CLIAMS:We claim:
1. A method for obtaining stimulated emission from a quantum dot, the method comprising:
identifying a medium;
synthesizing a plurality of quantum dots having a plurality of layers of semiconductor;
incorporating the synthesized quantum dots to form a gain medium;
exciting the quantum dot with a source to generate carriers; and
partitioning the generated carriers
Wherein partitioning of carriers into at least two distinct layers of the quantum dot results in controlled stimulated emission within the gain medium.
2. The method according to claim 1, wherein the medium is selected from the group comprising of a solution, a polymer, a glassy matrix, an inorganic matrix or a close packed quantum dot solid.
3. The method according to claim 1, wherein the semiconductor nanocrystal is at least one type-II semiconductor selected from the group comprising CdS, ZnS, ZnSe, ZnTe, CdSe, CdTe, PbSe, PbS, PbTe, InAs, InSb, GaAs InP, a semiconductor alloy or a combination thereof.
4. The method according to claim 1, wherein wherein the source is at least one selected from the group comprising of a continuous wave source, a pulsed wave source, a monochromatic collimated beam of electromagnetic radiation, a natural source of electromagnetic radiation, an amplitude modulated electromagnetic radiation, a semiconductor source of electromagnetic radiation or a combination thereof.
5. The method according to claim 1, wherein the partitioning of carriers occurs upon cooling of the quantum dot subsequent to the excitation.
6. The method according to claim 1, wherein the stimulated emission results in an amplification of the light in the gain medium.
7. The method according to claim 1, wherein the controlled spontaneous emission has a lifetime greater than 150ns.
8. A quantum dot light amplifier comprising:
A gain medium; and
A plurality of quantum dots with a plurality of layers of semiconductor formed in the gain medium,
Wherein partitioning of at least one carrier of the semiconductor nanocrystal into distinct layers occurs upon excitation with a source, the partitioning of carrier resulting in controlled stimulated emission within the gain medium.
9. A quantum dot light amplifier comprising:
A gain medium; and
A preformed plurality of quantum dots having a plurality of layers of semiconductor
Wherein partitioning of at least one carrier into distinct layers occurs upon excitation with a source, the partitioning of carrier resulting in controlled stimulated emission within the gain medium.
10. The quantum dot light amplifier according to claim 8 or claim 9, wherein gain is produced and/or sustained in the gain medium by pumping with an optical excitation source.
11. The quantum dot light amplifier according to claim 8 or claim 9, wherein at least one layer of semiconductor nanocrystal separates the two distinct layers having the generated carriers.
12. The quantum dot light amplifier according to claim 8 or claim 9, wherein the gain medium is selected from the group comprising of a solution, a polymer, a glassy matrix, an inorganic matrix or a close packed quantum dot solid.
13. The quantum dot light amplifier according to claim 8 or claim 9, wherein the amplification of the light occurs in the gain medium. ,TagSPECI:A QUANTUM DOT LIGHT AMPLIFIER

FIELD OF INVENTION
The invention generally relates to the field of semiconductor materials and particularly to synthesis of a semiconductor nanocrystal material capable of amplifying light through stimulated emission when it is irradiated by continuous wave or pulsed illumination.

BACKGROUND
Semiconductor nanocrystal, referred to commonly as Quantum Dot, exhibit large (typically ~10-16 cm2 or higher) absorption cross sections over a broad spectral region, as well as tunable emission spectra. This should in principle make them highly suitable laser materials. Semiconductor nanocrystals synthesized according to many known techniques, prevalent in the prior art, exhibit extremely short exciton lifetimes of less than 1 microsecond (?s), at room temperature, for most materials. An example of most commonly exhibited exciton lifetime is about 10 ns. The extremely short exciton lifetime makes it necessary to pump nanocrystals with other high power, short pulsed laser sources, including but not limited to, Ti:Sapphire and Nd:YAG lasers in order to induce lasing. Conventional Lasing materials such as Nd:YAG have much longer excitation lifetimes of about 200 microseconds that enable successful pumping from continuous wave sources such as diode lasers. There have been methods that have demonstrated pumping from quasi-continuous wave sources such as flash lamps. A significant disadvantage of pumping the semiconductor nanocrystal with the sources mentioned herein above is the extremely poor and narrow-band absorption cross sections. Further, nanocrystal lifetimes at room temperature are fairly low of the order of less than 100 ns in systems known to support amplification.

