METHODS FOR OBTAINING AN N-TYPE DOPED METAL CHALCOGENIDE QUANTUM DOT SOLID-STATE ELEMENT WITH OPTICAL GAIN AND A LIGHT EMITTER INCLUDING THE ELEMENT, AND THE OBTAINED ELEMENT AND LIGHT EMITTER

Abstract

The present invention relates to a method for obtaining an n-type doped metal chalcogenide quantum dot solid-state element with optical gain for low-threshold, band-edge amplified spontaneous emission (ASE), comprising: —forming a metal chalcogenide quantum dot solid-state element, and —carrying out an n-doping process on its metal chalcogenide quantum dots to at least partially bleach its band-edge absorption, which comprises: —a partial substitution of chalcogen atoms by halogen atoms, in the metal chalcogenide quantum dots, and/or —a partial aliovalent-cation substitution of bivalent metal cations by trivalent cations, in the metal chalcogenide quantum dots; and —providing a substance on the metal chalcogenide quantum dots, to avoid oxygen p-doping. The present invention also relates to the obtained n-type doped metal chalcogenide quantum dot solid-state element, a method for obtaining a light emitter with that n-type doped metal chalcogenide quantum dot solid-state element, and the obtained light emitter.

Claims

1. A method for obtaining an n-type doped metal chalcogenide quantum dot solid-state element with optical gain for low-threshold, band-edge amplified spontaneous emission (ASE), comprising: forming a metal chalcogenide quantum dot solid-state element, and carrying out an n-doping process on at least a plurality of the metal chalcogenide quantum dots of said metal chalcogenide quantum dot solid-state element, to at least partially bleach its band-edge absorption, wherein said n-doping process comprises: a partial substitution of chalcogen atoms by halogen atoms, in at least said plurality of metal chalcogenide quantum dots, and/or a partial aliovalent-cation substitution of bivalent metal cations by trivalent cations, in at least said plurality of metal chalcogenide quantum dots; and providing a substance on at least said plurality of metal chalcogenide quantum dots, wherein said substance is an oxide-type substance made and arranged to avoid oxygen p-doping of the plurality of metal chalcogenide quantum dots.

2. The method according to claim 1, wherein said metal chalcogenide is at least one of Pb-, Cd-, and Hg-chalcogenide, wherein said chalcogen atoms are at least one of sulphur, selenium, and tellurium atoms, and wherein said halogen atoms are at least one of iodine, bromine, and chlorine atoms.

3. The method according to claim 1, wherein said metal chalcogenide is at least one of Pb-, Cd-, and Hg-chalcogenide, wherein said bivalent metal cations are at least one of Pb, Cd, and Hg, in the +2 oxidation state, and wherein said trivalent cations are at least one of In, Bi, Sb, and Ga, in the +3 oxidation state.

4. The method according to claim 1, comprising providing said substance to: coat said metal chalcogenide quantum dot solid-state element to isolate the same from ambient oxygen; and/or infiltrate within the metal chalcogenide quantum dot solid-state element to react with oxygen present therein for suppressing their p-doping effect.

5. The method according to claim 1, wherein said substance is at least one of alumina, titania, ZnO, and hafnia.

6. The method according to claim 1, wherein said step of forming said metal chalcogenide quantum dot solid-state element comprises forming a blend with a host matrix of first metal chalcogenide quantum dots and, embedded therein, said plurality of metal chalcogenide quantum dots, which are second metal chalcogenide quantum dots having a smaller or equal bandgap, wherein said second metal chalcogenide quantum dots are larger than said first metal chalcogenide quantum dots, and wherein the method comprises applying said n-doping process at least on the second metal chalcogenide quantum dots so that they are heavily n-doped.

7. The method according to claim 1, comprising selecting the size of said plurality of metal chalcogenide quantum dots to obtain, after said n-doping process has been carried out thereon, an initial electron occupancy doping <N>.sub.D ranging from 1.4 to 5.4.

8. The method according to claim 7, wherein said step of selecting the size of said plurality of metal chalcogenide quantum dots comprises selecting quantum dot diameters ranging from 5.0 nm to 6.2 nm for PbS colloidal quantum dots.

9. The method according to claim 1, wherein at least part of said plurality of metal chalcogenide quantum dots are of a core-shell type, each including a core and at least one shell, wherein said core comprises a metal chalcogenide and said shell a distinct metal chalcogenide or an alloy of the metal chalcogenide of the core, and wherein the n-doping process is applied to either the core, the at least one shell, or both.

10. An n-type doped metal chalcogenide quantum dot solid-state element with optical gain for low-threshold, band-edge amplified spontaneous emission (ASE), comprising a plurality of metal chalcogenide quantum dots with its band-edge absorption at least partially bleached, wherein said plurality of metal chalcogenide quantum dots comprises: some chalcogen atoms substituted by halogen atoms, and/or some bivalent metal cations aliovalent-cation substituted by trivalent cations; and wherein a substance is provided on at least said plurality of metal chalcogenide quantum dots, wherein said substance is an oxide-type substance made and arranged to avoid oxygen p-doping of the plurality of metal chalcogenide quantum dots.

11. The n-type doped metal chalcogenide quantum dot solid-state element of claim 10, wherein said substance is at least one of alumina, titania, ZnO, and hafnia.

12. (canceled)

13. A method for obtaining a light emitter, comprising: providing a gain medium comprising at least one n-type doped metal chalcogenide quantum dot solid-state element obtained according to the method of claim 1; and providing an optical or electrical pump configured and arranged to excite said at least one n-type doped metal chalcogenide quantum dot solid-state element so that a population inversion is produced therein that generates an amplified spontaneous emission (ASE).

