Optoelectronics Lab

Introduction

The main research activity of this laboratory is based on the industrial and semi-industrial projects in the field of optoelectronic and semiconductor devices. These projects are done by the PhD and MSc students. Some of these works that will be explained in the section “Projects and Research Activities” are the design and simulation of photonic crystal devices, photonic crystal fibers, semiconductor lasers, photo detectors, solar cells and so on.
At present, the most researches in this laboratory are on the solar cells. One of the projects is being donein order to fabricate the laboratory sample of solar cells based on GaAs. Firstly, the design and simulation of the proper solar cell structures such as GaAs and AlGaAs/GaAs has been done. Then, the fabrication of the primitive sample is being donewith choosing the best structure and finally the final sample will be fabricated after the test and optimization of samples. Also, one of the main issues for improving the solar cell characteristics is utilizing the suitable antireflection coating and presenting the novel structures for it. In order to achieve these purposes, we are working on the antireflection coatings used in new generation solar cells such as thin film and quantum dot solar cells and cells based on the concentrators.

People

Projects

 Projects and Research Activities

 Design and Fabrication of GaAs Solar Cells :

As can be seen in the most papers, choosing the GaAs for achieving the best efficiency is the good choice because of its 1.424eV band gap energy. In the first step, we did a complete study on the types of solar cells. Then the design and simulation of the proper solar cell structures such as GaAs and AlGaAs/GaAs has been done. In the next step, the fabrication of the primitive sample is being donewith choosing the best structure and finally the final sample will be fabricated after the test and optimization of samples.

 Investigation of the Electrical and Optical Properties of the Antireflection Coatings in the Third Generation Solar Cells :

One of the main issues for improving the solar cell characteristics is utilizing the suitable antireflection coating and presenting the novel structures for it. In order to achieve these purposes, we are working on the antireflection coatings used in new generation solar cells such as thin film and quantum dot solar cells and cells based on the concentrators. We are studying on the several types of materials utilized for antireflection coatings, optical properties of these materials and the different structures. There is an expectation which the novel proposals for one layer and multilayer antireflection coating can be presented.

 Design and Modeling of WDM Photonic Crystal Integrated Devices Based on the Interaction of Cavity and Waveguide :

In summary, the resonance frequencies and the field distributions of 2D-PC were investigated. PC hybrid waveguides with quasi-flat and Lorentzian transmission spectrum were analyzed and modeled by using FDTD and CMT methods. The theoretical results derived by CMT were in good agreement with FDTD simulation results. A modified HW3 with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used in implementation of WDM filters. It was shown that in this case phase-shift is close to zero. Transmission of ultra-short pulses through the hybrid waveguide was also investigated. In this work, a low cross-talk and wideband waveguide intersection design based on two orthogonal hybrid waveguides in crossbar configuration was proposed and modeled by using the FDTD and CMT methods. In addition, it has been clearly proved that simultaneous crossing of ultra-short pulses through the intersection is possible with negligible interference. The proposed solution can be easily generalized to other 2D square as well as 3D cubic PCs. A modified HW2 with extra rods in both ends of the CCW and Lorentzian transmission spectrum was proposed, which can be used as the key element in implementation of WDM filters. It was shown that the phase-shift of the EM waves traveling between the modified HW2 cavities is close to zero. According to the theoretical theory using CMT in time, the performance of the proposed CDF was investigated and the conditions which lead to 100% drop efficiency were extracted.

