Shinichi Furuta

Semiconductor lasers with integrated plasmonic polarizers

The authors reported the plasmonic control of semiconductor laser polarization by means of metallic gratings and subwavelength apertures patterned on the laser emission facet. An integrated plasmonic polarizer can project the polarization of a semiconductor laser onto other directions. By designing a facet with two orthogonal grating-aperture structures, a polarization state consisting of a superposition of a linearly and right-circularly polarized light was demonstrated in a quantum cascade laser; a first step toward a circularly polarized laser.

High performance quantum cascade lasers based on three-phonon-resonance design

A quantum cascade laser structure based on three-phonon-resonance design is proposed and demonstrated. Devices, emitting at a wavelength of 9 μm, processed into buried ridge waveguide structures with a 3 mm long, 16 μm wide cavity and a high-reflection (HR) coating have shown peak output powers of 1.2 W, slope efficiencies of 1 W/A, threshold current densities of 1.1 kA/cm2, and high wall-plug efficiency of 6% at 300 K. A 3 mm long, 12 μm wide buried-heterostructure device without a HR coating exhibited continuous wave output power of as high as 65 mW from a single facet at 300 K.

Plasmonics for Laser Beam Shaping

This paper reviews our recent work on laser beam shaping using plasmonics. We demonstrated that by integrating properly designed plasmonic structures onto the facet of semiconductor lasers, their divergence angle can be dramatically reduced by more than one orders of magnitude, down to a few degrees. A plasmonic collimator consisting of a slit aperture and an adjacent 1-D grating can collimate laser light in the laser polarization direction; a collimator consisting of a rectangular aperture and a concentric ring grating can reduce the beam divergence both perpendicular and parallel to the laser polarization direction, thus achieving collimation in the plane perpendicular to the laser beam. The devices integrated with plasmonic collimators preserve good room-temperature performance with output power comparable to that of the original unpatterned lasers. A collimator design for one wavelength can be scaled to adapt to other wavelengths ranging from the visible to the far-IR regimes. Plasmonic collimation offers a compact and integrated solution to the problem of laser beam collimation and may have a large impact on applications such as free-space optical communication, pointing, and light detection and ranging. This paper opens up major opportunities in wavefront engineering using plasmonic structures.

High-performance, homogeneous broad-gain quantum cascade lasers based on dual-upper-state design

A broad-gain quantum cascade laser design with dual-upper-state is proposed to clarify its own feasibility. Devices employing the proposed active region design exhibit homogeneously wide (>330 cm−1) electroluminescence spectra of which shapes are insensitive to voltage changes. A buried heterostructure laser, emitting at λ∼8.4 μm, demonstrates a high continuous-wave output power of 152 mW together with a high constant slope efficiency of 518 W/A at room temperature. In addition, the device performance is observed to be very insensitive to temperature change; T0-values of ∼306 K and constant slope efficiency over the wide temperature range, 280–400 K.

Room temperature, continuous-wave operation of quantum cascade lasers with single phonon resonance-continuum depopulation structures grown by metal organic vapor-phase epitaxy

We propose a new quantum-cascade laser structure with single phonon resonance-continuum depopulation and demonstrate room temperature, continuous-wave operation of the proposed 7.9μm quantum-cascade laser. The laser grown by metal organic vapor-phase epitaxy emits a cw output power of 36mW at 30°C, exhibiting a threshold current density of 2.23kA∕cm2. The cw operation is reported for higher temperatures up to slightly above 60°C. The proposed structure may lead to a successful commercial mass production of the lasers.

Dipolar modeling and experimental demonstration of multi-beam plasmonic collimators

We designed a new class of plasmonic gratings that generate multiple free-space beams in arbitrary directions from a point source of surface waves, using a phenomenological model that accurately predicts their far-field, in amplitude, phase and polarization. We fabricated such gratings on the facets of semiconductor lasers. The plasmonic gratings proposed here are generally relevant to the interfacing of nanoscale optical components to free-space beams. The model introduced here can be used to design general two-dimensional plasmonic gratings.

Extremely high T0-values (∼450 K) of long-wavelength (∼15 μm), low-threshold-current-density quantum-cascade lasers based on the indirect pump scheme

The high device performance of a long-wavelength (∼15 μm), InGaAs/InAlAs, Fabry–Perot quantum-cascade laser based on the indirect pump scheme is reported. As a result of felicitous designing of the active region and waveguide structure, a low threshold-current-density of ∼3.5 kA/cm2, a high maximum output power of ∼216 mW, and a high slope efficiency of ∼346 mW/A, all at room temperature are obtained. The observed extremely high characteristic temperature of threshold current, T0∼450 K over wide temperature range, 320–380 K is ascribed to a strong suppression of electron populations in injectors, which are specifically visualized in the indirect pump scheme.

Broad-gain (Δλ/λ 0 ~0.4), 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.

High-performance quantum cascade lasers with wide electroluminescence (∼600 cm−1), operating in continuous-wave above 100 °C

The authors report high temperature continuous-wave (cw) operations of broad-gain quantum cascade lasers based on the anticrossed dual-upper-state to multiple-lower-state design. The devices exhibit extremely wide electroluminescence (>600 cm−1) and subthreshold amplified spontaneous emission (∼570 cm−1) spectra at room temperature. Despite showing such broad electroluminescence spectra, the high-reflection coated, buried heterostructure lasers operating at 6.8 μm demonstrate a low threshold current density of ∼1.5 kA/cm2 and a high power of >500 mW with a high slope efficiency of ∼1.6 W/A in cw mode at 300 K. The maximum cw operating temperature of above 100 °C is achieved.

Multi-Beam Multi-Wavelength Semiconductor Lasers

Multibeam emission and spatial wavelength demultiplexing in semiconductor lasers by patterning their facets with plasmonic structures is reported. Specifically, a single-wavelength laser was made to emit beams in two directions by defining on its facet two metallic gratings with different periods. The output of a dual-color laser was spatially separated according to wavelength by using a single metallic grating. The designs can be integrated with a broad range of active or passive optical components for applications such as interferometry and demultiplexing.