Feedback Gratings Research Articles

THz Difference-Frequency Generation in MOVPE-Grown Quantum Cascade Lasers

We report mass-producible room-temperature electrically-pumped THz sources based on intra-cavity difference-frequency generation in mid-infrared InGaAs/AlInAs/InP quantum cascade lasers. Devices are grown by a commercial foundry using metal organic vapor phase epitaxy. A dual-stack active region possessing giant optical nonlinearity for 3.5 THz generation and a non-collinear Cherenkov waveguide THz outcoupling scheme is employed. Fabry-Perot devices provided broad emission in the 3-4 THz range with a peak power of 5 μW. Single color THz sources were processed using surface distributed feedback gratings to produce narrowband emission at 3.5 THz with nearly 40 μW of peak power and a mid-infrared-to-THz conversion efficiency of 0.36 mW/W2. To better understand the dynamics of the DFG process, time gated spectral measurements of the mid-infrared pumps were performed and simultaneous lasing over the duration of the applied bias pulse was observed, thereby resulting in efficient difference frequency generation.

Distributed Feedback Gratings Based on Photo-Polymerization for Organic Solid-State Lasers

A novel preparation method of polymer DFB gratings has developed based on photo-polymerization for organic solid-state lasers. A photopolymer formulation sensitive to ultraviolet light is proposed for fabrication of polymer DFB gratings by spin-coating process. A very low surface relief depth ranging from 12.5 to about 1.0 nm has been demonstrated with a refractive index modulation of about 0.012. The experiment results indicate that the polymer DFB gratings are believed to have promising potentials in the fabrication of low-order DFB organic solid-state lasers.

Linearly polarized light from substrate emitting ring cavity quantum cascade lasers

The authors report on substrate emitting ring cavity quantum cascade lasers that feature linearly polarized emission beams at an emission wavelength of 8.8 μm. A central lobed far field is achieved by a modification of the distributed feedback grating, at which two π phase-shifts at an angular distance of 180° are applied. In this central lobe, 80% linear polarization is measured. In order to extend this polarization property to the whole far field, an on-chip wire grid polarizer is used. These devices show linearly polarized substrate emission with an extinction ratio higher than 1:16.

Electronically tunable aperiodic distributed feedback terahertz lasers

Focussed ion beam milling can be used to introduce aperiodic distributed feedback (ADFB) gratings into fully packaged, operational terahertz (THZ) quantum cascade lasers to achieve electronically controlled, discretely tunable laser emission. These aperiodic gratings—designed using computer-generated hologram techniques—consist of multiple slits in the surface plasmon waveguide, distributed along the length of the laser cavity. Tuning behaviour and output power in ADFB lasers operating around 2.9 THz are investigated with a variety of slit dimensions and grating scales. Mode selectivity and grating losses are found to be strongly dependent on milling depth into the upper waveguide layers, dramatically increasing as the metallic layers are penetrated, then rising more slowly with deeper milling into the laser active region. Grating scale and placement along the laser cavity length are also shown to influence mode selection.

Broadband photonic control for dual-mode terahertz laser emission

Short, holographically designed, aperiodic distributed feedback (ADFB) gratings are able to provide multi-band spectral filtering over arbitrarily wide bandwidths, offering a complimentary photonic technology to ultra-broadband terahertz quantum cascade lasers (THz QCLs). Using an ADFB grating, ion milled directly into the laser waveguide, high resolution spectral filtering is achieved in THz QCLs with heterogeneous active regions producing two distinct spectral gain peaks centred around 2.65 and 2.9 THz. Simultaneous dual-mode emission is achieved from a single section laser, with up to 20-dB side-mode suppression. Discrete electronic mode tuning occurs between ADFB bands, giving a switchable mode separation ranging from 163 to 267 GHz, along with continuous electronic and thermal tuning of up to ∼2 GHz.

