Mihail Dumitrescu - Academia.edu (original) (raw)

Papers by Mihail Dumitrescu

SPIE Proceedings, 2016

Dual-mode multi-section quantum-well distributed feedback lasers with surface gratings have been ... more Dual-mode multi-section quantum-well distributed feedback lasers with surface gratings have been fabricated, without regrowth, at 1310 and 1550 nm using UV nano-imprint lithography. Several laser and grating sections have been employed to control and stabilize the dual-mode emission and to reduce mode competition. Frequency differences between 15 GHz and 1 THz were achieved for different longitudinal structures. Frequency difference variations of several GHz have been measured under bias modulation with rates up to a few GHz. Higher frequency difference modulation rates are expected from improved measurement setups and from employing quantum dot active regions for further reduction of mode competition.

We have studied the influence of growth temperature (within the 410-470 ^oC range) on the nitroge... more We have studied the influence of growth temperature (within the 410-470 ^oC range) on the nitrogen incorporation into lattice-matched GaInNAs-on-GaAs epilayers grown by molecular-beam epitaxy under constant fluxes. It was found that, over the whole temperature range, nitrogen is incorporated both on substitutional sites and as dimers on Ga and As sites. On substitutional sites nitrogen is present in the form of N-Ga4 clusters and, to a lesser extent, in the form of N-Ga3In ones. Increasing the growth temperature reduces the amount of substitutional nitrogen and increases the ratio between the N-Ga3In and N-Ga4 clusters. At the same time, the band gap increases. The amount of nitrogen dimers also decreases with increased growth temperature but the ratio between nitrogen dimers and nitrogen substitutionals appears not to be affected by the growth temperature. The effects of annealing on the incorporated nitrogen are discussed in the paper.

Applied Physics Letters, 2010

Contactless electroreflectance (CER) has been applied to study band bending in Be-doped GaInNAs/G... more Contactless electroreflectance (CER) has been applied to study band bending in Be-doped GaInNAs/GaAs quantum wells (QWs). It has been shown that (i) the sign of GaAs-related CER signal changes from positive to negative due to Be-doping and (ii) QW-related CER resonances disappear for Be-doped QWs whereas they are clearly observed for un-doped QWs. The two observations indicate a Be-related shift in the Fermi level above the hole levels in the QW region, i.e., the change in band bending in this system. The results point out that the experimentally-observed enhancement in QW photoluminescence upon Be-doping is associated with a better collection/confinement of photogenerated carriers by the Be-doped GaInNAs QW.

Applied Physics Letters, 2002

We present a 1.3-μm GaInNAs/GaAs quantum-well heterostructure, which consists of a strain-mediati... more We present a 1.3-μm GaInNAs/GaAs quantum-well heterostructure, which consists of a strain-mediating GaInNAs layer grown between a compressive-strained quantum well and a tensile-strained GaNAs layer. Compared to a similar sample with no strain-mediating layer, this heterostructure exhibits improved material properties and remarkable redshift of emission with enhanced light intensity. The observations are based on photoluminescence spectra and x-ray diffraction data measured for the active region of the samples.

IEE Proceedings - Optoelectronics, 2004

The causes were investigated for the photoluminescence red-shift with increasing quantum well gro... more The causes were investigated for the photoluminescence red-shift with increasing quantum well growth temperature (T Q W ) reported in GaInNAs/GaAs quantum wells grown by molecular-beam epitaxy (Tournie et al., 2002; Chauveau et al., 2003). The phenomenon was found to be due to self-annealing, which occurred during the growth of layers on top of the quantum well at typical substrate temperatures T Q W for GaAs growth by molecular-beam epitaxy. This self-annealing induces a blue-shift of the quantum well emission, whose magnitude increases as T Q W decreases. The T Q W -dependent blue-shift is correlated with the presence of In and occurs without noticeable changes in macroscopic alloy composition or quantum well structure. The underlying cause for the increase in blue-shift with decreasing T Q W during self-annealing appears to be an increased number of In-N bonds due to point-defect-assisted diffusion. Possible defects involved are discussed in the paper on a qualitative basis.

conference on lasers and electro optics, Jun 21, 2015

Paper reports the DBR-RWG surface grating design, the fabrication process, and the output charact... more Paper reports the DBR-RWG surface grating design, the fabrication process, and the output characteristics of tapered DBR laser diodes for the applications, like for example LIDAR and range finding, that require eye-safe high-power single-mode coherent light sources. The fabricated regrowth-free DBR AlGaInAs/InP lasers exhibited a CW output power as high as 560 mW in single-mode operation at room temperature. At maximum output power the SMSR was 38 dB, proving the excellent behavior of the surface gratings. The tapered section enabled scaling the maximum CW power at room temperature from 125 mW to 560 mW, by increasing its length from 0.5 mm to 4.0 mm. The paper discusses the limitations and performance variation associated to the power scaling by using the tapered section length as a scaling parameter.

