Silicon Raman Laser Research Articles

Broadband high-Q multimode silicon concentric racetrack resonators for widely tunable Raman lasers

Multimode silicon resonators with ultralow propagation losses for ultrahigh quality (Q) factors have been attracting attention recently. However, conventional multimode silicon resonators only have high Q factors at certain wavelengths because the Q factors are reduced at wavelengths where fundamental modes and higher-order modes are both near resonances. Here, by implementing a broadband pulley directional coupler and concentric racetracks, we present a broadband high-Q multimode silicon resonator with average loaded Q factors of 1.4 × 106 over a wavelength range of 440 nm (1240–1680 nm). The mutual coupling between the two multimode racetracks can lead to two supermodes that mitigate the reduction in Q factors caused by the mode coupling of the higher-order modes. Based on the broadband high-Q multimode resonator, we experimentally demonstrated a broadly tunable Raman silicon laser with over 516 nm wavelength tuning range (1325–1841 nm), a threshold power of (0.4 ± 0.1) mW and a slope efficiency of (8.5 ± 1.5) % at 25 V reverse bias.

LED pumped Raman laser: Towards the design of an on-chip all-silicon laser

Design of an on-chip silicon Raman laser is proposed based on a slotted photonic crystal nanocavity. The slot has been considered to be filled with the silicon nanocrystal material having ultra-high Raman gain coefficient, which has further been fortified by the tight spatio-temporal optical confinement inside the nanocavity. Consequently, the threshold power as required for the lasing is miniaturized substantially, and a light emitting diode (LED) suffices for optical pumping. The LED is amenable for integration on the same platform as of the silicon nanocrystal embedded slotted photonic crystal nanocavity exploiting the efficient electroluminescence of the silicon nanocrystal material. The coupled mode equations for stimulated Raman scattering interactions, which govern the evolution of the pump and the Stokes photons, have been solved using a novel self-consistent algorithm to analyze the characteristics of the proposed laser. A lasing threshold-power, as small as, 5 μW has been achieved. Moreover, small-signal frequency response of the proposed LED pumped laser has also been evaluated in steady-state condition, and a modulation bandwidth as high as ∼1.6 GHz has been demonstrated which is almost two orders of magnitude higher than that of the conventional Raman lasers.

1.2-µm-band ultrahigh-Q photonic crystal nanocavities and their potential for Raman silicon lasers

Nanocavity devices based on silicon that can operate in the 1.2-µm band would be beneficial for several applications. We fabricate fifteen cavities with resonance wavelengths between 1.20 and 1.23 µm. Experimental quality (Q) factors larger than one million are obtained and the average Q values are lower for shorter wavelengths. Furthermore, we observe continuous-wave operation of a Raman silicon laser with an excitation wavelength of 1.20 µm and a Raman laser wavelength of 1.28 µm. The Q values of the nanocavity modes used to confine the excitation light and the Raman scattered light are about half of those for our Raman silicon laser operating in the 1.55-µm band. Nevertheless, this device exhibits an input-output characteristic with a clear laser threshold. Finally, we consider the effect of the higher scattering probability at shorter wavelengths on the Raman laser performance in the 1.2-µm band.

Widely tunable silicon Raman laser

Stimulated Raman scattering is an effective means of wavelength conversion and can largely extend the operating spectral range of an optical source. We demonstrate a high-performance tunable Raman laser on a sub-micrometer-thick silicon on insulator wafer using a standard foundry process. The key feature to this laser is the use of a tunable coupling mechanism to adjust both pump and signal coupling coefficients in the ring cavity, allowing demonstration of laser emission over a large wavelength tuning range of 83 nm. This Raman laser demonstrates efficient (slope of up to 26% and a maximum pump-to-signal power conversion efficiency of 10%) on-chip nonlinear wavelength conversion. Our results indicate great promise for substantially increasing the optical spectral resources available on a silicon chip.

Sub-100-nW-threshold Raman silicon laser designed by a machine-learning method that optimizes the product of the cavity Q-factors.