14. A light emitter, comprising: a gain medium comprising at least one n-type doped metal chalcogenide quantum dot solid-state element obtained according to the method of claim 1; and an optical or electrical pump configured and arranged to excite said at least one n-type doped metal chalcogenide quantum dot solid-state element so that a population inversion is produced therein that generates an amplified spontaneous emission (ASE).

15. A light emitter according to claim 14, wherein the light emitter is a superluminescence light emitter.

16. A light emitter according to claim 14, wherein the light emitter is a laser device further comprising a laser optical cavity and, optically coupled thereto, said gain medium, which is a laser gain medium, wherein said laser optical cavity is configured and arranged to provide optical feedback to said amplified spontaneous emission (ASE).

17. A light emitter according to claim 16, wherein said laser device comprises at least one of a vertical-cavity surface-emitting laser structure (VCSEL), a distributed feedback laser structure (DFB), and a whispering gallery mode laser structure (WGM).

18. A light emitter according to claim 17, wherein: said VCSEL structure comprises said laser gain medium with a thickness ranging from 200 nm to 1 μm, sandwiched between two Bragg reflectors forming a photonic bandgap ranging from 1000 nm to 2000 nm; said DFB structure comprises a waveguide resonator formed by: a corrugated substrate with corrugations implemented by periodically arranged structured elements forming a grating with a grating height ranging from 20 nm to 500 nm, and a periodicity ranging from 700 nm to 1400 nm, and said laser gain medium, with a thickness ranging from 20 nm to 1500 nm, arranged on top of said corrugated substrate over said corrugations; and said WGM structure comprises said laser gain medium with a thickness ranging from 10 nm to 2000 nm, optically coupled to one or more WGM resonators for single or multi laser mode, wherein the diameter of each resonator ranges from 50 μm to 1000 μm.

19. A light emitter according to claim 14, configured to emit light with a wavelength ranging from 800 nm to 2400 nm.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0069] In the following some preferred embodiments of the invention will be described with reference to the enclosed figures. They are provided only for illustration purposes without however limiting the scope of the invention.

[0070] FIG. 1. Doping in PbS CQDs films, according to the method of the first aspect of the present invention. a, Schematic representation of the S2-substitution to I— in (100) surface in large, cuboctaehedral-shaped PbS CQDs b,c, Calculated density of states (DOS) of the (100) surface before and after I-substitution showing that the Fermi level, EF is shifted to the conduction band d, Absorption spectra of two representative PbS CQD films, before and after doping. The CQD sizes are 5.5 nm and 6.1 nm with respective exciton peaks at 1480 nm and 1580 nm (solid black lines). After doping of the CQD films the absorption bleaches (red dash lines) e, The number of electrons in the conduction band, upon doping, depends on the size of the CQD due to the degree of (100) surface presence that enables doping. The dots represent the experimentally extracted number of electrons from measuring the bleaching of the absorption at the exciton peak overlaid with a sigmoidal function (black dash line) as a guide to the eye.

[0071] FIG. 2. Optical gain and transient absorption in PbS CQD solids. The gain spectra have been obtained in various probing wavelengths and different pump fluencies which are presented as exciton occupancy, <N> for both doped and undoped CQD films. The optical gain arises at the point that −Δα/α.sub.PbS>1 (horizontal black dash line). a, The gain spectra of the 6.2 nm PbS CQD (exciton peak at 1600 nm) shows gain threshold at <N>.sub.thr=4. b, The corresponding doped film exhibits a drastically reduced gain threshold at <N>.sub.thr=0.9 at 1650 nm. c, Comparison of gain spectra of various sized doped PbS CQDs films, possessing different initial doping values. The <N>.sub.thr reduces upon increasing initial doping. d,e, The transient absorption curves of the aforementioned doped and undoped samples at the probing wavelength of 1650 nm show that in the regime below optical gain the carrier dynamics follow a mono-exponential decay correlated with the band-edge relaxation. In the optical gain regime another fast component appears which is assigned to the gain lifetime. f, Average gain lifetime <τ.sub.gain> of different CQD sizes calculated from a range of pumping intensities while the error bars present the lowest and the highest measured values.

[0072] FIG. 3. ASE and VSL measurements in PbS CQD films. The present inventors collected the ASE and the VSL spectra in a series of PbS CQD films both doped (according to the method of the first aspect of the present invention) and undoped, while here the present inventors show selective data for two samples with <N>.sub.D of 2.7 and 4. The present inventors have excited the samples with a stripe-shaped laser beam at 800 nm with a Ti:Saphire femtosecond laser with pulse width of 80 fs and repetition rate of 1 KHz at ambient conditions. a,b, The ASE area has been calculated by fitting the ASE peak with a Gaussian function while the photon density it is shown in terms of exciton occupancy. The solid dot represent the integrated ASE area of doped and the hollow dots the undoped samples. The present inventors further confirm that the undoped samples show ASE threshold <N>.sub.thr=4, while the doped samples show <N>.sub.thr<4 while both ASE signal are saturated for a total occupancy of 8 c-f, The corresponding PL spectra of these sample in a range of pumping intensities with a threshold of 70 μJ/cm.sup.2 for the undoped and down to 25 μJ/cm.sup.2 for the doped films g, Summarizing ASE thresholds in terms of exciton occupancy in PbS CQD of different sizes h, The modal gain in the undoped films remains stable at 30 cm.sup.−1 while in doped samples is increased with a peak value of 114 cm.sup.−1 i, Collective ASE spectra from a series of PbS CQD films shows the tunability of the ASE peak from 1530 nm to 1650 nm, within the range of the fibre-optic communication bands.

