, nanolensing and nanoharvesting of light, nanotransmission lines and nano-antennas, single nanoparticle photonics, and quantum effects in nanophotonics (J. Opt. A 9 9.)

The collection of papers shows how much has already been achieved by these two communities, and illustrates the fascinating possible future directions and applications of nanophotonics and metamaterials.

New research is improving the performance of sensors based on fluid-core optical fibers, while also making them easier to fabricate. Tim Hayes reports.

Microstructured optical fibers have long held promise as sensitive and robust sensors for real-world applications such as bacteria detection and remote sensing, since their structure allows significant interaction between the light guided along the fiber and any chemical species filling the holes.

As a result they offer an attractive alternative to planar waveguides — where the sensing distance is usually limited to a few centimeters of free-space — and to standard fibers, where the guided mode is normally insensitive to the external environment.

Cristiano Cordeiro Image credit: Cristiano Cordeiro.

"Photonic crystal fibers (PCFs) can offer the best of both worlds," Cristiano Cordeiro of the University of Campinas, Brazil, explained to optics.org. "They can be very sensitive sensors, but also robust and very long. And their small hole diameters make it possible to fill a meter of fiber with only a few tens of nanoliters of fluid."

Practical difficulties remain though. The analyte must be able to enter the fiber in a way that does not damage the fiber's integrity, and the laser light must be as close to single-mode as possible to achieve the required sensitivity.

Single-mode fiber

Cordeiro's Brazilian team, which also included researchers from Universidade Presbiteriana Mackenzie, has therefore developed a new method to limit the number of guided modes inside the fiber. "Any interferometric measurement that is phase-sensitive will be difficult if several modes are present, since in a multimode fiber each mode travels a different path," noted Cordeiro. (Measurement Science and Technology, 18, 3075.)

The team simultaneously and selectively fills the core of a PCF with one liquid, and the cladding microstructure with a different liquid. Careful selection of the fluids allows the refractive index difference between core and cladding to be controlled, and so limits the number of guided modes inside the fiber. "To the best of our knowledge this selective filling of a PCF to limit the guided modes has not been demonstrated before," Cordeiro said.

Doubly selective filling method to insert one liquid into the cladding holes and another liquid into the core of a PCF.
Image credit: Cristiano Cordeiro.

The filling technique involves first closing the cladding holes at one end of the fiber with an electric arc using a fusion splicer. Reduced air pressure then draws a UV-curable polymer into the core of the fiber at the opposite end, where it is cured into a plug. The result is a fiber with the core blocked at one end and the cladding pores blocked at the other.

Syringes are then used to manually introduce the chosen fluids into the core and the cladding from the appropriate ends. Trimming leaves a length of PCF with liquid-filled cladding and a liquid-filled core.

Interferometric near-field images of light exiting a PCF with a 1.390-index core and (a) water-filled or (b) air-filled cladding. The fiber structure is superimposed for clarity.
Image credit: Mackenzie Photonics Group.

Cordeiro's team compared the behavior of PCFs having water-filled and air-filled cladding in an interferometer, using a 633 nm He-Ne source with the reference arm in free space. The core in both fibers was filled with a water-glycerin mixture. Tilting the reference beam produced interference fringes that indicated the extent of phase variance in the fiber. In the water-filled fiber the phase remained constant within the whole core area, suggesting the guidance of just the fundamental laser mode, while the air-filled fiber showed numerous fringe splittings.

Making PCF-based chemical or biological sensing practical for use by non-specialists, for example in a hospital environment, is the team's main challenge. "So far the process of inserting the two liquids is manual via syringes," explained Cordeiro. "Further development is needed to make it more practical for mass production and daily use. A second problem is evaporation of the liquids, which can empty the PCF in minutes. We're working on these issues."

Once the manufacture is optimized the team is looking at several future applications for fluid-filled PCFs, including all-fiber acetylene cells to be used as simple and cheap reference cells, bacteria detection, and characterization of liquid evaporation in PCFs.

Open access

Another key issue that researchers have been working on is making holes in the PCF through which the analyte can be introduced. "Lateral holes can be a major improvement in making PCFs into practical fluid sensors," Cordeiro said. "So far there is no standard way of creating these access points, and each process has its own advantages and disadvantages."

Rectangles milled into the fiber jacket using a focused ion beam. Milling time (bottom) 130 seconds and (top) 120 seconds.
Image credit: Cristiano Cordeiro.

The Brazilian team has used a focused ion beam (FIB) to locally mill holes through a fiber's external silica jacket, without compromising the fiber guidance or robustness. "We were the first group to propose an FIB for this purpose, which gives us full control over the size, shape and position of all lateral ports," Cordeiro said.