Beam and phase distributions of a terahertz quantum cascade wire laser

We report on both measurements and simulations of the beam profile and wavefront of a single-mode, 3.5 THz quantum cascade wire laser, incorporating a lateral corrugated metal-metal waveguide, 3rd-order distributed feedback grating. The intrinsic wavefront was measured by using a Hartmann wavefront sensor (HWS) without any optical components between the laser and HWS. Both beam profile and wavefront were simulated using an antenna array model, but taking the non-uniform electric field distribution along the waveguide into account. The results show that the non-uniform distribution along the wire laser plays a crucial role in realizing a nearly single-lobed narrow beam. The measured wavefront is spherical and agrees well with the simulation.

High performance terahertz quantum cascade laser sources based on intracavity difference frequency generation

We demonstrate high power, room temperature, single-mode THz emissions based on intracavity difference frequency generation from mid-infrared quantum cascade lasers. Dual active regions both featuring giant nonlinear susceptibilities are used to enhance the THz power and conversion efficiency. The THz frequency is lithographically tuned by integrated dual-period distributed feedback gratings with different grating periods. Single mode emissions from 3.3 to 4.6 THz with side-mode suppression ratio and output power up to 40 dB and 65 µW are obtained, with a narrow linewidth of 5 GHz.

Widely tuned room temperature terahertz quantum cascade laser sources based on difference-frequency generation

We demonstrate room temperature THz quantum cascade laser sources with a broad spectral coverage based on intracavity difference-frequency generation. Two mid-infrared active cores based on the single-phonon resonance scheme are designed with a THz nonlinearity specially optimized at the high operating fields that correspond to the highest mid-infrared output powers. A Čerenkov phase-matching scheme along with integrated dual-period distributed feedback gratings are used for efficient THz extraction and spectral purification. Single mode emissions from 1.0 to 4.6 THz with a side-mode suppression ratio and output power up to 40 dB and 32 μW are obtained, respectively.

Discrete mode tuning in terahertz quantum cascade lasers

A holographically designed, aperiodic distributed feedback grating is used as a multi-resonance filter and embedded within an existing Fabry-Pérot (FP) terahertz (THz) quantum cascade laser (QCL) cavity. Balancing the feedback strengths of the filter resonances and the FP cavity creates a system capable of a high degree of single-mode selectivity, which is sensitive to changes in driving current. Multi-moded QCLs operating around 2.9 THz are thus modified to achieve purely electronic discrete tuning spanning over 160 GHz with an average tuning resolution of 30 GHz. Applying the same multi-resonance filter to QCLs with gain peaks around 2.65 and 2.9 THz leads to dual-mode lasing with an electrically controlled frequency separation of between 190 and 267 GHz. A phase sensitive mode selection mechanism is experimentally confirmed by the observation of divergent fine-tuning of the lasing modes.

Gas chopping etching process for InP based nanostructures with high aspect ratios

The authors report a novel etching process for the fabrication of nanostructured III-V semiconductors with ultra-high aspect ratios. For the very first time, a time multiplexed etching process (gas chopping process) for an InP based material system has been successfully developed and demonstrated. The repetition of a Cl2/H2/Ar based etching step together with passivation and removal steps ensures very deep etching with straight walls without general restrictions by the etch depth. The newly developed process is very promising for the fabrication of deeply etched feedback gratings for lasers or nanophotonic devices requesting high aspect ratios, like lasers with surface defined distributed Bragg reflector/distributed feedback gratings or deeply etched photonic crystals. This new process allows the formation of vertical profiles with smooth surfaces and reduced footing effect. Using this gas chopping process for InP, a high mask selectivity of 15, good etching rate of 95 nm/min, and record values of the aspect ratio up to 41 could be demonstrated.

Low-threshold blue lasing from silk fibroin thin films

Silk is a natural biocompatible material that can be integrated in a variety of photonic systems and optoelectronic devices. The silk replication of patterned substrates with features down to tens of nanometers is exploited to realize highly transparent, mechanically stable, and free-standing structures with optical wavelength size. We demonstrate organic lasing from a blue-emitting stilbene-doped silk film spin-coated onto a one-dimensional distributed feedback grating (DFB). The lasing threshold is lower than that of organic DFB lasers based on the same active dye. These findings pave the way to the development of an optically active biocompatible technological platform based on silk.

