Masamichi Yamanishi

Directional emission and universal far-field behavior from semiconductor lasers with limaçon-shaped microcavity

We report experimental demonstration of directional light emission from limacon-shaped microcavity semiconductor lasers. Quantum cascade lasers (QCLs) emitting at λ≈10u2002μm are used as a model system. Both ray optics and wave simulations show that for deformations in the range 0.37<e<0.43, these microcavities support high quality-factor whispering gallerylike modes while having a directional far-field profile with a beam divergence θ∥≈30° in the plane of the cavity. The measured far-field profiles are in good agreement with simulations. While the measured spectra show a transition from whispering gallerylike modes to a more complex mode structure at higher pumping currents, the far field is insensitive to the pumping current demonstrating the predicted “universal far-field behavior” of this class of chaotic resonators. Due to their relatively high quality factor, our microcavity lasers display reduced threshold current densities compared to conventional ridge lasers with millimeter-long cavities. The perfor...

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 9u2002μm, processed into buried ridge waveguide structures with a 3 mm long, 16u2002μm 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 1.1u2002kA/cm2, and high wall-plug efficiency of 6% at 300 K. A 3 mm long, 12u2002μm 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.

Quantum cascade lasers with integrated plasmonic antenna-array collimators

We demonstrated in simulations and experiments that by defining a properly designed two-dimensional metallic aperture-grating structure on the facet of quantum cascade lasers, a small beam divergence angle can be achieved in directions both perpendicular and parallel to the laser waveguide layers (denoted as theta perpendicular and theta parallel, respectively). Beam divergence angles as small as theta perpendicular=2.7 degrees and theta parallel=3.7 degrees have been demonstrated. This is a reduction by a factor of approximately 30 and approximately 10, respectively, compared to those of the original lasers emitting at a wavelength of 8.06 microm. The devices preserve good room temperature performance with output power as high as approximately 55% of that of the original unpatterned lasers. We studied in detail the trade-off between beam divergence and power throughput for the fabricated devices. We demonstrated plasmonic collimation for buried heterostructure lasers and ridge lasers; devices with different waveguide structures but with the same plasmonic collimator design showed similar performance. We also studied a device patterned with a spiders web pattern, which gives us insight into the distribution of surface plasmons on the laser facet.

Indirect pump scheme for quantum cascade lasers: dynamics of electron-transport and very high T0-values.

An alternative pump scheme, named indirect pump one is proposed to clarify its own feasibility. The high device performances of 8 microm quantum cascade lasers with cavity lengths of 4 mm and 1.5 mm are demonstrated: low threshold current densities of 2.7 and 3.3 kA/cm2 and maximum output powers of 362 and 50 mW at room temperature, and high T0-values of 243 and 303 K around room temperature. The higher T0-value, 303 K is the highest record ever reported with quantum cascade lasers. The high stability for temperature changes is interpreted in terms of the indirect pumping model.

Measuring frequency noise and intrinsic linewidth of a room-temperature DFB quantum cascade laser

The frequency-noise power spectral density of a room-temperature distributed-feedback quantum cascade laser emitting at λ = 4.36 μm has been measured. An intrinsic linewidth value of 260 Hz is retrieved, in reasonable agreement with theoretical calculations. A noise reduction of about a factor 200 in most of the frequency interval is also found, with respect to a cryogenic laser at the same wavelength. A quantitative treatment shows that it can be explained by a temperature-dependent mechanism governing the transport processes in resonant tunnelling devices. This confirms the predominant effect of the heterostructure in determining shape and magnitude of the frequency noise spectrum in QCLs.

Intracavity quartz-enhanced photoacoustic sensor

We report on a spectroscopic technique named intracavity quartz-enhanced photoacoustic spectroscopy (I-QEPAS) employed for sensitive trace-gas detection in the mid-infrared spectral region. It is based on a combination of QEPAS with a buildup optical cavity. The sensor includes a distributed feedback quantum cascade laser emitting at 4.33u2009μm. We achieved a laser optical power buildup factor of ∼500, which corresponds to an intracavity laser power of ∼0.75u2009W. CO2 has been selected as the target molecule for the I-QEPAS demonstration. We achieved a detection sensitivity of 300 parts per trillion for 4u2009s integration time, corresponding to a noise equivalent absorption coefficient of 1.4u2009×u200910−8u2009cm−1 and a normalized noise-equivalent absorption of 3.2u2009×u200910−10 W cm−1u2009Hz−1/2.

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.

Small divergence edge-emitting semiconductor lasers with two-dimensional plasmonic collimators

Using quantum cascade lasers with a two-dimensional metallic aperture-grating structure defined on the facet the authors demonstrate a collimated laser beam with small divergence angle perpendicular and parallel to the laser waveguide layers (2.7° and 3.7°, respectively). These values represent a reduction by a factor of ∼30 and ∼10, respectively, compared to those of the original 8.06-μm- wavelength laser without plasmonic collimation. The devices preserve good room temperature performance with output power as high as 53% of that of the original unpatterned lasers.

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 (>330u2002cm−1) electroluminescence spectra of which shapes are insensitive to voltage changes. A buried heterostructure laser, emitting at λ∼8.4u2002μm, 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 ∼306u2002K and constant slope efficiency over the wide temperature range, 280–400 K.

Comb-assisted subkilohertz linewidth quantum cascade laser for high-precision mid-infrared spectroscopy

We report on the linewidth narrowing of a room-temperature mid-infrared quantum cascade laser by phase-locking to a difference-frequency-generated radiation referenced to an optical frequency comb synthesizer. A locking bandwidth of 250u2009kHz, with a residual rms phase-noise of 0.56 rad, has been achieved. The laser linewidth is narrowed by more than 2 orders of magnitude below 1u2009kHz, and its frequency is stabilized with an absolute traceability of 2×10−12. This source has allowed the measurement of the absolute frequency of a CO2 molecular transition with an uncertainty of about 1u2009kHz.