Raman silicon lasers based on photonic crystal nanocavities with a threshold of several hundred microwatts for continuous-wave lasing have been realized. In particular, the threshold depends on the degree of confinement of the excitation light and the Raman scattering light in the two nanocavity modes. Here, we report lower threshold values for Raman silicon nanocavity lasers achieved by increasing the quality (Q) factors of the two cavity modes. By using an optimization method based on machine learning, we first increase the product of the two theoretical Q values by a factor of 17.0 compared to the conventional cavity. The experimental evaluation demonstrates that, on average, the actually achieved product is more than 2.5 times larger than that of the conventional cavity. The input-output characteristic of a Raman laser with a threshold of 90 nW is presented and the lowest threshold obtained in our experiments is 40 nW.

Detection of negatively ionized air by using a Raman silicon nanocavity laser.

The performance of a Raman silicon laser based on a high quality-factor nanocavity depends on the degree of free-carrier absorption, and this characteristic may be useful for certain applications. Here we demonstrate that laser oscillation in a Raman silicon nanocavity laser stops abruptly after an exposure to a weak flux of negatively ionized air for a few seconds. Spectral measurements reveal that the laser interruption is mainly caused by the transfer of extra electrons from the negatively ionized air molecules to the silicon nanocavity. These electrons affect the efficiency of the Raman laser by free carrier absorption. We find that the laser output gradually recovers as the extra electrons escape from the nanocavity and confirm that such a detection of ionized air is repeatable. These results show that a Raman silicon nanocavity laser can be used for the detection of ionized air with a high spatial resolution.

Numerical investigation into the output performance of a silicon Raman laser based on a silicon-on-insulator waveguide pumped by 2 µm lasers

We numerically analyze the characteristics of a mid-infrared continuous-wave (CW) Raman laser in silicon-on-insulator (SOI) photonics circuits including standing-cavity and ring resonator configurations, which are pumped by lasers at 2 µm. The dependence of cascaded Raman laser performance on the cavity length, effective free carrier lifetime, and the power transmission is investigated for the optimization of CW Raman silicon standing-cavity lasers. In addition, the mid-infrared Raman laser in the ring resonator is also presented under different parameters; in particular, the coupling strengths between the bus waveguide and the ring resonator for both the pump and Raman lasers are crucial to the laser performance. The numerical simulation results also reveal that the position of the coupler for extracting the Raman light from the ring resonator plays an important role in optimizing the structures of such Raman lasers.

Raman silicon laser based on a nanocavity fabricated by photolithography

Fabricating silicon photonics devices by CMOS-compatible processes is important for applications. Here, we demonstrate a Raman silicon laser based on a heterostructure nanocavity that was fabricated by immersion photolithography using an argon fluoride excimer laser. The Raman laser confines the pump light and the Stokes Raman scattered light in two resonant modes of the nanocavity. By using the presented CMOS-compatible approach, sufficiently high quality-factors can be obtained for both modes. The sample whose frequency spacing of the two resonant modes closely matches the Raman shift of silicon, achieves continuous-wave oscillation with a lasing threshold of 1.8 µW at room temperature.

Cavity Engineering of Photon–Phonon Interactions in Si Nanocavities

The ever-increasing demand for faster, smaller, and energy-efficient devices has pushed the frontiers of research toward silicon photonics to meet the challenges for fabricating the next generation of computing systems. In order to design new devices at the nanoscale, it is important to understand and be able to control material properties, which may differ significantly from their bulk counterparts. Here, we demonstrate very large tunability of phonon-photon interactions in Si nanowire cavities by engineering the cavity mode at the emission wavelength. Raman scattering measurements performed to quantify these interactions reveal that the anti-Stokes to Stokes scattering ratio can vary from 0.035 to 0.405 in Si nanowires compared to a value of 0.1 for bulk Si, demonstrating tunability by over an order of magnitude. Moreover, a ratio of 0.85 was attained at a temperature of 580 K, which is the highest value ever reported for Si. Cavity modes that can be easily changed by changing the nanowire diameter, cavity geometry, or excitation wavelength provide efficient ways of tuning these interactions. Nanocavity engineering offers a new approach for tuning phonon-photon interactions in silicon and opens up new avenues of research and applications in the fields of silicon photonics, Raman lasers, telecommunication, and optical cooling.

