Μm Quantum Research Articles

Coherent phase lock of a 9 μm quantum cascade laser to a 2 μm thulium optical frequency comb

We demonstrate coherent phase locking of a room-temperature continuous-wave quantum cascade laser (QCL) at 9.1μm to a Tm-fiber laser frequency comb centered at 2μm, with an integrated residual phase error of 0.9rad (30mHz to 1.5MHz). This resulted in a QCL linewidth reduction from 525 to 25kHz at 1ms observation time, limited by the linewidth of the free-running frequency comb.

Wall-plug efficiency of mid-infrared quantum cascade lasers

We present a comprehensive analysis of the wall-plug efficiency of mid-infrared (λ ∼ 4–10 μm) quantum cascade (QC) lasers, by taking into account the transition efficiency of the active region, the light out-coupling efficiency from the emission facet, the voltage efficiency across the superlattice, and the current efficiency including the leakage and thermal backfilling current densities at threshold. All relevant parameters including their interdependencies have been taken into account and analyzed in order to optimize the wall-plug efficiency of practical QC lasers. Expressions for optimizing the extraction energy Δinj and the mirror loss αm are deduced, which are then used to maximize the wall-plug efficiency of QC lasers. Finally, predictions for the wall-plug efficiency of realistic QC lasers are made both at room temperature and at low temperature. Good agreement is found with corresponding experimental data from the literature.

5 μm laser source for frequency metrology based on difference frequency generation

A narrow-linewidth cw 5 μm source based on difference frequency generation of a 1.3 μm quantum dot external cavity diode laser and a cw Nd:YAG laser in periodically poled MgO:LiNbO(3) has been developed and evaluated for spectroscopic applications. The source can be tuned to any frequency in the 5.09-5.13 μm range with an output power up to 0.1 mW, and in the 5.42-5.48 μm range with sub-microwatt output. The output frequency is stabilized and its value determined by measuring the frequency of the two lasers with a remotely located frequency comb. A frequency instability of less than 4 kHz for long integration times and a linewidth smaller than 700 kHz were obtained.

Tapered 47 μm quantum cascade lasers with highly strained active region composition delivering over 45 watts of continuous wave optical power

A strain-balanced, Al0.7In0.22As/In0.72Ga0.28As/InP quantum cascade laser structure, designed for light emission at 4.7µm using the non-resonant extraction design approach, was grown by molecular beam epitaxy. Laser devices were processed in tapered buried heterostructure geometry and then mounted on AlN/SiC composite submounts using hard solder. A 10 mm long laser with 7.5µm-wide central section tapered up to 20µm at laser facets generated over 4.5W of single-ended CW/RT optical power at 283K. Maximum wallplug efficiency of 16.3% for this laser was reached at 4W level. Reliability of over 2,000h has been demonstrated for an air-cooled system delivering optical power of 3W in a collimated beam with overall system efficiency exceeding 10%.

Ridge-width dependence of the threshold of long wavelength (λ ≈14 µm) quantum cascade lasers with sloped and vertical sidewalls

We investigate the ridge-width dependence of the threshold of Quantum Cascade lasers fabricated by wet and dry etching, respectively. The sloped sidewalls resulting from wet etching affect the threshold in two ways as the ridge gets narrower. First, the transverse modes are deeper in the substrate, hence reducing the optical confinement factor. Second, more important, a non-negligible field exists in the lossy SiO2 insulation layer, as a result of transverse magnetic mode coupling to the surface plamon mode at the insulator/metal surface, which increases the waveguide loss. By contrast, dry etching is anisotropic and leads to waveguides with vertical sidewalls, which avoids the shift of the modes to the substrate layer and coupling to the surface plasmons, resulting in improved threshold compared with wet-etched lasers, e.g., for narrow ridge widths below 20 µm, the threshold of a 14 µm wide λ ≈ 14 µm laser by dry etching is ~60% lower than that of a wet-etched laser of the same width, at 80 K.

Singlemode quantum cascade lasers with power dissipation below 1 W

Presented are narrow-ridge, singlemode (λ=4.49 µm) quantum cascade lasers with low power consumption in continuous-wave operation. Episide up mounted 3 µm-wide and 1 mm-long devices with high-reflectivity backside coating show an electrical dissipation at laser threshold as low as 840 mW at 20°C. The devices can be tuned singlemode and mode-hop free between −30°C and +50°C and in a range of injected electrical power of 0.7 and 1.3 W with a sidemode suppression ratio better than 27 dB.

Proposal of High Performance 1.55.MU.m Quantum Dot Heterostructure Laser Using InN

This paper reports on a theoretical study and modeling of a 1.55µm quantum dot heterostructure laser using InN as a promising candidate for the first time. Details of design and theoretical analysis of probability distribution of the optical transition energy, threshold current density, modal gain, and differential quantum efficiency are presented considering a single layer of quantum dots. Dependence of threshold current density on the RMS value of quantum dot size fluctuations and the cavity length is studied. A low threshold current density of ∼51Acm-2 is achieved at room temperature for a cavity length of 640µm. An external differential efficiency of ∼65% and a modal gain of ∼12.5cm-1 are obtained for the proposed structure. The results indicate that the InN based quantum dot laser is a promising one for the optical communication system.