[0073] FIG. 4. Schematic representation of (111) surface with Iodine passivation (a) Calculated density of stated of the (111) surface (b).

[0074] FIG. 5. Series of absorption spectra of doped PbS CQD film for a period of 67 days (solid lines). The sample was kept in ambient conditions with continuous exposure of ambient light. The respective undoped films is plotted for comparison (dash line).

[0075] FIG. 6. The variation of photocurrent as a function of absorbed photon-flux in doped and undoped PbS CQD films treated with ZnI2/MPA and EDT for comparison.

[0076] FIG. 7. Transient absorption spectra of doped PbS CQD of 5 nm (a) 5.6 nm (b) and 5.9 nm (c) sizes. The lifetime traces have been fitted with mono-exponential decay below the gain threshold −Δα/α.sub.PbS=1, while at the gain regime the lifetimes have been fitted with bi-exponential decay (solid black line).

[0077] FIG. 8. Representative fitting and analysis of the undoped (a) and doped (b) samples at single exciton and multi-exciton regime.

[0078] FIG. 9. Amplified spontaneous emission measurements of doped and undoped PbS CQD solids at PbS CQD sizes of 5.5 nm (a)-(b) and 6.0 nm (c)-(d).

[0079] FIG. 10. Power dependence measurements of all PbS CQD films in terms of exciton occupancy versus the Integrated PL intensity (S-curves).

[0080] FIG. 11. ASE peak intensity versus stripe length for both doped and undoped CQD films of various PbS sizes.

[0081] FIG. 12 schematically shows a cross-section core-shell type structure (a) for a metal chalcogenide quantum dot n-doped according to the method of the first aspect of the present invention, and corresponding band alignment diagrams without (b) and with (c) gradual shelling.

[0082] FIG. 13 schematically shows the light device of the fourth aspect of the present invention, for an embodiment for which the device implements a laser device having a VCSEL structure.

[0083] FIG. 14 schematically shows the light device of the fourth aspect of the present invention, for an embodiment for which the device implements a laser device having a DFB structure, for an embodiment (top view) for which the DFB laser structure operates as first-order and an embodiment (bottom view) for which the DFB laser structure operates as mixed order.

[0084] FIGS. 15a and 15b schematically show the light device of the fourth aspect of the present invention, for an embodiment for which the device implements a laser device having a WGM structure in the form of a hollow cylinder/fibre filled with the gain medium (FIG. 15a), and in the form of a solid cylinder/fibre with its outer surface covered with the gain medium.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0085] In the present section, by means of several experiments detailed below, the present inventors demonstrate the feasibility and good results offered by the present invention, specifically for embodiments for which the metal chalcogenide quantum dot solid-state elements are PbS quantum dot solid-state elements, and sulphur atoms are partially substituted by iodine atoms.

[0086] Here the present inventors demonstrate infrared stimulated emission tuneable across the optical communication band based on PbS CQDs. The present inventors have employed robust heavily doped PbS CQD solid-state conductive elements that reach gain threshold at the single exciton regime, representing a four-fold reduction from the theoretical limit of an eight-fold degenerate system. They also exhibit room temperature stimulated emission near the single exciton regime, at a threshold two orders of magnitude lower than prior reports [11,12], and a net modal gain in excess of 110 cm.sup.−1, the highest reported to date in the infrared.

[0087] The present inventors posited that a CQD element, such as a film, robustly doped in the heavy doping regime, can address this challenge by utilizing the doping electrons present in the first excited state of the CQDs (conduction band) to reach the population inversion condition at reduced pumping fluence. To test the here proposed hypothesis, the present inventors employed a method to dope PbS CQDs in the heavy doping regime. The doping mechanism takes place by iodine substitution of surface sulphur sites on (001) exposed surface facets (FIG. 1a). As evidenced by DFT calculations on PbS (001) surfaces, there is evident n-type doping upon iodide substitution as illustrated in FIG. 1b. Iodide binding on (111) surfaces on the other hand serve as passivant without causing any strong doping effect [21] (see FIG. 4). In order to quantify the doping the present inventors monitored the exciton bleaching of the QDs (quantum dots) in absorption. Absorption spectra, as shown in FIG. 1d, illustrate that band-edge absorption of the doped PbS CQD films is bleached as a result of the population of the first excited state upon doping. Because (001) surfaces are progressively exposed with increasing PbS CQD size, the doping efficacy of the process increases with the size of the dots. FIG. 1e shows that particles smaller than 4 nm in diameter do not undergo this doping process due to the lack of (100) exposed facets, whereas in particles with an exciton peak of more than 1800 nm (˜7.5 nm) the conduction band is fully filled with 8 electrons (FIG. 1e). It is noteworthy that the doped CQD films are stable at room temperature and ambient conditions for a period of more than 2 months (see FIG. 5).