Design of a Monolithic Low-Threshold Narrow-Linewidth CW Mid-IR Silicon Raman Laser

We present the design and anticipated performance of an optically pumped monolithic all-silicon mid-IR (MIR) Raman laser. For the proposed 3.15 um cw laser design pumped at a 2.71 um pump wavelength, we estimate a threshold pump power of < 10 mW and an estimated output laser linewidth of <50 MHz. This design may pave the road toward demonstration of the first mid-IR silicon Raman laser.

Implementing a Raman silicon nanocavity laser for integrated optical circuits by using a (100) SOI wafer with a 45-degree-rotated top silicon layer

Raman silicon (Si) lasers based on high-quality-factor photonic crystal nanocavities are very compact and can operate at excitation powers of less than one microwatt. For the best performance, the nanocavity of the Raman Si laser has to be fabricated along the [100] crystal direction of the Si-on-insulator (SOI) wafer to enhance the Raman gain. On the other hand, Si photonic devices are usually fabricated along the [110] direction because crystalline Si can be easily cleaved along [110]. This rotation by 45 degrees of the nanocavity with respect to the cleavable direction can be problematic for various applications. Here we report a Raman Si nanocavity laser fabricated on a modified (100) SOI wafer in which the crystal orientation of the top Si layer is rotated in plane by 45 degrees relative to the crystal direction of the support substrate. We observe room temperature continuous wave laser oscillation with a sub-microwatt threshold and a maximum energy efficiency of 5.6%. It is found that the photonic-slab warpage induced by compressive stress is reduced in this 45°-rotated SOI wafer.

Strongly asymmetric wavelength dependence of optical gain in nanocavity-based Raman silicon lasers

While the realization of silicon lasers using interband transitions is still technically problematic, utilization of Raman scattering processes seems to be the most feasible alternative. Raman silicon lasers based on photonic crystal nanocavities provide sub-microwatt thresholds and CMOS compatibility. Therefore, this type of laser is suitable for dense integration in Si photonic circuits. However, details of the gain mechanism, which are important for improving laser performance, have rarely been discussed due to the lack of a suitable characterization technique. Here, we report on the excitation-wavelength dependence of optical gain in a high-quality nanocavity-based Raman silicon laser. For this, we employ a so-called stimulated-Raman-scattering excitation (SRE) spectroscopy, which allows us to reveal the range of excitation wavelengths enabling laser operation, the excitation condition for maximum output, shift of the gain peak, and enhancement of Raman gain including nonlinear optical losses. In particular, we find that laser output remarkably decreases in the long-wavelength region of cavity resonance as excitation power increases. Numerical simulations suggest that optical loss due to free-carrier absorption induced by two-photon absorption grows substantially above a certain threshold.

LED pumped micron-scale all-silicon Raman amplifier

A micron-scale all-silicon Raman amplifier has been proposed in this paper, exploiting the giant Raman gain of silicon nanocrystal material along with the extreme optical confinement of slotted photonic crystal waveguide. Light Emitting Diode (LED) has been considered here for low-cost optical pumping and the possibility of on-chip integration. At the same time, LED pumping eradicates the temporal impairment of output pulses which is otherwise unavoidable in case of continuous wave laser pumping. An overall gain of 3.22 dB has been achieved for a 400 Gbps input pulse train with a waveguide length of the amplifier which is as small as 4 μm. Moreover, the strong electroluminescence of silicon nanocrystal opens up the possibility of integrating the pump source on the same platform and, hence, expedites the future scope of realizing micron-scale silicon Raman laser without external pump source.

CMOS compatible high-Q photonic crystal nanocavity fabricated with photolithography on silicon photonic platform

Progress on the fabrication of ultrahigh-Q photonic-crystal nanocavities (PhC-NCs) has revealed the prospect for new applications including silicon Raman lasers that require a strong confinement of light. Among various PhC-NCs, the highest Q has been recorded with silicon. On the other hand, microcavity is one of the basic building blocks in silicon photonics. However, the fusion between PhC-NCs and silicon photonics has yet to be exploited, since PhC-NCs are usually fabricated with electron-beam lithography and require an air-bridge structure. Here we show that a 2D-PhC-NC fabricated with deep-UV photolithography on a silica-clad silicon-on-insulator (SOI) structure will exhibit a high-Q of 2.2 × 105 with a mode-volume of ~1.7(λ/n)3. This is the highest Q demonstrated with photolithography. We also show that this device exhibits an efficient thermal diffusion and enables high-speed switching. The demonstration of the photolithographic fabrication of high-Q silica-clad PhC-NCs will open possibility for mass-manufacturing and boost the fusion between silicon photonics and CMOS devices.