Second harmonic generation in quantum cascade lasers pumped by femtosecond mid-infrared pulses

Second harmonic generation (SHG) pulses at λ ∼ 2.25 µm have been obtained from a 4.5 µm quantum cascade laser (QCL) when it is resonantly pumped by transverse magnetic polarized 120 fs, λ = 4.5 µm pulses through the QCL’s front facet at room temperature. The measured SHG spectrum narrows when the bias across the QCL increases due to the electron population re-distribution and subband realignment. The expected quadratic dependence of the SHG with pump power is observed but saturates at higher pump powers. The linear to nonlinear power conversion efficiency is calculated (∼2 µW/W2) and compared with theoretical calculation. This experiment provides an alternative way of investigating the carrier dynamics in QCLs.

λ~71 μm quantum cascade lasers with 19% wall-plug efficiency at room temperature

Strain-balanced In0.6Ga0.4As/Al0.56In0.44As quantum cascade lasers emitting at a wavelength of 7.1 μm are reported. The active region is based on a three-phonon-resonance quantum design with a low voltage defect of 120 meV at injection resonance. A maximum wall-plug efficiency of 19% is demonstrated in pulsed mode at 293 K. Continuous-wave output power of 1.4 W and wall-plug efficiency of 10% are measured at the same temperature, as well as 1.2 W of average power in uncooled operation. A model for backfilling of the lower laser level which takes into account the number of subbands in the injector is presented and applied to determine the optimum value of the voltage defect to maximize wall-plug efficiency at room temperature, which is found to be ~100 meV, in good agreement with experimental results.

Temperature-insensitive long-wavelength (λ ≈14 µm) Quantum Cascade lasers with low threshold

We demonstrate high-performance, long-wavelength (λ ≈14 µm) Quantum Cascade (QC) lasers based on a diagonal optical transition and a "two-phonon-continuum" depletion scheme in which the lower laser level is depopulated by resonant longitudinal optical phonon scattering followed by scattering to a lower energy level continuum. A 2.8 mm long QC laser shows a low threshold current density of 2.0 kA/cm2, a peak output power of ~336 mW, and a slope efficiency of 375 mW/A, all at 300 K, with a high characteristic temperature T0 ~310 K over a wide temperature range from 240 K to 390 K.

Broad-gain (Δλ/λ_0~04), temperature-insensitive (T_0~510K) quantum cascade lasers

Broad-gain operation of λ~8.7 μm quantum cascade lasers based on dual-upper-state to multiple-lower-state transition design is reported. The devices exhibit surprisingly wide (~500 cm(-1)) electroluminescence spectra which are very insensitive to voltage and temperature changes above room temperature. With recourse to the temperature-insensitivity of electroluminescence spectra, the lasers demonstrate an extremely-weak temperature-dependence of laser performances: T0-value of 510 K, associated with a room temperature threshold current density of 2.6 kA/cm2. In addition, despite such wide gain spectra, room temperature, continuous wave operation of the laser with buried hetero structure is achieved.

Optimization of broadband (11–15 μm) optical coupling in Quantum Well Infrared Photodetectors for space applications

Large format detectors in the long wavelength region of the thermal infrared (TIR) spectrum ( λ ∼ 11–15 μm) with very high radiometric and imaging performances are of increasing scientific and operational interest for, e.g. meteorology and atmospheric chemistry in missions such as Meteosat Third Generation (MTG) or Planetary Science missions. A 30 months ESA project started in 2008, whose overall purpose is to expand and assess the performance of broadband 11–15 μm Quantum Well Infrared Photodetectors (QWIP) for spectro-imaging applications: Dispersive Spectrometers (DS) and Fourier Transform Spectrometers (FTS). The primary objective is the development of an optimized detection layer focusing on its technology and the necessary performance levels. In this paper, we present a theoretical and experimental study of the optical coupling inside the pixels of a QWIP focal plane array. The broadband 11–15 μm absorption spectrum is achieved by the use of a quantum well stack containing several kinds of quantum wells. The natural narrowband optical coupling of classical diffraction gratings doesn’t allow an efficient and uniform broadband optical coupling of the IR incident light over the whole 11–15 μm spectral band. Specific aperiodic or quasiperiodic optical coupling schemes have to be used. In this work we demonstrate the ability of obtaining a good spectral matching with various original broadband diffractive structures. An extensive experimental study is presented. Comparison with simulated spectra is performed.

