DESC:FIELD OF THE INVENTION
[0001] The present disclosure pertains generally to technical field of colloidal quantum dot (QD) as active laser medium. In particular the present disclosure pertains to QDs that are capable of providing gain under conditions of extremely low levels of continuous wave illumination and convert incoherent light to coherent light/lasers.

BACKGROUND OF THE INVENTION
[0002] Background description includes information that may be useful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication specifically or implicitly referenced is prior art.
[0003] Lasers (Light Amplification by Stimulated Emission of Radiation) are basically excited light waves. Lasers use different gases such as Helium, Neon, Argon, and Carbon Dioxide, Semiconductors such as Gallium Arsenide, solid-state materials such as Ruby, Glass, and chemicals such as Hydrofluoric acid in their operation. Lasers work by converting incoherent light to coherent light. Differing from incoherent light, coherent light is a beam of photons that have the same frequencies, wavelength, speed, and no or constant phase difference. Conventional light sources are incoherent sources which are converted to coherent light by Lasers.
[0004] Semiconductor quantum dots have also been in use as active laser medium in numerous applications in place of conventional materials. Due to strong confinement of charge carriers in quantum dots, they exhibit an electronic structure similar to atoms. Lasers fabricated from such an active media exhibit device performance that is closer to gas lasers, and avoid some of the negative aspects of device performance associated with traditional semiconductor lasers based on bulk or quantum well active media. Improvements in modulation bandwidth, lasing threshold, linewidth enhancement factor and temperature insensitivity have all been observed. In comparison to the performance of conventional strained quantum-well lasers of the past, the new epitaxial quantum dot lasers achieve significantly higher stability of temperature. The quantum dot active region may also be engineered to operate at different wavelengths by varying dot size and composition. Conventional colloidal Quantum dot lasers in contrast typically require pumping from high intensity laser sources that has limited their utility.
[0005] Despite overall success of QDs as emitters, impact of colloidal QDs as laser materials has been severely limited. This is primarily because eliciting stimulated emission from quantum dots requires excitation by intense short pulses of light typically generated using other lasers. Therefore, a new class of quantum dots that exhibit gain under conditions of conventional and low levels of continuous wave illumination would significantly contribute to enhance utility of QDs in laser applications.
[0006] There is therefore a need in the art to design and develop a new medium that exhibit gain under conditions of extremely low levels of continuous wave illumination and convert incoherent light to coherent light and lasers.
[0007] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application were specifically and individually indicated to be incorporated by reference. Where a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.
[0008] In some embodiments, the numbers expressing quantities of ingredients, properties such as concentration, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” Accordingly, in some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements.
[0009] As used in the description herein and throughout the claims that follow, the meaning of “a,” “an,” and “the” includes plural reference unless the context clearly dictates otherwise. Also, as used in the description herein, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise.
[0010] The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention.
[0011] Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience and/or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all groups used in the appended claims.

OBJECTS OF THE INVENTION
[0012] A general object of the present disclosure is to design and develop a new medium that exhibit gain under conditions of extremely low levels of continuous wave illumination and convert incoherent light to coherent light and lasers.
[0013] An object of the present disclosure to overcome limitations of colloidal QDs as active laser medium by enabling them to exhibit gain under conditions of extremely low levels of continuous wave illumination.
[0014] Another object of the present disclosure to provide improved means of generating coherent light from incoherent light by introducing incoherent light into a medium comprising of quantum dots. The medium provided by the present disclosure absorbs incoherent light in whole or in part, and converts it into coherent light.
[0015] Yet another object of the present disclosure is to provide improved means of increasing the power of a beam of light by absorbing light from different beams of light.
[0016] Yet another object of the present disclosure is to provide improved means of converting sunlight into coherent laser beam.
[0017] Yet another object of the present disclosure is to provide improved means of converting incoherent light from the conventional sources such as a light bulb, a flash lamp, an arc lamp or a Light Emitting Diode into laser radiation.
[0018] Yet another object of the present disclosure is to provide improved means of converting absorbed coherent light derived from a source such as but not limited to a laser diode, into a coherent beam.
[0019] Yet another object of the present disclosure is to provide improved means that absorbs a temporally continuous beam of light and emits a temporally pulsed beam.
[0020] Yet another object of the present disclosure is to provide improved means that absorbs a temporally continuous beam of light and emits a temporally discontinuous beam.
[0021] Yet another object of the present disclosure is to provide improved means that absorbs a temporally continuous beam and emits a temporally modulated beam.
[0022] Yet another object of the present disclosure is to provide improved means that absorbs a temporally pulsed beam of light and emits a temporally discontinuous beam.