BRIEF DESCRIPTION OF DRAWINGS:
So that the manner in which the recited features of the invention can be understood in detail, some of the embodiments are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.
FIG.1 illustrates comparison of absorbance cross section of quantum dot light amplifier to that of single component semiconductor nanocrystal, according to an embodiment of the invention.
FIG. 2 illustrates typical emission decay from quantum dot light amplifier, according to an embodiment of the invention.
FIG. 3 illustrates PL spectra of the quantum dot light amplifier, according to an embodiment of the invention.
FIG. 4 illustrates effect of addition of a broad-band absorber into a dispersion of the quantum dot light amplifier, according to an embodiment of the invention, according to an embodiment of the invention.
SUMMARY OF THE INVENTION
One aspect of the invention provides a method for obtaining controlled stimulated emission from a quantum dot. The method Including identifying a medium; synthesizing a quantum dot having a plurality of layers of a semiconductor nanocrystal; incorporating the synthesized quantum dot to form a gain medium; exciting the quantum dot with a source to generate carriers; and partitioning the generated carriers. The partitioning of carriers into at least two distinct layers of the formed quantum dot results in controlled stimulated emission within the gain medium.
Another aspect of the invention provides a quantum dot light amplifier. The quantum dot amplifier includes a gain medium. A plurality of layers of a semiconductor nanocrystal is formed in the gain medium, each layer of the semiconductor nanocrystal having two types of carrier. A partitioning of each type of carrier into distinct layers occurs upon excitation with an excitation source. The partitioning of carrier yields spontaneous radiative decay within the gain medium. The partitioning of the carrier occurs after cooling, to the band edge resulting in one type of carrier resident in a first layer of semiconductor and the second carrier resident in a second layer of semiconductor. A third layer of semiconductor separates each of the two layers. A small density of carriers leaks out into the intervening layers. The intermediate layer serves to tune the density of carriers. The quantum dot light amplifier can amplify light of a particular spectral band upon excitation from a continuous wave source or a pulsed source. The light that is amplified may originate from an external source or else from a combination of spontaneous and stimulated emission from quantum dots themselves.

DETAIL DESCRIPTION OF THE INVENTION
Various embodiments of the invention provide a quantum dot light amplifier. The quantum dot light amplifier exhibits optical amplification under very low continuous wave excitation fluences. The quantum dot light amplifier also exhibits amplified spontaneous emission under low levels of illumination. The quantum dot light amplifier, as described herein has a broad absorption band spanning the entire spectral region from the near infrared, about 700 nm, to the ultraviolet, about 280 nm. The quantum dot light amplifier synthesized herein includes a plurality of semiconductor layers or semiconductor alloy layers or combination thereof that are used to control electron-hole overlap of the band edge exciton as well as to reduce non-radiative decay rates. The synthesis of the quantum dots for the light amplifier along with experiments that illustrate the light amplification and the spontaneous emission shall be explained in detail herein below as embodiments of the invention.
Synthesis of quantum dots for the light amplifier: In one embodiment of the invention, the synthesis is achieved by mixing a first precursor and a second precursor. In one example of the invention, the first precursor is a group-II precursor. Group-II precursors include but are not limited to compounds wherein the element is in a +2 oxidation state and is associated with counter ions, which include but not limited to halides, an organic group such as a carboxylate, acetylacetonate, sulfate and nitrate; compounds wherein the element is in a zero oxidation state, which includes but is not limited to zinc alkyls and zinc aryls; and compounds in a different oxidation state which includes but is not limited to mercurous chloride.
The second precursor is a chalcogenide precursor. The chalcogenide precursor is usually the element itself or a compound where it may exhibit an oxidation state ranging from +6 to -2. Examples of the compound in a varying range of oxidation include but are not limited to H2S, NaHTe, Selenourea etc. Solutions of the two precursors are prepared in a solvent such as but not limited to octadecene, tributyl phosphene, trioctylphosphine etc. In some cases the solvent is solid at room temperature, e.g. Tetracosane or Trioctylphosphine oxide. The two precursors are mixed in presence of a solvent and/or one or more ligating molecule to initiate the reaction. In one embodiment a coordinating molecule is used as a solvent. After initiation, the reaction temperature is changed or kept the same as the mixing temperature to allow for nanocrystal growth. After nanocrystal growth, the zinc telluride particles are either separated from the reaction mixture or else used as is.