The team milled several 20 x 5 µm rectangles in the side of a PCF by using an FIB operating at 30 kV and 20 nA to open up access to the fiber interior. By finely controlling the milling time and FIB current it is possible to open up lateral access without disturbing the fiber core. Alternatively, by applying the technique to a fiber in which the cladding holes have been locally blocked using a fusion splicer, access to just the core of a hollow-core PCF can be achieved, allowing selective filling if desired.

Cordeiro is also part of a project involving researchers from the University of Sydney developing fibers in which one of the cladding holes is open to the environment over long lengths of fiber, exposing the core in what is claimed to be a unique optical fiber design.

The slotted structure presents advantages for sensing applications, since the time taken for chemicals to diffuse into the evanescent sensing region is minimal and the length of the sensing region is no longer limited. (Optics Express, 15, 11843.)

Slotted microstructured optical fibers with varying slot sizes. The slots were formed by drilling (left) 1 mm and (right) 2.5 mm holes into the intermediate perform or cane.
Image credit: Felicity Cox, University of Sydney.

But the novel design could have other applications too. "The key point is that we can bring anything into contact with the fiber core, not just liquids for chemical sensing," explained Felicity Cox of the University of Sydney. "We can evaporate metal coatings directly onto the core for plasmonic effects, since there is line-of-sight access to the core. We also want to try writing gratings without interference from the microstructure."

Fabricating the fibers required little deviation from standard fiber manufacture, involving only the drilling of holes into the side of the intermediate fiber perform before drawing to cane. The holes, carefully made so as to intersect with a single air hole and not impact the core, form slots along the fiber's length after drawing, with dimensions controlled by the size of the original hole. According to the team, the slot induced no additional losses to the fiber when in use.

The team is working towards using the sensors in practical environments where the fiber's robustness will be essential. "The exposed core provides an excellent sensing platform, although the sensitivity may be reduced by external contaminants being able to reach the core," said Cox. "But this is less of a problem in our slotted fiber than in existing evanescent wave sensors, where large areas of the core have to be deliberately exposed so as to make contact with the sample." In addition, the slotted fiber is made from plastic via a very simple process, so any sensors based on them could be disposable items, an additional practical benefit.

Slotted microstructured polymer optical fibers, approx. 140 micrometers diameter.
Left: 3-hole design
Right: 5-hole design
Image credit: Felicity Cox, University of Sydney.

Further work is now planned to investigate whether slotted fibers with hollow cores can be made, which could then be used in remote gas sensing. Biosensing applications could also benefit, as many of them involve substrate-bound sensing.

"The slotted fiber is truly a sensor, rather than a probe, since it can deliver real-time information," commented Cox. "It could also be a quasi-distributed sensor, because the analyte could enter the fiber anywhere along its length. But the most exciting thing is that we have a robust fiber with an accessible core, and chemical sensing is just one of many applications possible."

About the author

Tim Hayes is a reporter for optics.org and Optics & Laser Europe.

optics.org learns how a simple self-mode-locking design applied to ceramic lasers could aid other high-power lasers.

Researchers from Singapore, China and Japan have unveiled what they believe to be the first high-power self-mode-locked ceramic laser. The team says its simple approach can be applied to other high-power lasers, particularly those emitting at wavelengths where mode-locking elements such as semiconductor saturable absorber mirrors (SESAMs) are not available. (Optics Letters 32 2741)

"Our self-mode-locked ceramic laser is simple and compact," researcher Guoqiang Xie from Nanyang Technological University in Singapore told optics.org. "Our laser cavity uses just three mirrors and a ceramic rod. Compared to Kerr-lens mode-locking, the self-mode-locking is not so sensitive to cavity alignment and the mode-locking region is relatively long."

Self-mode-locking means that no additional active or passive mode-locking elements (such as SESAMs) are used in the laser cavity. The mode-locking is purely driven by optical and thermal effects in the laser material itself.

Xie says that his team's Yb:Y₂O₃ ceramic laser has an average output power of 2.7 W and emits 1.1 picosecond pulses at a repetition rate of 126 MHz. "The pulse energy reaches 21 nJ and a peak power of 19 kW," he said. "The pulses will be very useful for frequency doubling, frequency mixing and other nonlinear processes, or as seeding pulses for further amplifying large-scale ceramics."

The first step in the process is to pump the ceramic material with a 30 W fiber-coupled laser diode bar emitting at 937 nm. Xie explains that the diffraction loss induced by thermal lens aberration, in combination with the Kerr-lens self-focusing effect in the gain medium, results in the self-mode-locking.