SUMMARY
[0023] Aspects of the present disclosure relate to an active laser medium capable of providing gain under conditions of conventional and low levels of continuous wave illumination and convert incoherent light to coherent light/lasers. In particular, the present disclosure provides colloidal quantum dots (QDs) that exhibit gain under conditions of extremely low levels of continuous wave illumination and overcome limitation of colloidal QDs as active laser medium. Thus the disclosed QDs can convert coherent or incoherent light from various conventional sources such as sun, light bulb, a flash lamp, an arc lamp, a Light Emitting Diode or a laser diode in to coherent laser.
[0024] In an aspect, the disclosed QDs for use as active laser medium comprise two or more compositional layers where at least one of the layers is an alloy semiconductor, and exhibit an average spontaneous emission lifetime that is longer than 100 ns when held at room temperature in a dispersion with refractive index between 1 and 1.7 in absence of any other means to modify photonic density of states.
[0025] In an aspect, the two or more layers of the disclosed QDs can be of any one or combination of materials selected from group comprising ZnTe, ZnTeSe, ZnCdS, ZnTeS, CdZnTeSe, CdZnSeTeS.
[0026] In an aspect, the disclosed QDs exhibit a gain threshold under illumination of 1000 mW/cm2 or less, lasing threshold of less than 0.75 excitons per QD and localization of one or more of the photogenerated carriers within 100 ns of optical excitation.
[0027] In an aspect the disclosure provides a laser device comprising a plurality of QDs and a cavity arranged to provide optical feedback that allows the QDs to amplify an optical signal when they are optically pumped above the device lasing threshold by a pump source.
[0028] In an embodiment, the disclosed QDs comprise a colloidal nanocrystal of ZnTe or ZnTeSe forming a core; an intermediate shell of ZnSe, and an additional outer shell of material ZnCdS.
[0029] In an aspect, materials such as Zinc telluride (ZnTe)/Zinc selenide (ZnSe)/Zinc cadmium sulfide (ZnCdS) used for synthesis of the disclosed QDs provide band offsets such that holes relax to the ZnTe core, while electrons become localized to the outer ZnCdS shell after complete excitonic cooling. As a consequence, these QDs exhibit optical band gaps that are much narrower than the band gaps of their constituent semiconductor materials. Narrower band gaps result in the QDs exhibiting gain under conditions of continuous wave illumination as low as 74 mW/cm2.
[0030] Various objects, features, aspects and advantages of the inventive subject matter will become more apparent from the following detailed description of preferred embodiments, along with the accompanying drawing figures in which like numerals represent like components

BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The accompanying drawings are included to provide a further understanding of the present disclosure, and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the present disclosure and, together with the description, serve to explain the principles of the present disclosure.
[0032] FIG. 1A illustrates schematic of the core/multishell QD material (ZnTe/ZnSe/CdZnS) in accordance with embodiments of the present disclosure.
[0033] FIG. 1B illustrates TEM image of nanocrystals of the QD material (ZnTe/ZnSe/CdZnS) in accordance with an embodiment of the present disclosure on scale bar of 50 nm.
[0034] FIG. 1C illustrates XRD patterns observed during ZnSe shell growth, wherein ZnSe is used in accordance with an embodiment of the present disclosure.
[0035] FIG. 1D illustrates XRD patterns of a ZnTeSe core, wherein ZnTeSe is used in accordance with an embodiment of the present disclosure.
[0036] FIG. 1E illustrates XRD patterns of the CdZnS over layer on ZnSe, in accordance with an embodiment of the present disclosure.
[0037] FIG. 2A illustrates optical absorption of ZnTe, ZnTe/ZnSe, ZnTe/ZnSe/CdZnS QDs, in accordance with an embodiment of the present disclosure.
[0038] FIG. 2B illustrates Photoluminescence (PL) emission spectra observed during the course of growth of the CdZnS layer, in accordance with an embodiment of the present disclosure.
[0039] FIG. 2C illustrates lifetime of the ZnTe, ZnTe/ZnSe, ZnTe/ZnSe/CdZnS material in a dilute solution, in accordance with an embodiment of the present disclosure.
[0040] FIG. 2D illustrates behavior of the films with respect to emission pattern, in accordance with an embodiment of the present disclosure.
[0041] FIG. 3A and FIG. 3B illustrate emission pattern of a laser made in accordance with an embodiment of the present disclosure, wherein the laser is made by drop casting a ZnSeTe/ZnSe/ZnCdS QD film with lifetime 1.3 µs onto a partially reflecting (90%) mirror (FRQ-ND10, Newport).
[0042] FIG. 4A illustrates schematic of a QD solar laser made in accordance with an embodiment of the present disclosure.
[0043] FIG. 4B illustrates photograph of the laser made in accordance with an embodiment of the present disclosure.
[0044] FIG. 4C illustrates excitation spectrum of the QD sample, terrestrial solar spectrum and section of solar spectrum used for sample excitation, in accordance with an embodiment of the present disclosure.
[0045] FIG. 4D illustrates spectrum of solar QD laser, in accordance with an embodiment of the present disclosure.
[0046] FIG 5 illustrates (a) an exemplary image showing operation of a continuous wave QD laser having lasing QDs above lasing threshold inside a resonator and (b) a plot showing spatial coherence of QD laser as against that of a LED source in accordance with embodiment of the present disclosure.
[0047] FIG 6 illustrates an exemplary plot of results of a device stability test in accordance with embodiment of the present disclosure.
[0048] FIG 7 illustrates exemplary plots showing (a) early time bleach dynamics of the sample at the semiconductor band edge, (b) duration of electron residence in a delocalized semiconductor electronic state (c) emission lifetime and (d) sample absorbance in accordance with embodiment of the present disclosure.

