Optical Gain Measurements Research Articles

Scattered Emission Profile Technique for Accurate and Fast Assessment of Optical Gain in Thin Film Lasing Materials

Accurate measurement of optical gain is essential to screen materials as viable active media for thin-film laser applications. The net modal gain is typically measured using the variable stripe length (VSL) method, which has been extensively studied for the last few decades. In this work, we propose an alternative method, which we name scattered emission profile (SEP) method, to measure the net modal gain. It relies on the collection of amplified spontaneous emission (ASE) scattered from the surface of the film illuminated by a pump stripe. By using an appropriate setup, the new method results in a significantly faster measurement of net modal gain, while simultaneously providing a more accurate gain value. The setup and algorithm to extract the net modal gain are detailed in this paper and are demonstrated on lead halide perovskite films. The influence of the stripe length on the measured gain value is shown. Gain measurements performed over two different perovskite films, fabricated either via spin-coating or thermal evaporation, confirm the broad applicability of the SEP method. Finally, we show a quantitative comparison of the SEP method with VSL measurements and highlight the advantages and shortcomings of each method.

Optical net gain measurement on Al0.07Ga0.93N/GaN multi-quantum wells.

We report net gain measurements at room temperature in Al0.07Ga0.93N/GaN 10-period multi-quantum well layers emitting at 367 nm, using the variable stripe length method. The separate confinement heterostructure was designed targeting electron-beam pumped lasing at 10 kV. The highest net gain value was 131 cm-1, obtained at the maximum pumping power density of the experimental setup (743 kW/cm2). The net gain threshold was attained at 218 kW/cm2 using 193 nm optical pumping. From these experiments, we predict an electron-beam-pumped lasing threshold of 370 kW/cm2 at room temperature, which is compatible with the use of compact cathodes (e.g. carbon nanotubes). In some areas of the sample, we observed an anomalous amplification of the photoluminescence intensity that occurs for long stripe lengths (superior to 400 µm) and high pumping power (superior to 550 kW/cm2), leading to an overestimation of the net gain value. We attribute such a phenomenon to the optical feedback provided by the reflection from cracks, which were created during the epitaxial growth due to the strong lattice mismatch between different layers.

Independently programmable frequency-multiplexed phase-sensitive optical parametric amplification in the optical telecommunication band.

We experimentally demonstrate programmable multimode phase-sensitive amplification multiplexed in the frequency domain for flexible control of parallelly generated squeezed states. We utilize the unique phase-matching condition of a type-II periodically poled potassium titanyl phosphate (PPKTP) crystal and pulse shaping technique to fully control the frequency-domain parallel generation of squeezed states in the optical telecommunication band. We experimentally verify that the independent programmability of phase-sensitive optical parametric amplification (OPA) for the modes corresponding to different frequency bands can be achieved by shaping the pump laser pulse from optical parametric gain measurements using a coherent probe light generated by a degenerate synchronously pumped optical parametric oscillator.

Unveiling photophysical and photonic phenomena by means of optical gain measurements in waveguides and solutions

The increasing number of solution-processed laser compounds that can be implemented as low-cost, flexible, and/or integrated devices, makes necessary the development of reliable methods to delineate all their amplifying signatures and thus to open the door to appropriate cross-sample comparisons. Seeking to solve this problem, a new formalism to retrieve the losses and the optical gains from Amplified Spontaneous Emission (ASE) spectra as a function of the excitation density has been recently reported. In this manuscript, we explore the potential of this methodology to unveil relevant information on the photonic properties of the waveguiding devices and on the photophysics of the active materials. We demonstrate that the Variable Pump Intensity method opens the door to understand the relationship between the ASE thresholds and the optical gains and losses, it enables the extraction of the scattering/modal losses of the passive devices, and it can unveil the presence of leaky-modes and excited state absorption. In contrast, it does not perform too well in samples with multiple active species in its current implementation. We have substantiated all these findings using organic semiconductor thin films, several dye-doped polymer thin films and solutions of boron hydride.