"It was important to understand the thermal lens and its aberration value as well as having a suitable laser mode size when we designed the laser cavity," commented Xie. "We estimate the thermal lens aberration value and the diffraction loss induced by the aberration to obtain the nonlinear loss modulation required for mode-locking. In our laser, a laser mode radius of approximately 120 microns in the ceramic is used, which generates a nonlinear loss modulation of 2 x 10^-4."

The team is now investigating nonlinear processes such as multiple pulsing in the ceramic and hopes to extend the self-mode-locking technique to other high-power lasers. Xie added that there are no plans to commercialize the cavity design at the moment.

Author
Jacqueline Hewett is editor of Optics & Laser Europe magazine.

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Bacterial viruses can be made inactivate by a low-power visible femtosecond pulse, opening up new strategies for controlling micro-organisms.

The femtosecond laser renders viruses inactive.

A US group has found that firing a low-power femtosecond laser at bacterial viruses in solution can dramatically reduce the number of infectious viruses, in some cases by a factor of 1000. "I had to repeat the experiment several times to convince myself that the laser worked this well," said Shaw-Wei D Tsen of Johns Hopkins School of Medicine. (J Phys: Condens. Matter, 19, 322102.)

Tsen and the team, which also includes his father, believe that the pulses cause the viruses to vibrate by stimulating low-frequency Raman active vibrational modes. Under the right excitation, the vibration is sufficient to render the viruses inactive.

"The laser rapid pulses of the laser allow the solution surrounding the virus to cool off, reducing heat damage to the normal blood components," said Tsen.

The group used a diode-pumped continuous-wave mode-locked Ti-sapphire laser to fire femtosecond pulses at samples of M13 bacteriophage. Samples were typically irradiated for ten hours with 80 fs pulses each having an average power of 40 mW.

This method leaves the sensitive cells around the viruses unharmed and is much more selective than UV irradiation methods, which disinfect microorganisms but usually cause mutation. Lasers also pass through any water surrounding the viruses without absorption, unlike microwaves or ultrasonic vibrations.

Since the damage to the viruses is essentially mechanical, caused by structural vibrations rather than chemical attack, the technique should not trigger an immune response or evoke any drug-resistance. Consequently micro-organisms which are drug-resistant should be susceptible as well.

About the author

Tim Hayes is a reporter for optics.org and Optics & Laser Europe.

Optical components could benefit from a blue LED which offers enhanced coupling efficiency, stronger light emission, and a more uniform optical power output.

A team of researchers from the National Sun Yat-Sen University in Taiwan, Republic of China has fabricated a Fresnel microlens onto the sapphire substrate of a blue LED.

The researchers say that adding a Fresnel microlens enhances the optical power of the LED by around 1.68 times at an injection current of 20 mA.

Image Credit: Ming-Kwei Lee

The team

Researchers from the Republic of China say that fabricating a Fresnel microlens onto the sapphire substrate of a blue LED enhances the optical power by around 1.68 times compared to the same device without the lens at a 20mA injection current. (Applied Physics Letters 91 051111).

"The LED chip can emit 80 mcd at 450 nm under a current of 20 mA," Ming-Kwei Lee, researcher at the National Sun Yat-Sen University, Republic of China, told optics.org. "The optical power output of conventional LEDs exhibit a Gaussian distribution. Our LED gives a flat distribution which means a more efficient coupling is obtained."

Incorporating a textured surface into LEDs made from gallium nitride has already been found to enhance its external quantum efficiency, but it is difficult to obtain directional light emission. By fabricating a Fresnel microlens array instead, the researchers have obtained a more directional light enhancement. "The nearly flat surface of the Fresnel microlens also enhances the coupling efficiency with other optical components such as optical fibers without complicated alignment," added Lee.

The key to obtaining increased power output from the sapphire substrate is due to the difference in refractive indices of the materials. "There is a smaller difference in the refractive index between sapphire (1.76) and air, compared to between GaN (2.5) and air," explained Lee. "This means fabricating the microlens on the sapphire side provides a higher external quantum efficiency and greater emission area."

The researchers used a focused ion beam (FIB) milling technique to fabricate the Fresnel microlens structure onto the sapphire. The technique involves a one-step machining process by computer program.

"The sophisticated etching skill we have achieved cannot be obtained using other technologies," commented Lee. "The method uses accelerated gallium ions to etch the target. The path and energy of the charged gallium ions can be well controlled by the electric field."

The researchers will next investigate how the overall power output is affected by introducing a Fresnel lens to different faces of the LED. "We will fabricate a Fresnel lens onto the GaN face and onto the sides of a blue GaN to check the overall output power," concluded Lee. "We will also explore suitable packaging for this type of LED."

About the author

Marie Freebody is a reporter for Optics and Lasers Europe and optics.org.