DETAILED DESCRIPTION OF THE INVENTION
[0049] Unless the context requires otherwise, throughout the specification which follow, the word “comprise” and variations thereof, such as, “comprises” and “comprising” are to be construed in an open, inclusive sense that is as “including, but not limited to.”
[0050] In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the present invention. It will be apparent to one skilled in the art that embodiments of the present invention may be practiced without some of these specific details.
[0051] Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure or characteristic described in connection with the embodiment is included in at least one embodiment. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily all referring to the same embodiment. Furthermore, the particular features, structures, or characteristics may be combined in any suitable manner in one or more embodiments.
[0052] As used in this specification and the appended claims, the singular forms “a,” “an,” and “the” include plural referents unless the content clearly dictates otherwise. It should also be noted that the term “or” is generally employed in its sense including “and/or” unless the content clearly dictates otherwise.
[0053] The headings and abstract of the invention provided herein are for convenience only and do not interpret the scope or meaning of the embodiments. Reference will now be made in detail to the exemplary embodiments of the present invention.
[0054] Each of the appended claims defines a separate invention, which for infringement purposes is recognized as including equivalents to the various elements or limitations specified in the claims. Depending on the context, all references below to the "invention" may in some cases refer to certain specific embodiments only. In other cases it will be recognized that references to the "invention" will refer to subject matter recited in one or more, but not necessarily all, of the claims.
[0055] Various terms as used herein are shown below. To the extent a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.
[0056] The following is a detailed description of embodiments of the disclosure depicted in the accompanying drawings. The embodiments are in such detail as to clearly communicate the disclosure. However, the amount of detail offered is not intended to limit the anticipated variations of embodiments; on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the present disclosure as defined by the appended claims.
[0057] Embodiments explained herein relate to an active laser medium capable of providing gain under conditions of conventional and low levels of continuous wave illumination and convert incoherent light to coherent light/lasers. In particular, the present disclosure provides colloidal quantum dots (QDs) that exhibit gain under conditions of extremely low levels of continuous wave illumination and thus overcome limitation of colloidal QDs as active laser medium.
[0058] In an aspect, the disclosed QDs exhibit a gain threshold under illumination of 1000 mW/cm2 or less, lasing threshold of less than 0.5 excitons per QD and localization of one or more of the photogenerated carriers within 100 ns of optical excitation.
[0059] The disclosed QDs exhibit an average spontaneous emission lifetime that is longer than 100 ns when held at room temperature in a dispersion with refractive index between 1 and 1.7 in absence of any other means to modify photonic density of states.
[0060] Thus the disclosed QDs can convert coherent or incoherent light from various conventional sources such as sun, light bulb, a flash lamp, an arc lamp, a Light Emitting Diode or a laser diode in to coherent laser.
[0061] In an aspect, the disclosed QDs for use as active laser medium comprise two or more compositional layers where at least one of the layers is an alloy semiconductor. The two or more layers of the disclosed QDs can be of any one or combination of materials selected from group comprising ZnTe, ZnTeSe, ZnCdS, ZnTeS, CdZnTeSe, CdZnSeTeS.
[0062] In an aspect the disclosure provides a laser device comprising a plurality of QDs and a cavity arranged to provide optical feedback that allows the QDs to amplify an optical signal when they are optically pumped above the device lasing threshold by a pump source.
[0063] FIG. 1A illustrates schematic 110 of the multi-layer QD material having two or more layers - three layers to be specific – comprising a core, an intermediate shell and an outer shell in accordance with an embodiment of the present disclosure. In an embodiment, the disclosed QDs can comprise a colloidal nanocrystal of ZnTe or ZnTeSe forming a core; an intermediate shell of ZnSe, and an additional outer shell of material ZnCdS thus forming a ZnTe/ZnSe/CdZnS Quantum Dot.
[0064] It is to be appreciated that though the embodiments of the present disclosure have been explained with reference to ZnTe, ZnSe and CdZnS used for core, intermediate shell and outer shell of the QD, it is possible to get the same results using other materials such as but not limited to, any one or combination of materials selected from group comprising ZnTe, ZnTeSe, ZnCdS, ZnTeS, CdZnTeSe, CdZnSeTeS and all such variations are well within the scope of the present disclosure without any limitation whatsoever.
[0065] In an aspect, material for core, intermediate shell and outer shell of the QD are selected such that band offsets of the core, the intermediate shell and the outer shell are such that holes are localized to the core, while electrons are transferred to the shell. Zinc telluride (ZnTe)/Zinc selenide (ZnSe)/Zinc cadmium sulfide (ZnCdS) used for synthesis of the disclosed QDs in the exemplary embodiments of the present disclosure provide such band offsets. As a consequence, these exemplary QDs exhibit optical band gaps that are much narrower than the band gaps of their constituent semiconductor materials. Narrower band gaps result in the QDs exhibiting gain under conditions of continuous wave illumination as low as 74 mW/cm2.
[0066] In an aspect, the disclosed QDs exhibit a gain threshold under illumination of 1000 mW/cm2 or less, lasing threshold of less than 0.5 excitons per QD and localization of one or more of the photogenerated carriers within 100 ns of optical excitation.
