Wonill Ha

Integrated semiconductor vertical-cavity surface-emitting lasers and PIN photodetectors for biomedical fluorescence sensing

Vertical-cavity surface-emitting lasers (VCSELs), optical emission filters, and PIN photodetectors were fabricated as part of a monolithically integrated near-infrared fluorescence detection system. The integration of these micro-fabricated components with micro-arrays, flow channel arrays, and biochips can drastically reduce cost and enable parallel sensing architectures. An optoelectronic design is presented that integrates VCSELs, optical filters, and photodetectors through a modification to a typical VCSEL structure. System designs were simulated and compared, leading to several innovative approaches for integrated sensors. The laser and detector modules were characterized independently and subsequently integrated to form a complete sensor. VCSELs with oxidation apertures measuring 4, 7, 14, and 20 /spl mu/m showed a lasing wavelength of /spl lambda/=773 nm, threshold current densities from 6400 to 1300 A/spl middot/cm/sup -2/, and maximum output powers of 0.6-4 mW, with transverse single-mode and multimode operation. PIN photodetectors were fabricated with integrated emission filters. Quantum efficiencies above 85% were observed with a dark current of 500 fA/(mm detector diameter). Complete sensor units were tested and near-infrared fluorescent molecules (IR-800) were detected. A theoretical detection limit of 10/sup 5/ fluorophores//spl mu/m/sup 2/ was determined. The compact parallel architecture, high-power laser, and low-noise photodetector make this sensor a good candidate for biomedical fluorescence-based sensing applications.

Long-wavelength GaInNAs(Sb) lasers on GaAs

The boom in fiber-optic communications has caused a high demand for GaAs-based lasers in the 1.3-1.6-/spl mu/m range. This has led to the introduction of small amounts of nitrogen into InGaAs to reduce the bandgap sufficiently, resulting in a new material that is lattice matched to GaAs. More recently, the addition of Sb has allowed further reduction of the bandgap, leading to the first demonstration of 1.5-/spl mu/m GaAs-based lasers by the authors. Additional work has focused on the use of GaAs, GaNAs, and now GaNAsSb barriers as cladding for GaInNAsSb quantum wells. We present the results of photoluminescence, as well as in-plane lasers studies, made with these combinations of materials. With GaNAs or GaNAsSb barriers, the blue shift due to post-growth annealing is suppressed, and longer wavelength laser emission is achieved. Long wavelength luminescence out to 1.6 /spl mu/m from GaInNAsSb quantum wells, with GaNAsSb barriers, was observed. In-plane lasers from these samples yielded lasers operating out to 1.49 /spl mu/m, a minimum threshold current density of 500 A/cm/sup 2/ per quantum well, a maximum differential quantum efficiency of 75%, and pulsed power up to 350 mW at room temperature.

Multiple-quantum-well GaInNAs-GaNAs ridge-waveguide laser diodes operating out to 1.4 /spl mu/m

In this letter, results from a ridge waveguide laser diode (LD) structure, with three GaInNAs quantum wells (QWs) and GaNAs barriers, are presented. The sample was grown by solid source molecular beam epitaxy with an RF plasma nitrogen source. These devices differ from previously reported GaInNAs QWs LDs that used GaAs as the barrier material. The introduction of nitrogen into the barriers reduces the spectral blue shift caused by post-growth annealing. Long wavelength emission out to 1.405 /spl mu/m was observed. The devices exhibited threshold current densities as low as 1.5 kA/cm/sup 2/, high differential efficiency of 0.67 W/A, and a maximum output power of 350 mW.

GaInNAsSb for 1.3-1.6-/spl mu/m-long wavelength lasers grown by molecular beam epitaxy

High-efficiency optical emission past 1.3 /spl mu/m of GaInNAs on GaAs, with an ultimate goal of a high-power 1.55-/spl mu/m vertical-cavity surface-emitting laser (VCSEL), has proven to be elusive. While GaInNAs could theoretically be grown lattice-matched to GaAs with a very small bandgap, wavelengths are actually limited by the N solubility limit and the high In strain limit. By adding Sb to the GaInNAs quaternary, we have observed a remarkable shift toward longer luminescent wavelengths while maintaining high intensity. The increase in strain of these new alloys necessitates the use of tensile strain compensating GaNAs barriers around quantum-well (QW) structures. With the incorporation of Sb and using In concentrations as high as 40%, high-intensity photoluminescence (PL) was observed as long as 1.6 /spl mu/m. PL at 1.5 /spl mu/m was measured with peak intensity over 50% of the best 1.3 /spl mu/m GaInNAs samples grown. Three QW GaIn-NAsSb in-plane lasers were fabricated with room-temperature pulsed operation out to 1.49 /spl mu/m.

Integrated bio-fluorescence sensor.