Shell growth: Shells are grown by addition of Group-II and Group-VI precursors to a reaction mixture already containing nanocrystals described above. The growth conditions are adjusted to be slow enough to prevent independent nucleation of the shell material. The PL spectrum of the samples is monitored as a function of shell growth. As the shell grows thicker, the emission peak that is initially Gaussian becomes structured, and then becomes Gaussian again for thicker shells. Further studies concentrated on the structure that appears in these samples at intermediate shell thicknesses. An additional protective shell of a material which includes but is not limited to ZnS is overgrown through a similar approach to better control carrier lifetimes. The quantum dot thus formed is then incorporated into a medium. Examples of the medium include but are not limited to a solution, a polymer, a glassy matrix, an inorganic matrix, or a close packed quantum dot solid. Alternatively, the quantum dot obtained, according to the method as described herein above can be formed inside a medium.
The medium containing the quantum dot is excited by illumination from a source. Examples of source include but are not limited to a continuous wave source, a pulsed wave source, a monochromatic collimated beam of electromagnetic radiation, a natural source of electromagnetic radiation, an amplitude modulated electromagnetic radiation, a semiconductor source of electromagnetic radiation or a combination thereof. Upon excitation by the source, carriers are generated. The generated carriers then undergo relaxation and reside in two physically distinct regions of the quantum dot, upon cooling. Each of these physical regions comprises of at least one layer of semiconductor. In one embodiment, at least one layer of semiconductor separates the two distinct physical regions. In another embodiment, there is no intermediate layer of semiconductor that separates the two regions. In another embodiment, the quantum dot consists of three layers. After relaxation, one type of carrier is localized to the core and the outer layer while the other type of carrier is localized to the intermediate layer. The residing of the carriers in distinct layer of semiconductor results in partitioning of carrier. The carriers residing in the distinct layers not exhibit reduced, controllable overlap of electron and hole wave functions. This leads to control over spontaneous radiative emission lifetimes and thereby leads to control over cross sections of stimulated emission. The controlled stimulated emission is due to spontaneous emission lifetimes typically greater than 100ns. Stimulated emission in the medium results in gain and the amplification of light.
The synthesis of the quantum dot as described herein and the characterization of the synthesized quantum dot shall be explained in detail herein. Initially, precursor materials cadmium oleate, tellurium are synthesized. A 0.1M Cadmium Oleate is synthesized by dissolving .321g of cadmium oxide in a solution comprising 3ml of oleic acid and 3ml of octadec-1-ene. Oxygen present in the solution is removed through evacuation and followed by purging the solution with argon. Subsequent to removal of oxygen, the mixture is heated to about 250oC to obtain a colorless mixture. To the colorless mixture obtained, 18ml of octadec-1-ene and 1ml of oleylamine are added. Preparation of 0.1M sulphur in oleylamine: 64 mg of elemental sulphur is weighed and dissolved in a 10 ml of oleylamine to the flask containing sulphur. The mixture is stirred and evacuated. The mixture is then purged with argon. Subsequent to purging of argon, the mixture is heated to about 250OC under argon atmosphere until all the sulphur has dissolved completely. Upon dissolution of sulphur into the solution, the heating is stopped and 10 ml of oleylamine is added to obtain 0.1M sulphur in oleylamine. Preparation of 0.25M tellurium in trioctylphosphine: 0.3166g of metallic tellurium is weighed and transferred to a flask. A 10 ml of trioctylphosphine is added to the weighed tellurium and stirred overnight.
Example1: Preparation of ZnTe/ZnS/CdS heterostructure nanocrystal.
Oleylamine is used as ligand and octadec-1-ene is used as the solvent medium. The preparation of ZnTe/ZnS/CdS is carried out in 2 steps. First is the formation of Te2-, that is, reduction of tellurium to telluride ion. Second step is the formation of heterostructure nanocrystal.
In a clean round bottom flask, 94.5 mg of sodium borohydride is transferred. The flask containing sodium borohydride is then evacuated and purged with argon. This ensures the removal of oxygen that may reverse the conversion of telluride ion to tellurium. Two ml of 0.25 M tellurium in trioctylphosphine is injected into the flask. Catalytic amount (0.1 ml) of 1, 4-butanediol is then added and the mixture is stirred. Addition of 1, 4-butanediol provides proton that helps in the reduction. The reaction is carried out under argon atmosphere between 60-80oC. The kinetics of reduction is slow at room temperature and at high temperature the reverse reaction takes place. The mixture is cooled to room temperature and 0.5ml of butan-1-al is added. Butan-1-al consumes the unreacted sodium borohydride such that there is reduced evolution of hydrogen in the latter stage of the reaction procedure.