[0067] The disclosed QDs exhibit an average spontaneous emission lifetime that is longer than 100 ns when held at room temperature in a dispersion with refractive index between 1 and 1.7 in absence of any other means to modify photonic density of states.
[0068] FIG. 1B illustrates TEM image 120 of nanocrystals of the QD material (ZnTe/ZnSe/CdZnS) in accordance with an embodiment of the present disclosure on scale bar of 50 nm.
[0069] FIG. 1C illustrates XRD patterns 130 observed during ZnSe shell growth, wherein ZnSe is used in accordance with an embodiment of the present disclosure. Observed pattern for the ZnTe/ZnSe material was a straightforward sum of the patterns of individual patterns, which gives evidence of epitaxial growth with minimal interfacial alloying. Substrate artefacts at 33 and 38 degrees were removed.
[0070] FIG. 1D illustrates XRD patterns 140 of a ZnTeSe core, wherein ZnTeSe is used in accordance with an embodiment of the present disclosure. XRD patterns of a ZnTeSe core exhibits peak positions intermediate to ZnTe and ZnSe, consistent with a ZnTe0.87Se0.13 alloy.
[0071] FIG. 1E illustrates XRD patterns 150 of the CdZnS over layer on ZnSe, in accordance with an embodiment of the present disclosure.
[0072] FIG. 2A illustrates optical absorption pattern 210 of ZnTe, ZnTe/ZnSe, ZnTe/ZnSe/CdZnS QDs, in accordance with an embodiment of the present disclosure. FIG. 2A, shows absorption spectra observed for ZnTe core as well as for the ZnTe/ZnSe core/shell and quantum dots of these semiconductor materials after overgrowth of ZnCdS. The original ZnTe core as well as ZnTe/ZnSe core/shell structures show fairly wide band gaps. The overgrowth of ZnCdS causes the band edge to red-shift significantly, and the onset of absorption now occurs in the near-infrared. The band edge photoluminescence (PL) emission from these materials also shows a corresponding red shift. The mean emission lifetime of this particular sample is observed to be 0.8 µs that is expected to be optimal for the continuous wave excitation of Amplified Spontaneous Emissions (ASE) in QDs. This number has been extracted by fitting the decay curve to exponentials, and calculating an average lifetime through the equation: , where is the amplitude of each exponential and is the associated lifetime.
[0073] FIG. 2B illustrates Photoluminescence (PL) emission spectra 220 observed during the course of growth of the CdZnS layer, in accordance with an embodiment of the present disclosure.
[0074] FIG. 2C illustrates lifetime of the ZnTe, ZnTe/ZnSe, ZnTe/ZnSe/CdZnS material in a dilute solution, in accordance with an embodiment of the present disclosure. The material shows a lifetime of 814 ns in dilute solution.
[0075] FIG. 2D illustrates behavior of the films with respect to emission pattern, in accordance with an embodiment of the present disclosure. Films show a sharp threshold of 11 mW/cm2 above which the samples become highly emissive. The existence of a distinct, finite threshold gives evidence of the continuous wave pumping of optical amplification in these substrates.
[0076] A multimode laser can be made by drop casting a QD film (e.g. ZnSeTe/ZnSe/ZnCdS QDs) with appropriate lifetime onto a partially reflecting mirror e.g. FRQ-ND10, Newport. A dichroic mirror (e.g. 10B20UF.25, Newport) reflective over 700-930 nm can be placed at the other end. A continuous wave pump beam operative at an appropriate wavelength (e.g. at 405 nm) can be introduced from the side of the mirror. The output can be collected using a lens from the partially silvered end mirror. A long pass filter (e.g. 500 nm) can be used to separate the pump beam from the laser emission. The emission can be spectrally resolved using an un-cooled charge coupled device (CCD) (e.g. USB4000, Ocean Optics). FIG. 3A illustrates emission pattern 320 of a laser made in accordance with an embodiment of the present disclosure. It shows continuous wave QD laser spectrum (solid lines). The dashed curve shown is the spectrum of a QD film, without a resonator. Modes as narrow as 8 meV can be observed. The emission line shape is consistent with a multimode laser and also indicates the broad gain bandwidth afforded by the QD material. At increasing pump fluences, additional modes upto 400meV away from the central mode became observable. In contrast, to the observations made using a resonator configuration, the emission of a QD film in absence of a resonator was significantly broader (FIG. 3A, dashed curve). It could also be noted that the emission maximum does not correspond to the most intense modes observed in the presence of a resonator.
[0077] FIG. 3B illustrates emission pattern of a laser made in accordance with an embodiment of the present disclosure. It can be observed that the laser emission exhibits distinctly nonlinear initial rise (solid, blue curve) followed by a subsequent linear evolution (dashed). The data presented in this plot is for the mode at 1.53 eV. The laser operation can be associated with a rather low threshold that indicated operation under a single exciton gain regime. For example, the mode observed at 1.53 eV exhibits a pump threshold fluence of 74mW/cm2. Using the measured optical cross sections of QDs (8x10-14 cm2) at the wavelength of excitation, we can find the threshold fluence corresponding to the excitation of 1.2 x 104 excitons per QD per second. Since, the sample lifetime is 1.3 x 10-6 sec. this fluence causes the persistence of an average of 1.5 x 10-2 excitons per QD at the threshold fluence. This number is much lower than the number required by QDs to lase under the so-called single exciton regime (this regime requires an absolute minimum of 0.5 excitons per QD based on transparency criteria). The biexcitonic regime of lasing requires QD occupation as high as 1 exciton per QD or more. The origins of this low threshold can be attributed in part to the use of highly mismatched alloys (HMA’s). These systems provide high effective masses, making carriers (electrons and holes) susceptible to localization and collapse. In the