In situ etched gratings embedded in AlGaAs for efficient high power 970 nm distributed feedback broad-area lasers

We report optical nanostructuring technology, developed for distributed feedback gratings, broadly useable for many applications. The nanostructure is pre-structured into aluminum-free layers on top of AlGaAs then etched inside the epitaxy reactor and overgrown with AlGaAs. Oxygen contamination at the grating-interface is ∼3 × 1011 cm−2. These gratings introduce no extra internal optical loss and series resistance in broad-area lasers. Distributed feedback broad-area lasers using this technology achieve optical power >12 W, peak efficiency >60%, wide spectral locking range in current and heatsink temperature (over at least ∼30 °C) and operate at 10 W for >5000 h in a preliminary reliability test.

Distributed feedback quantum cascade lasers operating in continuous-wave mode at λ ≈ 7.6 μm

Distributed feedback (DFB) quantum cascade lasers (QCLs) in continuous-wave (CW) mode emitting at λ ≈ 7.6 μm are presented. Holographic lithography was used to fabricate the first-order distributed feedback grating. For a high-reflectivity-coated QCL with 14.5-μm-wide and 3-mm-long cavity, CW output powers of 300 mW at 85 K and still 10 mW at 270 K are obtained. Single-mode emission with a side-mode suppression ratio (SMSR) of about 30 dB and a wide tuning range of ∼300 nm in the temperature range from 85 to 280 K is observed.

Room temperature single-mode terahertz sources based on intracavity difference-frequency generation in quantum cascade lasers

We demonstrate room temperature single-mode THz emission at 4 THz based on intracavity difference-frequency generation from mid-infrared dual-wavelength quantum cascade lasers. An integrated dual-period distributed feedback grating is defined on the cap layer to purify both mid-infrared pumping wavelengths and in turn the THz spectra. Single mode operation of the pumping wavelengths results in a single-mode THz operation with a narrow linewidth of 6.6 GHz. A maximum THz power of 8.5 μW with a power conversion efficiency of 10 μW/W2 is obtained at room temperature.

λ∼3.36 μm room temperature InGaAs/AlAs(Sb) quantum cascade lasers with third order distributed feedback grating

We report on the development of strain compensated InGaAs/AlAs(Sb) quantum cascade lasers, incorporating a buried third order distributed feedback grating. Single mode operation with a side mode suppression ratio of ∼30 dB has been achieved in the wavelength range of 3.358–3.380 μm for temperatures between 270 and 360 K. The threefold increase in grating pitch size, compared with a first order grating, allows conventional photolithographic techniques to be used for single mode laser fabrication.

Optofluidic dye laser in a foil

First order distributed feedback optofluidic dye lasers embedded in a 350 microm thick TOPAS((R)) foil are demonstrated. They are designed in order to give high output pulse energies. Microfluidic channels and first order distributed feedback gratings are fabricated in parallel by thermal nanoimprint into a 100 microm foil. The channels are closed by thermal bonding with a 250 microm thick foil and filled with 5.10(-3) mol/l Pyrromethene 597 in benzyl alcohol. The fluid forms a liquid core single mode slab waveguide of 1.6 microm height on a nanostructured grating area of 0.5 x 0.5 mm(2). This results in a large gain volume. Two grating periods of 185 nm and 190 nm yield single mode laser light emission at 566 nm and 581 nm respectively. High emitted pulse energies of more than 1 microJ are reported. Stable operation for more than 25 min at 10 Hz pulse repetition rate is achieved.