Transition Absorption PbS CQD Films:

[0088] To verify the hypothesis of reaching single exciton gain threshold in doped PbS CQD films, the present inventors performed transient absorption (TA) studies in undoped PbS CQD films as well as a series of doped PbS CQD films with variable initial electron occupancy doping <N>.sub.D, determined by their size (FIG. 1d). The undoped PbS CQD films demonstrate optical gain threshold <N>.sub.thr—expressed in excitons per dot—of four, as expected from the 8-fold degeneracy (FIG. 2a). Upon doping, the <N>.sub.thr reduces and for the case of initial doping <N>.sub.D of 5.4 the <N>.sub.thr reaches a value of 0.9 excitons per dot (FIG. 2b). This four-fold reduction of the gain threshold upon doping outperforms the two-fold reduction reported in CdSe based CQD systems [14]. By varying the initial doping of the CQD films, according to the size-doping dependence shown in FIG. 1e, the present inventors have measured <N>.sub.thr values of 4.7, 2.3, 1 and 0.9 excitons per dot for QD sizes with diameter (initial doping) of: 5.0 nm (<N>.sub.D=1.4), 5.6 nm (<N>.sub.D=3.4), 5.9 nm (<N>.sub.D=4.4) and 6.2 nm (<N>.sub.D=5.4) respectively (FIG. 2c). This finding further corroborates the here proposed hypothesis of the effect of doping on the optical gain threshold.

[0089] The corresponding transient absorption of the undoped and doped films (FIG. 2d-e) shows a mono-exponential decay for <N> below the gain threshold while above the gain threshold the lifetime traces are fitted with a bi-exponential decay. The nearly mono-exponential lifetime below the gain threshold is ranging for both doped and undoped samples from 300-600 ps which the present inventors tentatively attribute to band-edge recombination. On the other hand, at the optical gain regime, a second (τ.sub.2) fast component is rising. The present inventors assign the fast τ.sub.2 component to the gain lifetime while the long τ.sub.1 component is likely due to band-edge recombination (100-300 ps). This can be further illustrated by FIGS. 7 and 8.

[0090] Particularly, the diagrams of FIG. 7 show the transient absorption spectra of doped PbS CQD of 5 nm (a) 5.6 nm (b) and 5.9 nm (c) sizes. The lifetime traces have been fitted with mono-exponential decay below the gain threshold −Δα/α.sub.PbS=1, while at the gain regime the lifetimes have been fitted with bi-exponential decay (solid black line).

[0091] FIG. 8 shows the representative fitting and analysis of the undoped (a) and doped (b) samples at single exciton and multi-exciton regime. The transient fitting was carried out with a mono-exponential decay I(t)=y.sub.0+A.sub.1e.sup.−(t-t0)/τ1 and for bi-exponential decay I(t)=y0+A.sub.1e.sup.−(t-t0)/τ1+A.sub.2e.sup.−(t-t0)/τ2 functions.

[0092] The fitting parameters of the transient absorption measurements are summarized at the table below:

TABLE-US-00001 A.sub.1 τ.sub.1 (ps) A.sub.2 τ.sub.2 (ps) Undoped <N> = 1 0.029 613 — — Undoped <N> = 9 0.049 490 0.073 38 Doped <N> = 1 0.0214 614 — — Doped <N> = 9 0.08 217 0.118 22

[0093] Table 1 Furthermore, extracting the gain lifetime from the transient absorption data at the probing wavelength of the highest gain value (FIG. 2f) the present inventors found an average gain lifetime of 27 ps in doped CQD films while the error bar represent the lowest and the highest values obtained in different pumping photon densities (see also FIG. 7). Slightly longer average gain lifetime of 35 ps is observed in undoped samples. It is noteworthy, that despite the presence of doping, Auger processes do not prevent reaching the gain regime. This can be due to the fact that gain lifetime is faster than the Auger and therefore competes favourably to it and/or Auger process is suppressed in this system.

Photoconductivity Measurement:

[0094] To shed further insights, having conductive films, the present inventors have performed transient photoconductivity measurements [22] that, as will be shown below, yield a very low value for the Auger coefficient of 10.sup.−31 cm.sup.6s.sup.−1, lower than prior reports for PbS CQDs [23]. This is likely due to the conductive nature of those films [22].

[0095] Indeed, the dependence of the photocurrent density on the excitation intensity was investigated to understand the mechanism of photo-carrier generation in the chemically treated elements following a previously reported procedure [33]. The photocurrent was collected at a voltage bias of 20 V. In FIG. 3 the present inventors show the variation of photo-current as a function of incident photon flux for different ligand treated PbS QD solids.

[0096] The standard EDT (1,2-ethanedithiol) treated and the according to the present invention ZnI.sub.2/MPA treated films showed different behaviour with high photon flux as the higher degree of photocurrent saturation observed in case of EDT treated film [33]. For doped ZnI.sub.2/MPA treated film, the photocurrent increased nearly 3 orders of magnitude due to the improvement in the mobility. The photon flux dependence of the photo-current showed a similar trend of the ZnI.sub.2/MPA treated undoped materials. To quantify the recombination dynamics involving more than one charge carrier taking place in the QD solids, the following equations have been considered,

[00001]$\begin{array}{cc}\frac{\mathrm{dn}}{\mathrm{dt}}=G-an-b{n}^2-c{n}^3& \left(1\right)\\ I\propto n& \left(2\right)\end{array}$

[0097] Where G is the generation rate, an is the carrier trapping rate, bn.sup.2 is the radiative recombination rate and cn.sup.3 is the Auger recombination rate. Photocurrent I is proportional to the charge carriers n. The plots are fit in FIG. 6 using equations 1 and 2 to get the idea of different recombination mechanisms in the QD solids. As the proportionality constant of the equation 2 is not known, proper fitting parameters to be found with b/a.sup.2 and c/a.sup.3 calculations. The fitting parameters are summarised in Table 2 below.

