Bandgap Theory and Quantum dots LASER

I have worked on quantum dots lasers grown by Phillip Poole and Pedro of Canada Research Council (NRC) in Ottawa to determine their operating characteristics using temperature and high hydrostatic pressure.

Temperature measurements help us to determine how the device inner workings cope with temperature. by the way my PhD research is aimed at finding ways to make semiconductor LASER devices temperature insensitive. Most of LASER devices in the market suffer from temperature dependence and hence require very expensive cooling systems. Meaning as temperature increase the performance of the devices decrease dramatically.

This effect can be seen by an increase in threshold current due to increase in Non-radiative recombinations of the carriers in the active medium. Ideally we should have only radiative recombinations which will be a result of injected photons exciting electrons to higher level and these excited electrons to release a photon on their way down to lower energy levels and hence perfect laser.

Sadly at room temperature the dominant process is non-radiative recombination named Auger recombination process which is thermally activated. this process is a major source of loss as the photon from de-excited electron is given to a third carrier which is turn excited further into higher level or vacuum. at room temperature radiative recombination could account to approx 6 percent of all threshold current. the rest is to just counter the effect of Auger at room temperature in devices i worked on.

Pressure measurement involves squeezing the device in high pressure about 10kbar, this is equivalent of standing five grown African Elephants on a 1 cm square area. this allows one to change the interatomic spacing between the atoms in the crystal lattice. In doing so the band gap changes as you increase pressure the band gap is increased in most four-five semiconductor devices.

more information can be found in my paper published by American Physics Letters (APL) in October 2010 title of the paper is Efficiency Limiting Processes in 1.55 micron InAs/InP-Based Quantum Dots Lasers.

The Promises of Quantum Dot Lasers

In 1982 Prof Arakawa and Prof Sakaki proposed that due to confinement nature of quantum dots, devices that will be made using them should be temperature insensitive.

Many researchers have tried to harness the power of 3D electron confinement but met with promising results using different techniques. The prediction that the devices will have low threshold current densities have yet to be proved. Temperature insensitivity comes with high threshold current densities and highly doped devices.

For undoped(Intrinsic) semiconductor devices temperature insensitivity and low threhold currents have not been materialised. The biggest obstacle in my view could be associateed with the fabrication process of these devices. Stranski-Krastanov method produce inhommogeneous dots and a wetting layer which could act as source of non-radiative recombinations centers.

Calculations still show if we can create a perfect quantum dot we should have a perfect LASER that will have both low threshold current densities and temperature insensitive.

The Effect of Anti-reflection coating on LASER facet

For Buried Heterostructure(BH) Quantum Well LASER fabricated on Indium Phosphide (InP) substrate the effective refractive index is about 3.48 from Fabry-Perot (FP) Measurement i had performed earlier. The as cleaved mirrors on this type of lasers has reflectivity of about 0.3 which was confirmed from the calculations using data obtained from FP measurements.

To reduce the reflectivity of the facet, anti-reflection coating was sputtered on the facet using Nordiko 2000 physical vapour deposition (PVD) machine.The devices used were 1mm long edge emmiting LASERS lasing wavelength at 1550nm. The results showed the improvement of external quantum differential efficiency. This allows the determination of internal losses on the device.

These are exciting results which can be used to measure internal losses and determine internal quantum differential efficiency as function of change in reflectivity of as cleaved facets. (I am expecting to write a paper on gain measurements on these devices using this technique am developing).

You can read my paper published on IEEE, titled Thermal Performance of 1.55μm InGaAlAs Quantum Well Buried Heterostructure Lasers which talks about efficiencies and losses of these devices.

Bands of Influence Theory Part 3

The famous Inverse Square Law allows us to know why the object is occupying the certain point in space and time (even though not precisely according to Heinseberg Uncertainty Principle) relative to another object but does not give us reason why one object can hold many other objects in different orbits like our sun and the planets.

We know that space is mysterious but it has a dimension of which can be observed by the effect has on light (Space can bend light) or the effect of planetary bodies as they pass closer to each other. I like to think about the space as a calm pond and planets and stars are like balls of different sizes floating on their heavenly journey, they disturb the calmness of the water and create waves.