In a second round bottom flask, 109.8 mg of zinc acetate dihydrate is taken. To this 8 ml of octadec-1-ene is added followed by addition of 2 ml of Oleylamine. Oxygen is removed by evacuation followed by purging with argon. The reaction mixture is then heated to 100o C. 40% of the content of the first flask. Telluride ion prepared as described herein above and is then quickly transferred to the second flask containing zinc acetate. Caution is exercised during transfer, due to evolution of hydrogen, the amount of which depends on the unreacted sodium borohydride that is left in the first flask. Remaining 60% of the contents of first flask are then transferred to the second flask drop wise, ensuring that the temperature of the mixture is below 140o C, since at a temperature above 150oC, ZnTe gets oxidized to form ZnO. The 40/60 partition ensures controlled and narrow size distribution. The temperature of the reaction mixture is then increased to 230o C followed by the addition of 0.1M zinc dimethylbutyrate and 0.1M sulphur in oleylamine. The addition is done slowly drop by drop. The temperature is then raised to 250o C. This helps to increase the kinetics of the formation of zinc sulphide shell on zinc telluride core and to avoid the formation of zinc sulphide nuclei in the reaction mixture. The temperature of the reaction mixture is then decreased to 230o C. Cadmium oleate and sulphur in oleylamine is then added drop wise to the reaction mixture. After addition of 0.5 ml of cadmium oleate and 0.5ml sulphur in oleylamine the temperature of the reaction mixture is raised to 280o C. When the colour of the reaction mixture turns brownish red, that is, when the PL maxima reaches 650 nm, 0.1 g of trioctylphosphine oxide and 0.01 g of tetradecyl phosphonic acid dissolved in 1 ml of octadec-1-ene is added. 1 ml of trioctylphosphine and 0.5 ml of cadmium oleate is also added to the reaction mixture. Trioctylphosphine oxide and tetradecyl phosphonic acid act as ligand and bind to the excess Zn2+ that is present in the reaction mixture. Trioctylphosphine binds to sulphur and thus prevents it from forming surface traps. Addition of excess of cadmium oleate increases the quantum efficiency of the quantum dots thus prepared. Drop wise addition of cadmium oleate and sulphur in oleylamine is continued till the PL maxima reached 800 nm.
Example 2: Preparation of ZnTe/CdTe/CdS heterostructure nanocrystal.
Oleylamine is used as ligand and octadec-1-ene is used as the solvent medium. The preparation of ZnTe/CdTe/CdS is carried out in 2 steps. The first step includes the formation of Te2-, that is, reduction of tellurium to telluride ion. The second step includes the formation of heterostructure nanocrystal. In a clean round bottom flask, 94.5 mg of sodium borohydride is transferred. The flask containing sodium borohydride is then evacuated and purged with argon. This ensures the removal of oxygen that may reverse the conversion of telluride ion to tellurium. Two ml of 0.25 M tellurium in trioctylphosphine is injected into the flask. Catalytic amount of 1,4-butanediol is then added and the mixture is stirred. Addition of 1,4-butanediol provides proton that helps in the reduction. The reaction is carried out under argon atmosphere between 60oC -80oC. The kinetics of reduction is slow at room temperature and at high temperature the reverse reaction takes place. The mixture is cooled to room temperature and 0.5ml of butan-1-al is added. Butan-1-al consumes the unreacted sodium borohydride so that there is reduced evolution of hydrogen in the latter stage of the reaction procedure.