TABLE-US-00002 TABLE 2 DEVICE b/a.sup.2 (s cm.sup.3) c/a.sup.3 (s.sup.2cm.sup.6) PbS_EDT treat 3.4 × 10.sup.−14   3 × 10.sup.−27 Undoped PbS_ZnI.sub.2/MPA 1.2 × 10.sup.−14 1.2 × 10.sup.−31 Doped PbS_ZnI.sub.2/MPA   9 × 10.sup.−15   1 × 10.sup.−31

[0098] The radiative (bi-molecular) rate parameters showed nearly similar values for all the cases whereas there is a distinct difference between EDT treated and ZnI.sub.2/MPA treated films in case of Auger coefficient. ZnI.sub.2/MPA treated doped and undoped both PbS QD solids showed a much lower Auger coefficient compared to the standard EDT treated solids. This confirms fast dissipation of charges in QD solids based with ZnI.sub.2/MPA treatment (both doped and undoped).

Power Dependence ASE Measurements:

[0099] Optical gain is a prerequisite for stimulated emission. Having achieved this, next the present inventors performed amplified spontaneous emission (ASE) measurements of thin films obtained according to the method of the first aspect of the present invention. In line with the TA measurements the present inventors observed stimulating emission from both doped and undoped samples (see FIG. 9). The present inventors calculated the stimulated emission occupancy threshold <N>.sub.thr by fitting the integrated PL spectra (see FIG. 10) from the power dependent S-curve of all the samples. In FIGS. 3a,b the present inventors plot the integrated ASE peak as a function of exciton occupancy and the respective power dependence measurement of two representative PbS sizes of 5.4 nm (<N>.sub.D=2.7) and 5.8 nm (<N>.sub.D=4.0) (FIG. c-f). All the undoped samples have an <N>.sub.thr of 4, in agreement with the TAS measurements of FIG. 2, and the ASE signal is saturated when 8 electrons have fully populated the conduction band. Power dependence measurement of those samples are shown in FIG. c-f positioning the stimulating emission peak in wavelengths above 1500 nm. The sharp ASE peak has an average FWHM of 14 meV, characteristic of a stimulated emission process and comparable with reported values in the visible from CdSe-based systems [14]. The stimulated emission threshold occupancy is summarized in FIG. 3g. The undoped samples preserve a constant value of <N>.sub.thr of 4 independent of their size, whereas in the case of doped CQDs increasing their size—and thereby the initial doping occupancy—the stimulated emission threshold decreases to a minimum value of 1.3 excitons in agreement with the transient absorption measurements.

[0100] A figure of merit of paramount importance for applications in optical amplification and lasing is the net modal gain of the material. The present inventors have experimentally measured the net modal gain g.sub.modal using the variable stripe length (VSL) technique, from the measured data shown in FIG. 11 for both doped and undoped CQD films of various PbS sizes.

[0101] The variable strength dependence data have been collected at the pump fluence in the saturation regime of the ASE signal. In order to extract the net modal gain values the data have been fitted with the following function:

[00002]$I\left(\lambda \right)=\frac{A\left(\lambda \right)}{G\left(\lambda \right)}\left(e^{G*L}-1\right)$

[0102] Where:

[0103] G: net modal gain coefficient

[0104] I: photoluminescence Intensity

[0105] L: stripe length

[0106] A: spontaneous emission growth parameter

[0107] The present inventors report an average g.sub.modal of 30 cm.sup.−1 nearly constant for all the undoped samples (FIG. 3h). Upon doping g.sub.modal increases up to a value of 114 cm.sup.−1 for an <N>.sub.D of 4, when the conduction band is half filled due to doping. This modal gain value outperforms prior reports from HgTe infrared CQDs with g.sub.modal of 2.4 cm.sup.−1 based on trap-to-band ASE [24], Er-doped fibre systems with values in the range of 0.01-0.1 cm.sup.−1 and compares favourably to costly epitaxial III-V multi quantum well and quantum dot systems [25]. For the first time, the present inventors demonstrate nearly full coverage in the whole optical fibre communication spectrum (FIG. 3i) from a solution processed material, extending beyond the spectral coverage of Er-doped fibre systems. The tuneable spectral coverage across the infrared taken together with the high gain values and the low threshold represent a significant advance towards the deployment of CQD solution processed lasers in the infrared. Most importantly, ASE has been demonstrated, for the first time, in conductive films of CQDs, of paramount practical importance for the realization of electrically pumped CQD lasers.

Calculating the Occupancy Per Dot:

[0108] In order to calculate the number of the occupancy per dot the present inventors carefully measured the thickness of each CQD film, H.sub.film using profilometry. Therefore, the number of QDs within the excitation area A.sub.exc is determined:

[00003]$\begin{array}{cc}{N}_{CQD}=\frac{A_{exc}{H}_{\mathrm{film}}0.74}{V_{\mathrm{CQD}}}\left(1\right)& \left(1\right)\end{array}$

Where V.sub.CQD is the volume of the quantum dots.

[0109] Next the energy required to generate one N=1 carrier per NC was calculated. For this reason, both the transmission (T) and the reflection (R) spectra of the film at 800 nm were measured.