So each of the bodies in the water will create a wave according to their size and these waves interact at some point in space and time. The points of interaction is where the Bands of Influences are formed. The interaction of these waves act as a path connecting all of these bodies together.

This is my theory of the bands of influences. It applies also to the configuration of atoms in the crystal, electrons in the atoms, planets on solar system, galaxies in the universe.

one example of these bands of influences is the influence of moon on earth tidal wave system. In my view these bands explain why planets like Jupiter can host 63 moons and planets like Saturn can have rings around it. It would not suprise me if the earth would have a second moon somewhere in one of its bands of influence.

One characteristics of these bands is that the bigger the object the more the effect of their influence on other objects.

More to come...Any questions so far don't hesitate to ask?

Band of Influence theory Part 2

So after categorising objects as Fermions or Bosons, this give a rise to question why is it objects have to be either of the two? The idea that laws of nature choose whether you become a Fermion or a Boson can not be far from truth.

On observations from the tiny electron going around a nucleus to the star like our sun and its planets going around it to our planet and its moon. Everything is connected in one way or another and the vast expanses of universe are also connected. This allows me to say there is a connection from the tiniest of atoms and their contents to the biggest objects in the universe to the universe itself and if there are other universes then all are connected.

The big question is how? My answer is connection through the bands of influences(energy). if we look at the atom we know that electrons occupy a certain (precise) energy levels and there are several bands of these energies. We can observe these bands more clearly when we create a quantum box (quantum well) where we can see that electrons will contain certain energy levels (using solutions to Shrodinger's equations) and there is a band of energy that these electrons will not dare to settle.. So Why can't they occupy anywhere they want?

if we go look at the solar system we see that planets have to orbit on well defined paths of which are critical to their presence. We all know about gravity and there are mathematical formula to describe that relationship between the sun and the planets. so the question again is why can't earth occupy any other path than this orbit.

Trivial example is why you(if you are Male) won't like it when another male come closer to your face (if you are fermion)..and that you need a personal space..where you feel comfortable for others to occupy..you have heard of space invaders is not for nothing it is all physics...just think about it..

This will continue next time...

Bands of Influence Theory according to Me

First of all i would like to start with background information..I categorise objects as either Fermions or Bosons. Fermions are all those objects that obey Pauli Exclusion Rule and Bosons are those which obey Einstein-Bose Statistics. Basically Fermions pair up only when there is a spin difference and Bosons just like each other and are not fussy in occupying same state.
The question is why do objects fall into those category types?

Scanning Electron Microscope (SEM)

Finally after lots of huffs and puffs i have managed to coat few devices with Anti-reflection coating (silicon dioxide). Results from L-I measurements show some effects of the coating.

As the coating lowers reflectivity of the mirror facet, we expect the external differential efficiency to increase (this is interesting because one would expect other way round). Finally i can enjoy the SEM image showing the coating onto the facet of the edge emitting LASER.

New Sample Holder Design

Frustrations can arise when you try and try to make something to work. Yes i have finally designed a clip that will make my life easier. I have devices which are 1mm long so it was tricky to hold them so that their 0.35mm facet can be coated. it is simplest but does the job with minimum annoyances. PhD is not all hanky-dory thats why when you hear African government leaders with fake doctor-hood from fake institute it tripple pisses you off.

There is alot of emotions involved in research there are days where you wonder whether you are in the right course or you made a right choice and other days where you feel like you reached heaven.

A week on Clean Room

Adorned in full body gear to rid of all falling particles like a surgeon ready to perform a complex procedure. The week spent in clean room was worth the hassle. i am now an expert in operating the Nordiko 2000 Physical Vapour Deposition machine. this ex-industry machine can be used to deposit thin layers of materials onto substrate placed above the target chamber where target materials are turned into plasma by application of RF power on a high vacuum chamber.

Dealing with laser devices of 1mm length and try to coat their facet which are about 350 micron wide is not trivial matter. I am hoping to perfect the technique in the coming weeks.