 Design of a Novel Structure for Photonic Crystal Fiber and Optimization of Its properties :

Photonics crystal fibers (PCFs) have attracted a great deal of attention in the optics community in recent years. In this work, new structures for PCFs are introduced. Also for the first time, a combination of intelligent methods is applied for optimization of these fibers.
PCFs have particular properties that are caused by their cladding structure and its ability of controlling the effective refractive index. The particular properties mentioned are ability to remain single mode at all wavelengths, great controllability in chromatic dispersion, low confinement loss, high values of birefringence, high nonlinearity, high numerical aperture and low bending loss.
The most common techniques for analyzing the PCFs are finite difference time/frequency (FDTD/FDFD) methods and the finite-element (FM) approach. In overall view, there is no complete analyzing method. In practice, the finite difference methods are very general and reliable (well established and tested) method. They are also accurate modal descriptors. But these conventional methods that simulate the optical characteristics of PCFs are computationally expensive and time consuming. In this thesis, an artificial intelligence method such as Neuro-Fuzzy system is used to establish a model that can predict the properties of PCFs. Simulation results show that this model is remarkably effective in predicting the properties of PCF such as dispersion, dispersion slope and confinement loss over the C communication band. The accuracy of this method is high about 98%.
Also this work proposes a combination of Differential Evolution (DE) and Estimation of Distribution Algorithm (EDA) (DE/EDA algorithm) to solve the optimization problem and to determine the PCF structure. This study is focused on the determination of the PCF structure that can lead to the minimum confinement loss and dispersion and nearly zero dispersion slope over C communication band. The optimized PCF exhibits a very low dispersion and ultra-low confinement loss at 1.55µm wavelength along with a nearly zero dispersion slope over the C communication band.

Equipments

Equipments of Optoelectronics Lab.

 Several Types of Gas and Semiconductor Lasers :

 Several Types of Photodiodes and Photo detectors :

 Optoelectronic Equipments Boxes :

 Frequency Meter :

 Other Equipments : 

Conferences & Journals

 Recent Conference and Journal Papers

Information

Useful Scientific Information

 Introduction to Solar Cells:

Nowadays, with the increasing energy consumption initiated mainly by scientific developments, aninevitable demand for new energy resources is sensed. This has made the human being to achieve the required energy by adopting different methods. Meanwhile environmental pollutions and limited natural resources are the dominant issue in front of energy production by conventional techniques. Hence the overall trend is to discover the renewable energies without the environmental pollution characteristics. The solar cell energy is one of the most important renewable energies which is in recent decades considered by human being. The sun born about 6000 million years ago and it is said that 4.2 million ton of its mass is converted to energy in each second. The core temperature of the sun is about 10 to 14 million centigrade degree while its surface energy with 5600 centigrade degree is propagated in electromagnetic wave form. The earth is located in 150 million kilometer distance from sun and the sun light takes about 8 minutes and 18 seconds to reach it. Although the amount of energy achieved by earth from sun is a small part of its radiation, but saved fossil energy in the earth, wind energy, and sea waves are the direct result of the sun energy. The amount of the sun energy which reaches the earth every day is sufficient for providing the one year necessities of all of the creatures on the earth.
Some of the advantages of photovoltaic cells can be expressed as follows:
• Sun energy resource is free and infinite.
• The cost of working with it is low.
• They have not any mobile parts.
• They have high reliabilities.
• High security
Solar energy can be converted directly or indirectly to the other types of energy such as heat and electricity. Photovoltaic effect can be used for converting the solar energy to electricity utilizing the solar cells. The basic structure of a solar cell is a p – n junction formed by semiconductor devices which produce the electrical energy with absorbing the light and migration of electrons between energy bands.
The principle of solar cells is the shining of light on a p – n junction and production of electron – hole pairs that makes the drift and diffusion currents. The condition of production of electron – hole pairs is that the photon energy be bigger than the band gap energy of the material used in solar cell. The band gap energy, Eg, is the energy level difference between the conduction and valence band of the semiconductor. Hence the band gap energy, Eg, is the necessary energy for transmitting the electron from the valence band to the conduction band and the additional energy is wasted in the heat form and the electron relaxation in the lowest point of the conduction bandtakes place. Therefore, the main condition for absorbing light by a material with the energy band gap Eg is as follows:

where h is the plank constant and ν is the frequency corresponding to the emitted light photon with wavelength λ = c/υwhich c is the light velocity.
The radial spectrum of sun in the surface of the earth is almost different from that in the space. Finally, the absorbed photons are in the wavelength range of :