2017 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC), 2017

Pulsed operation characteristics of a high-power 1550 nm tapered distributed Bragg Reflector (DBR... more Pulsed operation characteristics of a high-power 1550 nm tapered distributed Bragg Reflector (DBR) laser diode are described. The development targets applications such as LIDAR and range finding, which require eye-safe, coherent light sources with a high peak-power. In particular, we employ a regrowth-free technique and AlGaInAs/InP gain structure, and demonstrate a peak power of about 1.6 W (drive-current limited) and a CW power of 560 mW at room temperature.

IEEE Photonics Technology Letters, 2018

This letter presents narrow-linewidth 780-nm edge-emitting semiconductor DFB lasers fabricated wi... more This letter presents narrow-linewidth 780-nm edge-emitting semiconductor DFB lasers fabricated without re-growth using UV-nanoimprinted surface gratings. The third-order laterally coupled ridge-waveguide surface gratings enable single mode operation, excellent spectral purity (40-55 dB side mode suppression ratio and 10-kHz linewidth), and good light-current-voltage characteristics in continuous wave operation (∼112-mA threshold current, ∼1.55-V opening voltage, and 28.9-mW output power from one facet at 300-mA current for 2.4-mm-long devices), which are vital in various applications, such as rubidium spectroscopy and atomic clock pumping. The low fabrication cost, high throughput, structural flexibility, and high device yield make the fabrication method fully compatible with large-scale mass production, enabling the fabrication of low-cost miniaturized modules.

IEE Proceedings - Optoelectronics, 2005

The wafer of a 760 nm vertical-cavity surface-emitting laser (VCSEL), designed for oxygen sensing... more The wafer of a 760 nm vertical-cavity surface-emitting laser (VCSEL), designed for oxygen sensing up to high temperatures, is investigated using photomodulated reflectance (PR). By varying the angle of incidence, the VCSEL cavity mode (CM) wavelength is tuned through the positions of two excitonic quantum well (QW) transitions. The PR is also measured over a large temperature range to determine when the QW ground-state transition is tuned with the CM. When tuned, the QW/CM PR lineshape becomes anti-symmetric, as predicted by theory. This occurs at 388 K, where the CM and QW wavelengths coincide at 760.7 nm. It is also observed that when tuned, the CM width measured in the reflectance spectrum is maximised. Temperature dependent device studies are also conducted on a 760 nm edge-emitting laser containing a similar active region as the VCSEL. It is found that up to 250 K the device behaves ideally, with the threshold current being entirely due to radiative recombination. However, as the temperature increases, electron leakage into the indirect X-minima of the barrier and cladding layers becomes increasingly significant. At 300 K, approximately 25% of the threshold current is found to be attributed to electron leakage and this increases to 85% at 388 K. The activation energy for this leakage process is determined to be 255^5 meV, indicating that electron escape from the QWs into the X-minima of the barrier and/or cladding layers is chiefly responsible for the device's poor thermal stability. These results suggest that VCSELs containing this active region are likely to suffer significantly from carrier leakage effects.

2016 International Conference Laser Optics (LO), 2016

This paper reports the DBR-RWG surface grating design, the fabrication process, and the output ch... more This paper reports the DBR-RWG surface grating design, the fabrication process, and the output characteristics of tapered DBR laser diodes for applications such as LIDAR and range finding that require eye-safe high-power single-mode coherent light sources. The regrowth-free AlGaInAs/InP DBR lasers exhibited a CW output power as high as 560 mW in single-mode operation at room temperature. At maximum output power the SMSR was 38 dB. By changing the length of the tapered section from 0.5 mm to 4.0 mm, the maximum CW power could be scaled from 125 mW to 560 mW at room temperature.