I have mapped the deposition area using 6 inch silicon wafer. Using Ellipsometer i measured the wafer after an hour of exposure and the rate of deposition is 3.3 nanometer per minute. This information is crucial for better results in future operations.

Conference at Sheffield UK SEMICON

I have attended this conference for two years in a row. A lot of interesting research results have been presented. It was my first platform to ever present my work at a scientific audience. Semiconductor devices have many applications from communication to medical and environment to military.

Remote sensing of dangerous particle polutant in the air, water and soil. each particle have a unique signature in absorption spectrum which enables us to identify it.

Long distance communication by using optical fiber enables us to send huge amount of data at low loss of signals at the special communication windows 1.33 micron where dispersion of glass fiber is minimal and 1.55 micron wavelength where signal loss is minimum due to minimum absorption of light.

Hakki-Paoli Method to Measure Laser Gain

Hakki-Paoli Method is a common method to measure semiconductor laser gain in Fabry-Perot Cavities. This method estimates the gain by the depth of the modulations in the Amplified Emission Spectra(ASE) caused by the Fabry-Pérot resonances of the laser cavity below threshold. One factor affecting such measurement is the dependence on the Optical Spectrum Analyser (OSA) resolution to fully resolve the modes.

Why Sputtering Silicon Oxide on InP based device?

Previously i wrote about my efforts to deposit a thin layer of silicon dioxide on a laser device. The aim was to create an anti-reflection coating that will enable me to measure the gain of the laser device using a single pass method on segmented device.

The thickness of the coating will depend on effective refractive index of the device and other factors. The objective is to create quarter wavelength layer to prevent reflection of light in the cavity.

This gain measurement method is different from Hakki-Paoli Method. This method was pioneered by Prof. Smowton and Prof. Blood, where the device is divided into equal segments with longer absober segment on one end. When this segments are pumped separately can provide a way to measure absorption of the active medium and hence we can calculate modal gain. With this information we can gain knowledge on how to improve the device performance..that is the MAIN GOAL..

Laser Physics at Glance

Laser is an acronymn which stand for Light Amplification by Stimulated Emission Radiation. There are so many types of lasers ranging from gas lasers to semiconductor lasers. All use the same physics to achieve their intended functions. Lasers can be used to correct eye defects, but also Lasers can be used to cut diamond the hardest material on earth. Lasers are used in industries to cut metal like knife cuts butter. The most amazing thing is that they all use LIGHT.

How does Light become such formidable force? The answer lies on the properties of light. As i Have mentioned in earlier posting light travels with different wavelengths. From the light you can see to the invisible light (Infrared). These wavelength are directly connected to their energy. The smaller the wavelength the powerful it is. This is to say red light is less powerful than Orange light and orange light is less powerful than green light which is less powerful than blue light. I think you get the picture that when wavelength is getting smaller its energy increases linearly.

one can use this equation that Energy in eV is equal to 1240/wavelength in nanometres. The 1240 figure is from simplification of an equation containing mainly constants Plancks, Speed of light and etcs. This fact is used in your DVD technology since you can write and read using light. if you had a disc player it used red laser, you upgraded to DVD which uses green lasers meaning more information can be written and read by the laser.

Now Blue Ray uses blue lasers which has smaller wavelength compared to the previous technologies and hence can write more information on the same disc and also read it. If Ultra violet laser comes along Blue Ray will be obsolete too just like the VHS.

Next how does Semiconductor laser works? It should be fun and amazing at the same time.....

Wonders of Light Part 4

We can use light to communicate and also we can use the light to generate power. I am talking about Solar power. The light is powerful we can manipulate that power to generate electricity using solar panels. Solar panels are using the fact that light particle or a PHOTON can excite an electron from its ground state to upper state which is excited state. We know that electricity is simply the movement of electrons.

So if a photon can knock out electron forcing it to move to another state we have generated electricity. By using solid state physics we know that if we make a P-N junction using Silicon (sand) we can achieve that. We will let the photon to force the electrons in the junction and we will collect those electrons on the other side and that is electricity generation.

But solar power is not that powerful i hear you say!! Silicon based solar panels are not efficient. The Light spectrum is huge and silicon cover a tiny amount of that spectrum. It is all about how much the material can absorb that light. So new material with larger spectra range of absorption are being investigated to improve the solar capture.