[00004]$\begin{array}{cc}{N}_{\mathrm{photons}}^{<N>=1}=\frac{N_{\mathrm{CQD}}}{\left(1-T\right)\left(1-R\right)}& \left(2\right)\end{array}$

[0110] The photon energy at the excitation wavelength (800 nm) is given by:

[00005]$\begin{array}{cc}{E}_{\mathrm{photon}}^{800}=\frac{hc}{\lambda }=2.483{10}^{-19}J& \left(3\right)\end{array}$

[0111] Where h is the Planck's constant, c is the speed of light, and λ the wavelength of the radiation.

[0112] So the incident energy required for N=1:

[00006]$\begin{array}{cc}{E}_{CQD}^{<N>=1}=N_{\mathrm{photons}}^{<N>=1}\times 2.483{10}^{-19}& \left(4\right)\end{array}$

Methods:

PbS CQDs Synthesis:

[0113] PbS QDs synthesis was adapted from a previously reported multi-injection procedure. Briefly, 0.446 g lead(II) oxide (PbO, 99.999% Pb, Strem Chemicals), 50 mL 1-Octadecene (ODE, 90%, Alfa Aesar) and 3.8 mL oleic acid (OA, 90%, Sigma Aldrich) were introduced in 3-neck, round bottom flask and degassed overnight, under vacuum at 90° C. Then the reaction temperature was increased at 95-100° C. under Argon and 60 μL of Hexamethyldisilathiane ((TMS).sub.2S, Sigma Aldrich) diluted in 3 ml of ODE was swiftly injected. After 6 minutes, a second solution of 75 μL (TMS).sub.2S in 9 ml ODE was injected dropwise in a rate of 0.75 mL/min. The reaction was constantly monitored with aliquots and is was stop when at the desirable QD size. At that point both the heating and the injection was stopped and the solution was let cool down slowly at room temperature. QDs were purified three times by precipitation with anhydrous acetone and ethanol and re-dispersed in anhydrous toluene. Finally, the concentration was adjusted to 30 mg/mL and the solution was bubbled with N.sub.2 in order to minimize to oxidation of the QDs.

Doped PbS CQD Films:

[0114] The ad-hoc PbS CQDs (30 mg/ml) were spin-cast onto soda-lime glass substrates (1 cm×1 cm) at the speed rate of 2500 rpm for 20 s. The film was treated with ZnI.sub.2/MPA (7 mg/ml of ZnI.sub.2 dissolved in 0.01% of MPA in Methanol) solution for 5 s and the spin-coater was started again to dry the film, while 300 μL was MeOH were drop-casted to wash away the remain ligands. This procedure was repeated till the film thickness of ˜110 nm (4-5 layers). The film thickness was measured with profilometer. The PbS CQD films were doped after the capping with Al.sub.2O.sub.3 with atomic layer deposition (ALD).

Atomic Layer Deposition:

[0115] Al.sub.2O.sub.3 deposition was performed in a GEMStar XT Thermal ALD system. High-purity trimethylaluminium (TMA), purchased from STREM Chemicals Inc., was used as Al precursor. Pure H.sub.2O was used as 0 precursor. The deposition was carried out at 80° C. Before the process, the reaction chamber was pumped down and subsequently filled with pure nitrogen up to a pressure of approximately 0.56 mbar. The TMA and H.sub.2O manifolds were maintained at 150° C. during gas supply. Each layer of Al.sub.2O.sub.3 was formed by applying a 15-ms pulse of H.sub.2O at a partial pressure of 0.02 mbar, followed by a 50-ms pulse of TMA, at a partial pressure of 0.18 mbar. The waiting time between pulses was 15 s and 20 s, respectively.

Transmission and Absorption Measurements:

[0116] Room-temperature absorption measurements were taken under ambient atmosphere, using a Cary 5000 UV-Vis-NIR spectrometer.

Measurements of Doping Level by Optical Measurements:

[0117] Since, the 1S.sub.e states of PbS are eight-fold degenerated (including spin), the number of electrons in the CB per dot, n.sub.QD, can be calculated in a straightforward manner from the bleach of the first exciton transition. If I.sub.1 and I.sub.2 are defined as the integrated absorption strength of the excitonic transition of the undoped and doped samples, respectively, then n.sub.QD=8(1−I.sub.2/I.sub.1). Note that by saying undoped sample, the present inventors are assuming that the doping (whether p-type or n-type) of the samples without alumina is low enough to consider full valence band and empty conduction band.

Reflection Measurements of CQD Films:

[0118] Reflection measurements were obtained using a PerkinElmer Lambda 950 UV/Vis/NIR spectrophotometer equipped with a Universal Reflectance Accessory module.

Transient Absorption Measurements:

[0119] Transient absorption measurements were carried out using a titanium sapphire based ultrafast amplifier centred at 800 nm and generating 45 fs pulses at a repetition rate of 1 kHz. The optical setup utilized was a typical pump-probe non-collinear configuration. The main part of the fundamental energy from the amplifier was directed into a half wave plate and a thin film polarizer system to control the energy of the excitation pulse incident of the sample. The optical path of the pump beam included an optical chopper allowing the use of phase-sensitive detection thereby improving the signal-to-noise ratio. An optical parametric amplifier pumped with approximately 1 mJ of the fundamental 800 nm energy was used to generate the probe beam with wavelengths ranging from 1200 nm to 1700 nm. The probe beam optical path included a precise motorized translation stage to control the optical delay between the pump the probe beam. The probe beam was directed on the sample within the excitation area of the pump beam where changes in transmission and reflection were recorded simultaneously using lock-in amplifiers.