Influence of Al2O3 on the photoluminescence and optical gain performance of Nd3+ doped germanate and tellurite glasses

Abstract We investigated the influence of aluminum oxide, Al2O3, on the photoluminescence (PL) spectrum and optical gain (at 1064 m) of neodymium ion, Nd3+, doped tellurium and germanium oxide glasses with compositions TeO2 − ZnO and GeO2 − PbO, respectively. The PL properties of both materials were studied using a continuous-wave diode laser operating at 808 nm, in resonance with the Nd3+ transition 4I9/2 → 4F5/2. For samples containing Al2O3 it was observed PL intensity growth up to 30% in comparison with the samples without Al2O3. Luminescence decay (τm) from the 4F3/2 level was measured monitoring the 4F/32 → 4I11/2 transition. From calculations of the Judd–Ofelt parameters, the radiative lifetimes, the emission cross -section (σem) and the quantum efficiency of 4F3/2 → 4I11/2 transition were determined; the effective linewidth (Δλeff) was calculated to determine the gain bandwidth product (σem x Δeff), figure of merit (σem x τm) and saturation intensity (Is) parameters. Optical gain measurements were performed using a pump laser at 808 nm and a probe laser at 1064 nm. For the samples with Al2O3 the relative gain at 1064 nm reached ≈ 2.8 dB/cm (pump laser power: 400 mW) indicating an enhancement of about 65% for TeO2 − ZnO and 120% for GeO2 − PbO with respect to the samples without Al2O3. The optical gain enhancement is correlated to the PL intensity behavior and increases due to the presence of Al2O3 because the coordination environment of the Nd3+ changes and become more asymmetric leading to the growth of the emission cross-section. The present study shows that Nd3+ doped tellurium and germanium oxide glasses containing Al2O3 present particularly good prospects to be used as optical amplifiers at 1064 nm.

High-Gain Er3+:Al2O3 On-Chip Waveguide Amplifiers

In this work, high-gain on-chip Er3+ doped Al2O3 single-mode ridge waveguide amplifier device is reported. The growth of the active layer is realized using atomic layer deposition (ALD) compatible with Si processing technology. Optimization of the Al2O3 and Er2O3 growth conditions for fine-tuning the optical properties is systematically investigated. In order to optimize the active ion concentration, heterocycle structures consisting of separately optimized Al2O3 and Er2O3 are deposited. The structural and optical properties of the active layers were characterized by spectroscopic ellipsometry, X-ray photoelectron spectroscopy, time-resolved photoluminescence, and optical gain measurements. We demonstrated up to 2.74 ± 0.40 dB total gain and 13.71 ± 1.97 dB/cm internal optical net gain per unit length at 1550 ± 12 nm using a 2-mm-long device, pumped using a 980 nm laser diode at 23-mW. This work can be expanded to other rare-earth ion doping processes, paving the way to significant improvements of active on-chip applications.

Determination of the saturation length and study of its effects in optical gain measurements of organic semiconductors using the variable stripe length method

Three indicators for the determination of the saturation length limiting the validity of the variable stripe length method are presented. Depending on the maximum stripe length being used for the nonlinear fit, the resulting modal gain, its confidence interval, and the spectral full width at half maximum vary and give clear indications on the saturation length. Exemplarily, a blend of organic semiconducting spiro-compounds has been measured to present the importance of this length and demonstrate its determination. Subsequently, the optical modal gain is calculated with consideration of the saturation length. Both the saturation length and modal gain are shown to be dependent on the pump energy but result in a constant gain-length product of 2.2 ± 0.2, independent of the pump energy. Additionally, spectral gain profiles for different pump energies are presented with a maximum net modal gain of (128±4) cm−1 found at (2.98±0.09) mJ/cm2.

Optical gain in laser diodes with null reflectivity

Optical gain measurement is extended to chips designed for external cavity lasers, where facet reflectivity is intentionally kept null, which prevents from any application of the Hakki-Paoli method. The new method is first shown to be consistent with the standard methods, by applying it to a chip with Fabry-Perot resonances and comparing the results of the new and the Hakki-Paoli method. The validated new method is then applied to the non-reflecting cavity of a tunable laser diode based on external cavity.

Spectroscopic Evidence for the Contribution of Holes to the Bleach of Cd-Chalcogenide Quantum Dots.

In transient absorption (TA) measurements on Cd-chalcogenide quantum dots (QDs), the presence of a band-edge (BE) bleach signal is commonly attributed entirely to conduction-band electrons in the 1S(e) state, neglecting contributions from BE holes. While this has been the accepted view for more than 20 years, and has often been used to distinguish electron and hole kinetics, the reason for the absence of a hole contribution to the BE-bleach has remained unclear. Here, we show with three independent experiments that holes do in fact have a significant impact on the BE-bleach of well-passivated Cd-chalcogenide QD samples. Transient absorption experiments on high photoluminescence quantum yield CdSe/CdS/ZnS core–shell–shell QDs clearly show an increase of the band-edge bleach as holes cool down to the band edge. The relative contribution of electron-to-hole bleach is 2:1, as predicted by theory. The same measurements on core-only CdSe QDs with a lower quantum yield do not show a contribution of holes to the band-edge bleach. We assign the lack of hole bleach to the presence of ultrafast hole trapping in samples with insufficient passivation of the QD surface. In addition, we show measurements of optical gain in core–shell–shell QD solutions, providing clear evidence of a significant hole contribution to the BE transient absorption signal. Finally, we present spectroelectrochemical measurements on CdTe QDs films, showing the presence of a BE-bleach for both electron and hole injections. The presence of a contribution of holes to the bleach in passivated Cd-chalcogenides QDs bears important implications for quantitative studies on optical gain as well as for TA determinations of carrier dynamics.