case of delocalized levels that are populated by the exciton at the time of formation, oscillator strength is distributed over a certain energy window. Post-formation, electron-phonon and hole phonon interactions cause the localization of both carriers. This wave function collapse causes a major redistribution of oscillator strength. The oscillator strength of the collapsed exciton increases dramatically at energies lower than the absorption of the delocalized exciton. This implies that post excitation, the material effectively gains oscillator strength below the band edge. The QDs consequently behave like pseudo four level systems rather than three level materials unlike normal semiconductors. Independent confirmation of this mechanism comes from the observed depopulation of band edge states at early (< 2 ns) times. This suggests that the emission does not directly arise from the band edge states. The mode described above shows a nonlinear initial increase in intensity followed by a subsequent linear increase at higher pump powers, after the onset of stable laser operation. The low operation threshold suggests several potential applications for these materials. Given the broad pump bands that are naturally available to QDs, it is possible to harvest light from polychromatic sources such as light bulbs or the sun with great efficiency. The low threshold observed by us then permits the conversion of even low intensity, diffuse, polychromatic light into a laser. This aspect can be demonstrated by replacing the laser diode in the setup described above with the sunlight.
[0078] FIG. 4A illustrates schematic 410 of a QD solar laser made in accordance with an embodiment of the present disclosure. Order of occurrence from left to right include - L: Lens, LP: long pass filter DM: Dichroic mirror A: Aperture BP: band pass filter, SP: short pass filter HM: hot mirror. FIG. 4B illustrates photograph 420 of the laser made in accordance with an embodiment of the present disclosure. In operation, the NIR component of sunlight was removed using a hot mirror (50mm Dia., Edmund Optics) and the residual light was passed through short pass (500 nm) (FES0500, Thorlabs) and band pass filters to distinguish pump light from the sample response. The hot mirror end of this device was manually aligned to the sun using the alignment pinholes. Sunlight contains 32mW/cm2-eV at 2.5 eV. The filter combination employed by us allowed for the passage of only 24 mW of solar radiation. Concentration with a lens allowed for a fluence of 12 W/cm2 centered around 2.5 eV within the gain medium. While, we have used additional filters to limit amount of sunlight used in our experiment, the actual ability of QDs to harvest sunlight is far more superior, as indicated by their photoluminescence excitation (PLE) spectrum. FIG. 4C illustrates excitation spectrum (solid, red) of the QD sample, terrestrial solar spectrum (black, dotted) and section of solar spectrum used for sample excitation (blue, solid), in accordance with an embodiment of the present disclosure. FIG. 4D illustrates spectrum 440 of solar QD laser, in accordance with an embodiment of the present disclosure. Though, the design illustrated by us includes considerable simplification for the ease of understanding, significantly more efficient QD solar lasers can easily be contemplated by the person skilled in the art based on the disclosure made herein.
[0079] Similarly, other laser devices can easily be contemplated and
fabricated employing other light source means including but not limiting to light bulb(s), flash lamp(s), arc lamp(s), Light Emitting Diode(s) or any other light sources known to a person skilled in the art. Such laser devices can involve casting QDs onto glass or sapphire or any other substrates known to a person skilled in the art, or else, including them in a KBr matrix or any other suitable matrices known to a person skilled in the art. The QDs can then be held at a Brewster’s angle relative to the axis of a resonator formed out of two facing mirrors. The material can be pumped typically using an off-axis illumination. On-axis illumination can be employed with dichroic mirrors.
[0080] FIG 5 illustrates an exemplary image showing operation of a continuous wave QD laser having lasing QDs above lasing threshold inside a resonator and a plot showing spatial coherence of QD laser as against that of a LED source in accordance with embodiment of the present disclosure. Operation of a continuous wave QD laser includes (from left to right) 405 nm CW pump source, focusing lens, QDs in resonator, two collimating lenses (not visible), long-pass filter and Laser spot on screen. The figure width approximately corresponds to 30 cm. The generated laser beam is found to have a high degree of spatial coherence. Here a detector with no collection elements is moved away from the QD laser source (red circles). The distance is measured from the collimating optic closest to the detector. As can be seen from the plot on right side, no drop in signal intensity, that can be associated with beam divergence, is noticed for over 40 cm. In comparison, a LED source with a collimating paraboloid shows a rapidly decaying signal intensity that falls off roughly as square of the distance.
[0081] FIG 6 illustrates an exemplary plot of results of a device stability test in accordance with embodiment of the present disclosure. During the test the QD Laser device was illuminated by 586 mW/cm2 at 405 nm.
[0082] FIG 7 illustrates plots showing (a) early time bleach dynamics of the sample at the semiconductor band edge. A biexciton decay lifetime of 68 ps was observed, (b) duration of electron residence in a delocalized semiconductor electronic state was found to be as short as 650 ps, (c) sample emission lifetime was found to be as long as 1.3 microsec, suggesting a novel defect assisted emission mechanism, and (d) sample absorbance curve where arrow represents the probe energy.
[0083] The following examples further illustrate the invention. These examples are not intended to limit the invention in any manner.
[0084] QD films and Lasers were prepared according to procedure explained below.