Hopefully in the near future we can harvest the majority of solar light we will solve all our Energy problems....

Wonders of Light Part 3

The mighty sun is our close source of light, if it goes we go..scary isn't it. There are about 200 billions suns in our galaxy alone, that is almost to say there as many suns as the sand in the desert. the numbers are just humongous and that should give you a reflection on your daily life. Unfortunately or fortunately the distance between us and other SUN bodies are hundrend thousands, some millions, and some billions of light years away. Only light can travel that distance.

Knowing that fact is enough to send a chill down my spine. if our sun can swallow million earth size bodies and out there there are sun that are so humongous that they can swallow million bodies of our sun size. And if you think that is big there are even bigger objects in the universe.

To travel at the speed of light you have to be massless otherwise the equation E=mc2 will hold you back. Why? because for every kg will need astronomical amount of energy that does not exist. So it is wishful thinking about travelling at the speed of light.
so here i use the word PHOTON to mean a quanta of light.. in plain english a particle of light with well defined energy.

Wonders of Light Part 2

So we know a white light is made of countless number of individual lights of different wavelength. the difference in wavelength is the one that give the RAINBOW its colour. Isn't that AMAZING!!, if you think is not then i don't know what could be more amazing.

This what Isaac Newton saw and by using a prism he was able to create his own rainbow..I tell you people of that time were mesmerized. He went on to write alot about light how it was reflected and refracted. Einstein was also fascinated by light and his work on light gave us these modern technologies we have today.

Typically we use wires to transmit data information and radio waves. The problem is wires have a finite bandwidth and can only travel at the speed of propagating electrons. You may say but electrons are faster..the answer is yes but look at the medium it is travelling in..copper wire act as a resistor to the electrons and hence slow them down and data deteriorate along a short distance compared to light travelling on a fiber optic cable.

The light has huge bandwidth meaning many information bundles can be crammed into a fiber. So we create a light source which is coherent (LASER) and use a modulator and start chucking those 101001010100101010101 onto fiber optic cable from USA to Kawe Kinondoni at the speed of how fast we can modulate the light. hahaha you say to the internet cafe attendant, "net today is much faster". The guy is sweating vigorously and i wont say what he is watching on the net..

The wonders of the Light

Light has been the most fascinating entity in my life. Without light there would have been no significant life. Am saying this because light is the source of energy that advanced living things use to process their material needs.

Light is the only fastest entity in the universe. It travels for billions, trillions of miles until we run out numbers and hence we say some distance should be measured using the amount of distance travelled by light over the period of one year. A light year this is huge number. if the light from the sun takes about 8 minutes to reach earth, and you know the distance between the earth and the sun is about 100 million miles (it is not fixed due to the shape of the orbit). Just imagine the distance of one light year!! (you can do the maths).

What is more fascinating is how we can use light for communications.. thats is what am doing it is mind bongling when you open your laptop or desktop and Voiler the pictures appear before your eyes. Using fiber optic cables we can connect the world and you won't need to wait the downloading bar will be a thing of the past. You click and it appears.

Question is how do we do that? Light is made of many individual lights of different wavelength. When you see a rainbow it is the white light broken into its individual components..RED, YELLOW, BLUE beautiful scenery indeed. and that is one of the wonders of light.

More will be coming let me put my device on the probe station I am measuring light output versus injected carriers.

Big Bang Theory or Theories

Billions of years ago, the universe was no more than one entity of singularity. Then KABOOOOOOOOOM it exploded into smitherinies according to some big bang theory... anyway who am i to say it did not happen when all Astro-Physics evidence shows it happen that way. I agree with some difficult questions that gives me headache every time i think about plausible explanations.

Soon after the big KABOOOOOOOOM single atoms of hydrogen started forming and the force of the explosion proppelled these gaseous atoms in all directions. Some kind of force (some suggested that it is the gravity) forced these atoms to aggregate and the process is known as accretion. (I think is another type of phenomenon and not gravity, at this point gravity was almost insignificant to pull atoms together)

These atoms kept on assembling and the "force" kept on forcing them closer together until nuclear fusion started to take place. from single hydrogen atoms everything formed. hydrogen fused and make more heavier atoms and heavier they became the bigger the object grew until it became a huge star a nuclear fusion reactor...