But the photons out of this range couldn’t be absorbed and the crystal is transparent for them. The sun light that receives the earth is composed of these wavelengths: 47% infrared, 46% visible light and 7% ultraviolet. Hence the solar cells should have the higher absorption in the range of infrared and visible light. The optical path for light from a sky resource to the earth is called as the Air Mass (AM). The light is attenuated in passing from the aerospace in effect of scattering and absorption. The air mass in the out of aerospace (i. e. satellites) and the zenith at the sea surface is AM0 and AM1, respectively.In AM1, the sun light is normal to the surface of the cell. AM1.5 means that the sun is in the place that the radiation path length on the earth is 1.5 times of that in AM1 which is equivalent with the solar radiation spectrum with angle 48 degrees. The light intensity for AM0 and AM1 is almost equal to 1366W/m2 and 925W/m2, respectively.
The solar cells can be divided to some categories which is presented in follows in the time sequence:
• Single crystal silicon solar cells (c-Si)
• Polycrystalline (poly-Si) or multicrystalline (mc-Si) silicon solar cells
• Thin film amorphous silicon solar cells
• Organic solar cells
      Dye-Sensitized solar cells
      Polymeric solar cells
      Liquid crystal solar cells
Some of the last types of solar cells which the researches are done about them just in the primitive states are called the third generation solar cells.

 History of Solar Cells:

The photovoltaic effect was first recognized in 1839 by French physicist Alexandre Edmond Becquerel which observed the photovoltaic effect while experimenting with a solid electrode in an electrolyte solution when he saw a voltage develop when light fell upon the electrode. Becquerel whoseresearches was began from his childhood as a student, and then as an assistant, to his scientist father, observed this phenomenon in the age of 19. His father Antoine César Becquerel (1788-1878) was the discoverer of piezoelectricity. This effect firstly was studied by Heinrich Hertz on the solid materials such as selenium in 1870. But the first report of the PV effect in a solid substance appeared in 1877 when two Cambridge scientists, W. G. Adams and R. E. Day, described in a paper to the Royal Society the variations they observed in the electrical properties of selenium when exposed to light. However, in 1883 Charles Edgar Fritts, a New York electrician constructed a selenium solar cell that was in some respects similar to the silicon solar cells of today. It consisted of a thin wafer of selenium covered with a grid of very thin gold wires and a protective sheet of glass. But his cell was very inefficient. The efficiency of a solar cell is defined as the percentage of the solar energy falling on its surface that is converted into electrical energy. Less than 1% of the solar energy falling on these early cells was converted to electricity. Nevertheless, selenium cells eventually came into widespread use in photographic exposure meters. In 1887, Heinrich Hertz discovered that ultraviolet light altered the lowest voltage capable of causing a spark to jump between two metal electrodes.In 1888 Russian physicist AleksandrStoletov built the first photoelectric cell (based on the outer photoelectric effect discovered by Heinrich Hertz earlier in 1887). In 1904, Wilhelm Hallwachs discovered that a combination of copper and cuprous oxide is photosensitive. In the next year Albert Einstein published his paper on the photoelectric effect. The lower efficiency impasse was finally overcome with the development of the silicon solar cell by Russell Ohl in 1941. The modern photovoltaic cell was developed in 1954 at Bell Laboratories. The highly efficient solar cell was first developed by Daryl Chapin, Calvin Souther Fuller and Gerald Pearson in 1954 using a diffused silicon p-n junction.
The next researches after the paper of Einstein about the photoelectric effect can be followed as:

The best research cell efficiencies can be seen in this graph.

 From:

  www.wikipedia.org

  http://solar-module-panels.com/pv/history-of-photovoltaic-pv/

  www.eere.energy.gov

  www.britannica.com

 Types of Solar Cells: 

In the different sources, the various categorization saboutthe solar cell shave been made. Here, one of these divisions is presented.