Preparation of QD films:
[0085] Cleaned QDs were drop cast into films with 0.1 OD at 500 nm onto an indented glass substrate. These substrates were illuminated using a 405 nm continuous wave laser focused into a stripe with dimensions 300µm×2mm using a cylindrical lens. The excitation beam was chopped at 137 Hz. Sample emission was collected using the same lens and focused onto a silicon photodiode (PDA36A-EC, Thorlabs). The sample emission at 137 Hz was isolated using lock in detection.

Preparation of Laser device:
[0086] A multimode laser was made by drop casting a ZnSeTe/ZnSe/ZnCdS QD film with lifetime of 1.3 µs onto a partially reflecting (90%) mirror (FRQ-ND10, Newport). A dichroic mirror (10B20UF.25, Newport) reflective over 700-930 nm was placed at the other end. A continuous wave pump beam at 405 nm was introduced from the side of the 800 nm mirror. The output was collected using a lens from the partially silvered end mirror. A long pass filter (500 nm) was used to separate the pump beam from the laser emission. The emission was spectrally resolved using an uncooled charge coupled device (CCD) (USB4000, Ocean Optics).

Synthesis of QDs:
[0087] Chemicals required: Tellurium (Te, 99.99%), zinc acetate dihydrate (Zn(ac)2.2H2O, 99.0%), sodium borohydride (NaBH4 96%), trioctylphosphine (TOP, technical grade, 90%), 1-octadecene (ODE, technical grade, 90%), oleylamine (technical grade, 70%), oleic acid (technical grade, 90%), sulphur (S, < 99.5%), zinc stearate (purum, 10-12% zinc basis), selenium (Se, 99.99%), trioctylphosphine oxide (TOPO, technical grade, 90%), tetradecylphosphonic acid (TDPA, 97%), 1-dodecanethiol (DDT, >98%) and cadmium oxide (99.9%) were purchased from Sigma-Aldrich. 1,4-Butanediol (90%, AR) and dextrose (95% anhydrous, AR) were purchased from SD Fine Chemicals. All chemicals were used without further purification.