Research progress of silicon light source

To meet the requirements for high speed, low cost, and more information capacity, silicon photonics has been booming in recent years. Silicon photonics covers a very wide field. For the silicon photonics, researchers have successfully achieved silicon-based optical waveguides, switches, modulators, and detectors. But the problem of silicon based light source has not been really resolved, which has become a primary bottleneck for further developing the silicon photonics. The momentum of a phonon is required to allow an electron to transit from the minimum of the conduction band to the maximum of the valence band in Si because of the indirect bandgap. This two-particle process with a low probability makes it difficult to achieve high-efficiency silicon-based light source by itself. However, much effort has been made to characterize and understand the light-emission phenomena of silicon-based devices. Also, more attempts were made to enhance the emission efficiency of silicon. Practical silicon lasers are very important for silicon photonics and have been a long goal for semiconductor scientists. A number of important breakthroughs in the past decade have focused on silicon as a photonic platform thanks to the efforts of scientists. In this review, we introduce the recent progress of silicon-based luminescence materials, silicon light emitting diodes and silicon lasers. In the first part of this paper, common types of silicon-based light emitting materials, including porous silicon, silicon nanocrystals, rare earth-doped silicon, silicon defect emission, germanium on silicon and semiconducting silicides are comprehensively reviewed. Among them, the quantum effects and surface effects of low-dimensional silicon can greatly enhance the light emission efficiency. The erbium atoms in silicon-based rare earth materials can produce the light emission at communication wavelength band independently of the host. The transition from the lowest excited state to the 4f ground state yields light at 1.54 m. Moreover the emission energy is independent of the temperature due to the inner atomic transition. Group IV materials grown on silicon such as Ge and GeSn alloy can change from indirect bandgap into direct bandgap by introducing mechanically strain and modifying the component. Strong enhancement of photoluminescence and net gain emerging from the direct transition are very significant for fabricating the devices. In the second part, different light emitting diodes (LEDs) fabricated with above luminescent materials are introduced. The Si PN diodes were once popular at the earlier research stage. One approach was to modify the effective surface on high-purity single crystal silicon and the other idea was to use optically active defects in silicon. Ten years later, silicon LEDs in which the dressed-photon-phonons assisted method is used, made the Si PN diode rejuvenated. LEDs fabricated on nano-structured Si and silicon-based film were limited in the optoelectronic integration since the luminescence wavelength is not corresponding to the low-loss communication region. Although erbium-doped and Er silicate LEDs emit suitable light, their high turn-on voltage and low luminescence efficiency block the practical application. The researches of Ge-on-Si LED mainly focus on modifying the band structure by introducing strain and n-doping. In the third part, firstly we summarize the basic rules of the silicon laser. Then, we review the most recent progress in the field. Nanometer Si with periodic array can only behave unambiguous laser action using optical pumping and at very low temperature. Low threshold silicon Raman lasers with racetrack ring resonator cavities can only stop on paper also due to the difficulty in electrical pumping. The Ge-on-Si lasers operating at room temperature by optical and electrical pumping were accomplished in the past 5 years. The GeSn laser that is CMOS-compatible also came into being this year. Although so far, lasing has been implemented only by using pulsed optical pumping and stopped working at 90 K, this first demonstration of lasing in a direct-gap group IV alloy grown on standard silicon substrates is potentially an important step on the way to a platform of fully integrated silicon-based photonics. Hybrid III-V-on-Si lasers are considered as one of the most practical means due to the excellent photoelectric properties and mature preparation technology. Finally, current problems and future development direction in the silicon light source are also presented briefly.

Breakthroughs in Photonics 2013: A Microwatt-Threshold Raman Silicon Laser

A continuous-wave Raman silicon laser using a photonic-crystal high- Q nanocavity was developed in 2013, with a cavity size of less than 10 μm and an unprecedented ultralow threshold of 1 μW. This report describes how this breakthrough was accomplished by adopting a unique nanocavity design.