1. Crystalline Silicon Solar Cells

The most prevalent bulk solar cells are crystalline silicon (c-Si) solar cells. According to the type and size of the crystal, the bulk silicon crystal can be divided.

• Single-crystalline silicon (c-Si)
• Poly-crystalline (poly-Si) or multi-crystalline (mc-Si) silicon

2. Non-crystalline (amorphous) silicon thin film solar cells

The low cost is one of the advantages of solar cells based on the amorphous silicon (a-Si). Silicon and hydrogen are two components of the a-Si alloy. Moreover the characteristic of an a-Si alloy is that have a high absorption coefficient. Only a thin layer is required to absorb light and so the cost will be reduced.

3. Thin film GaAs Solar cells

The first condition for materials that should be used in a photovoltaic solar energy conversion device is matching the band gap with solar spectrum and having high mobility and high carrier lifetime.These conditions are met by many of compounds of II-VI, III-V and Si. The group of III-V despite the high cost of extraction and production of semiconductors, in space applications that cost is not important factor in them are used with great success. In 1961, Shockley and Queisser by considering a hybrid solar cellas a black body with thetemperature of 300 K showed that the highest efficiency solar cell,regardless of the typeof technologyused, is 30% and can be achieved for a cell with a band gap equal to 1.39eV.By considering that the band gap of GaAs is equal to 1.42eV, this material is suitable for solar cells. Thin film solar cells made of GaAs is named the second generation of solar cells.

4. Solar cells based on organic material

Solar cells made from organic materials are less efficient than their other counterparts. But due to low cost manufacturing capability and flexibility are suitable for nonindustrial usage. The solar cells based on organic materials include Dye-sensitized solar cells, polymer solar cells and solar cells based on liquid crystals.
• Dye Sensitized Solar Cells (DSSC)
The basic structure of a DSSC is importing of an optimized n-type transparent semiconductor (with wide band gap) in a lattice of nanoscale columns in contact with nanoparticles or coral-shaped blugings.

Schematic of DSSC
(S. J. Fonash, Solar Cell Device Physics (Second Edition) Elsevier, 2010.)

The surface of lattice is covered by a single layer of dye or quantum-dots that act as dye. Then electrolyte is used to achieve a channel between dye and anode. Dyes absorb the light and generate exciton, whichwill be separated in the interface of semiconductor and dye and cause to create electrons with photons for semiconductor and oxide dye molecules by electrolyte.

• Polymer solar cells

Polymer solar cells have special properties. Because of solubility of the active materials in this type of cells in most of organic solvents, polymer solar cells have the flexibility potential and ability to be fabricated in a printing process similar printing newspapers.

The ability to build of polymer solar cells as continuous manufacturing process
(Nat. Photonics, vol. 2, pp. 287–289, 2008.)

Recently power conversion efficiency about 6% is reported but this value is very less than the efficiency that is needed in typical applications.

• Solar cells based on liquid crystals

In some types of these solar cells, pillar liquid crystals are used for the fabrication. A group of liquid crystals can be in the pillar state. The pillar state is the case that the constitutive molecules of liquid crystal are placed like a disc and make some pillars. Previously, this group of liquid crystal called the disc liquid crystals.

5. Solar cells based on quantum-dots

A limiting factor for energy conversion efficiency of solar cells with a unit energy gap is that the energy of absorbed photon is higher than the band gap of semiconductor is wasted due to the interaction of electron-phonon until the carriers reach to the edge of band gap called relaxation.

Solar cell based on quantum-dots
(IEEE Transactions on electron devices, vol. 49, pp. 1632-1639, 2002.)

In recent years, new methods for reducing these losses are presented by using quantum structures like quantum dotes and quantum wells.

Improvingthe efficiency in photovoltaic solar cells by using quantum dot impact ionization
(Phys. Rev. B, vol. 60, pp. R2181-R2184, 1999.)