Then when the STAR goes Kaboooom tooo thats when the remnants form another star much smaller that previous and planets start forming from the left over materials. As time goes by these planets inhabit special orbits around the SUN which is the star and if the planet reaches a certain weight it too can force the left overs to form moon or moons that will be orbiting the planet.

I once suggested the existence of "bands of influence" on every object in this universe. One can observe this by looking at the influence of moon on earth and the influence of other bigger planets on small planets.

I am intending to explain my theory of the "bands of influence" in more details in the near future. Going back to KABOOM theory those particles from the explosions are the materials that made you and me and everything you see or not see. So you and I are part of the old hydrogen atoms that made the star. Thats why your body is made of Hydrogen, Carbon, Iron, Copper, some traces of Gold, Zinc, Calcium, and everything you don't know.....

A question to you..why sphere is the favourite shape in the universe?

Photon10 Conference in Southampton, UK

This was most interesting conference at last i had a chance to meet the greats in the world of intellectuals who pioneered semiconductor technologies. From theoreticians to fabricators to those who analyse ever complex world of quantum physics. I met Prof. P. Blood who is one of the giants i found out he was a friend to my Profs Profs, Prof. Adams who is in the list of 10 most influencial brits. He suggested the use of strain in semiconductor devices to improve their performance in 1980s which led to billion pound industry.

Yes Blood was a critic on my findings of characteristic features of Quantum dot lasers. "Using High hydrostatic pressure i am able to alter the atomic spacing in the crystal lattice" i said. In doing so am able to change the effective band gap of the device and hence change its operation parameters which in turn gives me an insight of what is really going on in the LASER device.

After my presentation he was happy and was looking forward to my paper that will be published by APL Journal. Anyway the paper was published two month ago..it is called "Efficiency Limiting Processes in 1.55 micron InAs/InP-Based Quantum Dots Lasers". You can google it!!!

My aim is create Temperature insensitive laser device that can be used in long transmission optical communication systems.. what i am doing in simple english is create a laser that will not be affected by changes in temperature for systems like SEACOM fiber. the SEACOM fiber depend on whole load of lasers and modulators to send data that you see on your computer screen faster.

Why are we going NANO?

Nano technology is the new envelop pusher for the existing advanced technologies. BASICALLY it is a technology that utilise the dimensions of individual atoms. its potentials are endless most are very useful and some very dangerous indeed. Applications ranging from medical to weaponry.

Yeah i vividly remember when i put on a graduation gown and the emcee called out my name for me to proceed and receive my degree certificate in Nanotechnology from the Chancellor who initiated as small talk as to what i was going to do with this knowledge, it downed on me that where I am from even the word NANO does not exist.

The technology that has led billions of dollars being invested by super rich countries seems to be invisible as it its nature suggest to African Universities i spare my thought on our political nearndathals.

This technology will make African produce obsolete as raw material. This technology which revolutionise the global trades and make poor countries even poorer has not registered in the mind of most third world intellectuals.

What am saying is instead of developed countries buying 100kg of cotton using NANOTECHNOLOGY they will only need 1kg. So the question is poor people of the world what are you going to do with 99Kg of your back breaking labour when you have not grasped the current manual technology?

Am sure you will be asking so what did you do.....I TRIED yes i tried to arrange some way for me to give a talk in one of the "prominent" university in east africa (UDSM) but they never replied...so hopefully someone will read this and spread the word that we should fully engage ourselves in new technologies to know their impact on our very life and the lives of our future generations......

IPRM Conference in Kagawa, Japan 2010

This conference was a highlight of 2010 for my PhD studies. Woke up early in the morning to catch a plane from Heathrow to Dubai (DXB) before boarding at early hours of morning for Narita Airport in Japan..it was looooong journey on the plane, reaching Japan in the evening mind you if travel from west to east you kinda lose a day along the way. Soon after touch down and take care of Immigration issues, boarded a train to a hotel nearby at Narita City center.