A 10.7 μm InGaAs/InAlAs Quantum Cascade Detector

A 10.7μm quantum cascade detector based on lattice matched InGaAs/InAlAs/InP is demonstrated and characterized in terms of responsivity, resistivity and detectivity. The device operates in the 8–14 μm atmospheric window up to 140K and shows a peak reponsivity of 14.4mA/W at 78K. With a resistance-area product value of 159Ωcm2, the Johnson noise limited detectivity D*J is 2.8 × 109 Jones (cmHz1/2 W−1) at 78K.

Bandgap engineering of 1.3 μm quantum dot structures for terahertz (THz) emission

Terahertz (THz) technology has attracted vast interests due to its wide applications. Quantum dot (QD) system is proposed to be the most suitable candidate for compact THz sources based on intraband transitions. However, transition energies of reported results still fall outside the THz range (with corresponding energy of ∼0.4–41meV). In this study, we investigate the effect of growth temperature and monolayer coverage on the transition energies of InAs/InGaAs/GaAs QDs and InAs/GaAs bilayer QDs, respectively. Consequently, the obtained energy difference was less than 40meV, thus demonstrating the feasibility of varying the QD growth parameters for bandgap engineering towards the THz emission range.

Application of quantum well-like thermocouple to thermoelectric energy harvester by BiCMOS process

This work aims at improving the energy harvester performance by using low-dimensional thermoelectric materials. A micro-thermoelectric generator (μTEG) with quantum well-like thermocouples is developed by state-of-the-art CMOS (Complementary metal-oxide semiconductor) process. A relaxation-time model is applied to analyze the characteristic length of silicon germanium quantum well, and a thermal model is also applied to calculate the thermocouple size for optimal performance by matching the thermal/electrical resistance. Analysis based on TSMC 0.35 μm 3P3M (3-poly and 3-metal layers) BiCMOS process shows that the maximum power factor and voltage factor of a μTEG is 0.241 μW/cm 2 K 2 and 10.442 V/cm 2 K. Design implementation validates that the μTEG with 60 μm × 4 μm quantum well-like thermocouples (0.05 μm Si 0.9Ge 0.1 quantum well on 0.300 μm P-thermoleg and 0.280 μm N-thermoleg) has the best performance compared with those reported in the literature.

Design and performance of 1.55 μm laser using InGaN

AbstractThis paper describes the theoretical design and performance of a novel InGaN‐based 1.55 μm quantum well laser. A careful analysis of the optical and modal gain, threshold behavior, optical output, and efficiency with cavity length has been carried out. The spectral range of the optical material gain is found to be narrow and modal gain varies sharply with changing the reflectivity of the partially transparent mirror. It is found that a very small voltage of 1.10 V is required to reach the threshold current. A very small threshold current of 5.1 mA is required to emit the light. The calculated values of threshold current and threshold voltage are lower than the reported values of the conventional 1.55 μm laser. An efficiency of around 59% is found for a cavity length of 300 μm keeping the reflectivity at 35% of one mirror and 100% of other. The above study indicates that the proposed InGaN‐based 1.55 μm laser is very promising for the fabrication of future high performance laser (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Dispersion measurements of a 1.3 μm quantum dot semiconductor optical amplifier over 120 nm of spectral bandwidth

Group delay and higher order dispersion measurements are conducted on a 1.3 μm quantum dot semiconductor optical amplifier at various injection currents. White-light spectral interferometry is performed, along with a wavelet transform to recover the group delay. The group delay, group velocity dispersion, and higher order dispersion terms are quantified. The measurement spans both ground state and first excited state transitions, ranging from 1200 to 1320 nm. The group velocity dispersion, β2, is found to be −6.3×103 fs2 (7.6 fs/nm) at an injection current of 500 mA.

GaAs/AlGaAs (~9.4 μm) quantum cascade lasers operating at 260 K

GaAs/AlGaAs (~9.4 μm) quantum cascade lasers operating at 260 K

Analysis of 1.2μm InGaAs∕GaAs quantum dot laser for high power applications

The effect of modulation p-doping on the characteristics of 1.2μm quantum dot lasers is reported. Compared to undoped devices, p-doped are shown to exhibit higher saturated gain, higher internal efficiency, improved T0, lower excited state lasing current densities, and higher internal loss. Both types of the device are analyzed with regard to high power applications in the extreme cases of complete and nonexistent gain clamping. Results from a laser optimized to have minimal threshold current are discussed.

Theoretical investigation of 1.3 μm dots-under-a-well and dots-in-a-well InAs/GaAs quantum dot vertical-cavity surface-emitting lasers

The threshold characteristic and output power of 1.3 μm quantum dot (QD) vertical-cavity surface-emitting laser (VCSEL) with dots-under-a-well and dots-in-a-well InAs/GaAs QD structures are investigated by using rate equation model and output power model. The influence of VCSEL and QD structures on the modal gain of VCSEL is analyzed. Threshold current density, quantum efficiency, and characteristic temperature are simulated for different QD structures. The dependence of output power of 1.3 μm QD VCSEL on the QD structure, threshold current, quantum efficiency, and oxide-aperture size is investigated in detail.