Highly Stable, Near-Unity Efficiency Atomically Flat Semiconductor Nanocrystals of CdSe/ZnS Hetero-Nanoplatelets Enabled by ZnS-Shell Hot-Injection Growth.

Colloidal semiconductor nanoplatelets (NPLs) offer important benefits in nanocrystal optoelectronics with their unique excitonic properties. For NPLs, colloidal atomic layer deposition (c-ALD) provides the ability to produce their core/shell heterostructures. However, as c-ALD takes place at room temperature, this technique allows for only limited stability and low quantum yield. Here, highly stable, near-unity efficiency CdSe/ZnS NPLs are shown using hot-injection (HI) shell growth performed at 573 K, enabling routinely reproducible quantum yields up to 98%. These CdSe/ZnS HI-shell hetero-NPLs fully recover their initial photoluminescence (PL) intensity in solution after a heating cycle from 300 to 525 K under inert gas atmosphere, and their solid films exhibit 100% recovery of their initial PL intensity after a heating cycle up to 400 K under ambient atmosphere, by far outperforming the control group of c-ALD shell-coated CdSe/ZnS NPLs, which can sustain only 20% of their PL. In optical gain measurements, these core/HI-shell NPLs exhibit ultralow gain thresholds reaching ≈7 µJ cm-2 . Despite being annealed at 500 K, these ZnS-HI-shell NPLs possess low gain thresholds as small as 25 µJ cm-2 . These findings indicate that the proposed 573 K HI-shell-grown CdSe/ZnS NPLs hold great promise for extraordinarily high performance in nanocrystal optoelectronics.

Real-time, dither-free Shack-Hartmann wavefront sensor optical gain measurement technique

Past research in adaptive optics (AO) has demonstrated the link between apparent beacon extent and wavefront gradient estimation sensitivity, or optical gain, of a classical Shack-Hartmann (SH) subaperture when using quad-cell detector regions. Pixel diffusion and residual wavefront error broaden the effective subaperture point spread functions as the atmospheric seeing varies in time. Although the AO community has generally shifted toward resolved subapertures to combat these interlinked issues, the quad-cell subaperture design offers efficient light usage for dim beacons, integrating less pixel noise while also reducing sensor readout latency. Particularly for telescopes in poor seeing conditions, in order to reduce beacon magnitude requirements, a quad-cell SH design, coupled with the proposed algorithm, can be an enabling solution. We present research conducted at the Starfire Optical Range over the past 8 years in implementing a robust approach that measures the real-time sensitivity on the site’s natural guidestar and laser beacon AO systems at the 3.5- and 1.5-m telescopes. Emphasis is given toward the practical aspects that must be considered beyond the pure theory, which has been presented in several prior works. A high-signal-to-noise strategy has been implemented that estimates the aperture-averaged subaperture sensitivity (related to beacon size) by exploiting the null space of the least-squares wavefront reconstructor. Careful consideration has gone into the implementation of this estimation method to avoid unintended effects, particularly at low-light levels. Unfortunately, this solution does not in itself address aperture-variant effects, such as sodium beacon elongation for extremely large telescopes.

Tm3+ doped Bi2O3-GeO2 glasses with silver nanoparticles for optical amplifiers in the short-wave-infrared-region

We synthesized the Bi2O3–GeO2 (BGO) glass doped with thulium ions (Tm3+) and investigate the influence of embedded silver nanoparticles (Ag-NPs) on the samples photoluminescence (PL) and on the process of energy transfer (ET) among bismuth ions and Tm3+. The samples were prepared by the melt-quenching method with and without Tm2O3 and/or AgNO3. Under excitation at 808 nm the pure BGO glass presented PL from 1100 to 1500 nm, characteristic of the bismuth ions, Bi+, while the samples with Tm3+ exhibited a strong PL from 1300 to 1600 nm. The Bi+ emission was quenched due to ET from the bismuth centers to the Tm3+ and the emitted intensity at 1470 nm increased by more than 1000% due to the presence of Tm3+. Moreover, the samples with Tm3+ and Ag-NPs presented further PL enhancement of ≈56% at 1470 nm for the samples heat-treated during 2 h. Optical gain measurements using 2 mm thick samples were performed with a probe beam at 1470 nm and the pump beam at 808 nm. A 4.5 dB/cm gain in the presence of Ag-NPs was measured when the pump laser power was 6 W. The Ag-NPs contributed to 500% of gain enhancement. The present results illustrate the appropriateness of the BGO glass with Tm3+ and Ag-NPs for obtaining intense PL and optical amplification in the near-infrared.