In these structures when carriers in semiconductor limited to certain areas by the potential barrier that are smaller or comparable to their de Broglie wavelength or Bohr radius in bulk semiconductor, relaxation dynamics will be quite different.

 Solar Cells Equivalent Circuit:

Solar cells can be modeled as a current source in parallel with a diode.  When there is no light present to generate any current, the solar cell behaves like a diode.  As the intensity of incident light increases, a current corresponding to light intensity is generated by the cell.In an ideal cell, the total current, I,is equal to the current, IL, generated by the photoelectric effect minus the diode current ID.A more accurate model will include two diode terms.
With these explanations and using the equations incorporated in solar cells, the following equation can be achieved for a solar cell.

where n is the diode ideality factor (typically between 1 and 2), and RS and RSH represents the series and shunt resistances that are described in further detail later. Also, the simplified equivalent circuit for a solar cell can be shown as follows:

Figure 1. Equivalent circuit of solar cells

During operation, the efficiency of solar cells is reduced by the dissipation of power across internal resistances.These parasitic resistances can be modeled as a parallel shunt resistance (RSH) and series resistance (RS), as depicted in Figure 1.For an ideal cell, RSH would be infinite and would not provide an alternate path for current to flow, while RS would be zero, resulting in no further voltage drop before the load.As shown in Figure 2, if RSH is decreased too much, VOC will drop, while increasing RS excessively can cause ISC to drop instead.At last, decreasing RSH and increasing RS will decrease the fill factor (FF) and maximum power.

Figure 2. Effect of diverging RS and RSH from ideality

It is possible to approximate the series and shunt resistances, RS and RSH, from the slopes of the I-V curve at VOC and ISC, respectively.  The resistance at Voc, however, is at best proportional to the series resistance but it is larger than the series resistance. RSH is represented by the slope at ISC.  Typically, the resistances at ISC and at VOC will be measured and noted, as shown in Figure 3.

Figure3. Obtaining Resistances from the I-V Curve

The series resistance is raised from the resistivity of the cell material in front of current flow, especially through higher surface to the contacts. This resistance is the main issue in higher density currents such as the situation in front of the focused light. Also, the parallel resistance can be created with the current leakage from the edges of the device and contacts of different poles.

From:
http://zone.ni.com/devzone/cda/tut/p/id/7230
www.wikipedia.org

 Antireflection Coatings:

The colorful world around us, or the science beneath, is heavilyabsorbed in ‘reflection’. However, the fact that the quantum concept, golden to the scientists, lay in a perfectly absorbing blackbody led us to look at things that suffer, if we may say so, from the lack of reflection. The anti-reflecting materials are interesting not only as high efficiency absorbers but also for the properties that lend them this character. To understand this, we need to know the production of colors from a surface and ways to control them. We can broadly classify the color production mechanism intodiffraction, interference, scattering, absorption and emission processes. For example, the color produced in thin film sorsoap bubble and that in a peacock’s tail feather has been attributed to  the interference phenomenon, the color of the sky to Rayleigh scattering  and the rainbow to diffraction. Assuming we have abroad working knowledge of these color production processes,we only need to study the ways to control them from leaving a surface so as to gauge its anti-reflecting properties. With the advent  of wave optics, when we characterize individual colors by wavelengths , these two phenomena, color production and antireflection become  closely associated. In most cases a ‘non-motion structural’ green object appears such due to the antireflecting property of that object to other non-green wavelengths. The green in the peacock’s feather appearing due to interference effects is also ‘non-motion structural’ or physical color.Suchphysical colors are common in nature and can be found in thebutterfly wings, the cuticle of arthropods such as beetle, feathers of peacock or hummingbird, Indigo snake skin,or crystalssuch as Opal. In contrast, the green or any othercolor of the Chameleon (family Chamaeleonidae) appears due tothe presence of skin chromatophores, which can be stimulatedphysiologically. Such colors or color changes that are taxonomicalor have behaviouristic nature are called ‘motion structural’colors and is not the subject of this review. Note, however, thatwe will be neglecting the emission process of color productionmechanism from our discussion.
If we study nature’s strategy for color production, in arange of species mentioned above, we observe a strangecommonality. That is, the colored surface is most often microoreven nanostructured and is not a flat or smooth one. It iscommon, real world knowledge that smooth surfaces shine morethan the rough ones. This simple sentence contains the very basicidea of anti-reflection (AR) that in order to reduce reflection or shine one has to impart roughness to the surface. The effect iscommon but the reason is not. Striving to understand this Newtoncoined the word ‘optical density’ which later became refractiveindex (RI, denoted by ‘n’) that characterizes a material mediumaccording to the velocity of light in it with respect to that in air. Anychange in the velocity of light, caused by two media havingdifferent RI, will then be detected as an optical disturbance by aneye or a photo detector. This optical disturbance or reflection willbe zero if the medium for light propagation does not change or thetwo media have the same RI. For example, a glass immersed inwater appears invisible, that is reflection-less, due to matching RI.Vacuum or, more practically, air being the all-pervading media, anobject becomes visible, or reflecting, due to a contrast in their RI. Inother words, a matching RI of an object with air (n = 1.0) makes itnon-reflecting and invisible. Hence for all practical purposes ofoptical AR, it is desirable to have a medium of low RI as close to 1.0as possible. Few examples of such media are MgF2, CaF2 and SiO2having n = 1.39, 1.44, 1.46, respectively. However, the problem of achieving low RI materials isnon-trivial and made more complicated because of the knownwavelength dependence of the RI. In principle, a matching RI in theultra violet (UV) part of the spectrum resulting in low reflectancedoes not ensure the same in the visible (VIS) or infra-red (IR)segments—that it is difficult to obtain broad band, over largewavelength regimes, AR using a uniquely designed material. Itsperformance will differ in different wavelength ranges. Lord Rayleighwasthe first to demonstrate, mathematically, that gradual transitionlayers with slowly varying density, which we can now correlate toRI, can work as AR coatings. Although such idea did work in deviceswith sound and light, it failed at grazing angles ofincidence (AOI) underlining the angle-dependence of ARproperties. For example, the color in the soap bubble or in thepeacock’s feather appears different when viewed from differentangles. It would not be difficult for the reader to convincethemselves that the interaction, and hence reflection or colorproduction, of a structured surface with electromagnetic wave will depend on the thickness of the surface and polarization of the lightalso as we will discuss later on. We complete our logical build upthat color production and AR effects are in fact related termscontrolled by RIs, which have a wavelength, viewing angle andpolarization dependence.
As it was mentioned the rate of absorption will determine the refractive index is because if the refractive index be closer to the environment then more light will be passed or absorbed. According to this in application of anti-reflections in solar cells try is to use a layer of the refractive index is closer to environment, but  not only matching with the refractive index is important but also there must be coordination with the substrate, so the anti-reflectionsdivided to different categories that include:
• Single layer
• Double layer
• Multi-layer

Single layer

Fabrication/application of single-layer AR coating is limited by the availability of proper material. For example, the RI of glass and transparent plastic substrates are nS~ 1.5, therefore RI of the thin film coating, with quarter-wavelength thickness, must require n ~1.22. Unfortunately, materials with RI~ 1.22 are rare. As depicted, the materials with low RI, such as magnesium fluoride (MgF2, n = 1.38), can be selected for AR coating to reduce the reflectance from 4.26% (uncoated glass) down to 1.5%. Nevertheless, it is still impractical to achieve exactly zero reflectance from a single-layer AR coating.