Quasi-periodic distributed feedback laser

Although lasers have found numerous applications, their design is often still based on the concept of a gain medium within a mirror cavity. Exceptions to this are distributed feedback lasers1, in which feedback develops along a periodic structure, or random lasers, which do not require any form of cavity2. Random lasers have very rich emission spectra, but are difficult to control. Distributed feedback devices, conversely, have the same limited design possibilities of regular lasers. We show, by making use of a quasi-crystalline structure in an electrically pumped device, that several advantages of a random laser can be combined with the predictability of a distributed feedback resonator. We have constructed a terahertz quantum cascade laser based on a Fibonacci distributed feedback sequence, and show that engineering of the self-similar spectrum of the grating allows features beyond those possible with traditional periodic resonators, such as directional output independent of the emission frequency and multicolour operation. Researchers have constructed a terahertz quantum cascade laser using quasi-periodic distributed feedback gratings based on the Fibonacci sequence. Features that go beyond traditional distributed feedback lasers are demonstrated, such as directional output independent of the emission frequency and multicolour operation.

Integration of organic semiconductor lasers and single-mode passive waveguides into a PMMA substrate

Efficiently combining active and passive elements in integrated optics is a key ingredient for their successful employment. Here, we present the fabrication of an optimized PMMA substrate structure for improved coupling of laser light generated by organic semiconductor distributed feedback lasers into single-mode deep ultraviolet induced waveguides. For production, electron beam lithography on an oxidized silicon wafer and subsequent reactive ion etching is used to form the feedback grating of the laser. Afterwards, an aligned second electron beam lithography step on top of the grating allows the fabrication of a topographical step of 1.67 μm on the edges of the grating area. Metal is evaporated on this resulting master structure serving as a plating base for electroforming of a Ni tool. The tool is then used for hot embossing of the structure into PMMA bulk material. On a length of 500 μm the imprinted grating lines, having a period of 200 nm, are 100 nm wide and 60 nm high. Aligned deep ultraviolet exposure to induce a passive single- or multi-mode waveguide and co-evaporation of the active material Alq 3:DCM finish the coupling region. This structure optimizes the coupling of laser light generated in the laser structure into the passive waveguide. In combination with microfluidic channels, the laser light can be considered for sensing applications on a PMMA lab-on-chip system.

A quantum dot nanoimprinted DFB laser

In this work, we investigate a composite of conductive polymer poly(2-methoxy-5-(2-ethylhexyloxy)-1, 4-phenylenevinylene) (MEH-PPV) and CdSe/ZnS quantum dots as a laser active medium for nano-imprinted devices. The material is found to exhibit strong amplified spontaneous emission at the resonances of both the quantum dots and the polymer (650 nm and 636 nm, respectively). To investigate the guiding properties of the material and the guided and the leaky modes of the structure, we performed photonic simulations using the Beam Propagation Method. Well resolved, low-threshold lasing (optical pumping 0.95 mJ/cm2) was detected after nanoimprinting of distributed feedback gratings on the surface of the composite layer. One-dimensional omnidirectional Bragg mirrors, specially designed for the region of the emission wavelength, prevent loss of radiation into the substrate.

Design for optimized coupling of organic semiconductor laser light into polymer waveguides for highly integrated biophotonic sensors

For downscaling of analytical processes to lab-on-a-chip systems the integration of optical components in microfluidic devices is an emerging issue. We are heading for an all polymer system, which combines optical waveguides with microfluidic channels in one monolithic device. Photodegradation of poly(methyl methacrylate) through deep ultraviolet radiation is used for both, processing of channels and waveguides, respectively. In addition the implementation of organic laser light sources on chip shall allow for the use of exclusively optical interfaces. Creating distributed feedback gratings by hot embossing in poly(methyl methacrylate) and subsequent evaporation of Alq 3:DCM as organic laser active material onto the pattern enables the fabrication of disposable laser light sources. The wavelength of the implemented laser may be tuned about 100 nm around λ ≈ 630 nm via variation of grating period and height of laser active material. Simulations of waveguide eigenmodes are presented in this paper in order to optimize the coupling of lasers to deep ultraviolet induced waveguides. A concept to realize the highly integrated optofluidic chip with butt coupling of the laser light source to the passive waveguides is shown.