The next morning boarded a train to Tokyo Train station for my booking with Bullet Train (Shinkassen), from Tokyo to Okayama, about six hours which means travelling the length of the country..I should have booked for Kansai City Airport in Osaka, anyway i wanted to see the country since i had a day before the conference.

So i stayed awake with huge jetlag hanging over my head. I admired the smooth ride in a world fastest train reaching speeds that will make Jumbo Jet fly away. Stopped ocassionally along the way, Yokohama, Kyoto, Osaka, to Okayama. The luck on my side did not wait long for a train "Mariner" that goes to Kagawa through some islands in the sea of Japan. Railroad on bridges with longest of spans, mind you it give you a thought of how much people laboured to put every bolt and a nut to hold this humongous structure. I came to a conclusion "smallest of living make the hugest of things".

Sunday passed as quickly as it came. On monday morning i had a presentation in the world stage. Room full of knowledgeable people (Professors and Doctors and PhD students) from all over the world sitting quitely listening to world novel research findings on Photonics. When my slot came, I climbed up the stage and presented my recent results on Quantum dot lasers operating at 1500nm wavelength grown on (100) facet of InP substrate. The Question came went and after final clapping had dissipated from the room i went to prepare for my next presentation that was on a poster. This was on Quantum well devices also operating at this 1500nm wavelength.


Days quickly passed and had time to travel to Osaka and stayed there for few days at Monterey Grasmere hotel in Osaka city center. The only negative about this trip was the food.. Plenty to eat if you don't have religious restriction..But God showed me a buffet restaurant at Osaka Port, the only problem was it Osaka Port was about three train changes from the hotel.

I had a two weeks Bullet Train Travel ticket so i went everywhere. Trip to Hiroshima was well worth it..I will never forget the bowl of rice and fish at Hiroshima Mall.

One thing i noticed, Apart from me I spotted two other africans in the whole conference..and none was representing African University or a african company..It is sad but true.

Why did I end up doing Quantum physics

It is a very long story, but in short i found a knowledge that makes sense of it all. The world and its contents, the universe and its wonders. basically the knowledge that tells you that our daily scrubblings are of least significant like the way you see a grain of sand in the desert. it is all about "WAVES", whether its oceanic waves or radio waves, microwaves, electro-magnetic waves, visible light waves, invisible light waves, X-ray waves, Gamma waves and whatever you call them and those other waves that we don't even know about. Your life is all about waves from the very atom that made the molecule that made the cell of your body to the car, house, computer and everything that you use or don't use.
Yeah why am i doing this while i know that i can not get work back in Africa, and here am saying Africa because to me Tanzania is no different to Kenya or any other African countries on their attitude to knowledge especially the one that they can not associate with huge amount of money. This is massive in terms of money for people of knowledge..I am doing it because i want to be selfish. Selfish in coming home and propagate the knowledge whether it be single or many handed.. because we are using it in our daily lives, if you use your mobile phone you are using quantum physics, when you surf on the net you are using it. therefore there is alot of money associated with it..if you have eyes to see greens from political blues.

Anti-reflection Coating using Nordiko 2000

Today, i tried to coat my laser devices with a thin layer of anti-reflection coating made of silicon dioxide..well the machine to do the job is called Nordiko 2000, physical vapour deposition. it is a big machine with pumps to create both low pressure and high vacuum needed for deposition of the material. when RF power is increased you can see a glow of plasma...WOOOOOOOOOOW. Plasma is a "fourth state of matter", where you have gas atoms stripped of their electrons. (this is what make your Plasma TV to work)...anyway after creating a plasma composed of mixture of Argon and Oxygen, that will evaporate your target material in my case silicon dioxide. then when the sample chamber is placed on top of this plasma, a thin layer of silicon dioxide is deposited at the rate of 3nm (nano metre) per minutes...which means an hour for me to wait for my calculated thickness to be achieved..oh there is a computer screen which is like a dino for its age..the menu is user friendly if you have Einstein memory..you need to make sure RF power is applied gradually and reflection is kept minimum and your efficient values are higher.