Preparation of precursors:
[0088] (a) 0.1M cadmium oleate: 0.3210g of cadmium oxide, 3 ml of oleic acid and 3 ml of ODE were added to a flask. The contents were held under vacuum at 100ºC for 1 minute and then the flask was filled with argon. Temperature was raised to 250ºC for 10 minutes to convert cadmium oxide to cadmium oleate. Mixture was cooled and 18 ml ODE and 1 ml of oleylamine were injected to produce 0.1M cadmium oleate solution.
(b) 0.1M zinc-oleate solution: 548.78 mg of zinc acetate dihydrate, 5 ml of oleic acid and 3 ml of ODE were added to a flask. The contents were held under vacuum at 100ºC for 1 minute and the flask was filled with argon. Temperature was raised to 250ºC for 10 minutes to convert zinc acetate to zinc oleate. Mixture was cooled and 16 ml ODE and 1 ml of oleylamine were injected to produce 0.1 M zinc oleate solution.
(c) 0.1M S in ODE: 64 mg of elemental S was added to 20 ml of ODE and stirred overnight at room temperature to produce 0.1M solution of S.
(d) 0.25M Te in TOP: 0.3166 g of Te pieces and 10 ml of TOP was stirred for 12 hours to produce a 0.25M solution of Te.
(e) Precursors for Zinc selenide growth: 1264.7 mg of zinc stearate, 156 mg of Se and 8 ml ODE were added to a flask and stirred for 10 hours to make 0.25M of zinc and Se precursors in the form of slurry.
(f) TOPO/TDPA in ODE: 0.1 g of TDPA and 0.3 g of TOPO was dissolved in 10 ml of ODE by stirring overnight.
The heterostructure nanocrystal was prepared under argon atmosphere. Oleylamine was used as ligand and ODE was used as the solvent medium. The preparation of ZnTeSe/ZnSe/CdZnS was carried out in two steps. The first step involved the formation of ZnTeSe or ZnTe core and the second step involved the formation of intermediate and outer shell.

Preparation of ZnTeSe core:
[0089] 94.5 mg of sodium borohydride and 7.8 mg of Se was transferred to a flask filled with argon. 1.6 ml of Te in TOP prepared as above is injected into the flask along with 100 µl of 1,4-butanediol and temperature of the reaction mixture was raised to 75ºC for 5 minutes. The flask was then cooled to room temperature and 90 mg of dextrose, dissolved in 1 ml of 1,4-butandiol, was added to it in order to consume any unreacted borohydride. Unreacted borohydride left in the flask decomposes at higher temperatures with the evolution of hydrogen gas.
[0090] 109.8 mg of zinc acetate dihydrate, 8 ml of ODE and 2 ml of oleylamine was taken in the second flask. Reaction mixture is held under vacuum at 100ºC and the flask was filled with argon. 40% of the contents of the first flask (Se-Te precursor prepared as mentioned above) was rapidly injected into the second flask containing zinc acetate. 60% of the remnant of flask 1 was then transferred to the second flask drop wise, while ensuring that the temperature of the mixture is at 100ºC.

Preparation of ZnTe core:
[0091] 94.5 mg of sodium borohydride was transferred to a flask filled with argon. 2 ml of Te in TOP prepared as above was injected into the flask along with 100 µl of 1,4-butanediol and the temperature of the reaction mixture was raised to 75ºC for 5 minutes. The flask was then cooled to room temperature and 90 mg of dextrose, dissolved in 1 ml of 1,4-butandiol, was added to it in order to consume any unreacted borohydride. Unreacted borohydride left in the flask decomposes at higher temperatures with the evolution of hydrogen gas.
109.8 mg of zinc acetate dihydrate, 8 ml of ODE and 2 ml of oleylamine were taken in a second flask. Reaction mixture was held under vacuum at 100ºC and the flask was filled with argon. 40% of the content in the first flask (Te precursor prepared as mentioned above) was rapidly injected into the second flask containing zinc acetate. 60% of the remnant of flask 1 was then transferred to the second flask drop wise, while ensuring that the temperature of the mixture was at 100ºC.

Preparation of ZnSe/ZnCdS shells on ZnTe and ZnTeSe cores:
[0092] ZnTeSe or ZnTe cores were prepared as mentioned above. The temperature of the reaction mixture was raised to 160ºC and 0.5 ml of zinc and Se precursor slurry prepared as aforementioned was added to it. Mixture was then annealed at 280ºC for ten minutes. Temperature of the reaction mixture was then lowered to 160ºC and 0.5 ml of zinc and Se precursor slurry was further added to it and mixture was again annealed at 280ºC for ten minutes. This procedure was repeated until ZnSe shell of desired thickness was obtained (3 to 4 monolayers). 100 µl of TOP was then added to the reaction mixture to dissolve excess Se. The reaction mixture was then cooled to room temperature and the quantum dots were cleaned using methanol alone. Cleaned dots (QDs) were then dispersed in minimum amount of hexane and transferred to a flask. 1 ml oleylamine and 4 ml ODE were added to the flask and the mixture was held under vacuum for 100 seconds at 100ºC and was then filled with argon. A 1:1 solution of 0.1M cadmium oleate and 0.1M Zn-oleate was prepared by mixing two cation precursors. 1 ml of 0.1M cation precursor and 1 ml of 0.1M S in ODE was added dropwise to the reaction mixture at 240ºC in presence of 1 ml of DDT. This was followed by addition of 0.5 ml of 0.1M cadmium oleate and 0.5M S in ODE. 1ml of TOPO/TDPA in ODE, 100 µl of TOP and 0.5 ml cadmium oleate was then added. Drop wise addition of cadmium oleate and S in ODE was continued until PL maxima reached 800 nm.
[0093] Quantum dots thus obtained were characterized using powder x-ray diffraction, TEM, and optical techniques.
[0094] While the foregoing describes various embodiments of the invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof. The scope of the invention is determined by the claims that follow. The invention is not limited to the described embodiments, versions or examples, which are included to enable a person having ordinary skill in the art to make and use the invention when combined with information and knowledge available to the person having ordinary skill in the art.