High-Qresonant modes in a photonic crystal heterostructure nanocavity and applicability to a Raman silicon laser

When a heterostructure is created at the center of a photonic crystal line-defect cavity to form a nanocavity, the photonic band gap contains several high-quality ($Q$) factor resonant modes. We have studied the optical properties of these modes to examine their applicability to Raman silicon lasers, which require two high-$Q$ resonant modes with a frequency spacing of 15.6 THz. Our experimental and numerical analyses reveal four types of resonant modes. We demonstrate that pairing the resonant mode originating from the first-order propagation mode with that arising from the second-order propagation mode is the most promising approach toward the realization of Raman silicon lasers.

A micrometre-scale Raman silicon laser with a microwatt threshold

The application of novel technologies to silicon electronics has been intensively studied with a view to overcoming the physical limitations of Moore's law, that is, the observation that the number of components on integrated chips tends to double every two years. For example, silicon devices have enormous potential for photonic integrated circuits on chips compatible with complementary metal-oxide-semiconductor devices, with various key elements having been demonstrated in the past decade. In particular, a focus on the exploitation of the Raman effect has added active optical functionality to pure silicon, culminating in the realization of a continuous-wave all-silicon laser. This achievement is an important step towards silicon photonics, but the desired miniaturization to micrometre dimensions and the reduction of the threshold for laser action to microwatt powers have yet to be achieved: such lasers remain limited to centimetre-sized cavities with thresholds higher than 20 milliwatts, even with the assistance of reverse-biased p-i-n diodes. Here we demonstrate a continuous-wave Raman silicon laser using a photonic-crystal, high-quality-factor nanocavity without any p-i-n diodes, yielding a device with a cavity size of less than 10 micrometres and an unprecedentedly low lasing threshold of 1 microwatt. Our nanocavity design exploits the principle that the strength of light-matter interactions is proportional to the ratio of quality factor to the cavity volume and allows drastic enhancement of the Raman gain beyond that predicted theoretically. Such a device may make it possible to construct practical silicon lasers and amplifiers for large-scale integration in photonic circuits.

Design of Silicon Photonic Crystal Waveguides for High Gain Raman Amplification Using Two Symmetric Transvers-Electric-Like Slow-Light Modes

We designed silicon photonic crystal (PhC) waveguides (WGs) for efficient silicon Raman amplifiers and lasers. We adopted narrow-width WGs to utilize two symmetric transvers-electric-like (TE-like) guided modes, which permit efficient external coupling for both the pump and Stokes waves. Modifying the size and shape of air holes surrounding the line-defect WG structures could tune the frequency difference between these two modes, at the Brillouin-zone edge, to match the Raman shift of silicon. Thus, small group velocities are also available both for pump and Stokes waves simultaneously, which results in a large enhancement of Raman gain. The enhancement factor of the Raman gain in the designed structure is more than 100 times that reported previously.

The physical principles of terahertz silicon lasers based on intracenter transitions

AbstractThe first silicon laser was reported in the year 2000. It is based on impurity transitions of the hydrogen‐like phosphorus donor in monocrystalline silicon. Several lasers based on other group‐V donors in silicon have been demonstrated since then. These lasers operate at low lattice temperatures under optical pumping by a midinfrared laser and emit light at discrete wavelengths in the range from 50 to 230 µm (between 1.2 and 6.9 THz). Dipole‐allowed optical transitions between particular excited states of group‐V substitutional donors are utilized for donor‐type terahertz (THz) silicon lasers. Population inversion is achieved due to specific electron–phonon interactions of the impurity atom. This results in long‐living and short‐living excited states of the donor centers. Another type of THz laser utilizes stimulated resonant Raman‐type scattering of photons by a Raman‐active intracenter electronic transition. By varying the pump‐laser frequency, the frequency of the Raman intracenter silicon laser can be continuously changed between at least 4.5 and 6.4 THz. The gain of the donor and Raman‐type THz silicon lasers is of the order of 0.5 to 10 cm−1, which is similar to the net gain realized in THz quantum cascade lasers and infrared Raman silicon lasers. In addition, fundamental aspects of the laser process provide new information about the peculiarities of electronic capture by shallow impurity centers in silicon, lifetimes of nonequilibrium carriers in excited impurity states, and electron–phonon interaction.