CLEO/Europe Conference on Lasers and Electro-Optics

Vertical-Cavity Surface-Emitting Lasers V, 2001

Visible vertical-cavity surface-emitting lasers (VCSELs) are potential light sources for polymer ... more Visible vertical-cavity surface-emitting lasers (VCSELs) are potential light sources for polymer optical fibre (POF) data transmission systems. Minimum attenuation of light in standard PMMA-POFs occurs at about 650 nm. For POFs of a few tens of meters in length VCSELs at slightly longer wavelengths (670 - 690 nm) are also acceptable. So far, the visible VCSELs have been grown by metal organic chemical vapour deposition (MOCVD). They may also be grown by a novel variant of molecular beam epitaxy (MBE), a so-called all-solid-source MBE or SSMBE. In this paper, we describe growth of the first visible-light VCSELs by SSMBE and present the main results obtained. In particular, we have achieved lasing action at a sub-milliamp cw drive current for a VCSEL having the emission window of 8um in diameter, while a 10um device exhibited an external quantum efficiency of 6.65% in CW operation at room temperature. The lasing action up to temperature of 45°C has been demonstrated.

Journal of Applied Physics, 2005

... Electronic mail: emil-mihai.pavelescu@orc.tut.fi. X-ray diffraction rocking curves of samples... more ... Electronic mail: emil-mihai.pavelescu@orc.tut.fi. X-ray diffraction rocking curves of samples S410 to S470 as well as for two reference samples grown at a of 410 and , respectively. The inset shows the dependence of the nitrogen concentration on growth temperature. ...

IEEE Photonics Technology Letters, 2005

Many applications require laser emission with stable single polarization. While stable transverse... more Many applications require laser emission with stable single polarization. While stable transverse-electric (TE) emission is favored by several mechanisms and, consequently, is relatively easy to obtain, achieving stable transverse-magnetic (TM) emission is more difficult. This letter proposes a method for suppressing the TE polarization by using a multiquantum-barrier. In our experiments, the multiquantum-barrier structure was inserted into the p-type cladding of tensile-strained GaInP-AlGaInP quantum-well lasers emitting at wavelengths shorter than 630 nm. As a result, the TE emission was suppressed over a wide range of injection levels, operating temperatures, and device lengths, proving that the method is effective for achieving stable TM emission.

Journal of Electronic Materials, 1999

Grothendieck proved in EGA IV that if any integral scheme of finite type over a locally noetheria... more Grothendieck proved in EGA IV that if any integral scheme of finite type over a locally noetherian scheme X admits a desingularization, then X is quasi-excellent, and conjectured that the converse is probably true. We prove this conjecture for noetherian schemes of characteristic zero. Namely, starting with the resolution of singularities for algebraic varieties of characteristic zero, we prove the resolution of singularities for noetherian quasi-excellent Q-schemes.

Novel In-Plane Semiconductor Lasers, 2002

We report on the growth of GaInNAs materials and lasers by molecular beam epitaxy (MBE) using a r... more We report on the growth of GaInNAs materials and lasers by molecular beam epitaxy (MBE) using a rf-plasma source. Optimal GaInNAs quantum well (QW) structures have been designed and grown in order to achieve the brightest and narrowest photoluminescence (PL) spectra beyond 1.30 um. State-of-the-art GaInNAs/GaAs SQW lasers operating at 1.32 um have been demonstrated. For a broad area oxide stripe, uncoated Fabry-Perot laser with a cavity length of 1600 um, the threshold current density is 546 A/cm2 at room temperature. Optical output up to 40 mW per facet under continuous wave operation was achieved for these uncoated lasers at room temperature.

SPIE Proceedings, 2004

We have explored the operation of resonant cavity light-emitting diodes (RC-LEDs) used as resonan... more We have explored the operation of resonant cavity light-emitting diodes (RC-LEDs) used as resonant cavity photodetectors (RC-PDs). Our experiments were conducted using a variety of components, operating in diverse wavelength ranges and having different structural ...

Optical and Quantum Electronics, 1999

The laser diode structures reported up to now in literature for the red wavelength range are stil... more The laser diode structures reported up to now in literature for the red wavelength range are still far from optimal – mostly because many of the desired characteristics are contradictory coupled. Some of the contradictory coupled laser diode characteristics are investigated and a novel transverse layer structure is proposed. Both optical simulation and a fully self-consistent model are used in

Optical and Quantum Electronics, 2008

Applied Physics Letters, 2009