ADVANTAGES OF THE INVENTION
[0095] The present disclosure provides a new medium that exhibit gain under conditions of extremely low levels of continuous wave illumination and convert incoherent light to coherent light and lasers.
[0096] The present disclosure overcomes limitations of QDs as active laser medium by enabling them to exhibit gain under conditions of extremely low levels of continuous wave illumination.
[0097] The present disclosure provides improved means of generating coherent light from incoherent light by introducing incoherent light into a medium comprising of quantum dots. The medium provided by the present disclosure absorbs incoherent light in whole or in part, and converts it into coherent light.
[0098] The present disclosure provides improved means of increasing the power of a beam of light by absorbing light from different beams of light.
[0099] The present disclosure provides improved means of converting sunlight into coherent laser beam.
[00100] The present disclosure provides improved means of converting incoherent light from the conventional sources such as a light bulb, a flash lamp, an arc lamp or a Light Emitting Diode into laser radiation.
[00101] The present disclosure provides improved means of converting absorbed coherent light derived from a source such as but not limited to a laser diode, into a coherent beam.
[00102] The present disclosure provides improved means that absorbs a temporally continuous beam of light and emits a temporally pulsed beam.
[00103] The present disclosure provides improved means that absorbs a temporally continuous beam of light and emits a temporally discontinuous beam.
[00104] The present disclosure provides improved means that absorbs a temporally continuous beam and emits a temporally modulated beam.
The present disclosure provides improved means that absorbs a temporally pulsed beam of light and emits a temporally discontinuous beam.
,CLAIMS:1. Quantum dots (QDs) for use as active laser medium, wherein the QDs comprise two or more compositional layers, wherein at least one of the two or more layers is an alloy semiconductor, and wherein the QDs exhibit an average spontaneous emission lifetime longer than 100 ns when held at room temperature in a dispersion with refractive index between 1 and 1.7 in absence of any other means to modify photonic density of states.

2. The QDs as claimed in claim 1, wherein the QDs exhibit gain under continuous wave illumination.

3. The QDs as claimed in claim 2, wherein the QDs exhibit a gain threshold under illumination of 1000 mW/cm2 or less.

4. The QDs as claimed in claim 1, wherein the QD is prepared from one or more of materials selected from the group comprising ZnTe, ZnTeSe, ZnCdS, ZnTeS, CdZnTeSe, CdZnSeTeS.

5. The QDs as claimed in claim 1, where the lasing threshold occurs at less than 0.75 excitons per QD.

6. The QDs as claimed in claim 1, where the lasing threshold occurs at less than 0.5 excitons per QD.

7. The QDs as claimed in claim 1, wherein the QDs have broadband absorption cross sections in excess of 10-16 cm2 per QD over at least a 0.5 eV range of energies over the 1 - 4 eV spectral region and the broadband absorption enables use of polychromatic light sources as pump.

8. The QDs as claimed in claim 3, wherein any or combination of conventional light sources selected out of sun, light bulb, a flash lamp, an arc lamp, a Light Emitting Diode or a laser diode is used as pump.

9. The QDs as claimed in claim 1, wherein a layer of the QDs placed between a partially reflective mirror and a dichroic mirror, and pumping from the dichroic mirror results in simulated emission of laser beam.

10. The QDs of claim 1 where the QDs exhibit localization of one or more of the photogenerated carriers within 100 ns of optical excitation.

11. A laser device comprising a plurality of QDs and a cavity arranged to provide optical feedback that allows the QDs to amplify an optical signal when they are optically pumped above the device lasing threshold by a pump source that has a temporally constant or a temporally variable power output, and wherein peak irradiance produced by the pump source in the QD laser medium is less than 105 W/cm2.

12. The device of claim 11, wherein operation threshold of the device occurs at apeak input pump optical irradiance of 1000 mW/cm2 or less.