Double layer

The application of a double-layer, with each layer of quarterwavelengththickness, can improve the performance of AR coatingif a high-index-layer (e.g., Al2O3, n1 = 1.69), while n1>nS, is coatedprior to the coating of low-index-layer (MgF2, n2 = 1.38

Reflectance as a function of wavelength for anti-reflection coatings: (a)single-layer AR coating:Air (n0 = 1.0)/0.25t-MgF2 (n = 1.38)/Glass (nS = 1.52); (b)two-layer AR coating: Air/0.25t-MgF2/0.25t-Al2O3 (n1 = 1.69)/Glass; (c) three layerAR coating: Air/0.25t-MgF2/0.5t-ZrO2 (nm = 2.05)/0.25t-CeF3 (n1 = 1.64)/Glass.
(Chattopadhyay et al,Materials Science and Engineering R 69 (2010), pp. 1–35.)

Multi-layer

The double-layer configuration can decrease the effectivereflectance at a particular target wavelength (λt), but simultaneouslyincrease the Reff on both sides of it, higher than that ofsingle-layer AR coating. Thus, the number of layershas to be increased if a lower Reff, on both sides of it, or a broaderAR region is required.The three-layer design of AR coating (quarter-half-quarterwavelength design), involves a thicker layer (M, λt/2 thickness)sandwiched between two λt/4 layers (H, L). The resultantstructure of the three-layer AR device can be represented assubstrate/H-2M-L/Air, where H and L are the high- and low-indexlayers, respectively. Simultaneously, the RI (nm) of the middlelayer, M, should satisfy nm> (n12/nS), in order to achieve thebroad-band AR effect. The effect of the 3-layer AR structure is shown in above figure.

Most of the information derived from:
Chattopadhyay et al,Materials Science and Engineering R 69 (2010), pp. 1–35.

 Solar Cell Scientists
    I. Alexandre Edmond Becquerel

Four successive generations of the Becquerel family were educated at the prestigious Frenchscientific institution of higher learning, the École Polytechnique, and became physics professors at the French Museum of Natural History. Alexandre Edmond Becquerel was the son of Antoine César (1788-1878), discoverer of piezoelectricity, father of Antoine Henri (1852-1908), who discovered radioactivity, and the grandfather of Jean Antoine (1878-1953), who is best known for his work on relativity and the discovery of polarization rotation in the presence of a magnetic field. Though all of these notable scientists carried out a variety of experiments related to optics, it was Alexandre Edmond Becquerel that made the greatest contributions to the field.
Born in Paris on March 24, 1820, Alexandre Edmond Becquerel became first a student, and then an assistant, to his father, and many of his investigations are associated with those of the elder Becquerel. Edmond was, however, particularly intrigued by light and embarked on in-depth studies of the subject. He examined the spectroscopic characteristics and the photochemical effects of the sun, and was especially interested in the phenomena of fluorescence and phosphorescence. To aid in his study of the latter, Becquerel invented the phosphoroscope, a device capable of measuring the duration of time between the exposure of a solid, liquid, or gas to a light source and the substance’s exhibition of phosphorescence. Through the use of the phosphoroscope, the physicist was able to more accurately determine whether or not certain materials exhibited phosphorescence or fluorescence, since one of the key differences between the phenomena is the duration of the excited state lifetime, which is the interval of time that passes before the effect occurs following the absorption of light (fluorescence occurs almost instantaneously after excitation, whereas phosphorescence is characterized by a much longer excited state lifetime). The phosphoroscope also enabled Becquerel to discover phosphorescence in a number of materials that were previously not believed to exhibit the effect.
Alexandre Edmond Becquerel’s work with fluorescence and phosphorescence led him in the late 1850s to develop the idea of utilizing these effects in light sources. He experimentally applied various luminescent materials as coatings to electric discharge tubes, and though he never succeeded in producing a commercial light, his work eventually led to the development of the fluorescent lamps that are widely used in modern times. Findings from many of Becquerel’s studies of luminescence and other areas of optics were published in 1867 and 1868 in the two-volume treatise entitled La Lumiere, ses causes et ses effets. His appointment at the Museum of Natural History occurred in 1878, so that he directly succeeded his well-known father, and he was a member of the Académie des Sciences from 1863 to his death on May 11, 1891.

From: http://micro.magnet.fsu.edu/optics/timeline/people/becquerel.html