Emission cross section and optical gain of 1.06mm laser Nd3+ doped borate glasses

In this paper we develop the optical pumping system based on 805 nm laser diode as photo-pumped source. Laser with a wavelength of 1064 nm is used as reference source and passed in a laser glass medium. Laser glass medium was synthesized from chemical compositions of (65-x)B2O3-15Na2O-10PbO-5ZnO-5Li2O-xNd2O3 (with x = 0.05, 0.1, 0.5, 1.0, 2.0 and 4.0). The emission spectra confirms that there are three emission peaks located at 946 nm, 1064 nm and 1350 nm corresponding to 4F3/2®4I9/2, 4F3/2®4I11/2 and 4F3/2®4I13/2 respectively. The emission cross section relatively higher for the 4F3/2®4I11/2 transition shows the compatibility of these glasses for near infra-red laser application. The measurements of optical gain had been carried out by using lock-in amplifier with voltage as output signal. From output voltage converted to optical gain in dB. The glass medium with 1.0 Nd3+ label was obtained the maximum optical gain with incident power 900 mW.

Intrinsic gain and gain degradation modulated by excitation pulse width in a semiconducting conjugated polymer

We have previously reported that substantially higher optical gain values can be achieved in the conjugated polymer poly[2-methoxy-5-(2′-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) through use of transient excitation conditions. In the present paper, we report on a systematic investigation of this behavior to elucidate the physical mechanisms involved, which enables us to distinguish between the fundamental intrinsic gain and an excitation induced degraded gain. Using pump laser pulses having temporal widths longer and shorter than the photoluminescence (PL) decay time of MEH-PPV, both quasi-steady-state (QSS) and transient excitation regimes are explored in our encapsulated waveguide heterostructures [Si(100)/SiO2/MEH-PPV/poly(methyl methacrylate)]. Under transient excitation (25ps pump pulses), extremely large optical gain is observed, reaching a value of 700cm−1 at a maximum pump energy density of 85µJ/cm2. However, under QSS conditions (8ns pulses), considerably lower gain coefficients are achieved with a maximum of ∼130cm−1 at an energy density of 2,000µJ/cm2; this factor of 5 decrease in optical gain performance is observed at the same excitation density as that for transient excitation using ps pulses. We have also employed unencapsulated waveguide structures [Si(100)/SiO2/MEH-PPV/air], which allows us to achieve additional insight on gain degradation under QSS conditions. It is clear that the gain measured under transient conditions is more representative of the intrinsic gain whereas that determined in the QSS regime is degraded by defect-mediated dissociation of emissive states due to localized thermal and oxidative damage to the films. It is in the QSS regime in which most optical gain measurements to date have been performed. These results suggest that further optimization of MEH-PPV – and most likely other conjugated polymers – as a robust optical gain medium can be achieved by consideration of the excitation pulse width.

Optical net gain measurement in n-type doped germanium waveguides under optical pumping for silicon monolithic laser.

Silicon (Si) monolithic lasers are key devices in large-scale, high-density photonic integrated circuits. Germanium (Ge) is promising as an active layer due to the complementary metal-oxide semiconductor process compatibility with Si. A net optical gain from Ge is essential to demonstrate lasing operation. We fabricated Ge waveguides and investigated the n-type doping effect on the net optical gain. The estimated net gain of the n-Ge waveguide increased from -2200 to -500/cm, namely reducing loss, under optically pumped condition.