Due to the recent explosion in optoelectronics for telecommunication applications, novel optoelectronic sensing structures can now be realized. In this work, we explore the integration of optoelectronic components towards miniature and portable fluorescence sensors. The integration of these micro-fabricated sensors with microfluidics and capillary networks may reduce the cost and complexity of current research instruments and open up a world of new applications in portable biological analysis systems. A novel optoelectronic design that capitalizes on current vertical-cavity surface-emitting laser (VCSEL) technology is explored. Specifically, VCSELs, optical emission filters and PIN photodetectors are fabricated as part of a monolithically integrated near-infrared fluorescence detection system. High-performance lasers and photodetectors have been characterized and integrated to form a complete sensor. Experimental results show that sensor sensitivity is limited by laser background. The laser background is caused by spontaneous emission emitted from the side of the VCSEL excitation source. Laser background will limit sensitivity in most integrated sensing designs due to locating excitation sources and photodetectors in such close proximity, and methods are proposed to reduce the laser background in such designs so that practical fluorescent detection limits can be achieved.

532-nm laser sources based on intracavity frequency doubling of extended-cavity surface-emitting diode lasers

We introduce a novel type of cw green laser source, the Protera 532, based on the intracavity frequency doubling of an extended-cavity, surface-emitting diode laser. The distinguishing characteristics of this platform are high compactness and efficiency in a stable, single-longitudinal mode with beam quality M2 < 1.2. The laser design is based on the previously reported NECSEL architecture used for 488nm lasers, and includes several novel features to accommodate different types of nonlinear optical materials. The infrared laser die wavelength is increased from 976nm to 1064nm without compromising performance or reliability. The intracavity frequency doubling to 532nm has been demonstrated with both bulk and periodically poled nonlinear materials, with single-ended cw power outputs of greater than 30 mW.

High-power high-brightness 980-nm lasers based on the extended cavity surface emitting lasers concept

We describe design and performance of novel, electrically pumped, vertical compound cavity semiconductor lasers emitting at 980 nm. The laser combines a vertical cavity semiconductor laser with a partially reflecting output coupler and an external cavity for mode control. The concept is scalable and has been demonstrated in monolithic low power (few miliwatts) devices all the way to high power extended cavity devices which generate over 950 mW CW multimode power and 0.5 W CW power in a TEM00 mode, the latter with 90% coupling efficiency into a single mode telecommunication fiber. The concept has been applied to the development of uncooled lasers, mounted in TO-56 cans, capable of producing 50 to 100 mW of fiber-coupled power. We have also demonstrated the extended cavity lasers at wavelengths of 920 nm and 1064 nm. We present reliability data for the chips used in the extended cavity lasers.

Operation of a passively mode-locked extended-cavity surface-emitting diode laser in multi-GHz regime

We report on passive mode-locking of vertical cavity surface-emitting diode lasers at 980-nm wavelength, applied to different extended resonator configurations. Stable mode-locking producing pulses of approximately 50 ps in duration at up to 6-GHz repetition rate has been achieved. The use of external feedback results in pronounced harmonic pulse generation, extending the operational range of these new devices.

50-nm E-mode In 0.7 Ga 0.3 As PHEMTs on 100-mm InP substrate with f max > 1 THz

We have demonstrated 50-nm enhancement-mode (E-mode) In<inf>0.7</inf>Ga<inf>0.3</inf>As PHEMTs with f<inf>max</inf> in excess of 1 THz. The devices feature a Pt gate sinking process to effectively thin down the In<inf>0.52</inf>Al<inf>0.48</inf>As barrier layer, together with a two-step recess process. The fabricated device with L<inf>g</inf> = 50-nm exhibits V<inf>T</inf> = 0.1 V, g<inf>m,max</inf> = 1.75 mS/µm, f<inf>T</inf> = 465 GHz and f<inf>max</inf> = 1.06 THz at a moderate value of V<inf>DS</inf> = 0.75 V. In addition, we have physically modeled the abnormal peaky behavior in Masons unilateral gain (U<inf>g</inf>) at high values of VDS. A revised small signal model that includes a shunting R<inf>gd-NDR</inf> with negative value successfully describes the behavior of the device from 1 to 67 GHz.

Laser sources at 460 nm based on intracavity doubling of extended-cavity surface-emitting lasers

Laser sources emitting at 460nm have been developed through intracavity doubling of an extended cavity, surface emitting semiconductor laser. These lasers are compact, spectrally pure, efficient, and have a high quality beam. The basic design is similar to previously reported work[1] at 488nm using Novalux Extended Cavity Surface Emitting Laser (NECSEL) structures. The choice of nonlinear material was found to be critical, with periodically poled materials providing distinct benefits over bulk materials. Output powers exceeded 20mW. The reliability of the completed lasers was found to be excellent.