In a second round bottom flask, 109.8 mg of zinc acetate dihydrate is taken. To this 8 ml of octadec-1-ene is added followed by addition of 2 ml of oleylamine. Oxygen is removed by evacuation followed by purging with argon. The reaction mixture is then heated to 100oC. Telluride ion prepared as mentioned above and 40% of the content in the first flask is then quickly transferred to the second flask containing zinc acetate. Caution should be taken since there would be evolution of hydrogen, the amount of which depends on the unreacted sodium borohydride that was left in the first flask. Remaining 60% of the contents of first flask are then transferred to the second flask drop wise, ensuring that the temperature of the mixture is below 140oC. This is because above 150oC, ZnTe gets oxidized to form ZnO. The 40/60 partition ensures controlled and narrow size distribution. The temperature of the reaction mixture is then increased to 230oC followed by the addition of 1 ml of 0.1M Cadmium oleate and 0.4 ml of 0.25 M Tellurium in Trioctylphosphine. The addition is done drop wise and slowly. The temperature is then raised to 250oC. The slow drop wise addition and the increase in the temperature of the reaction mixture is done to increase the kinetics of the formation of cadmium telluride shell on zinc telluride core and to avoid the formation of cadmium telluride nuclei in the reaction mixture. The temperature of the reaction mixture is then decreased to 230oC. Cadmium oleate and sulphur in oleylamine is then added drop wise to the reaction mixture. After addition of 0.5 ml of cadmium oleate and 0.5ml sulphur in oleylamine the temperature of the reaction mixture is raised to 280oC. When the colour of the reaction mixture turns brownish red, that is, when the PL maxima reaches 650 nm, 0.1 g of trioctylphosphine oxide and 0.01 g of tetradecyl phosphonic acid dissolved in 1 ml of octadec-1-ene is added. One ml of trioctylphosphine and 0.5 ml of cadmium oleate is also added to the reaction mixture. Trioctylphosphine oxide and tetradecyl phosphonic acid act as ligand and bind to the excess Zn2+ that is present in the reaction mixture. Trioctylphosphine binds to sulphur and thus prevents it from forming surface traps. Addition of excess of cadmium oleate increases the quantum efficiency of the quantum dots thus prepared. Drop wise addition of cadmium oleate and sulphur in oleylamine is continued till the PL maxima reached 800 nm. The progress of reaction is monitored by taking PL spectra and Absorption spectra of the quantum dots dissolved in hexane at various intervals during the course of the reaction.
Characterization of quantum dot light amplifier: FIG.1 illustrates comparison of absorbance cross section of quantum dot light amplifier to that of single component semiconductor nanocrystal, according to an embodiment of the invention. Some type-II nanocrystals (solid lines) can exhibit large cross sections that are similar or even larger than related single component semiconductor materials (dashed lines). In a type–II nanocrystal the absorption cross section increases exponentially with energy above the band gap. It is thus possible to absorb light strongly while still suppressing exciton radiative decay rates.
FIG. 2 illustrates typical emission decay from quantum dot light amplifier, according to an embodiment of the invention. The quantum dot light amplifier exhibits long term energy storage. 30% of the sample decays with time constants that are longer than or equal to 1 millisecond.
FIG. 3 illustrates PL spectra of the quantum dot amplifier, according to an embodiment of the invention. In left Panel of the figure all curves other than curve B are normalized to the emission at 1.82 eV. As the concentration of nanocrystals in hexane is increased, the shape of the PL spectrum is observed to change. At high concentrations (approximately OD 0.05 at the S exciton, curve A), the FWHM of the new peak is found to be ~2.8 kT at room temperature, much lower than the homogenous PL linewidths of most nanocrystals (~4 kT or more). For extremely dilute samples (curve B), the side band is absent. The growth of the side band is roughly linear with concentration. For very dilute samples (B), the exciton lifetimes are fairly long, ~20 microseconds. For concentrated samples (A), the lifetime is much shorter (~50 ns)
FIG. 4 illustrates effect of addition of a broad-band absorber into a dispersion of the quantum dot light amplifier, according to an embodiment of the invention. As the absorber concentration increases, amplitude of both emission bands decreases due to increased absorption of the emitted light. The emission from the lower energy band (1.65 eV) however decreases much more significantly relative to the regular PL emission at 1.8 eV. The emission spectra are recorded under continuous wave illumination from a laser pointer. The sideband observed here, are amplified spontaneous emission from the sample. Further, these amplified spontaneous emissions are also obtained by illuminating the sample with a pulsed source such as a flash lamp or by a pulsed laser diode (405 nm peak wavelength). Thus amplification obtained is broadly similar regardless of whether a continuous wave light source such as a xenon lamp, a laser pointer etc. are used or else if the source is a pulsed one such as a flash lamp, or a pulsed laser.
Amplified light sources are fairly ubiquitous. Such sources are used in data storage/retrieval, communications, sensing, machining, surgery, medicine etc. The invention described here represents the development of the most efficient optical amplifier that is possible by utilizing a three level system and a continuous wave pump source. The invention is further modifiable to be optimized to other pump sources. It will thus affect all applications that rely on amplified light.
The foregoing description of the invention has been set to merely illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the appended claims and equivalents thereof.