Optical Amplification at 1064 nm in Nd(TTA) 3(TPPO) 2 Complex Doped SU-8 Polymer Waveguide

Nd(TTA) 3 (TPPO) 2 complex (HTTA: 2-thenoyltrifluoroacetone, TPPO: triphenylphosphine oxide) doped SU-8 polymer waveguide amplifiers were demonstrated. The absorption and photoluminescence spectra were observed on a 100-μm-thick film of the Nd(TTA) 3 (TPPO) 2 complex doped SU-8 polymer. Under excitation at 808 nm, the fluorescence was obtained at 904 nm, 1068 nm, and 1346 nm wavelengths. The full-width at half-maximum bandwidth was about 25 nm wide around 1068 nm. Using reactive ion etching technology, optical waveguides were fabricated by taking the Nd(TTA) 3 (TPPO) 2 complex doped SU-8 polymer and polymethylmethacrylate (PMMA) material as the core and cladding layers, respectively. According to fundamental rate equations and optical power propagation equations, the gain characteristic of the polymer waveguide amplifier was simulated to be 5.1 dB/cm. An experimental setup for the optical gain measurement was established. A relative optical gain of about 2.4 dB/cm at 1064 nm was obtained in a 20-mm-long multimode waveguide for an input signal power value of 0.1 mW and pump power of 140 mW.

Modeling of Whispering Gallery Modes for Rare Earth Spectroscopic Characterization

A refined model of a mid-IR amplifier, constituted by a tapered chalcogenide fiber coupled to an erbium-doped chalcogenide microsphere, is integrated with a global solution search procedure based on particle swarm optimization approach. It is implemented in a computer code in order to obtain an inversion algorithm useful to evaluate the spectroscopic parameters of rare-earth-doped glass microspheres. The rare earth parameters can be recovered by means of the optical gain measurement. The error in evaluation of the erbium lifetime τ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">41</sub> is <;3.5%. It is <;0.5% for the other lifetimes and ion-ion interaction parameters. These excellent results are obtained since whispering gallery mode electromagnetic field interacts with rare earth for long effective distances.

Theory of the lifetime of an exciton incoherently created below its resonance frequency by inelastic scattering

When an exciton in semiconductor is scattered and its energy is decreased far below the resonance energy of the bare exciton state, it has been considered that an exciton-polariton is created immediately by the scattering process because there is no exciton level at that energy. However, according to the recent time-resolved measurements of P emission originating from inelastic exciton-exciton scattering, it looks rather natural to consider that the exciton-polariton is created in a finite time scale which is restricted by a coherence volume of the exciton after the scattering. In this interpretation, the exciton remains in this time scale far below its resonance energy as a transient state in a series of processes. We propose an expression of the P-emission lifetime depending on the coherence volume of the scattered excitons through the conversion process from them to the polaritons. The coherence volume of the scattered excitons appears in the calculation of the inelastic scattering process on the assumption of a finite coherence volume of the bottleneck excitons. Time-resolved optical-gain measurements could be a way for investigating the validity of our interpretation.

Gain spectrum measurement using the segmented contact method with an integrated optical amplifier

The measurement of optical gain utilising a segmented contact and integrated optical amplifier is reported. We show that in a direct comparison of methods, the use of the integrated amplifier allows the gain spectrum to be deduced over wider spectral ranges and to lower carrier densities, as compared to the conventional segmented contact technique.

Optical absorption of Mg-doped layers and InGaN quantum wells on c-plane and semipolar GaN structures

We studied optical absorption of Mg-doped AlInGaN layers using excitation-position dependent and polarization resolved photoluminescence from the slab-waveguide edge of a laser structure. The major absorption in the Mg-doped layers was found only when p-doping is activated. It increases with the removal of residual hydrogen, which in case of Mg doping is a p-type passivation impurity, and reversibly disappears after passivation by hydrogen. This absorption is weakly wavelength and temperature dependent, and isotropic. This can be attributed to acceptor-bound hole absorption, because those holes concentration is nearly equal to that of activated acceptors and weakly temperature dependent (unlike the free hole concentration, which is much lower and is an exponential function of temperature due to high ionization energy). The cross section of photon absorption on such activated acceptor was quantified to be in the order of 10−17 cm−2. The absorption cross section of free electrons was found to be at least one order of magnitude lower and below detection limit. The same technique was used to experimentally quantify band structure polarization components along basis directions for green InGaN quantum wells (QWs) grown on c- and semipolar planes. The A1 and B1 valence subbands of c-plane QW were found to comprise mostly |X⟩ and |Y⟩ states. There was rather minor amount of |Z⟩ states with average square fraction of only 0.02. In (20-21) plane, due to small band anticrossing near gamma-point, we observed highly polarized absorption edges of A1- and B1-subbands consisting mainly of |Y⟩ and |X⟩ states, respectively, and found their energy splitting to be ∼40 meV. For (11-22) plane with smaller band splitting and polarization, we observed polarization switching with indium (In) concentration greater than 30% in the QW (or photon energy less than 2.3 eV). We confirmed our study of valence band structures by optical gain measurements.