Amplified Spontaneous Emission Measurements:

[0120] For the ASE measurements the ultrafast laser pulse at 800 nm was directed through a cylindrical lens (focus length 15 cm) onto the sample at normal incidence. The stripe width was 700 μm while the stripe length was measured for every measurement in order to calculate the occupancy values (average value of 0.35 cm±0.05). The thickness the PbS CQD elements was ˜110 nm in order to avoid over-estimation of the occupancy per dot. The emission was collected perpendicular to the incident beam using 6 cm focusing lens (5 cm diameter) and coupled into an Andor spectrometer (Shamrock SR-303) equipped with an InGaAs camera (iDus).

Photocurrent Measurements:

[0121] The QD thin films were prepared on the top of the Si/SiO.sub.2 substrate with patterned Au electrode following the standard EDT (0.2%) and ZnI.sub.2/MPA ligand treatments. The distance between two Au electrodes was fixed at 10 μm. 637 nm wavelength continuous laser (Vrotran stradus 637) was used to excite the QD films. All the measurements were performed in ambient conditions using an Agilent B1500A semiconducting device analyser.

Computational Details:

[0122] Density functional calculations of PbS have been performed by periodic plane-wave code Vienna ab initio simulation package VASP [26]. All structures have been optimized using the Perdew-Burke-Ernzerhof (PBE) [27] exchange-correlation functional, one of the most widely employed functionals of the generalized gradient approximation (GGA) family. It is important to mention that pure GGA functionals tend to underestimate electronic properties of materials such as band gaps. In order to account for the best possible and detailed description of the electronic structure of PbS, single-point calculations using the Heyd-Scuseria-Ernzerhof (HSE06) [28] hybrid exchange-correlation functional containing a fraction of nonlocal Fock exchange has been applied on the preoptimized PBE geometries. A plane-wave basis set with a 315 eV cutoff for the kinetic energy and a projector-augmented wave description of core-valence electron interactions were employed [29]. The one-electron Kohn-Sham states were smeared by 0.1 eV using Gaussian smearing. Finally, converged energies were extrapolated to zero smearing. All calculations were performed using a k-point Monkhorst-Pack [30] mesh of 3×3×1 in the reciprocal space for the unit cell of PbS. Relaxation of all atoms in the calculated models was carried out during the geometry optimization until forces acting on each atom became less than 0.01 eV/Å. In addition, the electron density was converged using a threshold of 10-6 eV for the total energy. No corrections for the zero-point energies were applied.

[0123] Two slab models consisting on 2×2×1 supercells were chosen to study PbS, one for the (100) surface and another for the (111) surface. The model of the stoichiometric (100) surface contains 32 atoms arranged in 4 layers, each layer formed by combination of Pb and S atoms, resulting in a nonpolar surface. Meanwhile, the (111) surface has been modelled using 28 atoms arranged in 3 Pb—S bilayers and an extra layer of Pb atoms, giving rise to two Pb terminations. The present inventors considered the PBE optimized lattice parameter to model all slabs (6.004 Å) [31], which is slightly larger than the reported experimental one of 5.929 Å [32].

[0124] The interaction of iodine with PbS surfaces has been modelled in two different ways, doping and adsorption of I atoms, respectively. Doping of PbS by iodine was modelled by substituting one of the S atoms located on the outmost layer of the (100) surface by an iodine. In turn, adsorption of iodine on the (111) surface has been modelled by covering the two Pb termination with I atoms, leading to a 100% coverage situation. Only the hexagonal-close-pack site has been considered in the present study. In order to account for the possible electron transfers emerging due to these two different processes, all calculations were spin-polarized.

[0125] In this document, in a previous section, embodiments for which the n-doped metal chalcogenide quantum dots are of a core-shell type were described. One of those embodiments is illustrated by FIG. 12.

[0126] Specifically, FIG. 12(a) shows a cross-section of a core-shell type structure for a metal chalcogenide quantum dot comprising one core C covered by a shell Sh, while FIGS. 12(b) and (c) show two respective band alignment diagrams.

[0127] Particularly, diagram of FIG. 12(b) represents a core-shell quasi type-1 band alignment between the shell Sh and the core C, showing how the hole(s) is/are strongly confined in the core C, while the electron(s) are delocalized across the core-shell structure.

[0128] Similarly, diagram of FIG. 12(c) also represents a core-shell quasi type-1 band alignment between the shell Sh and the core C, but in this case with gradual shelling, i.e. for a shell Sh implementing a gradual change of the alloy making the same so that it causes a gradual shift of the energy levels and an increase of the bandgap of the shell Sh to provide strong confinement for the holes and weak confinement (if any confinement) to the electrons.

[0129] Finally, different structures for implementing a laser device according to corresponding embodiments of the light device of the fourth aspect of the present invention are shown in FIGS. 13, 14 and 15.

Vertical-Cavity Surface-Emitting Laser Structure (VCSEL):

[0130] Specifically, FIG. 13 schematically shows the light device of the fourth aspect of the present invention, for an embodiment for which the device implements a laser device having a VCSEL structure, which consists of two distributed Bragg reflector (DBR), M1 and M2, positioned in parallel for single wavelength emission or in a small angle (0.1°-5°) for spatial tuning of the principal laser mode across the area of the DBR. The gain material A, which in the present invention consist of the heavily n-doped colloidal quantum dot film, in this case in the form of a film, is deposited between the mirrors M1, M2 forming the DBR, thus forming a laser cavity.

[0131] The thickness L of the gain material is determined with the following function L=λ/2n, where n corresponds to the effective reflective index of the gain material A (e.g. PbS QD and Al.sub.2O.sub.3, PbS QD and Al.sub.2O.sub.3 and air). Taking under consideration that the refractive index of the QD medium, i.e. of the gain material A, ranges from 1.5 to 3, the thickness of the gain material for the telecom wavelengths spans from 200 nm to 1 μm.

[0132] The DBR mirror consist of two or more pairs of materials (e.g. SiO.sub.2, TiO.sub.2) forming a photonic band gap at the optical fibre optics communication wavelengths ranges from 1000 nm-2000 nm. The DBRs M1, M2 may have reflectivities within their photonic bandgap from 50% up to 99.999%. Preferably, the DBR through which laser emission is radiated has a lower reflectivity than the other DBR.

[0133] The optical excitation of the gain medium takes place through one of the DBRs, M1 or M2, outside their photonic bandgap (i.e. with light wavelength shorter that the low wavelength value of the DBR). Alternatively, optical excitation of the gain medium can take place from the side through a waveguiding mode within the gain medium. This waveguide can be implemented with the gain medium A, i.e. with the n-doped chalcogen quantum dots film, being integrated in a waveguide structure, for example embedded in a waveguide trench in a silicon substrate.

Distributed FeedBack Laser Structures (DFB):

[0134] As shown in FIG. 14, the DFB laser structure consist of a waveguide resonator formed by: [0135] a corrugated substrate S with corrugations implemented by periodically arranged structured elements (e.g. pillars, rectangles, trapezoids, etc.) in one or two dimensions resulting 1-D or 2-D interference grating respectively, where the periodic change of the refractive index that results the photonic band gap which depends on the height (H) of the structured element, periodicity (Λ) and the periodic change of the refractive index of the structured and the gain material; and [0136] the laser gain medium A arranged on top of the corrugated substrate S over the corrugations.

[0137] The waveguide resonator performs both functions, that of a waveguide and that of a resonator, as indeed light is wave-guided in the interface between the gain medium A and the substrate S, and in the presence of the corrugations this structure becomes resonant providing feedback (as a cavity).

[0138] The substrate S of the DFB laser structures is made of any of various materials including oxides, fluorides and/or doped oxides (e.g. SiO.sub.2, TiO.sub.2, MgF.sub.2, CaF.sub.2, ITO, FTO), Si, GaAs, and other organic polymeric materials like polyimide, PMMA (Polymethyl methacrylate), etc. The grating height can vary from 20 nm to 500 nm, while the periodicity (Λ) spans from 700-1400 nm. The DFB laser structures can operate as first-order (FIG. 14, top view), second-order or mixed order (second and first order, as shown in FIG. 14, bottom view), which is correlated to the periodicity or the mixed periodicity of the DFB structure.

[0139] The operating lasing wavelengths based on the aforementioned characteristics range from 1500 nm to 1800 nm. Moreover, the DFB structure with the use of conductive materials (e.g. ITO) can operate as electrically pumped laser. The DFB laser can be excited from one of the sides (up or down) or through a waveguide integration of the DFB structure in which light excitation is provided by coupling the light from the waveguide to the gain medium A, i.e. to the n-doped metal chalcogenide quantum dot solid-state element.

Whispering Gallery Mode Laser Structures (WGM):

[0140] For this kind of structure, the gain medium A comprising the heavily n-doped metal chalcogenide quantum dot solid-state element is coupled with Whispering Gallery mode (WGM) resonators Rs for single or multi laser mode at telecom wavelengths.

[0141] The structure of said WGM resonators Rs includes dielectric cylinders, disks, rings and fibre resonators, while the material from which they are made are oxides, fluorides and/or doped oxides (e.g. SiO.sub.2, TiO.sub.2, MgF.sub.2, CaF.sub.2, ITO, FTO).

[0142] The preferred operating lasing wavelengths are 1300 nm-1800 nm. The resonator diameter can vary from 50-1000 μm for the pertinent lasing wavelengths with Q-factors high as 105-106. The gain material A can be deposited on the ring, disk or sphere resonators Rs, or within or at the outer surface of the fibre and dielectric cylinders Rs.

[0143] Moreover, as shown in FIG. 15, the WGM resonators Rs (made of SiO.sub.2 for the illustrated embodiments) can be hollow fibres/cylinders with diameter 50 μm-10000 μm (R1) and thickness (W1) of 1 μm-500 μm.

[0144] The inner space can be fully filled with the gain medium A, as shown in FIG. 15a top views, for a hollow cylinder/fibre resonator Rs, or forming a ring or tubular element with thickness (W2) of 0.2 μm-100 μm, as shown in FIG. 15(a) bottom views. Inside that ring or tubular member forming the gain medium A, the space can be filled either with air or oxides, fluorides and/or doped oxides (e.g. SiO2, TiO2, MgF2, CaF2, ITO, FTO).

[0145] Alternatively, as shown in FIG. 15(b), the gain medium A can be deposited on the outer perimeter of an optical fibre or tubular resonator Rs.

[0146] At least for the embodiment of FIG. 15 top views, the n-doped metal chalcogenide solid-state element forming the gain medium A is not in the form of a film, but in the form of a solid cylinder filling the inner volume of the tubular resonator Rs.

[0147] A person skilled in the art could introduce changes and modifications in the embodiments described without departing from the scope of the invention as it is defined in the attached claims, such as providing any desired shape to the n-doped metal chalcogenide solid-state element (either in the form of a film or not) in order fit the requested application.

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